Seabed trenches are sometimes observed in front of suction pile anchors that are used as part of the mooring system for floating facilities, particularly for semi-taut configurations offshore West Africa. These trenches, due to chain movements on the seabed, can have a significant impact on the holding capacity of the suction pile. Better understanding of the trenching phenomenon is therefore required to anticipate, prevent and mitigate the impact of trenches on mooring systems. To develop this understanding, SBM has initiated model scale laboratory testing campaigns to simulate the trenching process. To complement this testing, a series of computational fluid dynamics (CFD) analyses have been conducted to determine if a limiting trench size and shape can be expected for an existing mooring system. In the CFD simulations, the trench shape is fixed (to approximate measured geometries from operating mooring systems in the field) and the shear stress on the trench walls is computed using 2and 3-dimensional CFD with an appropriate turbulence closure model. The CFD results are compared with the critical shear stress of the seabed (estimated based on literature and direct erosion testing on soils from the general region). The 3D simulations enable analyses to account for the potential effects of flow along the trench and non-constant chain geometry, and make it possible to study potential erosion under the lug due to vortex shedding caused by the chain. The resulting calculated trench wall stresses, when compared with the critical shear stresses, suggest that the modelled trench is unlikely to grow significantly below the anchor lug; thus, the modelled trench geometry appears to represent a limiting trench size and shape. These conclusions are confirmed by field measurements, which indicate that the trench geometry appears to be close to an equilibrium geometry.
Khalida Sassi, Gerard Fernandez, Joe Tom, Scott Draper and Ming Zhao
Keywords: moorings, trenching, CFD
Publisher: DFI