While subsea well intervention is necessary, traditional methods can make it a time-consuming and very costly activity, with drilling and semisubmersible rig costs running from US $1 million to $1.4 million a day. There are more than 4,000 oil and gas producing subsea wells worldwide, and this number is increasing at a rate of approximately 500 a year. With many of these wells more than a decade old, intervention is crucial to achieving maximum extraction.

The sustained rise in deepwater exploration has made the challenge of cost-effective well intervention even more pertinent. The challenging conditions experienced at depth in Asia, Brazil, West Africa, and the Gulf of Mexico mean many wells have been producing for several years without necessary intervention. This often results in suboptimum production and ultimate recovery reduction.

It was clear that the oil and gas industry required a step-change in technology, and it was at 3,000 m (10,000 ft) that the AX-S (“access”) challenge started.

The AX-S system, which can be deployed from a monohull vessel, is capable of operating at 3,000 m (10,000 ft) water depth. (Image courtesy of Expro)

Developed at 3,000 m

Following a seven-year development program involving the technical expertise of a range of partners – including input from more than 200 vendors – international oilfield services company Expro has introduced a technology that provides a cost-effective well intervention solution designed to close the value recovery gap between subsea and dry-tree fields by providing a safe, riserless, and remotely operated subsea solution that is at least one-third less than the cost of using a rig.

The AX-S subsea well intervention system is a life-of-field solution to well intervention and is designed to directly address some of the unique operating demands of the deepwater subsea environment.

The system, which is deployed from a monohull vessel, is the world’s first intervention technology that can operate in depths to 3,000 m, which means the technology is suitable for every subsea well in the world. The average intervention is six to eight days using AX-S, compared to a typical deepwater intervention time of 10 to 12.

To enable deployment of the system, Expro has entered into a multiyear charter party contract with TS Marine Asia Pacific to use its DP 2 multiservices vessel Havila Phoenix for worldwide operations. The Havila Phoenix is 110 m (361 ft) long and 23 m (75.5 ft) wide, with a moon-pool of 7.2 m by 7.2 m (23.6 ft by 23.6 ft) and a 250-metric-ton actively heave-compensated subsea crane. Two Schilling UHD workclass ROVs rated for 4,000 m (~13,120 ft) capable of carrying video cameras ensure operations are safe and effective.

Designed for deep water

The AX-S structure is 33.5 m (110 ft) tall and weighs 220 metric tons. It is deployed onto a subsea tree with an active heave-compensated fiber-rope winch from the vessel and is remotely controlled from the surface like an ROV. It consists of an integrated set of pressure-contained subsea packages comprising a well control package (WCP), tool storage package (TSP), wireline winch package (WWP), and fluid management package (FMP). A hydraulic plug-pulling tool overcomes the risks associated with pulling and setting tree crown plugs while a novel control umbilical overcomes the challenges of weight and subsequent deployment/handling system size.

The system has a fully enclosed pressure housing with no dynamic seals between the well bore and surrounding environments.

The WCP is a dual safety barrier containing industry-proven 7 3/ 8 -in. shear seal and gate valves. If a safety issue arises, the operator has time to identify the problem and isolate the well bore.

Positioned directly above the WCP is the TSP, which contains eight tool pockets located around the inner circumference of the package. The tools are swapped on the seabed (in minutes rather than hours), and as they are held in a pressure-retained housing, no pressure testing is required after each tool change. The pressure housing all but eliminates the possibility of hydrocarbons leaking into the surrounding water and water seeping into the well.

The tools are run in the well by the WWP, which has 7,620 m (25,000 ft) of monoconductor that conveys the various intervention tools into the well.

Expro, Deep Tek Ltd., and Parkburn have developed a fiber rope umbilical bundle and handling system to deploy the AX-S system. The bundle is made up of a fiber rope helically wrapped with three individual umbilicals, which provides greater strength and operational efficiency than wire rope alternatives. It is buoyant in water and adds no weight to the deployment system. The light weight of the rope also reduces winch power consumption, and there is no torque in the lifting line that could prove hazardous in managing vessel-based operations.

The final subsea section, the FMP, can deploy glycol fluid into the system to flush out hydrocarbons that are then circulated back into the well or subsea production system. Depending on the specific needs of the customer, seawater can be mixed with the fluid in variable ratios for pressure-testing and flushing.

A control cabin on the vessel has a computer generated interface to control the various packages on a fully automated basis. There is no requirement for any hydraulic lines going back to the surface.

The system is more cost effective than riser-based alternatives because it is supported from a monohull vessel, and it is faster to operate than wire-through-water solutions, especially on horizontal trees. Studies carried out by Expro indicate that AX-S is the only viable solution that is economically attractive for wireline intervention in deepwater wells.

Tried and tested

Expro completed the rollout of a three-phase wet testing process in September 2011. Starting at a depth of 115 m (375 ft), the company tested the active heave-compensation tool, winder, and umbilicals and deployed and recovered dummy packages onto the seabed in the Buchan Deep, East of Peterhead, Scotland.

These subsea packages were completed at a depth of 1,206 m (3,957 ft) in Sognefjorden, northeast of Bergen, Norway, and tested in 2,444 m (8,018 ft) water depth North of Shetland in the Norwegian North Sea. The final phase of testing, scheduled for completion in early 2012, is to install the subsea packages and run the final commissioning on the subsea wellhead.

AX-S aims to provide enhanced hydrocarbon recovery by providing wet trees with the same opportunity for intervention and management as dry trees. The system has the potential to transform the economics of subsea well production by providing a safe, cost-effective solution compared to traditional intervention methods.