Modularized Subsea Compressor Train and Method of Installation

Abstract

The invention relates to a method of installing a subsea system (1) comprising the steps of: —installing at least one first foundation structure (13′) on a seabed, wherein the first foundation structure (13′) comprises a connection interface (50′) connectable to a second foundation structure (13″), —installing a first compressor train on the foundation structure (13′), the first compressor train comprising at least a first compressor (8′), —connecting the first compressor train to at least one well flow line (2), —connecting a first compressed fluid line (9′) to an outlet (15′) of the first compressor (8′) and to a common outlet (16) for the compressed fluid in the subsea system (1), wherein the first compressed fluid line (9′) comprises a flow regulating device (24′), —connecting a first connection line (10′,12′) to the first compressed fluid line (9′) at a position upstream of the flow regulating device (24′) and/or to a line (2, 6′) at a position upstream of the first compressor (8), and wherein the first connection line (10′, 12′) is connectable to an additional compressor train positioned on the second foundation structure (13″), the first connection line (10′) comprising a flow regulation device (20′,22′), —connecting a second connection line (11′) to the first compressed fluid line (9′) at a position downstream of the flow regulation device (24′) and wherein the second connection line (11′) is connectable to the additional compressor train positioned on the second foundation structure (13″), the second connection line (11′) comprising a flow regulation device (21′). It is further described an associated a subsea system.

Claims

1. A method of installing a subsea system comprising the steps of: installing at least one first foundation structure on a seabed, wherein the first foundation structure comprises a connection interface configured to connect to a second foundation structure which is configured to at least partly support an additional compressor train on the seabed, and wherein the additional compressor train increases the subsea system capacity when needed; installing a first compressor train on the first foundation structure, the first compressor train comprising at least a first compressor; connecting the first compressor train to at least one well flow line; connecting a first compressed fluid line to an outlet of the first compressor and to a common outlet for the compressed fluid in the subsea system, wherein the first compressed fluid line comprises a flow regulating device; connecting a first connection line to the first compressed fluid line at a position upstream of the flow regulating device and/or to a line at a position upstream of the first compressor, wherein the first connection line is configured to connect to the additional compressor train supported at least partly on the second foundation structure, the first connection line comprising a flow regulation device; and connecting a second connection line to the first compressed fluid line at a position downstream of the flow regulation device, wherein the second connection line is configured to connect to the additional compressor train positioned at least partly on the second foundation structure, the additional compressor train thereby being connectable to the common outlet, and the second connection line comprising a flow regulation device.

2. The method according to claim 1, further comprising the step of: connecting a flow conditioning unit to the at least one well flow line upstream of the first compressor.

3. The method according to claim 1, further comprising the step of: connecting a first cooler upstream and/or downstream of the first compressor.

4. The method according to claim 1, further comprising the step of: starting up production using at least the first compressor train.

5. The method according to claim 4, further comprising the step of: determining a need for increased compression capacity.

6. The method according to claim 1, further comprising the steps of: installing the second foundation structure for supporting the additional compressor train on the seabed; connecting the second foundation structure to the first foundation structure via the connection interface on the first foundation structure and a complementary connection interface on the second foundation structure, the first and second foundation structures forming a common foundation structure; installing the additional compressor train on the common foundation structure such that the additional compressor train is supported partly by the first and second foundation structures; and connecting the additional compressor train to the first compressor train via at least the first connection line and the second connection line to thereby connect the additional compressor train to the common outlet, wherein the additional compressor train comprises an additional compressor.

7. The method according to claim 6, wherein the method further comprises moving or skidding components forming part of the subsea system into position relative each other on the common foundation structure.

8. The method according to claim 1, further comprising installing a second compressor train on the first foundation structure.

9. The method according to claim 6, further comprising: connecting the connection interface and the complementary connection interface to form a pivot connection between the first and second foundation structures such that the second foundation structure can be pivoted relative the first foundation structure and, wherein the pivot connection supports the second foundation structure when positioned on the seabed.

