METHOD OF BACK-PULSE FLUSHING CLOGGED PIPES, FOR EXAMPLE IN A HYDRAULIC PIPE SYSTEM

Abstract

Various embodiments of the present disclosure are directed to methods of removing liquid from a lumen of a hydraulic control line by a back-pulse flushing procedure. In one example embodiment, the back-pulse flushing procedure includes pressurizing the hydraulic control line to a pressure P1 by adding pressurized carbon dioxide into a first end of the hydraulic control line, where the carbon dioxide is in a liquid or supercritical state, maintaining the carbon dioxide in the liquid or supercritical state while the carbon dioxide diffuses through and accumulates in the matter, and depressurizing the hydraulic control line to cause the carbon dioxide to change into a gas state and press the matter out of the first end of the hydraulic control line.

Claims

1. A method of removing matter from a lumen of a hydraulic control line (2) by a back-pulse flushing procedure; wherein the back-pulse flushing procedure comprises: pressurizing the hydraulic control line to a pressure P1 by adding pressurized carbon dioxide into the hydraulic control line (2) at a first end of the hydraulic control line (2); adding the pressurized carbon dioxide at a temperature T, which at the pressure P1 is in a liquid state, LCO2, or in a supercritical state, scCO2; maintaining the carbon dioxide in a liquid state or in a supercritical state, respectively, by maintaining the hydraulic control line (2) in the pressurized state for a time t, while the LCO2 or scCO2 diffuses through the matter during the time t and the LCO2 or scCO2 accumulates inside the matter or on the opposite side of the matter or both; then, after the time t, depressurizing the hydraulic control line (2) at the first end to a lower pressure level P2<P1, for example atmospheric pressure, and causing the carbon dioxide to change into expanding gas inside the hydraulic control line (2) and to press the matter out of the hydraulic control line (2) through the first end of the hydraulic control line (2) by the expanding gas.

2. A method according to claim 1, the method comprises cyclically repeating the back-pulse flushing procedure multiple times.

3. A method according to claim 2, wherein the method comprises pressing the matter through the hydraulic control line (2) to the first end of the hydraulic control line (2) under turbulent conditions.

4. A method according to claim 3, wherein the method comprises adjusting the pressure P1 and the lower pressure level P2 to achieve a velocity of the matter in the hydraulic control line (2) that corresponds to a Reynolds number of at least 3000.

5. A method according to any preceding claim, wherein the method comprises selecting the time t to between 0.1 hour and 72 hours.

6. A method according to any preceding claim, wherein the method comprises pressurizing the hydraulic control line (2) to P1, wherein P1 is in the range of 10,000 kPa (100 bar) to 100,000 kPa (1000 bar).

7. A method according to any preceding claim, wherein the method comprises pressurizing the hydraulic control line (2) to a pressure P1 above the critical pressure, Pc, of carbon dioxide; adding the carbon dioxide as scCO2 at a temperature T in the range of 60 to 200 degrees centigrade.

8. A method according to any preceding claim, wherein the hydraulic control line (2) has a cross sectional area of less than 150 square mm.

9. A method according to any preceding claim, wherein the hydraulic control line is a hydraulic dead-end hydraulic control line for hydraulic actuation of an actuator in a valve of an offshore installation, the hydraulic control line having a cross sectional area of less than 150 square mm and a length of more than 100 m.

10. A method according to any preceding claim, where in the LCO2 or scCO2 is provided with a content of surfactant, wherein the method comprises adjusting the volume of the surfactant relatively to the volume of the LCO2 or scCO2 in the range of 1-5%.

11. A method according to any preceding claim, the method further comprising after removal of the matter, maintaining pressure in the hydraulic control line (2) and adding clean hydraulic liquid while under pressure and removing the CO2 by displacing it with the hydraulic liquid, and then lowering the pressure.

12. A method according to any preceding claim, wherein the method comprises selecting the time t to be between 2 hours and 72 hours.

