WO2013002644A1

WO2013002644A1 - Subsea compression assembly

Info

Publication number: WO2013002644A1
Application number: PCT/NO2012/050120
Authority: WO
Inventors: Kjell Olav Stinessen; Bianca Maria Mita NOGUEIRA
Original assignee: Aker Subsea As
Priority date: 2011-06-30
Filing date: 2012-06-27
Publication date: 2013-01-03
Also published as: NO20110946A1; AU2012276386B2; MY164751A; NO335450B1; BR112013033635A2; BR112013033635A8; AU2012276386A1

Abstract

Subsea compression assembly (100) adapted to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid from a subsea hydrocarbon well. The assembly (100) has a compressor (115) and a convection cooler (105) for cooling the fluid upstream of the compressor. The cooler (105) is a single flow channel cooler, cooling the fluid within the single flow channel by heat convection from the fluid to sea water through walls of a flow channel pipe (201) within which the single flow channel is arranged. It has a jacket pipe (207) arranged about the flow channel pipe, and a pump (215) adapted to pump sea water through an annulus (209) between the flow channel pipe (201) and the jacket pipe (207) from an annulus inlet (211) to an annulus outlet (213), along the axial extension of the flow channel pipe (201).

Description

Subsea compression assembly

The present invention relates to subsea compression of hydrocarbon-containing fluids. In particular, the invention relates to a subsea compression assembly adapted to compress a hydrocarbon-containing fluid flowing through the assembly. Such fluid will normally flow from a subsea well.

Background

When a fluid flow is conducted in a flowline subsea, a temperature difference between the surrounding seawater and the fluid within the pipe will result in heat transfer between the two through the pipe walls. Produced fluids from a subsea well, the wellstream, will typically be significantly warmer than the ambient seawater and will thus be cooled as it flows through the pipes. In some cases the operator employs a subsea processing plant into which the fluid is routed. In such cases, he may need an additional cooling of the fluid before it enters the processing plant. For this purpose it is known to arrange a subsea cooler upstream of the processing plant.

Such a subsea cooler is illustrated in patent application publication

US20100252227. This publication describes a subsea cooler having a number of coils which guides the flowing fluid which is to be cooled. Furthermore, the coils are arranged within a duct which is open in two opposite ends. At one of the ends the duct is provided with a propeller arranged to move seawater through the duct and thereby cool the coils and the fluid within them. Another subsea cooler is shown in international patent application publication WO201 1008101 . This cooler exhibits an inlet which is branched into a plurality of pipes of smaller cross section, and an outlet where the branches are collected.

When cooling a hydrocarbon-containing wellstream, hydrates may form within the flow path, and may narrow or even block the flow path. Hydrate formation will typically occur at about 25 °C and below. In order to avoid or reduce hydrate formation, it is common to add hydrate inhibitors to the flow, such as mono ethylene glycol (MEG), di-ethylene glycol (DEG), tri-ethylene glycol (TEG), and methanol. Due to the density differences between the liquid and gas phase of the inhibited wellstream, the inhibitor distribution within a cooler may in some cases become uneven. This is also the case if the liquid is inhibitor only. Such unevenness of distribution may lead to hydrate formation of parts of the cooler. This is typically the case for a cooler where the fluid flow is branched off into a plurality of parallel flow paths. Some branches will receive more than a sufficient amount of inhibitor, whereas other branches will receive an amount which is too small.

The object of the present invention is to provide a subsea compression assembly that comprises a novel subsea cooler that exhibits advantages with respect of known coolers. In particular it is an object that the novel subsea cooler is suitable for cooling inhibited hydrocarbon-containing fluids and that the pressure drop over the cooler is sufficiently small. The invention

According to a first aspect of the present invention, there is provided a subsea compression assembly adapted to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid produced from a subsea hydrocarbon well. The subsea compression assembly comprises a compressor as well as a convection cooler which is adapted to cool the fluid upstream of the compressor. According to the invention,

- the convection cooler is a single flow channel cooler, adapted to cool the fluid within the single flow channel by means of heat convection from the fluid to surrounding sea water through walls of a flow channel pipe within which the single flow channel is arranged. Moreover,

- the convection cooler further comprises a jacket pipe which is arranged about the flow channel pipe, and a pump which is adapted to pump sea water through an annulus between the flow channel pipe and the jacket pipe from an annulus inlet to an annulus outlet, along the axial extension of the flow channel pipe.

