A Stability Design Rationale: A Review of Present Design Approaches

Pipeline hydrodynamic stability is one of the most fundamental design topics which are addressed by pipeline engineers. In its simplest form, a simple force balance approach may be considered to ensure that the pipeline is not displacing laterally when exposed to the maximum instantaneous hydrodynamic loads associated with extreme metocean conditions. If stability can be ensured in a cost efficient way by applying a minimal amount of concrete weight coating only, this method when applied correctly, can be regarded as a robust and straightforward approach. However in many cases pipeline stabilisation can be a major cost driver, leading to complex and costly stabilisation solutions. In these circumstances, the designer is likely to consider more refined methods in which the pipeline is allowed to displace under extreme conditions. This paper discusses various design approaches and acceptance criteria that are typically adopted in pipeline stability design. Both force balance methods and calibrated empirical methods which are typically defined in modern design codes, are discussed in terms of their applicability as well as their limitations. Both these approaches are based on the assumption, directly or indirectly, that lateral displacement is a Limit State in its own right. It will be argued that this assumption may lead to unnecessary conservative design in many circumstances. It will be demonstrated that even if relatively large displacements are permitted, this may not necessarily affect the structural integrity of the pipeline. An alternative stability design rationale is presented which is based on a detailed discussion of the Limit States pertaining to pipeline stability. This approach is based on the application of advanced dynamic stability analysis for assessing the pipeline response. Pipeline responses obtained through advanced transient finite element analyses is used to illustrate how a robust design can be achieved without resorting to strict limits on the permissible lateral displacement.