10. A subsea system (1) comprising: a first foundation structure configured to be installed on a seabed, the first foundation structure comprising a connection interface configured to connect to a second foundation structure which is configured to at least partly support an additional compressor train on the seabed, wherein the additional compressor train increases the subsea system capacity when needed; a first compressor train supported by the first foundation structure the first compressor train comprising a first compressor with an inlet which is connectable to a well flow line; a compressed fluid line connected to an outlet of the first compressor and to a common outlet for compressed fluid in the subsea system, wherein the compressed fluid line comprises a flow regulation device; a first connection line connected to the compressed fluid line at a position upstream of the flow regulation device and/or to a line at a position upstream of the first compressor, wherein the first connection line is configured to connect to the additional compressor train supported at least partly on the second foundation structure, the first connection line comprising a flow regulation device; and a second connection line connected to the compressed fluid line at a position downstream of the flow regulation device, wherein the second connection line is configured to connect to the additional compressor train supported at least partly on the second foundation structure, the additional compressor train thereby being connectable to the common outlet, the second connection line comprising a flow regulation device.

11. The subsea system according to claim 10, wherein the first compressor train comprises a flow conditioning unit connected to the well flow line, and wherein the flow conditioning unit comprises at least a first outlet connectable to the inlet of the first compressor via at least one fluid line.

12. The subsea system according to claim 10, wherein the system comprises a first cooler positioned upstream and/or downstream of the first compressor.

13. The subsea system according to claim 10, further comprising: a second foundation structure configured to be installed on the seabed, the second foundation structure being connected to the first foundation structure via the connection interface and a complementary connection interface on the second foundation structure, the first and second foundation structures forming a common foundation structure positioned on the seabed; and an additional compressor train installed on the common foundation structure such that the additional compressor train is supported partly by the first and second foundation structures; wherein the additional compressor train comprises an additional compressor, wherein the additional compressor comprises an inlet and an outlet, and wherein the inlet is connected to the first connection line and the outlet is connected to a second compressed fluid line; and wherein the second compressed fluid line is connected to the common outlet via the second connection line.

14. The subsea system according to claim 13, wherein the additional compressor train comprises an additional cooler.

15. The subsea system according to claim 10, wherein the subsea system further comprises other necessary equipment for operating components of the subsea system, such as power equipment, wherein said equipment is located at the same foundation structure as the compressor train it shall operate.

16. The subsea system according to claim 13, wherein at least some of the components forming part of the subsea system are movable or skiddable on the common foundation structure.

17. The subsea system according to claim 13, wherein the connection interface and the complementary connection interface form a pivot connection between the first and second foundation structures such that the second foundation structure can be pivoted relative the first foundation structure, wherein the pivot connection supports the second foundation structure when positioned on the seabed.

18. The subsea system according to claim 11, wherein the flow conditioning unit is shared by the first compressor train and any additional compressor train(s).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0116] FIG. 1 shows an example of a prior art subsea system with a spool between components of the subsea system, and where the shape of the spool serves as a liquid trap;

[0117] FIGS. 2A-2F show a typical installation sequence according to the present invention;

[0118] FIGS. 2G-2J show different examples of modular components which may form part of the first foundation structure and the second foundation structure;

[0119] FIGS. 2K-2P show six different combinations of the modular components in FIGS. 2G-2J;

[0120] FIG. 3 shows an example of the components forming part of the first compressor train after the first compressor train has been installed;

[0121] FIG. 4 shows an exemplary setup of two compressor trains in series after the first and second compressor trains have been installed;

[0122] FIG. 5A shows an exemplary setup of two compressor trains in parallel after the first and second compressor train have been installed;

[0123] FIG. 5B shows an alternative exemplary setup compared to the solution in FIG. 5A of two compressor trains in parallel after the first and second compressor train have been installed;

[0124] FIG. 6 shows an exemplary setup of three compressor trains in parallel after the first, second and third compressor train have been installed;

[0125] FIG. 7 shows an exemplary embodiment where gas and liquid (water, oil and or condensate) are fed to the compressor trains in separate pipes based on the example setup in FIG. 5B;

[0126] FIG. 8A shows an example where a pump is connected to an outlet for heavier fluids from the flow conditioning unit for pumping liquids separated in the flow conditioning unit directly out through the common outlet;

[0127] FIG. 8B shows an example with one sole outlet from the flow conditioning unit, and where lighter and heavier fluids are mixed inside the flow conditioning unit;

