The state of knowledge of pipe-soil interaction for on-bottom pipeline design

The paper reviews recent advances in the understanding of pipe-soil interaction, and provides a updated knowledge on best practices for on-bottom pipeline design. Since the late 1990s, major programs of research work have been undertaken to develop appropriate models for pipe-soil interaction for seabed pipelines in challenging environments and operating at high temperature and pressure, to mitigate design issues associated with geohazards, hydrodynamic stability and thermal expansion management. Project-specific programs of work have been extended into industry-wide Joint Industry Projects, and operating pipelines are now providing field observations to validate and refine the design analyses. Much of this new knowledge is now maturing into best practices that can be presented in codes and standards. This paper synthesises that work, and provides recommendations of methodologies suited to codification that will guide future projects. The paper has been authored by a team of practitioners and researchers that comprise a Technical Panel working under the API/ISO geotechnical committee, and the paper sets out some of our views on future additions to the API/ISO codes. Recent advances in the treatment of pipe-soil interaction in pipeline design cover a range of aspects, including (i) quantifying subaqeous flow (submarine slide) geohazards - slide runout behaviour, pipeline impact loads and pipe deformation, (ii) predicting pipeline embedment, including the effects of the lay process, and through-life changes due to sediment transport, (iii) modelling axial pipe-soil interaction, including the strong influence of drainage and consolidation on soft soils, (iv) modelling lateral pipe-soil interaction, including cyclic effects such as the growth of soil berms beside the pipe, (v) modelling scour and self-burial, in regions of hydrodynamic activity, and the resulting changes in pipeline stability. Many of these effects are complex, involving temporal changes in seabed bathymetry and soil strength. However, they can also offer significant design efficiencies, providing a motivation to capture them accurately. For example, self-burial of a pipeline through seabed mobility may lead to an improvement in stability that reduces the requirement for weight coating or secondary stabilization works. Also, long-term changes in seabed friction due to consolidation following each cycle of expansion and contraction may lead to a progressive stabilization, reducing the need for anchoring. This paper includes examples where it has been possible for methods emerging from research to be applied in practical design, validated by observations from the laboratory or from operating pipelines. Many aspects of modern methods for pipe-soil interaction analysis are reaching a level of maturity that allows a consensus to be reached on best practices for design. This will unlock consistent and efficient approaches for future pipeline systems, and for management of existing systems.