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OTO 2001/056

The effects of local joint flexibility on the reliability of fatigue life estimates and inspection planning

A conventional, rigid-joint, spectral fatigue analysis has been carried out on Shell's Leman CD Platform. The platform was installed in the Southern North Sea in June 1970, in 34m of water. Conventional fatigue analysis indicates numerous joints with fatigue-lives below 10 years. Results from underwater inspection of 46 joints, however, show that only two joints have crack indications. The fatigue analysis was repeated with local joint flexibility explicitly modelled in the analysis. The resulting fatigue life predictions were then compared with the rigid-joint analysis and with the inspection data. The results show that, in all cases, the fatigue-life predictions have increased compared to the rigid-joint analysis. The implementation of LJF in the fatigue analysis reduces the requirement for underwater inspection by approximately 75%. Two joints have been found to have crack indications during underwater inspections of the Shell Leman platform. Both joints are identified as requiring inspection based on the LJF fatigue analysis. Concludes that conventional rigid joint fatigue analysis under-predicts fatigue life; also the use of LJF can assist in the overall optimisation of inspection planning, taken as one of a number of elements.

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Updated 2010-03-19