On Bottom Stability Analysis and Design of Submarine Pipeline

8/12/2019 On Bottom Stability Analysis and Design of Submarine Pipeline

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ON-BOTTOM STABILITY ANALYSIS AND DESIGN

OF SUBMARINE PIPELINE

MOHD. RIDZA BIN MOHD. HANIFFAH

UNIVERSITI TEKNOLOGI MALAYSIA


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PSZ 19:16 (pind. 1/97)

UNIVERSITI TEKNOLOGI MALAYSIA

BORANG PENGESAHAN STATUS TESIS

JUDUL: ON-BOTTOM STABILITY ANALYSIS AND DESIGN OF

SUBMARINE PIPELINE

SESI PENGAJIAN : 2006/2007

Saya MOHD. RIDZA BIN MOHD. HANIFFAH(HURUF BESAR)

mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah)* ini disimpan di PerpustakaanUniversiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut :

1. Tesis adalah hakmilik Universiti Teknologi Malaysia.2. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan

pengajian sahaja.3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara

institusi pengajian tinggi.

4. **Sila tandakan ()

(Mengandungi maklumat yang berdarjah keselamatan ataukepentingan Malaysia seperti yang termaktub di dalam

AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan

oleh organisasi/badan di mana penyelidikan dijalankan )

Disahkan oleh

( TANDATANGAN PENULIS ) ( TANDATANGAN PENYELIA )

Alamat Tetap: No 66, JLN SS 19/5, PM. Dr. Nordin Yahaya

47500 Subang Jaya, Nama PenyeliaSelangor Darul Ehsan.

Tarikh : 23 April 2007 Tarikh : 23 April 2007

TIDAK TERHAD

TERHAD

SULIT

CATATAN: * Potong yang tidak berkenaan** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak

berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis iniperlu dikelaskan sebagai SULIT atau TERHAD.

Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secarapenyelidikan, atau disertai bagi pengajian secara kerja kursus atau penyelidikan, atau

Laporan Projek Sarjana Muda (PSM).


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I hereby declare that I have read this project report and in my opinion this project

report is sufficient in terms of scope and quality for the award of degree of Bachelor

of Civil Engineering.

Tandatangan : ...........

Nama Penyelia : PM. Dr. Nordin Yahaya

Tarikh : 23 April, 2007


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ON - BOTTOM STABILITY ANALYSIS AND DESIGN OF SUBMARINE

PIPELINE

MOHD. RIDZA BIN MOHD. HANIFFAH

This thesis is submitted as a partial fulfilment of the requirements for the award of

the Bachelor Degree in Civil Engineering

Faculty of Civil Engineering

Univeristi Teknologi Malaysia

APRIL 2007


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I pledge that this thesis is my original work except the quotations and summaries

that I have stated the sources clearly

Signature : ____________

Authors Name : MOHD. RIDZA BIN MOHD.HANIFFAH

Date : 23 APRIL 2007


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Dedication

To my beloved parents who gave me the endless guidance and support and to my

lecturers and friends for giving me a wonderful campus life here in UTM.

Thank You


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ACKNOWLEDGEMENTS

I would like to present my sincere appreciation to my supervisor, PM. Dr.

Nordin Yahaya for his interest, help and encouragement throughout this study. His

advices during the preparation of this project are very much appreciated. Ive gained

a lot of knowledge not only about the study, but also other things in life from him.

I want to express my gratitude to my loving family especially my parents for

their encouragement and support. Finally, but by no means least, I would like to

thank my friends who were involved, directly and indirectly in helping me

completing this research.

All the people mentioned above have made this research a successful one and

contributed to a very memorable experience for me.


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ABSTRACT

All submarine pipelines should be stable under the combined action of

hydrostatic and hydrodynamic where the environmental conditions and the

hydrodynamic forces take place. On-bottom stability analysis is performed to ensure

the stability of the pipeline when exposed to wave and current forces. The

requirement to the pipeline is that no lateral movements at all are accepted, or

alternatively that certain limited movements that do not cause interference with

adjacent objects or overstressing of the pipe are allowed. A spreadsheet is developed

for the analysis of on-bottom stability of submarine pipeline. The analysis is based

on the DNV RP E305: On-Bottom Stability of Submarine Pipeline code and

guidelines from Petronas Standard PTS 20.196 and PTS 31.40.00.10. Data from a

case study in Baram, Sarawak will be taken as the input for the spreadsheet analysis.

The stability analysis of a pipeline is obtained based on the case study. Graphs are

then obtained to study the behaviour of pipeline under different environmental and

pipeline parameters such as wave height and thickness of concrete coating. The

minimum concrete coating for this case is 75 mm. The study provides understanding

of the mechanisms that are involved in analysis and design of submarine pipeline

especially on the aspects of stability.


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ABSTRAK

Semua paip dasar laut mesti berada dalam keadaan yang stabil daripada

tindakan hidrostatik dan hidrodinamik yang disebabkan oleh pengaruh alam sekitar.

Analisis kestabilan paip dasar laut dilakukan untuk memastikan paip adalah stabil

apabila terdedah pada daya yang terhasil disebabkan pengaruh ombak dan arus air.

Bagi mencapai kestabilan paip, pergerakan mendatar tidak dibenarkan berlaku.

Sebagai alternatif, pergerakan paip yang terhad dibenarkan dimana pergerakan

tersebut tidak akan menyebabkan paip mengalami tekanan yang berlebihan. Bagi

menganalisis kestabilan paip dasar laut, satu sistem mudah dibentuk menggunakan

perisianMicrosoft Excel. Analisis tersebut dilakukan berdasarkan kod DNV RP E305

: On-Bottom Stability Design of Submarine Pipeline dan garis panduan daripada

piawai Petronas, PTS 20.196 dan PTS 31.40.00.10. Data daripada kajian kes di

Baram, Sarawak diambil sebagai input untuk analisis tersebut. Keputusan analisis

kestabilan paip dari kajian kes tersebut diperoleh. Graf-graf diperolehi untuk

mengkaji kelakuan paip dengan nilai parameter-parameter alam sekeliling dan paip

seperti ketinggian ombak dan ketebalan penebat konkrit yang berlainan . Ketebalan

penebat konkrit minimum bagi kajian kes ini adalah 75 mm. Kajian ini memberikan

pemahaman kepada elemen-elemen yang terlibat dalam analisis dan rekabentuk paip

dasar laut terutama sekali pada aspek kestabilan.


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TABLE OF CONTENTS

CHAPTER CONTENT PAGE

TITLE OF PROJECT i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xiv

LIST OF APPENDIX xvi

I INTRODUCTION 1

1.1 Introduction 1

1.2 Background of Problem 2

1.3 Objectives 3

1.4 Scope of Study 3

1.5 Significance of Study 4


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II LITERATURE REVIEW 5

2.1 Introduction 5

2.2 Composition of a Pipeline 6

2.3 Types of Pipeline 7

2.4 Loads 7

2.4.1 Functional Loads 8

2.4.2 Environmental Loads 8

2.4.3 Accidental Loads 8

2.4.4 Installation Loads 9

2.5 Construction Practices and Equipments 10

2.6 Fundamental of Pipeline Design 11

2.6.1 Pipeline Design 12

2.6.1.1 Key Design Terms 13

2.7 Analysis and Design of Submarine Pipelines 16

2.7.1 Design Conditions 16

2.7.1.1 Codes and Standards 16

2.7.1.2 Serviceability Limit State (SLS) 17

2.7.1.3 Ultimate Limit State (ULS) 18

2.7.1.4 Accidental Limit State (ALS) 18

2.7.2 Wall Thickness Determination 18

2.7.3 On-Bottom Stability 19

2.7.4 Free Spanning 20

2.7.5 Corrosion Requirement 21

2.8 On-Bottom Stability Analysis and Design 23

2.8.1 General 23

2.8.2 Analysis Method Selection 23

2.8.3 Stability Criteria 25

2.8.3.1 Minimum Pipeline Submerged

Weight 25

2.8.3.2 Pipeline Submerged Weight 26

2.8.4 Environmental Parameter 29


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2.8.5 Hydrodynamic Forces 32

2.8.5.1 Drag Loads 33

2.8.5.2 Inertia Loads 33

2.8.6 Gravity Waves (Linear Wave Theory) 34

2.8.7 Assumptions for On-Bottom Stability

Analysis 36

III METHODOLOGY 37

3.1 Introduction 37

3.2 Flow Chart of Study 38

3.3 Background of Case Study 40

3.4 Data as an Input for the Spreadsheet 41

3.5 Findings of Study 43

IV ANALYSIS AND RESULTS 44

4.1 Introduction 44

4.2 Spreadsheet Development 44

4.2.1 Minimum Pipeline Submerged Weight

Spreadsheet 45

4.2.2 Calculation of Pipeline Weight

Spreadsheet 51

4.2.3 Graphs from DNV RP E305 for the

Calculation of Calibration Factor (Fw)

