NO335450B1 - Seabed compression device - Google Patents

Description

Seabed compression assembly

The present invention relates to seabed compression of hydrocarbon-containing fluids. In particular, the invention relates to a seabed compression assembly which is designed to compress a hydrocarbon-containing fluid that flows through the assembly. Such a fluid will normally flow from a seabed well.

Background

When a fluid is carried in a flow pipe on the seabed, a temperature difference between the surrounding seawater and the fluid inside the pipe will result in heat transfer between the two through the pipe walls. Produced fluids from a seabed well, the well stream, will typically be significantly warmer than the surrounding seawater and will thus be cooled as it flows through the pipes.

In some cases, the operator uses a seabed processing facility into which the fluid is fed. In such cases, he may need to further cool the fluid before it enters the processing plant. For this purpose, it is known to arrange a seabed cooler upstream in relation to the processing plant.

Such a seabed cooler is illustrated in patent application publication US201100252227. This publication describes a seabed cooler that has a number of windings that carry the flowing fluid to be cooled. Furthermore, the windings are arranged inside a channel which is open at two opposite ends. At one of the ends, the channel is provided with a propeller which is arranged to move seawater through the channel and thus cool the windings and the fluid inside them.

Another seabed cooler is shown in international patent publication WO2011008101. This cooler presents an inlet which is branched into a plurality of tubes of smaller cross-section, and an outlet where the branches are brought together.

Patent application US20080245098 describes a loop for a cooling medium for use in an air conditioner, a refrigerator, a display cabinet or the like. This publication is an example of how, for appliances placed in ambient air, such as in houses, a flow path for the refrigerant must be provided if the refrigerant is something other than the ambient air.

American patent publication US6434972 likewise describes an apparatus, more specifically an air conditioning apparatus (air conditioner), in which a cooling medium other than air is arranged in a provided flow path. The device has an internal heat exchanger with a multi-channel tube that is coiled into a spiral shape.

Patent application US2009020288 describes a cooling pipe which is arranged to form hydrates, then to remove the hydrates mechanically. The solution indicates a pipe-in-pipe solution which is adapted to the formation of a hydrate coating inside a pipe, after which the hydrate coating is removed using a "pig". The solution thus includes a "pig launching system" and a "pig receiving system".

When cooling a hydrocarbon-bearing well stream, hydrates can form within the flow path and can constrict or even block the flow path. Hydrate formation will typically occur at approximately 25 °C and below. To avoid or reduce hydrate formation, it is known to add hydrate inhibitors to the flow, such as monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG) and methanol. Due to the difference in density between the liquid and gas phases of the inhibited well flow, the inhibitor distribution inside a cooler can in some cases become uneven. This is also the case if only liquid is the inhibitor. Such uneven distribution can lead to hydrate formation in parts of the cooler. This is typically the case for a cooler where the fluid flow is branched into a plurality of parallel flow paths. Some branches will receive more than a sufficient amount of inhibitor, while other branches will receive an amount that is too small.

The purpose of the present invention is to provide a compression assembly which comprises a new seabed cooler which exhibits advantages compared to known coolers. It is a particular object of the invention that the new seabed cooler is suitable for cooling inhibited hydrocarbon-containing fluids and that the pressure drop across the cooler is sufficiently small.

The invention

In accordance with a first aspect of the present invention, there is provided a seabed compression assembly which is adapted to compress a fluid flowing through the assembly, whereby said fluid is an inhibited hydrocarbon-containing fluid produced from a seabed hydrocarbon well. The subsea compression assembly comprises a compressor and a convection cooler which is arranged to cool the fluid upstream of the compressor. In accordance with the invention - the convection cooler is a single-flow channel cooler, designed to cool the fluid inside the single-flow channel by means of heat convection from the fluid to surrounding seawater through the walls of a flow channel tube inside which the single-flow channel is arranged. Furthermore, - the convection cooler further comprises a casing pipe which is arranged around the flow channel pipe, and a pump which is arranged to pump seawater through an annulus between the flow channel pipe and the casing pipe from an annulus inlet to an annulus outlet, along the axial extent of the flow channel pipe.

