Power plants – Pressure fluid source and motor – Utilizing natural energy or having a geographic feature
Reexamination Certificate
1999-07-15
2001-02-27
Ryznic, John E. (Department: 3745)
Power plants
Pressure fluid source and motor
Utilizing natural energy or having a geographic feature
C060S413000
Reexamination Certificate
active
06192680
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a hydraulic control system for operating subsea equipment. More particularly, this invention relates to a hydraulic control system for operating subsea equipment at relatively deep water depths of 6,000 feet or more. The hydraulic control system is capable of supplying either an opening pressure or a closing pressure to the subsea equipment.
BACKGROUND OF THE INVENTION
Those skilled in the hydrocarbon recovery industry recognize that an increasing percentage of hydrocarbons are being recovered from offshore wells, including wells wherein the subsea wellhead is located in very deep water of 6,000 feet or more below the ocean surface. Subsea blowout preventers (BOPs) and related production control equipment rely upon a source of pressurized fluid to actuate the subsea equipment. Much of this equipment must be actuatable in at least two directions and thus is operated by supplying a hydraulic fluid pressure to either “open” or “close” the equipment. A reliable hydraulic control system to operate the equipment is particularly important in emergency applications wherein the equipment must be actuated to either the closed or the opened position in an emergency.
When subsea equipment is positioned in relatively shallow water of two thousand or three thousand feet, a reliable pressure source to operate the subsea equipment commonly is provided by a bank of accumulator bottles (accumulators), which are conventionally precharged with nitrogen. Each accumulator is thus a sealed container which houses pressurized nitrogen, and a bank of such accumulators may be fluidly interconnected to provide the power source for operating the subsea equipment. The nitrogen thus acts as an available spring force to operate the subsea equipment once hydraulic fluid under pressure is pumped into the accumulators from an external source at the surface. Once the subsea accumulators are activated, additional hydraulic fluid is conventionally transmitted from the surface to the subsea accumulators through hose ambilicals or relatively small conduit fill lines.
While the accumulator system as discussed above performs well on land operations and in relatively shallow subsea operations, significant problems are encountered using this accumulator system at water depths of more than 6,000 feet. The nitrogen precharge pressure must be increased to overcome the effects of hydrostatic head pressure for water depth of the control system. Nitrogen, like other gases, has a reduced expansion as the pressure to which it is subjected gets higher. Moreover, subsea equipment at 10,000 feet or more is inherently cool, and the combination of the cooled and high pressure nitrogen approaches saturation so that the nitrogen tends to lose its expanding characteristics and thus its pressurizing ability on the subsea equipment. As a consequence, numerous banks of accumulators are required to reliably supply activating fluid to a BOP at 10,000 feet, although the same BOP may be reliably controlled at the surface or in shallow waters with only a few accumulators.
At deep water depths, the hydraulic energy stored in the accumulators may be at a pressure of several thousand psi in addition to the hydrostatic head pressure of the surrounding sea water. At a depth of 10,000 feet, for example, the stored pressure would be approximately 5,000 psi plus 4,450 psi hydrostatic head, for a total of 9,450 psi. At this high pressure, the nitrogen in the accumulator is very inefficient since flow characteristics of the nitrogen become very sluggish. For the required capacity of fluid to reliably operate the subsea equipment, the quantity of accumulators required is thus significantly increased. This large number of accumulators represents a high cost to supply fluid to operate the subsea equipment, thereby increasing the overall cost of the hydrocarbon recovery operation.
The disadvantages of the prior art are overcome by the present invention, and an improved hydraulic control system for operating subsea equipment is hereinafter disclosed.
SUMMARY OF THE INVENTION
A system for supplying hydraulic pressure to operate subsea equipment utilizes the hydrostatic head pressure of the surrounding seawater to operate the subsea equipment. A subsea hydraulic fluid storage vessel is provided for storing a selected quantity of hydraulic fluid, which preferably is greater than 80 gallons. A piston or other fluid separation member separates the water from the hydraulic fluid in the fluid storage vessel and also pressurizes the hydraulic fluid in response to the pressure of the water at the depth of the fluid separation member, which in the exemplary application is in excess of 6,000 feet. A fluid supply line fluidly interconnects the fluid storage vessel and both first and second fluid input ports on the subsea equipment. A subsea hydraulic fluid reservoir vessel is provided for receiving hydraulic fluid from the subsea equipment, and a fluid exhaust line fluidly interconnects the subsea equipment with this fluid reservoir vessel. A vent line extends from the fluid reservoir vessel to the surface, which acts as a temporary reservoir for storing hydraulic fluid discharged from the subsea equipment without pressurizing the hydraulic fluid by the subsea hydrostatic head. A subsea pump exhausts fluid from the fluid reservoir vessel. In a preferred embodiment, the hydraulic fluid is a water based fluid, and the pump exhausts the fluid from the fluid reservoir vessel directly to the subsea water.
An electronic subsea control system may be used for selectively controlling the flow of hydraulic fluid from the fluid storage vessel to the first and second fluid input ports. A subsea regulator may be positioned along the fluid supply line to regulate the pressure of hydraulic fluid from the fluid storage vessel to the subsea equipment. An hydraulic fluid supply line may extend from the surface to the fluid storage vessel for initially supplying and resupplying hydraulic fluid to the fluid storage vessel.
According to the method of the invention, a selected quantity of hydraulic fluid is stored in a subsea hydraulic fluid storage vessel. The hydraulic fluid is separated from the seawater which automatically pressurizes hydraulic fluid in response to the hydrostatic head of the seawater pressure. The fluid storage vessel is fluidly interconnected with first and second input ports of the subsea equipment and, upon actuating the equipment, hydraulic fluid is transmitted to a subsea hydraulic fluid reservoir vessel. A vent line is provided for venting the fluid reservoir vessel to the surface, and hydraulic fluid in the fluid reservoir vessel is periodically exhausted, preferably directly to the seawater since the hydraulic fluid may be water based. In the case of an emergency, the energy of the hydraulic fluid storage subsea, which is pressurized by the hydrostatic head of the seawater, is thus available to actuate the subsea equipment, which may include pipe and shear rams.
It is an object of the present invention to provide a subsea hydraulic control system which utilizes a hydrostatic head of seawater to provide the hydraulic energy source which pressurizes fluid to operate the subsea equipment.
Another object of the invention is to provide a subsea hydraulic control system which utilizes a subsea hydraulic fluid reservoir vessel, and a vent line extending from the fluid reservoir vessel to the surface. As a control system is operated, fluid will be forced by seawater pressure to the subsea equipment, and fluid from the subsea equipment will be forced to the fluid storage vessel. As the fluid storage vessel becomes filled with subsea fluid, a pump may be used to empty the fluid back to the seawater. A supply line from the surface to the fluid storage vessel may be used to resupply hydraulic fluid to the fluid storage vessel.
It is a feature of the present invention to provide subsea hydraulic control system which is well suited for operating in very deep water depths of 6,000 feet or more.
It is a feature of the pre
Brugman James D.
Elkins Hubert L.
Merit Mark A.
Browning Bushman
Ryznic John E.
Varco Shaffer, Inc.
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