Static strength of fabricated trunnion and x joint under shear and in plane bending moment

An analytical approach to the trunnion design provides a complete table where pure pipe trunnion could beselected where failure mode is governed by shear and not chord indentation.. LIST

STATIC STRENGTH OF FABRICATED TRUNNION & X-JOINT UNDER SHEAR AND IN-PLANE BENDING MOMENT

QUAH CHIN KAU

NATIONAL UNIVERSITY OF SINGAPORE

2006

STATIC STRENGTH OF FABRICATED TRUNNION AND X-JOINT UNDER SHEAR AND IN-PLANE BENDING MOMENT

BY

QUAH CHIN KAU, M.Eng., B.Eng.(Hons) DEPARTMENT OF CIVIL ENGINEERING

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

NATIONAL UNIVERSITY OF SINGAPORE

2006

To God Be the Glory

I would like to express my sincere appreciation to my supervisors, Professor N E.Shanmugam, Associate Professor Choo Yoo Sang and Associate Professor Richard LiewJat Yuen for their invaluable support, guidance and encouragement throughout the course

of this study

Special thanks also to the staff from the former CAE/CAD/CAM Centre and Civil &Structural Engineering Laboratory, Faculty of Civil Engineering, for their help andsupport in the computational and experimental investigations respectively

I also gratefully acknowledge the National Science & Technology Board, Singapore(NSTB), Sembawang Marine & Offshore Engineering (SMOE) and the NationalUniversity of Singapore (NUS) for providing the necessary finances, equipment andresearch facilities to pursue the study

It was my mother, who struggled hard to put me through many years of education, that hasthe strongest desire to see her son succeed I dedicate this volume to her: for her untiringperseverance, patience and believe in her son

Last, but not the least, my dear wife, who makes the journey during the course of thestudy more bearable and enjoyable

Everything is made possible by the grace of God

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ……… …….… ……… i

TABLE OF CONTENTS ……… ………….… ……… ii

SUMMARY ……… ……….… ……… viii

LIST OF TABLES ……….… ……… xi

LIST OF FIGURES ……… ….… ……… xiii

LIST OF SYMBOLS ……….… ……… xx

CHAPTER ONE INTRODUCTION 1.1 Structural applications of hollow sections and trunnions ……… 1

1.2 Types of trunnions studied ……… ……… 8

1.3 Classification of joints ……… …… 14

1.4 Objectives and scope of research ……… 16

1.5 Survey of previous research ……… 19

1.5.1 Experimental research in trunnion ……… 21

1.5.2 Research in tubular X-joints ……….…… 25

1.5.3 Finite element method ……….…… 29

1.6 Current design recommendations ……… 31

1.7 Contents of Thesis ……… 35

CHAPTER TWO EXPERIMENTAL INVESTIGATION ON SMALL PIPE TRUNNIONS 2.1 Introduction ……… 36

2.2 Research programme ……… 38

2.2.1 Trunnion dimensions … … … 38

2.2.2 Fabrication of specimen … … … 41

2.2.3 6,000kN test rig … … … 44

2.2.4 Instrumentation … … … 46

2.3 Governing failure mode of trunnion … … … 48

2.3.1 Pure pipe trunnions … … … 48

2.3.2 Shear plate pipe trunnions … … … 51

2.3.3 Combined shear and pipe trunnions … … … … 54

2.4 Discussions of the test results … … … … 58

2.4.1 Design strength pure pipe trunnions … … … … … … … 58

2.4.2 Design strength of pipe trunnions with slotted shear plates only … … … 62

2.4.3 Design strength of pipe trunnions with shear plates and pipes … … … 65

2.5 Conclusions … … … … 67

CHAPTER THREE EXPERIMENTAL INVESTIGATION OF LARGE PIPE TRUNNIONS AND TUBULAR X-JOINTS 3.1 Introduction … … … … 69

3.2 Research Programme … … … 71

3.2.1 Trunnion dimensions … … … 71

3.2.2 Fabrication of specimen for test … … … 75

3.2.3 10,000kN test rig … … … … 77

3.2.4 Instrumentation … … … 87

3.3 Governing failure mode of trunnion … … … 90

3.3.1 Pure pipe trunnion … … … 90

3.3.2 Through pipe trunnion … … … … 95

3.3.3 Combined shear plate and pipe trunnion … … … … 97

3.3.4 Tubular X-joints … … … 101

3.4 Discussions of the test results … … … 105

3.4.1 Design strength of pure pipe trunnions … … … … … … 105

3.4.2 Design strength of through pipe trunnions … … … … 110

3.4.3 Design strength of combined shear plate and pipe trunnions … … … … 113

3.4.4 Transition of shear and bending moment … … … … 116

CHAPTER FOUR FINITE ELEMENT ANALYSES ON TUBULAR JOINTS 4.1 Introduction … … … 120

4.2 Finite element programs and hardware used … … … 122

4.3 Main characteristics of finite element work on tubular joints … 123 4.3.1 Finite element mesh and boundary conditions … … … … 123

4.3.2 Finite element types … … … … 125

4.3.3 Loading of the joints … … … 127

4.3.4 Modeling of the post-yield material property … … … … 128

4.3.5 Iteration procedure and convergence criteria … … … … 129

4.3.6 Numerical modeling of weld geometry … … … … … … 130

4.4 Numerical analysis for the experimental tests … … … 131

CHAPTER FIVE NUMERICAL SIMULATION OF THE EXPERIMENTS 5.1 Research programme and general finite element aspects … … … 134

5.2 Numerical analyses … … … 135

5.2.1 Pure pipe trunnions … … … … … 137

5.2.2 Shear plate pipe trunnions .… … … … 140

5.2.3 Through pipe trunnions … … … … 141

5.2.4 Combined shear plate and pipe trunnions … … … 143

5.2.5 Shear and bending loads on tubular X-joints … … … … 145

5.3 Comparison between the experimental and numerical results 146

5.3.1 Pure pipe trunnions … … … … 147

5.3.2 Shear plate pipe trunnions … … … … … … … 151

5.3.3 Through pipe trunnions … … … … … … … … … … … 152

5.3.4 Combined shear plate and pipe trunnions … … … … … 154

5.3.5 Shear and bending loads on tubular X-joints … … … … 156

CHAPTER SIX NUMERICAL PARAMETRIC STUDIES 6.1 Assumptions for the numerical models … … … 159

6.2 Pure pipe trunnions … … … … 161

6.2.1 Research programme … … … 162

6.2.2 Effective W ratio for trunnion design … … … … 170

6.2.3 Effective E ratio for trunnion design … … … 179

6.2.4 Selection and design approach for pure pipe trunnions 186

6.2.5 Proposed design formulation for pipe trunnions .… … 194

6.3 Through pipe trunnions … … … 196

6.3.1 Research programme … … … 197

6.3.2 Comparison of ultimate load capacity of through pipe trunnions … … … … … 201

6.3.3 Design approach of through pipe trunnions … … … … 205

6.4 Tubular X-joints … … … 208

6.4.1 Research Programme … … … 208

6.4.2 Interaction effects of shear and bending moment … … 209

6.4.3 Proposed interaction equation of shear and bending moment … … … 211

6.4.4 Effective width of trunnion brace … … … 216

CHAPTER SEVEN TRUNNION DESIGN CALCULATION 7.1 Design approach for fabricated trunnion … … … 218

7.2 Rigging arrangement and design loads … … … 220

7.3 Estimating lifting loads on the trunnion .… … … … … … 220

7.4 Pure pipe trunnion option … … … 221

7.4.1 Selection of pure pipe trunnion … … … 222

7.4.2 Verifying chord and brace sizes … … … 224

7.5 Through pipe trunnion option … … … 227

7.5.1 Selection of chord and brace sizes … … … 227

7.6 Combined pipe and shear plate trunnion option … … … 230

7.6.1 Design load considerations and layout … … … 231

7.6.2 Selection of main parameters … … … … … … … 234

7.6.3 Checks for trunnion brace … … … … … … 235

7.6.4 Checks for shear plate … … … … … … … 237

7.6.5 Checks for chord wall … … … … … … 239

7.7 Conclusions … … … … … … 240

CHAPTER EIGHT CONCLUSIONS AND RECOMMENDATIONS FOR

FUTURE WORK

8.1 Overview of experimental study … … … 241

8.2 Overview of numerical and parametric study … … … 243

8.3 Overview of trunnion calculations … … … 244

8.4 Main findings and conclusions … … … 244

8.5 Proposals for future research … … … … … … … 248

REFERENCES … … … … … … … 250

Current design receommendations define fabricated trunnions as consisting of aslotted shear plate and two side braces on each side of a main body It is used as alifting point in the installation of heavy offshore structures There is very limitedexperimental and numerical work that has been conducted to study the behaviour andstrength of trunnion subjected to shear and bending moment As a result, currentdesign recommendations for trunnion cover generic specifications on the geometricdimensions only Further, it does not include brace shear strength as contributing tothe overall static strength of a trunnion joint Thus, current trunnion joint designpractice is very conservative The detailed engineering designs for trunnion areempirical and dependent on the ingenuity of experienced engineers using empiricalequations for design checks This creates a big gap in the understanding of behaviourand strength of trunnion subjected to shear and bending moment

The objective of this study is to close the gap and provide a more rational designapproach through a comprehensive experimental and numerical research programme.There is a three fold approach used in this study The experimental programmeprovides the basis in benchmarking the ultimate strength of trunnion Numericalanalysis extends the validity range possible through tests An analytical designapproach is proposed based on experimental and numerical results The definition offabricated trunnion has been expanded to include pure pipe trunnion and through pipetrunnion while considering the shear strength contribution from brace

Due to the difficult nature of the test in simulating the behaviour of the trunnionsubjected to shear and bending moment, two experimental setup were performed.Twenty five large scale specimens were tested, majority on a completely new andspecially designed test rig that were capable of testing trunnion to failure without theuse of a grommet The use of grommet in the experiments posed many challenges andlimits the scale of specimens that can be tested The results obtained providesignificant knowledge on the ultimate failure behaviour of trunnion.

The large database from the experimental tests provide sufficient and valuableinformation to calibrate the numerical models Finite element models were created tosimulate the ultimate failure behaviour of the tests There is good agreement betweenthe experimental and numerical results and they provide the basis in performing anextensive numerical research programme on three types of trunnion

The results were collated and analysed The results have proven that the bracecontributes significantly to the static strength of the trunnion An analytical approach

to the trunnion design provides a complete table where pure pipe trunnion could beselected where failure mode is governed by shear and not chord indentation A newapproach was also introduced in using through pipe trunnion that extends theboundary by reducing the effects of chord indentation so that full static strength of thetrunnion joint can be mobilized In addition, an interaction equation was proposedbased on regression analysis of the results The allowable trunnion width isestablished

An example trunnion design calculations was presented to highlight the practicalnature of the proposed design approach that is applicable to the industry Threedifferent design approaches were used and their relative merits compared.

LIST OF TABLES

Table 2.1 Small pipe trunnion dimensions and non-dimensional geometric

parametersTable 2.2 Summary of the ultimate loads and displacement for specimen C1 to

CT8Table 2.3 Summary of the ultimate loads and displacement for specimen C1 to

C3Table 2.4 Summary of the ultimate loads and displacement for specimen C4 &

C5Table 2.5 Summary of the ultimate loads and displacement for specimen C6 to

C8Table 3.1 Large pipe trunnion dimensions and non-dimensional geometric

parametersTable 3.2 Summary of the ultimate loads and displacement for specimens CT1 to

CT5Table 3.3 Summary of the ultimate loads and displacement for specimens CT6

and CT7Table 3.4 Summary of the ultimate loads and displacement for specimens CT8 to

