| Developing renewable energy resources is considered as the best solution to copy with energy shortage and environmental pollution.Wave energy is an important marine renewable energy,which is widely distributed in the ocean.The wave energy resources in the deep sea are more abundant,with higher energy flow density and better quality.Offshore oscillating water column(OWC)wave energy conversion device has attracted worldwide attention due to its simple structure and the maintenance convenience.However,the hydrodynamic efficiency of the offshore OWC wave energy converter is low,and the effective frequency bandwidth is narrow;in addition,the mechanism of the nonlinear wave action on hydrodynamic load and motion response of offshore OWC wave energy device is not very clear.Therefore,this paper studies and optimizes the hydrodynamic performance of an offshore OWC wave energy device by combining theoretical analysis,numerical simulation and model test,in order to improve the energy conversion efficiency and survivability of offshore OWC wave energy device.Based on the potential flow theory,a second-order time-domain high-order boundary element model(HOBEM)is established to simulate the hydrodynamic performance of an offshore OWC wave energy device.In order to simulate the effects of Power Taken Off(PTO)system and fluid viscosity,a modified damping and secondary aerodynamic damping coefficients are introduced on the first and second order free surface boundary of the OWC chamber,and a nonlinear aerodynamic model of gas-liquid coupling is established.The numerical model can accurately simulate the hydrodynamic properties of the offshore OWC device,including the wave surface,air pressure,hydrodynamic pressure on the chamber wall,wave load and wave energy capture efficiency.A finite element model based on the slender rod theory is used to simulate the dynamic characteristics of the mooring line.The Newmark-βmethod is used to solve the coupled motion equations of the floating body and the dynamic equation of the mooring lines.The hydrodynamic properties of the single-chamber,dual-chamber and OWC device integrated with an offshore fixed wind turbine monopile are investigated by the methods of numerical predictions and experimental simulations,and the numerical predictions are compared with the experimental results.The effects of wave environment,chamber geometry(chamber width,draft,opening rate)and mooring stiffness on wave energy capture of the OWC wave energy device are discussed systematically.The simulation results show that the second harmonic wave not only reduce the efficiency of the wave energy capture at the resonance frequency,but also increase the horizontal load and moment under the action of low frequency waves;there is a resonance phenomenon that the incident wave with a specific frequency can stimulate the second-order sloshing resonance inside the OWC chamber,and the phenomenon leads to the increase of the hydrodynamic loads of the device;the wave energy capture efficiency of the OWC wave energy device decreases as the increase of the wave nonlinearity;the effective bandwidth of the single-chamber OWC device is widened by introducing an outer chamber wall.Based on the validated HOB EM,the hydrodynamic properties of different array forms of OWC devices are further simulated.The effects of incident wave direction and device spacing on wave energy capture efficiency,wave load and wave field distribution are analyzed.Due to the effect of multiple diffractions between OWC devices,the array arrangement can significantly improve the wave energy capture efficiency of the OWC device,especially front devices;the slit resonance between array devices increase the horizontal load of the OWC wave energy device.The hydrodynamic properties of three and four OWC devices integrated floating-moored platforms are analyzed.It is found that the coupling effect between mooring lines and floating integrated platform can enhance the capability of capturing wave energy for specific incident waves,which lays a foundation for the integrating investigation of the OWC wave energy device and floating marine structure. |
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