With the term single flow channel cooler is meant a cooler in which the flow path from its inlet to its outlet is not branched off into a plurality of flow paths, such as by arranging a plurality of parallel pipe. Thus, when entering the cooler at the cooler inlet there will exist only one flow path leading to the cooler outlet.

With the term "inhibited hydrocarbon-containing fluid" is meant a fluid to which an hydrate inhibitor is added. Such an inhibitor may for instance be in the form of mono ethylene glycol (MEG), di-ethylene glycol (DEG), tri-ethylene glycol (TEG), or other inhibitors which will be known by the person skilled in the art. Such inhibitors will prevent or reduce the formation of hydrates in the flow path of the hydrocarbon-containing fluid.

The flow channel pipe conducts the hydrocarbon-containing fluid which is to be cooled. The flow channel pipe may be constituted by one or more pipes assembled into one. Preferably, the sea water which is pumped through the annulus, along the axial extension of the flow channel pipe, will flow in the opposite direction than the hydrocarbon-containing fluid (counterflow). Furthermore, the flow channel pipe and the jacket pipe are preferably coaxially arranged. In one embodiment of the invention, the convection cooler occupies a horizontal area, wherein the flow channel pipe of the convection cooler is arranged with bends in such way that the flow path through the flow channel pipe of the convection cooler is at least fivefold the largest distance measured across said horizontal area. In this embodiment the cooler is shaped into a compact shape and is thus adapted for installation which requires little space. Various configurations of the cooler for such an embodiment will be discussed.

The flow channel pipe of the convection cooler can comprise a portion shaped into a spiral shape. This facilitates a particularly compact design of the cooler, as will be shown in the more detailed example description below.

The flow channel pipe of the convection cooler can also comprise a portion shaped into a serpentine shape or a spring shape. In some embodiments, the flow channel pipe can exhibit a plurality of portions shaped into a spiral shape or serpentine shape. Each of the shapes can extend in substantially one respective plane and the shapes can be arranged in a stack formation in such way that said respective planes are parallel with respect to each other.

The average inner diameter of the flow channel pipe can advantageously be within a region of 80 to 1 10 % of the average inner diameter of a flowline which guides 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 a somewhat smaller inner diameter than the upstream flowline. With a smaller inner diameter, one may reduce the size of the cooler, as well as ensure mist flow through the cooler, thereby ensuring an even distribution of liquid across the pipe cross section. In this way one may also obtain less pressure fluctuations over the cooler.

The convection cooler comprises a cooler flow inlet upstream of a cooler flow outlet, between which the annulus is arranged. From a position in a flowline upstream of the cooler flow inlet, which flowline connects to the cooler flow inlet, to the position of the cooler flow outlet, the flow path of the hydrocarbon- containing fluid exhibits a smallest curve radius which is larger than twice the inner diameter of the flow line at the position of the cooler flow inlet.

According to a second aspect of the invention, there is provided a subsea compression assembly adapted to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid produced from a subsea hydrocarbon well. The subsea compression assembly comprises a compressor as well as a convection cooler which is adapted to cool the fluid upstream of the compressor. According to the second aspect of the invention, the convection cooler is a single flow channel cooler, adapted to cool the fluid within the single flow channel by means of heat convection from the fluid to surrounding sea water. Furthermore, the convection cooler also comprises an inner pipe which is arranged within the flow channel pipe, and a pump which is adapted to pump sea water through the inner pipe, such that the fluid is guided in an annulus between the inner pipe and the flow channel pipe from an annulus inlet to an annulus outlet, along the axial extension of the flow channel pipe.

With an assembly according to the second aspect of the invention, the

hydrocarbon containing fluid is guided in an annulus and is thus cooled by the sea water-carrying inner pipe within the annulus as well as by the surrounding ambient sea water outside the flow channel pipe.

