FINITE ELEMENT ANALYSIS OF PIPELINE GLOBAL WALKING WITH SPANNING AND LATERAL BUCKLING

Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014 June 8-13, 2014, San Francisco, California, USA OMAE2014-24159 FINITE ELEMENT ANALYSIS OF PIPELINE GLOBAL WALKING WITH SPANNING AND LATERAL BUCKLING Gang Duan, Andy Tang, Xinhai Qi, and Jianxia Zhong GENESIS, Houston, Texas, U.S.A. ABSTRACT This paper investigates High Pressure and/or High Temperature (HPHT) design of a pipeline across an escarpment with significant seabed undulations and elevation change from drill center (DC) to riser touch down point (TDP). The pipeline has a tendency to walk towards the riser during pipeline start-up / shut-down (SD) operations due to seabed slope and riser bottom tension in a case study. A hold-back pile at the uphill pipeline end near DC to arrest walking, along with the riser bottom tension applied at the other end of the pipeline, results in significant tensions in the pipeline and substantial spans over escarpment. The pipeline spanning lengths, lateral buckling amplitudes and walking distances and their variations during the start-up / shut-down cycles are presented and discussed. Both wet-insulated single pipe (WISP) and pipe-in-pipe (PIP) options are studied and compared. Interactions among pipeline global walking, spanning and lateral buckling are observed. In-depth understanding of pipeline systematic response will help perform a safe and cost-effective pipeline design. 1. INTRODUCTION Deep water pipelines are commonly designed to transport HPHT multiphase fluids. HPHT pipeline design is a continuing and significant challenge [1]. When the pipeline has long free spans, the interaction of the spanning and lateral buckling with walking makes design more complicated. This paper presents a case study of interactions of pipeline walking with spanning and lateral buckling. and subsea structures at the pipeline ends. Lateral buckles can be initiated by pipeline out of straightness (OOS) features, uneven seabed bathymetry or engineered buckle initiators (e.g., sleepers) [2]. The observations show that, when free spans exist, pipeline thermal expansion can feed pipe into free spans besides lateral buckles and hence the pipeline overall behavior including walking will be affected. Pipeline walking is a phenomenon of the ratcheting response in the pipeline axial displacement due to the start-up / shut-down cyclic operation [3]. The short and hot pipelines have a greater tendency to undergo global walking. The long and hot pipelines are deemed as a series of short pipelines divided by the lateral buckles and/or spans. Therefore, the interactions among spans and buckles may impact the pipeline walking behavior. In a case study, the interactions of pipeline walking with spanning and lateral buckling are investigated by finite element analysis using the three dimensional (3D) finite element package Abaqus [4]. The results of the study include observations of variations of pipeline free spans, lateral buckles, thermal expansion and walking, as well as anchor pile reactions, over operational shutin cycles. The results of 8 WISP and 8X12 PIP responses are compared and discussed. 2. SYSTEM AND MODELING INPUT The key inputs of this study are presented in this section. The exposed hot pipelines may experience lateral buckling subject to high compressive force, because the thermal expansions are restrained by the soil underneath the pipeline 1 Copyright 2014 by ASME

2.1 Pipeline Route and Seabed Profile The pipeline is routed as a straight line directly from the DC to riser TDP. The total length of the pipeline is approximately 21000 ft. The water depths at DC and riser TDP are approximately 5000 ft and 7100 ft, respectively. The axial and lateral seabed slopes along the pipeline route are shown in Figure 1 and Figure 2, respectively, along with seabed elevation. FIGURE 1. SEABED ELEVATION AND AXIAL SLOPE FIGURE 2. SEABED ELEVATION AND LATERAL SLOPE 2.2 Pipe Geometry and Material Properties API SP 5L grade X-65 seamless line pipe is assumed in this study [5]. The design data of the pipeline and line pipe material properties are listed in Table 1. The nominal pipe outer diameter (OD) for the WISP pipeline is 8.625. For the PIP option, the nominal pipe OD of the inner pipe is 8.625, while the nominal OD for the outer pipe is 12.75. The pipe wall thickness for the WISP and PIP are presented in Table 2 calculated based on the water depth of pipeline route and design data [6]. A 5LPP insulation coating is added to the WISP pipeline to protect the production system from