Geohazard Study of Mudslide Event at a Subsea Escarpment and Structural Response of Crossing Flowlines: Part II – CFD and FSI Analysis

Mudslides can occur on the seafloor due to subsea earthquakes. These extreme events have a significant impact on the structural integrity of subsea flowlines and equipment. A numerical geohazard study was performed on a subsea mudslide event occurring at an escarpment. The study objective was to determine the structural response of two flowlines and one umbilical crossing an escarpment area subject to the extreme mudslide event and to assess requirement for additional stabilization ancillaries. The study utilized several numerical methodologies such as Computational-Fluid-Dynamics (CFD), Finite-Element-Analysis (FEA), and Fluid-Structure-Interaction (FSI). Different levels of CFD models were first considered (without flowlines) to show the effect of including additional physics with the final CFD model considering a viscous seawater column, viscoplastic multi-layered soil and high-mobility mud phases. The mudslide velocity field from this CFD simulation is extracted at every 1 s and applied to the Abaqus FEA simulation approach. This is presented in a separate paper [1]. The current paper utilized a FSI simulation approach where the evolution of mudflow debris is directly coupled with the FEA flowline model. This allows the FEA model to continuously update loading created by the mudslide motion. The flowline structural responses were monitored and assessed in terms of displaced shapes and structural loading. This methodology reduces conservatism over conventional approaches by considering viscous effects of the seawater, multi-layer soil and mud layers. Key findings from the study is that the mudslide event did not significantly displace flowlines over the escarpment. The two flowline responses were within structural design limits. It was also found anchoring did not significantly affect structural responses. The umbilical line, however, exceeded compression limits by four times the design limit. This is due to the high velocity mud-front interacting with an initially stationary umbilical and pushing it into compression during the initial stages of the mudslide event. The solution proposed was line rerouting which helped reduce compression levels to within design limits. This analysis assisted the project in CAPEX and installation cost reduction with no requirement of further umbilical compression mitigation components, and the omission of the pre-planned anchors.