National Oilwell Varco's Shaffer Spherical BOP is one of the first lines of defense in controlling a well. When actuated, hydraulic pressure operates the piston, and in turn closes the spherical. The Spherical's closure occurs in a smooth, simultaneous upward and inward motion, as opposed to horizontal motion..
Shaffer® Spherical blowout preventers are compact, annular type BOPs that seal reliably on almost any shape or size kelly, drill pipe, tool joint, drill collar, casing or wireline. Sphericals also provide positive pressure control for stripping drill pipe into and out of the hole. They are available in bolted cover, wedge cover, dual wedge cover and NXT models that contain each of the following special features:
- Rugged, reliable sealing element provides positive seal after hundreds of tests to full working pressure
- Strong, simple construction; only five major parts
- Compact body saves space; height is 15 to 20% less than height of some other annular BOPs
- Simple hydraulic system; only two hydraulic connections are needed
- Wear rings on moving parts prevent metal-to-metal contact; this feature prolongs preventer life
- Suitable for H2S service. Standard models are suitable for internal H2S service, simple bolt and lifting shackle changes convert them for external H2S service
- Servicing is easy; element can be changed without getting mud or grit into the hydraulic system
- Steel segments reinforce sealing element but do not protrude into well bore when element is open
- Element design provides long stripping life
- Manufactured in accordance with API Specification 16A, Third Edition, June 2004 and NACE MR0175 per API Specification 16A, Temperature T20, Standard nitrile packing element: 40°F to 170°F
Sphericals are generally 15 to 20% shorter than comparable annular preventers; this is a big advantage when installation space is limited. NXT design further reduces weight and height by eliminating the bolting flange through the base of the spherical bottom directly to the ram body. For applications that require two Sphericals, dual wedge cover preventers incorporate two sealing elements, adapter rings and pistons into one housing. Each sealing assembly operates independently of the other, yet the dual preventer is up to 20% shorter than two singles stacked one on top of the other.
All Shaffer Sphericals operate at 1,500 psi, whereas some
competitive units require much higher pressures, thus increasing accumulator size and cost to meet code requirements.
Wear rings prolong the life of the piston and the piston seals by eliminating metal-to-metal contact between the piston and the lower housing of the preventer. Wiper seals are used in places where debris might accumulate between metal parts and accelerate wear. Wide clearances allow any debris to flow off the wiper seals without damaging metal sealing surfaces. All dynamic seals are lip type, specially made to seal against moving parts with minimum wear.
Shaffer standard Sphericals meet all applicable American Petroleum Institute (API) and National Association of Corrosion Engineers (NACE) requirements for internal H2S service. Field conversion for external H2S service involves changing only the studs, nuts and lifting shackles. Arctic models which meet API 16A specifications for low temperature service also are available.
Shaffer Sphericals require only two hydraulic connections one for opening and one for closing. On some of the larger models, additional ports are provided for convenience when attaching hydraulic I Ines.
Servicing a Shaffer Spherical preventer in the field is simple. The upper housing lifts off for easy access to all internal components. The adapter ring protects the piston and hydraulic chamber from mud and grit while the element is being changed. Only one seal is disturbed. This is not true of some other annular preventers.
The success of NOV's Shaffer Sphericals is due primarily to the ruggedness and reliability of their sealing element. Hundreds of pressure tests to full working pressure have proven its ability to maintain a positive seal. Tests performed in an industry program sponsored by 22 companies including three major BOP manufacturers, proved the Shaffer Spherical element lasts two to four times longer than elements in other annular preventers. The element retains its ability to return to the opened position longer than other sealing packers do. The long sealing life of the element gives Shaffer Sphericals a high degree of reliability and keeps maintenance costs, as well as operating costs, to a minimum.
Steel segments molded into the element partially close over the rubber to prevent excessive extrusion when
sealing under high pressures. These segments always move out of the well bore when the element returns to the open position, even when the element is worn far beyond normal replacement condition.
Stripping operations are undoubtedly the most severe application for any preventer because of the wear the sealing element is exposed to as the drill string is moved through the preventer under pressure. To prolong sealing element life, it is important to use proper operating procedures when stripping. The recommended procedures are:
- Close the preventer with a maximum of 1,500 psi closing pressure.
- Just prior to commencing stripping operations, reduce closing pressure to a value sufficient to allow a slight flow.
- If conditions allow, stripping should be done with a slight flow to provide lubrication and prevent excessive temperature buildup in the element. As the sealing element wears, the closing pressure will need to be incrementally increased to prevent excessive leakage.
- If conditions will not allow leakage during stripping operations, the closing pressure should be adjusted to a value just sufficient to maintain a seal.
- As required closing pressures approach 1 ,500 psi, consideration should be given to switching to another preventer to complete stripping operations. The minimum pressure required to maintain a seal during stripping would vary depending on the particular element and its service history.
Smooth, Dependable Action
Stripping is smooth with a Shaffer Spherical because the element opens and closes so easily. The steel segments molded into the rubber make metal-to-metal sliding contact with the sphere of the housing. This provides a much lower coefficient of friction than rubber-to-metal contact. Well pressure assists will not cause it to oversea!, since the element's design and method of energization counteract the well bore pressure acting on the piston. During stripping out, well pressure and pipe movements are in the same direction. Rubber compresses firmly against the steel segments at the top of the element, but rubber at the bottom is only lightly compressed. This creates a "funnel effect" that makes it easy for a tool joint to enter the sealing element. Hydraulic pressure bleeds off, allowing the element to expand and accommodate the larger O.D. of the tool joint.
When stripping in, well pressure and pipe movement oppose each other. Well pressure compresses the rubber portion of the element against the steel segments as the tool joint moves down into the Spherical.
Choice of Rubber Compounds
Sealing elements are available in natural and Nitrile rubber compounds. This allows the Spherical to be fitted for any type of service water base mud, oil base mud, and low temperature. Natural rubber offers excellent fatigue and wear life in water base muds, and operates well in lower temperatures. This allows the Spherical to be fitted for any type of service; water base mud, oil base mud, and low temperature.
Single and Dual Snubber BOPS
With the single and dual snubbing BOPs operation, time is reduced by eliminating conventional ram-to-ram snubbing. Both single and dual snubber blowout preventers have a working pressure of 5,000 and 10,000 psi. And just like the Spherical BOPs, the snubbers utilize the Shaffer 10,000 psi spherical packing element with its field proven design. This explains why there is a long life under the harsh snubbing conditions of passing tool joints and couplings through the element.
In these single and dual snubbers, maintenance is also made faster and easier. The two-piece snubbing element enables easy replacement with pipe in the well. For more assistance in rapid removal of the element, there is a hydraulically operated quick-release bonnet and an easy lift upper housing. The easy-lift upper housing further assists in rapid snubbing element removal with hydraulic cylinders integral to the BOP housing. In all, only eight minutes are needed to replace a snubbing element.