Spreadsheet 54

4.3 Results for Minimum Pipeline Submerged

Weight Spreadsheet 57

4.4 Results for Calculation of Pipeline Weight

Spreadsheet 58

4.5 Parametric Analysis 60


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V DISCUSSIONS 63

5.1 Introduction 63

5.2 Discussions 63

5.2.1 The 3 Forces Involved and the Minimum

Pipeline Submerged Weight for a Cycle

Period of Time 63

5.2.2 Stability Analysis 64

5.2.3 Parametric Analysis 65

VI CONCLUSIONS 67

6.1 Conclusion 67

6.2 Recommendations 68

REFERENCES 69

BIBLIOGRAPHIES 70

APPENDIX 71

Appendix A 71


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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Weight of Pipe for Different Cases 28

2.2 Grain Size for Seabed Materials 30

3.1 Data for Minimum Pipeline Submerged Weight 42

3.2 Data for Pipeline Submerged Weight 42

4.1 Final Results for On-Bottom Stability Analysis of a

Submarine Pipeline for all 4 Cases 58


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LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Typical Cross-Section Through a Pipeline 6

2.2 Corrosion Requirement Coating for a Pipeline 22

2.3 Pipeline Cross Section 26

2.4 Determination of Significant Wave Velocity 29

2.5 Calibration Factor (Fw) 31

2.6 Rest Frame 34

3.1 Flow of Study 38

3.2 Flowchart for the Development of Spreadsheet 39

3.3 Location of Baram, Sarawak 41

4.1 Wave Profile and Environmental Data Inputs 45

4.2 Values for Horizontal and Vertical Velocities for a

Cycle Period of Time 46

4.3 Values for Horizontal Acceleration for a Cycle Period

of Time 47

4.4 Drag, Inertia and Lift Forces for a Cycle Period of Time 48

4.5 Minimum Pipeline Submerged Weight for a Cycle

Period of Time 49

4.6 Determination of Calibration Factor (Fw) and the

Outputs of First Spreadsheet 50

4.7 Pipeline Data Inputs 52

4.8 Final Outputs for the On-Bottom Stability Analysis

of a Submarine Pipeline 53

4.9 Graph from DNV RP E305 Fig. 2.1 54


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4.10 Table A1 - Grain Size for Seabed Materials from

DNV RP E305 55

4.11 Calibration Factor (Fw) from Figure 5.12 DNV RP E305 56

4.12 3 Forces involved over a Cycle Period of Time 57

4.13 Minimum Pipeline Submerged Weight for a Cycle

Period of Time 57

4.14 Pipeline Submerged Weight with Different Thickness

of Concrete Coating 59

4.15 Pipeline Specific Gravity with Different Thickness of

Concrete Coating 59

4.16 Minimum Pipeline Submerged Weight with Varying

Values of Wave Height (H) 60


Values of Mean Water Depth (d) 61


Values of Grain Size (d50) 61


Values for Diameter of Pipeline (D) 62


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LIST OF SYMBOLS

D - Hydrodynamic diameter of pipe

ID - Internal diameter of pipe

K - Keulegan-Carpenter number, K = Us Tu / D

M - Current to wave velocity ratio,M = Uc / Us

CD - Drag coefficient

CL - Lift coefficient

CM - Inertia coefficient

Dst - Steel pipe outside diameter (nominal)

FD - Drag force

FI - Inertia force

FL - Lift force

FW - Load factor

H - Significant wave height

Tn - Wave parameter, gdTn /=

T - Spectral peak period

Tu - Mean zero up-crossing periodUc - Steady current velocity at reference height zrabove seabed

Us - Significant wave velocity perpendicular to pipe ( no reduction factor

included)

Ws - Submerged pipe weight

d - Water depth

d50 - Mean grain size

g - Gravity constant

t - Wall thickness of steel


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tc - Concrete coating thickness

tcc - Corrosion coating thickness

tic - Insulation coating thickness

tmg - Marine growth thicknesstCA - Corroded material thickness

tL - Internal corrosion liner thickness

ca - Corrosion allowance usage factor

- Soil friction coefficient

c - Density of concrete coating

cc - Density of corrosion coating

i - Density of contents

mg - Density of marine growth

st - Density of steel

sw - Density of seawater

ic - Density of insulation coating

L - Density of internal corrosion liner

Wcs - Weight of carbon steel

WL - Weight of internal corrosion liner

Wcc - Weight of corrosion coating

Wic - Weight of insulation coating

Wc - Weight of concrete coating

Wmg - Weight of marine growth

Wi - Weight of contents

WCA - Weight of corroded material

W - Weight of pipe

B - Pipeline buoyancy

SG - Pipeline specific gravity

y - Negative distance between pipe and seawater level

Ur - Current velocity at bottom

L - Wave Length

Fw - Calibration Factor

zo - Roughness of seabed


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LIST OF APPENDIX

APPENDIX TITLE PAGE

A Hand Calculationfor On-Bottom Stability 71

Analysis of a Submarine Pipeline


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CHAPTER I

INTRODUCTION

1.1 Introduction

A pipeline system is defined as a pipeline section extending from an inlet point,

typically an offshore platform or an onshore compressor station, to an outlet point,

typically another offshore platform or an onshore receiver station.

The first oil-producing well at sea was drilled in 1947 (in the Mexican Gulf) , the

first pipelay barge commissioned in 1952, and the first pipeline laid on the seabed in

1954. It is estimated that close to 90 000 km of marine pipelines were installed for the

transportation of hydrocarbons during the following four decades, with approximately

5000 km being added each year [1]. The majority of the pipeline systems are located in

the heavily developed regions of the Arabian Gulf, the Gulf of Mexico and the North

Sea.

The objective of a marine pipeline is to transport a medium from one location to

another. Many different parameters such as economic, technical, environmental and etc.

determine whether or not a marine pipeline system will be installed.


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In analysis and design of marine pipelines, on-bottom stability analysis is one of

the scopes, besides determination of pipe size and wall thickness, free spanning and

corrosion requirement. On-bottom stability analysis is performed to ensure stability of

the pipeline when exposed to wave and current forces and other internal or external

loads [2].

1.2 Background Of Problem

All submerged pipelines, i.e. offshore pipelines and sections of onshore pipelines

in swamps, floodable areas, high water table areas, river crossings, etc., should be stable

under the combined action of hydrostatic and hydrodynamic where the environmental

conditions and the hydrodynamic forces takes place [3]. The hydrodynamic forces on the

pipeline and on the seabed are functions of the wave and current climate. It is important

to correctly predict the forces imposed on a pipeline since they have a direct bearing on

the safety and economy of the project. If the pipeline does not have enough submerged

weight to resist the hydrodynamic forces, the pipeline will be unstable, moving up and

down (due to lifting force) and back and forth (due to drag and inertia force). The

excessive pipe movement and oscillatory motions may cause high stress and fatigue

damage to the pipe [2]. Special considerations should be taken to pipelines installed in

liquefied seabeds which will have sinking depth and weak soils, where differential

settlements may lead to pipeline loss of integrity. In the design phase, the possibility of

local scouring under submarine pipelines must be taken into consideration. If local scour

occurs under the submarine pipelines, the pipelines may either vibrate due to the

hydrodynamic forces or induce additional static or dynamic loads due to self-burial.

Moreover, they might be destroyed partially or fully and thus be unable to perform their

functions. Pipeline stresses due to loss of support will occur by depressions or scour

below submarine pipelines


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1.3 Objectives

The objectives of this study are:

i. To look into the available procedure of pipeline analysis and designii. To identify the mechanisms and parameters involved in on-bottom stability

of pipeline

iii. To develop a spreadsheet on on-bottom stability in pipeline designiv. To obtain the stability analysis of a pipeline based on a case study

1.4 Scope of Study

The scope of this study is the design, analysis including the typical construction

of submarine pipelines . Generally, this study includes the history of the pipeline, the

composition and the types of load involved in analysis of a submarine pipeline.

However, focus will be on on-bottom stability of submarine pipeline based on code

DNV RP E305: On-Bottom Stability of Submarine Pipeline. Data from a case study

in Baram, Sarawak will be taken as the input for the spreadsheet analysis of stability .


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1.5 Significance of Study

This study provides understanding of the mechanisms that is involved in analysis

and design of marine pipeline especially on the stability. The identified mechanism will

be supported by findings from case study of on-bottom stability of a pipeline. Hence, the

safety of marine pipeline design is ensured and this will build up the confidence of

public on the development of offshore structures.


