Jørgen R Krokstad

Power Absorption Measures and Comparisons of Selected Wave Energy Converters

In this study, a selection of Wave Energy Converters (WECs) with different working principle is considered. It comprises a heaving device reacting against the seabed, a heaving self-reacting two-bodies device, a pitching device, and a floating OWC device. They are inspired by concepts which are currently under development. For each of these concepts, a numerical Wave To Wire (W2W) model is derived. Numerical estimates of the energy delivery which one can expect are derived using these numerical models on a selection of wave site along the European coast. This selection of wave site is thought to be representative with levels of mean annual wave power from 15 to 88 kW/m. Using these results, the performance of each WEC is assessed not only in terms of yearly energy output, but also in terms of yearly absorbed energy/displacement, yearly absorbed energy/wetted surface, and yearly absorbed energy per unit significant Power Take Off force. By comparing these criteria, one gets a better idea of the advantages and drawbacks of each of the studied concepts.Copyright

Extreme Response Statistics of Fixed Offshore Structures Subjected to Ringing Loads

This paper describes an efficient Monte Carlo based method for prediction of extreme response statistics of fixed offshore structures subjected to random seas. The nonlinear structural response known as “ringing” is studied, which is caused by the wave impact force on structural support units. Common challenge for design of such structures is a sound estimate of the hydrodynamic load including diffraction effects. The aim of the work was to develop specific methods which make it possible to extract the necessary information about the extreme response from relatively short time histories. The method proposed in this paper opens up the possibility to predict simply and efficiently both short-term and long-term extreme response statistics. The results presented are based on extensive simulation results for the large fixed platform operating on the Norwegian continental shelf. Structural response time histories, measured in MARINTEK (MT) wave basin lab, were used to validate numerical results.

Long-term fatigue damage sensitivity to wave directionality in extra-large monopile foundations:

The fatigue damage sensitivity to wave directionality on large-diameter monopile foundations for use in the offshore wind turbine industry is investigated. A frequency-domain approach with the Dirlik method is used to estimate fatigue damage and to evaluate the effect of wave spreading and swell separation compared to a total sea representation. To evaluate the long-term impact, 30 years of hindcast data from the Dogger Bank area are used. Furthermore, a computationally efficient time-domain model of a 10-MW offshore wind turbine is used to compare with the frequency-domain results. Results show that benefits in terms of structural fatigue can be obtained with directional considerations.