[0128] FIG. 9A shows an exemplary installation sequence for the first foundation structure;

[0129] FIG. 9B shows an exemplary installation sequence for the second foundation structure to the first foundation structure;

[0130] FIG. 9C shows an example of the connection between two consecutive foundation structures;

[0131] FIG. 10A is a perspective view of a subsea system comprising a first foundation structure installed subsea with a first compressor train and a second compressor train installed thereon;

[0132] FIG. 10B is a perspective view of the subsea system of FIG. 10A, as well as a second foundation structure connected to the first foundation structure;

[0133] FIG. 10C is a perspective view of the subsea system of FIGS. 10A and 10B and a second foundation structure connected to the first foundation structure and where a third compressor train and a fourth compressor train are installed on the second foundation structure;

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF EMBODIMENT

[0134] In the following, embodiments of the invention will be discussed in more detail with reference to the appended drawings. It should be understood, however, that the drawings are not intended to limit the invention to the subject-matter depicted in the drawings. Furthermore, even though some of the features are described in relation to the subsea system only, it is apparent that they are valid for the related method as well, and vice versa. Hence, any features described in relation to the method are also valid for the subsea system.

[0135] Furthermore, in the following specific description, reference is made to a first connection line, a second connection line and a third connection line. The first connection line (10′,10″,10′″) is described in relation to series operation of compressor trains, while the second connection line (11′, 11″, 11′″) is the same return line both for series and parallel operation of the compressor trains, and the third connection line (12′,12″,12′″) is described in relation to parallel operation of the compressor trains.

[0136] It is clear that in simplest form, the subsea system only requires either: [0137] 1) in case of series operation of compressor trains: the first connection line (10′,10″,10′″) and the second connection line (11′, 11″, 11′″), or [0138] 2) in case of parallel operation of the compressor trains: the third connection line (12′,12″,12′″) and the second connection line (11′, 11″, 11′″), or [0139] 3) to enable the possibility of series and parallel operation at a later stage, both points 1) and 2) above.

[0140] FIG. 1 shows an example of a prior art subsea system with a spool C between the first and second subsea components A, B of the subsea system, and where the shape of the spool C serves as a liquid trap (liquid indicated by solid part of spool C). Such spools C, if resting on the seabed, will be the lowest points of the subsea flow systems and consequently serve as liquid traps because the liquid flows to the lowest points by means of gravity. Thus, produced liquid and condensed liquid (during stand-still) will collect in these spools, and during start-up after stand-still or before production start, this collected liquid will enter the compressor as one liquid plug with a risk of damaging the compressor due to too high liquid ratio.

[0141] In FIG. 2A installation of first compressor train in a subsea system 1 is complete. The first compressor train is mounted on a foundation structure 13′. The foundation structure 13′ is supported on four foundation elements 17′,17″,17′″,17″″ (the foundation elements for first foundation structure can be mudmat, piles, suction anchors or a combination of these). The disclosed subsea system 1 comprises a first compressor train comprising a flow conditioning unit (FCU) 3. Well flow enters the flow conditioning unit 3 through a well flow line 2 connected to required pipes 27′ for the first compressor train. Compressed well flow with a higher pressure than the pressure of the well flow in the well flow line 2 exits the first compressor train through a common outlet 16 for the compressed fluid in the subsea system. The subsea system 1 further comprises a first compressor 8′ connected to the flow conditioning unit 3, a first cooler 30′ connected to an outlet of the first compressor 8′, wherein the first cooler 30′ is connected to the common outlet 16 for the compressed fluid in the subsea system. Required power to drive the first compressor 8′ (and possibly other electrically driven components of the subsea system) are submitted via electric cable 26 connected to a transformer 23 (which is a passive electrical device that transfers electrical energy from one electrical circuit to one or more circuits) and a first variable speed drive 19′ (which is a device used to control the speed of the first compressor 8′).

[0142] On the right-hand side in FIG. 2A the components forming part of the second compressor train are shown with the components separated from each other. The disclosed components include a second foundation structure 13″, two additional foundation elements 18′, 18″ for supporting the second foundation structure 13″, a second compressor 8″, a second cooler 30″ and required pipes indicated by element 27″.