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CHAPTER II

LITERATURE REVIEW

2.1 Introduction

A network of sophisticated pipeline systems transports oils, natural gas, and

petroleum products from producing fields around the world to consumers in every

nation. This network gathers oil and gas from hundreds of thousands of individual wells,

including those in some of the worlds most remote and hostile area. Pipelines bring oil

and gas produced from offshore wells to shore through water with certain depth. This

vast gathering and distribution system comprises hundreds of thousands of miles of

pipeline varying in size between 2 in. to 60 in. in diameter. Oil and gas pipeline systems

are remarkable for their efficiency and low transportation cost rather than using a barge,

rail and trucks. The link between pipeline size and economy is apparent as well as the

relationship between size and capacity. A 36-in. diameter line can carry up to 17 times

more than a 12-in. diameter pipeline, but construction and operating costs do not

increase at nearly the same ratio [1].


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2.2 Composition of a Pipeline

A typical cross-section through a large diameter pipeline is shown in Figure 2.1.

The pipe itself is most frequently manufactured of steel. Often not other materials are

used, e.g. reinforced plastic. Steel pipelines without protection would be susceptible to

corrosion in contact water. Therefore, the outer surface is provided with protective

coating. Efficiency of the coating is increased by electrochemical measures such as

cathodic protection [4]. Need for corrosion protection of inner surfaces depends on the

chemical characteristics of the transported fluid. An internal coating may be applied to

reduce wall roughness.

The external corrosion coating is protected by reinforced concrete cover which

also provides additional weight required for in-place stability (resistance against

flotation of a buried pipeline and resistance to unacceptable lateral motion of pipelines

on the seabed.

Concrete Cover

Pipe

Fluid

Figure 2.1 : Typical cross-section through a pipeline


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2.3 Types of Pipeline

Most oil and gas pipeline fall into one of three groups which are gathering, trunk

/ transmission or distribution. Small diameter pipelines within an oil or gas field, called

flowlines are usually owned by the producer. They connect individual oil or gas wells to

central treating, storage or processing facilities within the field. Another gathering

system made up of larger diameter pipelines, normally owned by a pipeline company

rather than the oil or gas producer, connects these field facilities to the large-diameter,

long distance trunk or transmission line.

Crude trunk lines move oil from producing areas to refineries for processing. Gas

transmission lines carry natural gas from producing area to city utility companies and

other customers.

2.4 Loads

Loads on a submarine pipeline can be divided into the following categories [5],

i. Functional loadsii. Environmental loads

iii. Accidental loadsiv. Installation loads


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2.4.1 Functional Loads

Functional loads are defined as actions that result from the operation of the

pipeline. The most significant force is caused by the pressure difference between

operation pressure inside and pressure outside. The pressure difference induces

tangential stress in the steel pipe.

2.4.2 Environmental Loads

Environmental loads are defined as actions resulting from the interaction of the

pipeline with its environment. In addition to gravity forces (self-weight, buoyancy and

hydrostatic pressure), environmental loads are primarily generated by wave and current

action. Other loads that can be characterised as environmental are soil pressure and other

natural actions, including the temperature of the surroundings.

2.4.3 Accidental Loads

Accidental loads are defined as loads which have a low probability of

occurrence. For submarine pipelines, such loads may be grouped into the following:


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i. Natural hazards such as earthquakes and mudslides.ii. Third party hazards such as dropped objects (near platforms), fishing activities

(trawling), shipping (anchoring, sinking) and military activities (firing).

2.4.4 Installation Loads

Installation of marine pipelines is to great extent weather dependant, and part of

the installation engineering is the determination of the acceptable limits (wind speed,

wave height, current) for the installation to take place. Apart from the pipeline self

weight and the normal environmental loads, specific actions during installation will

mostly be imposed static and dynamic force (from laybarge stingers, tie-in tools,

trenching equipment, etc.). The actions are ,

i. Installation of pipe strings (laying, reeling, towing, pulling)ii. Tie-in

iii. Trenching and backfillingiv. Hydrostatic testing

An exception is hydrostatic testing, where the test pressure is normally prescribed

by regulations, typically corresponding to 15% above the design pressure, although

substantially different values may be specified [2].


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2.5 Construction Practices and Equipments

Pipeline construction methods differ depending on the geographical area, the

terrain, the environment, the type of pipeline and the restrictions and standards imposed

by governments and regulatory agencies.

Construction costs also vary according to location, line size, environmental

conditions, equipment required and the construction schedule. Pipeline construction

projects have these features [1]:

i. Comprehensive environmental impact studies are required in many countriesbefore construction permits can be issued. Construction plans must provide for

the protection of scenery, wildlife and historic assets.

ii. Most oil and gas pipelines are constructed by welding short lengths, or joints, ofpipe together. There are a few exceptions to the use of welded connections, but

these are in short lines within a producing field or in similar application.

iii. Extensive testing of welders and the welds they produce is an important part ofthe construction of all long-distance petroleum pipelines.

iv. Most pipelines are buried below the sea bed for protection. There are cases inwhich large segments of a major pipeline are not buried, the most notable

example is the trans-Alaska crude pipeline where above ground sections were

installed.


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v. All pipelines are tested for leaks following construction before the line is put inservice. The hydrostatic testing is the most common techniques, filling the line

with water and subjecting it to a pressure greater than the design operating

pressure.

vi. The construction of all pipelines follows this general sequence, design and routeselection, obtaining right of ways, installation, tie-in to origin and destination

facilities and pumping or compressor testing and testing.

Submarine pipelines are built by welding individual pipes into a continuous line.

Quality of all welded joints are thoroughly by X-ray methods, sometimes also by

ultrasonic. Several construction methods can be used for submarine pipelines

construction, including the conventional lay barge method, the reel barge method, the

vertical lay method and the tow method. All require large sophisticated marine vessels.

2.6 Fundamentals of Pipeline Design

The amount of fluid that must flow through the pipeline is one of the first items

of information required for design. But a characteristic of many proposed pipeline

projects is that future capacity requirements are difficult to forecast. Determining the

capacity requirements for a pipeline gathering system to gather crude from producing

fields can be difficult. When oil and gas is discovered in an area, several years may pass

before the field is fully developed and maximum required capacity is known [4].

Additional capacity will be needed as more wells are put on stream, but the pipeline is

needed early in the fields life to transport production from the first wells.


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2.6.1 Pipeline Design

The most appropriate approach to pipeline design depends on the system, the

designer, the number of fixed variables, the availability of pipe and equipment and the

cost. Both installation (cost) and operating/maintenance expense must be considered in

choosing the optimum design. Often a design having a lower installation cost than

another alternative will be more expensive to operate. When compared based on

economic indicators over the life of the system, the design with the lowest installation

cost may not be the best solution.

One of the most design criteria which is the volume of oil or gas to be

transported is sometimes the most difficult to determine. There is often some uncertainty

in volume estimates, and making the best projection of volumes to be handled

throughout the life of the pipeline is the key to a profitable project. With projected

volumes and the origin and the destination of the pipeline known, pipeline design

typically follow these general steps [1]:

i. A required delivery pressure is determined at the pipelines destination. Thispressure may be set by the customers facilities.

ii. Pressure losses due to friction and the pressure required to overcome changes inelevation are added to the delivery pressure to determine the inlet pressure. In

single-phase flow, the pressure drop in the line must be overcome by pumps or

compressors is essentially the friction loss plus the pressure exerted by a liquid or

gas column whose height equals the difference in elevation between the ends of

the line.


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iii. With the line size and operating pressure determined, the pumping orcompression horsepower needed to deliver the desired volume of fluid at the

specified delivery pressure can be accurately calculated. If more than one pump

or compressor station is required, its location and size is set by calculating

pressure loss along the line and determining how much pump or compressor

horsepower is needed to maintain operating pressure.

iv. In most cases, it is necessary to perform economic calculations to compare thedesign with other combinations of line size, operating pressure and horsepower

in order to choose the best system.

This outline represents the basic steps involved in a preliminary design of a

single pipeline with no branch connections, no alternative routes and no significant

changes during its life. Few pipeline systems are that simple. Most have several branch

lines feeding into a main line that consists of more than one pipe size, beginning with a

smaller pipe at the inlet end and requiring larger pipe as flows from the branches feed in.

2.6.1.1Key Design Terms

It is important that term fluid includes both liquids and gases. Most of the

following fluid properties and other variables are considered in designing liquids or

natural gas pipeline.

i. Pipe diameter. The larger the inside diameter of the pipeline, the more fluid canbe moved through it, assuming other variables are fixed.


