Select Page

In the offshore construction industry, the connection between the newly installed pipeline and the riser is accomplished via a series of ‘spoolpieces’ (or spools). The spool is fabricated by welding pipe joints to form an Lshaped, Zshaped, or possibly a straight pipe. The figure below shows that four spools connect the riser flange to the pipeline flange, where the first, second, and fourth spools are L-shaped, whereas the third one is a straight spool.

The lifting analysis of a subsea spoolpiece typically yields the specifications of the lifting slings and the lifting configuration, which represents the location of the pick-up (rigging) points. In addition, the analysis checks the stresses in the spools resulting from the lifting forces. However, the pipe stresses would be of concern in limited cases where we have to use a small lifting angle (with the horizontal), and the bend angle is approximately 90 degrees, coupled with a small pipe (e.g., 8 inches or less). Otherwise, lifting the spool does not usually cause stresses exceeding the allowable limits.

In all cases, we must obtain the lifting configuration; thus the tension in the wires, to correctly choose the suitable slings. That means we must find/purchase the slings matching the analysis results, which may cost much. However, the main problem is the potential delay of providing the slings if we have a tight schedule, e.g., when changing the lifting configuration of the closing tie-in spoolpiece.

What if the lengths of the available slings become an input in the analysis procedure? In other words, the question becomes, “where can I install a sling of a particular length on the spool, with a particular hook height?” So if we were able to change the way we do the analysis, we would be using slings available in the stock, provided they have reasonable sizes and lengths, saving costs in many ways. Is that possible?

The answer is yes. The basic idea of obtaining the location of the pick-up points is that they represent the intersection between a sphere and a straight line, but how?

The sphere is centered at the hooking point, and its radius is the sling length. That means that the sphere surface represents all the possible points in space around the hooking point that are at an equal distance of the sling length from it. These points, of course, include the pick-up points we are looking for; thus, our job now is to find the intersection points between the sphere and the spool straight lines. See the below illustrating sketch.

Mathematically speaking, the sphere equation is defined by the center (hooking point) and the radius (sling length). And of course, the hooking point should be at a suitable vertical distance (hooking height) from the center of gravity, which should be calculated as well. Of course, that hook height should be reasonably shorter than the shortest sling. On the other hand, the spool equation is identified by the spool pipes lengths and bends’ angles. Consequently, the two equations must be solved simultaneously, yielding, ideally, two quadratic the root(s), representing the coordinates of the pick-up points of one particular sling length on a specific spool pipe.

Furthermore, the tension in the slings can be found by simple equilibrium equations of rigid body mechanics. So we have six equilibrium equations: three for summing forces in the three coordinate directions and three for summing moments. In that case, if we imagine that the max number of slings is six (which is practically too many), we can use as many equations as the number of slings we wish to use. Ultimately, we solve these equations simultaneously, get the tension, and subsequently choose the proper slings or probably check if the tension matches the slings we have in stock.

This procedure can be done by Excel VBA. A program has been developed (link at the bottom of this article) that performs the whole process from identifying the center of gravity, getting the pick-up points, and obtaining the tension. And, of course, the user can redistribute the slings and get new tension values if he wishes. Obtaining positive tension values is a sign that the slings distribution is physically possible and stable.

The following video explains the concept graphically.

#### Mohamed Hermas

Founder, Offshore Engineering Guide

Mohamed is an Offshore Pipeline Engineer and a CAE specialist, holding an M.Sc. in Subsea Engineering from the University of Aberdeen. He has been working in the offshore construction industry, with a focus on pipelines and cables, for 19 years.

Hermas has served in several capacities over the course of his career. He began as a field engineer, then project engineer, installation engineer, and pipeline engineer in many places worldwide.

In 2016, Mohamed was awarded the Chartered Engineer title from the Institution of Mechanical Engineers. In the same year, he launched a website for offshore construction professionals, with the following address: offshoreengineerguide.com

Mohamed’s interest in studying the finite element method began in 2017. By 2018, he has built his first mathematical model for stress analysis by Matlab language. Subsequently, he developed solvers serving many engineering purposes using Matlab, Wolfram language, and Excel VBA. He helps companies and individuals to make tailored programs/calculators for different engineering applications for which there is no software package available in the market.

Disclaimer:

The views, information, or opinions expressed in this article are solely those of the author and do not necessarily represent those of TheNavalArch Pte Ltd and its employees

## TRANSPORT VESSELS FOR FLOATING WIND

By Alan Crowle, BSc, MSc, CEng, CMarEng, FRINA, FMAREST, FSCMS Masters by Researcher, University of Exeter, College of Engineering, Mathematics and Physical Sciences, Renewable Energy Group SUMMARY Floating wind turbine construction is a large logistical exercise. The...

## Floating Rice Fields, the quest for solutions to combat drought floods and rising sea levels

by Lim Soon Heng, BE, PE, FSSS, FIMarEST Founder President, Society of FLOATING SOLUTIONS (Singapore) Abstract Amazing as it seems, there is a case for growing rice on floating platforms in the sea. The capital expenditure to develop this is offset by the opportunity...

## Using simple tools for efficient Passage Planning for seagoing vessels

Introduction Planning a vessel’s voyage is a critical detailed exercise, and the main goal is to ensure safe and efficient passage between two ports. The Master has the responsibility for the vessel voyage planning, but very often he delegates the actual voyage...

## Designing a closed chock as per IACS rules

Introduction   Chocks are used universally for mooring and towing operations on ships. For towing operations, Chocks are used for guiding the towing rope from the winch through the outer shell of the vessel to the tug. For mooring operations, the chock is used to...

## The Optim22 Method of Weight Optimization of Framed Steel Structures

1      Abstract A semi-automated structural weight optimization system is presented for framed structures of post and beam construction which is based on basic structural member design principles.   The approach is to adjust member properties in a manner that...

## The importance of clear decision support before marine operations

Our oceans are interspersed with human activities: be it fishing, marine transportation, or offshore operations. These operations usually involve assets worth multiple million dollars, be it the produce or the equipment or in most cases, the human life. Produce from...

## Sea Pressure Loads Calculation based on DNV Rules

Introduction   Sea pressure loads are an important factor in the structural design of a vessel. What is sea pressure load? As the term suggests, it is the external pressure on the vessel due to the surrounding sea. What kind of pressure it is, and how to...

## Selecting the right Capstan for berthing a vessel

Introduction Capstans are frequently deployed mooring equipment used on all types of vessels. Capstans are berthing/mooring equipment used to multiply the pulling force on mooring ropes. Traditionally, Capstans were operated manually but in modern ships, they are...

## Designing Fillet Welds for Symmetrical Joint Sections

Introduction Fillet welds are the most commonly used weld types in marine structures. A fillet weld is used when there are two pieces of metal that are joined perpendicular to each other or at an angle. In this article, we will explore how to select the right size...

## Designing a stanchion/stopper for sea-fastening of deck cargo

Introduction Stanchions – a familiar term for mariners and ship designers. What are Stanchions? A stanchion is generally a vertical pipe or beam which is used to support some structural item or provide support rails on the deck. In ships, the most common type of...

Please check out TheNavalArch’s product for planing vessel resistance estimation: