Abstract
Accurate load simulations are necessary in order to design cost-efficient support structures for offshore wind turbines. Due to software limitations and confidentiality issues, support structures are often designed with sequential analyses, where simplified wind turbine and support structure models replace more detailed models. The differences with an integrated analysis are studied here for a commercial OWEC Quattropod. Integrated analysis seems to generally predict less damage than sequential analysis, decreasing by 30-70 percent in two power production cases with small waves.Additionally it was found that using a different realization of the wave forces for the retrieval run in sequential analysis leads to an increase of predicted damage, which can be explained as the effect of applying two independent wave force series at the same time.The midsection of the detailed support structure model used shell elements. Additional analyses for a model with an equivalent beam model of the midsection showed only small differences, mostly overpredicting damage by a few percent. Such models can therefore be used for relatively accurate analysis, if carefully calibrated.
Highlights
The design of support structures for offshore wind turbines is based on accurate load calculations with numerical computer models
In this study we considered a commercial OWEC Quattropod R designed by OWEC Tower AS for a water depth of 26.1m in the Thornton Bank project
Two main environmental conditions were used for the power production loadcases that are relevant for fatigue lifetime estimation: (A) irregular waves with Hs = 1.0 m, Tp = 4.95 s and γ = 1.06 from a JONSWAP spectrum, and turbulent wind at U = 8 m/s from a von Karman spectrum with turbulence intensity I = 0.153; (B) irregular waves with Hs = 2.59 m, Tp = 6.99 s and γ = 2.13; turbulent wind at U = 20 m/s with I = 0.121
Summary
The design of support structures for offshore wind turbines is based on accurate load calculations with numerical computer models. Computer codes, and the way in which calculations are performed can significantly influence the results. This can potentially lead to overconservative designs, and a better understanding of the issues involved might offer the possibility of further optimizing support structures and reducing their costs. In this study we considered a commercial OWEC Quattropod R designed by OWEC Tower AS for a water depth of 26.1m in the Thornton Bank project. The load calculations and certification in this commercial project were performed in cooperation between the support structure designer OWEC Tower AS, the wind turbine manufacturer REpower Systems AG and the engineering consultancy TDA. Basis for most load calculations was a sequential analysis [9] that allows for cooperation between all parties involved without sharing detailed computer models
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