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ii. Pipe length. The greater the length of a segment of pipeline, the greater the totalpressure drop. Pressure drop can be the same per unit of length for a given size

and type of pipe, but total pressure drop increases with length.

iii. Specific gravity and density. The density of a liquid or gas is its weight per unitvolume. The specific gravity of a liquid is the density of the liquid divided by the

density of water and the specific gravity of a gas is its density divided by the

density of air. The specific gravity of water and air is therefore is 1.

iv. Compressibility. Because most liquids are only slightly compressible, this term isusually not significant in calculating liquids pipeline capacity at normal

operating conditions. In gas pipeline design, it is necessary to include a term in

many design calculations for the fact that gases deviate from laws describing

ideal gas behavior when under conditions other than standard or base

conditions. This term supercompressibility factor is more significant at high

pressures and temperatures. Near standard conditions of temperature and

pressure (60F and 1 atm), for example, the deviation ideal gas law is small and

the effect of the supercompressibilty factor on design calculations is not

significant.

v. Temperature. Temperature affects pipeline capacity both directly and indirectly.In natural gas pipeline, the lower the operating temperature, the greater the

capacity, assuming all other variables are fixed. Operating pressure also can

affect other terms in equations used to calculate the capacity of both natural gas

and liquid pipelines. For example, viscosity varies with temperature.

vi. Viscosity. The property of a fluid that resists flow, or relative motion betweenadjacent parts of the fluid. It is an important term in calculating line size and

pump horsepower requirements when designing liquid pipelines.


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vii. Pour point. The lowest temperature at which an oil will pour, or flow whencooled under specified test conditions. Oils can be pumped below the pour point,

but the design and operation under these conditions present special problems.

viii. Vapor pressure. The pressure that holds a volatile liquid in equilibrium with itsvapor at a given temperature. Vapor pressure is an especially important design

criterion when handling volatile petroleum products. The minimum pressure in

the pipeline must be high enough to maintain these fluids in a liquid state.

ix. Reynolds number. This dimensionless number is used to describe the type offlow exhibited by a flowing fluid. In streamlined or laminar flow, the molecules

move parallel to the axis of flow where else in turbulent flow, the molecules

move forth and back across the flow axis.

x. Friction factor. A variety of friction factors are used in pipeline design equations.They are determined empirically and are related to the roughness of the inside

pipe wall.

Other properties of the fluid and pipe may be used in specific calculations, but

these are the basic terms used to determine pressure drop and flow capacity. Many

system variables are interdependent. For example, operating pressure depends on

pressure drop in the line. Pressure drop, in turn, depends on flow rate and maximum

flow rate is dictated by allowable pressure drop.


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2.7 Analysis and Design of Submarine Pipelines

In order to analysis and design a submarine pipeline, the design conditions, wall

thickness determination, on-bottom stability, free spanning and corrosion requirement

are the aspects to be looked into.

2.7.1 Design Conditions

There are a few number of codes and standards that can be used to analyse and

design a submarine pipeline.

2.7.1.1 Codes and Standards

Pipeline design codes that are widely recognised include:

i. ASME B31.8-1999 Chapter VIIIii. BS 8010 Part 3

iii. ISO 13623iv. DNV OS-F101

A large number of pipelines have been successfully designed to the above codes.

In this research, DNV code is used. This is because the code has had international

approbation [2]. The DNV code is therefore considered the most appropriate standard

for future design. DNV code adopts the Load and Resistance Factor Design (LRFS)

format as a basis for the given structural limitations.


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The use of limit states in the LRFS format is detailed in the subsections below.

Traditionally the following different limit states are considered:

i. Serviceability Limit State (SLS)ii. Ultimate Limit State (ULS)

iii. Accidental Limit States (ALS)

2.7.1.2 Serviceability Limit State (SLS)

The SLS refers to a given load condition that, if exceeded can cause pipeline to

be unsuitable for continued operation. The following situations are normally considered:

i. Deformation and movement due to waves and currents (hydrodynamic stability)ii. Longitudinal deformations due to restrained temperature and pressure variations

(pipeline expansion)

iii. Lateral deformations due restrained temperature and pressure expansioniv. Blockage of the pipeline, due to hydrate formation or wax deposition

In the case of permanent local damage or permanent unacceptable deformation

the SLS is not the appropriate formulation and the ULS design factors shall be

introduced.


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2.7.1.3 Ultimate Limit State (ULS)

It shall be ensured that the pipeline has the required safety against failure in the

ULS, defined in terms of:

i. Plastic deformations (yielding)ii. Local instability (buckling)

iii. Crack instability (bursting)iv. Repeated loading (fatigue)

2.7.1.4Accidental Limit State (ALS)

The design of the pipeline is closely related to the risk analysis, in the sense that

scenarios entail a risk that is unacceptable, typically due to their high frequency of

occurrence, shall be considered in the ALS design.

2.7.2 Wall Thickness Determination

The primary objective of the pipeline design is to determine the optimal wall

thickness and steel grade of the pipeline. For the vast majority of existing pipelines the

wall thickness will have been selected following a simple hoop stress calculation.


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A usage factor applied to the Specific Minimum Yield Stress (SMYS) defines

the allowable stress which, when inserted into the hoop stress formula, determines the

minimum required thickness of the pipe. Then, select the nearest standard American

Petroleum Institute (API) wall thickness above the required minimum. For liquid or two-

phase pipelines a corrosion allowance may have been added. Calculations that aim at a

specified design life are often backed up by extensive testing, but the corrosion

allowance may also simply be based on experience with existing lines or on owner

preferences.

2.7.3 On-Bottom Stability

On bottom stability analysis is performed to ensure the stability of the pipeline

when exposed to wave and current forces and other internal or external loads (e.g.

buckling loads in curved pipe sections). The requirement to the pipeline is that no lateral

movements at all are accepted, or alternatively that certain limited movements that do

not cause interference with adjacent objects or overstressing of the pipe are allowed.

Hydrodynamic stability is generally obtained by increasing the submerged

weight of the pipe by concrete coating. There are other ways such as increasing the steel

wall thickness, placing concrete blankets or bitumen mattresses across the pipeline,

anchoring or covering it with gravel or rock. Alternatively, the hydrodynamic forces

may be reduced by placing the pipeline in a trench on the seabed, prior or subsequent to

installation. The natural backfilling of a pipeline depends on the environmental

conditions and the seabed sediment at the location.


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A pipeline on the seabed forms a structural unit where displacement in one area

is resisted by bending and tensile stresses. The real situation most probably involves a

great variety of pipeline-seabed interface conditions. Pipeline self lowering may result in

some sections of a pipeline being embedded to a larger degree than determined by

touchdown forces, and parts may even be fully buried. The embedment is influenced by

soil characteristics and phenomena such as scour, sediment transport and other seabed

instabilities. In other sections the pipe may be slightly elevated above the seabed due to

seabed undulations or scour processes. For both conditions, the hydrodynamic forces are

reduced relative to the idealized on bottom condition.

2.7.4 Free Spanning

The free span shall have adequate safety against failure modes and deformation

such as excessive yielding, fatigue, buckling and ovalisation. Free span analysis should

be based on generally accepted static and dynamic calculation methods, including non-

linear structure structural modeling, soil reaction description and deflection induced

axial forces.

The following pipeline conditions are considered:

i. Empty pipelineii. Water filled pipeline

iii. Pipeline during hydrotestingiv. Operating pipeline


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The analysis of free spans normally requires:

i. Static analysis for determining pipeline configuration, sectional forces andstresses under functional loads.

ii. Eigen value analysis for determining natural frequencies and modal shapesiii. Dynamic analysis for determining pipeline deflection, sectional forces and

stresses under combined functional and environmental loads or accidental

loading

iv. Fatigue analysis for determining accumulated fatigue damage due to cyclic loadsfrom wave action and vortex shedding

2.7.5 Corrosion Requirement

Corrosion is defined as a destructive attack on metal by a chemical or

electrochemical reaction with its environment. The driving force is the tendency for the

refined metal to return to a natural state characterized by a lower level of internal

energy. In the case of steel pipeline, the iron will tend to revert to its natural state as

ferrous oxide (iron ore).

Internal corrosion of pipelines depends upon the aggressiveness of the

transported medium and may be prevented by inhibitor injection, internal coating or use

of corrosion resistant alloys. The lifetime of the pipeline can also be extended by

introducing a corrosion allowance, i.e. an additional wall thickness over and above that

needed for pressure containment.


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External corrosion of a pipeline in seawater is an electrochemical process. A

galvanic element is created where an electric current flows between an anodic area and a

cathodic area, with the seawater acting as an electrolyte. Coating the steel surface

protects against corrosion by creating a physical barrier between the pipe and the

electrolyte, preventing oxygen from reaching the steel. Cathodic protection renders the

steel immune to corrosion by lowering the electrical potential.

A barrier coating is seen as the primary defense against corrosion with cathodic

protection being a back up measure against coating damage or breakdown. However,

cathodic protection might also be considered the principal corrosion prevention method

with the coating being introduced to reduce the necessary current consumption.

