Structural integrity assured for Liverpool Bay

When crack-like flaws are discovered in welded structures, an assessment on the necessity of repair is needed without delay. TWI engineers were called in to address this problem in an accommodation rig being built for the Liverpool Bay Development.

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 whilst non-destructive testing the side plates which 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 and worsen the problem. TWI needed to assess the situation - could structural integrity be maintained if nothing was done?

Engineers at TWI used the stress data supplied and results from special shallow crack fracture mechanics tests (conducted on test samples taken from the racks) to analyse the situation. By carrying out an Engineering Critical Assessment they were able to show that the flaws would remain stable with respect to failure by fatigue and fracture over the next 30 years. This being the design life of the platform.

Structural integrity was assured without repair.