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OC4 P1 LC5.7 Turbulent Wind Irregular Wave

The final load case in OC4 Phase I is the most complex, applying both turbulent wind and irregular wave loading to the fully flexible offshore wind turbine. The mean wind speed is 18m/s, and the turbulence description is based on the Kaimal spectrum and the normal turbulence model (NTM). The turbulent wind field definition is supplied by Risø DTU (Technical University of Denmark) in the form of HAWC2 format binary files. The ambient seastate is described by a Pierson-Moskowitz wave spectrum with Hs=6m and Tp=10s.

Popko et al. (2012) present results graphically in terms of probability density functions (PDF), power spectral densities (PSD) or damage-equivalent loads (DEL), for load cases with stochastic inputs such as this one. For conciseness here, a range of results are neatly summarised in tabular format instead. For each output parameter, the table presents:

•Mean value of the parameter predicted by Flexcom (e.g. mean generator power)

•Standard deviation of the parameter predicted by Flexcom (e.g. to quantify power fluctuation)

•Percentage difference between Flexcom’s mean value and the average mean of the other modelling tools

•Percentage difference between Flexcom’s standard deviation value and the average deviation of the other modelling tools

Wind speed and wave elevation are included in order to check consistency of inputs between Flexcom and the other models. The mean wind speed and the deviation in wind speed (which is a measure of the turbulence) are both consistent between all modelling tools. Likewise, the statistics of wave elevation are also consistent.

The mean values of generator power, rotor speed and lower speed shaft torque estimated by Flexcom all show very close agreement with the other modelling tools. However, Flexcom is underpredicting the deviations about the mean for each of these three parameters. These trends are consistent with OC4 P1 LC5.6, a more straightforward load case which considered steady wind and regular wave loading only. For reasons discussed previously, the simplified RNA model used in Flexcom 8.12.1 is unable to properly capture the characteristic fluctuations in loading and response at the 3P frequency. It is planned to address these limitations using a more advanced RNA model in the next version of the software.