Investigation of the Effect of Tuning Mechanisms on the Operational and Survival State of a Floating Oscillating Water Column Wave Energy Converter

This research was undertaken to determine the characteristics governing a thin-walled cylindrical, floating oscillating water column wave energy device with regards to power production and device integrity. This investigation considers how such an oscillating water column wave energy device can be optimised for power production yet still be robust enough to withstand unfavourable storm conditions by tuning mechanisms such as heave added mass changes, power take-off damping changes, and stiffness changes. The investigation also considers how the initial device and oscillating water column sizing affects performance in favourable and unfavourable sea conditions. This research was undertaken by utilising WAMIT and OrcaFlex. These are two industry accepted analysis tools. This thesis examines the power take-off efficiency of the device in moderate frequency waves and employs wave conditions from DNV standards during survival studies. It is conjectured that installation sites with moderate to low energy are more feasible for motion dependent wave energy converters than sites with higher concentrations of energy because the unfavourable storm conditions are not as severe. It was determined that a system with an oscillating water column natural frequency designed to match the mean peak wave frequency of the desired installation site, and operating within a structure with a natural frequency approximately 0.66 times the oscillating water column natural frequency, produced the most efficient system. This ratio ensured sufficient spacing between the natural frequencies. This spacing allows increased velocity differentials with a single forcing frequency. Achieving this ratio of natural frequencies is most feasible through tuning of the heave mass of the structure. This thesis concludes that such a device can withstand unfavourable storm conditions if the structure natural frequency can be altered. Adjusting the heave mass to move the natural frequency of the structure away from the peak wave frequency reduces the peak heave displacement and hence the peak mooring line tensions. Tuning during the operational and survival states is most feasible through changing the heave added mass of the device by employing or withdrawing heave plates.