Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design¹

Subsea pipelines are being designed to accommodate higher temperatures and pressures. Current modelling approaches that adopt constant lateral seabed resistance along the pipeline do not capture the high spatial variability in as-laid pipeline embedment from field observations, which strongly affects the lateral resistance. Ignoring spatial variability when designing pipelines with engineered buckles leads to higher predictions of axial force along the pipeline, with reduced likelihood of buckle formation. This can result in excessive mitigation measures being adopted, such as sleepers or counteract structures, which significantly increase project costs. Spatial variability of pipeline embedment is not currently handled rationally in design because an understanding of the physical mechanisms that cause as-laid embedment and methods for accurately predicting it have only recently emerged. This paper illustrates how the influence of these physical mechanisms that drive embedment can be extracted from field survey data and then modelled synthetically in design analyses. The impact of embedment variability and the resulting variation in lateral seabed resistance on the lateral buckling response is illustrated. The framework represents an improvement in the way geotechnical uncertainty and variability is handled in pipeline-seabed interaction analyses for use in pipeline design, and has already begun to be implemented in practice.