The post Cargo Stoppers – why they are critical and how to design them appeared first on TheNavalArch.

]]>A cargo transported on the deck of a ship is subject to many forces. These forces comprise of the inertial forces due to the ship motions – the three translations and three rotations – and the environmental force of wind.

Due to the combined effect of these forces, the cargo, if left unfastened on the deck of the vessel may not remain static at one location but may move around. To avoid this, it is critical that the cargo is restrained from movement. The design of all accessories to restrain the cargo is called Seafastening design.

A typical and simplest seafastening may comprise of lashing ropes (or wires) fastened to pad-eyes welded on deck of the vessel. However, for some heavy cargoes this may not be enough, and additional arrangements may be required for preventing both translational and rotational motion of the cargo on deck.

A Stopper is a simple structure which is welded to the deck of the vessel to prevent the cargo from sliding (i.e., the translational motion).

The very simplest stopper which can be imagined is simply a metal plate which is welded to the deck, and against which the cargo rests. The below figure should be able to provide a starting point.

*The simplest stopper comprising of a steel bar*

** **

Stoppers can be of many types. The choice of the right stopper depends on the geometry of the cargo, on the weight distribution of the cargo and on forces experienced by the cargo during the voyage. Stoppers can range from simple plate bars to complex H-beams (called Stanchions on ships).

Some of them are shown below:

**Types of Stoppers**

For example, the H-beam stoppers (also called stanchions) are mostly used for restraining pipes being transported. A box shaped cargo with most weight near its base will use a flat plate stopper (with or without face plate).

*H-beam stoppers (stanchions) used for restraining pipes*

The height of the stopper will depend upon the height where the cargo needs to be supported. For example, in the above pic showing stoppers for pipe transport, the stanchions need to be of a height more than the diameter of the pipe to secure the whole pipe bay adequately. Similarly, box shaped cargo like containers may find the simple steel bar stopper (or one with face plate) enough for restraining.

Whatever the shape and type of stopper selected, it should be structurally sound to perform its job – preventing the sliding of cargo, i.e., it should be able to overcome the forces which make the cargo slide. What are these forces?

he two forces which may lead to cargo sliding are:

- Transverse force, or the force which makes the cargo slide athwartship, i.e., Port and Starboard. This force arises due to sway and roll motion of the ship
- Longitudinal force, or the force which makes the cargo slide forward/aft. This force arises due to the surge and pitch motions of the ship.

The figure below demonstrates the two sliding forces.

To understand the structural design of a Stopper, let’s take an example of a box shaped cargo being restrained by a flat steel stopper with a face plate.

**Stopper with a Face Plate**

This kind of stopper has two components – a web plate and a face plate which are welded to each other. The face plate is what the cargo rests against, while the web plate is welded to the deck of ship. The details of the weld and configuration of the plate can be seen in the figures above.

Since the vessel experiences forces both in longitudinal and transverse directions, stoppers need to be installed on both longitudinal and transverse faces of the cargo. Taking a simple box shaped cargo as an example, a typical plan showing the stoppers is shown below. We can see that three stoppers on each side (Port/Stbd) are restraining the total Transverse force (F_{T}), while two stoppers on each side (Fwd/Aft) are restraining the Longitudinal force (F_{L}). The forces can be obtained from a motions analysis of the vessel, or using some empirical formulae (check out our products for calculating accelerations)

From the figure above, we can see that each transverse stopper takes one third of the total transverse force, while each longitudinal stopper takes one-half of the total longitudinal force.

Total force on each transverse stopper = F_{T }/ 3

Total force on each longitudinal stopper = F_{L }/2

Zooming down to a single stopper (whether transverse or longitudinal), it will experience a force which is F_{T }/ 3 for transverse stoppers and F_{L }/2 for longitudinal stoppers. Let’s call the force as F_{d} (Design Force). The design force acts on the face plate of the stopper. The location on the face plate where the force acts can vary depending on how the face plate is in contact with the cargo, but for simplification, we consider that the force is a point force acting at mid-height of the face plate. The figure below depicts this.

The force on the Face Plate is effectively transmitted to the web plate which is welded to the deck. This leads to a shear force on the web plate. Further, the force, acting at a height of H/2 (where H is height of face plate) leads to a turning moment which is transmitted to the base of the web plate, where the web plate is welded to the deck.

Thus, we need to analyze the web plate for:

- Shear Stress due to force F
_{d} - Bending Stress due to moment F
_{d}x H/2

*Weld Strength Check*

Since the weld is what affixes the web plate to the deck, the stresses in the weld are equally important to be evaluated.

Usually the weld type for welding the web to the deck is a fillet weld (two-sided). The weld profile for the stopper with face plate is shown below:

The force F_{d} and the resulting moment M are transmitted to the weld which is analyzed for:

- Shear stress on the weld
- Bending stress on the weld
- Total (combined) stress on the weld

With the above two types of checks (web plate strength check and weld strength check) performed, the stopper’s structural design is considered enough.

Similar principles can be applied to the design of other types of stoppers (H-beam stoppers or Stanchions).

To wrap up, stoppers are small structural items but play a vital role in retraining cargo during transport. Thus, their design should be done with due diligence and care to ensure a successful marine operation.

Please do check out our related products below:

- Stopper Design Spreadsheet

- Stanchion design spreadsheet

**3. Cargo Forces and Accelerations Calculator**

4. DNV Ship Motions Calculator

** **** **

**About the Author:**

**Prem Shankar, Naval Architect**

The author is the founder of www.thenavalarch.com, a website which aims to bring the marine and oil & gas industries to the internet through rich in quality but low in cost online content like useful calculators, video tutorials and articles. A graduate in Naval Architecture from Indian Institute of Technology (IIT), Kharagpur, he has more than 15 years of work experience in Singapore and India working with industry leaders like Mcdermott and Bumi Armada on the design, engineering, construction and installation of a variety of marine and offshore structures, ranging from ships to offshore platforms and FPSO’s. He also had the honor to serve as the Vice President of Society of Naval Architects and Marine Engineers Singapore from 2015-2017.

TheNavalArch is his endeavor to create a platform which brings out the knowledge currently lying with industry experts and places it on the web for everyone to benefit from. He’s constantly looking for collaborators who are interested in creating rich content and contributing to the growth of this platform and to the industry at large. Interested experts can contact him at info@thenavalarch.com

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for the accuracy of this article, or for any incidents/losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any activity which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

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]]>The post Are you chartering a tug bigger than necessary for your Barge? appeared first on TheNavalArch.

]]>

Barges are one of the most frequently used means for transporting deck cargo of different shape, size and weight. While some barges are self-propelled, the majority is towed by another vessel called a ‘Tug’.

Once an owner or charterer has decided on the Barge to be used for a transport operation (depending on the size and nature of cargo), the next step is to decide the tug which will be adequate to tow the barge.

From a very fundamental point of view, the tug should be able to overcome the weather forces which the Barge experiences when it is being towed in the sea. The weather forces are those of Wind, Current and Wave. Together, we call them the “environmental forces” or simply the “environment”.

A tug is considered adequate for a towing operation if it can HOLD the Barge in a prescribed environment. The word ‘prescribed’ carries huge significance. Who prescribes the environment? What is the prescribed environment?

**The effect of Environment on tug size**

The prescription for the environment can be found in guidelines prepared by organizations like Classification Societies or Marine Warranty Surveyors (MWS). For example, the DNV-ST-N-001 Section 11.12.2.4 gives the standard environment for open ocean tows as Wind 20 m/s

Current 0.5 m/s

Wave (Significant Height) 5 m

For standard OPEN OCEAN tows, the tug should be able to HOLD the barge static in the above-given environment.

In most cases, the standard weather criteria are applied to find out the environment forces which the barge will experience, and then select a tug big enough to overcome these forces.

What if the tow is not in the open ocean? What if it is a tow in a benign area?

Applying the OPEN OCEAN criteria indiscriminately for coastal or sheltered tows will result in unnecessarily high environment forces and selection of a tug bigger than required. After all, why select a bigger tug and pay more if a smaller tug could have done the job as well?

Benign tows will require much milder environment criteria, as specified in DNV-ST-N-001 Section 11.12.2.6

**DNV-ST-N001 Environmental Criteria for Bollard Pull Calculation – Standard vs Benign Criteria**

**How do I know the right sized tug for a benign area?** ** ** There are very simple steps to follow to find out the right sized tug for the vessel.

**Study the tow route**: First and foremost, study the tow route. Get the historical weather data and nautical charts to see the maximum wind, wave and current expected for the route for the period you’re planning to tow. It may also happen that the historical data for the whole year shows higher environment**Discuss the tow route with approval authority/MWS**: Once you’ve established that the weather qualifies for the ‘benign weather areas’ criteria using weather data from a confirmed source (like Metocean data or from Nautical Charts), send the details to the Marine Warranty Surveyor or the approval authority designated for the operation. Get approval from the MWS for using a ‘benign weather areas’ condition for calculating the required bollard pull. This may involve going back and forth with the MWS multiple times, supporting your case with data and arguments. Also, at times MWS may recommend a higher weather criterion than the one stated above for ‘benign’ condition to have some more margin of error.**Calculate the required bollard pull for the vessel being towed:**Once the environment is established and approved by the MWS, use this environment to calculate the required bollard pull. Bollard pull can be calculated using a detailed hydrodynamic analysis or using some simple calculators. TheNavalArch has some useful calculators which have been used by hundreds of our customers over the years to successfully obtain approvals for their tows. You can check them out below:

Some scenarios which can come up and possible actions are:

- MWS not willing to approve the lower environment than standard – this may happen if the Surveyor assigned is relatively inexperienced. Ensure that you have the right data. Escalate the matter with MWS and hold meetings in person if needed to obtain approval
- Selected tug claims a high bollard pull on paper but doesn’t deliver in practice. In such cases, a bollard pull test can be conducted to ascertain the claim.

To summarize, it is critical to establish the right environment (Standard or Benign) before selecting the tug. A little effort at the beginning stages while selecting the tug will ensure huge cost savings by selecting the optimum tug and avoiding an unnecessarily large tug for the same tow job.

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]]>The post Fendering – an Introduction appeared first on TheNavalArch.

]]>Source: www.trelleborg.com

In this two part article we will talk about Fendering, which is one of the basic but critical operations related to a ship. Fendering is, basically, protecting the ship’s sides from contact with another body (which can be another ship, jetty or quay wall). It can also mean protecting the jetty or berth from contact with ships. Thus, there are two situations for which fenders may need to be designed:

- For Ship-to-Jetty berthing
- For Ship-to-Ship berthing

This is a two part article. In this first part, we will cover the following:

- Berthing Energy
- Berthing types – side and end berthing
- Selecting the right fender
- Rules and standards

The second part will cover the detailed engineering calculation of Berthing Energy.

__Section A: Berthing Energy__

When a ship comes close to the berth or close to another ship, then there is a chance of the ship’s body impacting against the berth or other ship. This impact can damage the ship’s body or the berth. Fenders are provided to absorb the impact of berthing, and minimize the effect on the ship or berth/jetty.

If there is no fender, this energy KE (the Berthing Energy) will be completely transferred to the Jetty OR the other Ship.

Thus a fender is meant to absorb the impact during berthing. The energy of impact is called the *‘Berthing Energy’*

We will now discuss the mechanics of impact, and what is the berthing energy which the fender has to absorb.

When a body moves, it carries with it the kinetic energy of motion. During an impact, the kinetic energy is transferred to the body impacted. The kinetic energy is given by

**Kinetic Energy of Impact, KE = ½ x M x V ^{2}**

There are two scenarios possible

- Ship (say, Ship 1) berthing along a Jetty or Quay
- Ship (say, Ship 1) berthing along another Ship (say, Ship 2)

*Mass, M*

In case of a ship berthing along a Jetty or Quay, M is the mass of the Ship 1. In case of Ship 1 berthing along Ship 2, M is the ‘effective mass’ of the two-ship system. The Ship 1 moves ** relative** to the ship 2 as a body of mass

**M = M1 x M2/ (M1 + M2), **

where M1 = mass of Ship 1, M2 = mass of Ship 2

*Berthing Velocity*

The velocity of berthing, V, is a very critical input in calculating the berthing energy. The berthing velocity is determined based on many factors

- Ship size to be berthed.
- Environmental parameters at location of berthing – whether berthing is sheltered or exposed
- Berthing angle and ease of berthing

A guide for determining the berthing velocity is found in PIANC 2002. A graph which shows different curves considering different berthing scenarios is used to determine the berthing velocity.

Estimating the berthing velocity (source: www.trelleborg.com)

In fact, there are other factors which come into play in determining the Impact Energy. These are

- Added mass factor – due to mass of water moving along with ship
- Eccentricity factor – due to ship not berthing parallel to the jetty
- Softness Factor – whether the fender is soft or hard
- Berth Configuration factor – whether the jetty/quay is solid or open type

These will be taken up in detail in **Part 2** of this article.

*Normal and Abnormal Berthing Energies*

Berthing energy is classified as ‘Normal Berthing Energy’, and ‘Abnormal Berthing Energy’. Normal Berthing Energy is the energy capacity of the fender required for regular operations during the lifetime of the berth/vessel. Abnormal Berthing Energy is the energy capacity required to take care of rare incidents in which there can be significant fender damage, e.g., rare environmental hazards, or exceptionally large ships encountering the berth etc.

By now, we know that the fender is to be designed to absorb a Berthing Energy which depends on the size of the ship and other factors. Let’ see the concepts of Side and End berthing before we move on to see how to select the right fender.

__Section B: Fendering methods – Side Berthing or End Berthing__

A ship berthing with a jetty/quay can berth with either its side or its end aligned to the jetty/quay. There can be other types of berthing like Dolphin berthing or Locks, but we’ll limit ourselves to Side and End berthing for this article. The selection of berthing type will depend on factors like the ship size, berth type and size, and the ease of approach.