[0143] In FIG. 2B shows start of installation of the second compressor train. A second foundation structure 13″ supported on two additional foundation elements 18′,18″ are partly supported on two of the foundation elements 17′″, 17′″ (not shown in FIG. 2B) used in supporting the first foundation structure 13′ and on two additional foundation elements 18′, 18″ installed in connection with the installation of the second compressor train. The second foundation structure 13″ can be levelled relative the first foundation structure 13′ by raising or lowering the foundation elements 18′, 18″ (the foundation elements 18′, 18″ can be mudmat, piles or a combination of these).

[0144] In FIG. 2C installation of required pipes 27″ to the second compressor train, including first and second connection lines 10′,11′ (although not shown in FIG. 2C), has been done.

[0145] In FIG. 2D the installation of the second compressor train continues, including installing the second compressor 8″ on the second foundation structure 13″.

[0146] In FIG. 2E the installation of the second compressor train continues, where a second cooler 30″ is installed on the second foundation structure 13″.

[0147] In FIG. 2F the installation of the second compressor train continues, a second variable speed drive 19″ is connected to the transformer 23 and the second compressor 8″. As is clear from FIG. 2F, a common transformer 23 is used for the first and second variable speed drives 19′, 19″ as well as a common electric cable 26. Furthermore, the second compressor train utilizes the same flow conditioning unit 3, as well as the same well flow line 2 and the same common outlet 16 for the compressed fluid in the subsea system. The installation of the second compressor train, whether in series or in parallel connection with the first compressor train, is now complete once all the required pipes 27″ between the different components have been connected.

[0148] The second compressor train is preferably prepared for connection with a third compressor train in the event even further compression is required during the lifetime of the field. The installation of a third, and possibly an additional fourth, fifth, sixth, . . . , tenth) compressor trains, is performed similarly as described with reference to FIGS. 2A-2F and will not be further described herein. Such third and additional compressor trains will also preferable take advantage of the common components installed during the installation of the first compressor train, i.e. the flow conditioning unit 3, the transformer 23, the electric cable 26 and the connections to the well flow line 2 and common outlet 16 for the compressed fluid in the subsea system.

[0149] FIGS. 2G-2J show different examples of modular components which may form part of the first foundation structure 13′ and the second foundation structure 13″. The modular components in FIGS. 2G and 2H are examples of standard modular first foundation structures 13′ which can be stand-alone subsea systems 1 or be connected to any of the examples of second foundation structures 13″ disclosed in FIGS. 21 and 2J.

[0150] In particular, the subsea system 1 in FIG. 2G shows the first foundation structure 13′ with the first compressor train 8′ installed thereon. The foundation structure 13′ is supported on three foundation elements 17′,17″,17′″. The disclosed subsea system 1 comprises a first compressor train 8′ comprising a flow conditioning unit (FCU) 3. Well flow enters the flow conditioning unit 3 through a well flow line 2 connected to required pipes for the first compressor train (referred to as reference number 27′). Compressed well flow with a higher pressure than the pressure of the well flow in the well flow line 2 exits the first compressor train through a common outlet 16 for the compressed fluid in the subsea system. The subsea system 1 further comprises a first compressor 8′ connected to the flow conditioning unit 3, a first cooler 30′ connected to an outlet of the first compressor 8′, wherein the first cooler 30′ is connected to the common outlet 16 for the compressed fluid in the subsea system. The system in FIG. 2G may have similar power setup as described in relation to FIG. 2A. The first foundation structure 13′ is disclosed with a connection interface 50′ for connection to a complementary connection interface 51′ (not shown in FIG. 2G, see FIG. 2I).

[0151] The main difference between the first foundation structures 13′ in FIGS. 2G and 2H is that the first foundation structure 13′ in FIG. 2G has one compressor train 8, while the first foundation structure 13′ in FIG. 2H has two compressor trains 8′, 8″.

[0152] FIGS. 21 and 2J are examples of modular second foundation structures 13″ connectable to the first foundation structure 13′ in FIG. 2G or 2H.

