As subsea pipeline installation methods become increasingly sophisticated through use of increased vessel power and advanced stability and positioning controls, pipelines are likely to exhibit lower levels of horizontal out-of-straightness on the seabed than currently considered to occur during the lay process. This has resulted in an increased tendency for the problematic phenomenon of snap-buckling to occur as part of the controlled lateral buckling design.
A key parameter affecting the pipeline response during lateral buckling is the friction between the pipeline and the seabed, which is strongly influenced by the rate of pipe displacement. Under stable buckle formation, the relatively slow rate of pipe displacement is more predictable and leads to acceptable strains in the pipeline. During the formation of less stable snap-through buckles, the rate of pipe displacement can mobilize a wide range in friction factors that can cause high strains in the pipeline. Quasi-static methods cannot model the dynamic transition between a high friction (drained) response and a low friction (undrained) response. To circumvent this problem in design, a fully dynamic finite element approach is required which captures the rate-dependent frictional response.
This paper presents the results of a series of FE analyses that illustrate how the changing seabed friction affects the lateral buckling response of typical subsea pipelines. A contractile seabed is modeled, chosen to capture the general case where high values of slow drained friction are followed by low values of fast undrained friction, leading to changes in the buckle shapes compared to those obtained from static buckle modeling. The results are used to examine the conditions under which dynamic buckle analysis should be used and the effects of dynamic pipeline response on the levels of strain and deformation induced in buckles.