By the term single-flow channel cooler is meant a cooler in which the flow path from its inlet to its outlet is not branched into a plurality of flow paths, such as by arranging a plurality of parallel pipes. Consequently, upon entering the cooler at the cooler inlet, there will be only one flow path leading to the cooler outlet.

The term "inhibited hydrocarbon-containing fluid" refers to a fluid to which a hydrate inhibitor has been added. Such an inhibitor can, for example, be in the form of monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG) or other inhibitors that will be known to a person skilled in the field. Such inhibitors will prevent or reduce the formation of hydrates in the flow path of the hydrocarbon containing fluid.

The flow channel pipe carries the hydrocarbon-containing fluid to be cooled. The flow channel pipe can be made up of one or more pipes which are combined into one.

The seawater that is pumped through the annulus, along the axial extent of the flow channel pipe, will preferably flow in the opposite direction of the hydrocarbon-containing fluid (countercurrent). Furthermore, the flow channel tube and the casing tube are preferably coaxially arranged.

In one embodiment of the invention, the convection cooler occupies a horizontal area, whereby the flow channel pipe of the convection cooler is arranged with bends in such a way that the flow path through the flow channel pipe of the convection cooler is at least five times the largest distance measured across the horizontal area. In this version, the cooler is shaped into a compact shape and is thus designed for installation that requires little space. Different configurations of the cooler for such an embodiment will be described.

The flow channel tube of the convection cooler may comprise a portion formed into a spiral shape. This facilitates a particularly compact design of the cooler, which will be shown in the more detailed example description below.

The flow channel of the convection cooler may also comprise a portion shaped into a serpentine shape or a spring shape.

In some embodiments, the flow channel tube may exhibit a plurality of portions shaped into a helical or serpentine shape. Each of the forms can extend in essentially one respective plane and the forms can be arranged in a stack structure in such a way that said respective planes are parallel in relation to each other.

The average internal diameter of the flow channel pipe can advantageously be within a range of 80 to 110% of the average internal diameter of a flow pipe carrying the fluid from a remote location, such as a subsea well, to the subsea compression assembly.

The convection cooler can advantageously have the same or somewhat smaller internal diameter than the upstream flow pipe. With a narrower inner diameter, the size of the cooler can be reduced, as well as ensuring mist flow through the cooler, and thus ensuring an even distribution of liquid across the pipe's cross-section. In this way, lower pressure changes above the cooler can also be achieved.

The convection cooler comprises a cooler flow inlet upstream of a cooler flow outlet, between which an annulus is arranged. From a position in a flow pipe upstream of the cooler flow inlet, which flow pipe is connected to the cooler flow inlet, to the position of the cooler flow outlet, the flow path of the hydrocarbon-containing fluid exhibits a minimum radius of curvature greater than twice the internal diameter of the flow pipe at the position to the cooler flow inlet.

In accordance with a second aspect of the invention, there is provided a seabed compression assembly which is adapted to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid which is produced from a seabed hydrocarbon well. The seabed compression assembly comprises a compressor and a convection cooler which is designed to cool the fluid upstream of the compressor. In accordance with the second aspect of the invention, the convection cooler is a single-pass channel cooler which is designed to cool the fluid inside the single-pass channel by means of heat convection from the fluid to the surrounding seawater. Furthermore, the convection cooler also comprises an inner tube which is arranged inside the flow channel tube, and a pump which is arranged to pump seawater through the inner tube, so that the fluid is carried in an annulus between the inner tube and the flow channel tube from an annulus inlet to an annulus outlet, along the axial extent of the flow channel tube.

With an assembly in accordance with the second aspect of the invention, the hydrocarbon-containing fluid is carried in an annulus and is thus cooled by the seawater-carrying inner tube within the annulus as well as by the surrounding surrounding seawater on the outside of the flow channel tube.