[0153] The second foundation structures 13″ comprises a complementary connection interface 51′ for connection to the connection interface 50′ on the first foundation structure 13′ and required pipes for each compressor train (referred to as reference number 27″). The difference between the second foundation structures 13″ in FIGS. 21 and 2J being that the second foundation structure 13″ in FIG. 2I comprises one compressor train 8″;8′″ whereas the second foundation structure 13″ in FIG. 2J comprises two compressor trains 8″;8′″ and 8′″;8″″. The different denotations of the compressor trains 8″;8′″ and 8′″;8″″ on the second subsea structure 13″ is dependent on the whether there is one or two compressor trains 8′,8″ on the first foundation structure 13′. Similarly, the different denotations of the coolers 30″;30′″ and 30′″;30″″ on the second subsea structure 13″ is dependent on the whether there is one or two coolers 30′,30″ on the first foundation structure 13′.

[0154] FIGS. 2K-2P show six different combinations of the modular components in FIGS. 2G-2J, where FIGS. 2K, 2L and 2M show examples of a subsea system 1 with one compressor train, i.e. a first compressor train 8′, on the first foundation structure 13′, and FIGS. 2N, 20 and 2P show examples of a subsea system 1 with two compressor trains, i.e. a first and a second compressor train 8′, 8″, on the first foundation structure 13′.

[0155] In particular, FIG. 2K shows the first foundation structure 13′ with the first compressor train 8′ installed thereon with similar features as the subsea system 1 in FIG. 2G.

[0156] FIG. 2L is a subsea system 1 formed by combining the first foundation structure 13′ in FIG. 2G and the second foundation structure 13″ with one compressor train 8″ in FIG. 2I.

[0157] FIG. 2M is a subsea system 1 formed by combining the first foundation structure 13′ in FIG. 2G and the second foundation structure 13″ with two compressor trains 8″,8′″ in FIG. 2J.

[0158] FIG. 2N shows the first foundation structure 13′ with the first and second compressor trains 8′,8″ installed thereon with similar features as the subsea system 1 in FIG. 2H.

[0159] FIG. 2O is a subsea system 1 formed by combining the first foundation structure 13′ in FIG. 2H and the second foundation structure 13″ with one compressor train 8′″ in FIG. 2I.

[0160] FIG. 2P is a subsea system 1 formed by combining the first foundation structure 13′ in FIG. 2H and the second foundation structure 13″ with two compressor trains 8′″,8′″′ in FIG. 2J.

[0161] FIG. 3 shows an overview of the components forming part of the first compressor train of the subsea system 1 after the first compressor train has been installed. The setup in FIG. 3 is the same regardless of whether the next compressor train shall be arranged in a series connection or a parallel connection with the first compressor train. The first compressor train is supported by a first foundation structure 13′. Hydrocarbon fluids from e.g. a subsea well enter the flow conditioning unit 3 through well flow line 2. A flow regulation device 43 is arranged in the well flow line 2 upstream of an inlet 44 of the flow conditioning unit 3. The flow conditioning unit 3 comprises a first outlet 4 for lighter fluids connected to a fluid line 6′ for lighter fluids, which fluid line 6′ is connected to an inlet 14′ of a first compressor 8′ in a second end. The flow conditioning unit 3 further comprising a second outlet 5 for heavier fluids connected to a fluid line 7′ for heavier fluids, which fluid line 7′ comprises a flow regulation device 42′ and which is connected to the fluid line 6′ for lighter fluids upstream of the inlet 14′ to the first compressor 8′. Instead of connecting the fluid line 7′ for heavier fluids to the fluid line 6′ for lighter fluids it is possible to connect the fluid line 7′ for heavier fluids directly to the inlet of the first compressor 14′. A first compressed fluid line 9′ is connected to an outlet 15′ of the first compressor 8′ in one end thereof and to a common outlet 16 for the compressed fluid in the subsea system 1 in a second end thereof. The first compressed fluid line 9′ comprises a first cooler 30′. It is further disclosed a first recycle line 40′ with a first anti-surge valve 41′ recycling fluids from the first compressed fluid line 9′ downstream of the first cooler 30′ to the fluid line 6′ upstream of the first compressor 8′. The purpose of the recycle line(s) is to ensure that there is always sufficient flow through the compressor to avoid compressor-surge, even when there is little flow in the well flow line 2. The principle of the flow conditioning unit disclosed in FIG. 3 is described in Norwegian patent document NO 341968 B, which content is incorporated herein. The flow conditioning unit 3 further comprising an additional fluid line 6″ for lighter fluids from the first outlet 4 of the flow conditioning unit 3 and an additional fluid line 7″ for heavier fluids from the second outlet 5. The fluid line 7″ comprises a flow regulation device 42″ and is connected to the fluid line 6″ for lighter fluids upstream of the inlet 14″ to the second compressor 8″.