Example of embodiment

While 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 subsea compression assembly according to 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 coiled configuration of the inlet cooler;

Fig. 7 is a top view of another coiled configuration of the inlet cooler;

Fig. 8 is a side view of a coiled 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 subsea compression assembly 100 according to 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 flowline (not shown) or it is extending to a subsea well (not shown) or a manifold for several wells. The flowline carries hydrocarbon-containing fluids, containing both gas and liquid. The fluid entering the inlet end 101 of the assembly 100 can for instance have a temperature of 80 °C, if the assembly 100 is located close to a well template.

Between the inlet end 101 and outlet end 103, the assembly 100 further comprises an inlet cooler 105. Downstream of the inlet cooler 105 is a gas-liquid separator 107, out of which a liquid line 109 and a gas line 11 1 extend. A liquid line pump 1 13 is arranged to pump the liquid from the separator 107 towards the outlet end 103. Furthermore, the gas line 1 1 1 leads to a compressor 1 15 adapted to compress the gas leaving the separator 107. Downstream of the compressor 115 is a compressor discharge valve 1 17 and a discharge cooler 1 19. The discharge cooler 1 19 is needed in case the discharge temperature of the compressor 115 is higher than what the downstream flowline can tolerate.

From the downstream side of the inlet cooler 105 to the downstream side of the compressor 115 there is arranged an anti-surge line 121 . In the anti-surge line 121 there is arranged an anti-surge cooler 123 and an anti-surge valve 125. The anti-surge line 121 , with the associated cooler and valve, will protect the compressor 115 from surges. In some cases it will be favourable to combine the inlet cooler 105 and the ant- surge cooler 123 into one combined inlet and anti-surge cooler 105. That is, the anti-surge line 121 , which in that case would have no cooler, would be connected to the inlet upstream the inlet cooler 105. When hydrocarbon-containing fluids flowing in the flowline at say 35 °C and above, the inlet cooler 105 is justified as it will reduce the inlet gas volume entering the compressor 1 15 and thereby reduce the required compression power. Furthermore, employing the inlet cooler 105 will also provide a reduction of the compressor discharge temperature. This is favourable for the materials in the components downstream of the compressor 1 15. It may even eliminate the need for the discharge cooler 1 19. Of these reasons, the inlet cooler 105 is considered to be necessary when fluid in the flowline has a temperature of about 50 °C or 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 subsea compression assembly 100 shown in Fig. 1 is a typical example of a subsea compression assembly according to the present invention. It should be mentioned that the assembly shown in Fig. 1 is not a complete process flow diagram. However it shows the main components necessary for a subsea compression assembly. While the outlines of the subsea compression assembly 100 are described above, the inlet cooler 105, which is a subsea forced convection cooler, will now be described in more detail.

As mentioned above the inlet cooler 105 can also include the function of the antisurge cooler 123 and hence be a combined inlet and anti-surge cooler 105. In such an embodiment the anti-surge cooler 123 could be removed.

Fig. 2 shows a schematic illustration of the inlet cooler 105, which is a part of the subsea compression assembly 100 shown in Fig. 1 . The inlet cooler 105 is a forced convection cooler having a flow channel pipe 201 . The hydrocarbon- containing flow originating from the (not shown) subsea wells enters 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.

About the flow channel pipe 201 there is arranged a surrounding jacket pipe 207. The inner diameter of the jacket pipe 207 is larger than the outer diameter of the flow channel pipe 201 . Thus, between the two pipes there exists an annulus 209. Furthermore, at one end of the jacket pipe 207 there is arranged an annulus inlet 21 1 and at the opposite end of the jacket pipe 207 there is arranged an annulus outlet 213. A cooler pump 215 is arranged in association with the annulus inlet 21 1 to pump ambient seawater through the annulus 209 along the exterior face of the flow channel pipe 201 , and out of the annulus outlet 213. Preferably, the seawater in the annulus 209 will be pumped in the opposite direction

(counterflow) of the hydrocarbon-containing fluid flowing within the flow channel pipe 201 . The pump 215 can either be arranged at the annulus inlet 21 1 such that the 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 such that the seawater is moved through the annulus by the suction pressure of the pump.

