Subsea Rigid Pipelines – Methods of Installation

Copyright © 2019 Eng-Bin All rights reserved.

ISBN

978-1-5437-5144-4 (sc)

978-1-5437-5146-8 (hc)

978-1-5437-5145-1 (e)

Library of Congress Control Number: 2019905408

All rights reserved. No part of this book may be used or reproduced by any means, graphic, electronic, or mechanical, including photocopying, recording, taping or by any information storage retrieval system without the written permission of the publisher except in the case of brief quotations embodied in critical articles and reviews.

Because of the dynamic nature of the Internet, any web addresses or links contained in this book may have changed since publication and may no longer be valid. The views expressed in this work are solely those of the author and do not necessarily reflect the views of the publisher, and the publisher hereby disclaims any responsibility for them.

07/11/2019

CONTENTS

Foreword

Acknowledgements

Preface

Chapter 1 Introduction

1.1 General

1.2 Overview of Oil and Gas Field Development

1.3 Overview of Pipeline Installation Techniques

1.4 Typical Activities Carried Out on the Pipelay Vessel

1.5 How This Book Is Organised

1.6 Abbreviations

Chapter 2 Pipeline Installation by S-Lay Method

2.1 General

2.2 Types of Lay Vessels and Stingers

2.3 Evolution of S-Lay Vessels for Deep-Water Pipeline Installation

2.4 Installation by Moored Pipelay Vessel

2.5 Installation by DP Pipelay Vessel

2.6 Pipelay Start-Up/Initiation (Shallow Water)

2.7 Pipelay Start-Up/Initiation (Deep Water)

2.8 Pipelay Abandonment and Recovery (Shallow Water)

2.9 Pipeline Abandonment and Recovery (Deep Water)

2.10 Installation Analysis

Chapter 3 Pipeline Installation by J-Lay Vessel

3.1 General

3.2 Advantages and Disadvantages of J-Lay

3.3 Typical J-Lay Vessels

3.4 Normal-Lay Mode

3.5 Installation of Dead-Man Anchor (DMA) for Pipelay Initiation

3.6 Pipelay Initiation with Initiation Head (without PLET)

3.7 Pipelay Initiation with PLET

3.8 Pipelay Abandonment without PLET

3.9 Pipelay Abandonment with PLET

3.10 Installation Analysis

Chapter 4 Pipeline Installation by Reel-lay Vessel

4.1 General

4.2 Benefits and Limitations of Reeling

4.3 Types of Reel-Lay Vessels

4.4 Pipe Stalk Fabrication and Spooling

4.5 New-Generation Reel-Lay Vessel Utilising Potable Reel Concept versus Traditional Reel-Lay Methods

4.6 Pipelay Initiation, Normal Lay, Intermediate Abandonment, and Final Laydown

4.7 Engineering

Chapter 5 Pipeline Installation by Bottom Pull

5.1 General

5.2 Landfall-to-Landfall Bottom-Pull Operation

5.3 Bottom Pull from Offshore

5.4 Bottom Pull from Lay Vessel Offshore to Landfall

5.5 Engineering

5.6 Lessons Learnt

Chapter 6 Pipeline Installation by Bottom Tow, Off-bottom Tow, and Controlled-Depth Tow Method

