CFD Modelling coupled with Floating Structures and Mooring Dynamics for Offshore Renewable Energy Devices using the Proteus Simulation Toolkit

Abstract

With many countries showing a growing interest in offshore renewables, the development of floating support structures able to withstand extreme environmental loads is key to winning the race in offshore renewable energy deployment. Numerical modelling of these structures allows relatively inexpensive testing and selection of suitable designs. The research presented here focuses on the numerical analysis of the behaviour of different floating structure designs and the associated mooring dynamics under various wave conditions. Simulations are performed with Proteus, a relatively new and open-source computational fluid dynamics (CFD) software actively developed by the ERDC and HR Wallingford, using the Finite Element Method (FEM) to model two phase flows. In order to allow the simulation of moving bodies in Proteus, a mesh motion module has been developed. The mesh nodes in the fluid domain are moved using the equations of linear elastostatics while displacement of the nodes placed on the surface of the moving structure is imposed through boundary conditions (see Figure 1 for an illustrative example of the mesh motion for an oscillating floating body). The mesh motion is taken into account directly in the Navier-Stokes equation that are solved in the Eulerian frame. Using input forces and moments from the CFD solver, floating body and mooring dynamics are solved using the open source C++ library Project Chrono. This library allows a fully coupled simulation of rigid and flexible bodies with cable dynamics using FEM, where collision detection of the cables with structures is enabled using node clouds for seabed and other obstacles. Verification and validation of the coupled two-phase flow and body/moorings dynamics is conducted in this paper with the help of experimental data of floating bodies with varying constrains and degrees of freedom. Figure 2 shows results of one of the validation tests where the Response Amplitude Operator (RAO) was obtained with Proteus for the roll motion of a floating body under different wave loads and compared to experimental data [1] and to other numerical models [2]. Validating the model for these cases allows us to simulate with confidence more representational scenarios for offshore renewable energy devices using, for example, a selection of the following floating wind support structures: deep draught platforms (SPAR), tension-leg platforms (taut mooring), and semi-submersible (buoyancy stabilized)