A method for reliability analysis of submarine pipelines has been developed and applied in a numerical case study addressing three possible failure modes concerning on-bottom pipelines: excessive displacement, yielding and buckling. Different uncertainty sources relevant for these failure conditions are discussed and included in the analysis. The reliability calculation procedure can be applied directly representing a full probabilistic design or indirectly through a proposed calibration procedure.
The recent developments related to on-bottom submarine pipelines have lead to a redefinition of the design practice applied. The newly developed design method reported by Sotberg et al. (1988 and 1989), allows for small movements of the pipeline during the most extreme environmental conditions in the lifetime. The earlier pipeline stability design procedure for on bottom pipelines was normally based on a simple stability check requiring balance between the hydrodynamic forces and the pipe-soil resistance forces. The new design procedure has the potential to give a more economic pipeline design, with a prediction of the real safety level against relevant pipeline failure modes. When considering a pipeline design based on a dynamic stability criterion, i. e. allowed movements, the strength limit state needs to be evaluated. Allowing pipeline movements under extreme environmental conditions simples that the stress condition at constrained points along the pipeline has to be checked to verify a satisfactory design. This means that the dynamic stability design for on-bottom pipeline sections needs to consider several limit states (failure modes) such as pipeline movements, yielding and excessive straining which may cause local buckling or collapse. In this way, the relaxed design criterion with respect to pipeline stability, introduces a need for some additional design controls as compared to the traditional procedure. However, the benefit from this will be a more cost optimal design based on a thorough safety evaluation.
