Decommissioning is on the rise and the cost to industry will be about US$80 billion over the next 25 years to plug and abandon more than 5,000 wells, clear flowlines, and remove related structures. Is there a way to improve the decommissioning process, increase effectiveness, and improve worker safety? There are many steps in the decommissioning process and each one will be scrutinized and improved as decommissioning becomes routine.
Chet Morrison Contractors(CMC) has developed a piece of equipment that accomplishes part of the process: removing surface casing and embedded jacket legs. Once wells are plugged, the casings cut and pulled, and platform topsides and related in-water structures removed, then the remaining support leg stubs are cut below the mudline and extracted to leave an unobstructed seabed.
Fixed platforms are limited to continental shelf water depths with most structures in water depths less than 600ft. Standard operations to remove tubulars requires divers, breathing special air mixes under saturation conditions and working for extended periods at depth. They jet soils from around the legs until the required depth is reached, cut the stubs, and then backfill the excavation after stub removal. All this activity is done under the constant danger of potential burial from sloughing slopes. A standard operation can take four to five days to completely remove the stubs.
To make better use of divers, avoid slip-slope hazards, and reduce operational cost, CMC developed a subsea hydraulic abrasive rotating cutter (SHARC) that can remove well strings and stubs without the need for major soil removal. According to CMC, the tool can be positioned over the well or stub by divers or remotely by ROV. The tool is inserted into the hollow tube. It is then lowered to the required subsurface cutoff depth (15-25ft in the Gulf of Mexico), and then it executes the cut using high pressure water with entrained abrasive, a finely-ground coal slag.
Cutting efficiency is limited by the hydrostatic head that the jetting tool must overcome. The tool is designed to compensate for pressures down to 1000ft water depth and CMC says it can cut through 3in.-thick material as well as multiple pipe strings.
The tool and resulting cut is controlled from the surface vessel. Once the stub is cut, the tool is removed and the freed stub can be extracted from the seabed and the open hole filled.
Energy Resource Technology GOM, Inc. (ERT), former oil and gas subsidiary of Helix ERG, tested the tool at three fields in the Gulf of Mexico. According to Rod Hebert, ERT consultant, “The [crew] did outstanding work. Using this new cutting tool, they completed four wells for us at about half of the expected cost. Chet Morrison saved ERT and partners many millions of dollars.”
Bobby Lott, VP Well Services for CMC, noted that they had recently completed a P&A operation for a McMoran well in the Gulf of Mexico where three strings were cut and pulled: 9 5/8in., 13 3/8in., and 16in.-diameter. The deepest cut was 25ft below the mud line. With that length of cut tubular, he says that it usually requires pulling 40,000- 50,000lb more than the pipe weight to extract the tubular.
Lott noted that the tool will not work for every well, especially those with a longer reach or with multiple groutings. But, most subsea wells are completed with mudline hanger systems set 100ft below the mudline, so most casing strings are not grouted together. This makes the cutting task simpler. For caissons that are driven over surface casing (usually 30in.-diameter) to support a single well platform, CMC has added a feature to the tool that can extend the cutting head out from well center to cut 2.5in.-thick x 60in.-diameter caissons.
Most P&A operations can be completed in one day, according to Lott, saving significant time and expense using the Joanne Morrison, CMC’s diving support vessel. OE
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