Defects in a Platformer vessel

Cross section of boat sample


  • 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


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


  • 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


  • 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

Plant Integrity