Coiled Tubing Downlines Corrosion and Preservation Experience from Extended Riserless Light Well Intervention Campaigns

Abstract

Riserless Light Well Intervention (RLWI) is becoming a more prevalent means to complete needed intervention scopes in deepwater subsea well environments. This is enabled by application of coiled tubing (CT) as downline in a marine environment to facilitate fluid pumping operations versus conveyance through a riser-based system. However, this technique introduces additional factors that affect CT corrosion and service life. In collaboration with CT supplier Halliburton, bp decided to implement a conservative 6-month CT replacement schedule. The objective of this study was to inspect CT recovered from recent campaigns to evaluate if the 6-month frequency of replacement was fit for purpose. A coiled tubing (CT) downline inspection was performed on strings recovered from RLWI operations in deepwater Gulf of Mexico (GoM). The CT was sent to the manufacturer for both visual and Electromagnetic Inspection (EMI) analysis to understand the life degradation due to corrosion from exposure to the marine environment. The entire length of coiled tubing was run through the EMI inspection sensor to measure wall thickness and determine the pitting severity. The visual inspection indicated severe external pitting and corrosion at various sections of CT. Therefore, the pipe was taken out of service and sent for inspection. The initial assessment suggested the marine environment was primarily responsible for the degraded condition. Based on the results and comparison with other CT downline applications from RLWI operations in North Sea and West Africa, the CT replacement and inspection frequency was reevaluated for deepwater GoM. Additionally, improvement opportunities for CT preservation were considered. CT life degradation during unconventional and consistent use in open water is not widely understood in the industry. As of now, guidelines are not available for application as downline in RLWI operations. The results help to improve understanding, pursue improvement opportunities, and investigate currently conservative approaches while ensuring CT integrity and prevention of RLWI operational failures.