Assessment of Mass Gravity Flow Induced Seafloor Scour Impact on Stability of Mudmat Supported Subsea Structures

Abstract

Mass gravity flow phenomena resulting from mobilization of unstable sediments on submarine slopes are well recognized as a major geologic hazard for offshore infrastructure development. Previously published research pertaining to mass gravity flow impact on offshore infrastructure has focused on evaluating the direct impact loads generated by the flowing sediment mass in contact with the analyzed infrastructure component, such as a pipeline or a mudmat. In this study, the mass gravity flow impact on offshore infrastructure is addressed from a different perspective by investigating the potential for mass gravity flow induced seafloor scouring. Finite-element analysis is used to investigate the mobilization of near-seafloor sediments in a deepwater setting under the loads generated by a mass gravity flow event. Analysis results representative for a mass gravity flow in the far-field, near-field, and at the location of a mudmat supported subsea structure are presented and discussed in terms of failure mechanism, near-seafloor sediment deformation pattern and associated impact on stability of the subsea structure, in relation to flow thickness and position of the front of the flow relative to the subsea structure. For the case of a mass gravity flow in contact with the subsea structure, an example of failure envelope providing the ultimate lateral impact load from mass gravity flow required to cause instability of the structure, accounting for the flow induced mobilization of sediments underneath the mudmat, is also presented.