Structural integrity assured for Liverpool Bay

The Douglas Platform complex in the Liverpool Bay / Irish Sea showing the Accommodation Platform on the right. (Courtesy BHP Petroleum)

When crack-like flaws are discovered in welded structures an assessment on the necessity of repair is needed as soon as possible. TWI engineers were called in to address just this situation in an accommodation rig being built forthe Liverpool Bay Development Company.

Flaws had been detected in rack welds on the Morecambe Flame Jack-up Rig (now called the Douglas Accommodation Platform) during refurbishment. These were old fabrication flaws discovered while non-destructive testing the side plateswhich join the racks to the legs. Cracking had occurred at the weld toe of each rack.

The racks were fabricated from thick section high strength steel, so by trying to repair the old cracking more problems might be induced. TWI needed to assess the situation - could structural integrity be maintained if nothing wasdone?

Engineers at TWI studied the stress data supplied and the results from special shallow crack fracture mechanics tests (conducted on test samples taken from the racks). By carrying out an engineering critical assessment they wereable to show that the flaws would remain stable with respect to failure by fatigue and fracture over the next 30 years (the design life of the platform).

Structural integrity was assured without repair.