Condition Monitoring of Offshore Wind Turbines with Scour and Grout Damage in Monopile Foundations

Monopile foundations with grouted connections are used extensively for offshore Wind Turbine Generators. In recent years, the offshore wind industry discovered that the existing design methods for monopile to transition piece grouted connections may result in problems associated with grouted transition pieces. This has been evidenced by the settlement of the upper sections of tower structures onto the lower parts of the structures (i.e. the piles). In addition to grout damage, scour at the base of the turbine is another challenge. Excessive scour that occurs immediately adjacent to the monopiles can destabilise the turbine and affect its natural frequency. The problem of grout damage and scour can increase the likelihood of lost operation and structural failure and therefore timely detection of these problems is vital. This paper presents the derivation of a conditional monitoring strategy for detection of both issues. It has been shown that vibration monitoring of the structure alone cannot help in identifying possible scour and grout damage. It is illustrated that the forces/displacements across the new elastomeric bearings which are installed as a retrofit measure, in conjunction with long range stress/strain measurement can be used for grout damage detection as well as identification. Subsequently, vibration monitoring can be used to identify scour.