As part of the Joint Industry Project 'FPSO Fatigue and Fracture Capacity', a Special Task Group with nine participants performed investigations regarding finite element (FE) modelling and analysis of typical structural details in FPSO's (Floating Production, Storage and Offioading Units) as well as in ships. The purpose of this special effort was to develop recommendations on appropriate hot spot stress methods and S-N data for fatigue strength design. In total, five details with different characteristics, from both geometry and fatigue loading perspectives, were selected for which stress measurements and fatigue tests are available. Various finite element models were developed by participants, using different types and sizes of elements, modelling and stress evaluation techniques as well as FE programs. Three different stress extrapolation techniques for predicting hot spot stresses at the weld toes were investigated. The resulting hot spot stresses, together with the estimated fatigue lives, are compared against the existing design S-N curves published by the International Institute of Welding (IIW). It was concluded that the hot spot stresses predicted using the three stress extrapolation techniques, where the element sizes and stress evaluation points are determined by the plate thickness, can be used with the current design S-N curves. Most significantly, one of the recommended methods requires no stress extrapolation, which is considered an attractive and practical alternative to the existing practices developed by class societies.
The hot spot stress approach for the fatigue strength assessment of welded joints is based on the assumption that the local stress increase at the weld toe can be subdivided into two parts, one governed by the structural (i. e. macro-geometrical) stress increase and the other created by the localized notch stress due to the weld toe itself which is restricted to a region of approx. 2 – 3 mm around the toe.
