A methodology to estimate the risk of ringing on offshore wind turbine gravity based foundations

The offshore wind industry is following a fast increase in Europe, leading developers to consider sites with higher water depths, for which Gravity Based Foundations (GBF) could be a better option than monopiles. GBF are reported as potentially subject to “ringing” issues, and very high loads have been observed during basin tests on a generic foundation. The objective of this study is to propose a numerical methodology to estimate the probability of occurrence of such very high loads on a GBF. The hydrodynamic load model used here relies partially on potential flow theory and partially on strip theory, with Morison, Rainey and FNV terms applied on the smallest section of the foundation. In order to capture the wave asymmetry observed in high sea states, the free surface elevation is computed under irregular and nonlinear assumption, using the HOS method to solve the Zakharov’s equation. Results demonstrate the interest of using nonlinear free surface model as compared to first order model. The distribution of the overturning moment for three levels of Hs is also provided, making possible the evaluation of ringing occurrence.