The embodiment described with reference to Fig. 2 represents the simplest way of arranging the inlet cooler 105 upstream of the compressor 115 (and upstream of the separator 107). Enveloping the flowline along a distance typically in the range of 500 meters to a few kilometres can be sufficient to obtain the required cooling (the enveloped flowline then becomes 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, may reduce the required cooling area to about 30 % of what the necessary area would be with free convection cooling. Thus, arranging the jacket pipe 207 about the flow channel pipe 201 significantly reduces the required dimensions and weight, and can thus reduce cost. Compared to free convection cooling, i.e. having ambient seawater in direct contact with the walls of the flowline (or a flow channel pipe) without pumping, the forced cooling shows its main advantage at low temperature differences between the hydrocarbon-containing fluid and the ambient seawater. Such low

temperature differences can be at about 50 °C to 5 °C above seawater temperature. For such low temperature differences, free convection coolers would be unsatisfactory large.

In one embodiment, if there is a sufficient length of flowline upstream of the compressor, a part of the flowline may be employed as the inlet cooler 105 described above. In such an embodiment, a desired distance of the existing flowline (referred to as flow channel pipe 201 in Fig. 2) may be covered with a jacket pipe 207 and used for forced convection. That is, the desired length of the flowline will be surrounded by an annulus 209 within the jacket 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 flowline already installed. Of course, the flowline and jacket pipe 207 can be installed simultaneously when the possible need for cooling is known. However, in many cases arranging the jacket pipe 207 around an existing flowline is difficult or not feasible. If the flowline can be designed, fabricated and installed as a one-pipe cooler already at the time of installation of the flowline, this represents a favourable option. If the option of enveloping a flowline is not feasible, the inlet cooler 105 can be implemented with coiling, bending and/or spiral shapes, as will be described herein.

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

In some instances, there may not be a sufficient distance between subsea wells and the compressor 1 15 to employ the embodiment described above with reference to Fig. 2. That is, a distance between the subsea well and the compressor may be too short in order to achieve sufficient cooling of the hydrocarbon-containing fluid with the embodiment shown in Fig. 2. In other circumstances it may involve expensive challenges to arrange the jacket pipe in this manner on an already installed flowline. This could 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 pipe 201 and the surrounding jacket pipe 207 in a spiral configuration. The cooler pump 215 is not shown in Fig. 3, but is, nevertheless, arranged in order to pump seawater through the annulus 209 between the flow channel pipe 201 and the jacket pipe 207, such as shown in Fig. 2. Although schematically illustrating a separate embodiment including an elongated inlet cooler 105, Fig. 2 can also be appreciated as a principle sketch 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 pipe 201 which leads to a centre portion of the spiral shape. In this central portion the flow channel pipe 201 has an inclined section 217 with respect to a plane in which the spiral shape is formed. The inclined section 217 constitutes a transition between the straight portion and the start of the spiralling spreading of the flow channel pipe 201 . At the outer part of the spiral shape the cooler flow outlet 205 is arranged and connects to the downstream part of the subsea compressor assembly 100 (cf. Fig. 1 ). Along a distance of the straight portion 216 of the flow channel pipe 201 and almost to the shown cooler flow outlet 205, the jacket pipe 207 is arranged coaxially about the flow channel pipe 201 . Preferably there is some space between the turns of the jacket pipe 207 so that ambient water may flow freely between each turn. In this way, the seawater which is pumped through the annulus 209 and hence being heated by the warmer well stream within the flow channel pipe 201 is to some extent cooled by the ambient seawater outside the jacket pipe 207.

Of course, the direction of flow of hydrocarbon-containing fluid within the flow channel pipe 201 may be opposite of what described above. That is, the cooler flow inlet 203 and cooler flow outlet 205 may switch places. This is also possible for the flow of seawater within the annulus 209.

In a not-shown embodiment, a plurality of spiral forms as the one described above with reference to Fig. 2 is arranged in series. In such an embodiment, the spiral forms may be arranged such that the planes defined by each respective spiral are substantially parallel with respect to each other. The spiral forms can have a common centre axis along which they are arranged. Moreover they can have some axial distance between each spiral form in such a way that ambient seawater may flow in between each spiral form. Fig. 4 shows another configuration of the inlet cooler 105. In this embodiment the flow channel pipe 201 exhibits a spring shape. With such a configuration one may ensure a constant curvature along the curved part of the flow channel pipe 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 for pumping seawater through the annulus 209.

