Mudmat Design for Steel Jacket on-bottom Stability

MUDMAT DESIGN FOR STEEL JACKET ON-BOTTOM STABILITY

EDNA VILOD

UNIVERSITI TEKNOLOGI MALAYSIA

UNIVERSITI TEKNOLOGI MALAYSIA

BBOORRAANNGG PPEENNGGEESSAAHHAANN SSTTAATTUUSS TTEESSIISS♦♦♦♦

JUDUL: MUDMAT DESIGN FOR STEEL JACKET

ON-BOTTOM STABILITY

SESI PENGAJIAN: 2008/2009

Saya EDNA VILOD

(HURUF BESAR)

mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah)* ini disimpan di Perpustakaan Universiti

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 (��)

SULIT

(Mengandungi maklumat yang berdarjah keselamatan atau

kepentingan Malaysia seperti yang termaktub di dalam

AKTA RAHSIA RASMI 1972)

TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan

oleh organisasi/badan di mana penyelidikan dijalankan)

TIDAK

TERHAD

_____________________________

(TANDATANGAN PENULIS)

Disahkan oleh

____________________________

(TANDATANGAN PENYELIA)

Alamat Tetap:

KG.TERAWI, PENAMPANG

SABAH. DR. NORHAZILAN BIN

MOHD. NOOR

Tarikh: 4 MEI 2009 Tarikh: 4 MEI 2009

�

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 ini perlu dikelaskan sebagai SULIT atau TERHAD.

♦ Tesis ini dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara penyelidikan,

atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek Sarjana

Muda (PSM).

“I hereby declare that I have read this thesis and in my

opinion this thesis is sufficient in terms of scope and quality for the

award of the degree of Bachelor of Engineering (Civil)”

Signature : ……….……………………………………………………

Name of Supervisor: DR. NORHAZILAN MOHD. NOOR

Date : .……………………………………………………………

MUDMAT DESIGN FOR STEEL JACKET ON-BOTTOM STABILITY

EDNAVILOD

A report submitted in partial fulfilment of the

requirements for the award of the degree of Bachelor

of Engineering (Civil)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

MAY 2009

I declare that this thesis entitled “Mudmat Design for Steel Jacket On-Bottom Stability”

is the result of my own research except as cited in the references. The thesis has not been

accepted for any degree and is not concurrently submitted in candidature of any other

degree.

Signature : …………………………………………………

Name : EDNA VILOD

Date : …………………………………………………

ii

To my beloved father and mother, Milod Gololing and Jivina Sipanal, for their

unconditional love and support. To a special person, Henry Sam, for his love and

patience. To all my teachers and lecturers.

iii

ACKNOWLEDGEMENT

In preparing this thesis, I was in contact with many people, researchers,

academicians, and practitioners. They have contributed towards my understanding

and thoughts. In particular, I wish to express my sincere appreciation to my main

thesis supervisor, Dr. Norhazilan Mohd. Noor, for encouragement, guidance, critics and

friendship. Also to Dr. Airil Yasreen, for guidance, advices and motivation. Without

their continued support and interest, this thesis would not have been the same as

presented here.

My sincere appreciation also extends to all my course mates and others who have

provided assistance at various occasions. Their views and tips are useful indeed. My

fellow colleagues during my internship attachment with Sarawak Shell Bhd in Miri,

Sarawak who had shared their knowledge and experience that makes up most of the

content of this thesis.

iv

ABSTRACT

Mud mats are one of the temporary seafloor supports of jacket structure. The

function of a mud mat is similar to a shallow foundation onshore that is to provide

adequate contact area for load distribution to the soil. These temporary seafloor supports

are designed to support the weight of the jacket plus any additional loads imposed by

environmental or construction conditions before driving of the foundation piles. In this

study, mud mats design criteria is based on API-RP-2A Recommended Practice for

Planning, Designing, and Constructing Fixed Offshore Platforms. On-bottom refers to

the condition of the jacket during installation i.e. after jacket upending and flooding of

the jacket legs but before commencement of and during pile driving. Stability refers to

the mud mat support against overturning and sliding due to environmental loads. In

practice a trade-off will need to be made between the amount of sinking into ground and

the size of mud mats. Current design practices do not cover the settlement of mud mats

into the soil. This specific topic will be investigated, as part of the study.

v

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

TABLE OF CONTENTS vi

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS xii

1 INTRODUCTION

1.1 Problem Statement 3

1.2 Objectives of Study 3

1.3 Scope of Study 4

1.4 Importance of Study 4

1.5 Expected Findings 5

2 LITERATURE REVIEW

2.1 Offshore History 6

2.2 Oil and Gas Industry in Malaysia 9

2.3 Features of Offshore Structures 11

vi

2.4 Mudmat Design for Jacket On-bottom Stability 13

2.4.1 Case Consideration for Mudmat Design 14

2.4.2 Mudmat Shape Selection 17

2.5 Design Loads and Forces 18

3 METHODOLOGY

3.1 API-RP-2A Recommended Practice for 20

Planning, Designing and Constructing

Fixed Offshore Platforms

3.1.1 Shallow Foundation Capacity 22

3.1.2 Undrained Bearing Capacity 22

3.1.3 Drained Bearing Capacity 24

3.1.4 Correction Factors for Determining 25

Ultimate Bearing Capacity

3.1.5 Sliding Stability 28

3.1.6 Effective Area 29

3.2 Bearing Capacity of Shallow Foundation for 33

Specific Soil Condition

3.2.1 Linearly Increasing or Constant 33

Strength Soil

3.2.2 Layered Soil 37

3.2.3 Granular Soil 40

3.3 Settlement of Mudmat 41

3.4 Design Consideration 45

3.4.1 Static Load Condition 46

3.4.2 Including Environmental Load Condition 46

3.5 Alternative Design Criteria 47

vii

4 RESULT AND DISCUSSION

4.1 Design Criteria for F28DR-A Platform 48

4.1.1 Installation Sea States 48

4.1.2 Design Load Factors 50

4.1.3 Mudmat Design 51

4.1.4 Jacket Mudmat Structural Member 51

4.1.5 Soil Bearing Pressures 52

4.2 On-bottom Stability 54

4.3 Overturning Stability 54

4.4 Foundation Bearing 56

4.5 Sliding Stability 59

4.6 Mudmat Design 60

4.7 Mudmat Design Procedure 60

4.8 Relationship between Factor of Safety, 62

Mudmat Area and Mudmat Settlement

4.9 Relationship between Stability and Mudmat Shape 63

4.10 Mudmat Material Selection

4.11 Installation Sequence Influence on Jacket Stability 64

5 CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion 66

5.2 Recommendations 67

5.2.1 Emphasis on Soil Sample at Mudline 68

5.2.2 Addition of Mud Mat Settlement Analysis 68

and Calculation in Design Procedure

REFERENCES 70

APPENDICES

APPENDIX A 71

APPENDIX B 78

viii

LIST OF TABLES

TABLE NO. TITLE PAGE

4.1 Environmental Criteria 49

4.2 Wave, Current and Wind for Substructure 49

4.3 Load Factors for On-bottom Analysis 51

4.4 Overturning Utilization Factor 55

4.5 Foundation Bearing Safety Factor 58

4.6 Settlement of Jacket from Mudline EL(-)82.2m 59

4.7 Sliding Stability Maximum Utilization Factor 59

x

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.0 Foundation Elements Temporarily Supporting Jacket 2

2.0 World Map of Oil Producer Countries 8

2.1 Oil and Gas Field in Malaysia 9

2.2 Offshore Platform 12

2.3 Jacket at On-bottom Condition 14

2.4 On-bottom Stability 15

2.5 Stability from the Perspective of Wave Force Directions 16

2.6 Equivalent Triangular Area to Circular Area 17

2.7 Equivalent Triangular Area to Rectangular Area 17

3.0 Recommended Bearing Capacity Factors 29

3.1 Eccentrically Loaded Footings 31

3.2 Correction Factors for Shallow Foundation 35

3.3 Modified Bearing Capacity Factors, Nm 37

3.4 Bearing Capacity Factors 39

3.5 Relationship between Initial Shear Stress Ratio and Over 42

Consolidation Ratio

3.6 Relationship between Eu/su and OCR from CU Tests on 43

Three Clays

3.7 Relationship between Settlement Ratio and Applied Stress 44

Ratio for Strip Foundation on Homogenous Isotropic Elastic Layer

4.0 Environmental Direction 56

4.1 Relationships between Factor of Safety and Initial Settlement 62

4.2 Case Illustrations for Overturning Moment and Resisting Moment 63

xi

LIST OF SYMBOLS

F - hydrodynamic force vector per unit length acting normal to the axis of

the member

FD - drag force vector per unit length acting normal to the axis of the

member in the plane of the member axis and U

FI - inertia force vector per unit length acting normal to the axis of the

member in the plane of the member axis and ∂u/∂t

CD - drag coefficient

w - weight density of water

g - gravitational acceleration

A - projected area normal to the cylinder axis per unit length

V - displaced volume of the cylinder per unit length

D - effective diameter of circular cylindrical member including marine

growth

U - component of the water velocity vector (due to wave and/or current)

normal to the axis of the member

|U| - absolute value of U

Cm - inertia coefficient

��

�� - component of the local acceleration vector of the water normal to the

axis of the member

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1

CHAPTER 1

INTRODUCTION

Drilling for petroleum beneath the sea has increased tremendously in the last 50

years. Shortly after World War II, wells were drilled in shallow, protected water using

onshore methods, although these were inadequate. These inadequacies led to the growth

of an entirely new industry, which, with its many attendant service industries, has today

become one of the world’s most intensive concentrations of manpower, capital and

technology. Today, offshore activities take place in waters more than half the nations on

earth. And no longer do primitive, shore-bound wooden wharves confine offshore

operators. Instead they drill wells from modern steel or concrete structures.

Offshore jacket structures must be temporarily supported by the near-seafloor

soils before driving of the foundation piles. The foundation elements that bear on the

seafloor soils include the jacket leg extensions, lowest level of horizontal bracing and

mud mats (see Figure 1.0). All these foundation elements must be designed to support

the weight of the jacket plus any additional loads imposed by environmental or

construction conditions. To achieve a safe yet economical design, the structural and

geotechnical engineers must coordinate their activities in sizing the foundation elements.

The first step in the design sequence requires that the geotechnical engineer

explore soil conditions at the proposed structure location. By drilling and sampling a

boring, geotechnical data (strength and deformation properties) are determined for th

critical zone generally located to a 40 to 50 ft below the seafloor. After the structural

engineer provides sizes of the jacket leg extensions and lowest level of horizontal

bracing, the geotechnical engineer can then compute the soil resistance develop

these two foundation elements. Depending on the soil resistance, the length of leg

extensions may need to be shortened to assure full penetration. If the jacket leg

extensions and lowest horizontal bracing do not provide adequate temporary support,

mud mats must be provided and should be designed based on bearing capacity of the

seafloor soils as determined by the geotechnical engineer. Although mud mats only

comes later in consideration, almost all of steel jackets are build with mud mats as mean

of temporary support. This means that mud mats components are most reliable for on

bottom stability prior to pile installation.

Figure 1.0



boring, geotechnical data (strength and deformation properties) are determined for th



bracing, the geotechnical engineer can then compute the soil resistance develop







of temporary support. This means that mud mats components are most reliable for on

bottom stability prior to pile installation.

Foundation Elements Temporarily Supporting Jacket



boring, geotechnical data (strength and deformation properties) are determined for the



bracing, the geotechnical engineer can then compute the soil resistance developed on







of temporary support. This means that mud mats components are most reliable for on-

Foundation Elements Temporarily Supporting Jacket

2

1.1 Problem Statement

Mud mat is a component designed for most of steel jackets. It has become a

common practice among engineers to include mud mat in their jacket design even

though the support provided by jacket leg extensions and lowest level of horizontal

bracing is sufficient to stabilize the structure. The fundamental aspects that need to be

considered in mud mat design include the vertical resistance of soil provided by the

bearing capacity of soil, resistance of structure against sliding and overturning. Another

aspect rarely being considered correctly is the settlement of the mud mat. Although in

the design code such as the API does list a few guidelines on the mud mat design, there

are no documents that provide a proper and standard way of mud mat design.

Settlement of mud mat is often predicted by the method of comparison between

the applied pressure and the ultimate bearing capacity. This method is simple and easy

to apply in the analysis. In the actual case, settlement of mud mat could be very

important aspect to investigate especially when the topsoil is weak. Therefore, a proper

method should be identified in providing this information as part of the design practice.

1.2 Objectives of Study

The purpose of this study will serve to achieve the following:

1. to propose a proper and standard mud mat design procedure for on-bottom

stability of steel jacket using manual method

3

2. to make a comparison between the use of steel and timber mud mat

3. to investigate the significant effect of immediate settlement during on-bottom

condition.

1.3 Scope of Study

This study is based on two common types of mud mat, made of timber and steel.

The design code or guidelines is based on the API-RP-2A and other documents that refer

to the stated guidelines. The marine soils considered in this study are of cohesive soil

(clay) and cohesionless soil (sand). This study is based on several existing offshore steel

jacket structure used for oil and gas industry in the Malaysian waters. All analysis and

design works are done manually with the aid of computer software such as SACS

(Structural Analysis Computer System) and Microsoft Excel.

1.4 Importance of Study

This study will be useful for young and less experienced engineers in learning to

design mud mat and enhanced the design quality of experienced engineers. The content

on several important aspects that often overlooked in design will be a point to be

emphasis by designers in their future design. The comparison of material will not only

assist mud mat design but also in weight control management. The study will give a

clear perspective in selecting the more efficient and suitable material depending on the

project need and description.

4

1.5 Expected Finding

This study will anticipate the following result:

1. Proper mud mat design procedure that take into account all affecting aspects

of on-bottom stability.

2. The advantages and disadvantages of using steel and timber mud mat.

3. Identification of the significant effect of immediate settlement of mud mat

thus the need to be included as part of the mud mat design criteria and

procedure.

5

CHAPTER 2

LITERATURE REVIEW

2.1 OFFSHORE HISTORY

The offshore history begun in the late 1800’s, where the citizens of Summerland

California, began producing the numerous springs of crude oil and natural gas that

dotted their landscape. After drilling a large number of wells, these early oilmen noticed

that those nearest the ocean were the best producers. Eventually, they drilled several

wells on the beach itself. Then in 1887, one citizen, H.L Williams came up with the idea

of building a wharf and erecting the drilling rigs on it. His first offshore well extended

about 300 feet (90 meters) into the ocean. As expected it was a good producer and

before long entrepreneurs built several more wharfs. The longest stretched over 1200

feet (nearly 400 meters) in the Pacific Ocean.

By 1910, wells were being drilled in Ferry Lake in Caddo Parish, Louisiana.

Wooden derricks were erected on hastily built wooded platform constructed on top of

cypress tree pilings. In 1922, oil was discovered beneath the water of Lake Maracalbo,

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Venezuela. Drilling was begun from timber platforms erected in the shallow water of

this island. By 1930, the lake was a dense array of drilling rigs and production

platforms. It is estimated there are over 6000 platforms in the lake today.

California’s offshore development was pioneered by the Signal Oil and Gas

Company. Around 1930, the company was drilling wells in the tidelands at Elwood.

Marine drilling in the Gulf of Mexico began in 1930s with wells placed in the swamps

and marsh areas of Louisiana. Timber platform were used and canals to drills in the gulf

proper was 1933, off Creole in Cameron Parish. Approximately 25 wells were drilled

from pile foundations off the Gulf Coast from 1937 to 1942. These truly offshore

operations demonstrated the many difficulties that had to be overcome if drilling beyond

the shoreline was to continue with any efficiency.

World War II brought a temporary halt to offshore development, but advances in

technology, as a result of the war, allowed the industry to get a firm grasp on a strong

future. In 1946, Magnolia Oil Co. constructed a platform in 3 meters of water and

approximately 8 kilometers offshore. It was the first operation carried out this far from

shore. The platform was 53 by 23 meters. A total of 338 steel piles supported the

derrick. Communication was maintained by radio. The year 1947 saw the construction

of two platforms that become the design standard for many years. The new design called

for six steel templates or jackets fabricated onshore and carried to the site by barge.

They were then lowered into the water by a crane and fixed to the bottom using steel

piles driven through the jacket legs. In 1955, the first platform in over 30 meters of

water was in operation, with deck size of 67 by 32 meters. This new platform

introduced the use of skirt piles. The truss work consisted of three jackets of eight piles

each. Around 1950, while the developments mentioned previously were taking place in

the Gulf of Mexico and in the Santa Barbara Channel, the British Petroleum Company

was engaged in similar Exploration operation at Umm Shaif of the coast of Abu Dhabi

in the Arab Gulf.

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Though world economic recession in the 80s reduced the demand for oil,

technology pressed even further. The first offshore horizontal well was drilled and

drilling water depth reached 7500 feet or nearly 1.5 miles. Scientists began to get

clearer picture of the reserves beneath the seafloor through 3-D seismic imaging. In

1982, Unocal Corporation drilled the first horizontal well in North Sea an Shell installed

Bullwinkle, the world’s tallest standing structure from 1350 feet of water. New depth

records for drilling reached 7625 feet in the Gulf of Mexico and Shell Oil’s Platform

Troll which stands in the North Sea in 1000 feet of water of 1500 feet high became one

of two man-made objects visible with the naked eye from the surface of the moon. The

other is the Great Wall of China.

Figure 2.0 World Map of Oil Producer Countries

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Figure 2.1 Oil and Gas Field in Malaysia

2.2 OIL AND GAS INDUSTRY IN MALAYSIA

The oil venture in Malaysia, commenced in the late 1870s when Mr. Claude

Champion de Crespigny, the Resident of Baram discovered and recorded the existence

of oil in Miri, Sarawak. From this point, later on December 22, 1910, oil was stuck at

depth of 137 meters and was the first time commercially produced by Shell in Malaysia.

Malaysia contains oil reserves of approximately 3.4 billion barrels in January 2000.

Malaysia has a monthly production numbers range between 660000 and 730000 barrels

per day between 1996 and early 2001. In natural gas production, Malaysia contains 84.4

trillion cubic feet (Tcf) of proven natural gas reserves in January 2000. Production has

9

been rising progressively in recent years, reaching 1.45 Tcf in 1999, up from 1.37 Tcf in

1998.

Malaysia’s oil and gas production area covers the coast of Terengganu, Sarawak

and Sabah with 1307 kilometers pipeline networks for crude oil and 379 kilometers for

natural gas. Major oil fields can be found in Bekok, Bekor, Erb West, Bunga Kekwa,

Guntong, Kepong, Kinabalu, Pulai Samarang, Seligi, Semangkok, Tapis, Temana and

Tiong, while major oil terminals are located in Bintulu, Johor, Kerteh, Kuching, Melaka,

Penang, Port Dickson and Port Klang. Petronas has the licensing authority for all

upstream activities that consist of exploration, production and transportation of oil in

Malaysia. Most of the country’s oil fields contain low sulphur, high quality crude with

gravities in the 35 to 50 API range.

Esso Production Malaysia Inc. (EPMI) is the largest crude oil producer in

Peninsular Malaysia, accounting for nearly half of Malaysia’s crude oil production.

EPMI operates seven fields near the peninsular, and one-third of its production comes

from Seligi field. The Seligi-F platform, with its 28 wells, is the newest satellite in the

Seligi field, located 165 miles off the coast of Terengganu, Peninsular Malaysia.

Currently, Terengganu produce more than half of Malaysia’s total oil output with daily

production totaling 380000 barrels.

Besides Petronas Carigali (PCSB), five other companies are also involved in the

oil and gas exploration of Terengganu coast in PSC agreement. They are International

Petroleum Ltd., USA; Esso Production Malaysia; Western Mining Corporation,

Australia; Texaco and Penyu International Inc. The other major oil companies operating

in Sabah and Sarawak are Shell, Petronas Carigali, Amoco, Conoco, Enron, Exxon, IPC,

Mitshubishi, Mobil, Murphy Oil, Nippon Oil, Occidental, Statoil, Texaco, Triton and

Canadian Consortium led by Home Oil Ltd.

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Mudmat Design for Steel Jacket on-bottom Stability

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