Patent Description:
Hydraulic control pipe systems for controlling subsea valves in petrochemical industry are subject to accumulation of unwanted materials and impurities not only on the pipe walls but also in the valve itself, which can be detrimental for the functioning of valve. In the case where the hydraulic pipe system uses an oil-based fluid, the impurities or dirt that accumulated on the inside of the hydraulic pipe system is usually wax or grease. The impurities, including particulate matter, may also be accumulated in valves used in deep-sea installations with the risk of malfunctioning.

Malfunctioning of valves can lead to severe environmental accidents, for example when oil pipes are not closed properly and spilled into the sea water. As such unwanted materials or impurities result in reduced operational safety, there is a desire to provide a cleaning method.

The problem with hydraulic valve actuators in oil and gas production is discussed in <CIT> by Vickio, in which it is proposed to use hydraulic fluid at turbulent flow through the hydraulic system.

For the case that the valve can be opened, and liquids can be flushed through the valve, US patent application <CIT>et al. and assigned to Ocean Team Group A/S proposes a method where supercritical carbon dioxide, scCO2, or liquid carbon dioxide, LCO2, is flushed through the pipe under turbulent conditions.

<CIT> discloses use of scCO2 in oil pipes for removing fouling.

However, for control lines for valves that cannot be flushed through because there is no opening at the valve, this method does not apply. For cleaning, the valve would have to be demounted, for which typically the entire pipe system, typically having a length in the order of <NUM> to <NUM> meter, would have to be lifted up to the surface. This implies high costs and efforts, and the operation of the oil recovery system related to the valve would be halted, which is not desirable.

In order to clean pipe systems, it has been proposed to use turbulent flow in such pipes with a cleaning and flushing liquid. The turbulent flow assists in loosening contaminants that adhere to the inner wall of the pipes and flush away the contaminants. In the <CIT>, assigned to Ocean Team Scandina-via, a system is disclosed with fluid pipes are cleaned with a pulsated flow. In order to obtain a turbulent flow, a Reynolds number of at least <NUM> or at least <NUM> is mentioned.

When narrow pipe systems get very long, the pressure drop of the cleaning fluid throughout the pipe results in loss of turbulent flow, because the speed of the flow cannot be kept high enough. This problem is discussed in patent <CIT>; the pressure drop in pipes that are longer than <NUM> and with a narrow lumen of <NUM> prevent a flushing speed that creates a turbulent flow, because the pressure required at the entrance of the tube to compensate for the pressure loss along the pipe and for create the necessary flow speed would exceed the pressure that the pipes typically withstand. As a solution to this problem, <CIT> proposes filling the pipe system with flushing liquid as well as nitrogen gas such that number of portions of flushing liquid in the pipe is separated by gas portions. The gas in the alternating columns of oil and gas is compressed for subsequent expansion when a valve at the end of the pipe is opened in order to create a forceful flushing pulse through the pipe system.

For cleaning and flushing of pipe systems, heat exchangers, condensers and catalysers, liquid carbon dioxide (LCO2) or supercritical carbon dioxide (scCO2) has been proposed in <CIT> by Kipp. As illustrated in the figures of <CIT>, LCO2 or scCO2 is led into the bottom of a heat exchanger and extracted through a top valve before being filtered as gas and recirculated. In <CIT>, no details are given with respect to flow speed or pressure other than the pressure and temperature necessary to keep the carbon dioxide, CO2, in a liquid or supercritical state. It is explained that the LCO2 and the scCO2 would loosen the contamination from the inner walls.

Rinsing cavities with supercritical CO2 is disclosed in <CIT>. CO2 gas as a flushing in submarines is disclosed in European patent application <CIT>et al. <CIT> discloses CO2 for cleaning heat exchangers. <CIT>discloses scCO2 for cleaning a refrigeration system. <CIT> concerns scCO2 for cleaning of endoscopes. Use of hydrocarbon fluids for cleaning a chemical or hydrocarbon processing plant is disclosed in <CIT>. Substrate cleaning with scCO2 is disclosed in <CIT>et al. <CIT>discloses scCO2 for solubilizing a surfactant for enhanced oil recovery. <CIT>discloses scCO2 for mixing with heavy crude oil to reduce the viscosity and ease transportation of oil through pipes. <CIT> discloses cleaning of gas containers. <CIT> discloses flushing with CO2 of bore holes in work pieces in automobile industry.

