The worldwide inventory of deepwater wells increases weekly. The majority of these wells are high-volume producers and any interruption of production is extremely costly, as is the intervention necessary to bring the production back online. Interventions on deepwater subsea wells employ deepwater mobile offshore drilling units (MODU) whose cost per intervention in deepwater averages US $6 million to $10 million. Add to that the cost of 15 days deferred, or lost, production and the situation becomes serious indeed. To address this problem ExxonMobil has undertaken a 4-year development program to design a riserless, seafloor coiled tubing (CT) unit deployed from a specially designed vessel. Called SIM (subsea intervention module), the system (seafloor unit and dedicated vessel) will be licensed to SIM Ventures LLC, held by BJ Services and Otto Candies LLC. Plans call for the first unit to be deployed in 2008.

Mike Krall, vice president, drilling for ExxonMobil, said the system, "allows us, as an operator, to manage our reservoirs, maintain production and, ultimately, drive the profitability of our deepwater developments much more effectively than the industry has been able to do up to now." In the long term, ExxonMobil expects the system to cut the costs to diagnose, repair and stimulate deepwater wells by as much as 85%. "If you have to spend 15 days to shut a well in to get some data and maybe tweak that well, driving that cost down is a distinct advantage to an operator," observed Greg Browning, SIM project director.

Concept

Development work was carried out under SIMCO Ltd., a wholly owned subsidiary of ExxonMobil Corporation. SIMCO Ltd. funded the development team made up of industry experts in related fields.

Early on, the development team realized that the key to creating a viable seafloor system was the elimination of a riser for fluid returns to a vessel. According to Browning, "most people trying to design deepwater intervention vessels could not crack the nut of fluid returns to the vessel. We recognized that we put in a flow line that goes all the way back to a set of treatment facilities on the floating production, storage and offloading vessel - why not use that flow line as the return flow path?" Browning continued, "If you bring the flow path back to an intervention vessel you bring hydrocarbons onto the vessel. If you assess all the safety considerations you must address to make that happen, you end up with a MODU again." Thus, the vessel will supply the fluid but that fluid will either go to the reservoir or to the production facilities. While the concept dictated small additions to the production train (a scrubber tank, primarily), it was an order of magnitude cheaper than continued intervention via MODU.

The team further determined that the subsea components would have to be structurally self-supporting. Analysis indicated that the 500,000-lb, 80-ft (24.4-m) high system could be structurally supported as a freestanding unit by 36-in. structural casing. Combining that casing specification with a horizontal tree gave the system the free standing functionality required. These specifications will be incorporated into all of ExxonMobil's deepwater wells going forward, as they have mostly in previous developments. By licensing the system to an independent provider, SIM will be made available to all deepwater operators - however, these operators will also have to have 36-in. structural casing and a horizontal tree to deploy the system.

The concept also dictated a rapid deployment, dedicated vessel with a compensated deployment system, DP2 (dynamic positioning) capability and the pumps, tankage, and control systems necessary for CT operations in deepwater.

SIM system

The SIM system is comprised of two parts, the SIM vessel and the SIM tool. The two are connected by a fluid line and a power and control umbilical. A deployment line used to lower and land the tool is removed during operations. Beginning at the surface, the system looks like this.

SIM vessel. The SIM vessel is a 372-ft (113.46-m)-long, 79-ft (24-m)-wide, specially designed unit drawing 18 ft (5.49 m) of water. A 43-ft by 36-ft (13.115-m by 10.98-m) moon pool is incorporated in the middle of the vessel for SIM tool deployment. Above it sits a Huisman single mast deployment system with passive and active compensation. The deployment system is rated at 750 tons. The vessel will also deploy two work-class remotely operated vehicles (ROV). There are accommodations for 100 persons on board.

