Each rig was nearing the end of its original fatigue design life and replacement of the structures could not be economically justified. Life extension was therefore considered as a viable alternative.
The work varies slightly per rig but generally involves structural analysis, fracture mechanics based assessment of fatigue and fracture behaviour (using stress data supplied by the client) rig inspection, derivation of plant material and weldment properties and improvement of joint fatigue life by weld toe grinding.
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'.
For more information, please email contactus@twi.co.uk