Probabilistic demand models and fragility estimates for offshore wind turbine support structures

• We develop probabilistic demand models for wind turbine support structures.
• We assess the fragilities for serviceability, yield and ultimate performance levels.
• Activation of the power generation control system of turbine reduces the value of fragilities.
• The bending mode is found to control the fragilities for yield and ultimate performance levels.
• The effect of wave loading on the fragility is negligible especially for large wind speeds.

Probabilistic models are developed to predict the deformation, shear and moment demands on offshore wind turbine support structures subject to operational and environmental loadings (i.e., day to day wind, wave and current). An existing deterministic model is corrected by adding a correction term to capture the inherent bias, and model error arising from an inaccurate model form or missing variables. A database of structural responses is used to calibrate the proposed models. The database is obtained from detailed three-dimensional finite element analyses of a set of typical wind turbine systems with different design parameters. The finite element analyses account for the nonlinear soil-structure interaction. The proposed probabilistic demand models provide accurate and unbiased estimates of the demands on the support structure properly accounting for the underlying uncertainties. The models are then used to estimate the fragility of the support structure of wind turbines which is defined as the conditional probability of not meeting specified capacity levels.