New Subsea Technology Aims To Cut Well-Intervention Costs
Authors: Donnelly John 
Published: February 2015

Deepwater subsea well intervention must be done with mobile offshore drilling units (MODUs), which can be very expensive. The cost per intervention using the standard MODU averages up to U.S.$10 million, in addition to the lost or deferred production. With deepwater production poised to play an even greater role in meeting global hydrocarbons demand, those costs will become increasingly burdensome.

A new technology unveiled by ExxonMobil promises to mitigate this costly expense and significantly reduce the cost and time of downhole logging, acidizing, and other interventions in deepwater oil and gas wells. The subsea-intervention-module (SIM) technology offers a riserless way to deploy a seafloor coiled-tubing (CT) unit from a specially designed support vessel. ExxonMobil says the new technology, which it has been working on for the past 4 years, is advanced well beyond concept stage and has undergone significant high-risk engineering and component testing. The technology is now entering the detailed design and construction phase, and it will undergo further development to complete its design and its integration with subsea facilities.

The SIM system—compromising a subsea CT system and a purpose-built monohull vessel that provides deployment, control, and support—is designed for use in 2,000 to 6,500 ft of water and can be used in even deeper subsea production interventions. It will be able to accommodate well depth of up to 13,000 ft below the seafloor. The system can be used for intervention on any subsea well constructed with 36-in. structural casing and completed with a horizontal tree.

First use of the technology will be offshore west Africa in 2008. To commercialize the technology, ExxonMobil has licensed the SIM system to a joint venture between BJ Services Co. and Otto Candies LLC. The venture will operate the SIM system following the completion of design and construction.

SIM Support Vessel

The SIM system consists of the vessel—consisting of a dynamically positioned vessel, compensated deployment system, and controls—and the tool, which includes the CT module, blowout-preventer (BOP) module, one-way fluid-transfer lines, and remotely operated vehicle (ROV) with power and control umbilical. The SIM support vessel is a monohull vessel with double skin, a large working moonpool, and diesel/electric propulsion with a dynamic-positioning system. The vessel is the mother ship for the subsea CT unit.

The self-supporting vessel carries stores and supplies for a 30-day period to support multiple well-intervention missions and can be replenished at sea if needed. The vessel also contains a helicopter deck and mooring facilities on both sides for a supply vessel. The vessel is 372 ft long and 79 ft wide, with an 18-ft draft (Fig. 1). It can accommodate up to 97 people and operate in up to 6,500 ft of water depth. The moonpool is 43×36 ft and located in the middle of the vessel, with a 750-ton, single-wire Huisman deployment system above. The vessel also includes deployment capability of two ROVs. A state-of-the-art control room includes the SIM control station, two ROV control stations, and a system to control and monitor CT operations and deployment.