Crack tip constraint correction applied to probabilistic fracture mechanics analyses of floating production, storage and off-loading vessels

Revision 4 of the British Energy R6 document: “Assessment of the integrity of structures containing defects” provides methods to allow for loss of crack tip constraint for shallow weld flaws. The document also provides methods to estimate upper-bound values of the through thickness residual stress distribution for a range of common weld joint configurations. The present paper presents approaches to modify the R6 Option 1 failure assessment diagram (FAD) for loss of crack tip constraint pertaining to primary and non-uniform residual stress. The modified FAD was formulated for probabilistic fracture mechanics analyses of semi-elliptical surface cracks located at transverse deck welds of floating production, storage and off-loading vessels designed to operate in the North Sea. The objective was to study the influence on the failure probability of modifying the FAD for constraint and allowing for non-uniform residual stress. Another objective was to study the influence of constraint correction on the combined fatigue and fracture failure probability for the vessels subjected to wave loading. sults demonstrate clearly the importance of correcting for crack tip constraint pertaining to both primary and secondary stress and to allow for non-uniform residual stress for shallow surface flaws of known crack heights. However, in combination with fatigue crack growth the effects become less prominent as the failure probability is governed by the uncertainty in the parameters of the crack growth relationship and the long-term stress distribution.