Abstract
The US East Coast has several leased wind energy sites in water depths ranging from 60–100 m, where floating wind turbine systems are feasible. For these systems, designing suitable mooring systems for shallow water depths within the constraints of surviving hurricane conditions can be challenging. This study analyzed a mixed chain-synthetic mooring systems to predict the maximum extreme position and effective tension for different cases. Synthetic ropes, which offer a low cost and superior performance alternative compared to chain and wire ropes, were utilized dominantly in our mooring design. In this study, we simulated the IEA 10 MW reference wind turbine on a semi-submersible floating offshore structure deployed at two sites: Nantucket, MA (WD (Water Depth) = 60 m), and Monhegan, ME (WD = 100 m). For each depth and each mooring configuration, we conducted detailed numerical simulations for three different load design cases DLC 1.6 50-yr return period, DLC 6.1 50-yr return period, and SLC I.1 500-yr return period. The inflow turbulence was based on the Kaimal spectrum with the mean wind speed and turbulent intensity based on individual load cases and return periods. The JONSWAP spectrum was used to model waves with different significant heights and peak periods based on individual load cases. A uniform current was used for all simulation cases. For extreme value predictions, three 1-hour simulations with different seed numbers for the SLC I.1 500-yr return period were performed. The MPME (most probable maximum extreme) value is an extreme value statistic commonly used in the offshore industry. The top 10% of the peaks above a threshold value were further analyzed using a Weibull distribution to obtain MPME values. The effects of mooring material, number of lines per column and the water depth on the MPME as well as cost issues are discussed in the paper.
