An Improved Approach for Modelling Reliability of Buckle Formation for Subsea Pipelines

Abstract

Lateral buckling (LB) design for subsea pipelines generally requires reliability analysis to evaluate the probability that a lateral buckling mitigation system will behave as expected, which means that the engineered initiators will trigger buckles sequentially before any uncontrolled buckle can form in between. As various parameters involved in the buckle formation process are uncertain (soil characteristics, residual horizontal and vertical Out of Straightness (OOS) after pipelay), a probabilistic approach is used to assess the likelihood that the pipeline will buckle at the intended locations. The Safebuck JIP has developed a structural reliability model of the pipeline expansion process to calculate the probable virtual anchor spacing (VAS). This model can be used to assess the reliability of buckle initiation on the design of pipelines susceptible to lateral buckling. The buckling response of a pipeline will always be inherently uncertain because project specific OOS information is not known prior to pipelay. The Safebuck guideline focus is on horizontal OOS whereas the impact of bathymetry is addressed in a high level fashion, with no clear methodology. This paper presents an alternative approach to account for the effects of bathymetry on the reliability of buckle formation. The model is a significant advance on the current method for estimating buckle formation reliability and brings other important benefits such as capability for modelling any transient profiles of pressure and temperature. In this paper the difference between the horizontal residual OOS in pipelines for different pipelay methods is discussed, and it is shown that the method of pipelay can have a significant effect on pipeline horizontal residual OOS. The impact of change in horizontal OOS on reliability of buckle formation has been assessed.