hydrate and wax formation with a targeted overall heat transfer coefficient (OHTC) of 0.6 BTU/hr.ft 2.F. The insulation coating thickness and equivalent density are listed in Table 2. TABLE 1. PIPELINE DESIGN DATA AND LINE PIPE MATERIAL PROPERTIES Parameter Unit Data Design Data Design Pressure psi 9000 Design Temperature Ambient Temperature o F 230 o F 39 Operation Pressure psi 6500 Operation Temperature o F 160 Shut-down Pressure psi 1900 Shut-down Temperature o F 39 Contents Density pcf 52.4 Pipe and Material Properties Steel Density pcf 490 Young's Modulus psi 2.90E+07 Specified Minimum Yield Strength of Steel, SMYS psi 65300 Specified Minimum Tensile Strength of Steel, SMTS psi 77600 Poisson's Ratio - 0.3 Thermal Expansion Coefficient 1/ o F 6.50E-06 TABLE 2. PIPE GEOMETRY Parameter Unit Data 8 WISP Pipe OD inch 8.625 Pipe Wall Thickness inch 0.827 5LPP Insulation Thickness inch 3.41 5LPP Insulation Density pcf 45.01 8 Inner Pipe Pipe OD inch 8.625 Pipe Wall Thickness inch 1.063 12 Outer Pipe Pipe OD inch 12.75 Pipe Wall Thickness inch 0.827 2.3 Pipe-Soil Interaction Properties Pipeline spanning, lateral buckling and global walking behavior are strongly linked to the pipe-soil interaction. Accordingly, all pipe-soil interaction parameters (equivalent 2 Copyright 2014 by ASME

axial and lateral coefficients of friction and soil vertical stiffness) were properly defined, to characterize soil behavior commonly noted in the Gulf of Mexico. The soil axial resistance under pipeline global walking condition is primarily dominated by drained behavior during the entire design life, because the majority of the pipeline sections move slowly during each start-up and shut-down process [7]. The soil axial friction coefficient is assumed to be 0.5 in the study using the best estimate condition for the whole design life to simplify this case study. The soil lateral resistances including break-out and residual behavior are considered. The axial and lateral soil friction coefficients (pseudo friction factors) and their mobilization distances are presented in Table 3. TABLE 3. PIPE-SOIL INTERACTION FRICTION Parameter Data 8 WISP Friction Coefficients Axial Friction 0.50 Lateral Friction - Breakout 0.75 Lateral Friction - Residual 0.60 8X12 PIP Friction Coefficients Axial Friction 0.50 Lateral Friction - Breakout 0.80 Lateral Friction - Residual 0.65 8 WISP and 8X12 PIP Mobilization Distances Axial Friction Mobilization Distance Lateral Breakout Friction Mobilization Distance Lateral Residual Friction Mobilization Distance 1% Pipe OD 20% Pipe OD 100% Pipe OD 2.3 Pipeline End Boundary Conditions The DC end of the pipeline is assumed to be terminated at a PLET (pipeline end termination), and the other end of the pipeline is connected to a steel catenary riser (SCR). The pipeline SCR end is subjected to SCR bottom tension, while the pipeline end at PLET has two options of boundary conditions analyzed: Option 1: PLET resistance only. A 25-kip equivalent resistance opposite to the pipe end movement is assumed. Since the lower bound of PLET foundation resistance is slightly greater than 25 kip based on preliminary geotechnical analyses, this assumption is reasonably conservative to determine if walking control is required for the pipeline under investigation. Option 2: PLET resistance and hold-back anchor. Besides the PLET resistance, a suction pile is installed and connected to the PLET to prevent the pipeline from walking downhill. The maximum load capacity of the suction pile is sufficiently assumed to be 500 kips. The resistance / loading of the pipeline end boundary conditions are shown in Table 4. DC End TABLE 4. PIPELINE BOUNDARY CONDITIONS Parameter Unit Data Option 1:PLET Resistance Only kips 25 Nonlinear Force- Option 2:PLET with Suction Pile Displacement Curve up to 500 kips SCR End (Static SCR Bottom Tension) As Laid kips 84.3 Hydrotest kips 110.4 Design & Operation kips 105.7 Shut down kips 105.7 3. ANALYSIS METHODOLOGY This section discusses the non-linear finite element analysis methodology for pipelines using Abaqus software package [8]. The Abaqus model is applied to assess pipeline thermal expansion, lateral buckling, global walking, and free spanning, etc. 3.1 Pipe Element The Abaqus thick wall PIPE31H hybrid beam element with nonlinear material curve for API SP 5L X65 is used to model the pipeline. The element has the capacity to capture the pipeline axial stress, hoop stress, and radial stress over the pipe wall and can therefore include the general effects of internal and external pressure loadings on the stress state of the pipe. The hybrid element formulation is recommended as it improves convergence behavior. 3.2 Seabed Element The 4-node 3D bilinear quadrilateral rigid elements are utilized to construct a discrete rigid surface, which functions as the 3D uneven seabed. 3.3 Boundary Conditions The interaction between pipeline and seabed is defined as soft node to surface contact, and the effect of soil resistance on the pipeline is accounted for by the equivalent frictional coefficients in both axial and lateral directions (Table 3) through Abaqus FRIC user subroutine. The PLET resistance and SCR bottom tension are simulated with equivalent forces applied to the pipeline ends. 3 Copyright 2014 by ASME

