Modelling and analysis of riser seabed interaction

MODELLING AND ANALYSIS OF RISER-SEABED INTERACTION CHIEW GEOK HAR NATIONAL UNIVERSITY OF SINGAPORE 2007 _ MODELLING AND ANALYSIS OF RISER SEABED INTERACTION CHIEW GEOK HAR (B.Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2007 i _ ii ACKNOWLEDGEMENT I would like to express my utmost gratitude to my supervisors Professor Koh Chan Ghee and Professor Leung Chun Fai with whose research guidance and patience this thesis was made possible My sincerest thanks go out to research colleagues, Tho Kee Kiat and Teng Mingqing who had been helpful in offering technical assistance on ABAQUS Special thanks to Dr Hua Jun for his research inspiration and expertise I would like to express my heartfelt appreciation to my family and friends who stood by me during this period Lastly, I dedicate this thesis to my father and aunts for their unwavering support and understanding throughout all these years _ TABLE OF CONTENTS TITLE PAGE i ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iii SUMMARY v LIST OF FIGURES vi LIST OF TABLES x NOMENCLATURE xi CHAPTER INTRODUCTION 1.1 BACKGROUND……………………………………………………….1 1.2 SCOPE OF THESIS…………………………………………………… 1.3 LAYOUT OF THESIS………………………………………………… CHAPTER LITERATURE REVIEW 10 2.1 EARLIER WORKS…………………………………………………… 10 2.2 INTERNAL FLOW…………………………………………………… 12 2.3 RECENT NUMERICAL MODELS……………………………………… 14 2.4 RISER-SEABED INTERACTION……………………………………… 17 CHAPTER METHODOLOGY 34 3.1 DESCRIPTION OF PROBLEM………………………………………… 34 3.2 FINITE ELEMENT MODEL…………………………………………… 34 3.3 RISER-SEABED INTERACTION……………………………………… 35 3.3.1 Backbone Curve…………………………………… 39 iii _ 3.3.2 3.4 RISER-SEABED MODELLING…………………………………………42 CHAPTER 4.1 4.2 4.3 Suction Model……………………………………… 40 NUMERICAL VERIFICATION 50 UPLIFT EXPERIMENT……………………………………………… 50 4.1.1 Experimental Procedure…………………………… 50 4.1.2 Numerical Model………………………………… 51 4.1.3 Verification of Numerical Results………………… 52 LARGE-SCALE EXPERIMENT 54 4.2.1 Experimental Setup……………………………… 54 4.2.2 Experimental Procedure…………………………… 55 4.2.3 Numerical Model………………………………… 57 4.2.4 Verification of Numerical Results………………… 59 TOP RISER END SUBJECTED TO VERTICAL CYCLIC MOTION….…… 66 CHAPTER PARAMETRIC STUDIES 96 5.1 EFFECT OF SOIL STIFFNESS………………………………………… 96 5.2 EFFECT OF TOUCHDOWN GEOMETRY……………………………… 98 CHAPTER CONCLUSIONS AND RECOMMENDATIONS 103 6.1 CONCLUSIONS……………………………………………………… 103 6.2 RECOMMENDATIONS……………………………………………… 104 REFERENCES APPENDIX A 105 ABAQUS UMAT SOURCE CODE 108 iv _ v SUMMARY: This dissertation delves into numerical modelling of riser problems focusing on riserseabed interaction Recent findings showed that riser-seabed interaction which played an important role in riser bending fatigue was oversimplified by assuming linear elastic contact behaviour Soil suction as a stress-raising mechanism was ignored by assuming a stiffer seabed Hence, the research is focused on improving the riser-seabed contact model so as to better predict the riser bending stresses Numerical modelling was carried out in ABAQUS where the riser was discretised into 1-D beam elements The riserseabed contact is represented by plastic riser-seabed trusses to incorporate unloading and suction behaviour To accommodate uneven seabed profile or non-uniform geotechnical properties, seabed parameters can be calculated spatially in a pre-processor to be read into ABAQUS For riser-seabed interaction behaviour, bearing capacity theory and Bridge’s suction model are adopted To ensure a complete riser-seabed model, additional