Pipeline engineering experts, STATS Group, has completed an isolation project on behalf of AMEC to allow maintenance and replacement works on the North Everest platform in the central North Sea.
The workscope covered three maintenance valves and one ESD valve on a 20-inch gas import line onboard the BG-operated platform. After a detailed site survey and piggability study, STATS decided the best solution was to use a tethered Tecno Plug pushed by stem bar to location from a temporary launcher. As the pipeline had no pigging history, this method was considered as the least risk laden option rather than pigging a remotely operated plug from the pig trap.
This alternative method was proposed based on detailed analysis during the Front End Engineering Design (FEED) process, including the piggability study which revealed potential issues for the route of the pigging run which could have prevented the Tecno Plug™ from reaching the desired set location. The FEED process included generation of a project design premise, pipe stress analysis, piggability study, FMECA, operational procedure and risk assessment. This package identifies and mitigates operational risk and reassures clients by providing detailed analysis of the internal conditions and geometry of the pipeline prior to carrying out the isolation.
Prior to carrying out the isolation the pipeline pressure was reduced to 6 barg, with the ESD closed and sealant injected, the maintenance valve was then closed to provided temporary pipeline isolation. This allowed the temporary launcher to be installed inboard of the ESD valve and a team of STATS technicians to break out the piping. A weld repair scope was also required and the STATS team removed the first 90o bend with clam shell cutting tools.
STATS also provided gas bags to act as a vapour barrier while a new 1500# flange was welded into the line. The new flange was then pressure tested using a STATS Flanged Weld Test Tool before reinstating the new bend and launcher valves. With the temporary launcher now in place and fully pressure tested, the valves were opened and the Tecno Plug™ was deployed into position outboard of the valves and hydraulically set to provide pipeline isolation.
Once set and monitored the stem bar was disconnected and the valves were replaced. As the pipeline pressure was only 6 barg, the Tecno Plug™ was installed in the reverse direction, providing full isolation while also providing a test boundary to pressure test the integrity of the reinstalled valves.
A full pressure test at 208 Barg was then conducted between the temporary launcher and the Tecno Plug™ before the plug was unset and recovered back into the launcher. The valves were then closed and temporary launcher removed allowing the pipeline to be reinstalled and returned to operation.