Abstract

The complexity of the dynamic behaviour of offshore marine structures requires advanced simulations tools for the accurate assessment of the seakeeping behaviour of these devices. The aim of this work is to present a time-domain model for solving the dynamics of floating marine devices, subjected to non-linear environmental loads and paying special attention on the mooring dynamics. First, the formulation of the hydrodynamic approach for solving the wave-floater interaction is introduced. Second, the solver of the mooring dynamics, based on a non-linear Finite Element Method approach, is presented. Third, a procedure for coupling the hydrodynamic along with other external loads, with the floating structure and mooring dynamics is described. Fourth, some validation examples and comparisons among different mooring approaches are presented. Fifth, an analysis of the OC3 floating wind turbine concept is performed to study the influence of different mooring models, the effects of non-linear waves on the platform, and the tension in the mooring system. The dynamic mooring model along with the second-order wave model produce realistic simulations of the floating wind turbine performance.

Abstract

Floating Offshore Multi-Wind Turbines (FOMWTs) are an interesting alternative to the up-scaling of wind turbines. Since being new incoming concepts, there are few numerical tools for its coupled dynamic assessment at the present time. In this work, a numerical framework is implemented for the simulation of multi-rotor systems under environmental excitations. It is capable to analyse a platform with leaning towers handling wind turbines with their own features and control systems. This tool is obtained by coupling the seakeeping hydrodynamics solver SeaFEM with the single wind turbine simulation tool OpenFAST. The coupling of SeaFEM provides a higher fidelity hydrodynamic solution allowing the simulation of any structural design since using the Finite Element Method (FEM). Besides, a methodology is proposed for the extension of the single wind solver, allowing the analysis of multi-rotor configurations. To do so, the solutions of the wind turbines are computed independently by several OpenFAST instances, performing its dynamic interaction through the floater. The method is applied to the single turbine Hywind concept and the twin-turbine W2Power floating platform, supporting NREL 5-MW wind turbines. The rigid-body Response Amplitude Operators (RAOs) are computed and compared with other numerical tools. The results showed consistency in the developed framework. Agreement is also obtained in simulations with aerodynamic loads. This resulting tool is a complete time-domain aero-hydro-servo-elastic solver, able to compute the combined response and power generation performance of multi-rotor systems. 

Abstract

In this study wind-induced stresses and forces on a tensile membrane structure (TMS) and its supporting system are investigated using fluid-structure interaction. For the structural part geometrically nonlinear three-noded finite elements with the generalized-$\alpha$ time integration scheme is employed. For the fluid, the Navier-Stokes equations are solved using information from previous studies and others available in the literature. The coupling is achieved by a procedure known as the staggered partitioned scheme or weak coupling scheme. Results indicate that, by using common material properties for the fabric and supporting system, the fabric part of the structure may not be as critical as the cables and/or posts of the supporting system for withstanding wind-induced forces and stresses.