Virtual sensing on offshore wind turbines : long-term experience

Offshore wind turbines are dynamic structures that are typically designed for a finite fatigue life. However, the life-determining critical welds are often positioned below the water level and close to the mudline. This location makes regular inspection difficult and expensive. Nonetheless, to answer questions about life-time and structural health it is essential to have a clear picture on the accumulated fatigue life of these welds.
In this contribution we will explain and discuss the use of modal expansion to predict the strain history at the fatigue critical welds. The paper first discusses the properties of modal expansion based on the experience obtained over the past years and using a simple 4DOF model. In the second part we will summarize results from the long-term validation on an operational offshore wind turbine