Many units in the semi-submersible fleet have now reached such an age thattheir documented, minimum design fatigue life is either already exceeded, orwill, in the very near future be exceeded.
It is anticipated that these units will continue in their existing mode ofoperation for some time to come, and further, that some of these units will beconsidered as candidates for production units for benign waters, with theassociated requirement for extended lifetimes. In the case of an existing unitcontinuing in the same mode of operation, the owner may require to document, toa potential client, or national authority, that the fatigue life of the unit issatisfactory. In the case of a unit being converted the requirement for suchdocumentation will additionally be required by the Classification Society andthe method described in this may be one way to document extended fatigue lifefor these units.
In order to document satisfactory fatigue resistance it may therefore benecessary to undertake detailed evaluation of the fatigue reliability of theunits. One, or a combination, of the following detailed evaluations might beconsidered
Evaluate the original design utilisation factors.
Evaluate the operating history of the unit.
Perform detailed fatigue evaluation of specified joints.
Reliability analysis
This paper document best estimate levels of reliability of a typical‘old-age’ semi-submersible and demonstrate how the reliability varies whenClassification inspection routines are included in the analysis. In-servicedamage statistics (database) is utilised to identify the location that is mostcritical with respect to fatigue. Overview from the reported in-serviceinspection findings is given.
In addition to detailed spectral analysis, combined with a detailed FEanalysis of a critical joint, a reliability analysis has been undertaken inorder to evaluate if the probability of fatigue failure of the considered jointis at an acceptably level. The reliability evaluation accounts explicitly forprobability updating based upon information obtained from in-serviceinspection.
