Dynamic analysis of monopile supported offshore wind turbines

Abstract

This paper describes the stochastic dynamic response of National Renewable Energy Laboratory 5 MW offshore fixed-based wind turbine (OWT) under various soil conditions – medium dense sand, stiff clay and layered profiles in 20 m depth of water. The aerodynamic and hydrodynamic OWT loads are derived using the force-controlled approach. Usually the OWT generates power in an operational regime and survives at extreme wind speeds. Therefore, two met-ocean conditions adhering to the irregular Joint North Sea Wave Project spectrum are considered – one in an operational regime (average wind speed Vw = 12 m/s, significant wave height Hs = 4 m and peak spectral period Tp = 10 s) and another in a near cut-out regime (Vw = 22 m/s, Hs = 10 m, Tp = 14 s). The soil is modelled by way of a non-linear ground-to-spring model. For each sea state, time domain stochastic responses are calculated and the ensemble average response is calculated from 50 Monte-Carlo simulations. The change in ensemble average response due to changes in pile penetration depth and diameter of the piles for the three soil configurations is described. Results indicate that the dynamic response mainly depends on the stiffness of the soil and reiterate the need for detailed site-specific geotechnical investigations before designing OWT foundations.