CT11Table 3.5 Summary of the ultimate loads and displacement for specimens CT12

to CT17Table 3.6 Summary of the ultimate and design strength for CT1 to CT5

Table 3.7 Summary of the ultimate and design strength for CT8 to CT11

Table 5.1 Research programme for the numerical analysis

Table 6.1 Research programme of pure pipe trunnions (in plane only)

Table 6.2 Dimensions and non-dimensional geometric parameters for W = 0.46,

0.28 < E < 0.90, 10 < 2J <30Table 6.3 Dimensions and non-dimensional geometric parameters for W = 0.75,

0.28 < E < 0.90, 10 < 2J <30

Table 6.4 Dimensions and non-dimensional geometric parameters for W = 1.00,

0.28 < E < 0.90, 10 < 2J <30Table 6.5 Dimensions and non-dimensional geometric parameters for W = 1.25,

0.28 < E < 0.90, 10 < 2J <30Table 6.6a Results of the ultimate load capacity for pipe trunnions

Table 6.6b Results of the ultimate load capacity for pipe trunnions

Table 6.7 Results of the ultimate load capacity for test pipe trunnions

Table 6.8 Failure modes of pipe trunnions based on non-geometric parametersTable 6.9 Dimensions and non-dimensional geometric parameters for through

pipe trunnionsTable 6.10 Comparison of static strength with and without through pipe for 2J =

30Table 6.11 Comparison of static strength with and without through pipe for E =

0.70Table 6.12 Governing failure mode of through pipe trunnions

Table 6.13 Dimensions and non-dimensional geometric parameters for X-jointsTable 6.14 Shear and bending moment interaction from the numerical and

experimental resultsTable 7.1 Failure mode of pure pipe trunnions

Table 7.2 Failure mode of through pipe trunnions

LIST OF FIGURES

Figure 1.1 An effective use of a fabricated plate trunnions to transfer the loads

through a spreader beam for the lifting of a heavy topside moduleFigure 1.2 A fabricated pipe trunnion installed at an inclined position to transfer

the loads through a single lifting point for the lifting of a heavy topsidemodule

Figure 1.3 The installation of a topside above a platform by Heerema DB101

barge crane using fabricated pipe trunnionsFigure 1.4 The installation of a large offshore package requires fabricated pipe

trunnions during an upending operationFigure 1.5 The diagram above shows a typical trunnion reinforced with a stiffener

to enhance its carrying capacity for lifting operationFigure 1.6 A typical standard trunnion with a single brace attached to a chordFigure 1.7 Grommet (wire ropes) placed on both side of the trunnion during a

lifting operationFigure 1.8 The shear plate is inserted through the chord with the two-halves of the

brace welded over itFigure 1.9 The shear plate is inserted through the chord and welded on both sides

of the chord but the brace is not welded on the chordFigure 1.10 Brace inserted through the chord wall and welded on both sides of the

chordFigure 1.11 Configuration and dimensions of specimen of small pipe trunnionFigure 1.12 Configuration and dimensions of specimen of large pipe trunnionFigure 1.13 Recommended geometrical parameters for trunnion, Brown & Root

(1990)Figure 2.1 An elongated slot hole is made on the chord wall of the trunnion to

accommodate the shear plateFigure 2.2 A typical fabricated small pipe trunnion for test

Figure 2.3 Tensile coupon for plates and pipes

Figure 2.4 Test arrangement for small pipe trunnion (6000kN test rig)

