FAILURE INVESTIGATION OF TWO SEALS USED IN WATER BASED HYDRAULIC FLUIDS

摘要

Hydraulic seals are used extensively in collet type connectors for the installation of subsea wellhead equipment. These seals are exposed to the hydraulic fluid media and extremes of temperature including low sea water temperatures and elevated temperatures due to hot produced fluids or drilling fluids. The affect of seal exposure to a hydraulic fluid at temperature is that the seal material will degrade over time. It is important to understand the limitations and expected life of these seals, because a seal failure may result in inability to install or retrieve components. This paper is intended as a follow up to the 1996 paper, THERMOPLASTIC ELASTOMER ALLOY, TPA, SUBSEA HYDRAULIC SEAL DEVELOPMENT FOR SERVICE INCLUDING WATER BASED FLUIDS. The previous work showed that polyurethane seals have a lower life expectancy in water based hydraulic fluids than thermoplastic elastomer seals. It was determined that the failure mode of a polyurethane seal is by degradation due to hydrolysis as a result of exposure to the water based fluid. It was also determined that loss of interference due to compression set is the life limiting process associated with thermoplastic elastomer seals in water based fluid. In the current investigation, two seal failures were analysed. Both seals were of the same type, a lip seal energized with an elastomeric O-Ring. One seal was fabricated from polyurethane and was returned from the field after approximately 7 years service in water based fluids. The other seal type was fabricated from thermoplastic elastomer. The thermoplastic elastomer seals were leaking during testing prior to shipment of equipment. The results of both failure investigations will be presented. These results will be correlated with the life prediction data from the previous investigation. In the case of the thermoplastic elastomer, a change in formulation of the compound that led to a solution to the problem will also be discussed.