A Diverless Retrofit-Tee Hot-Tap GMA-Welding Method

Abstract

Technology Update The basic principle of hot-tapping is to establish a new branch pipeline connection to an existing pipeline while the primary pipeline is under full pressure. This involves connecting the branch pipe, including a valve, to the primary pipeline. Usually, this is accomplished by means of welding or a mechanical clamp connection—cutting a hole in the pipe wall with a machine connected to the valve, retracting the cutting head, closing the valve, and disconnecting and recovering the cutting machine. The pipe branch then may be extended by means of spools and a tie-in to a new pipeline in the normal way. This strategy has been shown to be very cost-effective, compared with alternative methods, including shutdown and tie-in at ambient pressure conditions. To date, for underwater applications, divers have been used to weld the branch pipe to the primary pipeline, and all installation and cutting operations also have been performed by divers. The cutting machines are essentially marinized versions of standard onshore hot-tap cutting machines and not optimized for the subsea application. This makes it diver-intensive, requiring the training and qualification of diver/welders in advance for the substantial welding job and operation of the cutting machine. The primary focus of the remote hot-tap project has been the development of a novel design for a diverless installation, combining the use of a remotely installed mechanical clamp, providing the structural integrity and interface with the isolation-valve module and hot-tap cutting tool, and a "seal weld" made by remotely operated hyperbaric gas metal arc (GMA) welding inside the branch pipe (Fig. 1). The intention of the seal weld is not to provide structural capacity as such, but to make a highly reliable, "metal-to-metal" seal meeting at least the same quality standard as that which traditional manually welded hot-taps are meeting today. The seal-welding qualification program has included work to demonstrate feasibility for up to 2000-m operation and a full, specific program for a potential 145-m application. The programs have included work on mechanical-property characterization and hydrogen pickup, and demonstration of repeatable hyperbaric-GMA-weld quality, even though it is not a structural weld as such. The new remote system will represent a substantial cost reduction because the vessel alone that is used for the remote equipment costs approximately half as much daily as a diver-supported vessel, even before diver-qualification costs are considered. More importantly, the system will extend the capability of the hot-tapping method beyond water depths limited by divers. The remote hot-tap welding tool is shown in Fig. 2.