Lateral buckling and on-bottom stability are often considered independently during pipeline design. However, it is expected that the interaction between lateral buckling and on-bottom stability will become an increasingly common and important pipeline design consideration; as projects move into higher temperature / higher pressure (HT/HP) design conditions in areas of cyclonic metocean environments. The interaction of lateral buckling and on-bottom stability occurs mainly at shallow water depths. This interaction is not expected in deeper water depths where the hydrodynamic loading from storms is less significant at the sea floor.
The main challenges faced in assessing lateral buckling and stability interaction include; bounding storm-induced horizontal out-of-straightness (HOOS), capturing the increased tendency to buckling due to lift and inline forces from hydrodynamic loading, and the post-buckle response at unplanned and planned initiation sites during a storm event.
In cases where pipeline absolute stability cannot be assured, storm-induced HOOS can be introduced into the pipeline. This HOOS can govern the buckling design; rather than HOOS due to pipelay, or vertical out-of-straightness (VOOS) due to bathymetric features. In addition, the lift and inline forces due to the storm loading increases the tendency for pipelines to buckle, and increases the likelihood of rogue buckle formation. Subsequently, storm loading may play a significant role in terms of initiation and response of uncontrolled lateral buckling, and therefore the extents of buckle control schemes (if required).
Furthermore, it is important to consider the response at planned initiation sites under storm loading. Severe storm loading can result in significant lateral displacement of planned buckles; affecting the strains and bending moments in the pipeline and influencing the buckling design.
This paper outlines the approach and knowledge gained from recent experiences in the design of pipelines subject to lateral buckling and severe cyclonic metocean conditions. The insight gained can be applied to other systems where the interaction of lateral buckling and on-bottom stability is of a concern. In order to perform appropriate assessments, Wood Group Kenny used a number of advanced finite element (FE) models which simulate buckle responses under various sea states for on-seabed and at snake bend location.