3.4 Loading Sequence The major loading sequences for the pipeline finite element analysis are presented in Table 5. Only pipeline full shut down cycles are simulated in this study because normally it dominates pipeline walking compared to other types of operational cycles. The riser bottom tension is applied to the pipeline riser TDP end from step 3 with the equivalent force listed in Table 4. The riser tension is updated accordingly under different load conditions, e.g., as-laid, hydrotest, operation, and shut-down. TABLE 5. LOADING SEQUENCE FOR THE PIPELINE FINITE ELEMENT ANALYSIS Step No. Description of Major Analysis Steps Figure 4 presents the case of the boundary condition option of a hold-back anchor connected to the PLET. A 25-kip end force is shown at the DC end in Figure 3, which is corresponding to the PLET resistance opposite to the pipeline axial movement (pipeline under compression in operation, pipeline under tension during shut down). Effective axial tensile force keeps increasing at DC end when shut-down condition is repeated (Figure 4). It is because the hold-back anchor prevents the pipeline DC end from walking downhill. Figures 3 and 4 show the WISP pipeline spans and lateral buckles locations for the two options of different boundary conditions based on Abaqus results. Details of lateral buckling amplitude and span length variations over loading cycles are shown in sections 4.3 and 4.4, respectively. As-Laid Condition 1 Establish pipeline on flat seabed 2 Apply residual lay tension to pipeline end 3 Lower the pipeline to uneven 3D seabed Hydrotest Condition 4 Flood the pipeline 5 Apply the hydrotest pressure 6 Return the pipeline pressure to flood condition Operation Condition (Start-up / Shut-down) 7 8 Apply uniform operation pressure / temperature Apply uniform pressure / temperature under shut-down condition 4. RESULTS AND DISCUSSIONS The results of the study are presented in this section, which include effective axial force, pipeline expansion and global walking, lateral buckling, free spanning, and anchor pile load, etc. FIGURE 3. EFFECTIVE AXIAL FORCE FOR WISP WITHOUT 4.1 Pipeline Effective Axial Force The effective axial force profile is the basics to understand the pipeline response to the applied load. The pipeline end effective axial force is consistent with the boundary conditions, while the reductions of the effective axial force along the effective axial force profile are caused by free spans or lateral buckles. Figures 3 and 4 show the WISP pipeline effective axial force under as-laid, hydrotest, operation cycles 1, 5, 10 (namely, OP-1, OP-5, and OP-10 heat-up), and shut-down cycles 1, 5, 10 (namely, SD-1, SD-5, and SD-10). Figures 5 and 6 show the combined effective axial forces of the PIP pipelines as those of WISP pipeline in the same loading conditions. FIGURE 4. EFFECTIVE AXIAL FORCE FOR WISP WITH The differences in the effective axial force shown between Figure 3 and Figure 4 are caused by the differences in boundary conditions at the pipeline DC end. Figure 3 covers the case for the boundary condition option of no hold-back anchor, while Figures 5 and 6 show locations of the PIP pipeline span. The PIP pipeline feeds thermal expansion into the corresponding span locations that results in decrease in the span gaps and lengths, but do not generate lateral buckling at the 4 Copyright 2014 by ASME

