Large-scale modelling of soil-pipe interaction during large amplitude cyclic movements of partially embedded pipelines

Abstract

As the development of offshore hydrocarbons moves into deeper water, pipelines form an increasingly significant part of the required infrastructure. High-temperature high-pressure pipelines must be designed to accommodate thermal expansion and potential lateral buckling. A novel design approach is to control the formation of pre-engineered lateral buckles to relieve the expansion. The amplitude of these buckles is typically several pipe diameters. Assessment of the force–displacement interaction between the on-bottom pipeline and the seabed is crucial for design. A series of large-scale plane strain model tests has been conducted to measure the response of a pipe segment partially embedded in soft clay, during large amplitude cyclic movements, mimicking consecutive thermal expansion and contraction at a bend in a pipeline. Four key stages in the force–displacement response have been identified: (i) breakout, (ii) suction release, (iii) resistance against a steadily growing active berm, and (iv) additional resistance during collection of a pre-existing dormant berm. A simple upper bound solution is proposed to model the observed response. This solution captures the experimental trends including growth of the active berm and collection of dormant berms. This approach is the first attempt to quantitatively model the mechanisms underlying the response during large-displacement lateral sweeps of an on-bottom pipeline, accounting for the growth of soil berms.