Figure 2.5 Loading Condition for 6000kN Test Rig

Figure 2.6 Support Condition for 6000kN Test Rig

Figure 2.7 Placements of transducer at top of chord

Figure 2.8 Placements of transducer at bottom of chord

Figure 2.9 Shear failure on specimen C1

Figure 2.10 Shear failure on specimen C2

Figure 2.11 Chord plastification and fracture on chord of specimen C3

Figure 2.12 Load deformation curves for specimens C1, C2 & C3

Figure 2.13 Shear failure on specimen C4

Figure 2.14 Shear failure on specimen C5

Figure 2.15 Load deformation curves for specimens C4 & C5

Figure 2.16 Shear failure on specimen C6

Figure 2.17 Shear failure on specimen C7

Figure 2.18 Shear failure on specimen C8

Figure 2.19 Load deformation curves for specimens C6, C7 & C8

Figure 2.20 Ultimate loads and shear capacity of specimens C1, C2 & C3

Figure 2.21 Ultimate loads and shear capacity of specimens C4 & C5

Figure 2.22 Ultimate loads and shear capacity of specimens C6 to C8

Figure 3.1 The process of cutting brace to shape and welding on the chord

Figure 3.2 The process of cutting hole on the chord and slotting the brace through

the chord before being butt welded together

Figure 3.3 The shear plate slotted through the vertical hole made on the chord and

the brace overlapping around the shear plateFigure 3.4 The length of the brace of the tubular X-joints as compared to the

normal trunnion where the brace arm is very shortFigure 3.5 A typical fabricated large pipe trunnion for test

Figure 3.6 A schematic isometric view of the 10,000kN test rig

Figure 3.7 Foundation support for the 10,000kN test rig with interconnecting

sections to provide a self actuating effect on the loading method

Figure 3.8 The loadings from the actuator is transmitted through the half spherical

ball to reduce bending effect from the loading endFigure 3.9 Loading condition for 10,000kN test rig

Figure 3.10 Key feature of the 10,000kN test rig consists of rotating saddle and

roller supportFigure 3.11 Support condition for 10,000kN test rig

Figure 3.12 Test Arrangement for large pipe trunnion (10,000kN test rig)

Figure 3.13 Placement of transducer in a typical specimen

Figure 3.14 Placement of transducer in a typical specimen

Figure 3.15 Deformation and governing failure mode of specimen CT1

Figure 3.16 Deformation and governing failure mode of specimen CT2

Figure 3.17 Deformation and governing failure mode of specimen CT3

Figure 3.18 Deformation and governing failure mode of specimen CT4

Figure 3.19 Deformation and governing failure mode of specimen CT5

Figure 3.20 Load deformation curves for specimens CT1 to C5

Figure 3.21 Deformation and governing failure mode of specimen CT6

Figure 3.22 Deformation and governing failure mode of specimen CT7

Figure 3.23 Load deformation curves for specimens CT6 and CT7

Figure 3.24 Deformation and governing failure mode of specimen CT8

Figure 3.25 Deformation and governing failure mode of specimen CT9

Figure 3.26 Deformation and governing failure mode of specimen CT10

Figure 3.27 Deformation and governing failure mode of specimen CT11

Figure 3.28 Load deformation curves for specimens CT8 to CT11

Figure 3.29 Deformation and governing failure mode of specimen CT12

Figure 3.30 Deformation and governing failure mode of specimen CT13

Figure 3.31 Deformation and governing failure mode of specimen CT14

Figure 3.32 Deformation and governing failure mode of specimen CT15

Figure 3.33 Deformation and governing failure mode of specimen CT16

Figure 3.34 Deformation and governing failure mode of specimen CT17

Figure 3.35 Load deformation curves for specimens CT12 to CT17

Figure 3.36 Design load for specimens CT1 to CT4

Figure 3.37 Design load for specimen CT5

Figure 3.38 Design load for specimens CT6 and CT7

Figure 3.39 Comparison load deformation plots for CT2 & CT6 and CT4 & CT7Figure 3.40 Load deformation curves for specimens CT8 to CT11

Figure 3.41 Load deformation curves for specimens CT12 to CT15

Figure 3.42 Load deformation curves for specimens CT16 to CT17

Figure 3.43 Comparison of the two series of load deformation plots

Figure 4.1 Quarter model of small pipe trunnion with boundary conditions shownFigure 4.2 Quarter model of large pipe trunnion with boundary conditions shownFigure 4.3 Effect of mesh on the finite element results