same locations as WISP pipeline. This is because the increased axial resistance prevents excessive thermal expansion and the increased lateral resistance makes lateral buckle initiation more difficult, for the heavier PIP pipeline with the higher bending inertia. 4.2 PIPELINE EXPANSION AND WALKING The WISP and PIP pipeline axial displacements along the pipelines are presented in Figures 7 to 10. Figure 7 shows the axial displacement of WISP pipeline without anchor. The DC end walks approximately 5 ft towards the riser end in the first ten cycles, while the riser TDP end walks slightly more than 10 ft. The extra 5-ft displacement at TDP is mainly because of the continuing pull out of spans under increasing effective tensions over thermal cycles, which indicates the interactions of pipeline walking and spanning. Figure 8 shows the axial displacement of WISP pipeline with anchor connected to the DC end PLET. The DC end walking is arrested, while the riser TDP end walks approximately 10 ft over ten thermal cycles. The 5-ft DC end axial displacement shown in Figure 7 does not contribute to the riser end walking. The pipeline displacement change near the DC end only affects the lateral buckle nearby, which is shown in Figure 13. This phenomenon indicates the interaction between pipeline walking and lateral buckling. FIGURE 5. EFFECTIVE AXIAL FORCE FOR PIP WITHOUT FIGURE 7. AXIAL DISPLACEMENT FOR WISP WITHOUT FIGURE 6. EFFECTIVE AXIAL FORCE FOR PIP WITH Span locations near the riser end for the WISP and PIP pipelines are close as shown in Figures 3 to 6. These spans are relatively long compared with the spans near the DC. For the span of the WISP pipeline near the DC, the span is dramatically shortened and even diminishes under operation. Then lateral buckle is formed due to increase effective compressive forces in the area. There are two lateral buckles occurring in WISP pipelines close to DC end initiated from two spans (Figures 3 and 4). The WISP pipeline feeds into the spans during the operation and fill the span gap, and the continuing feedings push the pipeline move laterally and generate two lateral buckles. FIGURE 8. AXIAL DISPLACEMENT FOR WISP WITH 5 Copyright 2014 by ASME

Figures 9 and 10 present the PIP pipeline axial displacements. It is observed that no lateral buckle occurs along the PIP pipeline, and the two spans near DC end for the PIP pipeline still exist; while the WISP pipeline has two lateral buckles due to excessive axial displacement shown in Figure 7 and 8. This is due to the increased axial and lateral resistance, and higher bending inertia making PIP more difficult to buckle laterally. restrained by the hold-back anchor when it is installed (Figure 11). Figure 12 shows that PIP pipeline has no walking tendency at SCR TDP end and the hold-back anchor has no effects on the riser end, while WISP pipeline riser end will walk axially about 25 ft under 30 start-up / shut-down cycles when no hold-back pile is installed. The hold-back pile at DC end has very small effects on the WISP pipeline riser end displacement in the first ten operation cycles, but will start to arrest the WISP pipeline riser end walking after ten operation cycles. FIGURE 9. AXIAL DISPLACEMENT FOR PIP WITHOUT FIGURE 11. PIPELINE END WALKING (DC END) FIGURE 10. AXIAL DISPLACEMENT FOR PIP WITH The WISP and PIP pipeline end expansion and walking at the DC end and the SCR TDP end are compared in Figures 11 and 12, respectively. Figure 11 shows that the WISP pipeline has more tendencies to walk downhill towards riser with a displacement rate of 6.4 in/cycle for WISP PLET. In contrast, the PIP pipeline displacement rate is 0.8 in/cycle for PIP PLET when no hold-back anchor pile is installed at the DC end. For both WISP and PIP pipelines the global walking propensity is FIGURE 12. PIPELINE END WALKING (SCR TDP END) 4.3 PIPELINE LATERAL BUCKLING The PIP pipeline does not have lateral buckles, which has been stated in Sections 4.1 and 4.2. The amplitudes of a lateral buckle over various cycles for WISP pipeline are presented in Figures 13 and 14 for the different boundary conditions. The close-up views of this lateral buckle are shown in Figures 13 and 14 to present the details of the buckling modes, amplitude 6 Copyright 2014 by ASME

variations for comparison. The other buckle nearby is minor and not presented in this paper. Both WISP pipeline with or without end anchor do not have lateral buckles under hydrotest condition, and the lateral buckling amplitude of the first operation (OP-1) is approximately the same, 10 ft, because the chains attached to the hold-back pile have not been fully stretched due to the slack in the as-laid condition. The pipeline section will keep feeding into the lateral buckle location near DC end when no hold-back pile is installed to arrest the walking. The lateral buckling amplitude increases from 10 ft to 18 ft in ten cycles due to the pipeline walking, which indicates the interaction between lateral buckling and walking. The increase of lateral buckling amplitude is constrained for the WISP pipeline when pipeline end at PLET is attached to a