Example of embodiment

As the invention has been described above in general terms, a more detailed, non-limiting example of embodiment will be described in the following with reference to the drawings, in which

Fig. 1 is a schematic illustration of a seabed compression assembly

in accordance with the present invention;

Fig. 2 is a schematic illustration of an inlet cooler which is part of

the compression assembly illustrated in Fig. 1;

Fig. 3 is a perspective view of a spiral configuration of the inlet cooler; Fig. 4 is a perspective view of a spring configuration of the inlet cooler; Fig. 5 is a top view of a serpentine configuration of the inlet cooler; Fig. 6 is a top view of a bent configuration of the inlet cooler; Fig. 7 is a top view of another bent configuration of the inlet cooler;

Fig. 8 is a side view of a bent configuration of the inlet cooler; and

Fig. 9 is a principle schematic diagram of a stacked spiral configuration of

the inlet cooler.

Fig. 1 shows a schematic illustration of a seabed compression assembly 100 in accordance with the present invention. The assembly 100 has an inlet end 101 and an outlet end 103. The inlet end 101 is in fluid communication with a flow pipe (not shown) or it extends to a subsea well (not shown) or a manifold for several wells. The flow pipe carries hydrocarbon-containing fluids containing both gas and liquid. The fluid entering the inlet end 101 of the assembly 100 can, for example, have a temperature of 80 °C, if the assembly 100 is close to a well template (eng: well template).

Between the inlet end 101 and the outlet end 103 the assembly comprises 100

further an inlet cooler 105. Downstream of the inlet cooler 105 there is a gas-liquid separator 107, from which a liquid pipe 109 and a gas pipe 111 extend. A liquid pipe pump 113 is arranged to pump the liquid from the separator 107 towards the outlet end 103. Furthermore, the gas pipe 111 leads to a compressor 115 which is arranged to compress the gas leaving the separator 107. Down-

upstream of the compressor 115 is a compressor outlet valve 117 and an outlet cooler 119. The outlet cooler 119 is needed if the outlet temperature of the compressor 115 is higher than what the downstream flow pipe can tolerate.

From the downstream side of the inlet cooler 105 to the downstream side of the compressor 115, a pump boundary pipe (eng: anti-surge line) 121 is arranged. 1 pump boundary pipe 121, a pump boundary cooler 123 and a pump boundary valve 125 are arranged. The pump boundary pipe 121, with the associated cooler and valve, will protect the compressor 115 against pressure fluctuations.

In some cases, it would be advantageous to combine the inlet cooler 105 and the pump boundary cooler 123 in one combined inlet and pump boundary cooler 105. That is, the pump boundary pipe 121, which in that case would have no cooler, would be connected to the inlet upstream of the inlet cooler 105.

When hydrocarbon-containing fluids flow in the flow pipe at, say, 35°C and above, the inlet cooler 105 is justified in that it will reduce the inlet gas volume entering the compressor 115 and thus reduce the required compression energy. Furthermore, using the inlet cooler 105 will also provide a reduction in compressor outlet temperature. This is beneficial to the materials in the components downstream of the compressor 115. It may even eliminate the need for the outlet cooler 119. For these reasons, the inlet cooler 105 is considered necessary when the fluid in the flow pipe has a temperature of about 50°C and above. If the subsea compression assembly 100 is arranged close to the subsea wells from which the hydrocarbon-containing fluid flows, its temperature may be 80 °C or even more.

The seabed compression assembly 100 shown in Fig. 1 is a typical example of a seabed compression assembly in accordance with the present invention. It should be mentioned that the assembly shown in Fig. 1 is not a complete process flow diagram. However, it does show the main components required for a seabed compression assembly. As the main features of the seabed compression assembly 100 have been described above, the inlet cooler 105, which is a seabed forced convection cooler, will now be described in more detail.

As mentioned above, the inlet cooler 105 can also include the function of the pump boundary cooler 123 and thus be a combined inlet and pump boundary cooler 105. In such an embodiment, the pump boundary cooler 123 can be removed.

