Petter Andreas Berthelsen

WINDOPT: An Optimization Tool for Floating Support Structures for Deep Water Wind Turbines

An integrated design tool for optimization of a floating wind turbine support structure of the spar buoy type, including mooring system and power takeoff cable, is described in this paper. The program utilizes efficient design tools for analysis of mooring system forces and vessel motions, and combines this with a gradient method for solution of non-linear optimization problems with arbitrary constraints. The objective function to be minimized is the spar buoy cost, and the mooring line and cable costs. Typical design requirements that may be included as constraints are: mooring line load limitations and minimum fatigue life, cable curvature radius, cable tension, tower top acceleration, and vessel motion and inclination. The spar buoy is modelled as composed of a set of cylindrical sections with different mass, buoyancy and cost properties, where each section is assumed to have a uniform mass distribution. It is assumed that a representative initial cost figure is available, and that it can be scaled in proportion with material mass. A simple relationship between mass and geometrical properties is proposed for both massive and thin walled tubular sections. Examples are included to demonstrate the various aspects of the optimization approach, including different parameterizations of the spar buoy.Copyright

Design and Aero-Elastic Simulation of a 5MW Floating Vertical Axis Wind Turbine

This paper deals with the design of a 5MW floating offshore Vertical Axis Wind Turbine (VAWT). The design is based on a new offshore wind turbine concept (DeepWind concept), consisting of a Darrieus rotor mounted on a spar buoy support structure, which is anchored to the sea bed with mooring lines [1]. The design is carried out in an iterative process, involving the different sub-components and addressing several conflicting constraints. The present design does not aim to be the final optimum solution for this concept. Instead, the goal is to have a baseline model, based on the present technology, which can be improved in the future with new dedicated technological solutions. The rotor uses curved blades, which are designed in order to minimize the gravitational loads and to be produced by the pultrusion process. The floating platform is a slender cylindrical structure rotating along with the rotor, whose stability is achieved by adding ballast at the bottom. The platform is connected to the mooring lines with some rigid arms, which are necessary to absorb the torque transmitted by the rotor. The aero-elastic simulations are carried out with Hawc2, a numerical solver developed at Riso-DTU. The numerical simulations take into account the fully coupled aerodynamic and hydrodynamic loads on the structure, due to wind, waves and currents. The turbine is tested in operative conditions, at different sea states, selected according to the international offshore standards. The research is part of the European project DeepWind (2010–2014), which has been financed by the European Union (FP7-Future Emerging Technologies).Copyright

Conceptual Design of a Floating Support Structure and Mooring System for a Vertical Axis Wind Turbine

This paper deals with the conceptual design of a floating support structure and mooring system for a 5MW vertical axis offshore wind turbine. The work is carried out as part of the DeepWind project, where the main objective is to investigate the feasibility of a floating vertical axis offshore wind turbine. The DeepWind concept consists of a Darrieus rotor mounted on a spar buoy support structure. The conceptual design is carried out in an iterative process, involving the different subcomponents. The present work is part of the first design iteration and the objective is to find a feasible floating support structure and mooring system for the DeepWind concept. The conceptual design is formulated as an optimization problem: Starting with an initial configuration, the optimization procedure tries to find a cheaper solution while satisfying a set of design requirements. This approach utilizes available response analysis programs for mooring system forces and vessel motions, and combines this with a gradient search method for solution of nonlinear optimization problems with arbitrary constraints. Two different mooring system configurations are considered: Chain systems with 3 and 6 lines, respectively.