The dynamic effects of marine growth on a tension moored floating wind turbine

As the offshore wind industry moves to water depths greater that 50m floating platforms will become the only cost effective solution for mounting turbines. Such platforms will be susceptible to bio fouling over their design life with marine growth capable of altering the hydrodynamic loading. Marine growth causes member effective diameter, mass, drag coefficients, force and hydrodynamic added mass to increase. In this paper, marine growth of various thickness and surface roughness is numerically modelled on a tension moored floating wind turbine under survival conditions using combined potential flow boundary element method and Morison equation viscous drag. The influence of time variant Reynolds number dependent drag coefficients is compared against time invariant drag coefficients. Marine growth thickness and surface roughness have a notable effect on the platform hydrodynamic forces. Surge, pitch motions, and nacelle accelerations decrease as surface roughness increases. Minimum tendon forces decrease. This increases the probability of a catastrophic tendon snap or slack event. The authors calculate the increase in displacement required to avoid this loss in tension. Detailed limits on the quantity of marine growth are suggested by the authors, above which the platform must be cleaned. The time invariant drag coefficient method has been found to give sufficiently consistent results to the time variant Reynolds number drag coefficient method.