Fig. 5 shows yet another embodiment of an inlet cooler 105, having the same basic components as the embodiment described with reference to Fig. 2, however with another configuration of the flow channel pipe 201 . In the embodiment shown in Fig. 5 the flow channel pipe 201 exhibits a serpentine shaped configuration. Corresponding to the embodiment shown with reference to Fig. 3, the serpentine-shape shown in Fig. 5 can also be arranged in one plane. In addition it may comprise a plurality of serpentine-shaped portions which are connected in series and stacked. Fig. 5 illustrates only the jacket pipe 207 of the inlet cooler 105. However, the flow channel pipe 201 is arranged within. The serpentine shape can be manufactured of a plurality of straight pipe pieces 219 and a plurality of 180° bends 221 .

Fig. 6 and Fig. 7 illustrate coiled embodiments. These figures show a top view of a coiled configuration of two different embodiments. In Fig. 6, each loop or coil comprises two straight pieces 219 of pipe (jacket pipe 207 shown enveloping a not shown flow channel pipe 201 ) connected into a coil by two 180° bends 221 , 223. Although not visible from the drawing of Fig. 6, one bend 221 connects the two shown straight pipe pieces 219, whereas the other bend 223 connects one straight pipe piece 219 to a straight pipe piece of another layer or another coil. That is, multiple layers of coils are stacked on top of each other. The same principle is shown in Fig. 7. However, the embodiment shown in Fig. 7 exhibits four straight pipe pieces 219 per coil and 90° bends 225, making the coiled shape into a rectangle or a square. The embodiments shown in Fig. 6 and Fig. 7 can be construed as alternatives to the spring shape shown in Fig. 4, wherein the spring shape comprises straight pipe pieces. Fig. 8 shows the embodiments of Fig. 6 and Fig. 7 in a side view. The transition pipe pieces (bends 223) between each layer are however not arranged at the edges (180° or 90° bends) of the coils. Instead, inclined transition pipe pieces are arranged as shown. Fig. 8 may also be construed 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 circle shapes connected with the inclined transition pipe pieces.

Fig. 9 shows how the coiled shape of Fig. 7 can be inwards spiralled and the stacked up layers with downwards pipe spools that connects the layers. Fig. 9 schematically illustrates three layers. The number of layers can however be selected according to need for cooling from case to case. The way of inwards spiralling and connecting layers shown in Fig. 9 is also relevant for the configuration illustrated in Fig. 6. The way connecting layers is also relevant for the shapes of Fig. 3 and 5. In Fig. 9 the downwards spool pieces that connects layers is shown to bend 90° form the horizontal plane, but it can however be more smoothly 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 pipes that are spiralled in planes are connected in layers or a stack which results in a compact cooler.

Fig. 9 illustrates three spirally shaped sections 300, 400, 500 of an inlet cooler 105 according to the invention. Each section is shown in a top view and a side view. Starting from the right hand side of Fig. 9, the first section (or layer) 300 has an inlet 301 at the periphery of the spiral shape. With a combination of straight pipe pieces and said 90° bends, the spiral shape ends at a central portion of the first section 300. 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 a perpendicular direction with respect to the plane of the first section 300. The transition piece 305 connects to the inlet 401 of the second layer 400. The inlet 401 of the second layer 400 is thus arranged at the central portion of the spiral shape of the second layer 400. In the second section 400 the flow direction is spiralling outwards towards the peripherally arranged outlet 403 of the second section. The outlet 403 of the second section 400 connects to a transition piece 405 which provides communication 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 pipe 201 having an inner diameter D. According to this embodiment, the flow channel pipe 201 is provided with curves, such as the curve of a spiral shape or the curve of a serpentine shape, having a curving radius of not less than twice the diameter D. Preferably the curving radius is bigger than three times the diameter D. This feature makes pigging operations more feasible (PIG - pipeline inspection gauge). This feature can also exist for the interface between the inlet cooler 105 and the upstream and downstream components, such as the flowline and the line between the inlet cooler 105 and the separator 107 (Fig. 1 ). An advantage of the subsea compression assembly 100 having an inlet cooler 105 according to the present invention is that the flow channel pipe 201 can be manufactured by standard flowline pieces. Since the flow channel pipe 201 exhibits dimensions in the same order as the flowline upstream of the inlet cooler 105, typically diameters in the range of 8" to 24", the inlet cooler can be fabricated by workshops or yards that fabricate the flowline. It is suitable for many offshore yards which are specialists in welding, inspecting, and testing of pipes of such dimensions. The material of the flow channel pipe 201 , which in fact is a flowline part which in some embodiments are coiled up at a limited area, can be the same as for the upstream flowline, typically carbon steel. Corrosion protection of the seawater exposed outer surface can be arranged in a well known way by sacrifice anodes and with surface coating. The cooler can also be made in stainless steel, i.e. 6Mo, duplex or super-duplex with or without surface coating.