6.1 General

6.2 Bottom Tow Method

6.3 Off-Bottom Tow Method

6.4 Controlled-Depth Tow Method (CDTM)

Chapter 7 Pipeline Installation by Surface Tow

7.1 General

7.2 Pipe String Fabrication and Supplementary Buoyancy Device Installation

7.3 Preparation for Launch and Tow

7.4 Launching of the Pipeline

7.5 Towing of Pipe String

7.6 Removal of Buoyancy Device and Laydown

7.7 Post-Installation Works

7.8 Installation Analysis

7.9 Lessons Learnt

Chapter 8 Pipeline Shore Approach Installation

8.1 General

8.2 Conventional Shore Approach Methods

8.3 Push-Pull Method

8.4 Horizontal Directional Drilling

8.5 Lessons Learnt

Chapter 9 Tie-ins for Rigid Pipeline

9.1 General

9.2 Shallow Water Tie-Ins

9.3 Deep-Water Tie-Ins

Chapter 10 Typical Methods of Riser Installation

10.1 General

10.2 Riser Installation by Stalk-On Method

10.3 Riser Installation by Flanged Tie-In (Shallow Water)

10.4 Riser Installation by Pull-In Method (Small-Diameter Pipelines)

10.5 Installation of Steel Catenary Risers (Deep Waters)

10.6 Installation Analysis

Chapter 11 Pre-commissioning of Pipelines

11.1 General

11.2 Basic Requirements for Pigging

11.3 Pipeline Filling, Cleaning, and Gauging

11.4 Pipeline Hydrotesting

11.5 Pipeline Dewatering

11.6 Pipeline Drying

Chapter 12 Seabed Intervention

12.1 General

12.2 Types of Seabed Intervention

12.3 Pre-Lay Intervention by Dredging

12.4 Pre-Lay Intervention by Drilling and Blasting

12.5 Pre-Lay Intervention by Placement of Supports

12.6 Pre-Lay and Post-Lay Intervention by Rock Dumping

12.7 Post-Lay Intervention by Jetting

12.8 Post-Lay Intervention by Subsea Ploughs and Trenchers

12.9 Post-Lay Intervention by Placement of Supports

12.10 Seabed Intervention for Span Corrections

12.11 Intervention for Crossings

12.12 Conclusion

Chapter 13 Potential Failure Modes and Considerations for Installation

13.1 General

13.2 Potential Failure Modes and Considerations for S-Lay Method of Installation

13.3 Potential Failure Modes and Considerations for J-Lay Method of Installation

13.4 Potential Failure Modes and Considerations for Reel-Lay Method of Installation

13.5 Potential Failure Modes during Bottom Tow, Off-Bottom Tow, and CDTM and Considerations for Installation

13.6 Potential Failure Modes and Design Considerations for Surface Tow (Rentis) Method of Installation

13.7 Potential Failure Modes and Design Considerations for Bottom-Pull Method of Installation

13.8 Potential Failure Modes and Considerations for HDD Operation

Chapter 14 Repair of Damaged Pipelines

14.1 General

14.2 Repairs during Pipeline Installation

14.3 Repairs of In-Service Pipeline

14.4 Emergency Pipeline Repair System

Chapter 15 Manufacture, Coating, and Transportation of Line Pipes

15.1 General

15.2 Line Pipe Manufacturing

15.3 Line Pipe Corrosion Coating

15.4 Sacrificial Anode Manufacturing

15.5 Line Pipe Anode Installation and Concrete Coating

15.6 Load-Out and Transportation of Line Pipe

AUTHOR’S BIOGRAPhY

RIGID SUBSEA PIPELINES

METHODS OF INSTALLATION

FOREWORD

When I first started working in an offshore oil and gas engineering company about 40 years ago and deciding whether to specialise in offshore pipeline engineering, my mentor, who was a sage of an engineer, told me something that stuck. He told me that pipeline engineering is more of an art than a science, and it would do me well to remember this if I choose to become a pipeline engineer. I think this observation is true now as it was then, which I found out over the years I have been involved with the offshore pipeline industry. Despite technology moving from the HP41CV as my trusty tool in the late ’70s to today’s high-speed computers capable of determining dynamic stresses and strains to accuracy of within a few percentage points, I think pipeline engineering remains very much of an art, a discipline that requires a good appreciation of the risks during both the operational and the construction phases. I believe one cannot truly be called an offshore pipeline design engineer without having also experienced line pipes manufactured in a pipe mill or pipelines installed from an offshore lay vessel. Over the years, I have come across many pipeline failures during installation and during operation that can be attributed to a lack of appreciation of the interdependency of uncertainties related to fluid mechanics, hydrodynamics, materials, corrosion, and geotechnics in the various phases of a pipeline life.

It was in the mid-80s that I became acquainted with Eng Bin (no relation of mine I must add). He was then working as pipeline engineer for McDermott; and I was with DNV, certifying an offshore pipeline he was involved with. And that’s how our long professional association and friendship started. Over the years, we have agreed and argued professionally over interpretation of code requirements, design methodologies, and shared knowledge. I have found Eng Bin to be not only very well grounded in the theoretical aspects but also very practical minded gleaned from his many years involved in managing offshore pipeline projects.

