NATURAL GAS IN A GAS PIPE

Technical Field

The present invention relates to an arrangement for removing liquid, especially hydrocarbons, but also water, from a natural gas flow.

When processing crude oil from a reservoir, the oil, gas and water are separated in several steps. After each separation step, the gas phase is processed further, often in repeated cooler, scrubber, and compressor steps to remove residual liquids and pressurize the gas for export. Scrubbers and compressors are often designed to handle a given load of liquid in the gas, 13 litres of liquid per million Sm ³ of gas being a generally accepted guideline as given by the NORSOK standard. Despite this relatively low specified amount of liquid, it has been experienced that scrubbers, compressors and other downstream equipment fail because of high amounts of liquid in the feed. This results in poor performance or downtime for the equipment and unnecessary loss of income for the operators.

It has been found that the amount of liquid transported in the gas pipes in practice is significantly higher than the specified 13 litres of liquid per million Sm ³ of gas. It has also been found that the major part of liquid in most processes are transported along the pipe walls. Liquid droplets will, due to the natural flow pattern in a gas pipe, be forced onto the walls of the pipe where the droplets coalesce with the liquid film on the wall and are then transported along the pipe wall by the gas flow.

The fact that the main part of the liquid transported in gas pipes is transported along the pipe walls makes it possible to transport significant amounts of liquid in gas pipes. This changes the working conditions for gas processing equipment, such as scrubbers, compressors, pumps and filters.

When the liquid film reaches the inlet of, e.g. a scrubber, this liquid film will break up and the liquid will be dispersed into the gas and hence increase the liquid load of the scrubber.

The present invention aims to remove a large portion of this liquid film that flows along the pipeline walls.

Background Art The arrangement of the present invention has, on the face of it, similarities with liquid removing arrangements for removing liquid water from steam before the steam enters a steam turbine. Such arrangements are known from GB 633607, filed in 1948 and CA 1239881 , filed in 1984. However, the principles of these prior art arrangements have not found their way into natural gas treatment.

GB 633607 shows an arrangement where a water trapping annulus is created by a tubular insert in the steam pipe. The annulus will trap water that is flowing along the walls of the pipeline, which will then be led first to a sump and then to a trap chamber.

Since the trap annulus also will trap a fair amount of steam, a sophisticated system of baffles and a conductor tube is arranged upstream of the trapping annulus to create a pressure that is lower than the pressure in the trap annulus, so that steam will flow backwards on the outside of the conductor tube and enter the steam flow through openings in the conductor tube upstream of the annulus trap.

The result is a complicated structure that may work well in a small steam plant, but in a huge gas recovery plant the structure would be susceptible to fouling and deposits, cavitation and damage. The arrangement would have to be cleaned frequently and it would not be possible to do this by pigging or similar methods. It would have to be disassembled. Hence, the down time could be just as much as the down time today.

GB 633607 is depending on a valve to control the water discharge from the trap body. If this valve is not set properly or if it fails, the water trap will not function properly, either resulting steam escaping through the water trap or the trapping of water ceasing.

CA 1239881 shows a somewhat newer arrangement for trapping water from steam. In a first embodiment, the trap is arranged in a T-junction of a steam pipe and comprises a 90° bend that is arranged in the transition between the incoming steam pipe and one of the arms of the T-junction. An annulus is created between the incoming steam pipe and the bend, so that the majority of the steam flows through the bend and the water near the walls of the steam pipe flows into the annulus. The water falls down to the bottom of the T-junction. The steam flowing through the bend flows over the water that has collected.

In another embodiment the incoming steam pipe has been closed off by an impingement plate. A sideways opening leads the steam sideways into one arm of the T-junction.

This arrangement is depending on being arranged in a T-junction of the steam pipe. As for GB 633607 it is susceptible to fouling and deposits, cavitation and damage. The downtime for maintenance would likely be as long as the current downtime.

WO 2009/097869 describes a downhole centrifugal separator, which sets the well flow, which is mainly gas, into rotation. Liquid droplets are flung against the wall of the production tubing. In one place, there is a section of inclined slits, through which the liquid is drained. The liquid is then conducted back to the formation, i.e., a water reservoir below the production zone.

A major problem with this device is that when the well flow is set into rotation the liquid will also be broken up into smaller droplets that are very hard to separate from the gas. The flow velocity must therefore be high in order to obtain enough energy to fling these small droplets against the wall of the production tubing.

US 2002/194988 describes an apparatus for removing one component, such as liquid from a stream of fluid. The stream is induced to flow at supersonic velocity and brought to swirl.

Also here, the liquid will be broken into smaller droplets, which requires a lot of energy, i.e., high velocity, to remove.

Summary of invention The present invention aims to solve the problem of removing liquid flowing along the walls of a pipeline without having the drawbacks of the above described prior art.

