Call in the experts - welding procedures and fracture toughness under scrutiny

A leading North Sea operator which found itself in the dark over welding procedures and interpretation of fracture mechanics test results recently turned to TWI for specialist technical advice.

It has also asked TWI to perform a number of additional confirmatory fracture mechanics tests.

The work formed part of an engineering critical assessment being made on 12 and 16 inch diameter offshore gas riser systems in the southern North Sea. The work comprised five objectives:

• Review the welding procedures used
  
  
• Review the welding parameters used to make the test welds and compare them with the procedures used to make the 16 inch diameter risers, and also examine the respective weld microstructures.
  
  
• Investigate the factors contributing to the low fracture toughness values recorded in the heat affected zones
  
  
• Comment on the extent to which the test welds were representative of the riser welds
  
  
• Conduct fracture mechanics tests on the heat affected zone of one of the test welds.

With work completed TWI was able to report to its client that it had found that arc energies and welding parameters, used to make the bulk of the welds, were within the ranges specified by the procedures.

On this basis it was considered that the range of mechanical properties achieved in the test welds closely reflected those in the girth welds of the production risers.

In particular the fracture toughness results were applicable to girth welds in the pipe bend sections of the riser and could be conservatively applied to the girth welds in straight sections of the riser.

Lastly, it was able to advise that the lowest HAZ fracture toughness values were found to be associated with the initiation of cleavage fracture at regions of intercritically reheated grain coarsened HAZ.