Experimental and Numerical Study of a Combined Offshore Wind and Wave Energy Converter Concept
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- Institutt for marin teknikk 
Clean and renewable energy is increasingly emphasized with the growing public attention to the environmental pollution problem. Among renewable energy resources, both offshore wind and wave energy are resources with great potential. However, the stages of technological development are quite different for wind and wave energy: wind energy technology has already been commercialized, while wave energy technology is still immature. To address the integration of wind and wave energy devices in one single offshore floating platform, a combined wind and wave energy converter concept named the spar torus combination (STC) concept was proposed under the European Commission FP7 Marine Renewable Integrated Application (MARINA) Platform project. The STC concept is composed of a 5 MW spar-type floating wind turbine and a torus-shaped wave energy converter. As a consequence, the investment cost can be reduced due to the common infrastructure, the augmentation of produced power, and the positive synergy between the spar and the torus regarding dynamic responses and power production. In the farm configuration, ocean space and energy can be better utilized. For the design of the STC concept, it is critical to have a feasible spar-torus interface including power take off (PTO) system under operational environmental conditions. However, it is challenging to ensure the structural integrity of the STC concept under extreme environmental conditions due to large wave forces on the torus. Numerical simulations considering linear hydrodynamic forces, aerodynamic forces, spar-torus interface, mooring system and PTO system were performed to predict the integrated dynamic responses of the STC concept under different working modes. Furthermore, model tests were performed with the focus on functionality and survivability, respectively, to validate the numerical model. However, strongly nonlinear phenomena, i.e., water entry and exit of the torus in the survivability model test was observed. A nonlinear numerical model based on the nonlinear potential flow solver with a local impact solution for bottom slamming events and an approximated model for the water shipped on the deck was used to investigate the water entry and exit processes. For the functionality model test, a roller-guide system was designed to model the spartorus interface; and the PTO system was modelled by two pneumatic dampers. However, with large PTO damping levels, strong air compressibility was observed. The numerical model for the functionality test provides satisfactory results compared with the model test when there is no strong air compressibility in the pneumatic dampers. In cases with strong air compressibility, an additional linear stiffness term in the PTO model was considered; and this additional factor improved comparisons between the numerical and experimental models. Wind effects were modelled by a simplified drag disc without considering centrifugal forces or gyroscopic effects. Wind forces significantly affect surge and pitch motions, but are only marginally important for the heave motion and wave power absorption. The wave power absorption is significantly affected by the PTO parameters. Reasonable PTO damping can boost the wave power absorption, while PTO with too high damping will restrain the wave power absorption because it reduces the relative heave motions. Survivability model tests were designed and performed with the focus on two survival modes of the torus: the Mean Water Level (MWL) survival mode and the Submerged (SUB) survival mode. For the MWL mode, the torus is locked to the spar, and the whole structure floats at the mean water level. For the SUB mode, the torus is locked to the spar at the mean water level and then is totally submerged to a specified position by additional ballast. The numerical model considering linear hydrodynamic properties predicts the dynamic responses well in cases with no water entering or exiting of the torus. The performance of the SUB survival mode is much better than the MWL mode in terms of both motions and interface forces, but complex mechanisms and remote operation procedures should be deployed to implement the SUB mode in the prototype. A wind drag disc was also used in the survivability model test, and the wind-induced mean drift for the SUB mode is dominant because the wave drift forces for the SUB mode are rather small. Two survivability model tests were performed respectively in the towing tanks of MARINTEK, Norway, and of INSEAN, Italy. The purpose of these two tests was to address the uncertainties and performances of different testing facilities. The physical models used in the two towing tanks have similar properties with slight differences. Consistent comparisons were achieved between the model tests in the two testing facilities. The differences and performances of the two models were also documented. In the MWL survivability test, water entering and exiting of the torus was observed to be accompanied by bottom slamming and water shipped on deck in cases with a wave height of 9m and an incident wave period close to 13s, which is the heave resonant period. In these strongly nonlinear cases, nonlinear hydrodynamic properties should be considered. A nonlinear numerical solver was used, and satisfactory comparisons were achieved between the solver and the model test. With water entry and exit, the spar-torus interface forces are strongly modified. Further nonlinear simulations showed that the slamming forces have insignificant effects on motions compared with the water on deck forces. The water on deck effects on heave motions are more significant than its effects on surge and pitch motions, and the effects on motions increase with increasing wave steepness and wave length due to the larger amount of water shipped on deck.