Figure 4.4 Solid model of trunnion compared with test specimen C1 at failureFigure 4.5 Solid model of trunnion compared with test specimen C5 at failureFigure 5.1 Comparison of experimental and numerical results of the ultimate

failure mode of small pipe trunnion C1Figure 5.2 Comparison of experimental and numerical results of the ultimate

failure mode of small pipe trunnion C3

Figure 5.3 Comparison of experimental and numerical results of the ultimate

failure mode of small pipe trunnion CT3Figure 5.4 Comparison of experimental and numerical results of the ultimate

failure mode of small pipe trunnion CT5

Figure 5.5 Comparison of experimental and numerical results of the ultimate

failure mode of shear plate small pipe trunnion C5Figure 5.6 Comparison of experimental and numerical results of the ultimate

failure mode of shear plate small pipe trunnion CT6Figure 5.7 Comparison of experimental and numerical results of the ultimate

failure mode of shear plate small pipe trunnion CT7Figure 5.8 Comparison of experimental and numerical results of the ultimate

failure mode of shear plate small pipe trunnion CT10

Figure 5.9 Comparison of experimental and numerical results of the ultimate

failure mode of tubular X-joints CT12, CT13 and CT14Figure 5.10 Load-displacement diagram for experimental and numerical results of

specimens C1 to C3Figure 5.11 Load-displacement diagram for experimental and numerical results of

specimens CT1 to CT5Figure 5.12 Load-displacement diagram for experimental and numerical results of

specimens C4 and C5Figure 5.13 Load-displacement diagram for experimental and numerical results of

specimens CT6 to CT7Figure 5.14 Load-displacement diagram for experimental and numerical results of

specimens C6 to C8Figure 5.15 Load-displacement diagram for experimental and numerical results of

specimens CT8 to CT11Figure 5.16 Load-displacement diagram for experimental and numerical results of

specimens CT3, CT15 to CT17Figure 6.1 Dimensions and non-dimensional geometric parameters of pipe

trunnionsFigure 6.2 Numerical load-displacement curves for 2J = 30

Figure 6.3 Numerical load-displacement curves for 2J = 25

Figure 6.4 Numerical load-displacement curves for 2J0 = 20

Figure 6.5 Numerical load-displacement curves for 2J0 = 15

Figure 6.6 Numerical load-displacement curves for 2J0 = 10

Figure 6.7 Ultimate load capacity of pure pipe trunnion for 2J0 = 30

Figure 6.8 Ultimate load capacity of pure pipe trunnion for 2J0 = 25

Figure 6.9 Summary of ultimate load capacity of pure pipe trunnions for 2J0 = 10

to 30Figure 6.10a Numerical load-displacement curves for W = 0.46

Figure 6.10b Numerical load-displacement curves for W = 0.46

Figure 6.11 Ultimate load capacity of pure pipe trunnion for W = 0.46

Figure 6.12a Numerical load-displacement curves for W = 0.75

Figure 6.12b Numerical load-displacement curves for W = 0.75

Figure 6.13 Ultimate load capacity of pure pipe trunnion for W = 0.75 over E ratioFigure 6.14a Numerical load-displacement curves for W = 1.00

Figure 6.14b Numerical load-displacement curves for W = 1.00

Figure 6.15 Ultimate load capacity of pure pipe trunnion for W = 1.00 over E ratioFigure 6.16 Numerical load-displacement curves for W = 1.25

Figure 6.17 Schematic representation of shear and in-plane bending momentsFigure 6.18 Statistical sampling data on ultimate strength of pure pipe trunnionFigure 6.19 Dimensions and non-dimensional geometrical parameters of pipe

trunnions with through pipesFigure 6.20 Ultimate failure of pipe trunnion with and without through pipe

Figure 6.21 Numerical load-displacement curves for through pipe trunnions

Figure 6.22a Comparison of load-deformation plots for through pipe trunnions

Figure 6.22b Comparison of load-deformation plots for through pipe trunnionsFigure 6.23 Schematisation of the through pipe trunnion