The figure below demonstrates the two different types of berthing:

Types of Berthing (Source: www.rubberstyle.com)

__Section C: Selecting the right fender__

Selecting the right fender requires a step-by-step approach which we delineate below:

*Step 1 – Homework*

Before evaluating the properties of fender needed, we need to do some background work to gather relevant information through a study of two broad aspects:

. Some important factors are –**Environment in which the fender is going to operate**- Berth construction
- Available space for fenders
- Seabed depth
- Tidal ranges
- Corrosion levels

. Some important factors are –**Range and characteristics of ships which it is going to serve**- Ship sizes to be served
- Ship types – a passenger vessel will have different fendering requirements than a Ro-Ro vessel
- Approach speeds
- of points of contact
- Frequency of berthing
- Mode – Side/End berthing
- Bow construction/flare angles of ships served

* *

*Step 2 – Estimating the berthing energy*

The next step is to estimate the berthing energy required to berth the vessel. The berthing energy is the most critical test of a fender. Whatever be the shape, size or material of the selected fender, it should be have the capacity to absorb the impact of the berthing energy. The fender’s capacity should be sufficient for it to absorb the Abnormal Berthing Energy. The detailed calculations for estimating the berthing energy shall be taken up in the **Part 2** of this article.

*Step 3 – Fender selection*

With the information from Step 1 available, and with the required berthing energy calculated, we can now select the right fender.

Selecting a fender is an exercise with multiple factors and constraints to be managed.

*Fender type and shape**–*Fenders can be fixed to the structure (e.g. in a berth/jetty) or floating (e.g., pneumatic fenders on a ship). This is best decided based on a case-to-case basis depending on the design requirements at hand. For example, for ship to ship berthing, pneumatic or foam fenders are generally used. These are usually hanging on the side of the vessel through chains and are lowered down when approaching the berth. On berth/jetty, a fixed type of fender is generally used, but it can also be floating one for berths with high tidal variation.

Fenders can also be of different shapes. Flat panel fenders are generally used on berths, while cylindrical/pneumatic fenders are used on ships. Pneumatic fenders are filled with air at high pressure to provide the absorption energy needed. They are generally used in ship-to-ship berthing and have a low deflection and high energy capacity. Foam fenders can be cylindrical or spherical and have a core of foam and an outer skin of a polymer. Donut fenders are designed to simply slip on a pile and float up and down on the pile with tidal variations. However, the selection really depends on the design needs of the case at hand.

* *

**Fixed fenders**– Cell fenders, V-type fenders and cylindrical fenders are generally used as fixed fenders.

V-type fender (source: www.theartofdredging.com)

Cell type fender (source: www.trelleborg.com)

Cylindrical fixed fender (source: eurotechbenelux.com)

**Floating fenders**– Foam fenders, donut fenders and pneumatic fenders are generally used as floating fenders.

*Foam Fender (source: **www.trelleborg.com**)*

*Pneumatic Fender (source: **www.airbag.cc**)*

*A donut fender (source: www.shibata-fender.team)*

– The fender should have energy capacity to absorb the Abnormal Berthing Energy. Allowances are required to be made for factors like temperature, velocity, compression angle, manufacturing tolerance etc. The fender’s energy capacity is measured in terms of RPD, or Rated Performance Data which is its capacity at a temperature of 23 degrees Celsius, 0.15 m/s impact velocity, 0 degrees compression angle and mid-tolerance. For values of temperature, velocity, compression angle and tolerance differing from these standard values, we need to apply allowance factors. The fender energy capacity is calculated as*Checking the energy capacity*

E_{F} = E_{RPD} x f_{TOL} x f_{ANG} x f_{TEMP} x f_{VEL}

Where

E_{F} is the minimum fender energy

E_{RPD} is the fender energy at Rated Performance Data (RPD)

f_{TOL} is the allowance factor for fender manufacturing tolerances

f_{ANG} is the allowance factor for compression angle. The minimum energy occurs at the maximum compression angle

f_{VEL} is the allowance factor for the impact velocity other than 0.15 m/s. The fender’s minimum energy occurs at the maximum impact velocity

f_{TEMP} is the allowance factor for temperatures other than 23 degrees C. The minimum fender energy occurs at highest temperature.

Each of the above factors can be determined from curves specific to the fender being selected.

– The fender is not only supposed to absorb the berthing energy, but it’s impact on the ship/berth structure too should be minimal. The reaction force is the force that the fender imparts on the vessel/berth. This reaction force must be less than the structural capacity of the vessel/berth. The maximum fender reaction is given as:*Checking the fender reaction*

R_{F} = R_{RPD} x f_{TOL} x f_{ANG} x f_{TEMP} x f_{VEL}

Where

R_{F} = maximum fender reaction

R_{RPD} = fender reaction at RPD

The rest of the terms are as described above.

– Spacing of fenders is specified by ‘fender pitch’ which is the distance between two adjacent fenders. The spacing of fenders is critical – fenders spaced too far apart may lead to the vessel hitting the berth. The spacing of fenders should be determined by studying the complete range of vessels expected to visit the berth. The spacing is determined from properties like bow radius and length of vessel. Generally, it is not recommended to exceed a spacing of 10 – 15 m. The figure below demonstrates the fender pitch (P).*Fender Spacing and contact*

Fender Pitch (source: www.trelleborg.com)

*Fender Contact*

The fender selection depends also on how many fenders are in contact with the vessel. The berthing energy is accordingly divided among the fenders in contact, depending on the extent of deflection of each fender.

Fender contacts – 2 Fender and 3 Fender contact cases (source: www.rubberstyle.com)

* *

* *

*Fender material and construction –*

** For panel fenders, **the panels are generally made of weldable structural steel. The actual material selected depends on the design requirements. For heavy duty environments, higher grade steels may be used. The panels are then covered with low-friction pads to minimize damage due to frequent encounter with the ship’s hull. Low friction material is important for longer life and lower maintenance cost of a fender. Generally, Ultra High Molecular Weight Polyethylene (UHMW – PE) pads are fitted to fender panels. They have good wear resistance, low-friction and long life.

** For pneumatic fenders, **generally rubber fenders are used. Natural rubber, synthetic rubber or a blend can be used. Synthetic rubber is generally more weather resistant than natural rubber. EPDM (Ethylene Propylene Diene Monomer) rubber is a special purpose rubber with weather resistance and long life. However, almost all rubber fenders have poor resistance to oil, fuels, hydraulic fluids, acids and most hydrocarbons.

– Depending on the shape and type of fender selected, there will be other factors which we will need to consider while selecting the right fender.*Other Factors*

** Accessories design **– Fender chains, shackles, nuts, pins, wheels, rollers, brackets etc. should also be designed to support the fender weight and prevent excessive movement of the fender.

** Hull pressure, belting and bending Moment **– The hull pressure against the fender affects the internal structure of fenders made of steel panels. For cell fenders installed on the berth, the bending moment imparted by the hull pressing against the fender should be within the strength of the fender. Different loading cases need to be analysed and the structural strength of the fender needs to be assessed for each case

*Paint and corrosion prevention** – *Fenders are required to function in harsh and corrosive marine environments. The fatigue due to frequent berthing adds to the corrosivity. Similarly, high temperatures in tropical zones and ship vibrations can be additional factors which lower the life and performance of fenders. The material of the fender must be reliable and corrosion resistant. Several methods can be used

- Galvanizing of all steel pads or accessories like chains, shackles etc
- Using sacrificial anodes
- Using protective paint coatings – the standard followed is ISO 12944
- Using stainless steel fixtures

** Availability, cost and spares **– the availability of the selected fender design, its cost, and availability of spare parts are other important factors to be considered.

** Testing **– Fenders can be tested in the factory before being purchased (Factory Acceptance Test or FAT). Tests can be setup to measure energy absorption and reaction force. Testing can be done in accordance with PIANC guidelines.

* *

*Section D: Standards and guidelines*

*ROM 2.0-11** Actions in the Design of Maritime and Harbor Works*

*ROM 3.1** Actions in the Design of Maritime and Harbor Works: this is the latest version of the Spanish ROM available in English*

*BS6349-4:2014** Code of Practice for Design of Fendering and Mooring Systems*

*EAU 2004** Recommendations of the Committee for Waterfront Structures*

*PIANC 2002** Guidelines for the Design of Fender Systems: 2002 Marcom Report of WG33*

*ISO EN 12944** Standard for Corrosion protection of steel structures by protective paint systems*

*ASTM** An international standards organization that develops and publishes voluntary **consensus technical standards for a wide range of materials, products, systems, and services*

*EN 10025** A set of European standards which specify the technical delivery conditions for hot rolled products of structural steels*

*JIS G-3101** A Japanese material standard for hot Rolled steel plates, sheets, strips for general structural usage*

*PIANC report WG121** Harbor approach changes design guidelines from 2014 incl. the latest design information on vessels*

*OCIMF Ship to Ship Transfers** – Considerations Applicable to Reverse Lightering Operations*

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for any incidents or losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any activity which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

The post Fendering – an Introduction appeared first on TheNavalArch.

]]>The post Loadouts – An introduction appeared first on TheNavalArch.

]]>*Image source: Flickr*

Loadout is a term oft heard of in the marine/offshore industry parlance. Loadout is generally referred to the operation of transferring a Cargo or a Structure from the place of fabrication to a sea-borne vessel, to be further transported on the vessel to the final destination. The Cargo or Structure can be as varied as a jacket, plant modules, steel structures, trusses or building structures, or even pipes etc. The place of fabrication can be a Yard, or a storage facility with access to sea. The vessel used can be a ship or a barge, depending on the demands of the transportation operation.

*Types of Loadouts*

Loadouts can be of different types:

A. Lifted Loadouts – In these kind of loadouts, the cargo to be loaded is simply lifted by a crane and placed in the right position on the vessel

*Lifted Loadouts (source: www.gspoffshore.com)*

B. Skidded Loadouts – In this loadout, skid beams/rails are constructed on the vessel. During the operation, the cargo is pulled onto the vessel using winches and it ‘skids’ to the vessel from the quay side.

*Skidded Loadout (Source: Flickr)*

C. SPMT Trailer loadouts – In this type of loadout, the Cargo is loaded onto a SPMT trailer. The SPMT trailer then moves from quayside to the vessel using link bridges (which are ramps connecting the Quay to the vessel).

*SPMT Trailer Loadout (Source: www.flickr.com)*

The selection of the right loadout method depends on the Cargo dimensions and weight, the limitations of the Quay side structure and cost effectiveness.

Generally, skidded/SPMT loadouts are done longitudinally, i.e., the Cargo is moved along the length of the vessel. However, in some cases, transverse loadouts are also carried out (cargo moving along the width of the vessel) – when longitudinal strength of the vessel may be a concern.

*Transverse Loadout (Source www.coscoht.com)*

**Challenges**

Skidded/SPMT loadouts are generally done in steps, and are calibrated exercises. The Cargo gradually moves from Quay side to the vessel, thus transferring the load. As the load transfers to the vessel, the vessel sinks and trims. The challenge is to maintain the vessel’s even keel and to keep it level with the quayside. This is done by calibrating the ballast in the tanks at each stage to maintain the vessel’s position. The longitudinal strength of the vessel too has to be maintained within limits throughout the exercise. Another layer of challenge is accounting for tidal variations during the loadout. The ballasting sequence has to take into account the rise/fall in tidal levels during the loadout operation itself.

**Engineering for loadouts**

Following engineering aspects are important for loadouts

- Study of Quay side structure – Involves study of tidal patterns and draft availability
- Vessel selection – depends on cargo size, quay side water depth
- Loadout Ballast Engineering – This is done by Naval Architects in consultation with the vessel master. It provides the patterns of ballast for different stages of loadout as the Cargo moves from the Quay side to the vessel in steps. The purpose is to maintain the vessel on level with quay, and on even keel, while ensuring longitudinal strength is within limits. The stability parameters too need to be checked and be within limits.
- SPMT Trailer selection and stability check – for SPMT loadouts
- Bridge plate/Ramp design – for SPMT and Skidded loadouts

Loadout engineering is an iterative process which takes multiple factors into account while arriving at the optimum design and ballasting pattern for a specific Cargo. Despite the best of engineering efforts, surprises during the actual operation are not unexpected, and the engineering team works closely with the operations team to ensure a successful loadout operation.

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for the accuracy of this article, or for any incidents/losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any activity which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

The post Loadouts – An introduction appeared first on TheNavalArch.

]]>The post Introduction to Pipe Transportation – Part 3 (Engineering) appeared first on TheNavalArch.

]]>In Part 1 we learnt about pipes, while in Part 2 we learnt about Planning and Scheduling of pipe transport operations. In this part, we will learn about Engineering.

__Phase 3 – Engineering.__

Engineering for pipe transportation is an intriguing subject. The shape of the cargo demands unique ways of stowage and lifting.

The whole exercise of engineering has two categories – 1. Calculations for pipes 2. Transportation Analysis for vessels

**Category 1 – Calculations and Analyses for the pipes.**

The pipes to be transported may be of varying dimensions and weight, and each pipe type needs to be studied for its own requirements of lifting and stowage.

Two major calculations related to the pipes are discussed below

**Stacking Height calculations**– for each type of pipe included in the project, the stacking height limit should be calculated. The stowage of each pipe type will be governed by its stacking height limit. We discussed in the section on ‘Pipes’ as to how stacking height limit is to be determined. The reader may also refer to our product ‘Stacking Height Calculator for Bare Pipes’

**Lifting plan and lifting calculations**

Pipes need to be lifted at the yard to be loaded on to the vessel. They also need to be lifted from the vessel to be fed to the lay barge. Thus, a lifting plan for the pipes needs to be in place.

A simple lifting configuration for pipes is to have a two-sling lifting arrangement. In this arrangement, one end of each sling is connected to the pipe while the other end goes to the crane hook. The end connected to the pipe has a soft eye which is large enough to accommodate the pipe. Masterlinks are used to connect to the hook end. In some cases, a spreader bar is used between the crane hook and the pipe. Separate slings are used to connect between crane hook & spreader, and between the spreader bar & pipe. Shackles are used to connect between the pipe and slings. Using a spreader leads to smaller slings being used which may be operationally more manageable. In some cases, a webbing sling may be used to go around the pipe. The lifting plan should be prepared in thorough discussions with the Operations team. In some cases, more than one lifting plan may have to be prepared to cater to different pipe types being lifted.

Whatever lifting arrangement is used, the lifting plan must clearly state the specifications and quantity of each item used in lifting.

Further, for each lifting plan prepared, a supporting lifting calculation must be prepared to demonstrate the adequacy of the lifting arrangement. GL Noble Denton Guidelines (Ref [2], ND-0027) is the guiding document for lifting design.