This book is truly a gem and one much needed for both the budding or experienced offshore pipeline engineer interested in knowing more about the practical aspects of offshore pipeline installation from a very experienced and well-regarded industry professional. It is by no means an authoritative handbook on the subject or a textbook covering all facets of offshore pipeline installation. Rather, it encapsulates Eng Bin’s more than 40 years of experience in the offshore pipeline industry; and he offers advice and practical tips, which no textbook can provide. I congratulate Eng Bin for this truly mammoth effort. I hope readers will not only find this book informative but will also come to more fully appreciate the art of pipeline construction.

Edwardus Ng

Sr Vice President, DNV GL

ACKNOWLEDGEMENTS

There are many people in my career who have shaped me. I remember some very fondly and some not so fondly. Of the people who have influenced me positively, I would like to single out two persons: Mr Jay Wilkins of McDermott and Mr Flip Geerling of Rockwater. These two gentlemen treated me with respect and gave me abundant opportunities to improve my skills and confidence.

Jay Wilkins hired me as McDermott’s first Singapore pipeline engineer in 1979; and even though he was the managing director of the company, he personally coached and guided me on how to perform pipeline installation engineering. I remember once when he was on the phone to discuss various aspects of pipelaying with the operations manager of McDermott (and this was the top position at the operations division at that time), he would address me as his ‘expert’ and would give me credit for much of the recommendations given by himself. So when I actually went to the operations division to discuss the matters further, I was given the respect of an ‘established’ pipeline engineer and abundant cooperation. This helped boosted my confidence and helped pave the way for me to be the key pipeline engineer in the company.

Similarly, Flip Geerling gave me the opportunity to be the first engineering manager for Rockwater (now renamed Subsea 7) in the Eastern Hemisphere. Like Jay Wilkins, he was approachable, helpful, and full of positive advice and encouragement. For a relatively young engineer at that time, I received genuine show of concern from my bosses early in my career; and it etches deep in my memory. Ever since I became a manager, I have genuinely tried to emulate the fine behaviour of these two gentlemen when I deal with my younger colleagues and subordinates.

I would also like to thank Dr Andrew Palmer, whom I have long regarded as the ‘maha guru’ of pipeline engineering, for being my unofficial mentor. I had brief encounters with Andrew when I was a young pipeline engineer in McDermott and he was the VP of engineering at RJ Brown and Associates. I also had some dealings with Andrew when I was working for Rockwater. Although I have been mulling on whether to write this book or not for many years, it was Andrew who finally persuaded me to proceed with my dream of putting my experience on paper and sharing it with the younger generation. Hence, I would like to thank Andrew for prodding me on to write and complete this book.

Finally, I would like to thank the following companies that have given permission to use their company information for this book and to the persons listed below for their advice and contribution to this book:

□ Mr Edwardus Ng of DNV GL, for agreeing to write the foreword for my book

□ Subsea 7, for generously granting me permission to use their photos and, in particular, to Ms Jasmine Kim, Ms Aoibheann Rogers, and Mr Adzariat Monergi, for facilitating my request

□ Hyundai Heavy Industries, for generously granting me permission to use their photos and, in particular, to Mr Taewon Lee for granting me the permission and to Mr Ju-Yeob Lee, for linking me up with Hyundai

□ EMAS AMC, for granting me permission to use their photographs and, in particular, to Mr David Lowther

□ ShawCor, for generously providing photographs for various types of pipe coating and for their permission to use these photographs and, in particular, to Mr Theng-Kie Goh and Ms Amelia Chee

□ Tri-Star Industries, for generously providing information and photographs for sacrificial anode manufacture and for permission to use these photographs and, in particular, to Ms Huijun Li and Ms Evonne Teo

□ Van Oord, for granting me permission to use photographs taken of their project and, in particular, to Mr Joop Rijkers

□ SK E&C and SK Innovation for granting permission to use photographs taken of their project, and in particular, to Mr Jin-Kwang Kim.

□ Saipem, for granting me permission to use some of their photographs and, in particular, to Mr Arnaldo Racca and Ms Linda George

□ INTECSEA, for granting me permission to use their materials on rock berm design and, in particular, to Mr Hong-Kiat Chia and Dr Xuexin Dai

□ Mr Abdul Hadi Masron, for his great efforts in making sketches and illustrations for this book

□ Dr Sir-Hoon Yeo and Ms Elsie Ng, for generously providing material information for the book, for proofreading, and for giving advice

□ Mr Chong-Fong Lee, Mr Yong-Say Kueh, and Mr Chee-Siong Yong, for proofreading the drafts and providing highly constructive comments

