Session: 09-01-04 Wind Energy: Aero-hydrodynamics 4

Paper Number: 125820

125820 - Relationship Between Waves and Operating Offshore Wind Turbine Response 

The offshore wind turbine industry has been actively working toward the goal of carbon neutrality by 2050. Technological advancements in large-scale fixed-bottom offshore wind turbines (OWTs) have significantly improved power generation and economic efficiency. However, the dynamic response characteristics of these turbines, especially in the presence of waves, remain largely unknown. In this study, we conducted a series of numerical simulations using a wind-wave load coupled analysis method to understand the relationship between wave action and the dynamic response of a 15MW monopile-type OWT in wind and waves.

We selected a 15 MW monopile-type OWT model, which was adopted for analysis code comparison and validation within the framework of the International Energy Agency Wind Technology Cooperation Program (IEA Wind TCP). This OWT model is a three-bladed upwind type with a variable-speed wind turbine featuring pitch control. Its operational wind speeds include cut-in at 3 m/s, rated at 10.6 m/s, and cut-out at 25 m/s. In our study, we assumed that the OWT model is situated in coastal waters at a water depth of 20 m. The analyses were conducted using OpenFAST, a time-domain solver developed by the National Renewable Energy Laboratory (NREL) in the U.S., which serves as a wind turbine load analysis code. This code is a comprehensive coupled wind-wave-structure analysis code, encompassing aerodynamic analysis based on blade elementary momentum theory, hydrodynamic analysis employing the modified Morison equation, and structural analysis for both the wind turbine tower section based on the modal method and the monopile section based on the finite element method. Note that, In the hydrodynamic analysis, wave breaking forces were not considered. The only drag and inertia forces were considered to calculate wave forces. Therefore, our focus in this study was on understanding the impact of non-breaking waves on the dynamic response of the OWT. For our input data, we characterized fluctuating winds using a Kaimal spectrum (mean wind speed at the hub height ranging from 4 to 24 m/s, wind direction at 0 deg), and uni-directional irregular waves using a JONSWAP-type spectrum (wave height ranging from 2 to 8 m, period from 6 to 16 s, wave direction at 0 deg). The data was sampled at a frequency of 20 Hz, and the analysis spanned a period of 600 s.

As a result of our investigation, in wind and waves, wave loads have a substantial impact on the dynamic response characteristics of the OWT at the mudline. The bending moments can locally increase, especially in steep wave conditions, depending on the wave steepness and relative water depth. From the perspective of designing and constructing monopile-type OWTs, it is crucial to accurately evaluate the amplification effect of the bending moment due to waves. Furthermore, when dividing the waves into wind waves and swell and investigating the specific impact of each on wind turbine responses, it was found that bending moments can be locally amplified due to wind waves and swell under specific conditions during strong winds and high waves, such as in typhoons and strong low-pressure systems.  Similarly, swell can lead to bending moment amplification under weak wind conditions.

Presenting Author: Hiroaki Kashima Port and Airport Research Institute

Presenting Author Biography: Hiroaki Kashima is a wave professional with over a decade of practical experience in the field of ocean and coastal engineering. In recent years, He has expanded his research activities into the fields of wind engineering and vibration engineering, focusing on research on the behavior of offshore wind turbines with respect to wind and waves.

Authors:

Hiroaki Kashima Port and Airport Research Institute
Haruo Yoneyama Port and Airport Research Institute