Following points may be helpful which doing the lifting calculations

- Dynamic Loads – The loads on the slings will comprise of the self-weight of the pipe, as well as any dynamic loads expected. Usually, dynamic loads are higher offshore compared to yard. A rule of thumb is to take a factor of 1.3 for dynamic load. If a spreader bar is present, its weight must be included in calculations for upper slings and fastenings
- Strength check – Each item of the lifting arrangement must be checked for its strength. These include the shackles, pad-eyes, slings, spreader bars, webbing slings, masterlink, delta plate etc. Usually a check against WLL is sufficient, but in some cases Shear and Bending Stresses need to be checked too.
- Lifting multiple pipes – If the pipes are of smaller size and weight, multiple pipes may be lifted at a time by using multiple webbing slings (see picture below)

- MWS Approval – The Lifting plan and calculations should be submitted to the MWS for approval before any operation commences.

__Category 2 – Transportation Analysis for the vessel(s).__

Once the pipe calculations and lifting plan are available, the next step is to carry out the detailed Transportation Analysis for the vessel(s) involved in the transport. The purpose of this Analysis is to demonstrate the suitability of the vessel(s) for the transport. We will etch out a step-by-step process below for carrying out a Transportation Analysis

*Step 1 – Prepare the vessel’s stowage plan (stowage on deck)*

The first action required is to prepare a stowage plan for the selected vessel. For simplicity, we will limit discussion to vessels which can stow only on the deck. Stowage in Cargo holds will not be covered in this article.

What is a stowage plan? It is a plan showing the way pipes are going to be loaded on the vessel. Let’s take an example of a simple barge of length 76 m and width 30 m. Let the clear available deck space on the vessel be 66 m x 26 m. Pipes are to be loaded on this vessel. Pipes are generally stacked with their lengths oriented along the length of the vessel. This is called Longitudinal stacking. Pipes can also be stacked with their lengths along the vessel’s width (Transverse stacking) but it is not a preferred method.

A simple way to prepare a stowage plan can be as below:

- Load pipes on the forward end of the available deck space. The bottom tier will span 26 m wide. Keep loading tiers above the bottom tier till the stacking height limit is reached. This is the first stack of pipes. It is also called a ‘Bay’. Its length is the length of one pipe = 12.2 m.
- Start loading pipes behind (or aft of) the fwd Bay. For lifting and manual handling of pipes, we need to keep a clear spacing between the fwd Bay and the second bay (minimum 400 mm). In a similar fashion, keep loading pipes in second bay till the stacking height limit is reached.
- Load the next Bay aft of the previous one. Keep repeating this process till the aft end of the available deck space is reached.

Loading in the above fashion, we will arrive at a stowage plan as shown in the figure below.

__A simple calculation for number of bays and their spacing__

Usually the number of bays possible to be loaded on the vessel can be decided from some initial calculations. We will derive the formula for the number of bays possible.

Let the available length of deck space be L_{d}, the spacing between bays be s, and the length of a bay (same as pipe length) be l_{p}. Let the number of bays be n_{B}. The number of spacings between bays is one less than the number of bays.

From the figure shown, we can write a simple relationship

Length of bay x number of bays + spacing x number of spacings = Length of deck

The number of spacings is one less than the number of bays.

Thus, l_{p} x n_{B} + s x (n_{B} – 1) = L_{d}

- s = (L
_{d}– n_{B}x l_{p})/ (n_{B}-1) …………………………..Eq(1) - Let the minimum spacing required be smin. Thus s >= s
_{min} - (L
_{d}– n_{B}x l_{p})/ (n_{B}– 1) >= s_{min} - n <= (L
_{d}+ s_{min})/ (l_{p}+ s_{min}) …………………………….Eq(2)

In the present case, L_{d} = 66 m, l_{p} = 12.2 m, s_{min} = 0.4 m. This gives n <= 5.26. Since n must be an integer, n <= 5. Thus, the maximum number of bays possible is 5. Using this n_{B} = 5 in the Eq(1), we get the actual spacing between bays as s = (66 – 5 x 12.2)/(5-1) = 1.25 m or 1250 mm.

__Excel Spreadsheet__

One good way of representing the stowage plan is to use an excel spreadsheet. Once the number of bays is known, we can create a spreadsheet showing all the bays, and the number of pipes stacked in each bay. The spreadsheet can also be used to calculate the total weight of pipes in each bay and on the vessel.

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Here it is useful to highlight some points regarding preparing a stowage plan

- Numbering of bays – Usually bays are numbered beginning from fwd end. The foremost bay is Bay No. 1, the one aft of it is Bay No. 2 and so on.
- Deck Loading Limit – The deck of the vessel will have a limit to the load it can take. It is usually specified in tons per sq. m. In the spreadsheet, the deck loading of each bay must be calculated, and it should be ensured that the deck loading limit of the vessel is not breached. It may so happen that we may not be able to load pipes upto the stacking limit because the deck loading is reached earlier.
- Height restrictions for manual handling – For manual handling of pipes, there are restrictions on the maximum height of a bay of pipe on deck. Generally, a maximum height of 2 m is followed. It may happen that the stacking height limit of the pipe is more than 2 m. In such a case, the maximum tiers possible will be dictated by the height restriction of 2 m, and not by the stacking height limit.
- Trim by aft – Usually in marine transportation, a slight trim by aft is preferred when the vessel is in sea (see Ref[2] ND 0030). This can be achieved by ballasting the vessel. However, trim by aft can also be achieved by arranging the pipes and adding more pipes aft than forward. This is a sensitive exercise and should be done with caution.
- Vessel’s capacity – We may be able to load pipes on all bays upto the stacking height limit, but the total weight of pipes loaded must be less than the maximum Deadweight (carrying capacity) of the vessel. If the total weight of pipes is exceeding the deadweight, then pipes should be removed till the total weight of pipes is less than the deadweight.
- Multiple pipe types in holds and deck – If two or more different types of pipes are being loaded in a vessel which has loading in both holds and deck, then the pipes which are to be unloaded first should be loaded on deck, and those to be unloaded later should be loaded in holds.

From the above we can see that preparing a stowage plan is an exercise fraught with multiple constraints, and requires caution and experience. thanavalarch has its own spreadsheet developed for pipe stowage plans on vessels with only deck loading. You may check it here.

*Step 2 – Study the route and environmental Parameters*

Source: pixabay

The next step is to collect the data on the environment which the vessel will experience along the route. The data includes the wind, wave, and current parameters. These are very critical inputs in the analysis, because they will determine the forces which the vessel and the pipes experience in the sea.

Environment data can be obtained from Nautical Charts, or from a data provider like Metocean. The data for the relevant window of transport should be obtained, and the most extreme environment during the window should be used for further analysis. Reading and extracting environment data is a separate exercise, and will not be dealt with in this article. We will move on to the next step – the Ship’s trim, stability, and strength evaluation.

*Step 3 – Evaluating Vessel’s Trim, Stability and Strength*

Now we have a stowage plan at hand, we would like to know what will be the floating condition of the vessel when loaded with pipes. We would also like to check the vessel’s stability and strength in this loading condition. For this, we need to know the vessel’s hydrostatics and other geometric properties like cross curves etc. Depending on the data availability, different approaches may be employed:

- If the vessel has a loading computer onboard, then the stowage plan can be sent to the vessel’s Master who performs the stability and strength evaluation and sends back the report. Generally loading computers are installed onboard all merchant vessels, but not on unmanned barges.
- If the vessel doesn’t have a loading computer, but if it has stability booklet with all required information (like trimmed hydrostats and cross curves), then hand calculations can be performed to check trim and stability. However, hand calculations are not always accurate and dependable.
- If the vessel doesn’t have loading computer or reliable data, then the stability can be evaluated by modeling it in a computer software like GHS, Maxsurf, NAPA, Autohydro etc. For this, the linesplan of the vessel should be available. Generally, this method is adopted for barges, for whom credible data or loading computer is not available.

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__Preparing the loading plan__

The loading plan of the vessel will include, apart from the pipes to be stowed, the amount of liquids like ballast, fuel oil etc. which will be used onboard. When preparing a loading plan, following points are helpful:

- Draft restrictions – There may be draft restrictions at the loading/unloading locations. The stowage plan and loading plan should be prepared keeping these in mind
- Trim by aft – Loading plan should preferably be prepared to provide a slight trim by aft. In any case, a trim by fwd is not acceptable, and should be avoided at any cost
- Hog and Sag – Tanks should be ballasted to avoid making the vessel experiencing too much hog or sag. For example, if the pipes have been loaded more around the midship, the vessel will tend to sag. In such case, ballast tanks of aft and forward should be utilized to correct the sag. If ballast tanks around midship are filled, the sag will increase and longitudinal strength may be compromised.

__Checking Floatation, Stability and Strength__

Once the Loading plan is prepared, we can check the equilibrium floating condition of the vessel. The floating hydrostatics will provide the draft, heel, trim, VCG, KM, GM etc. The floating condition stability should be evaluated against the standard rules of IMO Stability. Conventional IMO rules relate to sea going merchant vessels as per IMO RESOLUTION A.749 (18) Ch 3.

There are separate IMO rules for Stability of non-self-propelled Barges. These are in IMO Res A 749(10) Sec 4.7. These can be followed for barges.

For conventional ships, both departure and arrival conditions should be checked. For towed barges, only one loading condition needs to be checked.

The longitudinal strength of the vessel should be checked using a computer software. The Shear Force and the Bending Moments resulting from the check should be within the limits of the vessel. In a vessel with loading computer, the limits are available. However, for vessels like Barges, these limits are not available, and they can be calculated from some Class rules available (e.g., ABS Rules for Barges).

If the longitudinal strength requirement is not met, then the loading condition is not appropriate. It needs to be modified. For example, if the vessel is failing in sagging condition, this means there is too much load near midship. The load needs to be transferred to fwd and aft parts of the vessel. This can be done using ballast tanks. In some cases, rearranging the pipes (provided there is space) can also achieve the desired results.

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*Step 4 – Motions Analysis*

At this stage, the stowage plan has been prepared and stability of the vessel evaluated. Next step is to do a motions analysis of the cargo (i.e., pipes). What is a motions analysis and why do we need it?

A motions analysis is an analysis done in a computer software to calculate the motions and accelerations which the vessel and its cargo will experience in the sea in the given environment. The results of motions analysis are the forces/accelerations acting on the cargo in the three directions – Longitudinal, Transverse and Vertical. It also gives the maximum magnitude of the six motions of the vessel – roll, pitch, heave, surge, sway and yaw.

Motions analysis requires sophisticated computer software. In case a motions analysis cannot be performed, the alternative is to follow industry guidelines like Nobledenton Guidelines for Marine Transportations (See Ref[1] ND-0030, Sec 7.9).

From motions analysis, we will know the motions and accelerations at the different bays of pipes. The most conservative results will further be utilized for seafastening calculations. Readers may like to check thenavalarch’s product ‘Cargo Forces and Accelerations’ based on Ref[1] ND-0030.

Considering its vastness, a detailed discussion on motions analysis is out of scope of this article. We will move on to the next step which is seafastening calculations for the pipes.

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*Step 5 – Seafastening Calculations*

Perhaps the most niche section of the Transportation Analysis is the seafastening calculations for the pipes. Due to the unique shape and stowage plan of the cargo, unique engineering solutions have to be deployed for the seafastening of pipes. First, what do we mean by seafastening calculations?

Seafastening is the way the cargo will be ‘fastened’ or secured to the deck of the ship during transport. It is obvious that we cannot be transporting the stacked pipes on the ship without somehow securing them to the deck. This is because the pipes are subject to forces and motions (see previous section), and they may be lost to the sea if not secured. Talking about securing pipes to the deck, the first solution which comes to mind is to tie ropes around them and secure these ropes to the deck of the ship. This actually is the way pipes are secured to the deck of the ship. However, this is not the complete picture. Let us consider one-by-one all the different forces and motions which the pipes experience, and what securing solution to devise for them.

- Rolling motion of the ship (pipe overturning) – when the ship rolls to one side, the pipes loaded on the ship tend to roll over. This is called the overturning of pipes, and is depicted below.

Rolling leads to an overturning moment on the pipe. To prevent this overturning, lashing ropes can be utilized. These ropes run across the bay of pipe and are secured to the deck. From engineering calculations, it should be demonstrated that the ropes are strong enough to contain the overturning of a complete tier of pipes. These are called pipe overturning calculations. Pad-eyes are welded on the deck to secure these ropes to the deck. The calculations should also demonstrate the adequacy of the strength of pad-eyes.

- Pitching motion of the ship and longitudinal forces (pipe sliding) – when the ship pitches, the pipe tilts along its length. Due to this tilt, there is slipping between two tiers of pipes. The top tier of the pipe may slide over the tier below. Slipping also depends on the magnitude of pitching experienced. The longitudinal force experienced by the vessel adds to this sliding force.

In most cases if the pipes are concrete coated, the friction between two tiers of pipes is sufficient to prevent the slipping due to pitching. However, in cases of bare pipes being shipped, or when the pitching is high enough to cause sliding, then arrangements have to be made to prevent the sliding of pipes. One method is to design longitudinal stoppers to hold the pipes longitudinally. Other method is to use anti-friction rubber mats placed between tiers of pipes to improve friction between the tiers. Engineering calculations are required to be done for checking the sliding of pipes and present solution required. This is called ‘Pipe sliding verification’

- Transverse forces on ship and stanchions – the transverse acceleration of the ship results in the bay of pipes swaying to the side of the vessel. While lashing across the bay prevents the overturning of the pipes, it may not be sufficient to hold the pipes from swaying. This transverse motion of the pipes is contained by providing ‘stanchions’ on the ship. A stanchion is a vertical beam which is welded to the deck of the ship. Mostly for pipe transport operations, an I-beam stanchion is the most preferred design. The pipe bays rest on the stanchions on the sides (see figure below). All the transverse forces on the pipe bays are taken by these stanchions. A vessel may have existing stanchions which may be utilized, or new stanchions may be welded, depending on the requirements and forces experienced. In any case, engineering calculations and analyses are required to demonstrate that the stanchions are fit for the purpose. Sometimes a FE Analysis may also be needed.

