MARIN Model Tests Breaking Waves
Unique model testing at the MARIN facilities of breaking waves against an offshore wind turbine, has confirmed that breaking waves can induce significant oscillations and accelerations in the turbine. Together with a number of other partners, MARIN and ECN are in the process of starting up a Joint Industry Project (JIP) with the acronym WiFi: Wave impacts on Fixed turbines.
MARIN has a lot of expertise in the field of wave impacts against ships and offshore structures. Dr Bas Buchner, Vice President of MARIN and leader of its Renewable ENergy Team (RENT): “MARIN wants to contribute to the development of reliable offshore renewable energy with this expertise from the maritime industry. Breaking waves can occur quite often in the shallow water areas of fixed wind turbines. Considering the relatively flexible foundations and towers of offshore wind turbines, we realised that the impulse load of breaking waves might result in resonant vibrations of the structure. This can have significant effects on the loads in the tower and accelerations in the critical mechanical systems at nacelle level . But we did not want to state this as a problem without a confirmation, so we developed these pilot model tests within our own R&D.”
For this purpose a special model was developed by MARIN’s specialists in the field of hydrodynamic loading and structural response. The model material and set-up was chosen such that the model had the correct flexibility and natural periods, a technique recently developed to investigate the vibrations of ships in extreme waves. The initial results of the tests confirm that the tower can vibrate due to the breaking waves. Erik-Jan de Ridder, the responsible MARIN Project Manager: “When a large wave hit the foundation and tower, the tower started to vibrate at its natural periods. At nacelle level this resulted in large horizontal accelerations, in one case even up to 0.5g. Although these results need further analysis, the first observation is that these values are large and will impact the design and operation of the turbine.”
The test results will now be further analysed together with MARIN’s partner ECN based on its extensive expertise in turbine design, mechanics and dynamics. Aart van der Pal, Manager Integral Wind Turbine Design of ECN: “Breaking wave loads can affect the extreme and fatigue loads on the foundation, tower, turbine blades, shaft, gearbox and generator, aspects that have not been studied in detail so far. This will need to be taken into account in the design process.”