Hydraulic rams

The present invention relates to hydraulic rams and in particular, though not exclusively, to an actuator for delivering a supplementary force to the piston of a shear ram in a blowout preventer.

There are a number of applications where hydraulic rams are used. Each ram typically includes a piston which is driven forward by a hydraulic force. In oil well drilling, hydraulic rams are located in blowout preventers. In an emergency procedure when a well is required to be shut in to prevent a blow-out whilst drilling, two opposing rams are brought together to seal the well bore. These rams are typically referred to as shear rams as they include a shear blade on the front face of the piston used to sever drill pipe or casing in the wellbore.

When the shear rams are actuated by a hydraulic force, the opposing shear blades are brought together to interact, with the blades being driven by the hydraulic pistons. The shear blades first crush and then shear the drill string, casing or other tubular in the well. As wells are drilled to greater depths the tubulars are of increasing diameter, wall thickness and increased steel grades. Consequently shearing the tubular in the well requires more hydraulic force. This calls for a larger actuator piston, hence larger operating fluid volumes and higher closure pressures.

For shear rams deployed onshore or on fixed offshore installations, the increasing size is inconvenient but can be accommodated. For shear rams that are deployed subsea, the volume of hydraulic fluid that must be stored under pressure in fluid/gas accumulators increases exponentially with water depth. The volume and weight of these accumulators makes the subsea blowout preventer or shut off system much heavier and therefore more difficult to deploy.

It is amongst the objects of embodiments of the present invention to provide a supplementary force actuator for use on a hydraulic ram to increase the applied force without substantially increasing the volume of hydraulic fluid required.

It is also amongst the objects of embodiments of the present invention to provide a two stage method of operating a hydraulic ram.

It is also amongst the objects of at least one embodiment of the present invention to provide a blowout preventer having a two stage actuator mechanism for improved shearing of drill pipe, casing or other tubular.

According to a first aspect of the present invention, there is provided a supplementary force actuator for use on a hydraulic ram, the actuator comprising:

an actuator body including fixation means to connect the body to a hydraulic ram;
first and second chambers located in the body, the chambers isolated from each other by an actuator piston;
a rod adapted to be connected to an operating piston of the hydraulic ram, pass through the first chamber and the actuator piston, and extend into at least a portion of the second chamber;
the actuator piston being releasably engageable to the rod by gripping means; and
wherein the hydraulic ram is operated by a force from movement of the operating piston and by a supplementary force from movement of the actuator piston.

Thus once the normal operating piston has moved the ram to crush an object, the actuator piston can be released to provide a supplementary force on the ram.

Preferably the actuator includes a release mechanism to operate the actuator piston. In this way the actuator piston can be released at or near the end of the stroke of the operating piston and thereby provide the supplementary force where it is most required.

Preferably a separation element is located between the hydraulic ram and the body. More preferably one or more seals are arranged on the separation element to act upon the rod and prevent the release of hydraulic fluid from the body into a hydraulic fluid chamber of the hydraulic ram.

Preferably the actuator includes energy storage means arranged to provide a force to act upon the actuator piston.

In a first embodiment the energy storage means is a mechanical means. Preferably the mechanical means is one or more springs held in compression. Advantageously the mechanical means is a plurality of Bellville springs as are known in the art.