13. A method according to claims 8 and 12.

14. A method according to claims 9 and 12.

1. A method of removing matter from a lumen of a hydraulic control line by a back-pulse flushing procedure; wherein the back-pulse flushing procedure comprises: pressurizing the hydraulic control line to a pressure P1 by adding pressurized carbon dioxide into a first end of the hydraulic control line; adding the pressurized carbon dioxide at a temperature T, which at the pressure P1 is in a liquid state, LCO2, or in a supercritical state, scCO2; maintaining the carbon dioxide in a liquid state or in a supercritical state, respectively, by maintaining the hydraulic control line in the pressurized state for a time t while the LCO2 or scCO2 diffuses through the matter during the time t and the LCO2 or scCO2 accumulates inside the matter or on the opposite side of the matter or both; then, after the time t, depressurizing the hydraulic control line at the first end to a lower pressure level P2<P1, for example atmospheric pressure, and causing the carbon dioxide to change into expanding gas inside the hydraulic control line and to press the matter out of the first end of the hydraulic control line by the expanding gas.

2. The method according to claim 1, the method further includes cyclically repeating the back-pulse flushing procedure multiple times.

3. The method according to claim 2, wherein the method further includes pressing the matter through the first end of the hydraulic control line under turbulent conditions.

4. The method according to claim 3, wherein the method further includes adjusting the pressure P1 and the lower pressure level P2 to achieve a velocity of the matter in the hydraulic control line that corresponds to a Reynolds number of at least 3000.

5. The method according to claim 1 wherein the method further includes selecting the time t to between 0.1 hour and 72 hours.

6. The method according to any preceding claim, wherein the method further includes pressurizing the hydraulic control line to P1, wherein P1 is in the range of 10,000 kPa (100 bar) to 100,000 kPa (1000 bar).

7. The method according to claim 1 wherein the method further includes pressurizing the hydraulic control line to a pressure P1 above the critical pressure, Pc, of carbon dioxide; and adding the carbon dioxide as scCO2 at a temperature T in the range of 60 to 200 degrees centigrade.

8. The method according to claim 1 wherein the hydraulic control line has a cross sectional area of less than 150 square mm.

9. The method according to claim 1 wherein the hydraulic control line is a hydraulic dead-end hydraulic control line configured and arranged for hydraulic actuation of an actuator in a valve of an offshore installation, the hydraulic control line having a cross sectional area of less than 150 square mm and a length of more than 100 m.

10. The method according to claim 1, where in the LCO2 or scCO2 is provided with a content of surfactant, wherein the method further includes adjusting the volume of the surfactant relatively to the volume of the LCO2 or scCO2 in the range of 1-5%.

11. The method according to claim 1 the method further including the step of, after removal of the matter, maintaining pressure in the hydraulic control line [[(2)]]and adding clean hydraulic liquid while under pressure and removing the CO2 by displacing it with the hydraulic liquid, and then lowering the pressure.

12-14. (canceled)

15. The method according to claim 1, wherein the method further includes selecting the time t to be between 2 hours and 72 hours.

16. The method according to claim 8, wherein the method further includes selecting the time t to be between 2 hours and 72 hours.

17. The method according to claim 9, wherein the method further includes selecting the time t to be between 2 hours and 72 hours.

Description

DESCRIPTION OF THE DRAWING

[0053] This invention will be described in relation to the drawings, where:

[0054] FIG. 1 shows a sketch of an offshore installation

[0055] FIG. 2 is a diagram showing Reynolds number from flushing contaminations in an oil pipe;

[0056] FIG. 3 is a diagram showing the gradual cleanliness of the pipe in terms of the NAS1638 standard:

[0057] FIG. 4 is a table for the definition of the NAS 1638 standard;

[0058] FIG. 5 is a diagram Reynolds number during filling of the pipe with scCO2.