- Vertical forces and dunnage crushing – In addition to the self-weight of the pipes, heave forces due to ship motion also act on the deck of the vessel. The deck of the vessel takes this combined force. Usually, the pipes are not loaded directly to the deck, and there are wooden planks (called dunnage) which are placed between pipes and deck. The dunnage needs to be thick enough to take the dynamic vertical loads. The contact area between pipes and dunnage is small (considering pipes have circular section), and so the compressive stress on dunnage is expected to be high. This stress needs to be evaluated, and dunnage accordingly selected to fit the requirements. This is called ‘Dunnage crushing verification’

*Step 6 – Lashing Design*

Based on the results of Seafastening design, the whole lashing plan to secure the pipes to the vessel needs to be prepared. The following items are part of the lashing plan of the vessel.

- Lashing ropes to prevent overturning – Wire ropes are generally used for this purpose. Generally, wire ropes with both ends soft-eye are utilized.
- Pad-eyes or D-rings – Pad-eyes or D-rings of adequate strength need to be welded to the deck of the vessel to tie the lashing ropes. In some cases, pad-eyes are welded to the top of stanchions.
- Shackles – to connect the pad-eye to the lashing wire rope
- Turnbuckles – may be needed to secure wire rope to deck
- Wire rope clips – for soft end of ropes
- Timber dunnage (planks) – to be placed between pipe bays and deck, and also between stanchion and pipes
- Gunny sacks or rubber hose – To protect the pipe from damage by wire rope, gunny sacks can be placed between pipe and rope, or rubber hoses can be fitted to the wire ropes.

The lashing plan should clearly show the locations, specification and quantity of each lashing item utilized. A simple lashing plan is shown below (only for reference)

**Lashing in Cargo Holds**

- For lashing in cargo holds, there is no need for stanchions, since the walls of the hold provide the securing against transverse forces.
- To check overturning on partially loaded holds, a few pipes on top tiers may be bundled together using rachet straps to prevent the overturning.
- If sliding occurs, the stoppers/rubber mats are required in holds too
- Holds may also have tween decks. In such cases, the loading on the tween decks should be carefully planned so as not to exceed the loading limit on the tween deck pontoons

Besides the stowage plan, the lashing plan is the most important document for the operations team. They follow it religiously, and so it should be prepared with great care and caution so as to avoid any hassles in operations.

*Step 7 –Bollard Pull Calculations and Towing plan *

*Image Courtesy – www.pixabay.com*

This step is applicable only when the vessel is a towed vessel, e.g., a non-self-propelled barge. For such vessels, a tug needs to be engaged to pull the vessel to location. The size of the tug will depend on the force required to pull the vessel in a given environment. The calculation to determine this force is called Bollard Pull calculation. Following are the steps to carry out this calculation

- Get the environment parameters. These are the wind speed, current speed, and significant wave height of the tow route. In case the environment data are not available, the standard open ocean conditions as specified in Ref [1] ND-0030 may be used. It specifies a wind speed of 40 knots, current speed of 1 knot and significant wave height of 5 m.
- Calculate the environmental forces on the vessel due to the wind, current and wave.
- Wind force calculation will require transverse windage area of the cargo and vessel.
- Current force is calculated from the underwater area of the vessel and current speed
- Waves lead to an added wave resistance force on the vessel. This can be computed using standard references like Ref [5] DNV-OS-H103 Sec 7.2.6

- Once total force is calculated, divide it by towing efficiency of the tug to obtain the required bollard pull.
- The selected tug’s bollard pull must be greater than the required bollard pull.

thenavalarch has its own spreadsheets for Bollard Pull calculations for Barges and Ships. You can check them here.

Once the Bollard Pull is known, a towing plan of the vessel is to be prepared. The guidance to be followed is in Ref [1] ND-0030. A standard towing arrangement with two smit brackets, two towing bridles, a delta plate and a towing rope is shown below. The towing plan should clearly specify the items to be used for towing, their ratings and quantity.

A typical towing plan for a barge looks like this (source, Ref [1] ND-0030)

*Image Source – GL Noble Denton Guidelines for Marine Transportation, ND-0030*

Similarly, an emergency towing plan needs to be prepared as per requirements in ND-0030 (Ref [1]). This should also specify all the required items for emergency towing of the vessel.

A typical Emergency towing plan for a Barge may look like this (source ND-0030, Ref [1])

*Image Source – GL Noble Denton Guidelines for Marine Transportation, ND-0030*

That brings us to the end of this section on Transportation Analysis. The entire transportation analysis document needs approval from the Marine Warranty Surveyor (MWS) before the actual operations can begin. The engineering team should work actively with the MWS, resolving any outstanding comments from MWS to ensure that all approved documents are in place for the operations team to start their work.

*Phase 4 – Execution of actual operation.*

*Image Courtesy www.pixabay.com*

With Engineering done and approved documents in hand, the operations team will execute the entire pipe transport operation involving lifting, loading, securing, transportation and unloading. However, the engineering team is required to be actively involved throughout the operation of the project to support the operations team. By their very nature, these operations have a knack of springing up surprises every now and then, and the engineering team will most likely be more occupied during the actual operation of the project than during the engineering phase.

Hope this article was helpful to you. If yes, please do remember to share it with others who can benefit from it.

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for any incidents or losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any activity which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

The post Introduction to Pipe Transportation – Part 3 (Engineering) appeared first on TheNavalArch.

]]>The post Introduction to Pipe Transportation – Part 2 (Planning & Scheduling) appeared first on TheNavalArch.

]]>In Part 1, we looked at the properties of pipes. In this section, we will be looking at the Planning & Scheduling of a pipe transportation operation (see below, Phase 1 & 2)

**SECTION 2: PLANNING, SCHEDULING, AND ENGINEERING FOR A PIPE TRANSPORT PROJECT**

*Source: pixabay.com*

Now that we know about pipes and how to calculate their stacking limit, what next? We need to find out how to transport them. The whole process of Pipe Transportation and its Engineering can be divided into the following Phases

**Phase 1**– Establish the scope of transportation. This includes the total pipes to be transported, their properties and their weights. This would also entail the route of transportation.**Phase 2**– Vessel selection and schedule preparation**Phase 3**– Engineering. This includes engineering analyses and calculations for the pipe types and the vessels selected.__For the pipes__**,**we need to carry out stacking height calculations, and prepare detailed lifting plans for each pipe type (to lift them from Quay side to the vessel)engineering includes detailed plans and analyses demonstrating the suitability of the vessel for pipe transport. This document is called a ‘Transportation Analysis’, and includes__For the vessels,__- Vessel stability and strength check
- Motions analysis as per the environment
- Preparation of stowage plans
- Seafastening design for the pipes
- Preparation of lashing plan
- Bollard Pull calculations, if the vessel is a towed barge
- Towing/Emergency Towing Arrangement as applicable

**Phase 4**– Execution of the actual transportation operation. This includes- Lifting of pipes from quayside to the vessel
- Loading of pipes on the vessel as per the stowage plan and schedule
- Transporting pipes to final location of discharge
- Unloading of pipes at the final location

*Phase 1 – Establish the scope of work.*

Usually, the requirement of a project comes with a list of pipes to be transported. This list will contain the different pipes and their quantity to be used for a stretch of a subsea pipeline. Let’s consider a hypothetical stretch of a subsea pipeline shown below.

Each color depicts a different pipe type to be utilized for that stretch. The first pipe type (in Yellow color) is Type 1, and stretches for the first 0.122 km. The second is Type 2 pipe which stretches for the next 0.2196 kms, and the third is Type 3 pipe which spans the longest segment of 3.318 kms. If we list it in a tabulated form, the Table looks like below:

The table provides the details of all pipe types to be utilized in laying this section of the Pipeline. It gives their diameter, wall thickness, corrosion coating, concrete coating, weight, and Quantity.

We can see that the last column of the above table is “Lay Rate”. The Lay rate is nothing but the planned rate at which the pipe has to be laid on the sea bed. In above table, the Type 1 pipe has a lay rate of 70, i.e., 70 Nos. of Type 1 pipes must be laid on the sea bed per day. This implies that at least 70 pipes must be transported and supplied per day to the pipe-laying vessel. The lay rate assumes great significance in preparation of the schedule of the pipe transportation.

This phase also provides the details of the location where the pipes are to be loaded from and the location where they are to be supplied to. Usually, pipes are loaded from a Yard/Quayside to the transport vessel, and they are delivered to a pipe laying vessel at the other end.

*Phase 2 – Selection of vessels and preparation of schedule.*

*Source: pixabay.com*

In this phase, the pipe transportation contractor will select appropriate vessels suitable for the transportation and present a detailed plan to the client showing the sequence in which the vessel(s) will be deployed for transportation. This is not a linear process, and the vessel selection and schedule affect each other. Multiple vessels may be deployed to ensure that the lay rate is maintained. At times, a bigger transport vessel may be selected to transport the pipes to a location close to the pipelay vessel, and smaller barges deployed to trans-ship the pipes from the transport vessel to the pipelay vessel. This kind of operation is called Trans-shipment. The number of vessels selected also affects the costs of the entire operation, and it is important to strike the right balance between the number & types of vessels selected and meeting the lay rate.

Let’s take a very simple example with this hypothetical scenario:

We need to transport 1200 pipes to a pipelay vessel from a yard. The journey time from yard to pipelay vessel is 1 day. The loading rate of pipes is 200 per day. The lay rate is 100 pipes per day. We need to select vessels and plan a schedule for the transportation.

For selecting the vessels and preparing the schedule, we can have various options. We will explore three options below. The selection of the best option depends on the feasibility and costs.

**Option 1** – Select a vessel big enough to transport all 1200 pipes in one go.

The entire schedule can be as follows:

- Days 1 through 6 – Vessel loads 200 pipes per day
- Day 7 – Vessel departs.

- Day 8 – Vessel reaches the lay barge and unloads 100 pipes to the lay barge.

- Days 9 to 19 – Vessel discharge 100 pipes per day to lay barge
- Day 20 – Vessel departs and finishes its contract.

Thus, total number of days of contract of Vessel = 20 days

Project duration = 20 days

If the contract rate of the vessel is USD 10000 per day (assumption), then total cost of contracting = USD 200000

**Option 2 **– Select two smaller vessels (say barges) of capacity 600 pipes each and make them work in parallel. Both barges depart together and arrive at the same time to the lay barge. However, the lay barge may have a limitation to allow only one barge at a time. So, while the first barge is unloading to the lay barge (5 days), the second barge is idle, waiting for its turn.

The entire sequence can be

- Days 1,2 & 3 – Vessels 1 & 2 loads 200 pipes each per day
- Day 4 – Both vessels depart.
- Day 5 – Both vessels arrive. Vessel 1 discharges 100 pipes to lay barge. Vessel 2 is idle
- Day 6 –Vessel 1 discharges 100 pipes to lay barge. Vessel 2 is idle
- Day 7 –Vessel 1 discharges 100 pipes to lay barge. Vessel 2 is idle
- Day 8 –Vessel 1 discharges 100 pipes to lay barge. Vessel 2 is idle
- Day 9 –Vessel 1 discharges 100 pipes to lay barge. Vessel 2 is idle
- Day 10 –Vessel 1 discharges 100 pipes to lay barge. Vessel 2 is idle
- Day 11 –Vessel 1 departs. Vessel 2 discharges 100 pipes to the lay barge
- Day 12 –Vessel 2 discharges 100 pipes to lay barge. Vessel 1 reaches yard and finishes contract.
- Day 13 –Vessel 2 discharges 100 pipes to lay barge
- Day 14 –Vessel 2 discharges 100 pipes to lay barge
- Day 15 –Vessel 2 discharges 100 pipes to lay barge
- Day 16 –Vessel 2 discharges 100 pipes to lay barge
- Day 17 – Vessel 2 departs and finishes its contract.

Total duration of contract of Vessel 1 = 11 days

Total duration of contract of Vessel 2 = 17 days.

Total project duration = 17 days.

If the contracting cost per vessel is USD 7000 per day, then total contracting cost = (17 + 11) x 7000 = USD 196000

**Option 3** – Select two smaller barges and make them work in sequence. Let’s say the capacity of each vessel is 400 pipes.

The entire schedule with these two vessels can be

- Days 1 & 2 – Vessel 1 loads pipes (200 per day)
- Day 3 – Vessel 1 departs.
- Day 4 – Vessel 1 reaches the lay barge and unloads 100 pipes.
- Day 5 – Vessel 1 discharges 100 pipes to lay barge. Vessel 2 starts loading pipes at yard.
- Day 6 – Vessel 1 discharges 100 pipes to lay barge. Vessel 2 finishes loading pipes at yard.
- Day 7 – Vessel 1 discharges 100 pipes. Vessel 2 leaves yard.
- Day 8 – Vessel 2 arrives and discharges 100 pipes to lay barge. Vessel 1 departs.
- Day 9 – Vessel 2 discharges 100 pipes to the lay barge. Vessel 1 reaches yard and starts loading
- Day 10 – Vessel 2 discharges 100 pipes to the lay barge. Vessel 1 finishes loading
- Day 11 – Vessel 2 discharges 100 pipes to the lay barge. Vessel 1 leaves yard
- Day 12 – Vessel 1 arrives and discharges 100 pipes to the lay barge. Vessel 2 departs.
- Day 13 – Vessel 1 discharges 100 pipes to the lay barge. Vessel 2 arrives at yard and finishes its contract.
- Day 14 – Vessel 1 discharges 100 pipes to the lay barge.
- Day 15 – Vessel 1 discharges 100 pipes to the lay barge.
- Day 16 – Vessel 1 departs and finishes its contract.

Total duration of contract of Vessel 1 = 16 days

Total duration of contract of Vessel 2 = 9 days.

Total project duration = 16 days.

If the contracting cost of each vessel is USD 5000 per day, then total contracting cost = (16 + 9) x 5000 = USD 125000.

A table summarizing the time and cost outcomes of all three options is presented below:

Thus, we can see that from the point of view of both cost and time effectiveness, the Option 3 is the best option. Thus, **bigger is not always better in pipe transportation**.

*****A note of Caution!

The above example was a highly simplified one, and in real cases, the scheduling and selection of vessels is a complicated exercise. There are other factors and costs involved which should be accounted for, and the voyage times of different vessels may be different. It is not the intention of this article to get too deep into this subject. Let’s move on to Phase 3 – engineering.

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for any incidents or losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any operation which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

The post Introduction to Pipe Transportation – Part 2 (Planning & Scheduling) appeared first on TheNavalArch.

]]>The post Pipe Transportation – An Introduction (Part 1) appeared first on TheNavalArch.

]]>(To read Part 2, click here)

This is the first part in the 3-part series on Pipe Transportation. In this part we will discuss about pipes and their properties.

