Steel is the most common construction material in the Oil & Gas industry, and it begins to rust the day it is cast. This observation raises several challenges for the asset integrity and life extension of all offshore units. Therefore, for the ageing FPSOs (>10 years), frequent and costly structural maintenance operations in hazardous environment may be required to repair their hull. While a standard ship can go to dry-dock for “crop and renew” maintenance operations through standard hot works techniques, a permanently moored asset has to be maintained in situ. The challenge is to perform these offshore structural maintenance operations with no production disruption while maximizing safety even for the most stressed areas such as a mid-ship deck plating.
COLDSHIELD, a co-development between COLD PAD, TOTAL and IFP Energies nouvelles, has been set up to meet this very challenge. This innovative alternative to “crop and renew” is covered by approval certificate from Class. COLDSHIELD stops the corrosion and restores hull structural strength. Two industrial applications have been performed so far. Among the steps to be taken to demonstrate that such repair can be considered as permanent, it was mandatory to characterize the fatigue behavior of this reinforcement solution.
While the fatigue behavior of steel has been an important research topic since the early eighties of the last century, its understanding for bonded composite materials is still at its infancy. Indeed, for the time being there is — to our knowledge — no S-N curve for structural bonded reinforcements. Full scale coupon specimen fatigue tests of the structural bonded reinforcements were conducted by COLD PAD, third-partied by Bureau Veritas and in collaboration with two laboratories (private and public) in order to study the adhesive fatigue. A statistical analysis was performed according to international standards. The resulting S-N curve is fit for industrial fatigue design. It demonstrates a comfortable fatigue strength (S-N curve presenting a slope of 9).
This paper presents the results of a fatigue life assessment campaign of COLDSHIELD including the experimental setup, the fatigue test results, and the numerical analyses. It details the reason why the design is compatible with a stress-based approach. It also explains the conclusions that can be derived in terms of fatigue life for a deck repair campaign.