load paths are proposed For verification purposes, numerical results are compared against test data from both laboratory and large-scale experiments The laboratory tests investigated the case of a pipe embedded in offshore clay for 72 hours before being pulled out at constant velocity For the large-scale experiment, the riser anchored to the seabed was subjected to vertical actuation displacements to simulate the lay-down and lift-up motion of a riser Numerical analyses with and without suction are carried out to investigate the influence of suction which has been largely ignored in previous interaction models Parametric studies are carried out to analyse the effect of seabed stiffness and touchdown geometry Keywords: riser, suction, touchdown zone, seabed interaction, modelling _ LIST OF FIGURES Figure 1.1: Flexible Riser Configurations……………………………………… Figure 1.2: Lifting up and Laying down of Riser……………………………… Figure 1.3: Schematic Diagram of Problem…………………………………… Figure 2.1: Deep Trenches at Touchdown Zone after Installation…………… 28 Figure 2.2: Full depth FEM Model…………………………………………… 28 Figure 2.3: Reduced Experiment Model……………………………………… 28 Figure 2.4: Uplift Experimental Results (Willis 2001)………………………… 29 Figure 2.5: Suction model for Quasi-static and Fatigue loading (Theti 2001)… 29 Figure 2.6: Adopted Re-contact Point after separation (Theti 2001) ………… 30 Figure 2.7: Bending moment for strain gauge on soft clay seabed (Bridge 2002)……………………… …………………………… 30 Figure 2.8: Suction model for Back-Analysis (Bridge 2002)………………… 31 Figure 2.9: Comparison of Test and Numerical Bending Moment Envelope (Bridge 2002)……………………………… …………………… 31 Figure 2.10: Effect of Pull-out Velocity on Suction Behaviour (Egil 2004)…… 32 Figure 2.11: Bridge Soil Model (Bridge 2004)………………………………… 32 Figure 2.12: Adopted Re-contact Point after Separation (Bridge 2004)…………………………….………………………… 33 Figure 2.13: Displacement-controlled Pipe-penetration Test (Clukey 2005)…… 33 Figure 3.1: FEM Model……………………………………………………… 45 Figure 3.2: Soil Model including Suction……………………………………… 45 Figure 3.3: Load Paths at Different Stages of Interaction……………………… 46 vi _ Figure 3.4: Bearing Width of Riser Pipe……………………………………… 48 Figure 3.5: Truss Force-displacement Behaviour……………………………… 49 Figure 3.6: Truss Stress-strain Behaviour……………………………………… 49 Figure 4.1: Setup for Uplift Experiment……………………………………… 68 Figure 4.2: Experimental Diagram………………………………………………68 Figure 4.3: Numerical Model………………………………………………… 69 Figure 4.4: Experimental and Numerical Results of Uplift Test……………… 69 Figure 4.5: Actuator Unit (Willis 2002)…………………… ………………… 70 Figure 4.6: Top Riser End Connection (2H Offshore 2000)… ……………… 70 Figure 4.7: Plan View of Anchor Arrangement (2H Offshore 2000)………… 70 Figure 4.8: Bearing Width for Trench………………………………………… 71 Figure 4.9(a): Riser Profile during Pull-up……………………………………… 72 Figure 4.9(b): Riser Profile during Lay-down…………………………………… 72 Figure 4.10: Bending Moment across riser during Pull-up……………………… 73 Figure 4.11: Bending Moment across riser during Lay-down………………… 74 Figure 4.12: Comparison of Bending Moment Envelopes……………………… 75 Figure 4.13(a):Comparison of Bending Moment for Strain Gauge A………………75 Figure 4.13(b):Comparison of Bending Moment for Strain Gauge D…………… 76 Figure 4.13(c):Comparison of Bending Moment for Strain Gauge F……………… 76 Figure 4.13(d):Comparison of Bending Moment for Strain Gauge J……………… 77 Figure 4.13(e):Comparison of Bending Moment for Strain Gauge K………………77 Figure 4.14(a):Truss Strains at D and F during Actuation………………………….78 