In this article, we will talk about the transportation of pipes on ships. Pipes are needed for various uses in both onshore and offshore environments. They are extensively used in laying pipelines on ground and sea bed. In this article, we will limit ourselves to pipes required for laying pipelines on the sea bed for transporting oil and other fluids. However, the pipe transportation principles remain the same and can be easily applied to any other scenario.

Pipes may be fabricated at location A, while they may need to be laid on the sea bed at location B. If the number of pipes to be transported is high, then ships or barges need to be deployed for transporting these pipes. To imagine it in the simplest way, it involves just loading pipes on the deck or cargo holds of a ship (the picture above has deck loading), and taking the ship to its final location. In real terms, though, it is a niche operation requiring experienced personnel and skilled engineering to drive it to success.

This article gives an insight into the mechanisms involved in large scale pipe transportation operations, and the engineering which goes behind it.

We will divide this article into sections

- SECTION 1 – We will talk about pipes and their properties
- SECTION 2 – We will talk about a pipe transportation project’s planning and engineering. It will be further subdivided into
- Section 2.1 – Establishing the scope
- Section 2.2 – Planning and Scheduling
- Section 2.3 – Engineering
- Section 2.4 – Operation

In this Part, we will cover Section 1. In Part 2, we will cover Sections 2.1 & 2.2, while in the Part 3, Section 2.3 & 2.4 will be covered.

*References*

Following references have been used in this article

[1] GL Nobledenton Guidelines for Marine Transportation, ND-0030

[2] GL Nobledenton Guidelines for Marine Lifting and Lowering Operations, ND-0027

[3] ABS Rules for Building and Classing of Steel Barges

[4] Roark’s Formulas for Stress and Strain

[5] DNV-OS-H103 Modelling and Analysis of Marine Operations

**SECTION 1 – PIPES**

Let’s begin by talking about pipes. In the pipe transportation industry, they are also called ‘joints’.

*Pipe Sizes – single, double, triple, and quad joints*

Pipes can be of different sizes. Their size is defined by following two parameters (see Figure) –

- Outer diameter
- Length
- Wall thickness

Pipes can be of different diameters and wall thicknesses depending on their usage. In terms of length of pipes used in pipelaying, there is some standardization. A single joint is generally 40 Ft (12.2 m) in length. There can also be double, triple, and quad joints with lengths 80, 120 and 160 Ft respectively.

* *

*Pipe Coatings – Bare Pipes, CWC and Corrosion Coating*

Pipes are usually made of steel. Pipes made of only steel without any additional coatings are called ‘bare pipes’. However, a steel pipe may be coated with concrete for providing negative buoyancy to the pipeline on the sea bed. This provides submerged weight stability to the pipeline, and protects the corrosion coating of pipes. The concrete coating is called as Concrete Weight Coating, abbreviated as CWC.

Pipes may additionally be coated with a thin anti-corrosion coating to protect them from the highly corrosive marine environment. There may be additional layers of other kinds of coatings on pipes depending on the usage.

*Stacking of pipes – Nested Diameter, Pipe Yield Stress and Stacking height limit *

The next important aspect of pipes is the way they are stacked for storage or transport. Pipes can be stacked on above another forming different tiers or ‘stacks’. The pipes of the second tier (the one just above the bottom tier) will sit in the grooves created in the bottom tier. Similarly, the third-tier pipes will sit in the grooves of the second tier and so on. If we continue like this, a trapezium shaped stacking will be created, as shown below:

* *

*Nested OD*

This type of stacking is called ‘nested’ stacking (pipes sitting in grooves). With the pipes sitting in the grooves, the total height of a stack of pipes is less than the number of stacks times the diameter. This gives rise to the term ‘Nested Diameter’. Nested outer diameter (Nested OD) is the total height of the stack divided by the number of stacks. This is lesser than a pipe’s outer diameter.

Nested Diameter (Nested OD) = Total Height of Pipe Stack / Number of stacks

Below we will derive a simple formula for Nested OD.

In the above figure, R is radius of pipe = Pipe OD/2.

h = vertical distance between two tiers = √3/2 x pipe OD

If the number of tiers is n, then

Stack Height, H = R (for bottom tier lower half) + (n-1) x h + R (for top tier upper half)

*H = pipe OD + (n-1) x √3/2 x pipe OD**Nested OD = H/n*

*Stacking Height Limit*

This naturally raises a question: how high can we keep stacking pipes safely? As we keep stacking, we realize that the bottom tier of the pipe takes increasingly higher weight. The stacking height is determined by the maximum number of stacks which we can add before the bottom tier yields. How do we measure this yielding?

- For bare pipes, the yield limit is the yield stress of steel of pipe
- For concrete pipes, either the concrete or the steel may yield. The first one to yield governs the yield limit
- For pipes with anti-corrosion coating, the coating may also yield, but it is usually not governing in comparison to steel and concrete yielding.

Let’s talk a bit more in detail on calculating the stacking limit by taking the case of bare pipes. For the bottom tier of pipes, the following loads are acting –

- Load due to pipes above
- Self-weight

These two are depicted below

From the above, and based on Roark’s formulae for the two load cases above, the limiting number of tiers can be calculated. For more on Stacking Height Calculation for bare pipes, you can see thenavalarch’s product Pipe Stacking Height Calculator.

That brings us to the end of Part 1. In Part 2, we will discuss planning and scheduling of pipe transport projects.

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for any incidents or losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any operation which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

The post Pipe Transportation – An Introduction (Part 1) appeared first on TheNavalArch.

]]>The post Ship Corrosion – Cathodic Protection and Sacrificial Anodes appeared first on TheNavalArch.

]]>** **

Image Source: pixabay

**SECTION 1 – INTRODUCTION**

Due to the environment in which they operate, ships are among the structures most exposed to environmental corrosion. The sea water is a very corrosive environment because the salt present in it makes it a very good conductor of electricity. It creates a lot of free ions which accelerate oxidation of iron (mild steel) which ships are made of. This oxidation creates what we know as rust.

Almost every part of the ship is subjected to corrosion, with varying intensity. Parts of the vessel underwater or exposed to water (e.g. ballast tanks and pipes) are more affected by corrosion. Some of the parts highly exposed to corrosion by sea water are

- Ship’s external hull – exposed to water
- Rudder
- Propeller shaft
- Bilge Keel
- Bow Thruster
- Cargo Tanks
- Ballast Tanks
- Other tanks
- Pipes carrying ballast/cargo

There are various methods of protecting the ship hull and other areas from corrosion. The selected method depends on the area to be protected, its shape and its environment.

There are three major methods of corrosion protection

- Anti-corrosion paints – metallic/organic

- Cathodic Protection – ICCP (Impressed Current Cathodic Protection)

- Cathodic Protection – Sacrificial Anodes

In this article, we will discuss one of such methods: Cathodic Protection using Sacrificial Anodes

*Anodes on a hull and rudder (source www.cathodicme.com)*

** **

** **

** **

**SECTION 2 – CATHODIC PROTECTION USING SACRIFICIAL ANODES: THE BASICS**

**2.1 What is Cathodic Protection?**

Cathodic protection is a mechanism which is employed to protect the ship’s surface from corrosion. As the name suggests, it has something to do with ‘Cathode’. What is a Cathode, and how does it protect a ship from getting corroded? For this we will have to get a little bit into the mechanism of corrosion.

How does corrosion take place in ships?

Ships are made of steel; whose main component is iron. Iron is an electrochemically positive element, i.e., it has a tendency to give up electrons to become a free ion. Sea water is composed of oxygen and hydrogen, and it produces electrochemically negative hydroxyl ions which can accept the electrons given by Iron. This way the Iron ions combine with the hydroxyl ions of water to form Iron Hydroxide. This is called the oxidization of Iron, and this oxide is what we call as the brown color rust.

*The mechanism of Corrosion (source www.zoombd24.com)*

This youtube video gives a good picture of how corrosion occurs:

https://www.youtube.com/watch?v=cIGJX3PfIsY

**2.2 The galvanic series**

The galvanic series is a series which rates metals based on how readily they give up electrons to become ions. This is measured in terms of ‘Electrode Potential’. The table below shows the rating of different metals. The ones with more negative electrode potential are more likely to give up electrons and get corroded.

*The galvanic series (Source www.nordhavn.com)*

**2.3 The BIG idea!**

Looking at the table above gives us an idea. The elements like Aluminium, Zinc and Magnesium are higher up than Steel on the scale (i.e., more negative). This means they are more ready to give up electrons and get corroded compared to Steel.

What if we introduce a Zinc bar and place it on the steel? The Zinc will get corroded first instead of the Steel, protecting the Steel, right?

This is the entire concept of Cathodic protection. When such an arrangement is used, the item which is being protected (i.e., ship’s steel) is called Cathode, and the one which sacrifices itself to protect the cathode is called Anode. Anodes are more electrochemically negative, and they save the Steel by getting corroded first.

The Steel is converted to a ‘Cathode’, and so this method of protecting the Steel from corrosion is called ‘Cathodic Protection’.

**SECTION 3 – ABOUT SACRIFICIAL ANODES**

Now let’s move on to learn about these anodes and how they are used for Cathodic Protection. Till now we know that we can use Zinc, Aluminium and Magnesium for becoming sacrificial anodes. Generally, for ships, Aluminium and Zinc are used.

How do these anodes look? Where do we place them on the ship? How many of them are needed?

**3.1 Sacrificial Anodes – the basics**

The basic idea of using sacrificial anodes is to use a metal like Zinc/Aluminium ** and create its contact** with the surface to be protected.

The simplest picture which comes to mind is simply using a flat bar of the metal and fix it to the surface to be protected. This is actually the method commonly used to protect the outer ship’s hull.

We will next discuss the geometry and classification of anodes

*3.1.1 Geometry of an anode*

A simple anode will have two parts: the anode body and the anode insert. The anode body is the actual sacrificial material of the anode (Zinc or Aluminium), while the insert is generally flat bar or tubular, and made of steel. The insert is used to secure the anode to the surface to be protected using welding or bolting. Following figure illustrates the parts:

*Geometry of an anode (source www.stoprust.com- edited by thenavalarch)*

*3.1.2 Anode Classification*

We will discuss now the classification of anodes. Anodes can be classified based on their shape, size, material, mounting method and method of securing to the surface to be protected.

__3.1.2.1 Anode Shape__

Following are some widely used shapes for anodes

- Flat or block shaped
- Cylindrical or semi-cylindrical
- Tear-drop anodes
- Bracelet anodes
- Disc shaped
- Tubular anodes

Anodes can be of different shapes based on their applicability. The selection of the shape of anode depends on several factors. Some of these factors are:

- shape of the surface to be protected,
- availability of space,
- accessibility,
- ease of installation
- special considerations, e.g., effect on resistance for small boats

For example, flat anodes are used mostly for flat, large surfaces like the ship’s hull. Tear-drop anodes are used in high speed boats where streamlining of water is important as flat anodes will increase the boat’s resistance. Bracelet anodes are used for pipelines and propeller shaft, while tubular anodes are used for cables. There are no fixed rules here though, and the choice depends on the availability, cost and flexibility in design. For example, cylindrical anodes can also be used to protect pipelines, and it is not necessary to use bracelet anodes if they are costlier.

*Different anode shapes*

__3.1.2.2 Anode Size__

Anodes can be big or small sized, and this affects their weight and the overall weight of the structure to be protected. What size anode to select also depends on many factors, some of them being

- Size and shape of area to be protected – the hull can take large sized anodes, while a small rudder may not be able to accommodate the same sized anodes
- Space availability and accessibility – for example, the web or flange of a girder has less space available, and it cannot take big sized anodes
- Structural strength considerations – for a longitudinal, installing a single big sized anode may lead to a point load if the anode is too big, compared to several small sized anodes which will apply a distributed load

__3.1.2.3 Anode Material__

Usually for marine applications, Zinc or Aluminium anodes are deployed. Zinc has been traditionally used for corrosion protection, though Aluminium is now widely used. The two properties which measure performance of an anode are listed below.

- Closed Circuit Potential – the first parameter, Closed Circuit Potential signifies the ease with which the anode will be corroded. The more negative the value, the more readily the anode will get corroded. Generally, a potential of less than -0.08 Volts is required for cathodic protection of shipbuilding steel to be effective.
- Electrochemical Capacity (in Amp-hr/kg) – The second parameter, the Electrochemical Capacity, signifies the rate at which the anode material will be consumed.

The two parameters for Zinc and Aluminium are listed in the table below:

Parameter |
Aluminium |
Zinc |

Closed Circuit Potential | -1.1 V | -1.05 V |

Electrochemical Capacity (Ah/kg) | 2000 | 780 |

*Properties of Anode Materials (Source: DNV RP-B401)*

We can see from the above table that Aluminium has a higher closed circuit potential – so it will more readily start working compared to Zinc. It also has higher Electro-chemical capacity compared to Zinc, and will be longer lasting for the same anode size.

Further, in fresh water application, Zinc tends to develop a calcareous coating on the anode surface, which prevents their effective working.

However, Zinc anodes have sometimes been found more reliable in environments with low oxygen, e.g., marine sediments or areas with high bacterial activity. Thus, while Aluminium is the more efficient one, Zinc may be more effective in some cases.

Further, Aluminium anodes, if falling from a height on oxidized steel, can create sparks. Thus they are nor recommended to be used inside cargo tanks of tankers. The maximum height above tank bottom which they must be placed is 28/W meters, where W is the weight of the anode in kgs.

Hence, the selection of the material depends on the type of environment it is going to be used, and should be carefully carried out.

__3.1.2.4 Anode Mounting Method__

The next important consideration for installation of anodes is the mounting method, i.e., the configuration of the tubular insert, and the positioning of the anode vis-à-vis the surface to be protected.

Based on mounting technique, there are two major types of anodes which are used in ships:

- Flush mounted anodes – in this type of anode, the anode material (Aluminium or Zinc) is in direct contact with the surface to be protected. The insert is generally a flat bar which can be welded or bolted to the surface.

*A Flush Mounted Anode (source archive.hnsa.org)*

- Slender stand-off anodes – In these types of anodes, the anode material is not in direct contact with the surface to be protected, and there is a gap (hence the name stand-off). The insert is generally a tubular one which can be welded or bolted to the surface.

*A stand-off anode (source www.acp.no)*

One question arises, why do we need stand-off anodes, and why not flush anodes everywhere? What is the benefit of stand-off design?