The present invention is based on maintaining a stratified flow as far as possible in order to maintain the liquid united, i.e., in larger blobs or droplets. These larger volumes of liquid can be removed using substantially less energy and hence at a much smaller velocity than if the well flow is set into rotation, or a swirl or other disturbance is created. To achieve this, the invention has as an object to influence as little as possible on the flow before the liquid has been separated from the gas. This is achieved by an arrangement for removing liquid from a flow of natural gas in a gas pipe, comprising a straight pipe section with a uniform cross- section, said pipe section being perforated along a substantial part of the circumference of the pipe section wall, said perforations being in communication with a chamber arranged outside the inner wall of said pipe section, said chamber collecting liquid that is flowing along the inside wall of the gas pipe and out through said perforations; said chamber being fluidly connected to a liquid tube; said liquid tube in turn being fluidly connected to a processing unit or receiving pipe section that is capable of receiving liquid; and said liquid tube having a liquid lock.

The collected liquid will be conducted to an appropriate place for further handling.

A convenient arrangement is achieved by said perforations being in the form of a plurality of openings arranged in at least one circle along the wall of the pipe section. These openings will capture the majority of the liquid flowing along the pipe wall.

Alternatively, the perforations are in the form of at least one continuous slit arranged in a circle along the wall of the pipe section. This will also ensure that a majority of the liquid is captured.

If the perforations are flush with the inner wall of said pipe section, they will not disturb the gas flow to any significant degree.

By arranging a liquid stopper adjacent the perforations to hold back the liquid flowing along the inside wall about the perforations, will be ensured that no significant amount of liquid can pass over the perforations.

Preferably, the processing unit or receiving pipe section has a lower pressure than the gas pipe. This will ensure that the liquid is transported to the

appropriate place for further handling.

Further preferably, said liquid lock is in the form of a U-shaped tube section. This will ensure that gas will not be transported with the liquid to any significant degree.

By adjusting the liquid level in the liquid lock by positioning the inlet to the liquid tube relative to the outlet from the liquid tube, a self-regulating device is achieved.

In one embodiment, said gas pipe is an inlet pipe to a scrubber.

In one embodiment, the process unit is a scrubber.

In another embodiment, said gas pipe is an outlet gas pipe from a scrubber.

In a further embodiment, said receiving pipe section is a liquid outlet pipe from a scrubber.

In another embodiment, said gas pipe is an outlet gas pipe from a production separator.

In another embodiment, said gas pipe is an outlet gas pipe from a gas-liquid separation unit.

The invention provides a non-invasive arrangement that will not have any significant negative influence on the gas flow pattern or gas flow direction. The gas will flow undisturbed past the arrangement without any change in direction. Hence, the risks of fouling, deposits, and cavitation are insignificant.

The arrangement can be installed in any straight section of the gas pipe, vertically or horizontally, both on topside and subsea installations and downstream any process equipment experiencing liquid carryover to the gas phase and thereby to the gas pipe.

The invention provides a simple and robust arrangement that is only marginally susceptible to damage, e.g., by high velocity droplets or particles in the gas flow. If the perforations should become clogged, the liquid collection will of course be reduced or stop, but this will have no influence on the gas

transportation. This means that the gas can be processed while maintenance is prepared.

The arrangement is easy to install. It only requires the insertion of a relatively small size spool piece at a convenient place somewhere in the gas pipe, and the hook up of a liquid tube to a process unit that can handle the collected liquid.

The arrangement requires no regulation. It can be made without any moving parts, where the design, such as size of perforations, collection chamber, liquid pipe and liquid lock as well as the positioning of the process equipment to receive the liquid relative to the lock will define the functional parameters, such as capacity.

Brief description of drawings

The invention will now be described in detail, referring to a preferred

embodiment, where:

Figure 1 shows a scrubber for separation of gas and liquid with an arrangement according to the invention placed immediately before the scrubber,

Figure 2 shows the arrangement according to the invention incorporated in a gas pipe,

Figure 3 shows a detail of the arrangement of the invention in cross-section, and.

Figure 4 shows schematically the arrangement according to the invention incorporated in a sub-sea installation.

Detailed description of the invention Figure 1 shows a scrubber 1 of a per se known type. A gas inflow pipe 2 is connected to the scrubber 1 approximately at the middle of the height of the scrubber. At the same height a plurality of vanes 3, are arranged to slow down the flow of the gas. At an upper part of the scrubber is arranged a mist mesh 4, on which small droplets of liquid can collect and fall back downwards in the scrubber 1 , as indicated by the arrow 5. At the top of the scrubber 1 , is a gas outlet 6. At the bottom of the scrubber 1 , is a liquid outlet 7. Liquid will collect in the lower part of the scrubber 1 , as indicated by a sump 8.

