To make offshore wind energy cost effective there is a need to reduce the Cost of Energy (CoE). The aim of work package 3 of the D4REL project is to investigate where cost reductions in the support structure are possible while keeping a sound and safe design. Probabilistic design methods (structural reliability methods) are used to study whether there is any conservatism in the design of support structures.
For the probabilistic analysis, a modern 4MW offshore wind turbine is selected. In the probabilistic design approach, the limit state condition is adopted. The limit state function g is the difference between the resistance R and the load L. For a safe design the limit function should be larger than zero. In structural reliability the design point is determined where the limit state function is zero. Three limit state functions are considered:
1. A fatigue limit state;
2. An ultimate limit state on the yield stress;
3. An ultimate limit state on the global buckling stress.
A generic reliability analysis approach is taken by coupling the aeroelastic simulation tool TURBU with the open source reliability analysis tool FERUM. The First Order Reliability Method (FORM) is used since it calculates the influence of the stochastic variables with respect to the variance of the limit state function.
Based on the selected stochastic variables, the fatigue and ultimate buckling limit state show a smaller probability of failure than the target probability of 10-4 according to the standard. It is shown that the translation of this smaller probability of failure to a reduced partial safety factor is not straightforward.
In the ultimate limit state the uncertainty of the soil stiffness has a large influence. As requested by Van Oord, the uncertainty of the soil properties of the different soil layers is investigated. The contribution of the clay layer to the uncertainty of the limit state function is with 80-90% the largest.