Intervention operations will be run from the vessel's state-of-the-art control room where all operations are monitored simultaneously. Included are two ROV control stations, a SIM control station, and systems to monitor and control the deployment system and CT operations for stimulation and logging. A central supervision station provides a birds-eye-view of all operations.

SIM tool. Likened to a non-moving ROV by Browning, the SIM tool is comprised of two systems, the lower blowout preventer (BOP) module and the upper CT module that both receive power, fluids and control from the SIM vessel. Together, the package measures 80 ft by 23 ft by 20 ft (24 m by 7 m by 6.1 m). The modules are deployed together. The unit is designed to operate in water depths exceeding 6,500 ft (1,982 m). and well depths below the mudline of 13,000 ft (3,965 m)(bearing in mind that the mudline is RKB (rotary kelly bushing) for SIM). The CT module is rated to 6,000+ psi wellhead pressure while the BOP module is rated at 10,000 psi.

The upper, CT module is comprised, where possible, of proven equipment. However, some items, such as the injector head, had to be redesigned for operation in deepwater seafloor conditions. But, in all cases, this is a fully functional CT unit capable of undertaking all normal CT work.

The unit deploys 2-in. CT with an internal wireline for logging. A 23⁄8-in. by 1-in. concentric CT string provides two, separate fluid paths from the vessel to the SIM tool for supply of treatment fluids and methanol for hydrate prevention. No fluids are returned to the vessel.

Tool storage and retrieval is via a 24 slot carousel which houses six in/out tools and 18 job-related tools providing a complete tool set, with four back ups, for most jobs. Should additional tools be needed, two of the slots allow for tool interchange with the vessel. The system accommodates BHAs up to 24 ft (7.32 m) long. The module can deploy tools for logging, pumping, acidizing and scale treatments.

The stripper stack within the CT module is unique. Of the six stripper elements - each made of a new elastomer - four are backups. In addition, the two elements in operation are separated by pressurized fluid to ensure no flow-by leakage. The unit utilizes BJ's standard deployment rams.

An electric over fiber-optic system controls the module. The CT module may be retrieved independently of the BOP module by means of an H-4 connector.

The lower, BOP module contains triple redundancy well control equipment. Mounted atop an H-4 connector, the module contains, from the bottom up, a safety head with a shear/seal ram, a lower gate valve, a pipe/slip ram, a shear/seal ram and a second gate valve. Only one gate valve is used during normal operations.

Current status

The team of 35 engineers and designers supported by SIMCO Ltd. has put every piece of unproven technology through rigorous testing over the last 2 years. According to Browning, "At this time we are passing a milestone event. The technology is now viable enough that the service providers can now take the risk of construction and operation." Dave Dunlap, vice president and president, International Division of BJ, concurs, noting "BJ is committed to bringing this technology to deepwater operators following ExxonMobil's innovative development work." Under a recently signed agreement SIM Ventures LLC will begin construction of the first unit next year, allowing time for changes in detail design, especially with regard to input from other deepwater operators. SIMCO Ltd. will retain a status as co-funder for a period after which 100% ownership will be assumed by SIM Ventures LLC.

When the first SIM system becomes operational in 2008, it will be available to anyone who can meet the 36-in. structural casing and horizontal tree specifications.

Benefits

"The savings that SIM will create for deepwater operators should stimulate development in a number of vital areas," according to Dunlap. The system should cost 50% of the cost of a typical deepwater MODU and will be up to three times faster per job, including mob and de-mob time. That will bring a well intervention down from today's average of $6 million to $10 million and 15 days for a MODU to half the cost over 5.6 days average for the SIM system. At market saturation, that translates into a potential savings of $250 million per year per unit over MODUs. Perhaps as important as cost savings will be the ability to periodically log and monitor reservoir performance, leading to enhance production and recovery factors. SIMCO Ltd. also estimates that the system will allow development of more marginal reserves as the cost of deepwater intervention falls.

Browning believes that the time is now right for developing special deepwater tools. He sees SIM as just the first of these.

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