Figure 4.14(b):Truss Stresses at D and F during Actuation……………………… 78 Figure 4.14(c):Stress-strain Relationship of riser-seabed truss at D……………… 79 vii _ viii Figure 4.14(d):Stress-strain Relationship of riser-seabed truss at F……………… 79 Figure 4.15: Actuated Cyclic Motion of Top Riser End………………………… 80 Figure 4.16: Test Locations in Riser Profile…………………………………… 80 Figure 4.17(a):First Actuation Cycle for P………………………………………… 81 Figure 4.17(b):Second Actuation Cycle for P………………………………………81 Figure 4.17(c):Third Actuation Cycle for P……………………………………… 82 Figure 4.17(d):Fourth Actuation Cycle for P……………………………………… 82 Figure 4.17(e):Fifth Actuation Cycle for P………………………………………… 83 Figure 4.18(a):First Actuation Cycle for Q…………………………………………84 Figure 4.18(b):Second Actuation Cycle for Q…………………………………… 84 Figure 4.18(c):Third Actuation Cycle for Q……………………………………… 85 Figure 4.18(d):Fourth Actuation Cycle for Q……………………………………… 85 Figure 4.18(e):Fifth Actuation Cycle for Q……………………………………… 86 Figure 4.19(a):First Actuation Cycle for R………………………………………… 87 Figure 4.19(b):Second Actuation Cycle for R…………………………………… 87 Figure 4.19(c):Third Actuation Cycle for R……………………………………… 88 Figure 4.19(d):Fourth Actuation Cycle for R……………………………………… 88 Figure 4.19(e):Fifth Actuation Cycle for R……………………………………… 89 Figure 4.20(a):First Actuation Cycle for S………………………………………… 90 Figure 4.20(b):Second Actuation Cycle for S………………………………………90 Figure 4.20(c):Third Actuation Cycle for S……………………………………… 91 Figure 4.20(d):Fourth Actuation Cycle for S……………………………………… 91 Figure 4.20(e):Fifth Actuation Cycle for S………………………………………… 92 Figure 4.21(a):First Actuation Cycle for T………………………………………… 93 _ Figure 4.21(b):Second Actuation Cycle for T…………………………………… 93 Figure 4.21(c):Third Actuation Cycle for T……………………………………… 94 Figure 4.21(d):Fourth Actuation Cycle for T……………………………………… 94 Figure 4.21(e):Fifth Actuation Cycle for T…………………………………………95 Figure 5.1: Assumed Touchdown Geometry……………………………………100 Figure 5.2: Effect of Seabed Stiffness on Moment Envelope (Flat Seabed)…… 100 Figure 5.3: Effect of Seabed Stiffness on Moment Envelope (0.6m-deep trench)………………………………………………… 101 Figure 5.4: Effect of Seabed Stiffness on Riser Profile (0.6m-deep trench)…… 101 Figure 5.5: Effect of Trench Depth on Moment Envelope…………………… 102 Figure 5.6: Effect of Trench Depth on Riser Profile…………………………… 102 ix _ 101 Effect of Seabed Stiffness on Moment Envelope (0.6m-deep Trench) 15 10 Bending Moment (kNm) 0 20 40 60 80 100 120 -5 -10 -15 -20 -25 -30 -35 Span (m) lower limit with suction(Suo=0.4kPa) typical with suction(Suo=0.87kPa) upper limit with suction(Suo=1.27kPa) Figure 5.3 lower limit without suction(Suo=0.4kPa) typical without suction(Suo=0.87Pa) upper limit without suction(Suo=1.27) Effect of Seabed Stiffness on Moment Envelope (0.6m-deep Trench) Effect of Seabed Stiffness on Riser Profile (0.6m-deep Trench) 0.6 0.4 Height (m) 0.2 0.0 20 40 60 -0.2 -0.4 -0.6 -0.8 Span (m) lower limit with suction(Suo=0.4kPa) typical with suction(Suo=0.87kPa) upper limit with suction(Suo=1.27kPa) Trench profile Figure 5.4 lower limit without suction(Suo=0.4kPa) typical without suction(Suo=0.87Pa) upper limit without suction(Suo=1.27) Effect of Seabed Stiffness on Riser Profile (0.6m-deep Trench) 80 _ 102 Effect of Trench Depth on Bending Moment Envelope 15 10 Bending Moment (kNm) 0 20 40 60 80 100 120 -5 -10 -15 -20 -25 -30 -35 Span (m) Trench depth=0.0m Figure 5.5 Trench depth=0.2m Trench depth=0.4m Trench depth=0.6m Effect of Trench Depth on Moment Envelope Effect of Trench Depth on