Inspection-Based Life Extension of Offshore Jacket Fleets

Abstract

Over the years, underwater inspections of analytically “fatigued” tubular joints have mostly concluded with lack of weld cracks or indication thereof. Life extension of platforms, however, continues to honor fatigue results by considering all “fatigued” joints as severed in the end-of-life models. With many severed joints, linear design assessments seldom work, typically triggering nonlinear pushovers, with fewer qualified contractors and software, higher running times, leading to risk assessments, fit-for-risk conclusions, and mitigation measures such as downgrading platform manning and additional underwater inspections. All these struggles continue today while underwater inspections continue to negate analytical fatigue results. This paper proposes two approaches to reconcile this conflict. The first is based on use of underwater inspections of “fatigued” joints to reset their lives, and also to regress an update for similar uninspected yet “fatigued” joints. The second approach is based on leveraging the statistics of the underlying S-N curves to suggest scenarios of severed joints. In the first approach, only one end-of-life extension model is retained albeit with fewer “fatigued” joints. In the second approach, several end-of-life extension models are developed albeit each has fewer severed joints consistent with S-N statistics. In both cases, the conflict between end-of-life models and underwater inspections is addressed, linear design checks become more successful, nonlinear pushovers are mostly avoided as are the ensuing risk assessments and mitigation measures. The two approaches were developed internally in ExxonMobil and successfully applied to its offshore fleets worldwide. The two methods with example applications are presented in this paper for industry consideration.