Fig. 2 shows a schematic illustration of the inlet cooler 105, which is part of the seabed compression assembly 100 shown in Fig. 1. The inlet cooler 105 is a forced convection cooler having a flow channel tube 201. The hydrocarbon-containing flow coming from the (not shown) seabed the wells enter the inlet cooler 105 at a cooler flow inlet 203, at one end of the flow channel pipe 201. At an opposite end of the flow channel pipe 201, the hydrocarbon-containing flow exits the inlet cooler 105 at a cooler flow outlet 205.

A casing pipe 207 is arranged around the flow channel pipe 201. The inner diameter of the casing pipe 207 is larger than the outer diameter of the flow channel pipe 201. Consequently, there is an annular space 209 between the two pipes. Furthermore, at one end of the casing pipe 207 an annular space inlet 211 is arranged and at the opposite end of the casing pipe 207 an annular space outlet 213 is arranged. A cooler pump 215 is arranged adjacent to the annular space inlet 211 to pump ambient seawater through the annular space 209 along the outer the surface of the flow channel pipe 201, and out of the annulus outlet 213. The seawater in the annulus 209 will preferably be pumped in the opposite direction (countercurrent) of the hydrocarbon-containing fluid flowing inside the flow channel pipe 201.

The pump 215 can either be arranged at the annulus inlet 211 so that seawater is moved through the annulus by the discharge pressure of the pump, or the pump can be arranged at the outlet 213 of the annulus so that the seawater is moved through the annulus by the pump's suction pressure.

The embodiment described with reference to Fig. 2 represents the simplest way to arrange the inlet cooler 105 upstream of the compressor 115 (and upstream of the separator 107). Enclosing the flow pipe along a distance typically in the range of 500 meters to a few kilometers may be sufficient to achieve the necessary cooling (the enclosed flow pipe is then the flow channel pipe 201 illustrated in Fig. 2).

As a rule of thumb, forced cooling, such as in the embodiment described above with reference to Fig. 2, can reduce the required cooling area to approximately 30% of the area that would be required with free convection cooling. Arranging the casing pipe 207 around the flow channel pipe 201 consequently reduces the required dimensions and weight considerably and can thus reduce cost.

Compared to free convection cooling, i.e. having ambient seawater in direct contact with the walls of the flow tube (or a flow channel tube) without pumping, forced cooling exhibits its main advantage in low temperature differences between the hydrocarbon-containing fluid and the surrounding seawater. Such low temperature differences can be at approximately 50 °C to 5 °C above the seawater temperature. For such low temperature differences, free convection coolers would be unsatisfactorily large.

In one embodiment, if there is a sufficient length of the flow pipe upstream of the compressor, a part of the flow pipe can be used as the inlet cooler 105 described above. In such an embodiment, a desired length of the existing flow pipe (referred to as flow channel pipe 201 in Fig. 2) can be covered with a jacket pipe 207 and be used for forced convection. That is, the desired length of the flow pipe will be surrounded by an annulus 209 inside the casing pipe 207, through which cooling seawater will be pumped. In this way, additional cooling can be achieved for a well stream flowing in an existing flow pipe that has already been installed. The flow pipe and casing pipe 207 can of course be installed at the same time when the eventual need for cooling is known.

In many cases, however, arranging the casing pipe 207 around an existing flow pipe is difficult or not feasible. If the flow pipe can be designed, made and installed as a one-piece cooler already at the time of installation of the flow pipe, this represents an advantageous possibility. If the possibility of enclosing the flow pipe is not feasible, the inlet cooler 105 can be implemented with windings, bends and/or spiral shapes, which will be described herein.

It should be noted that the inlet cooler 105 described herein does not exhibit any header/manifold. The existing flow pipe can be part of the inlet cooler 105 or an additional flow pipe can be installed, for example directly to an existing flow pipe.