Figure 2.2 : Corrosion requirement coating for a pipeline


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2.8 On-Bottom Stability Analysis and Design

2.8.1

General

Subsea pipelines resting on the bed or placed in the trench are subjected to

lateral instability due to environmental loads comprising of wave and current forces. The

lateral instability is countered by lateral soil frictional resistance due to submerged

weight of pipeline. If the submerged weight is inadequate, the increase in submerged

weight is normally achieved by increasing weight of the pipeline or else reducing the

environmental loads by trenching or burial. In present analysis the stability is presumed

to be achievable by adding sufficient weight in form of concrete coating.

The purpose of design for on bottom stability is determination of wall thickness

of steel pipe and the pipeline submerged weight required to withstand action of

functional loads combined with environmental loads. While functional loads are

important only for steel pipe thickness, environmental loads are most decisive for the

pipeline submerged weight and less important for steel pipe thickness.

2.8.2 Analysis Method Selection

Pipeline stability analysis shall be carried-out in accordance with DNV RP E305.

Three methods are provided for the stability check [6]:


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i. Dynamic Analysis Method.ii. Generalized method and

iii. Simplified Method,

Dynamic analysis involves dynamic simulation of a section of pipeline under the

action of waves and current. The dynamic analysis is to be used in specialized

circumstances. Generalized pipeline stability analysis is based on generalization of the

results from Dynamic Analysis, through the use of a set of non-dimensional parameters

and for particular end conditions.

The simplified method is suitable for most of the design cases [4]. The DNV RP

E305 Simplified Static Stability method is based on a quasi-static equilibrium approach.

The calibration factor, Fw, ties the classical static design approach to the generalized

stability method. A safety factor of 1.1 is inherent in the calibration factorFw.

The equilibrium condition in vertical direction is not always studied. The

equilibrium condition is of interest for finding the expected penetration of a pipeline

only in the case of a very soft seabed. Thus, it is restricted to examination of equilibrium

in the horizontal direction.


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2.8.3 Stability Criteria

2.8.3.1 Minimum Pipeline Submerged Weight

The minimum submerged weight required to prevent any horizontal movement

of the pipeline under the extreme environmental loading, is calculated by a simple static

force balance of the horizontal hydrodynamic and soil frictional forces. The stability

criteria may be expressed as based on DNV RP E305.

WLID

sub FFFF

W

++=

)( (2.1)

where:

Wsub = submerged weight of pipeline; (N/m)

FL = hydrodynamic lift force per unit length (N/m)

FD = hydrodynamic drag force per unit length (N/m)

FI = hydrodynamic inertia force per unit length (N/m)

= Coefficient of friction between pipe and soil from

Cl 5.3.3 DNV RP E305.

FW = Calibration factor from Cl 5.3.7 DNV RP E305.

The static stability design is based on the following main assumptions:

i. Pipe movements are not allowed, requiring equilibrium between loads(hydrodynamic forces) and reactions (soil resistance forces)

ii. Near bed wave flow is time varying and only the component perpendicular to thepipe axis is considered

iii. Soil resistance is calculated based on two-dimensional assumptions, and mayinclude simple friction as well as passive soil resistance


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2.8.3.2 Pipeline Submerged Weight

Submerged weight shall consider the weight of the following components [7] :

i. Steelii. Internal corrosion liner (if any)

iii. Corrosion coating (if any)iv. Insulation coating (if any)v. Concrete coating (if any)

vi. Marine growth (if any)vii. Internal contents

viii. Metal loss through internal/external corrosion

Internal Contents

Internal Corrosion

Liner

Steel

External Corrosion

CoatingInsulation Coating

Concrete Weight Coating

Marine Growth

Figure 2.3 : Pipeline cross section


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The hydrodynamic diameter of the pipe is given by,

D = Dst+ 2(tcc+ tic+ tc+tmg) (2.2)

The weight of the components (in air) are calculated as follows:

i. Carbon Steel Weight Wcs= (Dstt)t st (2.3)ii. Internal Corrosion Liner Weight WL = (Dst 2t - tL)tLL (2.4)

iii. Corrosion Coating Weight Wcc = (Dst+ tcc)tcccc (2.5)iv. Insulation Coating Weight Wic= (Dst+ 2tcc+ tic)ticic (2.6)v. Concrete Coating Weight Wc= (Dst+ 2tcc+ 2tic+ tc)

tcc (2.7)

vi. Marine Growth Weight Wmg= (Dst+ 2tcc+ 2tic+ 2tc+ tmg) tmgmg (2.8)

vii. Internal Diameter of Pipe ID = Dst 2t 2tL (2.9)viii. Weight of Contents Wi= /4(ID)2i (2.10)

ix. Weight of Corroded Material Wcorr= ((D 2t + tCA)tCA)stCA (2.11)

where, CA= corrosion allowance usage factor


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Four cases are of interest [8]:

i. Operational Pristine no marine growth or metal loss to corrosionincluded.

ii. Operational End of Life marine growth included and CAof corrosionallowance (as an annular area) has been lost to corrosion.

iii. Installation pipeline empty, no marine growth and no loss of corrodedmaterial.

iv. Hydrotest as for installation but pipe full of hydrotest water.

Table 2.1 : Weight of pipe for different cases

Case Outer Diameter, OD Weight of Pipe, W

Operational

Pristine

D+ 2tcc+ 2tic+ 2tc Wcs + WL+ Wcc + Wic + Wc + Wi

Operational End of

Life

D+ 2tcc+ 2tic+ 2tc+

2tmg

Wcs + WL+ Wcc + Wic + Wc + Wmg +Wi Wcorr

Installation / Hydrotest D+ 2tcc+ 2tic+ 2tc Wcs + WL+ Wcc + Wic + Wc + Wi

Pipelines buoyancy, submerged weight and specific gravity are calculated as follow [7],

Pipeline buoyancy, B= /4 OD2

sw (2.12)

Pipeline submerged weight Ws= WB (2.13)

Pipeline specific gravity SG = W/B = Ws/B + 1 (2.14)


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2.8.4 Environmental Parameter

The defining sea-state parameters areHsand Tp, which are used to calculate the

significant wave velocity perpendicular to the pipe (Us).

Figure 2.4: Determination of Significant Wave Velocity (DNV RP E305)


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The Tnand Tpare determined using following relationship.

g

dTn = and gHT sp 250= (2.15)

From Figure 2.4, (Us*Tn)/Hsis determined for the given Tn/Tp. Then, significant

wave velocity perpendicular to the pipe (Us) is determined.

Table 2.2 :Grain size for seabed materials (DNV RP E305)

Seabed

Grain

Size Roughness

d50

(mm) zo (m)

Silt 0.0625 5.21E-06

Very Fine Sand 0.125 1.04E-05

Fine Sand 0.25 2.08E-05Medium Sand 0.5 4.17E-05Coarse Sand 1 8.33E-05

Very Coarse Sand 2 1.67E-04

Gravel 4 3.33E-04

Pebble 10 8.33E-0425 2.08E-03

50 4.17E-03

Cobble 100 8.33E-03250 2.08E-02

Boulder 500 4.17E-02


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Grain size (d50) and roughness (z0) of the seabed is determined based Table 2.2.

+

+

+

= 11ln1

1ln

1

o

o

o

r

avg

z

D

D

z

zz

U (2.16)

= UavgU D/Ur

Ur = Current velocity at reference height

Zr, = reference height for the current velocity, assumed 3 m

z0 = bottom roughness parameter

d50 = mean grain size

Figure 2.5 :Calibration Factor,Fw (DNV RP E305)


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Current to wave velocity ratio, M= UD/ Us

Keulegan Carpenter number, K = (UsTp) /D

From Figure 2.5, the calibration factor can be obtained after determining the

value of M and K. Calibration factor is used in the calculation of pipeline submerged

weight.

2.8.5 Hydrodynamic Forces

When using the calibration factorFwto calculate Wsubthe hydrodynamic loading

on the pipe is determined using the following relationship.

Drag Force, FD= CDUnUnD (2.17)

Inertia Force, FI= CMD2n/ 4 (2.18)

Lift Force, FL=CLUn2

D (2.19)

Un= (Un2+ Vn2) (2.20)

where, D = the total outside diameter

= density of seawater

CL = 0.9, is the lift force coefficient

CD = 0.7, is the drag force coefficient

CM = 3.29, is the inertia force coefficient

Un = water particle horizontal velocity

Vn = water particle vertical velocity

n = water particle horizontal acceleration


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2.8.5.1 Drag loads

As fluid passed over a body a shear layer develops in the fluids flow. The body

experiences a force caused by skin-friction due to the tangential viscom shear layer

between body and flow. In addition, it experiences a pressure or form drag from the

pressure on the body.