The benefit of a stand-off design is that it is a more compact design, and the anode material is better utilized in a stand-off design. This is quantified by a parameter called ‘anode utilization factor’. This is the fraction of the anode material which is * actually* utilized over the lifetime of the anode. For flush anodes, this is around 80%, while for stand-off anodes it is 85 to 90%. Thus, stand-off anodes are better utilized over their lifetime.

Further, in case of flush anodes, due to constant contact between the anode material and the surface, the surface may suffer from embrittlement caused by deposition of ions from the anode material to the cathode (the protected surface).

That said, stand-off anodes protrude from the surface on which they are installed. When used on external hull of a vessel, these affect the streamlined shape of the vessel, and lead to increased drag and higher powering requirements. In comparison, flush anodes are closer and more compliant to the vessel’s geometric shape and have lower effect on resistance. Thus, flush anodes are usually preferred on outer hull due to their low drag properties.

Both Flush mounted and slender stand-off anodes are further classified into Short and Long, depending on their ratio of length to width. The length affects the resistivity of the anode and thus its current capacity.

__3.1.2.5 Method of Securing the anode to the surface to be protected__

There are three basic methods of securing the anode to the surface which is to be protected. They are

- Welding
- Bolting
- Using studs/brackets

Welding ensures the closest electrical contact between the anode and surface to be protected, thus ensuring good conductivity between anode and the surface through the insert material. However, due to issues of accessibility, some locations (e.g., stringers, girders etc.) may not be conducive to welding, and bolting or bracket installations may be preferred. Additionally, if the anodes have to be replaced relatively frequently, then bolted ones are relatively easier to replace compared to welded ones. Anodes can also be bolted to small studs or brackets which in turn are welded to the hull.

*Welded Anode (source firtech-marine.com)*

*Bolted Anode (source www.boatstasmania.com.au)*

*Bracketed anodes in tanks (Source cathwell.com)*

Now that we know about anodes and their basic properties, in the next section we will discuss about how to estimate the number of anodes required for protecting a surface (e.g., the ship’s hull or tanks)

**SECTION 4 – HOW TO CALCULATE THE QUANTITY OF ANODES REQUIRED**

In this section, we will see how we can calculate the number of anodes needed for protecting a surface. For this calculation, we will be following the DNV-RP-B-401, which details the procedure.

Before we get into actual formulas, it will be pertinent to understand what the anode is doing and how it is protecting the surface. Some concepts are presented below.

**4.1 Current demand of surface to be protected**

The anode, when connected to a surface, sets up an electrical circuit and current flows from cathode to the anode.

Each surface to be protected will need a minimum amount of current to flow for adequate protection. This is called the ‘current demand’ of the surface to be protected.

It is measured in terms of the amount of current required for protection of a unit area of the surface, also called as ** current density**. The current demand of a surface depends upon many factors, like

- Dissolved oxygen content in water
- Marine growth
- Temperature
- Salinity

If the required current density for the Surface to be protected is i_{C}, and the area of the surface is A_{S}, then the total current demand of the surface will be

I_{C} = i_{C} x A_{S}

__4.1.1 Initial, Final and Mean Current Demands __

The current demand of the protection surface also varies during its lifetime. Initially, when anodes are installed, then the surface metal is bare and fresh. The initial current demand will be the amount of current required to effect polarization of the *bare metal surface* in a short time-frame for protection to begin. This is called the *initial current demand*.

However, overtime, the surface develops calcareous deposits (due to cathodic protection), and also marine growth. These act as a deterrent to corrosion and thus reduce the current demand. When the anodes are close to depletion, then the current required to initiate protection in a short time-frame is called the *final current demand*.

Once Cathodic Protection is in action over a long time, then the Cathode potential becomes more negative, and the Cathodic Protection (CP) system is said to have reached a steady-state. This is called cathodic polarization, and it reduces the current demand over the operational life of the structure. The current demand during the steady state is called the *mean current demand*.

Here we need to note the difference between the initial/final and mean current demands. While initial/final current demands are the currents required to initiate Cathodic Protection, the mean current demand is the current required for the Cathodic Protection to keep operating during the lifetime of the anode. The mean current demand is around 50% of initial/final current demands, since the Cathodic Polarization leads to more negative cathodic potential, reducing the current needed for CP to keep working.

Since current demand is measured in terms of current density, we will adopt the following symbols and formulae for the initial, mean and final current demands

Initial Current Demand, I_{ci} = i_{ci} x A_{C}, where i_{ci} = Initial Current Density, A_{C} = Area of protection surface

Mean Current Demand, I_{cm} = i_{cm} x A_{C}, where i_{cm} = Mean Current Density, A_{C} = Area of protection surface

Final Current Demand, I_{cf} = i_{cf} x A_{C}, where i_{cf} = Final Current Density, A_{C} = Area of protection surface

DNV RP-B401 provides the recommended values of the Initial, Final and Mean current densities as per the table below

*Recommended Initial, Final and Mean Current Densities as per DNV RP-B401*

**4.2 Coating Breakdown Factor**

The second important concept here is the coating breakdown factor. When a surface is coated with an electrically insulating coating (epoxy, polyurethane or vinyl based), then this provides additional protection against corrosion and reduces the current demand.

The factor by which the coating reduces the current demand of a structure is called coating breakdown factor. Its value lies between 0 and 1. A value of 0 means that the coating is 100% insulating, and a value of 1 means that the coating provides no current reduction.

The extent of reduction in current demand is dependent on the type of coating and the water depth at which the structure is installed.

There are different types of coatings as prescribed in DNV-RP-B401

*Category I* One layer of epoxy paint coating, min. 20 μm nominal DFT

*Category II* One or more layers of marine paint coating (epoxy, polyurethane or vinyl based), total nominal DFT min. 250 μm.

*Category III* Two or more layers of marine paint coating (epoxy, polyurethane or vinyl based), total nominal DFT min. 350 μm.

*DFT = Dry Film Thickness

The coating breakdown factor if given by

f = a + b.t

where t is the coating age and a, b are factors determined from DNV-RP-B401

*Coating breakdown factors (source DNV-RP-B401)*

The coating breakdown factor is different for initial, final and mean phases (since ‘t’ is different for each), and is to be calculated separately for each stage.

__ __

**4.3 Current demand including coating breakdown**

After incorporating the coating breakdown factors, the initial, mean and final current demands can be written as

Initial Current Demand, I_{ci} = i_{ci} x f_{ci} x A_{C}, where f_{ci} = initial coating breakdown factor

Mean Current Demand, I_{cm} = i_{cm} x f_{cm} x A_{C}, where f_{cm} = mean coating breakdown factor

Final Current Demand, I_{cf} = i_{cf} x f_{cf} x A_{C}, where f_{cf} = = final coating breakdown factor

At this stage, we know what current demand of the surface to be protected is, and how we calculate this current demand. Let’s now look into the critical properties of anode which help us in the calculation of number of anodes needed.

**4.4 Anode Properties and Concepts**

__4.4.1 Resistance of Anode__

The resistance of anode is simply a function of the anode dimensions, the anode geometry and the resistivity of the seawater. Seawater resistivity depends on the temperature and salinity of the sea water. A graph for determining Seawater resistivity is provided in DNV-RP-B401

*Seawater Resistivity as function of Temperature for salinity between 30-40% (Source DNV-RP-B401)*

Depending on the type of anode, there are different formulae to calculate the resistance of anode

*Resistance formulae for different anode types (Source: DNV RP-B401)*

From the above formulae, we can see that the resistance depends on the anode dimensions. Now, as the anode is consumed by usage, its mass depletes, and the final dimensions of a completely used anode will be smaller compared to the time it was installed new. Thus, the resistance in the initial and final conditions of the anode will be different.

__4.4.2 Current output of anode__

The current output of anode is the amount of current which one anode produces. From basic electricity concepts, the current produced by one anode when it is connected to the surface (the cathode), is given by

I_{A} = (Potential difference)/Resistance of anode = ∆E/R_{a}

Here, potential difference is the electrochemical potential difference between the anode and the surface which it protects. For example, if the surface material is mild steel and the anode is Zinc, then the potential difference is

∆E = (Design potential of mild steel) – (Design potential of Zinc)

Design potential of mild steel = -0.8 V

Design potential of Zinc = -1.05 V

I_{A} = ∆E/R_{a} = (1.05 – 0.8) /R_{a}

As explained above, since the resistance of the anode is different in initial and final conditions, this implies that the current output of the anode will also be different in the initial and final conditions. If the resistance of the anode in initial and final conditions is represented by R_{ai} and R_{af} respectively, then the initial and final current capacity are given by

Initial Current Capacity of Anode, I_{ai} = ∆E/R_{ai}

Final Current Capacity of Anode, I_{af} = ∆E/R_{af}

__4.4.3 Anode utilization factor__

The whole mass of an anode may not be really utilizable for cathodic protection. After the anode depletes to a certain extent, its effectiveness becomes unpredictable. Thus, only a net mass of the anode can be utilized for cathodic protection. The fraction of anode mass which is actually usable is called the anode utilization factor. This factor depends on the geometry and shape of the anode, and recommended values are provided in the DNV-RP-B401

*Recommended Anode Utilization Factors (Source: DNV-RP-B401)*

__4.4.4 Anode Electrochemical Capacity and Closed Circuit Potential__

These properties have been discussed before. Electrochemical capacity signifies the rate at which the anode material will be consumed. It is measured in Ampere-hr/kg. Basically, it measures the amount of material which will be consumed to produce a one Ampere current for 1 hr.

Closed circuit potential is the potential at which the anode will be when connected in a circuit to the cathode. Basically, the Steel surface is at a potential of -0.8 V while the anode is at a more negative potential.

The Electrochemical capacities and closed circuit potentials of Aluminium and Zinc anodes can be taken from DNV-RP-B401 and presented below.

*Anode electrochemical capacity and closed circuit potential (Source: DNV-RP-B401)*

__4.4.5 Current capacity of anode__

Now we will discuss a property called the current ‘capacity’ of the anode. This is different from the current output of the anode. Current capacity is the amount of current which the anode can produce over its lifetime of usage. Thus, it depends on the amount of material the anode carries, i.e., its weight. We know by now that the net anode mass usable for cathodic protection is determined by the anode utilization factor.

If the mass of one anode is m_{a} kgs, and its utilization factor is u, then the net usable mass of the anode will be m_{a} x u (kgs)

Now, if the electrochemical capacity of the anode is designated as Ԑ, then Ԑ amperes of current per hour will be generated by per kg mass of the anode.

Thus, the current generated by the net mass of the anode will be

C_{a} = m_{a} x u x Ԑ

This is called the current capacity of the anode, and denotes the amount of current per hour it can produce over its lifetime.

__4.5 Calculation of number of anodes needed__

Now that we are through with the concepts above, we can get into the actual calculation of number of anodes. Understanding the calculation is relatively simple.

Let the minimum number of anodes required be N.

- Step 1 – On one hand, we have a structure to be protected. We know the area of the structure. Depending on the coating properties and environment, we can get the coating breakdown factor and the current density for the structure from DNV-RP-B401
- Step 2 – Next, we can calculate the total current demand of the structure. There is Initial, Final and Mean current demand. They are denoted as I
_{ci,}, I_{cf }and I_{cm } - Step 3 – calculate initial and final anode current outputs. Earlier we calculated the initial and final current outputs of the anode, denoted by I
_{ai }and I_{af } - Step 4 – The initial current demand is basically current required to initiate Cathodic Protection in the new structure. Thus, all the anodes combined together should produce enough current to overcome the current demand. To overcome initial current demand, the total initial current output of N anodes should be more than I Putting this in a relation form, we can write

**N x I _{ai} >= I_{ci}**

- Step 5 – Similarly, the final current demand is basically current required to initiate Cathodic Protection in the structure with depleted anodes. Thus, all the anodes (in depleted condition) combined together should produce enough current to overcome the current demand. To overcome final current demand, the total final current output of N anodes should be more than I Putting this in a relation form, we can write

**N x I _{af} >= I_{cf }……………………….Equation 2**

- Step 6 – Calculate individual anode current capacity. The current capacity of the anode is given by

**C _{a} = m_{a} x u x Ԑ (in Ampere-hr)**

m_{a} can be obtained from the anode’s catalog, while u (anode utilization factor) and Ԑ (electrochemical potential) can be obtained from the DNV-RP-B401.

- Step 7 – Calculate the total
*required*current output of anodes. The required mass of anodes should be sufficient to supply the mean current demand over the design life of the anodes.

Let the design life of anodes be t_{f} years. In hours it comes to t_{f} x 8760 (1 yr has 8760 hrs)

The mean current demand of the structure is **I _{cm}** Amperes.

Thus, total *required* current demand over the design life of anodes = **I _{cm} x t_{f} x 8760**

- Step 8 – Calculate the total
*required*anodes to meet required mean current demand.

The total current capacity from all anodes over design life of anodes = **C _{a} x N**

The total current capacity should be more than the demand. Thus,

**C _{a} x N >= I_{cm} x t_{f} x 8760 ……………………….Equation 3**

Thus, we see that the total number of anodes should be able to satisfy the equations 1, 2 and 3 simultaneously.

- The total initial current output of anodes should be more than the initial current demand of the structure
- The total final current output of anodes should be more than the final current demand of the structure
- The total mean current capacity of anodes should be more than the mean current demand of the structure

Based on above guidelines, we can calculate the minimum number of anodes required, N.

**Section 6 – Step-by-step guide for selecting Anodes for Cathodic Protection of your structure**

In this section, we summarize all the steps which will be needed for you to plan the cathodic protection of your structure using sacrificial anodes.

