A hybrid wind and wave energy converter concept, which consists of a spar floating wind turbine and a coaxial wave energy converter named the STC (Spar Torus Combination) concept, has been proposed to achieve better utilization of the ocean space and energy, synergy of the wind and wave conversion, and to reduce the energy cost. The torus moves along the spar cylinder in operational conditions. The critical component of this concept is the interface between wave energy converter and spar floater. Nonlinear local structural modelling of the interface is performed and presented in this paper. The wave energy converter is designed to be connected mechanically to the spar floater by a series of rollers through hydraulic supports. Rubber rollers are used to reduce the contact stress. Different configurations of the spar structure at the interface with the torus are investigated. Quasi-static analyses are carried out. Numerical and experimental studies of the global behavior have been performed, and the results are used as input to the local structural analysis.


Offshore wind energy and wave energy are two significant offshore renewable energy resources. To address the integration of wind and wave energy devices on a single platform with focus on floating concepts for deep water application and to achieve a better utilization of the ocean space, the European Commission FP7 Marine Renewable Integrated Application Platform project (Sojo and Auer, 2014) was established. Under this project, several hybrid concepts of wind and wave energy converter were proposed, and three of them were narrowed down. The spar torus combination (STC) is one of the three concepts (Muliawan et al., 2012). In addition, there are several other hybrid concepts proposed (Perez-Collazo et al., 2015). Cost of the facilities of these concepts could be reduced due to the sharing of the same floater, mooring system, cables and power substations.

The STC concept is composed of a spar floating wind turbine (FWT) and a torus shape wave energy converter (WEC) as shown Figure 1. The WEC is installed on the spar cylinder. Wave power can be absorbed through a hydraulic power take off (PTO) system by the relative heave motion between spar and WEC, and the PTO system damping can be adjusted to control the performance of the WEC under different sea states. In emergency situations, the PTO system should be able to be released and the mechanical system should have the ability to brake the relative heave motion between the two bodied. Positive synergy between the spar and the torus has been shown by the integrated numerical simulations with respect to the power and motion (Muliawan et al., 2013b) under operational conditions, and model tests on the functionality of the STC have been performed and numerical validation was carried out (Wan et al., 2016b). However, under extreme sea states, the survivability of the STC is challenging, and several survival modes (Muliawan et al., 2013a) have been proposed to ensure the structural integrity. Model tests and numerical simulations on the selected survival modes have been carried out to investigate the feasibility and performance of the survival modes under different environment conditions (Wan et al., 2014; Wan et al., 2016a).

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