Validation of a Frequency-Dependent Morison Force Formulation for a Large Monopile in Severe Irregular Seas

Motivated by the increased interest in renewable energy and the need for validated hydrodynamic load models, a rigid monopile and a fully flexible large monopile wind turbine have been tested experimentally at a 1:50 model scale in irregular waves. Furthermore, a new engineering load model combining the conventional Morison equation for slender bodies with a frequency-dependent mass coefficient based on the first-order MacCamy and Fuchs solution has been developed and compared to Rainey’s load model and the model test results. Nonlinear wave kinematics based on the results of the nonlinear potential code REEF3D::FNPF have been applied as input to the numerical models. The new model better estimates the response of the monopile in the frequency range relevant for ringing events. The experimentally obtained 90th percentile bending moment response near the first natural frequency of the monopile is estimated within 2% by the new model with frequency-dependent mass coefficient, while a traditional Morison approach or the Rainey model overestimate the response by 47% and 74%, respectively.