Fitness-for-Service Assessment of In-Service Wind Turbine Towers

This paper presents a fitness-for-service (FFS) assessment of wind turbine towers currently in service. The turbines had been in service for several years before the manufacturer determined that the towers were inadvertently manufactured with crack-like defects that were not identified during quality control. This revelation resulted in the possibility of crack propagation over time, which could remain unnoticed in between infrequent visual inspections during routine maintenance. Cracks could grow long enough to threaten the serviceability or even the survivability of the towers. While it was possible, using the principles of deterministic fracture mechanics, to determine the maximum allowable crack length before fracture is expected to occur, and hence the remaining life in years, these types of calculations may be excessively conservative, depending on the material properties, and loading parameters assumed. A much more definitive quantification of failure risk was determined, by evolving deterministic fracture mechanics into probabilistic fracture mechanics. This approach runs tens of millions of individual fracture calculations by addressing the inherent spread in material behaviors with return periods of turbine specific load cases, culminating in the determination of a probability of failure over time. The manufacturer could then compare these probabilities with industry allowables, helping to manage their risk. These risk measurements also helped determine appropriate frequencies of inspection, helping to further alleviate manufacturer and owner concerns. This paper will benefit the engineering community by introducing them to state of the art analytical tools available for determining the status of damaged infrastructure, helping to make informed repair/replacement decisions.