Achieving Economically Successful Deployment of Coiled Tubing at Sea as Downline to Repair a Gas Pipeline

摘要

Offshore pipeline pre-commissioning activities normally involve the use of downline to permit the communication between surface equipment (normally rigged up in a vessel or rig) and the subsea pipeline injection/vent point. Recently, a major operator in Brazil deployed a 1 3/4-in. coiled tubing (CT) string as downline at sea using a methodology to maintain a smooth bend radius to help reduce fatigue issues during operations. Using CT as downline for this operation saved the operator USD millions by eliminating the need to replace a pipeline section. The 40-kilometer (km) pipeline had a blockage composed of pigs, debris, and hydrates. After two failed interventions, the operator decided to install a subsea hot tap on each side of the plug, displace the hydrocarbon, and clean the pipeline to cut the section and replace it with new pipe. The blockage was located 6 km from the center of the pipeline. CT string was deployed to the seabed (1400 m) and was attached using soft sling in a deployed 8-ton clump weight positioned next to the pipeline. A 25-m flying lead hose was used in the bottomhole assembly (BHA) to connect the CT to the pipeline subsea hot tap near the plugged section and compensate vessel movement. After attaching the CT string on the clump weight, the vessel was moved away from this point to deploy 40 m of the CT on the seabed, forming a smooth bend radius. A Nitrogen generator unit (NGU) was used to displace gas in the pipeline to the production unit flare. Cleaning operations were completed using a pig propelled with filtered seawater provided by pumping equipment located on the production unit. The execution plan required that all fluids be recovered onboard the support vessel through the CT attached to the subsea hot tap. Using CT and established best practices, the team was not only able to clean the pipeline section as planned but also eliminate the need to cut and replace the pipeline section. The CT string was kept under water for more than 170 hours, demonstrating its capability to satisfactorily function as downline equipment during this operation.