In some cases, it may be that there is insufficient distance between seabed wells and the compressor 115 to use the embodiment described above with reference to Fig. 2. That is, a distance between the seabed well and the compressor may be too short to achieve sufficient cooling of the hydrocarbon-containing fluid with the embodiment shown in Fig. 2. In other cases, it may entail expensive challenges to arrange the casing pipe in this way on an already existing flow pipe. This may be due to seabed conditions.

As an alternative to the embodiment described above with reference to Fig. 2, another embodiment will now be described with reference to Fig. 3. Fig. 3 shows the flow channel tube 201 and the surrounding casing tube 207 in a spiral configuration.

The coolant pump 215 is not shown in Fig. 3, but is nevertheless arranged to pump seawater through the annulus 209 between the flow channel tube 201 and the casing tube 207, as shown in Fig. 2. Although it schematically illustrates a separate embodiment comprising an elongated inlet cooler 105, Fig. 2 can also be understood as a schematic diagram of the components included in the other described embodiments. The cooler flow inlet 203 is arranged at the end of a straight portion 216 of the flow channel tube 201 leading to a center portion of the spiral shape. In this central part, the flow channel tube 201 has a section 217 which is inclined relative to the plane in which the spiral is formed. The inclined section 217 constitutes a transition between the straight part and the start of the spiral spread of the flow channel tube 201. At the outer part of the spiral shape, the cooler flow outlet 205 is arranged and connected to the downstream part of the subsea compressor assembly 100 (see Fig. 1). Along a distance of the straight portion 216 to the flow channel pipe 201 and almost to the cooler flow outlet 205 shown, the casing pipe 207 is arranged coaxially about the flow channel pipe 201. There is preferably some space between the windings of the casing pipe 207 so that the surrounding seawater can flow freely between each winding. In this way, the seawater that is pumped through the annulus 209 and is thus heated by the warmer well flow in the flow channel pipe 201 is cooled to a certain extent by the surrounding seawater on the outside of the casing pipe 207.

The direction of the flow of hydrocarbon-containing fluid inside the flow channel tube 201 can of course be opposite to that described above. That is, the cooler flow inlet 203 and the cooler flow outlet 205 can switch places. This is also possible for the flow of seawater inside the annulus 209.

In an embodiment not shown, a plurality of spiral forms such as that described above with reference to Fig. 2 are arranged in series. In such an embodiment, the spiral shapes can be arranged so that the planes defined by each respective spiral are essentially parallel in relation to each other. The spiral shapes can have a common central axis along which they are arranged. Furthermore, they can have some axial distance between each spiral shape in such a way that ambient seawater can flow between each spiral shape. Fig. 4 shows another configuration of the inlet cooler 105. In this embodiment, the flow channel tube 201 exhibits a spring shape. With such a configuration, one can ensure a constant curvature along the bent part of the flow channel tube 201. In contrast, with the spiral shape shown with reference to Fig. 3, the curvature is constantly decreasing or increasing. A cooler pump 215 (not shown in

Fig. 4) is arranged to pump seawater through the annulus 209.

