Joint and flange failures can occur. When they occur offshore, and particularly underwater, they pose additional challenges and potential safety and environmental risks, as well as causing lost production. TotalEnergies E&P Nigeria was faced with this scenario when, during a routine inspection, two flex joints at the top of the water injection riser systems connected to the FPSO Akpo were found to be leaking. Without intervention, either of the leaks could have led to a catastrophic failure. An innovative, breakthrough solution was chosen; it addressed the root cause of the problem and enabled repairs to be made safely, without production losses or harm to the marine environment.
A flange leak at the FPSO Akpo
The FPSO Akpo stands in 1,300 meters of water offshore Nigeria. The subsea pipelines are linked to the FPSO using steel catenary risers (SCRs) connected to the topsides using flex joints. The joint supports the SCR weight of about 200 tons, as well as transferring liquid at a pressure of about 25 MPa. During a routine underwater inspection in 2016, leaks were discovered at two of these flex joints. After investigation, the cause was found to be incorrectly sized ring seals (gaskets). Further investigation, including an ultrasonic inspection method specially developed with IS Industrie, identified the source and cause of the leaks — and, critically, that ongoing leakage was causing cavitation erosion that would ultimately lead to a catastrophic failure if the situation were not corrected. The investigation established that the same non-conforming gaskets had also been installed on all of the water injection flex joints, but fortunately not on the production, gas injection, or gas export riser flex joints.
An Innovative Repair Carried Out In Situ and Without Lost Production
The investigation concluded that an intervention would be needed to correct and/or repair all four water injection flex joints. The aggressive nature of the leak meant this was time critical; therefore, a six-month target was put in place, even though a technical solution had yet to be developed. Under normal circumstances, replacing a gasket would be a routine task. However, the additional cavitation damage to the metal flange surfaces would require replacement or repair. This was not straightforward: the upper portion of the joint, which was part of a replacement spool, could be changed out, but the lower portion was fixed to the flex joint and would require an in-situ repair.
A technical solution was studied involving underwater machining by divers, but after several months of planning and testing it was concluded that this would not be feasible as the machining tool was not suitable for use by divers.
The technical solution ultimately chosen involved using a composite epoxy repair together with a hybrid steel elastomer gasket. A hybrid gasket was selected since the steel interleaf would limit stress relaxation (creep) at very high pressures. The gasket would have to be robust enough to accommodate some surface anomaly. It would be installed underwater, and would therefore need to expel any water from its ring groove during tightening in order to prevent trapped water from resulting in overpressure and gasket failure (incompressibility of the water).
TotalEnergies was able to contract its affiliate Hutchinson to design this highly specialized custom gasket.
The hybrid gasket was demonstrated to be sufficient for surface anomalies up to 1 mm x 1 mm. Beyond this, the epoxy material would fill additional anomalies and distortion caused by the cavitation erosion. A simple, inexpensive technology recently developed on the Moho Nord project, dubbed “underwater plasticine,” had been used to measure the cavitation erosion. TotalEnergies contracted 3X Engineering to develop the composite epoxy mastic, a material usually used for pipeline repair. However, the approach would require subsea shot blasting first in order to facilitate proper adhesion of the mastic. The mastic would then need to be applied by divers with a sufficient curing time of one hour to remain soft enough for application, yet be cured in under eight hours to limit the length of the operation, after which it would have to withstand a compressive strength of 70 MPa imposed by the gasket.
In just a few months, TotalEnergies developed an innovative repair procedure to resolve a major integrity threat, without lost time injury or any impact on the FPSO’s production. The integrity of the repair has been inspected regularly: there are no signs of degradation. It is worth noting that other companies that have experienced the same problem have since contacted TotalEnergies with a view to applying this solution.
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