Thus,

( ) PFD FFDkKcF +/,Re, (2.21)

Total drag Friction Pressure

drag

2.8.5.2 Inertia Loads

A body immersed in fluid is generally associated with an entrained mass of watt

called the added mass. In some cases the added-mass is directly proportional to the

immersed of the body. This is often assumed.


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2.8.6 Gravity Waves (Linear Wave Theory)

The simplest mathematical description of a gravity ( ocean ) wave is given by

linear wave theory [10].

Important parameters in linear wave theory,

i. wave height,H all other value may be calculated usingii. mean water depth, d these three

iii. wave period, T

H

d

Figure 2.6 :Rest frame ( waves moving to left to right )

A few assumptions have to be made which are,

i. Ignore surface tension and viscosityii. Two-dimensional small amplitude waves permanent formiii. Wave propagate through initially still wattiv. Irrotational and incompressible


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The wavelength,Lis calculated based on the formula below [7],

=

L

dgTL

2tanh

2

2

(2.22)

In order to calculate the force on a structure immersed in moving fluid we need

to determine the particle kinematics. ( i.e. Velocity and accelerations ).

i) Particle velocities

Horizontal Velocity, U = H cosh[ 2 (y + d ) /L ] . cos( 2 (x t ) ) (2.23)

T sinh (2 d /L ) L T

Vertical Velocity, V = H sinh [ 2 (y + d ) /L ] . sin ( 2 (x t ) ) (2.24)T sinh (2 d/L ) L T

ii) Particle Accelerations

Horizontal Acceleration, = 2H 2cosh [ 2 (y + d ) /L ] . sin ( 2 (x t ) ) (2.25)T2 sinh (2 d / L ) L T

Therefore linear wave theory is most suitable for dealing with deep water wave

of small amplitude.


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2.8.7 Assumptions for On-Bottom Stability Analysis

The following assumptions have been made in the pipeline on-bottom stability

analysis:

i. No pipe burial has been consideredii. No water absorption on the concrete is considered

iii. No marine growth on the pipeline is taken into considerationiv. Current and wave acting perpendicular to the pipelinev. No pipe burial has been considered

vi. No water absorption on the concrete is consideredvii. No marine growth on the pipeline is taken into consideration

viii. Current and wave acting perpendicular to the pipelineix. The soil friction for clay is calculated based on fig 5.11 in DNV RP E305x. The 1 year significant wave height and peak period plus 1 year current

are considered for the installation condition. Pipeline is assumed to be

empty during this condition

xi. The 100 year significant wave height and peak period plus 100 yearcurrent are considered for the operating conditions. Minimum internal

product density of 733 kg/m3has been used

xii. For the immediate and long term vertical settlement calculation, the mostconservative case of hydrotest and operating (max product density of

1025 kg/m3) conditions respectively are considered


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CHAPTER III

METHODOLOGY

3.1 Introduction

This chapter is about the methods to achieve the objectives of the study such as

acquiring the data, determination of code to be used and developing a spreadsheet. Data

and information is required through books, internets and journals on pipeline and

submarine pipelines. Studying and understanding about the study itself is a part of this

study.

A part of that, a spreadsheet is developed based on the code DNV RP E305 for

analysis of on bottom stability of a submarine pipeline. A case study from Baram,

Sarawak will be used with the data acts as the input to the spreadsheet analysis of

stability developed.


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3.2 Flow Chart

Collecting and reviewing of

information and data

Study and understanding of

information and data gathered

Determining the code that will be

used as a guide in stability analysis

Developing a spreadsheed for on

bottom stability analysisof a

pipeline

Requiring the relevant datas as an

input for the spreadsheet

Analysing and collection of results

Interpretation of results achieved

and conclusion as a whole for the

study

Figure 3.1 :Flow of study


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Figure 3.2 : Flowchart for the development of spreadsheet.

After the determination of DNV RP E305: On-Bottom Stability Design of

Submarine Pipeline as the code to be referred to, the development of the Microsoft Excel

Spreadsheet starts with the determination of pipeline and wave conditions.


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Pipeline conditions consist of the thickness and density of the steel and other

coatings used for the submarine pipeline. From here, the pipeline weight can be

calculated and the pipeline submerged weight can be determined as well as the pipeline

specific gravity.

Wave conditions consist of the wave height, wave period, the mean depth of

seawater and etc. In order to calculate the forces acting on the pipeline, the wave

kinematics have to be determined based on the wave conditions. Then, the minimum

pipeline submerged weight can be determined and compared for on-bottom stability.

3.3 Background of Case Study

The Baram Field is located approximately 25 km offshore Lutong, Sarawak with

the water depths ranges up to 61 metres. Some of Baram facilities and pipelines are

aging and susceptible to corrosion due to inadequacy of corrosion management

programme. As a mitigation measure in addressing the risk, some of the aging pipelines

have to be replaced. On-bottom stability of the BARAM Pipeline Replacement Project

pipelines has to be checked.


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Figure 3.3 : Location of Baram, Sarawak

3.4 Data as an Input for the Spreadsheet

Data are taken from Baram Pipeline Replacement Project in Baram, Sarawak.

The 100 year significant wave height and peak period plus 100 year current are

considered for the operating conditions. Minimum internal product density of 733 kg/m3

has been used.

The 1-year significant wave height and peak period plus 1-year current are

considered for the installation and hydrotest conditions. Pipeline is assumed to be empty

during installation and filled with hydrotest water (assumed seawater) during hydrotest.


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Table 3.1 : Data for minimum pipeline submerged weight

Parameters Symbol(Unit) Case 1 & 2 Case 3 Case 4

Gravity g(ms-2) 9.81 9.81 9.81Wave Height H(m) 2.7 2.2 2.2

Spectral PeakPeriod T

(s) 11.6 9.9 9.9

Mean Water

Depth

d (m) 4.4 4.1 4.1

Distance between

pipe and SWL

y (m) -4.19 -3.89 -3.89

Diameter of Pipe OD (m) 0.6174 0.6174 0.6174Density ofSeawater

P (kgm-3) 1025 1025 1025

Zero Up Crossing

Period

Tu(s) 9.16 7.91 7.91

Current Velocityat Bottom

Ur (ms-1

) 0.5 0.4 0.4

FrictionCalibration Factor

u 1.3 0.9 1.3

Grain Size d50(mm) 0.5 0.5 0.5

Table 3.2 : Data for pipeline submerged weight

Diamater of Steel,Dst(mm) 406.4

Thickness of , (mm)1) Steel, t 14.32) Internal Corrosion Liner, tL 03) Corrosion Coating, tcc 5.54) Insulation Coating, tic 05) Concrete Coating, tc 856) Marine Growth, tmg 07) Corroded Material, tCA 0

Density of , (kg/m3)

1) Steel,pst 78502) Internal Corrosion Liner,pL 03) Corrosion Coating,pcc 12804) Insulation Coating,p ic 05) Concrete Coating,pc 30446) Marine Growth,pmg 10257) Content,pi 7508) Seawater,psw 1025

Corrosian Allowance Usage Factor, uCA 3


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3.5 FINDINGS OF STUDY

This study will provide an overview of the analysis in design of submarine

pipeline as it is a fairly new field. Besides, a further understanding will be developed on

the procedure of pipeline design. Through the spreadsheet analysis of on bottom

stability, the results that can be expected are minimum concrete coating thickness,

calculated submerged weight of the pipeline which should be bigger than the minimum

submerged weight of the pipeline.


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CHAPTER IV

ANALYSIS AND RESULTS

4.1 Introduction

This study focuses on the on-bottom stability of a submarine pipeline which sits

on an even seabed without any trenching and burial. The stability is calculated based on

the spreadsheet developed using Microsoft Excel Spreadsheet. The analysis is done

based on data from the project in Baram, Sarawak. Four cases are being analyzed which

are operational pristine, operational end of life, installation and hydrotest.

4.2 Spreadsheet Development

The spreadsheet is developed using formulas from DNV RP E305 - On-Bottom

Stability Design of Submarine Pipeline and Linear Wave Theory.


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Literature review has to be made to understand the nature and symbols used in

the formulas. Below are the results of the spreadsheet developed.

4.2.1 Minimum Pipeline Submerged Weight Spreadsheet

Figure 4.1 shows the 2-Dimensional Wave Profile and its parameters and also

the inputs to be entered in the spreadsheet. The inputs consists of wave height (H),

spectral peak period (T), mean water depth (d), distance between pipeline and seawater

level (y), outside diameter of pipe (OD), zero up crossing period (Tu), and current

velocity at bottom (Ur). The wave length (L) requires try and error method to determine

its value and is calculated using Equation 2.22 in Chapter II.

