Component:

• Platformer vessel fabricated from 1¼Cr½Mo steel, with a history of internal and external cracking in weld and HAZ material
• The largest defect currently detected is known to be associated with low chromium levels in the inner part of the weld metal

Objective:

• To determine whether the vessel is fit to continue in service, to the satisfaction of the regulatory authorities

Approach:

• Review of previous inspection reports and defect assessments
• Consideration of the susceptibility of both standard and sub-standard weld metals to hydrogen attack, in conformity with API RP540
• Fracture mechanics calculations compatible with API RP579, using an ultra-conservative 16" long half-wall defect, so as to obviate the need to determine properties of the sub-standard inner weld metal
• Allowance on fracture toughness and crack growth rates made for the effects of the hydrogen environment
• Allowance made for weld residual stresses
• Sensitivity analysis to variation in operating conditions

Benefits:

• Leak-before-break demonstrated
• Critical defect sizes determined, based on normal operation and on all credible fault conditions
• Failure times by crack growth, from the mid-wall position outwards, determined for current and design operating conditions
• Safe operating temperature limits specified to allow operation for a further 30,000h
• Safe operating practices specified for pressurisation and cool-down