DETAILED DESCRIPTION OF THE INVENTION

[0059] FIG. 1 shows a sketch of an offshore installation 1, which is an oil or gas rig in sea water 4. Oil or gas from a well 7 is pumped through a tube 3 to the rig 1 and pumped from there through an umbilical to an accumulator, for example a vessel. The tube 3 can be closed off by a valve 6, which is important for safety reasons, especially environmental protection in case of problems. The valve 6 comprises a hydraulic actuator that is operated by hydraulic fluid in hydraulic pipe 2. In contrast to the oil transporting tube 3, the hydraulic pipe 2 has a much smaller diameter, typically in the order of 5 mm to 13 mm, such a quarter inch pipe or a half inch pipe, which is a commonly used pipe size for this purpose.

[0060] With time, the hydraulic fluid, for example oil, in the hydraulic pipe 2 increases in viscosity and sludge may be deposited not only on the walls of the pipe but also in the valve, especially in the actuator, in addition to particles from the hydraulic fluid or from the mechanical components in the tube and valve system. Sludge can plug the lines such that transport of liquid through the pipe is no longer possible or at least not possible to a level that ensures proper functioning of the equipment. Also, particulate matter can become part of the sludge. Another risk is accumulation of sludge and/or particulate matter in equipment that is connected to the pipe and driven by the hydraulic fluid. For example, hydraulic valve systems are at risk for being clogged and malfunctioning due to sludge and particulate matter.

[0061] As the hydraulic pipe 2 for controlling the valve cannot be flushed through due to being a dead end pipe, a cleaning method is used in which matter is removed from a lumen of a pipe by a back-pulse flushing where carbon dioxide in liquid state LCO2 or supercritical state scCO2 is added to a pipe for the CO2 to diffuse into and through the matter, after which the pressure is reduced. The pressure reduction changes the CO2 into expanding gas that presses the matter out of the pipe at the same end into which the CO2 was inserted. In addition, flushing the pipe 2 when filling CO2 into the pipe is additionally cleaning the walls inside the pipe.

[0062] The method is useful for cleaning long dead-end pipes, for example hydraulic control pipes for valves in offshore installations, especially in oil and gas industry. It is advantageously applied in cycles to remove the matter in portions from the pipe.

[0063] FIG. 2 is a diagram showing Reynolds numbers from cyclic flushing contaminations in an oil pipe. Due to the Reynolds number of more than 5000, the flushing has been turbulent with a very good cleaning efficiency.

[0064] FIG. 3 is a diagram showing the gradual cleanliness of the pipe in terms of a National Aerospace standard (NAS 1638), which is an international standard used for defining cleanliness and the definitions of which is shown in FIG. 4.

[0065] FIG. 5 is a diagram Reynolds number during filling of the pipe with scCO2. It is seen that the Reynolds numbers are above 30000, which indicates turbulent flushing with scCO2.

[0066] The use of SCCO2 for flushing pipes is superior to flushing with LCO2. This is due to the fact of the lower viscosity as well as for the higher diffusivity. The lower viscosity allows higher flow speed at reduced pressure loss as compared to LCO2. The lower diffusivity results in better penetration of the matter. However, especially for underwater pipes, the temperature cannot always be maintained above the critical temperature of Tc=31° C. why LCO2 may be used instead. Experimentally, useful results have also been obtained with LCO2.

[0067] For instances where a pipe is placed in sea water and cooled through the pipe wall by the sea water, the temperature may drop such that a supercritical state cannot be preserved along the entire pipe. In such case, where the CO2 changes into liquid form, variations with respect to pressure loss and speed inside the lumen would occur. However, the flushing would still be possible, although parameters would have to be adjusted. For example, the pressure loss would be higher due to the higher viscosity, and the entrance pressure would have to be chosen correspondingly higher. In order to keep the CO2 in a supercritical state for as much of the pipe length as possible, the flow speed should be adjusted relatively high.

TABLE-US-00001 Item Reference Offshore installation/rig 1 Hydraulic pipe 2 Tube 3 Sea water 4 Valve 6 Well 7