Engineering critical assessment of mooring system casting welds

Critical assessment work carried out by TWI on behalf of an Industrial Member company confirmed the structural integrity of a mooring system for a floating production, storage and offloading facility.

Following discovery of unexpected heat affected zone cracking after welding of structural castings into the system the company instigated the engineering assessment as an alternative to repairing the welds.

TWI carried out fracture toughness testing on test welds made in the same heat of cast material as the production castings. Metallographic examination of these specimens demonstrated that the fracture initiated in a microstructure that would be expected to exhibit lower bound fracture toughness.

Fracture mechanics analyses were carried out in accordance with PD6493:1991 (a forerunner of BS 7910) to calculate limiting flaw sizes which would not cause failure of the structure and to determine the likely amount of fatigue crack growth.

It was concluded that the system was not susceptible to brittle fracture or fatigue failure and TWI joined with the company to present the fitness-for-purpose method and conclusions from the investigation to the operator.