Dynamic Mudline Damping for Offshore Wind Turbine Monopiles

Abstract

Fatigue is often a design driver for large (e.g. 5–10 MW) offshore wind turbines (OWTs), necessitating a thorough examination of damping sources: aerodynamic, hydrodynamic, structural, and soil. Of these sources, soil damping has been least considered by researchers with respect to OWTs. Aeroelastic programs, such as the National Renewable Energy Laboratory (NREL) code FAST, are typically used for time history analysis of aerodynamic and hydrodynamic loads experienced by OWTs. To take into account foundation flexibility while minimizing computational expense, reduced-order foundation models such as the mudline stiffness matrix are often used. Mudline stiffness and damping matrices are derived here for the NREL 5MW reference turbine. By recompiling FAST with mudline stiffness and damping matrices, the contribution of soil damping to OWT dynamic behavior is then quantified by comparing time history analysis results including and excluding soil damping.