Cyclic loading response of offshore pipelines using simple shear tests

Offshore pipelines are increasingly being adopted at elevated temperatures and pressures as more petroleum deposits are explored deep offshore. These extra-long pipelines are subjected to frequent heating and cooling cycles that can lead to ‘pipeline walking’, resulting in serviceability failures during service life. Understanding the behaviour of the shearing zone beneath pipelines provides essential information to support reliable predictions of pipe axial-soil interaction. This study examines the cyclic shearing response of soil underneath axially moving pipelines using large-scale experiments and cyclic simple direct shear tests. Large- scale pipe-soil tests were first conducted to understand the shearing mechanism and zone of shearing influence of the soft clay underneath the pipeline. Having characterised the shearing mechanism using pipe-soil tests, cyclic direct simple shear tests were then performed for further characterization of the residual shearing resistance under various practical drainage conditions of pipe-soil interaction. The results of the simple shear tests show reliable prediction of axial walking response compared with large-scale test data. The results of the present study were also used to identify drainage limits and residual resistance using dimensionless charts which can contribute to the design practice of offshore pipelines when considering on-bottom stability in axial direction.