It appears from the above prior art that cleaning with CO2 in liquid form or in supercritical state is common practice for hydraulic pipes when the CO2 is inserted at one end and released at the opposite end.

However, these methods are not applicable for hydraulic pipes to actuators in control valves, where the pipe has a dead end which is not accessible. Accordingly, there is a need for further improvement in the art.

It is an objective for the present invention to provide an improvement in the art. In particular, it is an objective to provide a method for cleaning fluid control pipes, for example hydraulic pipes, which are dead-end pipes or where release of the fluid, for example hydraulic fluid, at the remote end cannot be released, for example due to environmental reasons. In particular, it is an objective to provide a cleaning method for hydraulic pipes to actuators in subsea control valves, especially in oil and gas industry.

This objective is achieved with a method in which matter, such as clogging matter, is removed from a lumen of a pipe, such as a clogged pipe, by a back-pulse flushing where carbon dioxide in liquid or supercritical state is added to a pipe for the CO2 to diffuse into and through the matter in the pipe, 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.

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 repeated cycles to remove the matter from the pipe in portions. The method is useful for other types of pipes, in particular other types of fluid control pipes and also for chemical injection pipes. For example, the pipe is part of an umbilical, in particular offshore umbilical, optionally of the type used for subsea industry.

For example , the matter in the pipe, typically, contains viscous solid, for example wax or grease, and potentially also solid particles, optionally also liquid, such as hydraulic fluid. In hydraulic pipes, the hydraulic liquid, for example oil, may have changed into sludge, also called grease or wax. This can range from solid over viscous solid to liquid state. Sludge can clog the lines such that transport of liquid through the pipe is no longer satisfactory, for example not any longer possible or at least not possible to a level that ensures proper functioning of the equipment.

Also, particulate matter can be 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.

In more detail, carbon dioxide, CO2, is provided at a pressure and a temperature, where the carbon dioxide is in a liquid state, LCO2, or in a supercritical state, scCO2. In order to maintain the CO2 in a liquid or supercritical state, the pressure of the pipe is adjusted correspondingly, for example to the same pressure or only slightly lower pressure, or even a higher pressure. Important is that the pressure level P1 in the pipe is not causing the CO2 to change into a gaseous state when entering the pipe and flowing to the position of the matter that is to be removed.

The LCO2 or scCO2 is diffusing through the matter along a part of the pipe and accumulates inside the matter and/or on the other side of the matter, the latter being a special situation if the matter is a plug of grease that is clogging the pipe. The diffusion may be assisted by gravity. By sufficiently depressurising the pipe, the CO2 changes into a gaseous state, where it builds up pressure inside the matter or on the other side of the plug. The pressure causes expansion of the gas and presses the matter out of the first end, especially if the pipe is a dead end pipe or if the pipe is very long such that displacement of the material to the other end and out of the other end is much harder than pressing the matter out of the first end. The method is useful for cleaning pipes from the first end only.

The cleaning from one end only has a great advantage in offshore installation for oil and gas recovery in that the operation of the oil or gas plant is not necessary to stop, which saves high costs.

Experimentally, satisfying results have been achieved with both LCO2 and scCO2. However, the selection of either of the states depends on the circumstances. If the pipes are cold, for example in deep seawater, it can be difficult to keep the supercritical state, which requires a temperature above the critical temperature Tc=<NUM> (degrees centigrade). In such cases, use of LCO2 can be advantageous over scCO2. However, in oil pipes during pumping operation, the temperature can be above <NUM>, why scCO2 can be used with success. For example, the scCO2 is added to the pipe at a higher temperature than the pipe has itself, optionally at a temperature in the range of <NUM> to <NUM> degrees centigrade. As compared to LCO2, the supercritical state has lower diffusivity and viscosity and tend to penetrate the matter easier and faster. Also, in the case that the matter to be removed is far down in a narrow tube, the scCO2 flows easier and faster through the tube.

The latter is of high interest when the back-pulse flushing procedure for removing matter is repeated cyclically multiple times, for example in the range of <NUM>-<NUM> times, for removing matter in minor portions step by step. For example, the CO2 may penetrate the matter over a distance of a few meter and be used to remove portions of matter from the pipe where each portion corresponds to a volume that fills a few meter of the pipe.

