Richard T. Kendall
When drilling for oil, small amounts of high-pressure natural gas trapped in oil, shale, and other geological formations must be safely vented or burned off. In some cases, the oil contains dangerously high amounts of gas essentially an explosive bubble which cannot be safely trapped or burned. This is the major cause of oil-well "blowouts."
To prevent such catastrophes, drillers install enormous valves collectively called a blowout preventer or BOP on all wellheads. BOP control-system power units deliver large volumes of high-pressure fluid instantaneously during emergencies. Because wellhead pressures can exceed 15,000 psi, with drill-pipe diameters to 22 in., it is essential that BOP valves close quickly. Electromechanical devices require at least a momentary start-up and are much too slow.
Bladder accumulators, which react in less than a second, have proven a reliable means to operate the valves and close off the main pipe. The accumulators feature enormous power reserves, flow rates, and fluid volumes. A standard blowoutpreventer control unit consists of 22 or more 11 or 15-gallon bladder accumulators. They are typically precharged to 1,000 psi, then hydraulically charged to 3,000 psi by electric-motor-driven piston pumps or air-operated diaphragm pumps. They must operate in a variety of harsh environments, from land-based rotary rigs to offshore drilling platforms. Accumulators may be required to remain charged for months, or even years, before being activated.
Subsea accumulators require special materials, such as steel shells either electroless-nickel plated or internally phenolic coated with a marine-epoxy OD. The oil port and other wetted surfaces are predominantly 316-stainless steel. The units are submerged to an average depth of 1,500 ft with accumulators operating to 10,000 ft and drilling to 20,000 ft. Bladder accumulators rated to 9,000 psi are now available to overcome the extreme hydrostatic head pressures.
BOP control units typically use 95/5 high-watercontent fluid because it is safe, economical, readily available, and environmentally acceptable. This fluid often becomes highly contaminated but, contrary to popular belief, bladder accumulators tolerate dirt. Most bladders fail because of an inadequate nitrogen gas precharge that lets the bladder extrude through the gas valve. However, once an accumulator is properly precharged and checked, nitrogen rarely leaks out. Periodic inspection and preventative maintenance ensures long life and reliable service.
The basic bladder accumulator design has been around since 1946, but there have been significant enhancements over the years. These include better elastomeric compounds, the availability of stainless steels, and shell manufacturing methods such as spun tubing, pierced billets, and deep drawing.
However, one challenge for oilfield-equipment manufacturers is operating control units in extremely cold environments, such as northern Canada and the North Sea. Bladder compounds are generally limited to 40 to 50°F. Below these temperatures, bladders lose elasticity and can become brittle and crack.
One alternative is the buoyant-float accumulator. This special nonseparator-type accumulator has no bladder: the nitrogen-gas precharge is in direct contact with the fluid. Today's buoyant-float accumulators have a special syntactic-foam float that attaches to a poppet valve. As fluid discharges, the float lowers and closes the valve, preventing nitrogen from escaping. It differs from previous versions in that the float only travels about 3 in. In the past, the float traversed the length of the unit and was prone to jam and remain open.
Charging techniques differ slightly from those used with bladder accumulators. First, technicians pump approximately one gallon of fluid into the unit, then precharge with nitrogen and wait about 10 min for the gas and liquid to separate. Then the remaining fluid is pumped in.
This design yields blowout-prevention control systems that survive cold environments and extreme subsea depths. They also tolerate dirty fluid and are field repairable. The units have been used successfully worldwide, particularly in the former Soviet Union. Buoyant-float accumulators are also adaptable to excessively hot conditions.
Accumulators Inc., (713) 465-0202, accumulators.com