Dynamic analysis of an offshore wind turbine including soil effects

Offshore wind turbines (OWTs) offer an attractive, sustainable solution to the impending global energy crisis. A major challenge in fixed-bottom OWT design is accounting for soil-structure interaction (SSI) under the influence of random dynamic loading from wind, waves and currents. Usually, SSI is either ignored in OWT studies or is incorporated by means of simplified foundation concepts like the apparent fixity model. OWTs in shallow water depths (less than 30 m) are mostly supported on monopiles - large diameter steel pipe piles driven into the subsoil. Monopiles transfer the dynamic lateral loads into the soil by bending action. The present work deals with the dynamic analysis of the NREL 5MW OWT on a monopile foundation, in Indian waters. It involves parametric studies on various clayey soil profiles - soft, medium stiff and stiff clay. An operational
wind speed of 12 m/s and a sea state of 4 m significant wave height and 10 s spectral peak period are considered. The OWT design should ensure that the natural frequency is away from the forcing frequencies of wind, wave and rotor. A water depth of 20 m is considered. Hub-height aerodynamic loads are obtained using the NREL-FAST code, which is based on the bladeelement momentum (BEM) theory. The hydrodynamic time domain analyses are performed in the FEM based coupled hydrodynamic - geotechnical software, DNV-GL - USFOS. USFOS makes use of the JONSWAP spectrum to generate irregular waves. Soil is represented by means of p-y, Q-z and t-z curves. Results indicate the significance of including SSI in OWT studies. Variation in response due to change in pile penetration depth and pile diameter are also highlighted. Stiffness of clay is the design driver for OWTs.