Temperature compensation of deepwater accumulators

Abstract

The method of providing an accumulator for the storage of pressurized liquids by the use of a pressurized gas, comprising providing for the storage of said pressurized gas, providing for the storage of said pressurized liquids which are pressurized by said pressurized gas, moving said accumulator to a location of lower environmental temperatures, and increasing the temperature of said pressurized gas to increase the pressure of said gas and therefore to increase the pressure of said pressurized liquid.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Ser. No. 10/314,361

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

N/A

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISK

N/A

BACKGROUND OF THE INVENTION

The field of this invention is that of deepwater accumulators for the purpose of providing a supply of pressurized working fluid for the control and operation of equipment. Typical equipment includes, but is not limited to, blowout preventers (BOP) which are used to shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, or hydraulically actuated connectors and similar devices. The fluid to be pressurized is typically an oil based product or a water based product with additives to enhance lubricity and corrosion protection.

Currently accumulators come in three styles and operate on a common principle. The principle is to precharge them with pressurized gas to a pressure at or slightly below the anticipated minimum pressure required to operate equipment. Fluid can be added to the accumulator, increasing the pressure of the pressurized gas and the fluid. The fluid introduced into the accumulator is therefore stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work.

The accumulator styles are the bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and the float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of the charging gas.

Accumulators at the surface typically provide 3000 p.s.i. working fluid maximum pressure. As accumulators are used in deeper water, the efficiency of conventional accumulators is decreased. In 1000 feet of seawater the ambient pressure is approximately 465 p.s.i. For an accumulator to provide a 3000 p.s.i. differential at 1000 ft. depth, it must actually be precharged to 3000 p.s.i. plus 465 p.s.i. or 3465 p.s.i.

At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 p.s.i., so the precharge would be required to be 3000 p.s.i. plus 2000 p.s.i. or 5000 p.s.i. This would mean that the precharge would equal the working pressure of the accumulator. Any fluid introduced for storage would cause the pressure to exceed the working pressure, so the accumulator would be non-functional.

Another factor which makes the deepwater use of conventional accumulators impractical is the fact that the ambient temperature decreases to approximately 35 degrees F. If an accumulator is precharged to 5000 p.s.i. at a surface temperature of 80 degrees F., approximately 416 p.s.i. precharge will be lost simply because the temperature was reduced to 35 degrees F. Additionally, the rapid discharge of fluids from accumulators and the associated rapid expansion of the pressurizing gas causes a natural cooling of the gas. If an accumulator is quickly reduced in pressure from 5000 p.s.i. to 3000 p.s.i. without chance for heat to come into the accumulator (adiabatic), the pressure would actually drop to 2012 p.s.i.

A fourth type accumulator has been developed which is one which is pressure compensated for depth, and is illustrated in the U.S. Pat. No. 6,202,753. This style operates effectively like a summing relay to add the nitrogen precharge pressure plus the ambient seawater pressure to the working fluid. This means that irrespective of the seawater depth (pressure), the working fluid will always have a greater pressure available for work by the amount of the nitrogen precharge.

This “pressure compensated” style has numerous advantages in addition to the pressure compensation. It allows lower gas pressures with associated safety, eliminates the need to recharge the system for differing operational depths, and eliminates expensive mistakes in setting the charge pressures.

Although the pressure compensated type has advantages over the other types of accumulators, it is still impacted by the change in temperature as the seawater and therefore nitrogen becomes colder with depths.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide a pressure compensated accumulator for deepwater ocean service which compensates for the decrease in temperature due to operating in ocean depths.

A second object of the present invention is to provide an accumulator which can have the gas of the accumulator cooled at the surface to the temperature of the subsea environment to allow realistic operational tests at the surface without having to change the charge of the accumulator before deploying it to ocean depths.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partial section thru a subsea blowout preventer stack showing applications of principles of this invention.

FIG. 2 is a half section of an accumulator of the present invention.

FIG. 3 is a partial section of the top portion of the accumulator of this invention.

FIG. 4 is a partial section of the accumulator of this invention showing means to exhaust accumulated liquids from the nitrogen chamber.

FIG. 5 is a partial section of the accumulator of this invention showing the lower portion of the vacuum portion of the accumulator.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a blowout preventer (BOP) stack 10 is landed on a subsea wellhead system 11, which is supported above mudline 12. The BOP stack 10 is comprised of a wellhead connector 14 which is typically hydraulically locked to the subsea wellhead system 11, multiple ram type blowout preventers 15 and 16, an annular blowout preventer 17 and an upper mandrel 18. A riser connector 19, and a riser 20 to the surface are attached for communicating drilling fluids to and from the surface.

Blowout preventer 16 shows that an accumulator 40 of this invention being connected to one of the outer cavities 41 thru line 42 and valve 43. If the valve 43 is opened, fluid pressure from accumulator 40 will move the ram 45 toward the center of the vertical bore (and seal against an opposing ram similarly moved). Accumulator 40 can be any of the types described in the description above.

