Wind–Wave Loading and Response of OWT Monopiles in Rough Seas

Monopile-supported OWTs are prone to ringing – a dangerous nonlinear resonant phenomenon which is still not fully understood. Both wind and waves have a major influence on it, wind with its aerodynamic damping in the case of an operating wind turbine, and waves due to the fact that only in the case of fully nonlinear wave kinematics such phenomenon has been observed numerically. This study is interested in both of these influences, especially in the case of misaligned wind and waves – when the nonlinearities of waves are coming from a direction of reduced aerodynamic damping. However, for the influence of wave nonlinearities to be fully understood and the most accurate loading model to be used in the study, an intermediate investigation is conducted comparing the loading from combinations of six models for wave kinematics in increasing nonlinearity and three hydrodynamic loading models – Morison equation, slender-body theory in its two used formulations, and FNV perturbation theory on a fixed cylinder representative of an OWT monopile. It has been found that wave kinematics from fifth order could already be sufficient to capture the nonlinear loading in such steep waves as long as a suitable more sophisticated loading model is used. Moreover, none of the considered combinations in this two-dimensional study manage to capture the secondary load cycle, which used to be directly linked to ringing, therefore in the next phase a moving cylinder is to be considered to analyse the potential of capturing ringing in the most efficient manner before moving to the full dynamic OWT in wind and waves.