Griet De Backer

Experimental investigation of the validity of linear theory to assess the behaviour of a heaving point absorber at the Belgian Continental Shelf

The results of an experimental investigation on a heaving point absorber are presented. The physical tests are used to validate numerical simulations of the behaviour of the point absorber based on linear theory in the frequency and time domain. Floater response and power absorption are evaluated in regular and irregular waves representing a mild wave climate. A good correspondence is found between the physical and numerical test results. In irregular waves the difference between numerical and experimental power absorption is generally smaller than 20%. In regular waves the correspondence is good as well, except in the resonance zone; i.e. when the natural frequency of the buoy is tuned towards the resonance frequency of the incident wave. In this case, non-linear effects such as viscosity and a non-linear hydrostatic restoring force become important due to the high velocities and motion amplitudes of the point absorber. However, because of these large amplitudes, pure resonant cases are often not preferred in practical applications. In general it is concluded that the numerical results are in good accordance with the experimental results and can be used to predict the point absorber behaviour in mild energetic waves in non-resonance conditions.Copyright

Investigation of vertical slamming on point absorbers

This paper focuses on the impact of vertical slamming on floating point absorber systems and the associated pressures that might be expected when these phenomena occur. In a first part it will be shown how the occurrence probability of slamming can be reduced by implementing a slamming restriction, i.e. by controlling the motion of the point absorber. The impact of these slamming restrictions on power absorption will be discussed. Secondly an investigation is made of the pressures that occur when the buoys are subject to vertical bottom slamming. Analytical results are presented, which give a pressure prediction of an impacting body with conical and hemispherical shape, using Wagner theory. Laboratory experiments have been carried out at Ghent University. Impact pressures were measured during drop tests with both hemispherical and conical buoy shapes. These pressures were measured by ICP pressure sensors with a range up to 345 kPa with small membrane and very high resonance frequency (> 250 kHz). Analytical and physical results are compared and conclusions are drawn.Copyright