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.