□ Mr Yu-Yeob Lee and Mr Anthony Wong, for unselfishly sharing their arsenal of construction photographs with me

□ Mr David Ng, for his advice and information on reel-lay installation and engineering

□ Mr Lip-Wee Soh and Mr Colin Cheshire, for their advice and contributions on the chapter pertaining to pre-commissioning

□ Mr Dave Foster and Mr Iain Pierce of Propipe and Mr Sachin Sanghai of Trans Asia Pipeline, for generously providing photographs of pre-commissioning equipment and granting me permission to use them

□ Mr Syed Alsagoff, for his advice, review, and comments on the chapter pertaining to pipe manufacturing and coating

□ Dr Andrew Palmer, Dr Jessica Zheng, Dr Xuexin Dai, Mr Choon-Hau Lua, Mr Kia-Hock Goh, Dr Amit Dutta, Mr Darren Wong, Dr Zhiwei Huang, Ms Yufeng Guo, Mr Joshua Tan, Mr Santosh Kurundwade, Mr. Donikon Fajar, Mr. Chaiyong Zheng, Sivakaanthan Ambedkar, and Mr Noordin Choterai, for their encouragement, contributions, and advice

□ Ms Radiah Jamid, for helping me prepare the final manuscript for submission to the publisher

PREFACE

Oil and gas have been the most-sought commodities as affordable source of energy for decades, and offshore exploitation for such energy sources was conducted since the twentieth century, with increased vibrancy since early 1970s. Subsea pipelines, also known as submarine pipelines, constitute a major means of gathering and transporting hydrocarbons and associated products from an oil/gas field to storage plants offshore and processing plants onshore, including import and export offloading lines from onshore storage tanks to loading/offloading facilities (e.g. SPMs) nearshore. Pipelines are analogous to arteries in the human body. Without pipelines, the extracted hydrocarbons from the ground cannot be transported to the various intended destinations and, as such, is one of the most important investments for an oil/gas field.

I am one of the many engineers who have, either deliberately or by chance, entered the field of submarine pipeline engineering; and I have thoroughly enjoyed working in this field. I came into this field after graduating from Cranfield Institute of Technology (renamed Cranfield University) in UK with a master’s degree in offshore structures in 1978 and was recommended by my supervisor, Dr Colin Kirk, to RJ Brown and Associates in Singapore. After a short stint in RJ Brown, I was hired by McDermott Engineering (Singapore) as their first newly recruited pipeline engineer in Singapore; and it was here that I spent ten foundational years learning and practising pipeline engineering. I am very grateful to RJ Brown and Associates for providing me the initial foundation in pipeline detailed engineering and to McDermott for providing me the opportunity to develop both detailed and installation pipeline engineering, as well as the opportunities to gain actual field experiences. It was through the ten-year stint in McDermott that I have gained tremendous experience in design and installation of submarine pipelines and associated facilities using traditional lay barges, including riser installation and subsea intervention.

In addition to McDermott, I was also privileged to have worked for other contractors as well as various consultants, where I learnt that it was possible to install pipelines without using traditional lay barges. It was in these companies that I realised that pipelines can be installed purely with onshore equipment and tugs, and I was privileged to have had the opportunity to work directly on a few of such projects.

I am taking this opportunity in my golden years to share my varied experiences with the younger generations and with anyone who is interested to learn from my experience. This book is targeted at students and newcomers to the oil and gas industry who have little or no knowledge of pipeline construction. Unlike other technical books on pipelines, it does not address the detailed design of pipelines but instead provides an overview of construction methodologies for subsea pipelines. As such, I am providing a different perspective to students of pipeline engineering, which is by providing a practical and illustrative approach to explain and illustrate how submarine pipelines can be installed by various methods. Some contractors may choose to utilise a variation of some of the methods illustrated in this book, but it is not the intention of this book to cover all possibilities.

I have used examples from my past projects, where available, to highlight the various aspects of the work. However, although I have been in the industry for four decades, I profess that there are many areas of pipeline construction where I do not have actual project or field experience. As such, I have relied upon materials and pieces of advice from my colleagues, ex-colleagues, friends, and contractors in the industry for the necessary input. Sources of illustrations and photos are indicated below the photos/illustrations or in the references after each chapter.

Although the topics have been written to the best of my knowledge, there may be areas that may not have been accurately or adequately covered; and for this, I take full responsibility. I hope that the readers will tell me about this so that such inadequacies or inaccuracies can be amended or improved in the next revision, if any.