Figure 6.24 Numerical load-displacement curves for X31 and X41 series

Figure 6.25 Numerical moment-rotation curves for X31 and X41 series

Figure 6.26 Interaction curves of shear and bending moment

Figure 7.1 Reactions acting on the pure pipe trunnion brace by the grommetFigure 7.2 Reactions acting on the through pipe trunnion brace by the grommetFigure 7.3 Schematic view of the trunnion configuration in the design

Figure 7.4 Section X-X view of the trunnion configuration in the design

Figure 7.5 Reactions acting on the trunnion brace by the grommet

Figure 7.6 Distribution of forces on both sides of the trunnion brace

Figure 7.7 Distribution of width after the grommet is flattened

Figure 7.8 Bearing stress distribution on brace and stiffeners

Figure 7.9 Forces acting through the shear plate

Figure 7.10 Section properties of the shear plate

LIST OF SYMBOLS

A 0 area of chord cross section

A 1 area of brace cross section

C constant

E young’s modulus

I moment of inertia of member cross section

f yi yield stress of material (the subscript i represents “0” for chord

material, “1” for brace material and “s” for shear material)

f ui ultimate stress of material (the subscript i represents “0” for chord

material, “1” for brace material and “s” for shear material)

T angle between brace and chord

t w thickness of weld

t m thickness of main plate

d 0 outer diameter of chord

t 0 wall thickness of chord

d 1 outer diameter of brace

t 1 wall thickness of brace

d m1 mean diameter of brace

d m1 = d 1 – t 1

A 1 shear area of brace

A p = ½ S d m1 t p

d s depth of shear plate

t s thickness of shear slate

A s shear area of shear plate

A s = d s t s

E diameter ratio between brace and chord, d 1 / d 0

2J diameter to thickness ratio of chord, d 0 / t 0

W thickness ratio between brace and chord, t 1 / t 0

the point where the yield curve starts to deviate from elastic linearity

corresponding to the point where the yield curve starts to deviate fromelastic linearity

F yura load predicted by the numerical method corresponding to Yura

deformation limit of 80Vy E

which corresponds to the deformation at experimental failure load

M bending moment

M u ultimate in-plane bending moment

M u,ip * joint design resistance for in-plane bending moment

M p plastic moment capacity of joint

F ip * applied force to effect the joint design resistance for in-plane bending

moment

S i elastic section modulus of member (the subscript i represents “ 1” for

brace material and “ s” is shear material)

V i shear force (the subscript i represents “ 1” for brace material and “ s” is

shear material)

l distance from chord wall to point to loading

' displacement of trunnion brace

'yura displacement of trunnion brace based on Yura’s deformation limit

I rotation of trunnion brace

Iyura rotation of trunnion brace based on Yura’s deformation limit

N hardening constant used in the Ramberg-Osgood relationship

Hr reference strain used in the Ramberg-Osgood relationship

Vr reference stress used in the Ramberg-Osgood relationship

H true strain

V true stress

d sl grommet sling diameter

w 1 distance from chord wall face to point of loading

w t trunnion brace width

f trunnion flange, 0.75d sl

c clearance distance from keeper plate, 1.5d sl

F v vertical lifting point force of trunnion

F h horizontal component of the lifting point force of trunnion

R i reaction force acting on the chord wall (the subscript i represents “ A”

and “ B” for two adjacent sides of the chord wall)

CHAPTER ONE INTRODUCTION

1.1 Structural applications of hollow sections and trunnions

There are many applications of hollow sections as structural elements Havinginherent properties in resisting compression, tension, bending and torsional forces aswell as low drag coefficients, hollow sections are used widely in offshore structures.Offshore jackets, mobile drilling units, tension leg platforms and even large semi-submersibles use various forms and combinations of tubular structures as theirprimary load carrying members The predominantly axial loads on the tubularbracings and secondary moment at the joints are well studied and provide greaterconfidence in advancing such designs to massive structures as used in offshorestructures nowadays However, there is always an incentive for designers to achievegreater optimisation through extending the use of tubular members and joints forvarious applications in tubular structures In fact, very thick walled hollow sectionshave become more acceptable as the need for larger structures continues to increase.Recently, it has also found wide application in many commercial buildings as

structural elements as well as in airports Thus, hollow section joints continues to bestudied extensively to further enhance its application in many other areas like bridges,towers and offshore platforms Research has been conducted on uni-planar as well asmulti-planar hollow section joints Kurobane (1989) and Lalani (1994) provideinteresting overviews of the recent developments in the research of tubular structuresand joint strength technology

