Numerical modelling of the HAB Energy Buoy: Final report

This report presents a complete summary of the project "Numerical modelling of the HAB Energy Buoy". The HAB Energy Buoy is a self-reacting wave energy device consisting of two concentric heaving bodies, where energy is absorbed through relative motion between the bodies. The project is organized into two stages. In the first stage, a numerical frequency-domain model of the device is developed and used to investigate a number of variations of the device geometry, in order to arrive at a design optimized for the target deployment site. The model is capable of predicting the power capture, motion response, and power take-off loads of the device. The model is further able to give an estimate of the power production of the device in a given wave climate as well as other statistical estimates of the device motions and loads. The performance of different device shapes and dimensions has been evaluated, where displacement limits appropriate for each configuration are imposed to give a more realistic prediction of the power capture and help ensure a fair comparison. The process has led to a better understanding of the behaviour of the device. The adopted design, which has a gradually enlarged tube at the bottom, is found to be superior to a design with a uniform tube. In the second stage, the frequency-domain model is developed further to include losses due to drag, using a linearized drag model. The model is validated by small-scale model tests in Aalborg University's wave tank, where measurements of the device displacements, power capture, and mooring loads are shown to be generally in good agreement with the numerical predictions. The tests however also revealed the occurence of Mathieu instability, where significant pitch and roll motions were observed having an oscillation period twice that of the wave. A time-domain model of the device is subsequently developed, and is able to predict the occurence of this behaviour. Some strategies to avoid the occurence of Mathieu instability are considered, and lowering the centre of gravity is found to be an effective strategy. The inclusion of power take-off stiffness in addition to damping, where each is optimised for each sea state, is not found to improve the mean power capture for the given wave climate. A particular strategy of latching is considered in this study, which does not require future wave prediction. The strategy is able to somewhat increase the power capture in regular waves, but when applied in irregular waves produces similar result to the original case without latching.