Calibration of design fatigue factors for offshore structures based on fatigue test database

Highlights

• A database of steel fatigue test grograms from the 1970s to year 2016 has been presented.

• Statistical analysis of the fatigue test data shows good agreement of in-air S-N curves for D curve and T curve but disagreement for F curve especially in corrosive environment with current DNVGL fatigue design standard.

• FORM analysis based on the fatigue test database gives substantial reduction of DFFS for F S-N curve especially for corrosion environment to the order of 3 times in the most conservative case.

Abstract

The concept of Design Fatigue Factors (DFFs) was introduced for providing desired level of safety in structural fatigue design in offshore structures, often associated with damage calculated from S-N curves. Calculation of fatigue damage from S-N curves can be affected by multiple factors, e.g. types of weld class, corrosion condition, loading conditions, stress concentration on different geometries etc. Each of them can be subject to different level of uncertainties. This study investigates probabilistic models of uncertainty of the fatigue strength in the S-N curves frequently used in offshore oil and gas, and the offshore wind industry based on a comprehensive fatigue test database. It then recalibrates the DFFs from a detailed reliability analysis. The results have shown that with the same target reliability while the DFFs for in air condition in T curve and D curve can remain the same as those from current standards, the DFFs for F curves are reduced especially for the corrosive environmental conditions which has the maximum reduction up to about three times.