Temporal changes in pipeline-seabed condition, and their effect on operating behaviour

It is increasingly recognized that the state of the seabed surrounding an on-bottom pipeline may change during the operating life of the pipeline. For seabed sediments that are soft and fine-grained, the strength may vary through episodes of pipeline movement due to consolidation effects. For seabed sediments that are mobile due to waves and currents, the burial state and the adjacent seabed topography may vary due to sediment transport and scour. These changes in the strength and topography of the surrounding seabed alter the exposure of the pipeline to hydrodynamic loads and ambient cooling, as well as the level of geotechnical support and insulation provided by the seabed. The design relevance of these changes in seabed condition is amplified by modern design approaches in which the pipeline itself can be tolerably mobile-for example in a dynamic onbottom stability approach or through engineered schemes of global buckling and axial walking. This paper illustrates the interactions between the geotechnical and sediment transport processes and the resulting global pipeline behaviour. Two interactions are considered: the long-term axial walking behaviour on soft soil, and the longterm insulation and temperature profile on a mobile seabed. The examples highlight the potential for over or underestimation of various inputs to a pipeline design when these temporal changes in pipe-seabed condition are overlooked. Emerging analysis methods for pipeline-seabed interaction that incorporate these temporal effects can lead to more reliable and cost-effective design.