The influence of slope on the stability of pipelines subjected to horizontal and vertical loading on clay seabeds

Stability under horizontal (H) and vertical (V) loading is an important design consideration for seabed lain subsea pipelines. From initial installation, extending through their design life, these pipelines are subjected to a combination of H and V loading due to various external and internal driving mechanisms. These mechanisms include self weight loading, forces applied during installation, temperature and pressure driven expansion and in some cases significant hydrodynamic loading. All these aspects need to be considered in design, requiring an understanding of the interaction between the pipeline and the seabed it rests upon, i.e. pipe-soil interaction. In the past, emphasis has been placed on empirical methods of considering pipe-soil interaction. Typically model testing has been used to consider vertical and subsequent horizontal load response. Recently, progress has been made away from purely empirical methods to include the derivation of theoretical upper bound solutions for combined H-V loading and the use of numerical analysis to calculate H-V stability envelopes. This and previous work relating to the H-V stability of pipelines has largely been confined to the case of a flat seabed with no consideration of the influence of seabed slopes. This paper presents a suite of finite difference analysis undertaken to investigate the influence of a seabed slope on pipeline H-V stability envelopes. Initially modeling considers a slope on weightless soil with subsequent modeling considering the additional influence of soil weight. Analysis comprised a series of displacement controlled excursion through H-V load space, defining stability envelopes for a range of conditions. For a weightless soil the pipeline stability envelope was seen to rotate through H-V load space reducing vertical resistance to penetration and producing significant asymmetry in the horizontal load response. In addition to this rotation consideration of soil weight introduced a change in the geometry of the stability envelope. The results presented in this paper suggest that the influence of seabed slope on pipe soil interaction and the stability envelope in H-V load space is significant and worthy of consideration in design.