Bureau Veritas & Cold Pad push the boundaries of Fatigue life assessment for bonded assemblies

Do you know the similarity between an aircraft window corner and a bracket toe within the bottom of an FPSO’s hull?  The answer is fatigue caused by repeated cycles of stress due to loading, offloading, wave, turbulences, maneuvers, and more.

With the latest progress of composite and other non-metallic materials, the industry has pushed the limits of conventional construction and repair techniques. The safe adoption of these materials, especially the adhesive-bonded units, required naval architects to develop a fatigue life assessment methodology.

This methodology was one of many questions raised by Bureau Veritas to Cold Pad during a midship deck repair campaign of a large FPSO moored in offshore West Africa. Lately, our question has been answered through simulations and full-scale tests, which resulted in the world first fatigue S-N curve for a bonded hull reinforcement. This achievement was possible thanks to the patented ColdShieldTM technology featuring capped stresses on edges, allowing a stress-based approach.

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After an intensive test campaign witnessed by Bureau Veritas and statistical treatment of the collected data following ISO 12107:2012, Cold Pad issued an SN curve directly applicable for design.  On the practical side, the fatigue life assessment of an actual repair project led to a design life of 70 years with a safety factor of 10. The failure mode was very repeatable with a debonding occurring at the bondline (base metal/adhesive interface). We should also note that for some of the tests at 1 million cycles, the base metal steel substrate cracked before the debonding of the reinforcement!

These results confirm the favorable fatigue behavior of bonded assemblies accompanied by the development [or assesment] of the tools used for the fatigue life assessment on the most severe location of an FPSO (i.e., the midship section).

This breakthrough allows stakeholders to safely consider a wide range of offshore applications for hull repair and beyond.

The study leading to the S-N curve was presented at the latest OMAE conference in Glasgow along with a Bureau Veritas co-authored paper. (see section Materials Technology; 3-3-1 Fatigue Improvement and Repairs; page 60 of )

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