Referring now to FIG. 2, accumulator 50 has an upper plate 51, a lower plate 52, a first cylinder 53, a second cylinder 54, a third cylinder 55, a fourth cylinder 56, connecting bolts 57, connecting nuts 58, and lifting eye 59.

First cylinder 53 has an upper bore 70, a lower bore 71, a bulkhead 72, a cylinder rod 73, an upper piston 74, and a lower piston 75. Fourth cylinder 56 has an upper bore 80, a lower bore 81, a bulkhead 82, a cylinder rod 83, an upper piston 84, and a lower piston 85.

Second cylinder 54 is charged with a pressurized gas, has a valve assembly 90 near the bottom, and a heating element 130. Third cylinder 55 is charged with a pressurized gas and has a heating element 131.

Chambers 100, 101, 102, and 103 are pressurized with a gas such as nitrogen or helium. Chambers 115 and 116 contain a working fluid accessible thru ports 117 and 118.

Chambers 120 and 121 contain sea water or oil at seawater pressure and the resultant sea water pressure which comes in thru ports 122 and 123 respectively. Chambers 125 and 126 contain a vacuum or may simply be allowed to have atmospheric pressure at the surface at assembly which will effectively be a vacuum in deep water.

Electric heating elements 130 and 131 have terminals 132, 133, 134, and 135 which penetrate the upper plate 51. Electric heating elements 130 and 131 are suspended within second cylinder 54 and third cylinder 55 respectively. These chambers house the majority of the nitrogen gas which acts as the energy storage “spring” to give the accumulator a pressure drive.

If an accumulator has a precharge of 3000 p.s.i. and the temperature of the accumulator is dropped from 84 to 34 degrees F., the pressure will drop by 275 p.s.i. to 2725 p.s.i., giving an automatic loss of efficiency of 29.5%.

The electricity it takes to heat one gallon of nitrogen gas 1 degree at approximately 2000 p.s.i. is about 2.08 watt-hours. For a 100 gallon system to raise the temperature 50 degrees F., it will take about 2867 watt-hours total. At the 480 volts in a typical deepwater drilling control system, this means a total of approximately 358 amp-minutes. If a typical system can send 200 amps down the dual control and power cables, this means that it will take about 2 minutes to heat the gas to compensate for the temperature differential.

This means when a substantial withdrawal occurs from the accumulator banks, the power umbilicals can be utilized to restore the equivalent of the surface temperature within a couple of minutes to give the full operating capacity back to the accumulators. After the operations are completed, the temperature will return slowly to the deepwater ambient temperature (34 deg. F.) as the accumulators are trickle charged back to their full capacity.

Referring now to FIG. 3, upper plate 51 has port 140 communicating the top of first cylinder 53 with second cylinder 54, port 141 communicating fourth cylinder 56 with second cylinder 54, and port 142 communicating third cylinder 55 with second cylinder 54. As the top of all four cylinders are interconnected, the volumes of the four cylinders are combined to provide a gas spring on the top of the two pistons 74 and 84. Pistons 74 and 84 contains seals 152 and 153 respectively to seal between the gas chamber 100 and 103 and the working fluid chambers 115 and 116.

Recesses 160 and 161 on the upper sides of pistons 74 and 84 serve to hold fluid 165 and 166. The retention of the fluid 165 and 166 in the recesses 160 and 161 serves to prevent the pressurized gas at 100 and 103 from contacting and thereby tending to leak past the seals 152 and 153. As liquids are characteristically easier to seal than gasses, the insurance of liquids on both sides of the seal will improve the quality of the sealing.

If not for the recess, as piston 74 goes to the top of the stroke of cylinder 53, all of the liquid might be expelled thru port 140 and dumped into second cylinder 54. Likewise the liquid in the top portion of fourth cylinder 56 might be expelled thru port 141 into second cylinder 54.

Alternately, if during the service life of the accumulator, an excess amount of liquid from chamber 115 passes by seal 152 into chamber 100, the excess amount of liquid will be expelled into the second chamber 54 and excess liquids from fourth cylinder 56 will also be expelled into second cylinder 54.

Referring now to FIG. 4 a lower portion of second cylinder 54 is shown. When an excess amount of fluid is vented into second cylinder 54, float 170 is raised pulling pin 171, link 172, and pin 173 up while pivoting up on shoulder 174. As pin 173 is pulled up valve 175 moves up and opens against spring 177. At this time the high gas pressure in chamber 101 pushes the excess liquid out until the float 170 lowers and allows the valve 175 to close. The excess liquid moves out through check 180 to vent out port 182 to the ocean. The check 180 will then be closed by spring 181. In this way, a single valve assembly 90 can remove any excess fluids which may be vented past the seals on either piston 74 or 84.

Referring now to FIG. 5, a partial section of the bottom of cylinder 56 is shown. In this case a check valve 190 is provided with a spring 191. If the piston 85 is simply lowered to the bottom of the stroke by the pressure of the gas from the top of the upper piston 84, a high pressure will be generated in any liquid trapped at the bottom of the cylinder. The pressure will approximately be the sum of the pressure of the seawater entering port 122 plus the pressure of the gas in chamber 100. As the total pressure will exceed the seawater pressure (i.e. at port 123), any liquids in chamber 126 will be expelled past check valve 190.