Whereas the pressure within the flowline and the connected flow channel pipe 201 can be significantly, for instance 200 bar or more, the required pressure within the annulus 209 is rather low, typically less than 10 bar above the seawater pressure. It needs only be sufficient to move the pumped seawater through the annulus 209, from the annulus inlet 21 1 to the annulus outlet 213. A possible leakage through the jacket pipe 207 would of course not be an environmental concern. Furthermore, only a significant leakage would reduce the cooling performance of the inlet cooler 105 to a substantial extent. Thus, material and assembly of the jacket pipe 207 may be chosen correspondingly. One may for instance use a polymeric material. On the other hand, if one desires to achieve a cooling effect on the seawater within the annulus from the ambient seawater outside the jacket pipe 207, one should choose a heat-conductive material, such as a metal.

The material of the jacket pipe can also be selected of a metal or alloy that acts as sacrifice anodes for the cooler pipe (flow channel pipe) or sacrifice anodes may be attached to the inner side of the jacket pipe.

In one embodiment, the inner diameter of the flow channel pipe 201 is reduced along the flow path of the hydrocarbon-containing fluid. This is in order to ensure a turbulent flow regime within the flow channel pipe 201 which is favourably to achieve high heat transfer from the wellstream to the inner pipe wall. A turbulent flow regime, e.g. mist flow, will also ensure an even distribution of the inhibitor and contribute to preventing hydrate formation within the flow channel pipe 201 . Additional advantages are that the turbulent flow also counteracts depositions, e.g. wax and particles, and fouling and scaling.

In one embodiment, the pump 215 of the inlet cooler 105 can be automatically controlled on basis of measured outlet temperature of the hydrocarbon- containing fluid exiting the inlet cooler 105.

Case illustration

In the following, a case is described with realistic parameters for an inlet cooler 105 suitable for employment with the subsea compressor assembly 100 according to the present invention. Table 1 below describes the existing parameters as well as the desired output parameters of the inlet cooler 105 of this case example.

Table 1 Total Duty 8300 kW

Hot side Cold side

Fluid Gas Seawater

Pressure bar 21 60

Inlet Outlet Inlet Outlet

Temperature °C

90 20 10 20

Actual flow m³/h 6242 5507 735 735

Pressure drop bar 2.79 2.24

OHTC 741 W/m² K

The task is to cool gas from 90 °C down to 20 °C. The gas flow is 2700000 Sm³/day, which at inlet conditions (90 °C and 21 bar) results in an actual inlet flow of 6242 m³/h. The resulting overall heat transfer coefficient (OHTC) is 741 W/m² and calculated required cooler area is 363 m³ with no surface coating. In this calculation is not taken into consideration that the wellstream of a gas field normally includes a small fraction of liquids, say 0.1 to 3 weight-%. The calculation is however still accurate enough to give a representative picture of the cooler performance. Needed cooler area is 363 m².

For comparison, OHTC for a passive cooler is 262 W/m²K and required cooler area is 1027 m². In this case example, a serpentine shape (cf. Fig. 5 described above) is chosen for the inlet cooler 105. Table 2 below shows the resulting dimensions

characteristics of the cooler 105. The resulting inlet cooler 105 is rather compact with regards to its performance. Table 2