For example, the pipe is pressurized to a pressure P1 above the critical pressure, Pc=<NUM> MPa, of carbon dioxide. The carbon dioxide is then added as scCO2 at a temperature T above the critical temperature Tc=<NUM>, for example in the range of <NUM> to <NUM> degrees centigrade. Typically, the pressure P1 is far above the critical pressure, for example in the range of <NUM> MPa (<NUM> bar) to <NUM> MPa (<NUM> bar). After the dwell time of t, in which the LCO2 or scCO2 has diffused into and through the matter, the pressure is lowered at the first end to a level P2, for example to atmospheric pressure (<NUM> bar), in order to press the matter out of the pipe by the expanding gas.

In experiments, where hydraulic pipes under seawater have been cleaned with CO2, each flushing cycle can have a dwell time t of the CO2 which varies broadly, For example, for a clogged hydraulic line, the clogging may take up to three days to penetrate. On the other hand, if the hydraulic fluid is still liquid, especially if the clogging has been removed, the dwell time t is in the order of minutes. The time t therefore is in the range of <NUM> hour to <NUM> hours, typically however in the range of <NUM> hour to <NUM> hours. For example, the first cycle implies a dwell time t in the range of <NUM> to <NUM> hours and the subsequent cycles a time t in the range of <NUM>-<NUM> hours, potentially in the range of <NUM> to <NUM> hours.

The method can be used to clean and empty even very long pipes of narrow diameter, for example several kilometers long and with a diameter of less than <NUM>.

Useful when flushing such pipe that contains liquid, for example hydraulic liquid, such as oil, is a turbulent flushing. In order to press the the matter through the pipe to the first end of the pipe under turbulent conditions, the related Reynolds number has to be high enough, for example at least <NUM>. However, experiments have been made, where the Reynolds number was above <NUM>, for example in the range of <NUM> and <NUM>.

The Reynolds number is defined as Re=density*velocity*diameter/viscosity and can correspondingly be calculated for the matter during the back-pulse flushing and also for the LCO2 or scCO2 travelling down the pipe towards the matter in the cycles.

For example, the Reynolds number can be determined in the following procedure. By measuring the volume of matter that has been removed from the pipe for each of the multiple back-pulse flushing cycles and knowing the pipe diameter, the length of the already flushed part of the pipe can be calculated, where the flushed part is that part of the pipe from which matter has been removed during the corresponding cycles. The lengths of the flushed part as summed from all the already performed cycles is yielding the depth inside the pipe at which the next cycle has to remove matter. The depth gives the distance from the first end to the matter that is to be removed in the next cycle. With the calculated distance and a measured time lag between the depressurization of the pipe and the arrival of the matter at the first end of the pipe, an average velocity of the matter can be calculated. By also determining or estimating the density and the viscosity of the matter, the Reynolds number can be calculated on the basis of the average velocity.

Already when filling CO2 into the pipe, it is advantageous to create turbulence for the CO2, as this turbulence cleans the pipe walls. For LCO2, turbulent flow is expected for a Reynolds number of at least <NUM>, for example at least <NUM>. This number is very much like the corresponding estimate for flushing oil. For SCCO2, the Reynolds number for turbulent flow is about an order of magnitude higher, for example at least <NUM>,<NUM> or at least <NUM>,<NUM> or thus at least <NUM>,<NUM>.

For example, the speed of the CO2 through the lumen is at least <NUM>/sec, for example at least <NUM>/sec or at least <NUM>/sec or at least <NUM>/sec. However, this also depends on the cross section in the tube, and turbulent speed can potentially be achieved with speed as low as <NUM> or <NUM>/sec.

However, in case that the SCCO2 is filled into a lumen of a pipe that is very long, for example more than <NUM> long, and extends into sea water, the temperature of the sea water would result in a temperature drop inside the tube which may cause a change of the supercritical state into a liquid state. As there is an interest of keeping the CO2 in a supercritical state for relatively long inside the lumen, the speed should of the CO2 not become too low. A speed of at least <NUM>/sec has been found to be a good selection in such cases, although the speed may be lower or higher in dependence of the surrounding conditions, for example cold sea water, which influence the temperature drop. The advantage of SCCO2 as compared to LCO2 is the lower viscosity, which allows a higher flow rate at relatively low pressure drop through the tube. The higher flow rate is a good measure against early temperature decrease below the critical temperature.