In this way, the manufacturing convenience of a four cylinder accumulator bank is complimented with the ability to remove any collection of liquids by a single valve assembly 90, and each of the lower vacuum chambers can be purged by a simple check valve assembly.

In addition to the ability to bring the temperature of the pressurized gas back to the temperature at the surface, a considerable efficiency can be obtained by increasing the temperature of the gas to elevated temperatures.

By selecting appropriate gasses, when the gas reaches a temperature like 34 deg. F. at the bottom of the ocean, it may become a liquid and functionally collapse in volume, allowing the chamber of the gas to become smaller. This can cause a chamber of liquids to become larger, or in other words can recharge the accumulator with liquid. When the gas chamber is then reheated, the gas in the liquid state can be evaporated and thereby repressurized. In this manner a relatively simple system for recharging an accumulator can be located subsea.

The heating coils of this preferred embodiment illustrated can likewise be replaced by cooling elements. The cooling elements can cool the temperature of the gas at the surface to the temperature of the seawater in ocean depths (typically 34 deg. F.). This will allow gas to be charged to the full pressure of 3000 p.s.i. at the surface for testing and then be normally operational at 3000 p.s.i. when it reaches subsea.

The foregoing disclosure and description of this invention are illustrative and explanatory thereof, and various changes in the size, shape, and materials as well as the details of the illustrated construction may be made without departing from the spirit of the invention.

Claims

1. The method of providing an accumulator for the storage of pressurized liquids by the use of a pressurized gas, comprising

providing for the storage of said pressurized gas,

providing for the storage of said pressurized liquids which are pressurized by said pressurized gas,

moving said accumulator to a location of lower environmental temperatures, and

increasing the temperature of said pressurized gas to increase the pressure of said gas and therefore to increase the pressure of said pressurized liquid.

2. The invention of claim 1, further comprising said pressurized gas and said pressurized liquid are in separated chambers.

3. The method of claim 1, further comprising using electricity to increase the temperature of said pressurized gas.

4. The method of claim 3, further comprising using the umbilical power cable of a subsea blowout preventer stack to provide the electric power to increase the temperature of said gas.

5. The method of claim 1, further comprising that said location of lower environmental temperatures is near the floor of an ocean.

6. The method of claim 1, further comprising that said location of lower environmental temperatures is in outer space.

7. The method of claim 5, further comprising that the temperature of said pressurized gas in the subsea or outer space location is increased to a temperature higher than the temperature of the environment.

8. The method of recharging the pressure of an accumulator with a pressurized gas pressurizing a liquid comprising,

allowing the temperature of said gas to cool to a first temperature and liquefy and therefore be of a smaller volume,

allowing the smaller volume of said liquefied gas to increase the volume available for said liquid and therefore draw a greater volume of said liquid into said accumulator, and

increasing the temperature of said liquefied gas to a second temperature to evaporate said gas, increase its pressure, and thereby pressurize said liquid.

9. The method of claim 8 further comprising said first temperature is the temperature near the bottom of an ocean.

10. The method of claim 8, further comprising using electricity to increase the temperature of said pressurized gas.

11. The method of claim 10, further comprising using the umbilical power cable of a subsea blowout preventer stack to provide the electric power to increase the temperature of said gas.

12. The method of compensating for the loss of gas pressure in an accumulator due to the reduction of the temperature of the gas, comprising

providing a first chamber for the storage of pressurized liquids,

providing a second chamber for the storage of said pressurized gas,

increasing the temperature of said pressurized gas to increase the pressure of said gas and therefore to increase the pressure of said pressurized liquid.

13. The method of claim 12, further comprising using electricity to increase the temperature of said pressurized gas.

14. The method of claim 13, further comprising using the umbilical power cable of a subsea blowout preventer stack to provide the electric power to increase the temperature of said gas.

15. The method of claim 12, further comprising that the method for increasing the temperature of said pressurized gas is by using heating elements within said second chamber.

16. The method of claim 12, further comprising that the method for increasing the temperature of said pressurized gas is by using heating elements outside of said second chamber.

17. The method of providing an accumulator for the storage of pressurized liquids by the use of a pressurized gas, comprising

providing for the storage of said pressurized gas in a first chamber,

providing for the storage of said pressurized liquids which are pressurized by said pressurized gas,

reducing the temperature of said gas at the surface to the anticipated temperature of the deepwater environment,

setting the pressure of the gas at the surface to the desired subsea pressure,

lowering the accumulators to the subsea environment, and

allowing the accumulators to become the same temperature as the subsea environment.

18. The method of claim 17, further comprising that the reducing of the temperature of said gas at the surface is accomplished by using cooling coils within said first chamber.

19. The method of claim 17, further comprising that the reducing of the temperature of said gas at the surface is accomplished by using cooling coils external to said first chamber.