hold-back anchor pile because the anchor prevents the pipeline walking at DC and feeding thermal expansion into the lateral buckling area as shown in Figure 14. The lateral buckle is initiated from the applied load and the three dimensional seabed terrains as shown in Figures 13 and 14. A hold-back pile at the uphill pipeline along with the riser bottom tension applied at the other end of the pipeline, results in significant tensions in the pipeline as shown in this study. The right sides of the lateral buckles are pulled unsymmetrically, which is due to the extensive tensions generated from the SCR bottom tension transferred all the way from riser end under shut-down condition in combination with the hold back anchor as shown in Figure 14. The compressive mechanical strains at the buckle crest are listed in Table 6 for different loading conditions. The strains in Case 1 are slightly larger than those in Case 2 due to the pipeline walking feeding into the lateral buckling location. Figure 13 shows the increased buckling amplitude over operation cycles. FIGURE 14. WISP PIPELINE LATERAL BUCKLING (PLET END WITH ) TABLE 6. LONGITUDINAL STRAINS (COMPRESSION) OF PIPELINE AT BUCKLE CREST Load Longitudinal Mechanical Strain Condition Case 1 Case 2 OP-1-0.13% -0.13% OP-5-0.14% -0.11% OP-10-0.14% -0.10% SD-1-0.11% -0.11% SD-5-0.11% -0.08% SD-10-0.11% -0.07% Note: Case 1: WISP Pipeline without Anchor Case 2: WISP Pipeline with Anchor 4.4 PIPELINE SPANNING There are more than ten spans along the pipeline route trigged by the seabed undulations. The maximum span lengths under as-laid, operation cycles 1, 5, 10 (OP-1, OP-5, and OP- 10), and shut-down cycles 1, 5, 10 (SD-1, SD-5, and SD-10) are listed in Table 7. FIGURE 13. WISP PIPELINE LATERAL BUCKLING (PLET END WITHOUT ) The pipeline thermal expansion feeds into the span locations as the pipeline expands under operation condition, so the maximum span length under operation condition is shorter than pipeline under as-laid condition, as shown in Table 7. The pipeline is subjected to excessive effective axial tension under shut-down condition, especially when a hold-back anchor is installed at DC end. Normally the pipeline span lengths under shut-down conditions are slightly longer than that in the previous operation state, which is consistent with the PIP pipeline span lengths (Table 7, Cases 3 and 4). The maximum 7 Copyright 2014 by ASME

span length for PIP under operation gradually decreases over thermal cycles because the pipeline continuously feeds into the span location. The maximum span length for WISP pipeline keeps increasing as the shut-down condition is repeated (Table 7, Cases 1 and 2), due to excessive effective axial tension along the pipeline, and the stretched pipeline sections ratchets towards the riser TDP end, which results in more pipeline end displacement. This shows the interaction between pipeline spanning and walking. The maximum span lengths shown in Table 7 are obtained along the entire pipeline route, which may not occur at the same location for WISP and PIP pipeline. A local span for WISP pipeline and its length variation during start-up / shut-down cycles are illustrated in Figure 15. Moreover, the span lengths at operation are similar for the cases with and without anchor, because the anchor mainly affects the effective tension near the DC during shut-down as shown in Figures 3 to 6. TABLE 7. PIPELINE FREE SPAN LENGTH UNDER DIFFERENT LOADING CONDITIONS Load Condition Maximum Span Length, ft Case 1 Case 2 Case 3 Case 4 As-Laid 324 324 291 291 OP-1 297 297 150 150 OP-5 306 306 90 90 OP-10 SD-1 309 309 312 309 87 162 87 162 SD-5 336 366 135 135 SD-10 480 486 111 132 FIGURE 15. WISP PIPELINE FREE SPANS UNDER DIFFERENT LOADING CONDITIONS 4.5 PIPELINE LOAD Figure 16 shows anchor loads for WISP and PIP pipeline if the hold-back pile installed. Both anchor loads show the trend of stabilization after a number of thermal cycles. The anchor load for PIP increases faster with operation cycles, but it stabilizes sooner also because the PIP pipeline has milder tendencies to walk than WISP pipeline shown in this study. PIP will need an anchor pile with minimum load capacity about 350 kips (with safety margin), while the anchor capacity for the WISP pipeline should be slightly larger than that of PIP based on the trend of the stabilization. Note: Case 1: WISP Pipeline without Anchor Case 2: WISP Pipeline with Anchor Case 3: PIP Pipeline without Anchor Case 4: PIP Pipeline with Anchor Figure 15 shows the local pipe bottom-line profiles of the longest span for the WISP pipeline. The