Fig. 5 shows yet another embodiment of an inlet cooler 105 which has the same basic components as the embodiment described with reference to Fig. 2, however with a different configuration of the flow channel tube 201. In the embodiment shown in Fig. 5, the flow channel tube 201 exhibits a serpentine-shaped configuration. Corresponding to the embodiment shown with reference to Fig. 3, the serpentine form shown in Fig. 5 can also be arranged in one plane. In addition, it may comprise a plurality of serpentine-shaped parts which are connected in series and stacked. Fig. 5 only illustrates the casing pipe 207 of the inlet cooler 105. However, the flow channel pipe 201 is arranged inside. The serpentine form can be made from a plurality of straight pipe parts 219 and a plurality 180 of bends 221. Fig. 6 and Fig. 7 illustrate coiled embodiments. These figures show a top view of a wound configuration of two different embodiments. In Fig. 6, each loop or winding comprises two straight sections 219 of pipe (cap pipe 207 shown enclosing a flow channel pipe 201 not shown) connected to a winding of two 180° bends 221, 223. Although it cannot be seen in the drawing in Fig. 6, one bend 221 connects the two shown straight pipe pieces 219, while the other bend 223 connects one straight pipe piece 219 to a straight pipe piece of another layer or winding. That is, several layers of windings are stacked on top of each other. The same principle is shown in Fig. 7. However, the embodiment shown in Fig. 7 presents four straight pipe pieces 219 per winding and 90° bends 225, so that the wound shape is made into a rectangle or a square. The embodiments shown in Fig. 6 and Fig. 7 can be understood as alternatives to the spring form shown in Fig. 4, whereby the spring form comprises straight pipe pieces. Fig. 8 shows the embodiments of Fig. 6 and Fig. 7 in a side view. However, the transition pipe pieces (bends 223) between each layer are not provided at the edges (180° or 90° bends) of the windings. Instead, inclined transition pipe pieces are arranged as shown. Fig. 8 can also be understood as a side view of layers having a circular shape, connected by a transition pipe piece. This would be somewhat similar to the spring shape shown in Fig. 4, however with circular shapes connected to the inclined transition pipe pieces. Fig. 9 shows how the wound form in Fig. 7 can be inwardly spiral and the stacked layers with downwardly directed tube coils connecting the layers. Fig. 9 illustrates three layers schematically. However, the number of layers can be chosen depending on the need for cooling from case to case. The method of inward spiraling and connecting the layers shown in Fig. 9 is also relevant for the configuration shown in Fig. 6. The method of connecting layers is also relevant for the shapes in Fig. 3 and 5.1 Fig. 9 is the downward-directed pipe pieces which interconnects the layers shown bent at 90° from the horizontal plane, however, it may be more evenly curved to reduce pressure drop. Preferably, the pressure drop of the inlet cooler 105 should be less than 3 bar. The main point of Fig. 9 is to demonstrate how layers of tubes that are helical in plan are interconnected in layers or a stack resulting in a compact cooler. Fig. 9 illustrates three helical sections 300, 400, 500 of an inlet cooler in accordance with the invention. Each section is shown in a top view and a side view. From the right side of Fig. 9, the first section (or layer) 300 has an inlet 301 at the periphery of the helix. With a combination of straight pipe pieces and said 90° bends, the spiral shape ends at a central part of the first section 300.1 in this central position, the outlet 303 of the first layer 300 is arranged. The outlet 303 is connected to a first transition piece 305 which extends, in this embodiment, in an orthogonal direction with respect to the plane of the first section 300. The transition piece 305 is connected to the inlet 401 of the second layer 400. The inlet of the second layer 400 is accordingly arranged at the central part of the spiral shape of the second layer 400.1 the second section 400, the direction of flow is spiral outwards towards the circumferentially arranged outlet 403 of the second section. The outlet 403 of the second section 400 is connected to a transition piece 405 which provides connection to the inlet 501 of the third section 500.

In one embodiment of the present invention, the inlet cooler 105 is provided with a flow channel 201 having an inner diameter D. In accordance with this embodiment, the flow channel pipe 201 is provided with bends, such as the bend of a spiral shape or the bend of a serpentine shape, which has a bending radius which is not less than twice the diameter D. Preferably, the bending radius is greater than three times the diameter D. This feature makes pigging operations (eng: pigging operations) more feasible (PIG - pipeline inspection gauge). This feature may also exist for the interface between the inlet cooler 105 and the upstream and downstream components, such as the flow pipe and the pipe between the inlet cooler 105 and the separator 107 (Fig. 1).