CHAPTER 1

Introduction

1.1 General

This book is about how rigid steel pipelines that lie beneath the sea can be installed. These pipelines are broadly known as subsea pipelines or submarine pipelines. Subsea pipelines can be further classified as trunk lines, infield pipelines, and flowlines; and the classification is based on their purpose and location.

A flowline usually refers to a pipe that transfers hydrocarbon from an oil or gas well to a processing facility. It might also transfer hydrocarbon from a smaller facility to a larger one within the same oil/gas field. These flowlines are generally short and lie within the same oil field.

A pipeline usually refers to a pipe that transfers processed hydrocarbon from a processing facility or storage facility to another location, which could be another processing facility or an onshore plant or to the end user. Pipelines can be further classified as infield pipelines and trunk lines.

A trunk line is typically a long and relatively large diameter pipeline that transfers processed hydrocarbon from a processing facility offshore (e.g. offshore platform) to an onshore receiving facility whilst infield pipelines refer to pipelines that connect one facility to another within the same field.

It should also be noted that flowlines and small-diameter infield pipelines could be either rigid steel pipelines or flexible pipelines. However, since this book focuses solely on rigid pipelines, installation of flexible pipelines will not be addressed.

This book provides an overview of various construction methodologies for subsea pipelines using an illustrative approach to explain how subsea pipelines can be installed by various methods. Readers should be aware that construction methodologies for any particular type of method vary amongst contractors based on economic considerations and their equipment availability and preferences. Hence, what I have written is based on my personal experience as well as from literature with which I have accessed to. I have also used examples from my past projects, where available, to highlight the various aspects of the work. Otherwise, examples are taken from associates and from various contractors, as well as from the public domain.

1.2 Overview of Oil and Gas Field Development

This section provides a brief overview of the general activities that are carried out for an oil and gas field development, which typically involves pipeline construction.

1.2.1 General Phases of a Typical Field Development Project

A typical oil and gas field development project is normally very complicated and involves substantial investment, planning, risks, and time. The following provides an overview of the general phases involved in such a field development, which typically span several years:

• Exploration survey and evaluation of field potential (performed by the operator/owner of the field).

• Exploratory drilling to estimate field’s reserve and determine viability of developing the field (performed by the operator/owner of the field).

• Field layout design after a decision has been made to develop the field (performed by the operator/owner or subcontracted to a specialist consultant).

• Front-end engineering studies (typically performed by the operator/owner or through a specialist consultant, although it is not uncommon for the operator/owner to pass this work to the installation contractor). This activity may be combined with field layout design by the same party or performed concurrently by different parties.

• Tender preparation, floating of tenders, and award of contracts (performed by the operator/owner of the field although sometimes part of this work may be subcontracted to a specialist consultant). The contract for developing the entire field is typically awarded to more than one contractor, e.g. production drilling to a drilling contractor, supply of subsea trees to a specialist subsea tree manufacturer, procurement and installation of production facilities and pipelines to an installation contractor, etc.

• Award of contracts to the successful tenderers (by the operator/owner of the field).

• Execution of the various contracts (by the various contractors)

• Taking over of completed facilities from the contractors upon successful completion of the respective contracts (by the operator/owner of the field)

• Commissioning and start-up of production (by the operator/owner of the field)

1.2.2 General activities carried out by the contractor for the installation of a subsea pipeline would include but not limited to:

As this book is about installation of rigid subsea pipelines, the author will concentrate on summarising the activities required for an EPCI subsea pipeline installation contract, which encompasses the following general activities undertaken by the pipeline installation contractor (or its designated subcontractor(s)):

• Detailed engineering design of the pipeline, including flow assurance studies, to confirm/revise the pipeline properties such as pipeline diameter and wall thickness, concrete coating requirements (for on-bottom stability), sacrificial anode requirements (for cathodic protection of pipeline during its operating life), expansion spool requirements, seabed intervention requirements, etc. (done either by contractor or subcontracted to a specialist consultant)

• Procurement of permanent pipeline materials, field joint coating materials, and installation aids

• Manufacture of line pipe (subcontracted to pipe mills) and anodes (subcontracted to anode manufacturer)

• Coating of line pipe and installation of anodes on line pipe (subcontracted to a specialist coating yard)

• Installation analysis and drafting of installation procedures

• Transportation of coated line pipe and other materials to the field

• Mobilisation of installation vessel/equipment to the field

• Installation of pipeline(s) (as well as other facilities, such as subsea production facilities, fixed platforms, floating platforms, etc.).

