Around the world, operators are bolstering their offshore E&P efforts to meet growing oil and gas demand, which requires a substantial investment in infrastructure. Whether the equipment resides above the water on a platform, on the seafloor, or beneath the mud line, it will eventually reach the end of its productive life cycle.

Dismantling a subsea well and its supporting infrastructure must be carefully executed to ensure that no threats to personnel safety or the environment are encountered and that usable components are salvaged.

Plug and abandonment operations are viewed as a necessary part of doing business, even though they offer no return on investment. Weatherford has developed a technology designed to lower the cost and risk of subsea well abandonment and suspension. The external-latch, Mechanical Outside Single Trip (MOST) system is designed to cut and retrieve subsea wellheads or associated downhole tubulars and equipment in one trip without the use of mobile drilling units or explosive severance devices.

The technology is suitable for single-trip cutting and retrieval of a variety of subsea wellheads with 18/ 4 -in. high-pressure housings and for cutting and retrieving multiple cemented or uncemented strings of various sizes from 13 3/ 8 in. through 36 in. The system has a maximum 400,000 lb pull capacity and a lift capacity of 200,000 lb and, depending on the type of wellhead to be retrieved, is operable in one of three cutting modes: compression cut with marine swivel (or top-drive rotary), compression cut with mud motor, and tension cut with mud motor.

BHA schematics for the three MOST tool configurations: These include compression cutting with marine swivel (left), compression cutting with mud motor (center), and tension cutting with mud motor (right). (Images courtesy of Weatherford)

Greater stability

By latching on to the external profile of the wellhead high-pressured housing, the system prevents wellhead turning and listing for greater stability. And unlike an internal latch mechanism, which risks damaging the internal sealing areas of the high-pressure housing and thus making the wellhead unusable, the technology's grapple arms never contact the internal seals.

This design also provides greater clearance to allow the cuttings to flow out of the ports and away from the working mechanism, preventing a swarf buildup. The external latch provides support to the wellhead assembly to eliminate any lateral whipping that might impede cutting and helps increase the likelihood that the retrieved wellheads can be reused with minimal damage.

The unlatched (left) and latched (right) views of the MOST tool are shown against the external view of the high-pressure wellhead housing.

Unlike abrasive cutting tools, the MOST system works reliably at any well-bore depth. In addition, it eliminates the need for explosives or chemical agents. By avoiding the preblast protocol that must be followed when using explosives, the system also can improve wellsite safety and efficiency.

In addition, shape charges may not completely sever the pipe on the first explosion, necessitating a second blast and raising further safety and environmental risks. By contrast, once the cut is made with the MOST system, positive affirmation on complete separation is achieved by simply pulling up on the tool.

Operational principles

The system was specifically designed for deployment from a semisubmersible rig or monohull vessel. The technology is easy to apply and set, by lowering it into position as part of a bottomhole assembly (BHA). The grapple arms are activated by moving the mandrel body upwards. The grapple arms then close and engage the external profile on the subsea wellhead.

The latch and unlatch can be visually confirmed using an ROV at the seafloor. The mandrel is rotated into the locked position inside the grapple housing, which is also visually confirmed via the ROV.

When the cut is made in tension, the grapple arms are already engaged and latched onto the high-pressure wellhead housing with an overpull of 20,000 lb to 30,000 lb. Once the cut has been completed, the entire assembly is pulled out of the hole and landed in the rig's moonpool.

Field-proven system

This system has been deployed in more than 1,500 wells globally, including in the US Gulf of Mexico (GoM), Australia, North Sea, Norway, and West Africa.

In the world's first rigless subsea abandonment, the system was deployed from a monohull vessel to decommission five subsea wells in the North Sea region. The operation marked the world's first drillpipe-deployed surface recovery and storage of oil-based drilling fluids.

Crews recovered the 18 3/ 4 -in. subsea wellheads, 30-in. conductor casing, 20-in. casing, and the retrievable guidebase from the seabed in a single trip. This project was completed in 40 days with no accidents, environmental incidents, or nonproductive time.

The tool also was used to recover an exploratory subsea wellhead off the coast of western Australia in approximately 1,186 m (3,914 ft) water depth. After cutting and pulling a 9 5/ 8 -in., 53.50-lb/ft production casing string using a BHA consisting of a cut and pull spear and the M-23 cutter, a second BHA, including the MOST system, was run to cut and pull 20-in. by 36-in. conductor casing strings and recover the subsea wellhead.

The second BHA, consisting of a 12-in. outer diameter M-24 casing cutter, non-rotating 17 1/ 4 -in. stabilizer, and the MOST tool, was lowered in the hole and stabbed into the wellhead at 1,186 m (3,891 ft) with an ROV. The tool latched on to the high-pressure wellhead housing and locked with 30,000 lb overpull. The conductor casing was cut at 1,191 m (3,907 ft) using sea water and a flow rate of 900 gal/min with 1,600 to 1,840 psi pump pressure. The cut was completed in four hours, and the wellhead was released and pulled to the rig floor and laid out.

GoM subsea abandonment

An operator in the GoM used the system to help permanently abandon a 4,267-m (14,000-ft) subsea well in 95 m (313 ft) of water, fulfilling regulatory requirements for protection of the environment and clearing the subsea floor. Two separate BHAs were run to cut and retrieve 9 5/ 8 -in. and then 13 3/ 8 -in. casing strings. After the proper plugs were set, the riser and BOPs were pulled.

The compression-cut MOST tool with the M-24 cutter dressed to cut 20-in. by 30-in. conductor was deployed. An ROV was used to guide the assembly into the wellhead, and the MOST tool engaged the wellhead externally with its grapple arms locked onto the outer profile. An overpull of 40,000 lb was applied to ensure engagement, and once confirmed, a 10,000-lb down weight was set on clamps prior to cutting.

Cutting parameters of 900 gal/min at 2,758 psi were established, and the cut was made 6 m (20 ft) below the mud line. Maximum pump pressure of 3,800 psi was recorded. Mud coming out from around the outside of the 30-in. casing, lateral movement on the wellhead, and a drop in pressure (to 2,850 psi) all gave positive indication that the cut was completed. With an overpull of 55,000 lb, the wellhead and conductor started to come out of the seabed. The wellhead and the guide base were successfully recovered without any damage to the wellhead, allowing it to be reused. The cutting operation was completed in just over three hours.

These jobs highlight the various options and flexibility that the technology is capable of handling. The shallow-water GoM job was performed with a smaller, inexpensive semisubmersible rig, and the production string cuts were done using mud motors as the rig had limited top-drive capabilities. The deepwater Australian job used a larger semisub with a top drive system of greater capability. Both these jobs were completed without any safety or environmental incidents accidents, allowing the wellheads to be efficiently recovered for reuse.

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