| Offshore pipelines are the important component of offshore oil and gas recovery system,and the pipe-soil interaction is the key factor for controlled lateral buckling design of pipelines.Accurate assessment of pipe-soil behaviour is of great significance for the design,to ensure that lateral buckles form in a safe manner.This paper presents a numerical investigation into the large deformation behavior of seabed during pipe laying and lateral buckling,as well as the pipe-soil interaction for deep-water pipelines on soft clay,using a sequential limit analysis approach developed recently.Analytical models are presented to predict the pipe dynamic embedment and soil resistance during lateral movement,based on which both pipe-soil interaction model and engineering measures are put forward to guide the deep-water pipeline design.The main contributions of this work include:(1)SLA models that considers large soil deformation,strain softening and rate effect of soil strength are established,and are proved to be valid to simulate both the monotonic and cyclic pipe-soil interaction by comparison with centrifuge tests data.Based on Tresca yield criterion,the soil undrained shear strength can be correlated to depth,cumulative plastic shear strain and strain rate via VUSDFLD subroutine of ABAQUS platform,and CEL models are established to analyze the cases where soil berm collapses onto the pipe crown,which cannot be simulated by SLA models.(2)Equations are developed to calculate pipe penetration resistance,and also to describe the backbone curve for cyclic penetration cases,taking pipe-soil roughness,soil unit weight,strength gradient,strain softening and rate effect into consideration.The decreasing velocity and stable value of penetration resistance depend on cyclic amplitude and failure mode.(3)The influence of cyclic displacement during pipe laying on embedment is quantified.It is found that small-amplitude lateral pipe movement reduces the soil strength to remolded strength,meanwhile,pushes soil aside,creating a narrow trench which can lead to additional penetration up to 70%-230%.The embedment of pipelines is closely related to the total horizontal displacement experienced by the pipe,while the soil rate effect has limited effect on the final depth.For shallowly embedded pipes(w?0.5D),the dynamic embedment is dominated by buoyancy,rather than soil strength;for deeply embedded pipes(w>0.5D),the influence of strain softening on cyclic pipe embedment is not as significant as expected,although it does greatly affect the soil resistance.(4)For the pipe displaced under fixed vertical load,the pipe trajectory,soil resistance and failure mechanism of pipe with various pipe weight,initial embedment and other parameters are studied in detail.V-H yield envelopes and analytical models are proposed to predict the breakout resistance and lateral steady-state residual resistances,respectively.(5)For the pipe displaced under fixed vertical displacement,explicit models have been proposed to calculate both the lateral and vertical soil resistance for shallowly embedded pipes(w?0.5D)during lateral movement.The evolution of V-H yield envelopes is investigated and a normalised model is established to predict the pipe-soil interaction force.For deeply embedded pipe(w>0.5D),the soil berm is prone to rolling over the pipe crown,thus introducing severe numerical instability,and the SLA analysis cannot proceed further.Therefore,CEL analyses are performed to investigate the evolution of soil berm,and it is found that the soil resistance tends to be stable and can be regarded as full-flow soil resistance once the soil berm rolls over the pipe crown. |
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