Typical cross sectional sizes of pipes for underwater hydraulic pipes in gas and oil industry are less than <NUM> mm2 (square millimeter) and typically at least <NUM> mm2. For example, the pipe is a hydraulic dead-end pipe for hydraulic actuation of an actuator in a valve of an offshore installation, the pipe having a ross sectional area of at least <NUM> mm2 and less than <NUM> mm2 and a length of more than <NUM>, typically in the range of <NUM>-<NUM>, although even longer lengths are possible.

For example, experimentally a quarter inch lumen of a <NUM> long pipe was cleaned with such method. The pressure used was <NUM> bar, and the temperature <NUM>.

In another experiment, a chemical injected fluid had become very thick and sticky, and the hydraulic line could not be used. During this back-flushing experiment, <NUM>,<NUM> liter of matter was removed from the one-way line. This volume was equivalent to <NUM>,<NUM> line that had been back-flushed, out of a total line of <NUM> with and inner diameter of <NUM>. When the CO2 is flushing the matter out of the pipe, the CO2 can easily be recovered and used in subsequent back-pulse flushing cycles.

In some embodiments, the LCO2 or scCO2 is provided with a content of surfactant, wherein the volume of the surfactant relatively to the volume of the LCO2 or scCO2 is typically in the range of <NUM>-<NUM>%. For example surfactants with long-chained hydrocarbons are used or surfactants with aromatic rings. Possible surfactants are cyclic hydrocarbon solvent, dipropylene glycol mono n-butyl ether, alcohol ethoxylate, or ethoxylated alkyl mercaptan.

For refilling hydraulic liquid back into the pipe, after removal by the method as described above, in some embodiments, pressure is maintained at elevated level in the pipe and the clean hydraulic liquid is added while the pipe is kept under pressure. The CO2 is then removed displacing it with the hydraulic liquid before the pressure is lowered again.

For example, the cross sectional area of the lumen is <NUM> square mm and the length more than <NUM>; the speed of the CO2 through the pipe during the flushing step is at least <NUM>/sec, optionally at least <NUM>/sec, and the Reynolds number is at least <NUM>,<NUM> if the CO2 is in the liquid state and at least <NUM>,<NUM>, optionally at least <NUM>,<NUM>, if the CO2 is in the supercritical state.

The cross section of the pipe system is in one simple case circular with a given diameter. Alternatively, the cross section can be shaped as an ellipse, a curved free form, or a polygon or even a combination thereof. The cross section can be uniform or nonuniform along the whole length of the pipe, although, typically, it will be uniform. The pipe can be straight or curved, for example having one or more bends. For example, the pipe is made of metal, such as stainless steel or nickel alloys, or a polymer/metal combination. Optionally, it has a uniform circular cross section with an inner diameter in the range of <NUM> to <NUM> and a length of at least <NUM>.

To enable the pressurizing of the CO2, a compressor or pump is connected to the first end of the pipe by fittings. Typically, the system is configured for recycling the CO2 after flushing of the pipe. The system comprises the following elements:.

According to an embodiment of the invention, the flushing system further includes a system of sampling filters placed after the return point of the CO2 and is configured for cleaning the CO2 from impurities and for check of the cleanliness by a particle counting method.

This invention will be described in relation to the drawings, where:.

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

With time, the hydraulic fluid, for example oil, in the hydraulic pipe <NUM> 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.

As the hydraulic pipe <NUM> 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 <NUM> when filling CO2 into the pipe is additionally cleaning the walls inside the pipe.

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.

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

<FIG> is a diagram showing the gradual cleanliness of the pipe in terms of a National Aerospace standard (NAS <NUM>), which is an international standard used for defining cleanliness and the definitions of which is shown in <FIG>.

<FIG> is a diagram Reynolds number during filling of the pipe with scCO2. It is seen that the Reynolds numbers are above <NUM>, which indicates turbulent flushing with scCO2.

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=<NUM> why LCO2 may be used instead. Experimentally, useful results have also been obtained with LCO2.

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.

Claim 1:
A method of removing matter from a lumen of a hydraulic control line (<NUM>) 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 (<NUM>) at a first end of the hydraulic control line (<NUM>);
- 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 (<NUM>) 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 either inside the matter or on the other side of the matter when the matter is a plug clogging the hydraulic control line or both;
- then, after the time t, depressurizing the hydraulic control line (<NUM>) 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 (<NUM>) and to press the matter out of the hydraulic control line (<NUM>) through the first end of the hydraulic control line (<NUM>) by the expanding gas.