Effect of environmental parameters on the amount of scour in marine structures

Scour is a phenomenon that occurs as a result of natural erosion by ground water flow and transport of seabed material. In this research, scour around the legs of marine structures is simulated using the FLOW3D.V.9.3™ commercial software. The general scour model consists of two components, namely, the lift and thrust forces. The drag force, that is inserted to the structure, is a combination of those two components. As the sediment concentration equals the volume fraction of cohesion, the drag force that determines the rigidity of the sediment model, is activated. Also, as the sediment concentration changes, the viscosity and density of the fluid changes. The numerical model used in this research is similar to the experimental set-up that was performed in a sedimentation tank two meters long, 1.5 m wide and 0.3 m high; while the sandy sediment particle diameter was 0.084 cm. The numerical model implements an RNG turbulence model. After the studies, the time to reach equilibrium in the model was 100 seconds. The relative error between experimental and numerical scour depth is about 10 percent. Scour depth was determined for legs with circular and elliptic as well as for rectangular base shapes with different aspect ratios. Numerical results indicate that the scour depth decreases for the more streamlined elliptic base shapes. Also, the effect of water depth on scour depth, while all other parameters are kept constant, is a linear relationship. Finally, as sediment particle diameter increases, scour depth decreases.