Riser Profile 1.2 Height (m) 0.8 0.4 0.0 20 40 80 60 -0.4 -0.8 Span (m) Flat Surface trench depth=0.0m Figure 5.6 0.2m-deep profile 0.2m 0.4m-deep profile trench depth=0.4m 0.6m-deep profile trench depth=0.6m Effect of Trench Depth on Riser Profile _ 103 CHAPTER CONCLUSIONS AND RECOMMENDATIONS In this dissertation, the velocity-dependent Bridge’s suction model is incorporated numerically into the material model for plastic seabed trusses Additional load paths have been proposed to complement the existing model A brief conclusion on the feasibility of this approach as well as issues involving riser-seabed interaction is made Recommendations for further studies are also suggested 6.2 CONCLUSIONS The modelling of riser-seabed interaction using user-defined plastic trusses is simple to implement and robust enough to incorporate suction as well as soil degradation in future Through laboratory and large-scale experiments, the accuracy of the Bridge’s suction model is verified By characterising re-penetration behaviour from suction which is common in typical riser scenarios, the proposed load paths have ensured the completeness of the interaction model to describe riser-seabed behaviour especially for cyclic riser motions Several improvisations are made regarding the bearing width of riser resting on the trench and the touchdown geometry assumed A major contribution is the numerical incorporation of Bridge suction model whereby pullout velocity is updated at every riser-seabed truss at each time-step during unloading Through the numerical analyses, suction which causes peak local sagging moment is found to make a significant contribution to riser bending fatigue The simplified _ 104 approach of assuming higher seabed stiffness to ignore suction is not reliable as it may underestimate or overestimate riser fatigue For the parametric study, numerical findings indicated higher bending fatigue with stiffer seabed especially for uneven seabed With the development of trenches at the touchdown geometry, bending moment envelope widens with time Hence, the deep trenches formed with time accelerated riser fatigue and should be accounted for in riserseabed interaction 6.2 RECOMMENDATIONS This dissertation has verified the numerical model against experimental results in static analysis Further developments are possible The case of varying soil strength across the seabed can be carried with the existing numerical model The numerical model can be easily extended to dynamic analysis in ABAQUS to include wave, current forces and internal flow For dynamic analysis, heave motion at the top riser end that is critical for riser fatigue at TDP can be considered Alternatively, the critical case of transverse motion at the riser top end in three-dimensional modeling can be carried out to investigate the bending fatigue caused by riser-seabed interaction with the trench walls in the transverse direction For experiments, most pipe pull-out tests are conducted using artificially large loading in order to capture the pipe-soil interaction behaviour However, risers are generally light in water and pull-out tests using the self-weight of the pipe would be closer to actual riser scenarios _ 105 REFERENCES S Ahmad and T K Datta (1989) “Dynamic response of marine risers”, Engineering Structures, 11(3), pp 179-188 S Ahmad (1992) “Nonlinear response analysis of marine risers”, Computers and Structures, 43(2), pp 281-295 P Bar-Avi (2000) “Dynamic response of risers conveying fluid”, Journal of Offshore Mechanics and Arctic