When an oil or gas well is formed, a borehole is drilled into the subterranean formation and extended to the location of an oil or gas deposit. At the surface of the borehole, a wellhead may be used. The wellhead may provide pressure containment capabilities for controlling pressure developed within the borehole. The wellhead may also provide a physical, structural interface for drilling and production equipment operating within the borehole.
A well may be abandoned when the oil or gas reserves of a well are depleted, or when the production costs exceed the expected returns. At that time the well may be plugged in accordance with environmental and regulatory requirements. For instance, a cement material may be flowed into a well for formation of a cement plug. After testing the structural integrity of the cement material, the wellhead may also be removed. Various techniques may be used to remove the wellhead. On land, for instance, a cutter may be placed within the wellhead and used to cut the casing below the ground surface. Once the casing is cut, the wellhead can be lifted and removed. The wellhead may then be reused at another well site.
Removal of a wellhead for a subsea well often uses a different process. For instance, following plugging of the borehole, an explosive charge may be located within the well casing below the subsea surface. Upon detonating the charge, the well casing may be cut to allow removal of the wellhead assembly. In other cases, a mechanical or hydraulic cutting apparatus may be lowered from the surface towards the wellhead. Underwater divers or a remotely operated vehicle may be used to locate the cutter in the borehole or to secure the cutting apparatus to the wellhead. Once the cutting process is completed, the cutting apparatus can be disconnected and removed. A wellhead removal device may then be lowered and connected to the wellhead to allow the wellhead to be lifted from the subsea location.