Design approaches for avoiding PWHT of North Sea offshore platforms

This paper presents a review of the application of fracture mechanics to offshore platforms designed for the British sector North Sea. Since the earliest days post weld heat treatment (PWHT) has been extensively applied, in order to ensure adequate resistance to brittle fracture. The alternative approach, permitted by the regulations, of justifying leaving joints as-welded by the use of fracture mechanics, is also extensively practiced. This paper describes one design approach which is considered to account for the real behavior of engineering materials and which takes account of the reality of low toughness found in heat affected zones, the uncertainties of NDT practice and defect sizing and the pattern of residual stresses and fatigue crack growth behavior. The difficulties which arise are presented and the paper illustrates the dilemma that modern weight efficient structures are more likely to require PWHT than heavier structures with lower design stresses and longer fatigue lives.