Validation of a Combined Wind and Wave Power Installation

To meet the increasing energy demand of the world it is important to develop technology for harvesting energy from renewable sources. One of the largest renewable energy sources is the world s oceans, where wind, wave and thermal energy are considered the main sources. Offshore wind and wave technology is emerging; several designs are under development and prototype testing of some technologies has given positive results. It is beneficial to install wind turbines and wave converters on the same foundation as this can reduce costs and facilitate connection to shore.A validation of the combined wind and wave power production platform W2Power has been carried out in this thesis. W2Power is a triangular semi-submersible platform with two wind turbines and ten wave energy converters along the sides. The work has consisted of planning and performing an experimental campaign and making numerical simulation models of the platform. For the experimental campaign only one side of the platform has been considered, this to reduce costs and increase test model accuracy. The main objectives of the experimental campaign were estimation of power production and investigation of the interaction between the wave energy converters. The wave energy converters were tested for operational conditions, both regular and irregular waves. A total of three simulation models have been made using the softwares GeniE, HydroD (Wadam) and SIMA. Of these, two models are of the experimental set-up, where one is tuned to produce the same results as the model tests and one is untuned for comparison, and one model of the entire platform. The two wind turbines integrated in the W2Power design have not been taken into account. The model tests were not performed with optimal load resistance in the air cylinders due to a calculation error made in the beginning of the experimental campaign. This resulted in the air cylinders acting more as springs than dampers, overestimating the actual forces and slightly underestimating the responses of the buoys. Some measurements with optimal load resistance exist, and the estimation of produced power was done based on these. An electricity production of 10 400 GWh per year was estimated as a total for all ten wave converters connected to the platform. The expected electricity output of the platform should be in the range of 1 10 GWh per year, when compared with other wave energy devices, making the estimations unexpectedly high. The RAOs computed in Wadam for the buoys alone, i.e. not connected to the platform framework, concord with the calculated RAOs from the model test. The heave peak period in Wadam is 6 seconds versus 5.5 seconds for the measured results, while the surge peak periods are 5 versus 5.5 seconds. The heave peak amplitudes varies from 1.9 m/m for 0o incoming angle to 1.3 m/m for 90o and from 0.7 m/m to 0.4 m/m in surge. This concordance implies that the hydrodynamics of the buoys is correctly modelled. The responses measured during the model test and the calculated RAOs revealed that a shadowing effect between the buoys exist: the buoy interacting with the incoming waves first have higher response and forces than the other buoys.The analyses done with the numerical simulation model in SIMA did not give satisfactory results. The rotation point was modelled so that the rotation arms of the buoys were flexible instead of stiff arms rotating about a point. To achieve correct responses large forces were applied to the fixed elongation couplings representing the air cylinders. This led to large vertical forces on the rotation arm that were not consistent with the measured forces. However, the simulated horizontal forces acting on the platform framework were of equal magnitude as the measured for most conditions analysed. The response of the buoys had to be reduced to simulate correct vertical forces, making the numerical model inconsistent with the measured responses. Based on the model test and the estimation of produced power, it was concluded that the W2Power design is feasible.