- Step 1 – Study the surface to be protected. The shape, size and geometry of the surface affects the anodes to be selected. Get the following parameters of the surface
- Area of the surface
- Material of the surface
- Closed circuit potential of the surface material. Different categories of steel may have different potentials
- Coating category. See DNV-RP-B401

- Step 2 – Study the environment in which the surface is going to experience corrosion. Following parameters of the environment should be obtained
- Salt water or fresh water
- Salinity of water – affects resistivity
- Water depth – affects current demand
- Temperature of environment in which structure will operate – affects the resistivity of anodes and current demand

- Step 3 – Select the anode type to be used
- Select Anode material
- Zinc and Aluminium are widely used.
- Aluminium has better anode properties compared to zinc
- However, zinc is more effective in certain environments with low oxygen
- Aluminium has height restrictions when used in cargo tanks of tankers

- Select anode shape and size – it can be rectangular, tear shaped, cylindrical or other shape based on the operational requirements of the structure
- Accessibility, availability of space and ease of installation are important factors
- For example, for a ship hull, Tear shaped anodes will have lower drag compared to block shaped ones
- The number of anodes will be less for bigger anodes, but installation may be difficult, and they may lead to high point loads. Accessibility and availability of space are equally important considerations

- Select anode mounting method
- Can be slender stand-off, flush mounted, or bracelet etc.
- Slender stand-off anodes have better utilization factor compared to flush mounted.
- Flush mounted anodes may be more preferable for outer hull due to relatively better drag properties.

- Select Anode material

- Step 4 – Get all the anode properties for the selected anode
- Design life
- Electrochemical Capacity
- Closed Circuit Potential
- Anode dimensions

- Step 5 – Calculate the initial, final and mean current demands of the structure to be protected
- Get coating breakdown factors from DNV-RP-B401 based on coating type
- Get the current densities from DNV-RP-B401
- Calculate the initial, final and mean current demands from the current densities and coating breakdown factors.

I_{ci} = i_{ci} x f_{ci} x A_{C}, I_{cm} = i_{cm} x f_{cm} x A_{C}, I_{cf} = i_{cf} x f_{cf} x A_{C}

- Step 6 – Calculate the initial and final current
**output**of each anode- Get anode utilization factor and resistivity from DNV-RP-B401
- Get the anode dimensions from anode catalog
- Calculate the resistance of each anode
- Calculate the initial and final resistance of the anode, R
_{ai}& R_{af} - Calculate the difference in electrochemical potential between anode material and the protection surface material, ∆E
- Calculate the initial and final current outputs of anode using formula

I_{ai} = ∆E/ R_{ai} , I_{af} = ∆E/ R_{af}

- Step 7 – Calculate the current
**capacity**of each anode- Get anode utilization factor and electrochemical capacity from DNV-RP-B401
- Calculate current capacity of each anode as

C_{a} = m_{a} x u x Ԑ (in Ampere-hr)

- Step 8 – Calculate the mean current demand of the structure
- Get the design life of anodes from the catalog, t
_{f} - Get the mean current density from DNV-RP-B401, I
_{cm} - Mean current demand =
**I**_{cm}x t_{f}x 8760

- Get the design life of anodes from the catalog, t
- Step 9 – Calculate the number of anodes needed (N), by satisfying all the following conditions

**N x I _{ai} >= I_{ci}**

**N x I _{af} >= I_{cf}**

**C _{a} x N >= I_{cm} x t_{f} x 8760**

There are other important topics related to anodes, viz.

- Installation of anodes
- Replacement of anodes – when and how?
- Anode Quality testing

Considering the vastness of the material to be covered in these topics, we will cover them in later articles.

__References and Links:__

- DNV RP-B-401
- http://www.sintef.no/globalassets/upload/materialer_kjemi/anvendt-mekanikk-og-korrosjon/faktaark/corrosion-protection-web.pdf
- http://www.performancemetals.com/anodes/AnodeFAQs.shtml
- http://www.cruisingworld.com/how/zinc-and-aluminum-sacrificial-anodes
- http://www.mcpsltd.com/tankanodes.html
- http://www.etc-cps.com/app_marine.htm
- http://www.amteccorrosion.co.uk/cathprotguide.html
- https://www.eagle.org/eagleExternalPortalWEB/ShowProperty/BEA%20Repository/Rules&Guides/Archives/2_SteelVesselRules2000/Part5VesselTypesCh1_6
- http://www.calqlata.com/productpages/00057-help.html
- http://www.pangolin.co.nz/node/19

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for any incidents or losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any operation which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

PS: www.thenavalarch.com has its own spreadsheet app for calculating the number of anodes needed for protection. It is based fully on DNV-RP-B401. You may wish to check it out below.

The post Ship Corrosion – Cathodic Protection and Sacrificial Anodes appeared first on TheNavalArch.

]]>The post Longitudinal Strength of Ships – an Introduction appeared first on TheNavalArch.

]]>*Source: http://worldwideflood.com*

Talking about strength of a ship, the picture that comes to mind is that of a ship being subject to rough weather of the sea, and trying her best not to crack or capsize.

A ship with sufficient strength should be able to bear its self-weight, the weight of its cargo, and also the forces which the sea exerts upon it.

**Abbreviations**

*SF – Shear Force*

*BM – Bending Moment*

**Longitudinal/Global vs Local Strength**

At the outset, it is useful to know the difference between global and local strength of ships. Longitudinal strength is also called as global strength. Global strength pertains to assessing the strength of the entire ship when it is floating in still water or in waves. Local strength, on the other hand, is about assessing the strength of a localised structure, like a girder or a longitudinal for loads experienced locally. In this article, we will talk about global (or Longitudinal) strength only.

**The basic premise – ship as a beam**

So, how do we go about assessing the longitudinal strength of a ship?

*Here’s the problem statement*

I have a ship which is floating in water. It is loaded with cargo, and its tanks are filled depending on the operational requirements (called the loading condition, e.g., ballast departure/arrival OR fully loaded departure/arrival etc.). In the open ocean, it will also experience waves. I want to know whether the ship has sufficient strength to withstand this loading. What do I calculate? What do I check it against?

Put simply, I would like to calculate the forces and moments acting on the ship’s structure because of this loading condition, and check them against the maximum which the ship can take.

Complicated as it may sound, the basic premise is actually quite simple. The whole ship is considered to behave like a simple beam, and principles similar to a beam are applied to evaluate her strength.

** **

**Comparisons to a beam**

When we analyse the strength of a beam which is under some load, our final target is to calculate the following forces in the beam

- Shear Force
- Bending Moment

From basic theory of beams, it can be known that the Shear force distribution is a mathematical integration of the load distribution along the length of the beam, while the Bending Moment distribution is the mathematical integration of Shear force distribution along the length of the beam. We will discuss what is integration and how it is done in later sections. To know more about this theoretical formulation, see here.

If the Load, Shear Force and Bending Moment are designated by P, SF and BM respectively, what we have is

SF = ʃ P.dx and BM = ʃ SF.dx (these fancy symbols mean ‘integration’, and are nothing to be overwhelmed with.)

** **

**Load, Shear Force and Bending Moment**

Similar to a beam, to find out the Shear Force/Bending Moment of the ship, we follow the steps below:

- Find out the load distribution of the Ship along its length
- Integrate the load distribution along the ship’s length to get the Shear Force distribution
- Integrate the Shear force distribution along the ship’s length to get the Bending Moment

At this juncture, several questions arise in our mind –

- What are the loads on the ship?
- What do we mean by load distribution? How to get it?
- What is meant by integrating the load/shear force and how do we do it?

We’ll take them up one by one.

*Loads on a ship*

If we think from a very basic viewpoint, there are only two kinds of loads on the ship (ignoring external wind/current forces)

- Weight of the ship – acts downwards and distributed over entire length of ship
- Buoyancy force which the water exerts on the ship’s underwater body – acts upwards and distributed over the length of underwater portion of ship

The buoyancy, on the other hand, is the upwards force exerted by water on the ship. When the ship is in equilibrium, its Weight is equal to its buoyancy. The buoyancy force distribution depends on the underwater profile of the ship, which keeps changing because the ship keeps encountering waves of different sizes.Coming to the Weight of a ship – there are different types of weights on the ship. First is the self-weight of the ship, also called the lightweight. It comprises of the ship hull’s structural weight, the weight of machinery and the weight of outfitting (basically, all the items which are unchanging are part of Lightweight). The other type of weight is the Deadweight (DWT). It is the weight of all the changeable items like Cargo, Fuel, Ballast Water, Fresh Water, and all the other items in the ship’s tanks. Together, Lightweight and Deadweight add up to the total weight of the ship called displacement.

* *

*Hogging and Sagging*

* *

Imagine a ship as a long structure with hollow compartments in between. What happens if I load more at the ends of the ship and less on the midship part? The ends of the ship will bend down, while the middle part will be pushed up, leading to the ship taking the shape of an arch, with the deck being convex, and bottom of the ship being concave. This is called ‘Hogging’ of the ship. Similarly, if the ship is loaded more on the middle and less at the ends, then the midship will go down, while ends will go up, leading to a condition called ‘Sagging’. Hogging or sagging can also be induced by the wave which a ship is encountering. A long wave with crest at the midship and troughs at ends will increase the hogging of the ship (by increasing buoyancy in midship), and a wave with crest at ends and trough at midship will increase Sagging. Hogging and sagging are depicted below. Needless to say, these deflections put stress on the structure of the ship, and the ship structure should be strong enough to bear them.

*Image Courtesy – Wikipedia*

* *

**Load Distribution**

For the strength calculation, what is more important is not the total load on the ship (which is Total Weight minus Total Buoyancy, and is zero for a ship in equilibrium), but the * Load Distribution along the length of the ship*. To elaborate, this means how the weight and buoyancy are distributed along the ship’s length. For example, if the machinery of the ship is located aft, then the weight distribution will show heavier weights towards aft. Similarly, if the ship has a fuller bow, then the forward portion of the ship carries more buoyancy, and so the buoyancy distribution will show higher buoyancy in the fwd of the ship.

The load distribution is nothing but the net load plotted at each point along the length of the vessel. Load is the Weight minus Buoyancy at any point along the length of the ship.

Load = Weight – Buoyancy

So, how do we plot the Load Distribution along the length? The three-step process is below:

Step 1 – Plot the weight distribution along the ship’s length

Step 2 – Plot the buoyancy distribution along the ship’s length

Step 3 – Find out the load distribution by subtracting the buoyancy from weight along the length of the ship

* *

**Plotting the Weight Distribution**

The basic idea is to plot the load due to weight at each point along the ship’s length. This involves plotting weight distribution of each individual item comprising the ship’s weight at its location along the length of the ship. Now, each item spans over a distance along the ship’s length. Thus, it’s weight is spread over a length. To plot its weight distribution, we need to plot its weight per unit length over the item’s length. The total area of this plot of weight per unit length should be equal to the weight of the body.

The following method is followed to add an item to the Weight distribution

**Method to add an item to Weight Distribution**

- Find out the Weight (w) and length (l) of the item
- Find out the LCG (Longitudinal Center of Gravity) of the item along the length of the ship
- The weight of the item is considered to be distributed in the shape of a trapezium.
- Find out the ordinates of the trapezium considering following two facts
- The total area of the trapezium should be equal to the weight of the item
- The Centroid of the trapezium should be equal to the LCG of the item

- Plot the trapezium by
**adding**it to the existing Weight distribution - Keep adding items to the load curve one by one to give the final Weight distribution of the ship

To illustrate the above, let’s assume that we want to add a Deck Cargo which has a weight of 100 tons, is 10 m long, and its LCG is located at midship. Also, its LCG lies 6 m from its own aft end (see figure).

Let’s plot its trapezium. Let the ordinates of the trapezium be a and b. The calculation is demonstrated below

Now that we know how to plot the weight distribution of an item on the ship, let’s take up the individual components of weight of the ship and see how we can plot them.

The Weight of the ship comprises of Lightweight and Deadweight. We’ll take them up one by one

*The scare of Lightweight distribution and the way out*

The Lightweight, or the self-weight of the ship is comprised of the structural steel weight of the ship’s body, the weight of machinery, and the weight of outfitting (accommodation included).

There are numerous items on the ship, and taking into account each and every item of the ship to prepare the lightweight distribution is a tedious task. The good news is, there are some approximate methods available. These methods create the lightweight curve based on the usual lightweight distribution expected from a regular ship-shaped vessel. Generally, a ship is fuller in the midship region and slimmer at ends. So, the weight distribution is also expected to follow a similar trajectory. A typical lightweight distribution curve is shown below:

We can see that the curve is constant for midship region, while it tapers towards the ends where weight is less. Lightweight can be plotted using one such simpler method as demonstrated above. For more about methods for lightweight distribution, see here.

* *

*Deadweight distribution and adding it to lightweight curve*

Once we have the lightweight curve from approximate methods as described above, we need to add the items of deadweight to it to get the total weight distribution curve.

Deadweight comprises of cargo, fuel oil, ballast, fresh water etc (i.e., the variable loads on the ship). To add a deadweight item, we just follow the procedure as described in the beginning of this section by creating the trapezium and adding it to the existing weight curve. In most cases, the load distribution will be a rectangle (both the ordinates of the trapezium will be same), since most deadweight items are uniformly distributed along their geometrical length.

The following pictures illustrate this process of adding the trapezium of an item to the weight curve.

Once the distributions of all items of deadweight have been added to the Lightweight curve, then we will arrive at the final Weight Distribution Curve. It may look something like this:

*Buoyancy Distribution Curve*

Once we have the Weight distribution, the next step is to create the buoyancy distribution curve. The basic idea is same – to plot the load due to buoyancy at each point along the length of the ship.

The buoyancy force is determined by the shape of the underwater hull, and it is the weight of the water displaced by the underwater hull. With this understanding, we can see that the buoyancy distribution is same as the volume distribution of the underwater portion of the hull. If the underwater volume is divided into sections of unit length along its length, then the volume of the underwater hull is nothing but an integration (or sum) of the areas of these sections along the ship’s length. Thus, the buoyancy distribution is a plot of the section areas of sections along the length of the underwater body of the hull.

*Still Water condition vs Wave conditions*

Here it is useful to take note of the wave condition in which the ship is sailing. The underwater body of the ship is variable, as it keeps encountering waves of different sizes. However, when there are no waves present, then the water can be considered still, and the waterline is horizontal. This floating condition of the ship is called Still Water condition.

How do we take care of the waves? They are of different sizes. For design purpose, we consider that the vessel should be able to take a wave whose wavelength is equal to the length of the vessel itself. There can be two cases for such a wave – one case when the crest of the wave is at midship (and troughs are at ends of the ship), and the other when the trough is at midship (and crests at ends of the ship). The Still Water case and the two wave cases are depicted below. We can see that the effect of the wave cases is to increase the hogging or sagging of the vessel.

Whatever the underwater profile of the vessel (whether still water or wave cases), we need to plot the buoyancy distribution curve accordingly.