Parameter/Component Dimension or other characteristic

Diameter of flow channel pipe (primary 10"

pipe), inches

Diameter of jacket pipe (secondary pipe), 14 "

inches

Footprint of a layer, m x m 4.9 x 2.8

Bend radius = 2 pipe diameters, m 0.508

Cooling area per layer, m² 16.51

Height of cooler, m 7.8 Number of layers 14

Resulting total cooler area, m² 363

The resulting inlet cooler 105 in this example case will have 14 layers of serpentine shapes, stacked vertically on top of each other. The needed 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 carried in an annulus between an inner pipe and the walls of the flow channel pipe 201 '. That is, compared to the embodiment described with reference to Fig. 2, the pumped sea water and the hydrocarbon- containing fluid have changed places. With such an embodiment, the larger pipe, i.e. the flow channel pipe 201 ' must be dimensioned to the pressures within the hydrocarbon-containing fluid. The flow channel pipe 201 ' must also be large enough to contain the sea water-carrying inner pipe 207'. Furthermore, the inner pipe 207' needs to constitute a pressure barrier between the hydrocarbon- containing fluid and the pressure of the cooling sea water in the inner pipe 207'.

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

Claims

1 . Subsea compression assembly (100) adapted to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid produced from a subsea hydrocarbon well, wherein the subsea compression assembly (100) comprises a compressor (1 15) as well as a convection cooler (105) which is adapted to cool the fluid upstream of the compressor, characterized in

- that the convection cooler (105) is a single flow channel cooler, adapted to cool the fluid within the single flow channel by means of heat convection from the fluid to surrounding sea water through walls of a flow channel pipe (201 ) within which the single flow channel is arranged; and

- that the convection cooler (105) further comprises a jacket pipe (207) which is arranged about the flow channel pipe (201 ), and a pump (215) which is adapted to pump sea water through an annulus (209) between the flow channel pipe (201 ) and the jacket pipe (207) from an annulus inlet (21 1 ) to an annulus outlet

(213), along the axial extension of the flow channel pipe (201 ).

2. Subsea compression assembly according to claim 1 , characterized in that the convection cooler (105) occupies a horizontal area, wherein the flow channel pipe (201 ) of the convection cooler (105) is arranged with bends in such way that the flow path through the flow channel pipe (201 ) of the convection cooler is at least fivefold the largest distance measured across said horizontal area.

3. Subsea compression assembly according to one of the preceding claims,

characterized in that the flow channel pipe of the convection cooler comprises a portion shaped into a spiral shape.

4. Subsea compression assembly according to one of the preceding claims,

characterized in that the flow channel pipe of the convection cooler comprises a portion shaped into a serpentine shape.

5. Subsea compression assembly according to one of the preceding claims,

characterized in that the flow channel pipe of the convection cooler comprises a portion shaped into a spring shape.

6. Subsea compression assembly according to one of the claims 3 and 4,

characterized in that the flow channel pipe exhibits a plurality of portions shaped into the spiral shape or serpentine shape, wherein each of the shapes extend in substantially one respective plane and wherein the shapes are arranged in a stack formation in such way that said respective planes are parallel with respect to each other.

7. Subsea compression assembly according to one of the preceding claims,

characterized in that the average inner diameter of the flow channel pipe (201 ), is within a region of 80 to 1 10 % of the average inner diameter of a flowline which guides the fluid from a remote location, such as a subsea well, to the subsea compression assembly.

8. Subsea compression assembly according to any one of the preceding 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 flowline upstream of the cooler flow inlet (203), which flowline connects 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 smallest curve radius which is larger than twice the inner diameter of the flow line at the position of the cooler flow inlet (203).

9. Subsea compression assembly (100) adapted to compress a fluid flowing through the assembly, said fluid being an inhibited hydrocarbon-containing fluid produced from a subsea hydrocarbon well, wherein the subsea compression assembly (100) comprises a compressor (1 15) as well as a convection cooler (105) which is adapted to cool the fluid upstream of the compressor, characterized in

- that the convection cooler (105) is a single flow channel cooler, adapted to cool the fluid within the single flow channel by means of heat convection from the fluid to surrounding sea water through; and

- that the convection cooler (105') further comprises an inner pipe (207') which is arranged within the flow channel pipe (201 '), and a pump (215') which is adapted to pump sea water through the inner pipe (207'), such that the fluid is guided in an annulus (209') between the inner pipe (207') and the flow channel pipe (201 ') from an annulus inlet (21 1 ') to an annulus outlet (213'), along the axial extension of the flow channel pipe (201 ).