Engineering, Transactions of the ASME, 122(3), pp 188-193 C Bridge and N Willis (2002) “Steel catenary risers - Results and conclusions from large scale simulations of seabed interaction”, Proceedings of the 14th Annual Conference Deep Offshore Technology C Bridge et al (2003) “Full-scale model tests of a steel catenary riser”, Advances in Fluid Mechanics, 36, Fluid Structure Interaction II, pp 107-116 C Bridge et al (2004) “Steel catenary riser touchdown point vertical interaction models”, Proceedings of the Offshore Technology Conference, 16628 C Bridge and N Willis (2004) “SCR seabed interaction experiments”, Journal of Offshore Technology, 12(3), pp 23-37 Y T Chai, K S Varyani and N D P Barltrop (2002) “Three-dimensional LumpMass formulation of a catenary riser with bending, torsion and irregular seabed interaction effect”, Ocean Engineering, 29(12), pp 1503-1525 _ 106 E Clukey et al (2005) “Model tests to simulate riser-soil interaction effects in touchdown point region”, International Symposium on Frontiers in Offshore Geotechnics, pp 651-658 B M Das (1993) Principles of soil dynamics, PWS-Kent Publication Company, Boston E Giertsen, R Verley and K Schroder (2004) “CARISIMA a catenary riser/soil interaction model for global riser analysis”, Proceedings of the 23rd International Conference on Offshore Mechanics and Arctic Engineering, pp 633-640 S Kao (1982) “Assessment of linear spectral analysis method for offshore structures via random sea simulation”, Journal of Energy Resources Technology, Transaction of the ASME, 104(1), pp 39-46 C L Kirk (1985) “Dynamic response of marine risers by single wave and spectral analysis methods”, Journal of Applied Ocean Research, 7(1), pp 2-13 J E Kokarakis (1987) “Nonlinear three-dimensional dynamic analysis of marine risers”, Journal of Energy Resources Technology, Transactions of the ASME, l09(3), pp l05-111 L P Krolikowski and T A Gay (1980) “An improved linearization technique for frequency domain riser analysis”, Proceedings of the Offshore Technology Conference, 3777 R S Langley (1984) “The linearization of three dimensional drag force in random seas with current”, Applied Ocean Research, 6(3), pp 126-131 _ 107 B B Mekha (2001) “New Frontiers in the Design of Steel Catenary Risers for floating production systems”, Journal of offshore mechanics and arctic engineering, Transactions of the ASME, 123, pp 153-158 P J O’Brien and J F McNamara (1989) “Significant characteristics of threedimensional flexible riser analysis”, Engineering Structures, 11(4), pp 223-233 M H Patel (1984) “Finite-element analysis of the marine riser”, Engineering Structures, 6(3), pp 175-184 M H Patel and F B Seyed (1989) “Internal flow-induced behaviour of flexible risers” Engineering Structures, 11(4), pp 266-280 W Raman-Nair and R E Baddour (2003) “Three-dimensional dynamics of a flexible marine riser undergoing large elastic deformations”, Multibody System Dynamics, 10(4), pp 393-423 F B Seyed and M H Patel (1992) “Mathematics of flexible risers including pressure and internal flow effects”, Marine Structures, 5(2-3), pp 121-150 F B Seyed and M H Patel (1995) “Review of flexible riser modelling and analysis techniques”, Engineering Structures, 17(4), pp 293-303 A W Skempton (1951) "The bearing capacity of clays", Proceedings Building and Research Congress, 1, pp 180-189 R Theti (2001) “Soil interaction effects on simple catenary riser response”, Pipes and Pipelines International, 46(3), pp 15-24 2H Offshore Engineering Ltd (2002) “Effects of riser/seabed interaction on