• Pre-commissioning of laid pipelines

• Handing over of the completed and pretested pipeline(s) to the operator/owner of the field

1.3 Overview of Pipeline Installation Techniques

Exploration for oil started onshore in the early twentieth century; and by 1940s, oil exploration had extended to shallow depths offshore in the Gulf of Mexico. The first oil well offshore was drilled in the Gulf of Mexico in 1947, 17 km from shore and at a depth of 6m (p. 6, ref. 1). Since then, remarkable improvements in technology has enabled exploration to go beyond depths exceeding 2 km. To complement the offshore oil exploration, technology for subsea pipeline installation was developed, commencing with the laybarge method of installation in the Gulf of Mexico. This technology was eventually exported to the North Sea and the Arabian Gulf.

Installation of subsea pipelines is very different from installation of onshore pipelines. For onshore pipelines, line pipes are delivered to the site by land-based transport (e.g. trailers) and placed along the ‘right of way’. They are subsequently welded to form the pipeline and progressively lowered into a trench by onshore equipment and backfilled with indigenous soil. For traditional subsea pipeline installation, the line pipes are delivered to the lay vessel, which is then used as a welding factory to weld the pipes into a pipe string. Unlike onshore pipelines, the lowering of the pipeline to the seabed is more complicated as the bending stress incurred in the pipeline is far greater and buckling of the pipeline will occur if not executed properly. The lay vessel moves forward as more and more pipes are welded on the vessel until all the required line pipes are welded out. Installation of subsea pipelines in the open water is subject to dynamic effects of the vessel movements as well as the effects of the wind, waves, and current forces on the vessel and the pipeline. All these factors add more risks to the installation of subsea pipelines than when installing onshore pipelines.

If the water is very shallow indeed—say, not more than a very few metres deep—then the pipe is welded together and laid down to seabed under its own weight until it reaches the bottom. In this case, the bending stress in the unsupported span between the vessel and the seabed may be acceptable. However, as the water depth increases, the bending stress increases; and the pipeline cannot be laid without doing something to reduce the stress. Engineers have come up with a solution for overcoming this problem, and that is through the following scheme:

• Introduction of a supplementary support structure, called stinger, which is hinged to the stern of the lay vessel and extend some or all the way to the bottom (depending on water depth)

• Introduction of tension to reduce the ‘sag’ in the pipeline and thereby reduce bending moment to an acceptable level

• Introduction of lay initiation methodologies to induce tension in the pipeline as the pipeline leaves the lay vessel at the initial stage, and this includes the adjustment of lay tension and stinger profile as the line pipes are gradually welded on the lay vessel to form the pipeline until the first length of pipeline is laid on the seabed

• Similarly, the introduction of laydown procedures to allow the pipeline to be lowered to seabed in a controlled manner and within the allowable stresses after the entire length of pipeline has been welded on the lay vessel

Introduction of the above measures result in the pipeline assuming an S shape as it is being laid. The upper part of the S curve is labelled as overbend whilst the lower part near the seabed is labelled as sagbend. Simply put, the weight of the pipeline in the overbend is supported by the stinger, and the weight of the pipeline in the sagbend is supported by the tension. The tensioner system needs to be highly reliable; otherwise, any failure of the tensioner will result in damage to the pipeline, and this can occur within seconds.

As offshore exploration enters into deeper water in search of more hydrocarbon deposits, it became necessary to install pipelines in deep waters. The traditional S-lay method of pipeline installation became increasing difficult as the stresses in the overbend could not be managed. As a result, engineers came up with a solution to remove the ‘overbend’. This is done through the removal of the overbend section of the pipeline during pipelaying. Hence, the birth of the J-lay method of installation whereby the pipeline only has a sagbend during pipelaying (see chapter 3).

During the initial period of pipelaying, pipeline stresses were limited to the elastic range of the pipe properties. Eventually, multiple researches were carried out on various aspects of pipelaying; and codes of practices were updated to allow pipelines to be laid in the ‘plastic’ mode. For example, DNV’s original code of practice for pipeline installation (1976 and 1981) recommended pipeline strain to be