*Steps for plotting the Buoyancy Distribution Curve*

The buoyancy curve in the plot of section areas of the different sections along the length of the vessel. Usually the sections are taken at the stations of the lines plan of the vessel. The body plan is used to get the section areas of the stations. This process is shown below:

*Plotting the final Load Distribution*

Once we have the Weight and Buoyancy distributions with us, we superimpose them, and at each point of the weight curve, we deduct the corresponding buoyancy value to obtain the point on the load curve. This process is demonstrated below:

We can see that the load curve has both positive and negative ordinates. Put simply, the weight is higher than buoyancy at some places which buoyancy is higher at other places. When the ship is in equilibrium, the total Weight and total Buoyancy are equal. Thus, the positive and negative areas of the load curve cancel out and the net area of the load curve is zero for vessels in equilibrium.

* *

*Integration of the Load Curve to get Shear Force Curve*

Now that we have the load curve, we need to integrate it to get the Shear Force Curve. What is this integration, and how is it carried out?

The Shear Force at any point along the length can be found out by adding the area under the load curve * up to that point*. For example, if we want to find out the Shear Force on the vessel at a location x = L/4 along its length (where L is the total length of the vessel), then we need to add-up the area under the load curve from x = 0 (aft end) to x = L/4.

The above process of finding the shear force is done at different locations (usually the stations) along the length of the vessel. This gives us the Shear Force values at these locations. If we plot all these Shear force values along the length of the vessel, then we obtain the Shear Force Curve, which looks like the green curve in the above picture

*Integration of the Shear Force Curve to get Bending Moment Curve*

In a similar fashion as done with the load curve, we use the Shear Force curve to obtain the ordinates of the Bending Moment at different locations along the ship’s length, and plot these points to obtain the Bending Moment Curve of the ship. It looks something like this:

** **

**Comparing the results against allowable values**

So now we have the Shear Force and Bending Moment distributions along the length of the ship. What next? We need to check if these are within the allowable limits of the vessel. How do I find out the allowable limits?

The allowable limits of Shear Force and Bending Moment are the values which the structure of the ship can take. Like in the case of a beam where the beam’s cross sectional properties determine the limit of the forces and bending moments it can take, the Ship’s capacity to take shear and bending depends on its sectional properties. By ‘section’ here we mean the structural section of the ship, which looks something like this:

Since the ship’s section shape keeps varying along its length (though it is same for the parallel mid body), the section properties are also different at different locations along the length of the ship. Thus, the capacity of the ship to take shear and bending is also different at different longitudinal locations. The limits at a particular location along the ship’s length are calculated from the following relations

SF_{allowx} = τ_{allow} x A_{X}

BM_{allow} = σ_{allow} x SM_{X}

Where

SF_{allowx} is the Shear Force allowed at a particular location X along the length of the ship

τ_{allow} is the allowable shear stress of the ship’s material

A_{X} is the Area of the ** structural section** of the ship at location X.

BM_{allow} is the allowed Bending Moment at a particular location X along the length of the ship

σ_{allow} is the allowable bending stress of the ship’s material

SM_{X} is the Section Modulus of the ** structural section** of the ship at location X.

The values of τ_{allow} and σ_{allow} can be obtained from Class Rules. For example, ABS rules require σ_{allow} to be 17.5 kN/cm^{2} (or 175 MPa) and τ_{allow} to be 11.0 kN/cm^{2} (or 110 MPa) for ordinary steel (See Ref 1, Part 3-2-1/9.3). For higher strength Steel, these may be reduced by factors provided in Class Rules.

*Section Area and Section Modulus*

Section Area and Section Modulus are the properties of the structural section of a ship at the chosen location along its length, which is the actual cross section of the ship showing all structural elements like plates, stiffeners, girders, brackets etc. See the figure above.

The calculation of section properties (Area, Inertia and Section modulus) is a separate study and will not be dealt with in detail here. To know more about calculating the Area and Section Modulus of a structural section of a vessel, do check out our app Midship Section Modulus Calculator

The section properties need to be calculated for different locations along the length of the vessel. For each of these locations, the allowable bending moment and shear force need to be calculated. Once we have the allowable values at different points along the length of the ship, we compare them to the actual obtained values of SF and BM at these points. If the SF or BM is exceeding the allowed values at even a single point along the length of the ship, then the ship’s structure is NOT adequate in the proposed loading condition.

**Practical importance and implications of Longitudinal Strength Calculations**

*During design stage*

Longitudinal strength calculation forms an integral part of the stability booklet of the vessel. All the loading conditions (e.g., fully loaded departure/arrival or ballast departure/arrival) in the stability booklet should have an accompanying longitudinal strength calculation. This establishes the suitability of the vessel from a strength point of view in all common loading conditions expected.

*During actual vessel operation*

Longitudinal strength becomes all the more critical during actual vessel operations. For vessels with a loading computer, each loading condition should be evaluated in the loading computer for longitudinal strength before commencing loading. For vessels without a loading computer, like dumb barges, the longitudinal strength must be evaluated in a standard software (like GHS/Sutohydro/NAPA etc.) before loading is commenced. For smaller ships, longitudinal strength calculations are not mandatory and may be exempted.

*What to do if the Longitudinal Strength is not adequate for a loading condition?*

If the longitudinal strength is not within allowable limits, following are some useful tips

- If the vessel is in design stage, then structural modifications may be done to improve the Section modulus (e.g., by enlarging a girder, or increasing plate thickness etc.) and thus improve the allowable Bending Moment/Shear Force.
- If the vessel is in operation and software calculations demonstrate a failure of longitudinal strength, loading should not be commenced.
- First, check the location at which the bending moment is exceeding the allowable limit. Usually the location of failure is near midship, since the moment is highest around midship region.
- If the failure is in sagging, then there is high load around midship region. This can be corrected by shifting ballast from midship region to aft/fwd ends (subject to availability). If the failure is in hogging, then there is high load at the ends. This can be corrected by shifting ballast from ends to midship region. At times, relocation or rearrangement of deck cargo also helps in reducing the bending moment.
- The distribution of cargo is also critical in determining longitudinal strength. If the vessel has, say, seven holds, and if four holds are to be filled with cargo (and rest empty), then there are many ways of doing it. Each method will have a different effect on longitudinal strength
- We can fill the 2
^{nd}, 3^{rd}, 4^{th}and 5^{th}The loading will be more in the midship region and lead to higher sagging - We can fill the 1
^{st}, 2^{nd}, 6^{th}and 7^{th}This will lead to higher loads at ends, and thus higher hogging - Fill the 1
^{st}, 3^{rd}, 5^{th}and 7^{th}This way, the load is evenly distributed along the length of vessel, neither leading to too much sag or hog.

- We can fill the 2
- Thus, different approaches may be applied to deal with a situation in which longitudinal strength limit is exceeded.

**Conclusion**

That brings us to the end of this discussion. Longitudinal strength of ships is a critical factor which affects both the design and operation of the ship. Careful planning and supporting calculations during the design stage should be carried out to ascertain the vessel’s suitability from strength point of view. During actual operations, loading conditions should be created keeping in mind the allowable limits and longitudinal strength capacity of the vessel.

We hope this post was of use to you. Do let us know your thoughts in the comments section

References

- ABS Rules for Classification of Steel Ships, Part 3
- http://www.mathalino.com/reviewer/mechanics-and-strength-of-materials/relation-between-load-shear-and-moment
- https://www.sawe.org/files/Methods%20of%20Longitudinal%20Weight%20Distribution.pdf

*Disclaimer: This post is not meant to be an authoritative writing on the topic presented. thenavalarch bears no responsibility for the accuracy of this article, or for any incidents/losses arising due to the use of the information in this article in any operation. It is recommended to seek professional advice before executing any activity which draws on information mentioned in this post. All the figures, drawings and pictures are property of thenavalarch except where indicated, and may not be copied or distributed without permission.*

The post Longitudinal Strength of Ships – an Introduction appeared first on TheNavalArch.

]]>The post Bollard Pull Calculations – an Introduction (Part II) appeared first on TheNavalArch.

]]>__Bollard Pull Calculations for Towing Operations__

__Part II – Finding out the maximum feasible tow speed__

**(To read Part I, please click here)**

__Introduction__

This is Part – II of the two part article on Bollard Pull calculations. In the Part I we saw how to calculate the required Bollard Pull to select a tug. At this stage, we have accomplished the following

- Calculated the required Bollard Pull (BP) to tow the vessel (barge or ship)
- Selected the tug based on the required Bollard Pull

In this part we will see how we can estimate the maximum safe towing speed for the vessel to ply in a given sea state. Before we get to the actual methodology, let’s look at the concept of Available Pull of the tug at non-zero speed.

__Concept – Available pull of the tug at non zero-speed__

By now we have selected the tug, and it has its rated bollard pull, which is its maximum pulling force at zero forward speed. Let’s call it BP_{max}. As we saw in Part I, this BP_{max} should be more than the total required towing force for the STALL condition. However, the actual towing scenario (called the TOW condition) is different in two ways

- First, the tug is not static but towing the vessel forward at a non-zero speed. When the tug moves forward, part of the tug’s power is used in overcoming the resistance of the tug itself, and the rest is actually available to tow the vessel. Let’s say, if F
_{tug}is the force which the tug utilizes for itself, then the available power for towing (BP_{available}) will be

BP_{available} = BP_{max} – F_{tug}

At zero speed, F_{tug} is zero, and whole BP_{max} is available for the holding the towed vessel. At the maximum free-running speed of the tug, the whole BP_{max} is utilized for the tug’s own resistance, and the available power for towing is zero. Thus, if we plot the available towing power of the tug vs speed, we will get a curve like below. The actual curve is not exactly a straight line, but we will assume a straight line to keep things simple (it also leads to more conservative results).

In the example above, we can see that at 6 knots towing speed, the maximum available power for towing is only 40% of the BP_{max}, while 60% goes to overcome the tug’s own resistance

- Second, the environment under which towing is performed is not the same as the STALL environment scenario. Usually, the STALL sea state is a harsher one in which the requirement is to HOLD the tow, and not move it forward. The towing will be done is a comparatively milder environment.

Our objective is to find out the maximum speed which the tug can make in a given environment.

**Methodology**

The methodology which we are going to follow for the above exercise is outlined below

- Step 1 – Get the environmental parameters under which the towing will be done
- Step 2 – Get the tug particulars
- Bollard Pull at 100% and 85% MCR or other MCR values as required
- Tug maximum forward free run speed

- Step 3 – Plot the tug performance curves for different MCR values
- Step 4 – Calculate the total environmental forces (on the towed vessel) for different speed of towing, beginning from zero speed up to the maximum tug speed
- Step 5 – Plot the curve of Total Environmental force (F
_{TOT}) vs towing speed on the same graph as the tug performance curve. - Step 6 – The intersection of the curves for total environmental force and tug performance will give the limiting towing speed

Let’s now see each step in detail

*Step 1 – Get the environmental parameters*

The environmental parameters for towing in this case are the maximum wind, current and wave in which the towing operator proposes to tow the vessel. The towing operator has to advise the safe limiting environment in which she/he plans to tow. For example, the operator may decide that a wind of 20 knots, wave of 3 m and current of 1 knots is the limiting environment in which she/he plans to tow the vessel. The operator would like to know the maximum speed she/he can make with this limiting environment.

*Step 2 – Get the tug particulars*

We saw that the available power for towing keeps reducing with speed of tow. The performance curve of the tug is an important input for determining the available power. For plotting the performance curve, we need the following:

- The free running maximum speed of the tug – this is obtained from the tug’s specifications sheet
- The Bollard Pull of the tug at 100% MCR and 85% MCR. Usually the tug will be operated at 85% MCR. 100% MCR is more significant for the STALL condition to calculate the maximum required Bollard Pull. If the tug is operating at some other MCR value, then the Bollard Pull for that value should also be obtained.

**Step 3 – Plot the tug performance curve**

Once we have the above data, we can plot the tug performance curves for 100% & 85% MCR (or any other value) using the method described in Section 2.

**Step 4 – Calculate the total environmental forces for different tow speeds**

- Wind and current forces are calculated using standard formulas described in Part I.
- The wind and current forces will keep increasing as the tow speed increases. This is because the effective (or relative) wind/current speed against the vessel increases as the vessel moves forward. If the speed of the tug is V
_{tow}, and speed of the wind is V_{wind}, then the effective wind speed is

V_{effective} = V_{tow} + V_{wind}

The wind force will have to be calculated for the above effective speed. Same holds true for the current force

- The wave force is calculated based on DNV-RP-H103 Sec 7.2.6. The wave force also increases with the towing speed, and is calculated in accordance with the relevant sections of DNV.

- Once we have the wind, wave and current forces, we calculate the total environmental force, F
_{TOW}by adding them up. F_{TOW}is calculated for different tow speeds from zero to maximum tug speed.

We can do the calculations and present them in a spreadsheet format. Once such calculation table is presented below (speed means towing speed)

**Step 5 – Plot F _{TOT} against tug speed on the same plot as for the tug performance curve. **

The curve will then look something like the below (the tug’s bollard pull is 150 MT)

**Step 6 – Finding the limiting towing speed**

The intersection of the curves for total environmental force and tug performance will give the limiting towing speed. In the above plot, the point ‘X’ is the intersection of the two curves. We can see that at all points to the left of the point ‘X’ (i.e., speeds less than 2.9 knots marked by blue zone), the available towing pull of the tug exceeds the required towing force, thus making the towing feasible. However, to the right of point ‘X’ (the red zone), the available tug power is less than the towing force required and towing is not feasible. Thus the maximum towing speed feasible for this case is 2.9 knots.

Thus we saw how the maximum feasible towing speed can be calculated for towing a vessel under a given environment. This exercise becomes useful when the Client or MWS requires that a minimum towing speed be achieved and demonstrated through calculations.

That brings us to the end of this two-part article on Bollard Pull. We hope it has been useful to our readers. Do let us know your thoughts in the comment section. Happy Towing!!

**References**

- DNV RP-H103 Modelling and Analysis of Marine Operations
- ABS, 2016, Rules for Building and Classing Mobile Offshore Drilling Units, Part 3 Chapter 1,Section 3, ‘Environmental Loadings’, p.11.
- An Approximate Power Prediction Method, J.Holtrop and G.G.J.Mennen, 1982
- BV – Towage at Sea of Vessels or Floating Units
- OTC 3220, Vol 4, Resistance of Offshore Barges and Required Tug Horsepower

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