Figure 4.1 :Wave profile and Environmental Data Inputs


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Figure 4.1 also describes the use of colour fonts. Blue fonts means that a value is

to be inserted in the cell. Green fonts are for excel calculated values, black fonts are for

fixed values and red fonts are for important excel calculated values.

Figure 4.2 and Figure 4.3 shows the calculated values of wave water particle

kinematics, which are horizontal and vertical velocities as well as horizontal

accelerations based on the values of the inputs and are calculated using Equations 2.23 -

2.25 . The values are plotted on graphs as shown in the same figure.

Figure 4.2 :Values for Horizontal and Vertical Velocities for a Cycle Period of Time


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Figure 4.3 :Values for Horizontal Acceleration for a Cycle Period of Time

The drag, inertia and lift forces values for a cycle period of time are as shown in

Figure 4.4. The values are calculated using Equations 2.17 - 2.19 based on the input

values and the wave water particle kinematics determined earlier. The three forces are

then plotted in a single graph as shown in the figure. The fixed values shown in the

figure are drag force, inertia force and lift force coefficients with the value of 0.7, 3.29

and 0.9 each respectively.


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Figure 4.4 :Drag, Inertia, and Lift Forces for a Cycle Period of Time

The minimum pipeline submerged weight values over a cycle period of time are

calculated using Equation 2.1 and are shown in Figure 4.5. These are important values as

it is the output needed for this particular spreadsheet. The values are then plotted on a

graph as shown in the figure. From Figure 4.5, a new input must be inserted which is the

friction calibration factor (u). But for calibration factor (Fw), the value has to be

determined first as in Figure 4.6.


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Figure 4.5 :Minimum Pipeline Submerged Weight for a Cycle Period of Time


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Figure 4.6 :Determination of Calibration Factor and the Outputs of MIN Weight

Spreadsheet

Figure 4.6 shows the calculation of calibration factor and also the output, which

is the maximum value between the minimum pipeline submerged weights. In calculating

calibration factor, the value of Us*Tn/Hsis taken from DNV RP E305 Figure 2.1. As for

the grain size, the value is taken from DNV RP E305 Table A1 and lastly the value of

calibration factor itself is taken from DNV RP E305 Figure 5.12. These figures and table

from DNV RP E305 are inserted in the Microsoft Excel Spreadsheet developed but in

the third spreadsheet (Graph Spreadsheet). Hyperlinks from this spreadsheet are used to

link with the Graph Spreadsheet. The values taken from Graph Spreadsheet are inserted

there and will automatically be shown in this spreadsheet.


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The minimum pipeline submerged weight required for on-bottom stability is only

for a case. The value is then inserted in the specified blue cell. New environmental

inputs are then inserted and the same processes as before is undertaken to get the

minimum pipeline submerged weight required for the other three cases and are then

inserted according to their case in the blue cells. These four values will be automatically

shown in the second spreadsheet which is CALC. Weight Spreadsheet. Click the To

Calculating Weight of Pipe hyperlink to move to the second spreadsheet.

4.2.2 Calculation of Pipeline Weight Spreadsheet

Figure 4.7 shows the input for this particular spreadsheet. These are the diameter

of steel (Dst), thickness of materials such as steel (t), internal corrosion line (tL),

corrosion coating (tcc), insulation coating (tic), concrete coating (tc), marine growth (tmg)

and corroded material (tCA) as well as the density of the materials which are steel (st),

internal corrosion liner (L), corrosion coating (

cc), insulation coating (

ic), concrete

coating (c), marine growth (mg), content (i) and seawater (sw). A corrosion allowance

usage factor (uCA) value is to be inserted.


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Figure 4.7 : Pipeline Data Inputs

The values of internal diameter of pipe (ID) and the weight of materials such as

carbon steel (Wcs), corrosion coating (WL), corrosion coating (Wcc), insulation coating

(Wic), concrete coating (Wc), marine growth (Wmg), content (Wi) and corroded material

(WCA) are calculated using Equations 2.3 - 2.11 based on the inputs inserted as shown in

Figure 4.8.


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Figure 4.8 : Final Outputs of On-Bottom Stability Analysis of Submarine Pipeline

Figure 4.8 also shows the final outputs of on-bottom stability analysis of a

submarine pipeline. The outer diameter (OD), weight of pipeline (W) and pipeline

buoyancy (B) are calculated automatically for all four cases with the same value of

inputs. The main outputs are the pipeline submerged weight (Ws) and the pipeline

specific gravity (SG) which are calculated each using Equation 2.13 and Equation 2.14

of Chapter II. The pipeline submerged weight values for all four cases are compared

with the minimum pipeline submerged weight values calculated in the first spreadsheet

and the stability is determined whether its stable or not to withstand the forces exerted

to the pipeline.


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4.2.3 Graphs from DNV RP E305 for the Calculation of Calibration Factor (Fw)

Spreadsheet

In order to get the value of significant wave velocity perpendicular to the

pipeline (Us), the values of Tn and Tn/Tp is calculated in the first spreadsheet and are

transferred in this spreadsheet for the user to use the graph as shown in Figure 4.9. The

value of Us*Tn/Hsis then inserted in the cell provided and is automatically shown in the

first spreadsheet.

Figure 4.9 : Graph from DNV RP E305 Figure 2.1


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Figure 4.10 shows the grain size (d50) and roughness for seabed (zo). The type of

seabed is determined and the grain size is inserted in the cell provided. The values of

roughness will be automatically shown in this spreadsheet. Both of the values will

appear in the first spreadsheet for further calculations.

Figure 4.10 : Table A1- Grain Size for Seabed Materials from DNV RP E305


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In order to get the value of calibration factor (Fw), the values of M and K is

calculated in the first spreadsheet and are transferred in this spreadsheet for the user to

use the graph as shown in Figure 4.11. The calibration factors value is then inserted in

the cell provided and is automatically shown in the first spreadsheet and is used to

calculate the minimum pipeline submerged weight values for a cycle period of time.

Figure 4.11 :Calibration Factor from Figure 5.12 DNV RP E305


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4.3 Results for Minimum Pipeline Submerged Weight Spreadsheet

The following results are based on data inputs from case 3, which is the

installation phase. Figure 4.12 shows the drag, inertia and lift forces acting on the

submarine pipeline over a cycle period if time while Figure 4.13 shows the minimum

pipeline submerged weight required to withstand the stated forces over a cycle period of

time.

Forces vs t/T

1500

1000

500

Force (N/m) Drag0 Force0.0 0.1 0.2 1.00.3 0.4 0.5 0.6 0.7 0.8 0.9

Inertia

-500 Force

Lift

-1000

-1500t/T

Force

Figure 4.12 :3 Forces involved over a Cycle Period of Time

Pipeline Subm erged Weight (Ws) vs t/T

-150

-100

-50

0

50

100

150

200

250

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

t/T

W

sub(kg/m)

Figure 4.13 :Minimum Pipeline Submerged Weight over a Cycle Period of Time


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4.4 Results for Calculation of Pipeline Weight Spreadsheet

From the calculation of pipeline weight spreadsheet, the final result whether a

submarine pipeline is stable or not for all for all 4 cases are summarized in Table 4.1

below.

Table 4.1 : Final Results for On-Bottom Stability Analysis of a Submarine Pipeline for

all 4 cases

Outer Weight Of Pipeline

Pipeline

Submerged Stability

Case Diameter Pipe, W Buoyancy,B Weight, WsOD(mm) (kg/m) (kg/m) (kg/m)

1) Operational

Pristine 587.4 639.84 277.77 362.08 OK

2) Operational End

of Life 587.4 639.84 277.77 362.08 OK

3) Installation 587.4 555.77 277.77 278.00 OK

4) Hydrotest 587.4 670.67 277.77 392.90 OK

Pipeline Specific Min SG

Case Gravity, SG

1) Operational Pristine 2.30

2) Operational End of

Life 2.30

3) Installation 2.00 1.1

4) Hydrotest 2.41


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Figure 4.14 and Figure 4.15 show the pipeline submerged weight and pipeline

specific gravity with different thickness of concrete coating for the 4 cases.

232.02

Pipeline Submerged Weight (Ws) Vs Thickness of

Concrete Coating (tc)

0

100

200

300

400

500

600

10 20 30 40 50 60 70 80 90 100

110

120

tc (mm)

W

s(kg/m)

Case 1

and 2Case 3

Case 4

Figure 4.14 :Pipeline Submerged Weight with Different Thickness of Concrete Coating

Pipeline Specific Gravity (SG) Vs Thickness of

Concrete Coating (tc)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

10 20 30 40 50 60 70 80 90 100

110

120

tc (mm)

SG

Case 1

and 2

Case 3

Case 4

Figure 4.15 :Pipeline Specific Gravity with Different Thickness of Concrete Coating


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4.5 Parametric Analysis

Parametric analysis is done by varying an input parameter and the others are

fixed. In this study, 4 input parameters are varied which are the wave height (Figure

4.16), mean water depth (Figure 4.17), grain size (Figure 4.18) and diameter of pipeline

(Figure 4.19). This is done to see the behavior of structure in terms of the minimum

pipeline submerged weight (Ws) to varying values of input parameters stated above with

the other environmental parameters are fixed.

