Cofferdam and Hyperbaric “Live” Repair of Gas Pipeline Leaks

Offshore pipelines may face many threats. Apart from internal and external corrosion third party threats represent major hazards to submarine pipelines. Recent pipeline leaks caused by third party as well as construction and installation have been repaired online in a two-step process involving installation of a leak clamp followed by a permanent repair by installation of a welded stand-off sleeve. The welding has depending on the water depth been executed in a hyperbaric habitat or in shallow water using purpose built cofferdams. A concept proposed by DNV GL has been successfully applied to repair of leaking submarine pipelines. To ensure the safety of the repair crew the concepts involves using a gas containment barrier installed over a traditional mechanical leak clamp. The gas containment barrier is either purged with inert gas or nitrogen or it is maintained with a constant inert gas pressure that is monitored continuously during the repair. In the event of a sudden gas leak into the gas containment barrier a pre-set pressure relief valve will open and dump the gas leak outside the habitat. This paper describes the details of a few cases of leaking submarine gas pipelines and the immediate causes of the leak, the repair method selection, the repair method details, cofferdam or hyperbaric welding qualification and execution. The paper also describes the various steps in the process to ensure that the pipeline damage is stable and that the repairs can be safely undertaken to restore the pipelines to their original design condition without reduction of pressure or flow rate. The paper describes the method of global and local finite element analyses as well as fracture mechanics assessment by FEA to assess the stability of the flaws causing the gas leaks. The pipelines in question have all been gas transmission lines carrying gas to gas fired power plants for which gas pressure reduction or shutdown were completely unacceptable. Future development is expected to involve development of remotely controlled repairs using similar concepts at water depths where diver/welders cannot be employed due to the various country regulations or simply because the water depths are too deep for saturation divers. Methodology according to DNV RP-A203 [1] is described for qualification of new technology for underwater pipeline repairs. Further references are made to the recent updates to the DNV RP-F113 Pipeline Subsea Repair [2] with regards to requirements for “live” pipeline repairs. The DNV RP-F113 refers to the PRCI Weld Thermal analyses [3] and requirements to perform full scale mock-up tests of the repair as part of the repair method qualification based on DNV OS-F101 Submarine Pipelines [4].