COT Transverse Bulkhead Steel Renewal Assessment
Significant corrosion of transverse bulkhead and vertical stiffener webs was identified during a periodic inspection of a Cargo Oil Tank. In many locations the corrosion was of Substantial or Renewable levels.
Typical IACS / Class repairs for these structures would have been a crop and renew of the affected stiffener webs. For efficiency, a shipyard would typically crop and renew all affected stiffeners. This was estimated to be a steel renewal scope of approximately 5-10 tonnes.
An engineering analysis was performed which assessed the future predicted capacity of the bulkhead stiffeners should calculated corrosion rates for these structures persist. This analysis determined that when the next periodic inspection was due (also deemed to the be facility end of field life), the stiffeners would still have sufficient capacity to comply with classification requirements.
The calculation was submitted to Class for review, approved and the repairs were removed from the scope.
By performing an assessment to determine the future capacity of the structure on the basis of consistent corrosion rates, it was demonstrated that the structure had sufficient capacity, even though the steel thicknesses were less than the renewable limit according to IACS guidelines. A significant steel renewal scope of 5-10 tonnes was saved, and this had a measurable impact on the shipyard works critical path as there were other significant repairs to do in the same COT that could not be performed concurrently.