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Life extension of offshore rigs

A new lease of life has been given to a North Sea structure with the help of TWI's Structural Integrity team.

A number of Sedco Forex's fleet of semi-submersible drilling rigs were destined to reach the end of their original fatigue design lives. Replacement of the structures could not be economically justified so life extension was investigated.

TWI was called in to carry out the extension work at various sites worldwide including Rotterdam, Cape Town and Singapore. This involved site work within the specified dry docking period and desk based fracture/fatigue analysis.

The following life extension strategy was adopted by TWI in conjunction with Sedco Forex:

  • stress analysis to identify highly stressed joints
  • fatigue analysis to select joints with lowest fatigue lives
  • design and supervision of an inspection programme
  • mechanical testing of samples extracted from the rig
  • on-site implementation of fatigue life improvement techniques (e.g. weld toe grinding)
  • calculation of fatigue life for the structure after repairs and modifications

Commenting on the first semi-submersible rig treated in this way, Pierre Morvan, Sedco Forex Engineering Manager, said: 'Back in the 1970s when this rig was designed, today's knowledge of fatigue simply did not exist. With recent studies and subsequent upgrading, we now believe that these rigs can go on for 40 years'.

TWI's efforts over a seven-day period saw the operation completed and passed by the client several days ahead of float out.

For more information please email