Pipeline-Seabed Interaction in Soft Clay

Offshore pipelines laid on the seabed in a snake configuration and transporting hydrocarbon products under high pressure/high temperature are becoming a cost effective alternative to trenching and burial. However, there appears to be a major disparity between the level of sophistication and accuracies inherent in the structural FE models used for expansion and lateral buckling analysis of pipelines, and the degree of crudity in adopting and using Coulomb friction values. This Paper reports the findings of a programme of geotechnical finite element analyses performed for a project where some 91km of 26” gas pipeline was designed to be laid in a snake configuration. The seabed soils were predominantly very soft clay. The ABAQUS/Explicit finite element program was used with an adaptive meshing technique to analyse the embedment and large lateral ploughing movements of the pipelines by a distance of several diameters. It was found that the FE model predicts the initial pipeline embedment into soil accurately and rectifies the inaccuracies inherent in published plasticity-based closed form solutions. A new non-dimensional relationship is proposed for estimating pipeline embedment in soft clays. The effect of important parameters such as the soil-pipeline interface friction, operating submerged weight and initial embedment, were all captured. Predicted cyclic lateral ploughing showed similarities to the observed response in reported model tests. The results were used in the structural FE model of the pipelines to analyse the expansion and lateral buckling problems and hence design the number and critical lay curvature of snakes as well as other important features.