Subsea well intervention is a time-consuming and costly activity, with rig costs running up to US $1 million a day. A newly introduced riserless intervention system aims to close the value recovery gap between subsea and dry-tree fields by providing a safe, riserless, remotely operated subsea well intervention solution that costs significantly less than using a rig.

A control cabin on the vessel has a computer-generated interface that is used to instruct the various packages on a fully automated basis. (Images courtesy of Expro)

Partnering for progress

Expro initiated discussions with a number of major operators in 2002 to determine the possibility of developing a cost-effective method of subsea well intervention that could operate down to 10,000 ft (3,000 m) water depth. As the discussions progressed and industry demand increased, it became clear that a solution was required, and in 2003, Expro began the AX-S research and development program with part-funding from three major operating companies. A feasibility study was completed in 2004. Since then, design and testing of key components has been ongoing.

Development continues

AX-S is designed to deliver a full range of wireline intervention services in deepwater wells at substantially less than the cost of using a rig.

The TSP contains eight tool pockets that are located around the inner circumference of the package. Tools are swapped on the seabed, and because they are held in a pressure retained housing, no pressure testing is required after tool changes.

The AX-S system is deployed onto a subsea tree with an active heave-compensated fiber rope winch from a monohull vessel and is remotely controlled from the surface like a remotely operated vehicle (ROV). It consists of an integrated set of pressure-contained subsea packages comprising well control, wireline tools, wireline winch, and fluid management functions. An integrated hydraulic plug-pulling tool overcomes the risks associated with pulling and setting tree crown plugs. A novel control umbilical overcomes the challenges of weight and size.

Expro is now at the end of the sixth year of a seven-year development and qualification program costing $130 million.

The technology development is scheduled for completion by May 2010, and the system is being constructed at several locations throughout Scotland. The first field test is scheduled to take place in a Norwegian fjord in June 2010 before the system is placed in its first field operational use on a North Sea well. Once the system is fully commercialized, key target markets will include the deep offshore waters of West Africa, the Gulf of Mexico, and Brazil. As the deepest monohulled electric-line deployment currently stands at 1,319 ft (402 m), there has been significant industry demand, and there are plans to construct a number of AX-S systems within the next 10 years.

The new design

The AX-S system is 110 ft (33.5 m) tall, weighs 220 metric tons, and is rated for operations to 10,000 ft. DNV has independently provided hazard assessment and technical qualification throughout the development and testing program. Qualification testing at every step of development is critical to mitigate the risk of such a ground-breaking project.

The WCP is the dual safety barrier containing industry-proven 7 3/8-in. shear seal and gate valves. It is shown here in final assembly.

The well control package (WCP)

is the dual safety barrier containing industry-proven 73?8-in. shear seal and gate valves. If a safety issues arises, the operator has time to identify the problem before it becomes dangerous as the system is a fully enclosed pressure housing with no dynamic seals between wellbore and surrounding environments.

Positioned directly above the WCP is the tool storage package (TSP). The TSP contains eight tool pockets that are located around the inner circumference of the package. Tools are swapped on the seabed, and because they are held in a pressure retained housing, no pressure testing is required after tool changes.

Tools are run in the well by the wireline winch package. The pressure housing all but eliminates issues such as hydrocarbon leaking into the surrounding water and water seeping into the well. The winch has 25,000 ft (7,620 m) of mono-conductor that conveys the various intervention tools into the well.

The fluid management package is the final subsea section and can deploy glycol fluid into the system to flush out hydrocarbons, which are circulated back into the well or subsea production system. Depending on the specific customer needs, seawater can be mixed with the fluid in variable ratios.

A control cabin on the vessel has a computer-generated interface that is used to instruct the various packages on a fully automated basis. All of the hydraulic power is generated in the AX-S system, so there is no requirement for a hydraulic line going back to surface. To ensure operations are safe and effective, video cameras and an ROV are an integral part of the system.

Expro, Deep Tek Ltd. and Parkburn have jointly developed a fiber rope umbilical bundle and handling system for deploying the AX-S system. This comprises three individual umbilicals helically wrapped around a main fiber rope. This construction provides greater strength, and as it is buoyant in water, it adds no extra deployment weight. Having 10,000 ft of traditional wire rope would require the handling system to be more than double the size at 190 metric tons. The low weight allows a smaller handling system and smaller deployment vessel to be employed.