An advantage of the subsea compression assembly 100 having an inlet cooler 105 in accordance with the present invention is that the flow channel pipe 201 can be made from standard flow pipe pieces. Since the flow channel pipe 201 exhibits dimensions in the same order of magnitude as the flow pipe upstream of the inlet cooler 105, typically diameters in the range of 8" to 24", the inlet cooler can be made by workshops or yards that make the flow pipe. It is suitable for many offshore yards that specialize in welding, inspection and testing of pipes of such dimensions. The material of the flow channel tube 201, which is actually a flow tube section that in some embodiments is coiled up in a reduced area, may be the same as that of the upstream flow line, typically carbon steel. Corrosion protection against the seawater that is exposed on the outer surfaces can be arranged in a known manner by means of sacrificial anodes and with surface coatings. The cooler can also be made in stainless steel, for example 6Mo, duplex or super-duplex with or without surface coating.

While the pressure inside the flow tube and the associated flow channel tube 201 can be significant, for example 200 bar or more, the required pressure in the annulus 209 is rather low, typically less than 10 bar above seawater pressure. It only needs to be sufficient to move the pumped seawater through the annulus 209, from the annulus inlet 211 to the annulus outlet 213. A possible leak through the casing pipe 207 would of course not be an environmental concern. Furthermore, only a significant leak would significantly reduce the cooling capacity of the inlet cooler. Accordingly, the material and assembly of the casing tube 207 can be selected accordingly. You can, for example, use a polymer material. On the other hand, if you want to achieve a cooling effect on the seawater inside the annulus from the surrounding seawater outside the casing pipe 027, you should choose a heat-conducting material, such as metal.

The casing tube material can also be chosen from a metal or alloy that acts as sacrificial anodes for the cooler tube (flow channel tube) or sacrificial anodes can be attached to the inner side of the casing tube.

In one embodiment, the inner diameter of the flow channel tube 201 is reduced along the flow path of the hydrocarbon containing fluid. This is to ensure a turbulent flow regime inside the flow channel pipe 201 which is advantageous for achieving high heat transfer from the well flow to the inner pipe wall. A turbulent flow regime, for example mist flow, will also ensure an even distribution of the inhibitor and help prevent hydrate formation inside the flow channel tube 201. Further advantages are that the turbulent flow also counteracts deposits, for example wax and particles, and fouling and scaling.

In one embodiment, the pump 215 of the inlet cooler 105 may be automatically controlled based on measured outlet temperatures of the hydrocarbon-containing fluid flowing from the inlet cooler 105 .

Illustration of embodiment

In the following, an embodiment is described with realistic parameters for an inlet cooler 105 which is suitable for use with the seabed compressor assembly 100 in accordance with the present invention. Table 1 below describes the existing parameters as well as the desired output parameters of the inlet cooler 105 of this example embodiment.

The task is to cool gas from 90 °C down to 20 °C. The gas flow is 2700000 Sm<3>/day, which at the inlet conditions (90 °C and 21 bar) results in an actual inlet flow of 6242 m<3>/h. The resulting overall heat transfer coefficient (OHTC) is 741 W/m<2>and the calculated cooler area is 363 m<3>without any surface coating. In this calculation, no account has been taken of the fact that the well stream from a gas field usually comprises a small fraction of liquids, let's say 0.1 to 3% by weight. However, the calculation is still accurate enough to give a representative picture of the cooler performance. The required cooler area is 363 m<2.>

For comparison, the OHTC for a passive cooler is 262 W/m<2>K and the required cooler area is 1027 m<2>.

In this embodiment, a serpentine shape (see Fig. 5 described above) is chosen for the inlet cooler 105. Table 2 below shows the resulting dimensions that are characteristic of the cooler 105. The resulting inlet cooler 105 is relatively compact in relation to its performance.

The resulting inlet cooler 105 in this example embodiment will have 14 layers of serpentine forms, stacked vertically on top of each other. The required area for the cooler is 4.9 x 2.8 meters and its height is 7.8 meters.

Alternative embodiment

In a not shown embodiment of the second aspect of the invention, the hydrocarbon-containing fluid is conducted in an annular space between an inner tube and the walls of the flow channel tube 201'. That is, compared to the embodiment described with reference to Fig. 2, the pumped seawater and the hydrocarbon-containing fluid have changed places. With such an embodiment, the largest pipe, that is to say the flow channel pipe 201', must be sized for the pressures inside the hydrocarbon-containing fluid. The flow channel pipe 201' must also be large enough to contain the seawater-carrying inner pipe 207'. Furthermore, the inner tube 207 needs to form a pressure barrier between the hydrocarbon-containing fluid and the pressure of the cooling seawater in the inner tube 207'.

