Advancements in vortex-induced vibration fatigue prediction for subsea pipeline free spans: Research progress from the tide research hub

Free spans pose a major challenge when designing and maintaining subsea pipelines. These spans can form due to natural seabed irregularities, engineered irregularities (such as buckle initiator or crossing structures) and sediment mobility. When ocean waves and currents interact with a free spanning pipeline section, alternate vortex shedding may develop, potentially leading to vortex-induced vibration (VIV) and fatigue damage. The pipeline response, the ensuing amount of fatigue, and consequently whether the free span requires mitigation, is governed by complex coupled interactions between the pipeline, the seabed, and the flow itself. Given the complexity of these interactions, code based methods to assess VIV-induced fatigue (such as DNV-RP-105) are necessarily simplified and therefore have potential to be conservative. To investigate potential conservatisms in design practice, and how these may be adopted to reduce operational cost, this research systematically considers a number of separate but interrelated factors (such as hydrodynamics and the effect of near-seabed turbulence, local seabed irregularities, marine growth on spans, non-linear soil springs, etc.) and how these can influence the structural and fatigue behavior of a free spanning pipeline. This research is being undertaken within the Transforming energy Infrastructure through Digital Engineering (TIDE) Research Hub (https://tide.edu.au/) at The University of Western Australia. The project team comprises academics, students and industry collaborators, with activities that include (1) reviewing historical survey data across multiple pipelines and from installation to operation stages, to quantify changes in free span geometry, (2) conducting laboratory model tests and high-fidelity numerical simulations to investigate VIV, (3) analyzing span propagation due to local scour and studying bedform migration, and (4) studying pipe-soil interaction at free span shoulders. The research aims to generate design advice for individual components of the free span system, which can be integrated into structural dynamic models to enable improved fatigue prediction and this paper provides an overview of the current progress of the research program.