A liquid collection arrangement according to the invention is incorporated in the gas inflow pipe 2 and denoted by reference number 9. From the collection arrangement, a liquid tube 10 carries liquid to the sump 8 in the scrubber 1 . The liquid tube has a liquid lock 1 1 , formed by arranging the tube 10 in a u-shape. The significance of this liquid lock 1 1 will be explained later. Instead of the tube 10 conducting the liquid to the sump 8, it may also be connected to the liquid pipe (not shown) that is connected to the liquid outlet 7. The liquid tube 10 with the liquid lock 1 1 may, in fact, according to the invention, be connected to any suitable vessel or piping upstream or downstream the arrangement.

Figure 2 shows the liquid collection arrangement of the invention in detail. It preferably comprises a short pipe section or spool piece 12 that has been mounted between two parts of a gas pipe 2. This can be done by flange connections or in other suitable ways. Preferably, the spool piece 12 should be easily removable for maintenance and cleaning.

The spool piece has a plurality of openings 13 through the wall of the spool piece. The openings are preferably arranged in a circle along the perimeter of the spool piece 12, but may also be arranged in a plurality of circles. The openings in different circles may be staggered from the openings in adjacent circles.

Alternatively, one or more slits extending along the whole or a substantial part of the perimeter of the spool piece may replace the openings.

On the outside of the spool piece 12 is mounted a chamber unit 14. The chamber unit covers the openings 13 or the at least one slit, so that the inside of the spool piece is in communication with a chamber 15 (see figure 3) in the chamber unit 14. The chamber unit fits tightly against the outside of the spool piece so that no liquid or gas can escape to the outside of the chamber 15. The chamber 15 is preferably a continuous chamber extending around the whole circumference of the spool piece 12, but it may also be divided into two or more sections.

If the perforations is in the form of a continuous slit, the chamber unit 14 may form a structural bridge between the parts of the pipe section12 on either side of the slit.

The chamber 15 is in communication with the liquid tube 10, which, as explained above, has a liquid lock 1 1 , such as a U-shaped section. If the chamber 15 is divided into sections, there may be one liquid tube for each chamber section. These tubes may be connected to a common manifold before the scrubber. The tube should be at or close to the lowers point of the chamber 15 or chamber section.

The function of the liquid collection arrangement of the invention is as follows:

Liquid that adheres to the pipe wall when gas is transported to the scrubber 1 will flow along the pipe wall driven by the flow of gas. When the liquid that is transported along the wall reached the openings 13, or slit, the liquid will escape through the openings 13 and reach the chamber 15. The liquid inside the chamber will run towards the lowest point and enter the tube 10. The scrubber 1 has a slightly lower pressure than the gas pipe 2, so that the liquid will be transported to the scrubber 1 . The liquid lock 1 1 will prevent gas from being blown into the scrubber 1 via the tube 10. The size, i.e. height, of the liquid lock 1 1 is adapted to the pressure difference between the gas pipe 2 and the scrubber 1. The diameter of the tube 10 is also adapted to the pressure difference and the amount of liquid expected. The pipe diameter 10 and the height of the liquid lock 1 1 is chosen so that the lock 1 1 on the one hand will always be filled with liquid, but on the other hand the pressure difference is able to draw excess liquid, i.e. liquid that is not needed to fill the lock, into the scrubber. A drain valve (not shown) may conveniently be arranged in the lock 1 1 , preferable at the lowest point of the U-bend. Through this valve, the lock may be drained, and the valve may be used to flush the tube 10 to remove clogging of the tube 10.

The tube 10 may also be connected to other types of equipment than a scrubber, such as a pipe or another separator vessel.

The openings may be circular, oval, square, rectangular, or any other convenient shape. The openings 13, or slits, are preferably flush with the inside wall of the gas pipe 2, so that there are no obstacles inside the gas pipe.

However, there may also be arranged a liquid stopper 16 downstream of the openings 13 or slit. The liquid stopper 16 may be in the form of a ring with an L- shaped cross-section, where one of the legs of the L-shape is attached to the pipe wall and the other leg extends upstream. Thereby, liquid will be prevented from flowing over the openings or slit, but held back so that it will have to escape through the openings or slit. The liquid stopper preferably has a height from the pipe wall corresponding to about 5% of the diameter of the pipe 2. The height will typically be in the range 7-17 mm depending on pipe diameter. This small height will not influence significantly on the gas flow.

The arrangement according to the invention can be installed on both topside and sub-sea installations. Figure 4 shows schematically the arrangement as a part of a sub-sea installation. The well fluids are pumped from a well 17 up to a manifold 18 situated at the seabed. The oil, water and gas mixture is

transported in a subsea pipe 19 to a gas-liquid separation unit 20 where the free gas is separated from the liquids. The gas is separated and transported in a gas pipe 21 and the liquids are separated and transported in a liquid pipe 22. In this example the liquid collecting arrangement according to the invention, denoted by reference number 23, is arranged at the gas pipe 21 . A liquid return tube 24, or liquid receiving pipe section, with a liquid lock (not shown) is connected to the liquid pipe 22. The gas-liquid separation unit can be any gas-liquid separation device available in the marked or new to the marked based on gravity, cyclonic principles, pipe arrangements etc.