Reduction in Mooring System Lifetime due to Corrosion and Severe Storms

Abstract

Abstract We report on a multi-year investigation into mechanisms of fatigue, corrosion, and abrasion on floating offshore wind turbine (FOWT) mooring systems funded by the U.S. Bureau of Safety and Environmental Enforcement (BSEE). The project team examined the efficacy of standards-based design procedures as they relate to fatigue damage and material degradation. Best practice design encourages applying 50% of a mooring chain corrosion allowance to fatigue calculation. However, the project team concludes that this practice may underestimate fatigue damage when the corrosion rate is high relative to the component diameter. A more robust accounting of corrosion rate in fatigue calculations decreased the predicted lifetime of a mooring system by over 80% in one small scale field deployed case study. This discrepancy was reduced in a similar comparison of a full-scale system. Best practice design focuses on operational and normal sea states for fatigue damage calculation. The project team has concluded, through simulation of validated case studies, that severe storms may contribute significant fatigue damage to chain moorings. Over 50% of the fatigue on a prototype FOWT mooring was caused by storms which occur less than 0.2% of the time. A single 20-year storm produced over 15% of the anchor fatigue of a theoretical full-scale FOWT. By augmenting standards-based predictions of the degradation rate of chain-based mooring systems through better accounting for corrosion and abrasion rates over the full lifetime of the mooring system and including rare, extreme storms in fatigue calculations, the engineer or developer can plan ahead and reduce the risk of dangerous and expensive failures.