2D nonlinear hydrodynamic model for support platforms of offshore floating wind turbines
Date of Issue2016
School of Mechanical and Aerospace Engineering
Renewable energy is a vital option for our sustainable future. With its high source availability and relative technology maturity, wind energy has been extensively investigated and widely used since the 1970s. In the past decade, offshore wind started to draw a lot of attention and was regarded as one of the most promising renewable alternatives among all existing green technologies. In order to extract the offshore wind energy, wind turbines have to be implemented in the sea environment. They are supported either by fixed substructures in shallow water or by floating platforms in deep water. Compared to their land-fixed counterparts, offshore floating wind turbines are subjected to additional wave loads and mooring forces, making their design, manufacturing, installation, and maintenance much more challenging and hence resulting in much higher costs. It is therefore quite important to fully understand the hydrodynamics of offshore floating wind turbines and have the capability of fast and correctly predicting their performance in different sea conditions. Various hydrodynamic models have been proposed and applied to the analysis and design of offshore floating wind turbines. These models, either in the frequency domain or in the time domain, are based on linear assumptions, and hence cannot deal with nonlinear dynamics as well as transient events for the floating wind turbine system, both of which are, however, of great importance in the study of design load cases. Moreover, linear assumptions may not be valid for certain cases such as with steep waves or with large motions of the floating platform. As specified in IEC (International Electrotechnical Commission) 61400-3 design standard, a nonlinear wave model is required in several design load cases in order to determine the extreme loads expected over the life cycle of offshore floating wind turbines. Therefore, the analysis and design of offshore floating wind turbines is in urgent need of a fast hydrodynamic model that can take wave and body nonlinearities into account. This research aims to develop an efficient and high-fidelity hydrodynamic model for the analysis of offshore floating wind turbines. As the main component of this hydrodynamic model, a two dimensional, fully nonlinear numerical wave tank (NWT) is established first. In this NWT, time-dependent boundary value problems (BVPs) are formulated for both the velocity potential and the acceleration potential. On boundaries, the fully nonlinear free-surface and body-surface boundary conditions are satisfied at their exact positions. The free surface is updated using the mixed Eulerian-Lagrangian scheme. At each time step, the BVPs of both potentials are solved using the desingularized boundary integral equation method (DBIEM). By tapping the strength of the DBIEM, a novel, efficient acceleration-potential solving method is proposed. This method turns the original unknown t into t in the implicit boundary condition on the body surface, and hence determines the acceleration potential on the body surface by solving just one BVP, making it much more efficient compared to other existing methods. With this new fully nonlinear NWT, investigations are conducted on several fundamental problems in a sequence from simple to complex, i.e., wave propagation, wave diffraction, wave radiation, wave diffraction-radiation, and transient wave-body interaction. These studies not only validate the new NWT, but also reveal more physical insights into generic wave-body interaction problems, especially when the nonlinearity is significant. After being validated, this NWT is coupled with a quasi-static mooring model to form a new hydrodynamic model, which is further coupled with a simple aerodynamic model and a rigid-body dynamic model to form a preliminary integrated analysis tool for offshore floating wind turbines. To demonstrate its capability, this integrated analysis tool is then applied to a barge-type offshore floating wind turbine - the NREL offshore 5-MW baseline wind turbine - for a case study. The effects of wave frequency and wave height on the wind turbine's power generation, mean surge position, wave loads, mooring tensions, and tower-root bending moment are investigated and discussed in detail. Although still being in its infancy, this research pushes the frontiers of the analysis and design of offshore floating wind turbines from linear to nonlinear hydrodynamics.