Well slot abandonment in offshore marine facilities is complex, with numerous challenges and interfaces that must be managed effectively to safely execute the project. A significant challenge is in the cutting, hoisting and handling of nested tubulars and conductor sections when they are retrieved to surface. The structural integrity of the tubulars can vary, owing to marine interactions and prevalence of extensive corrosion and, in some cases, connectivity between the annuli and the marine environment. As such, extensive pre-operational engineering and stringent operational control measures are required.

The paper presents a case study of the permanent well slot abandonment of fifteen producing wells and two disposal wells. The facility is located offshore Japan, in a water depth of approximately 500 ft, with the distance from drill floor to mudline being 637 ft. The safe execution of well clean out, tubular-conveyed rotary cutting, and hoisting was undertaken through extensive onshore engineering and project preparation, to prevent personnel or environmental incidents.

The engineering challenges of hoisting over 650 ft. of nested tubulars and conductor are discussed. The mudline adhesion interactions with the conductor requires additional over pull, above the force of the nested string end load. Bespoke hoisting equipment was developed for the project and used upon slipping the nested strings in the rotary, to safely hoist the cut strings, and allow for handling during laydown operations.

Challenges in the structural impairment of the nested strings, due to annular cement being poor or not present are discussed, allied to localized issues such as boring and cutting operations through casing collars or centralizers. An audit of operation times is presented and the incremental efficiency gains and time reductions due to increasing integration of the service company and operator teams, allied to competency increases, are presented.

The project involved the recovery and lay down of over 29,500 ft. of nested tubulars and conductor, during which there were 52 subsurface rotary cuts, 245 boring and pinning operations with 241 surface cuts performed. The project was executed in 62 days, 32% ahead of the operator's project approval for expenditure. The project involved over 26,000 man hours, with no personnel or environmental incidents throughout the project duration.

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