When an offshore foundation is exposed to waves and currents, local scour could be developed around a pile and even lead to a structural failure. Therefore, understand and prediction for the scour due to sediment transport around foundations are important in the engineering design. In this study, flow and scour around a monopile foundation exposed to a current were investigated by using the computational fluid dynamics (CFD) and discrete element method (DEM) coupling method. The open source computation fluid dynamics library, OpenFOAM, and a sediment transport library were coupled in the OpenFOAM platform. The results of simulations regarding the incipient motion of the particle were presented. The flow fields and sediment transport around the monopile were simulated. The scour depth development was simulated and compared with existing experimental data.
Most of offshore wind turbine farms constructed under 60 m depth are installed as bottom-fixed structures. It is important to install a foundation structure that can stably support the load of the superstructure in a bottom-fixed offshore wind turbine. Bottom-fixed offshore wind turbines consider several design factors depending on the seabed ground and marine environment. Among them, a scour can be defined as the phenomenon that the seabed particles around the foundation structure are transported due to the interaction of the fluid flow and the structure. The scour is a cause of deterioration of the stability of the structure which must withstand the large turnover moment acting on the turbine.
As the operation period of bottom-fixed offshore wind turbines increase, the researches on the scour problem have been done (Whitehouse, 1998; Sumer and Fredsoe, 2002). In particular, experimental and numerical studies on the scour around the monopile, which is the simplest foundation of bottom-fixed structures, have been performed (Dargahi, 1989; Pang et al., 2016). Park et al. (2017) predicted the scour using the bed shear stress by CFD. However, the Eulerian-based CFD approach does not sufficiently take into account the influence of the soil. For the soil transportation simulation, the DEM approach is used. Cundall and Strack (1979) was first presented the basic concept of the discrete element method, which simplified the collision between particles using the spring-dashpot model. In the DEM, small size overlap between the particles is allowed, and the behavior of the particles is analyzed by the repulsive force generated by the particle overlap. The advantage of this method is that it can analyze a large amount of particles based on a simple collision model and analyze the exact behavior of the particles. Thus, to consider the interaction between the fluid flow and the soil, a CFD and DEM coupling method is needed. Recently, studies on the sediment transport using the CFD-DEM coupling method have been carried out (Schmeeckle, 2014; Sun and Xiao, 2016), but there have been only a few studies to realize the scour phenomenon.