SCRs”, Report No 1500-RPT-008 _ 108 2H Offshore Engineering Ltd (2000) “STRIDE JIP PHASE III”, SUT Talk (http://www.2hoffshore.com/technical_papers/papers/2000/pap046.pdf) N R T Willis and P T J West (2001) “Interaction between deepwater catenary risers and a soft seabed: Large scale sea trials”, Proceedings of the Offshore Technology Conference, 13113 Yong Bai (2001) Pipelines and risers, Elsevier Science Ltd, New York _ 109 APPENDIX A ABAQUS INPUT FILE SUBROUTINE UMAT(STRESS,STATEV,DDSDDE,SSE,SPD,SCD, RPL,DDSDDT,DRPLDE,DRPLDT, STRAN,DSTRAN,TIME,DTIME,TEMP,DTEMP,PREDEF,DPRED,CMNAME, NDI,NSHR,NTENS,NSTATV,PROPS,NPROPS,COORDS,DROT,PNEWDT, CELENT,DFGRD0,DFGRD1,NOEL,NPT,LAYER,KSPT,KSTEP,KINC) C INCLUDE 'ABA_PARAM.INC' C CHARACTER*80 CMNAME DIMENSION STRESS(NTENS),STATEV(NSTATV), DDSDDE(NTENS,NTENS),DDSDDT(NTENS),DRPLDE(NTENS), STRAN(NTENS),DSTRAN(NTENS),TIME(2),PREDEF(1),DPRED(1), PROPS(NPROPS),COORDS(3),DROT(3,3),DFGRD0(3,3),DFGRD1(3,3) C adaptation from elastictrussmat4.f C Declaration of variables INTEGER:: OPENSTATUS,INPUTSTATUS,STAT,MEMSTATUS,CABLE_ELE,NUMBER,prevstat REAL(8):: STRESSMIN,STRAINMIN,ELELENGTH,ALPA,STRAINMIN2 REAL(8):: STRESSINTER,LEINTER,LEMAX,STRESSMAX REAL(8):: PI,NC,SUO,SUG,D,X,L,V,W,Y,LE,GRAD,STRAND,FORCE,DISP,B,DTXDE REAL(8):: E1,E2,E3,E4,E5,E6,sigmau,C,dstressde,dbde,dncde,A4,A5,A6 REAL(8):: kc,kv,kt,V,kf,nf,kdv,kdt,kd,nd,Q,BREAKOUT,tim,fc,T_D,T_X,R1,R2 C Initialisation LEINTER=0.0D0 STRESSINTER=0.0D0 STRESSMAX=0.0D0 LEMAX=0.0D0 GRAD=0.0D0 Q=0 BREAKOUT=0 FC=0 V=0 NC=PROPS(1) SUG=PROPS(3) D=PROPS(4) T_D=0.5D0*D SUO=PROPS(2)+T_D*SUG R1=3.0D0*D/2 R2=D/2.0D0 L=PROPS(5) ELELENGTH=PROPS(6) FORCE=PROPS(7) DISP=PROPS(8) IF (KSTEP==1 and KINC==1) THEN STATEV=0 ENDIF STRESSMIN=STATEV(1) STRAINMIN=STATEV(2) STAT=STATEV(3) SLOPE=STATEV(5) LEMAX=STATEV(6) STRESSMAX=STATEV(7) _ 110 kf=0.98D0 nf=0.21D0 kd=0.83D0 nd=0.19D0 TIM=72.0D0*60/(365.0D0*24*60) PI=3.141592654 PREVSTAT=STAT IF (KSTEP0) STOP "CANNOT OPEN FILE" NUMBER=NOEL-100000 READ(10,FMT='(ES17.9)',REC=NUMBER,IOSTAT=INPUTSTATUS) STATEV(4) CLOSE(10) X=STATEV(4) DDSDDE(1,1)=FORCE*(L)/(D*(DISP+T_D+X)) END IF IF (KSTEP==2 or kstep==4) THEN IF (KINC==1) THEN DDSDDE(1,1)=SLOPE ENDIF END IF IF (KSTEP==3) THEN IF (KINC==1 AND STRESS(1) 0.000000000000001) THEN STRAN(1)=STRAN(1)+DSTRAN(1) STRAND=(-1)*STRAN(1)*(L)-X-T_D IF (STRAN(1) >= E1) THEN STRESS(1)=0.0000000001*(STRAN(1)-E1) DDSDDE(1,1)=0.0000000001 STAT=0 STATEV(8)=0.0D0 STRESSMIN=0.0 STRAINMIN=0.0 ELSE IF (STRAN(1)D OR T_X[...]... inadequacy of existing riser models in modelling riser- seabed interaction The riser at the touchdown zone was recently identified as a fatigue hotspot which substantially reduced the riser fatigue life The source of bending fatigue was the repetitive motions of lifting up and lying down on the seabed causing the riser to experience contact and separation from the seabed This cyclic riser- seabed interaction. .. regarding the riser limits in certain extreme scenarios 1.2 SCOPE OF THESIS The main aim of this thesis is to adopt a more realistic riser- seabed interaction model for riser analysis by considering suction Part of the research scope involves the incorporation of the velocity-dependent Bridge suction model into the material model for riser- seabed trusses representing the riser- seabed interaction in... vertical cyclic motion of the top riser end Parametric studies are then conducted to investigate the effect of seabed stiffness and touchdown geometry especially deep trenches on riser bending stresses 1.3 LAYOUT OF THESIS Chapter 1 gives a brief