Dynamic responses of a scaled tension leg platform, wind turbine support structure in a wave tank

Understanding the dynamic behaviour of offshore wind and wave energy devices is extremely important in relation to the maintenance and management of these marine assets. Since experimentation in sea may often be difficult, expensive or unnecessary at a conceptual or at precommercial stage, scaled experimentation in wave tanks can be central to the understanding and assessment of their dynamic responses. This paper presents scaled dynamic testing in a wave tank of a tension leg platform for supporting a wind turbine for different regular wave conditions and sea states characterised by the Bretschneider spectra. The dynamic response of the device was monitored at different locations on the device using load cells and with a camera based motion recognition system. For the analysis, a frequency response function based approach was considered to illustrate the response of the device under varied wave loading conditions. The experimentation does not assume any underlying theoretical model, apart from the consideration of Froude scaling conditions to attempt to link experimentation conditions with actual conditions. The results are important for characterisation of theoretical models and fitting parameters of such theoretical models, while identifying the limits and potential challenges of applying linear dynamic analysis in these situations. The findings are also important for establishing system identification and control strategies for undesirable levels of dynamic response. The fitted models based on physical modelling, when combined with scaled experimental responses, can also be helpful in better estimating extreme responses of the devices.