Innovative Subsea Repair of a Leaking Flexible Joint Using Hybrid Gasket and Composite Material

Abstract

During a routine subsea inspection of the Akpo subsea assets, leaks were identified on two of the flexible joints at the top of the water injection steel catenary risers. These leaks were categorised as the highest integrity threats for the district and a task force was formed in order to determine the cause of the leaks and thereby, the most appropriate means to resolve the issue. A phased array UT inspection method has been specially developed to perform underwater inspection of the gasket groove. The nature of the leak was deemed time-critical. The leak was of relatively high volume (the leak rate was estimated at 5 000 cubic meters per day), was worsening and was inducing structural damages to the flexible joint by cavitation. A technical solution was studied involving underwater machining using divers. The upper joint surface of the gasket groove was part of a replaceable spool but the lower joint surface was integral to the flexible joint and therefore had to be repaired in-place. After several months of planning and testing, it was determined that the machining tool was not suitable for the operation by divers. With only a few months remaining in the 6-month target delivery, a second solution was proposed using a composite epoxy mastic repair together with a hybrid steel-elastomer gasket. The use of such a solution would be a first-of-its-kind for this application and would involve underwater grit blasting to achieve proper adhesion of the mastic. It would also involve precise bolt torquing and tensioning in very restricted work space around the flexible joint receptacle and ultrasonic acoustic bolt length measurements, in order to measure the stress in the bolts.