Hydrodynamic Module Coupling in the Offshore Wind Energy Simulation (OWENS) Toolkit

When considering the future of offshore wind energy, developing cost effective methods of harnessing the offshore wind resource represents a significant challenge which must be overcome to make offshore wind a viable option. As the majority of the capital investment in offshore wind is in the form of infrastructure and operation and maintenance costs, reducing these expenditures could greatly reduce the cost of energy (COE) for an offshore wind project. Sandia National Laboratory and its partners (TU Delft, University of Maine, Iowa State, and TPI Composites) believe that vertical axis wind turbines (VAWTs) offer multiple advantages over other rotor configurations considering this new COE breakdown. The unique arrangement of a VAWT allows the heavy generator and related components to be located at the base of the tower as opposed to the top, as is typical of a horizontal axis wind turbine (HAWT). This configuration lowers the topside CG which reduces the platform stability requirements, leading to smaller and cheaper platforms. Additionally this locates high maintenance systems close to the ocean surface thus increasing maintainability. To support this project and the general wind research community, the Offshore Wind ENergy Simulation (OWENS) toolkit is being developed in conjunction with Texas A&M as an open source, modular aero-elastic analysis code with the capability to analyze floating VAWTS. The OWENS toolkit aims to establish a robust and flexible finite element framework and VAWT mesh generation utility, coupled with a modular interface that allows users to integrate easily with existing codes, such as aerodynamic and hydrodynamic codes. Current efforts to include a hydrodynamic module are focused on coupling WavEC2Wire with OWENS. WavEC2Wire is a wave-to-wire numerical model developed by Marco Alves at the Insituto Superior Tecnico for the analysis of wave energy converter devices. It has been adapted from its original form and restructured for use as a hydrodynamic module capable of providing OWENS with necessary floating platform dynamics. Hence, WavEC2Wire functions as a rigid-body solver designed to calculate the platform motion due to wave loads, moorings, and the influence of the attached VAWT and tower. This paper presents the WavEC2Wire module and details the OWENS coupling method. Additionally, planned improvements in the WavEC2Wire module as well as future development in OWENS are presented. INTRODUCTION Although offshore wind resources make offshore wind energy an attractive opportunity, the cost of energy (COE) of offshore wind projects must be reduced to make offshore wind a viable option. As over half of the capital investment in offshore wind is in the balance of station costs (Figure 1), reducing these expenditures could greatly reduce the COE for these projects. For a land based wind farm the turbine contributes 68% of the installed cost, whereas it is only 32% of the total for an offshore wind project [15]. Therefore, it is more important to consider turbine designs that lower the balance of station costs rather than trying to decrease the cost of the turbine itself.