On the other hand, one will achieve cooling of the hydrocarbon-containing fluid from both radial sides of the annulus.

Claims (9)

1. Seabed compression assembly (100) arranged to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid produced from a seabed hydrocarbon well, whereby the seabed compression assembly (100) comprises a compressor (115) and a convection cooler ( 105) which is designed to cool the fluid upstream of the compressor, characterized in that - the convection cooler (105) is a single-flow channel cooler, designed to cool the fluid inside the single-flow channel by means of heat convection from the fluid to surrounding seawater through the walls of a flow channel tube (201 ) inside which the one-way channel is arranged; and - that the convection cooler (105) further comprises a casing pipe (207) which is arranged around the flow channel pipe (201), and a pump (215) which is arranged to pump seawater through an annulus (209) between the flow channel pipe (201) and the casing pipe ( 207) from an annulus inlet (211) to an annulus outlet (213), along the axial extent of the flow channel tube (201). 2. Seabed compression assembly in accordance with patent claim 1, characterized in that the convection cooler (105) occupies a horizontal area, whereby the flow channel pipe (201) of the convection cooler (105) is arranged with bends in such a way that the flow path through the flow channel pipe (201) of the convection - the cooler is at least five times the largest distance measured across said horizontal area. 3. Seabed compression assembly in accordance with one of the preceding patent claims, characterized in that the flow channel pipe of the convection cooler comprises a part shaped into a spiral shape. 4. Seabed compression assembly in accordance with one of the preceding patent claims, characterized in that the flow channel pipe of the convection cooler comprises a part shaped into a serpentine shape. 5. Seabed compression assembly in accordance with one of the preceding patent claims, characterized in that the flow channel pipe of the convection cooler comprises a part shaped into a spring shape. 6. Seabed compression assembly in accordance with one of patent claims 3 and 4, characterized in that the flow channel pipe exhibits a plurality of parts shaped into the spiral shape or the serpentine shape, whereby each of the shapes extends mainly in one respective plane and whereby the planes are arranged in a stack formation in such a way that said respective planes are parallel to each other. 7. Seabed compression assembly in accordance with one of the preceding patent claims, characterized in that the average internal diameter of the flow channel pipe (201) is within a range of 80 to 110% of the average internal diameter of a flow pipe carrying the fluid from a remote location, so as a subsea well, to the subsea compression assembly. 8. Seabed compression assembly in accordance with one of the preceding patent claims, characterized in that - the convection cooler (105) comprises a cooler flow inlet (203) upstream of a cooler flow outlet (205), between which the annulus (209) is arranged; - that from a position in a flow pipe upstream of the cooler flow inlet (203), which flow pipe is connected to the cooler flow inlet (203), to the position of the cooler flow outlet (205), the flow path of the hydrocarbon-containing fluid exhibits a minimum radius of curvature greater than twice the the inside diameter of the flow tube at the position of the cooler flow inlet (203). 9. Seabed compression assembly (100) arranged to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid produced from a seabed hydrocarbon well, whereby the seabed compression assembly (100) comprises a compressor (115) and a convection cooler (105) which is arranged to cool the fluid upstream of the compressor, characterized in that - the convection cooler (105) is a single-pass channel cooler, arranged to cool the fluid inside the single-pass channel by means of heat convection from the fluid to the surrounding seawater; and - that the convection cooler (105) further comprises an inner tube (207) which is arranged inside a flow channel tube (201), and a pump (215) which is arranged to pump seawater through the inner tube (207), so that the fluid is led in an annulus (209) between the inner tube (207) and the flow channel tube (201) from an annulus inlet (211) to an annulus outlet (213), along the axial extent of the flow channel tube (201).