While extensive applications and research have been conducted on hollow sectionconnections, it is pertinent to point out that the fabricated pipe trunnions has not beeninvestigated Only small-scale research was carried out on fabricated plate trunnionrecently as reported by Choo et al (1995), Choo et al (2001a), and Choo et al (2001b).These studies provide some guidance on the possibility in extending the researchwork to cover a wider spectrum of trunnion behaviour A fabricated trunnion consists

of short braces connected to a main chord, and is used as a lift point instead of eyes for very heavy lifts, e.g offshore jackets and topsides As pointed out earlier, aslarger structures are being built for the offshore industry, installation of suchstructures is becoming a critical issue A fabricated trunnion, being a lift point, may

pad-be one of the weak links in the successful installation of offshore structures The mainchord of a fabricated trunnion is usually welded directly onto the main structure sothat it can provide a strong point as well as a pivot for upending sequence ofoperations for offshore jacket installation or very tall towers in refinery plants Thus agood understanding of the structural behaviour of fabricated trunnions, especially itsultimate carrying capacity and failure mechanism, is important and critical in thesuccessful installation of new and larger structures

In order to appreciate the scale and importance of fabricated trunnions, the followingdescriptions of lifting methods and associated use of trunnions in the industry areprovided for reference Figure 1.1 shows an effective use of a fabricated platetrunnion to transfer the loads through the spreader beam for the lifting of a heavytopside module The hook load is directly transferred through the spreader beam andthen down to the fabricated plate trunnion and the module structure

Figure 1.1 An effective use of a fabricated plate trunnions to transfer the loadsthrough a spreader beam for the lifting of a heavy topside module

Figure 1.2 shows a fabricated pipe trunnion installed at an inclined position due to therequirement and design of the structure to transfer the loads through a single liftingpoint for the lifting of a heavy topside module The fabricated pipe trunnion isinstalled on the deck of the topside as shown in the picture insert The picture insertalso shows the large wire ropes that were used to wrap around the brace pipe of thetrunnion Due to the inclined angle from a single point lifting of the Asian Hercules

Enlarged Area

of Plate

crane, there is flexibility where the trunnion can be designed to handle the inclinedloads It is noted also that the trunnions were designed to be installed to orientatealong different directions to accommodate the required lifting arrangement There isgreat feasibility in the design of trunnions for such use, especially in offshoreinstallation where there is greater complexity and unknowns from the workingenvironment Also the simplicity in the overall shape and components of the trunnionmakes it a very suitable lifting point scheme for such offshore structures

Figure 1.2 A fabricated pipe trunnion installed at an inclined position to transfer theloads through a single lifting point for the lifting of a heavy topside module

The lifting of offshore packages, especially topside modules, onto pre-installedjackets at offshore locations is common in the offshore industry Figure 1.3 shows theinstallation of a topside above a jacket by the Heerema DB101 barge crane using

Enlarged Area of

Pipe Trunnion

fabricated pipe trunnions Such topside module could be in the range of 5,000 to10,000 ton in a single lift This coupled with the harsh environment that the offshorestructures are installed; the use of fabricated trunnions can enable consistent safety inthe lifting operation, and provide higher carrying capacity while permitting flexibility

in the arrangement as dictated by the offshore installation requirements

Figure 1.3 The installation of a topside above a platform by Heerema DB101 barge

crane using fabricated pipe trunnions

Another important use of fabricated trunnions relates to its characteristic pipe bracearrangement that provides a pivot point for a heavy offshore structure to be upended

in water As offshore structures are usually dry towed to site on barges lying on itsside, there is a need for the structure to be upended before being installed on location,