longest span under aslaid condition has the maximum span gap up to 16 ft. Although long sections of pipeline feed into the span location and sag loosely inside the span area during the first operation state (OP1), the pipeline span section still separates from the seabed, and the span length decreases 27 ft compared to that in the as-laid condition. The span length increases under shut-down (SD-1) condition due to the increase in effective axial tension. The span length keeps increasing over the operation cycles as shown in Table 7. The stretched pipeline section walks towards the riser end, which indicates the interactions between pipeline spanning and walking. FIGURE 16. WISP AND PIP PIPELINE LOAD 5. OBSERVATION OF INTERACTIONS OF WALKING, SPANNING, AND LATERAL BUCKLING The pipeline global walking, lateral buckling and spanning behavior are investigated with a case study with finite element analysis. It is observed that the pipeline walking is linked to the lateral buckling and spanning. Besides the observations 8 Copyright 2014 by ASME

discussed in previous section, some preliminary findings are summarized below. Figures 7 to 10 show that pipeline section feeds into span locations near the DC end. For the lighter WISP pipeline, excessive feeding fills the span gap and pushes the pipeline to move laterally, which generates two lateral buckles indicating the interaction between spanning and lateral buckling. Figures 7 and 11 show the WISP pipeline DC end walking towards the lateral buckling location. Figure 13 shows the lateral buckling amplitude keeps increasing contributed from the pipeline ratcheting into the lateral buckling location, which indicates the interaction between pipeline walking and spanning / lateral buckling. The results shown in Figures 11 and 12 indicate that PIP pipeline has less walking potential compared to the WISP pipeline. Table 7 and Figure 15 show that the WISP pipeline span length increases significantly over the shut-down cycles, which results from the increasing effective tension under walking. The straightened pipeline sections in spans move toward the riser end resulting in more walking at the riser TDP end as shown in Figure 12, which shows the interaction between pipeline walking and spanning. Pipeline end walking can be controlled by installation of an anchor pile, but the hold-back pile combined with the riser bottom tension result in significant tensions (Figures 4 and 6) and substantial spans along the pipeline especially for WISP pipeline. The PIP pipeline span lengths and their variations over thermal cycles are less than those of WISP pipeline (Table 7). Based on these observations, this study will be furthered to quantify the observed phenomena later and explore the mechanism of the interactions of pipeline walking with spanning and lateral buckling. 8. REFERENCES [1] DNV-RP-F110, Global Buckling of Submarine Pipelines, 2007. [2] Harrison, G.E, Brunner, M.S., Bruton, D.A.S., King Flowlines - Thermal Expansion Design and Implementation, OTC Paper 15310, Offshore Technology Conference, Houston, May 2003. [3] Carr, M., Sinclair, F., and Bruton, D., Pipeline Walking - Understanding the Field Layout Challenges, and Analytical Solutions Developed for the SAFEBUCK JIP, OTC Paper 17945, Offshore Technology Conference, Houston, May 2008. [4] ABAQUS/Standard User s Manual, Version 6.12, Simulia Inc., 2013. [5] API Specification 5L, Specification for Line Pipe, fortyfifth edition, 2012. [6] API RP 1111, Design, Construction, Operation and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design), Fourth Edition, 2009. [7] White, D.J., Ganesan, S.A., Bolton, M.D., Bruton, D.A.S., Ballard, J-C., Langford, T., SAFEBUCK JIP - Observations of Axial Pipe-soil Interaction from Testing on Soft Natural Clays, OTC Paper 21249, Offshore Technology Conference, Houston, May 2011. [8] Bruton, D.A.S, Carr, M., Overview of the SAFEBUCK JIP, OTC Paper 21671, Offshore Technology Conference, Houston, May 2011. 6. CONCLUSIONS In this paper, a case study of WISP and PIP pipelines with significant seabed undulations and slope is presented. The preliminary results show the co-existing phenomena of pipeline walking, spanning, lateral buckling, and their interactions. PIP pipeline shows milder tendencies of walking, stronger bending capacity, shorter and more stable spans, and less anchor pile load capacity requirement, which might be a cost effective option in the design to be considered. 7. ACKNOWLEDGEMENTS The authors acknowledge the management of GENESIS for permission to publish this paper. 9 Copyright 2014 by ASME