Structural condition assessment of offshore wind turbine monopile foundations using vibration monitoring data

Abstract

The offshore wind farm industry has substantially grown in recent years. There is a substantial number of wind farms in the waters offshore Europe with several others proposed for regions in North America and Asia. A vast majority of the existing offshore wind turbines are supported on monopile foundations. The presence of a monopile in a flowing body of water causes the water to accelerate around the monopile, which can result in seafloor scour. Scour can significantly reduce the soil support thereby reducing the monopile stability and strength. Scour can also reduce the fundamental natural frequency of the turbine structure. If the natural frequency of the support structure approaches the turbine rotor frequency, a resonant condition will develop that can damage the turbine or its support structure. To avoid this situation, a monitoring system consisting of two strategically located bidirectional accelerometers was installed on the towers of four turbines of an existing offshore wind farm. The objective of the monitoring campaign was to correlate frequency measurements with the overall structural performance of the monopile foundations. The developed approach was based on operational modal analysis and system identification techniques using acceleration response data and calibrated computer models. Identified frequencies were correlated to specific structural performance limit states including; vibrating resonance of the tower with the turbine rotor, lack of foundation stability, and yielding of the monopile foundation material. This paper presents the findings of this work, which can assist owners and operators who may benefit from similar structural health monitoring systems.