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.

Effect of foundation modelling on the fatigue lifetime of a monopile-based offshore wind turbine

“…The viscous dampers have been calibrated to give a foundation damping factor of approximately 1 % near rated wind speed conditions (load case 6). This is considered reasonable and in line with studies from literature (Shirzadeh et al, 2013;Carswell et al, 2014). The foundation damping factor of 1 % expresses the hysteretic energy loss in the soil as a percentage of the total elastic strain energy of the soil.…”

Geotechnical aspects of offshore wind turbine dynamics from 3D non-linear soil-structure simulations

“…A number of studies have been carried out to fill this gap, in most cases by performing emergency OWT shut-downs, i.e. rotor-stop tests (Tarp-Johansen et al, 2009;Versteijlen et al, 2011;Damgaard et al, 2012Damgaard et al, , 2013Shirzadeh et al, 2013;Carswell et al, 2014). Table 4 provides a synopsis of methodologies and inferences from the cited literature studies, as well as a solid ground to assess this study's results -extracted from the most meaningful cases D r = 60-80%.…”

response to referee 1

“…They found that a 50 % reduction in the soil's Young modulus increased the fatigue damage equivalent moment at mud-line by approximately 12 %; and a 50 % reduction of the soil damping properties increased the fatigue damage equivalent moment by 25 %. Carswell et al (2015) studied the effect of soil damping for an OWT with a monopile foundation subjected to extreme storm loading. The hysteretic damping was computed using a non-linear elastic two-dimensional finite element model and was included in the foundation model by a viscous rotational damper at mud-line.…”

“…5, where K is a stiffness matrix, d is a displacement vector, and C is a damping matrix. In Model 3, damping coefficients are only applied in rotational degrees of freedom (rotation around x and y axes), as moments typically dominate mud-line loading for OWT monopiles (Carswell et al, 2015). The moment response for a single degree of freedom is given by…”