Abstract
The oscillating-water-column (OWC) spar-buoy is a type of wave energy converter that may exhibit undesirable large roll and pitch amplitudes caused by a dynamic instability induced by parametric resonance. The occurrence of this phenomenon not only reduces the power extraction but significantly increases the structural loads on the buoy, the turbine rotor and on the mooring system. The paper compares the parametric resonance behaviour of two configurations of an OWC spar-buoy using experimental data obtained in a wave flume at a scale of 1:100. The configurations investigated were: (1) closed and (2) fully open-air chamber. The experimental tests covered a wide range of regular and irregular waves, as well as in free decay experiments. Results showed that opening the air chamber reduces the coupling between the buoy and the OWC within, thus shifting the damped natural heave frequency of the system in comparison with the closed chamber configuration. This effect changes parametric resonance characteristics of the two configurations due to the coupling between roll/pitch and heave modes. Moreover, for specific wave frequencies, the occurrence of parametric resonance observed when the chamber is closed do not occur while the air chamber is fully open. These results suggest the possibility of controlling a pressure relief valve installed on top of the device to reduce parametric resonance whenever this dynamic instability is detected.
Highlights
An oscillating water column (OWC) spar buoy, is a type of wave energy converter (WEC) that utilises the relative heave motion between an outer spar hull and an inner moon poollike water column to drive the air, enclosed in a chamber above the OWC, through a turbine installed at the top of the buoy and connected to the atmosphere
The purpose was to assess the possibility of applying an active control method to a pressure relief valve installed in parallel with the power take-off system to reduce large pitch and roll motions due to parametric resonance
The present study explores a dual purpose for these relief valves, to mitigate the occurrence of parametric resonance, since opening a relief valve reduces the coupling between the spar buoy and inner water column, altering the device motion and providing a means to detune the resonant dynamics
Summary
An oscillating water column (OWC) spar buoy, is a type of wave energy converter (WEC) that utilises the relative heave motion between an outer spar hull and an inner moon poollike water column to drive the air, enclosed in a chamber above the OWC, through a turbine installed at the top of the buoy and connected to the atmosphere. Like many offshore spar structures [1,2,3,4], this device is prone to large amplitude pitch and roll motions, caused by parametric resonance, when the frequency of the waves is around twice the pitch/roll natural frequency, see Figure 1. Parametric resonance is a dynamic instability caused by the time-varying changes in the parameters of a system [5], which manifests in floating offshore structures, due to the wave-induced heave motion of the structure varying the metacentric height. The occurrence of parametric resonance in the OWC spar buoy severely reduces the heave motion and energy capture. Large-amplitude pitch/roll motions can lead to structural and safety issues.
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