The use of fully remotely installed retrofit tees for hot-tapping applications is an extremely attractive economic option for maximising the potential of oilfields. A diverless hot-tap tee system is in the process of being built and commissioned, with hyperbaric GMA welding selected as the internal saddle seal welding method. Process capability has previously been demonstrated in the range of 145 to 1,000 m using a specially developed 1.0-mm metal-cored wire. This paper considers the performance and application of a 0.9-mm Inconel 625 wire for hot-tap welding. A direct comparison of the performance of the 2 wires is made: process and electrical stability; metallurgical analysis; and macro quality and mechanical properties.
Hot-tap technology has for many years been used onshore and in process plant applications in order to connect branch pipelines to production pipeline systems without stopping production. The majority of hot-taps is based on welding the branch pipe onto the pipeline and then tapping by hydraulically operated drilling machines. Retrofit tees are being developed for diverless subsea installation onto existing pipelines. This paper describes the hyperbaric GMA (gas metal arc) welding development required for the internal seal weld between the retrofit tee and the mother pipe. To date, a steel metal-cored wire has been selected and qualified as the consumable of choice for the seal weld. This paper goes on to consider and describe the potential use of an Inconel 625 filler wire, a material known for its good corrosion resistance and weld metal ductility characteristics.
In the North Sea fewer than 10 hot-taps have been carried out on subsea pipelines so far, of which 2 were on the Stat pipe system. These have provided very cost-effective solutions, but they are based on an approach using divers and thus limited to waterdepths where diving is possible