Abstract
Consequences of failure of offshore wind turbines (OWTs) is in general lower than consequences of failure of, e.g., oil & gas platforms. It is reasonable that lower fatigue design factors can be applied for fatigue design of OWTs when compared to other fixed offshore structures. Calibration of appropriate partial safety factors/Fatigue Design Factors (FDF) for steel substructures for OWTs is the scope of this paper. A reliability-based approach is used and a probabilistic model has been developed, where design and limit state equations are established for fatigue failure. The strength and load uncertainties are described by stochastic variables. SN and fracture mechanics approaches are considered for to model the fatigue life. Further, both linear and bi-linear SN-curves are formulated and various approximations are investigated. The acceptable reliability level for fatigue failure of OWTs is discussed and results are presented for calibrated optimal fatigue design factors. Further, the influence of inspections is considered in order to extend and maintain a given target safety level.
Highlights
Consequences of failure of the support structure of an offshore wind turbine are in general lower than consequences of failure of, e.g., oil & gas platforms
The results shows that required Fatigue Design Factors (FDF) values determined for cumulative reliability index equal to 2.5 and 3.1 and for an annual reliability index equal to 3.1 and 3.7 covering the range of reliability levels implicitly given in relevant standards
For offshore wind turbines lower fatigue safety factors can be expected compared to those required for offshore oil & gas platforms since the consequences of failure are in general lower
Summary
Consequences of failure of the support structure of an offshore wind turbine are in general lower than consequences of failure of, e.g., oil & gas platforms. For offshore wind turbines a reliability level corresponding to unmanned structures or even lower can be relevant. If inspections are performed even lower material costs can be obtained, but they have to be balanced with the costs to inspections Such a reliability- and risk-based approach has been used for offshore oil & gas steel structures, see, e.g., Faber et al [2] and Moan [3]. Requirements to fatigue design factors for design of welded details in support structures for offshore wind turbines are presented, both without and with inspections Requirements to fatigue design factors for design of welded details in support structures for offshore wind turbines are presented in Section 5, both without and with inspections
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