Large deformation finite element analysis of partially embedded offshore pipelines for vertical and lateral motion at seabed

Dutta, Sujan (2012) Large deformation finite element analysis of partially embedded offshore pipelines for vertical and lateral motion at seabed. Masters thesis, Memorial University of Newfoundland.

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Abstract

Subsea pipelines play a significant role in transporting hydrocarbon from offshore. For both shallow and deep water, an effective means of hydrocarbon transportation is the usage of pipeline. However, deep water pipelines are expensive to bury and an economic way is to lay the pipelines on seabed. Due to pipe installation procedures (e.g. wave action, pipelines self weight etc.), pipelines could penetrate into the seabed a fraction of its diameter. Pipelines might experience thermal expansion (due to low ambient and high internal temperature) during operation cycles which can cause pipelines to expand axially. But due to restraining conditions from accumulated soil/pipe interaction and effective longitudinal force along the pipeline, bending moments can develop in the pipelines, which cause pipelines to buckle laterally. This lateral buckling is resisted mainly through soil/pipe interaction. In addition, the berm formed around the pipe (during installation period) plays a vital role in resisting the lateral pipe movements. Thus, accurate prediction of soil/pipe interaction of an as-laid pipeline is very important for the development of pipeline design guidelines. To address this critical phenomenon, the first step is to capture the soil behaviour during pipe vertical penetration along with the berm formulation mechanism. This is a large deformation problem. To solve the problem numerically, a large deformation numerical tool is required. In this study, the Coupled Eulerian Lagrangian (CEL) finite element method is used for analysis of partially embedded pipelines. Analyses are performed using ABAQUS 6.10-EFl software. In the deep sea, the undrained shear strength of clay typically increases with depth. In addition, the undrained shear strength is strain rate dependent. Moreover, strain softening behaviour of clay is another critical phenomenon that should be considered. The standard von Mises yield constitutive model available in ABAQUS cannot capture this clay behaviour. Therefore, in this study an advanced soil constitutive model that considered these phenomena is implemented in ABAQUS using user subroutines programmed in FORTRAN. Results from the analysis are compared with centrifuge test results and other available solutions in the literature. [t is shown that the Coupled Eulerian Lagrangian (CEL) approach together with the advanced soil constitutive model is a very effective tool for modelling large deformation behaviour of partially embedded pipelines in seabed both for vertical penetration and lateral movement.

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