Minimum Pipeline Submerged Weight (Ws) Vs Wave

Height (H)

0

100

200

300

400

500

600

1.00 1.50 2.00 2.50 3.00 3.50

H (m )

Ws(kg/m)

Figure 4.16 :Minimum Pipeline Submerged Weight With Varying Values of Wave

Height


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Minimum Pipeli ne Submerged Weight (Ws) Vs

Mean Water Depth (d)

0

20

40

60

80

10 20 30 40 50 60

d(m)

Ws(kg/m

)

Figure 4.17 :Minimum Pipeline Submerged Weight With Varying Values of Mean

Water Depth

Minimum Pipeline Submerged Weight (Ws ) Vs Grain

Size (d50)

210

220

230

240

250

260

270

0.001 0.01 0.1 1 10 100

d50 (mm )

Ws(

kg/m)

Figure 4.18 :Minimum Pipeline Submerged Weight With Varying Values of Grain Size


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Minimum Pipeline Submerged Wei ght (Ws) Vs

Diameter of Pipeline (D)

0

100

200

300

400

500

0.50 0.60 0.70 0.80 0.90 1.00

D (m )

Ws(kg/m

)

Figure 4.19 :Minimum Pipeline Submerged Weight With Varying Values for Diameter

of Pipeline


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CHAPTER V

DISCUSSIONS

5.1 Introduction

This study focuses on the development of spreadsheet for on-bottom stability

analysis of a submarine pipeline. The data inputs are inserted in the spreadsheet and the

behavior of submarine pipeline are analysed. Analysis is done after the tables and graphs

as in Chapter IV are obtained.

5.2 Discussion

The examples are based on data inputs from case 3 which is the installation

phase.

5.2.1 The 3 Forces Involved and the Minimum Pipeline Submerged Weight for a

Cycle Period of Time

From Figure 4.12, the drag, inertia and lift forces are a function of sinus and

cosines. The drag and inertia forces can be positive or negative depends on its horizontal

movement, whether to the right or left.


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But lift force consists of positive values only. This is because lift force is a one

way direction, which is upwards. The combination of these 3 forces and using a formula

with the value of calibration factor, the minimum pipeline submerged weight is

determined as shown in Figure 4.13. Negative value means that the combination of the

forces causes the pipeline to be more stable because it is pushed downwards towards the

seabed. From the graph, the minimum pipeline submerged weight required to withstand

the forces is 232 kg/m. The actual weight of the pipeline must be more than this value to

ensure stability.

5.2.2 Stability Analysis

The stability has to be checked for all 4 cases. From Table 4.1, the actual

pipeline submerged weight is bigger than the minimum pipeline submerged weight

required for all 4 cases. So, the pipeline is stable. This is supported by the fact that the

pipeline specific gravity is more than 1.1 for all 4 cases [6].

From Figure 4.15, the minimum thickness of concrete coating is around 75 mm,

based on the installation phase. Installation phase in considered the critical phase. If the

value is less than 75 mm, the pipeline would probably fail due to high stress and fatigue

damage during installation. The minimum concrete thickness is based on the minimum

pipeline submerged weight required, which is 232kg/m.


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5.2.3 Parametric Analysis

Parametric analysis is done by varying an input parameter and the others are

fixed. In this study, 4 input parameters are varied which are the wave height, mean water

depth, grain size and diameter of pipeline.

From Figure 4.16, the minimum pipeline submerged weight (Ws) increases with

the increment of wave height (H). When the wave height increases, the water particles

kinematics increases (velocity and acceleration). This will increase drag, lift and inertia

forces which contribute to the minimum pipeline submerged weight.

As for Figure 4.17, the minimum pipeline submerged weight (Ws) decreases with

the increment of mean water depth (d). When the mean water depth increases, the wave

length (L) increases. This reduces drag, lift and inertia forces because the water particle

kinematics decrease, which in turn contributes to the decrement of minimum pipeline

submerged weight.

If the grain size of seabed (d50) is increased, the pipeline submerged weight

decreased. The maximum pipeline submerged weight is 263 kg/m. This is because it

depends on the calibration factor (Fw) taken from Fig 5.12- DNV RP E305 with the

maximum value of Fw is 1.62. When grain size is increased, the roughness of seabed

increased but the value of Fw decreases. So thus the minimum pipeline submerged

weight. This is shown in Figure 4.18.


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From Figure 4.19, when the pipeline diameter (D) is increased, the pipeline

submerged weight also increased. Pipeline diameter is not involved in the calculation of

water particle kinematics. It affects the drag, lift and inertia forces directly. The forces

increase with the increment of pipeline diameter.


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CHAPTER VI

CONCLUSION

6.1 Conclusions

Based on the analyses, concrete coating and thickness of steel pipeline

contributes the most to the stability of the pipeline. This is because of their high density

that is involved in the calculation of the pipeline weight. With the given case study, theminimum concrete coating is 75 mm. This is based on Case 3 (Installation), which is the

installation phase because it gives the minimumpipeline submerged weight between thefour cases. Application of concrete coating is the primary means of achieving stability.

A minimum concrete thickness of 38 mm must be adopted. From the spreadsheet

calculations, the pipeline on-bottom stability of the case study is ok.


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6.2 Recommendations

The wave water particle kinematics are calculated using Linear Wave Theory.

The values are different if calculated based on DNV RP E305 Section 2. A study can be

done to see the comparison of the two stated methods. Possibility of sinking should be

checked for an exposed submarine pipelines resting directly on the seabed.


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REFERENCES

[1] John L. Kennedy. Oil and Gas Pipeline Fundamentals , PenWell Tulsa,

Oklahama. 1993.

[2] Michael W.Braestrup ed., Jan Bohl Andersen, Lars Wahl Andersen, Mads

Bryndum, Curt John Christensen. Design and Installation of Marine

Pipelines, Blackwell : Science Ltd. 2005.

[3] Iwan R. Soedigdo, K.F. Lambrakos, Billy L. Edge. Prediction of Hydrodynamic

Forces on Submarine Pipelines Using an Improved Wake II Model, Ocean

Engineering 26. 1999.

[4] B.K Marzurkiewicz. Offshore Platforms and Pipelines, Trans Tech Publication.

1987.

[5] Det NorskeVeritas.DNV OS F101-Submarine Pipeline System, Veritec. 2000.

[6] Det NorskeVeritas.DNV RP E305 - On-Bottom Stability Design of Submarine

Pipeline, Veritec. 1998.

[7] Muhamad Hazlalin Ibrahim. Fundamentals of Pipeline Design: Pipeline On-

Bottom Stability, Seminar UTM City Campus Kuala Lumpur Malaysia. 2005.

[8] Detailed Design Services For Baram Pipeline Replacement Project: Pipeline

On-Bottom Stability Report,PCSB. 2005.

[9] JAE Young Lee, P.E. Stability of Pipeline Under Oblique Waves, CSO Aker

Engineering, Houstan. 2001.

[10] Dr. Nordin Yahaya.Marine Structures, UTM .

[11] PTS 20.196. Pipeline Engineering, Petronas. 1994.


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BIBLIOGRAPHIES

(1) PTS 31.40.00.10. Pipeline Engineering, Petronas. 1994.

(2) PTS 20.088. Pipeline Engineering, Petronas. 1994.

(3) S.W. Gong, K.Y. Lam, C. Lu. Structural Analysis of a Submarine Pipeline

Subjected to Underwater Shock, International Journal of Pressure Vessels and

Piping 77. 2000.

(4) Fuping Gao, Dong-Sheng Jeng.A New Design Method for Wave-Induced

Pipeline Stability on a Sandy Seabed, Research Report No R860, The University

of Sydney. 2005.

(5) Kevin C. Ewans,Journal of Offshore Mechanics and Arctic Engineering,

Transactions of the ASME. Vol.125. 2003.

(6) Guidelines for the Design of Buried Steel Pipe, American Lifelines Alliance,

ASME. 2001.


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APPENDIX

APPENDIX A

HAND CALCULATION

ON-BOTTOM STABILITY ANALYSIS OF A SUBMARINE PIPELINE BASED

ON A CASE STUDY FROM BARAM, SARAWAK

Example Based on Case 3 (Installation Phase)

Inp

Documents

On Bottom Stability Analysis and Design of Submarine Pipeline