Subsoil strain measurements on an operational wind turbine for design validation and fatigue assessment

Measuring on offshore wind turbines is a challenging and cost intensive work especially when aiming for poorly accessible parts of the structure. While it is desirable to replace these measurements by indirect methods uncertainty exists about the necessary assumptions in environmental properties (e.g. soil conditions) and the structural dynamics. This paper presents preliminary results of a measurement campaign of subsoil strains on three operating offshore wind turbines located in the Belgian North Sea. The measurement data enables to check design assumptions e.g. concerns are raised about geometric constraints of the original design. Furthermore the dynamic behavior of the structure is analyzed in order to identify main fatigue contributions. Sensor data revealed a high impact of wave load for welds low on the monopile as well as notable contribution of the second order mode on fatigue damage below the mudline.