background history of the riser The forces acting on the riser and the issues influencing riser design are introduced Areas of concern affecting riser fatigue life... both ends, a riser with stiffness and pinned at seabed and a riser with stiffness fixed at seabed end A similar result for the last two cases showed that the boundary condition at the seabed end was not significant In the fourth case, the riser was pinned at seabed end and the motion of the top end was specified with heave and surge to model vessel motion 2.4 RISER- SEABED INTERACTION Despite accounting... environmental loadings A better understanding of the riser behaviour allows a more accurate prediction of the fatigue life of the riser as well as more appropriate design codes for riser implementation Furthermore, the identification of local failure zones in the riser can facilitate modifications of the riser to improve its fatigue performance Numerical modelling of the riser before installation also allows... loopholes of the existing interaction model and the need for load paths proposed to complete the riser- seabed interaction model In Chapter 5, parametric studies are carried out to investigate the effect of touchdown geometry and seabed stiffness A flat seabed profile with progressively deepened trenches at the touchdown area is analysed to investigate the impact of deep trench formation on riser stresses... floater motions by change of geometry without the use of heave compensation equipment There exist several riser configurations for the flexible riser system, such as free hanging catenary riser, lazy S riser, steep S riser, lazy wave riser, steep wave riser and pliant wave riser as shown in Figure 1.1(b)-(f) The earliest riser configuration used was the free hanging catenary riser with great resemblance... displacement lB : Level of seabed truss’s bottom end lR : Level of riser at initial undeformed configuration lSEABED : Seabed level LELE : Length of beam element on seabed LT : Length of seabed truss NC : Non-dimensional shape and depth factor Nx : Drag force along x-axis Nz : Drag force along z-axis PI : Internal pressure of riser PO : Outer pressure of riser xii _ xiii... caused the riser at touchdown zone to oscillate between hogging moments upon contact and sagging moments when suspended as shown in Figure 1.2 causing bending fatigue Unaware of the significance of riser- seabed interaction behaviour and touchdown geometry on riser fatigue, existing riser models unrealistically simplified the riser- seabed behaviour by assuming rigid or linearly elastic flat seabed (Chai... Current practice of assuming a stiffer seabed to overlook stress-raising mechanisms is too simplified to capture the complexity of the interaction or give an accurate prediction of riser stresses Attempts have been made by Theti (2001) and Clukey (2005) to describe actual riserseabed behaviour and trench formation Main contributor to riser fatigue is the frequent vertical riser movements of the pipe responsible ... _ MODELLING AND ANALYSIS OF RISER SEABED INTERACTION CHIEW GEOK HAR (B.Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE... delves into numerical modelling of riser problems focusing on riserseabed interaction Recent findings showed that riser- seabed interaction which played an important role in riser bending fatigue... displacement lB : Level of seabed truss’s bottom end lR : Level of riser at initial undeformed configuration lSEABED : Seabed level LELE : Length of beam element on seabed LT : Length of seabed truss NC