[0162] It is further disclosed a (third) connection line 12′ connected to the fluid line 6″ for parallel connection of the first compressor train with the second compressor train. The third connection line 12′ comprises a flow regulation device 22′.

[0163] The compressed fluid line comprises a flow regulation device 24′ downstream of the first cooler 30′ and the connection to the first recycle line 40′. A first connection line 10′ for connecting the first compressor train with the second compressor train is connected to the first compressed fluid line 9′ upstream of the flow regulation device 24′ and downstream of the first cooler 30′ and the connection to the first recycle line 40′. The first connection line 10′ comprises a flow regulation device 20′ which is closed when no additional compressor trains are connected. A second connection line 11′ for connecting the first compressor train with the second compressor train is connected to the first compressed fluid line 9′ downstream of the flow regulation device 24′ and upstream of the common outlet 16 for the compressed fluid in the subsea system 1. The second connection line 11′ comprises a flow regulation device 21′ which is closed when no additional compressor trains are connected.

[0164] FIG. 4 shows an exemplary setup of two compressor trains in series after the first and second compressor trains have been installed. The components of the first compressor train are similar to the components described in relation to FIG. 3 and will not be repeated herein. The second compressor train is supported partly by the first foundation structure 13′ and partly by the second foundation structure 13″. The inlet 14″ of the second compressor 8″ is connected to the first connection line 10′ connected to the first compressed fluid line 9′ in the first compressor train.

[0165] A second compressed fluid line 9″ is connected to an outlet 15″ of the second compressor 8″ in one end thereof and to a common outlet 16 for the compressed fluid in the subsea system 1 in a second end thereof. The second compressed fluid line 9″ comprises a second cooler 30″. A second recycle line 40″ with a second recycle valve 41″ (regulated type) ensuring a minimum allowed flow in the second compressor 8″ and the second cooler 30″ extends from first connection line 10′ upstream of the second compressor 8″ to the second compressed fluid line 9″ downstream of the second cooler 30″. The second compressed fluid line 9″ comprises a flow regulation device 24″ downstream of the second cooler 30″ and the connection to the second recycle line 40″. A first connection line 10″ for connecting the second compressor train with a third compressor train is connected to the second compressed fluid line 9″ upstream of the flow regulation device 24″ and downstream of the second cooler 30″ and the connection to the second recycle line 40″. The first connection line 10″ comprises a flow regulation device 20″ which is closed when no additional compressor trains are connected. A second connection line 11″ for connecting the second compressor train with a third compressor train is connected to the second compressed fluid line 9″ downstream of the flow regulation device 24″ and upstream of the common outlet 16 for the compressed fluid in the subsea system 1. The second connection line 11″ comprises a flow regulation device 21″ which is closed when no additional compressor trains are connected.

[0166] In order to ensure that the same well fluid is compressed both in the first compressor 8′ in the first compressor train and the second compressor 8″ in the second compressor train, the flow regulation device 24′ in the first compressed fluid line 9′ is closed such that compressed well fluids in the first compressed fluid line 9′ is directed to the inlet 14″ of the second compressor 8′ (except any fluid directed through the first and/or second recycle line 40′, 40″) and exit the subsea system through the common outlet 16.

[0167] Series operation of the first and second compressors 8′, 8″ in the first and second compressor trains, respectively, can be achieved by operating the following valves to be in a closed or an open position: [0168] flow regulation device 24′ in the first compressed fluid line 9′ is closed, [0169] flow regulation device 20′ in the first connection line 10′ is open, [0170] flow regulation device 25′ is closed [0171] flow regulation device 24″ in the second compressed fluid line 9″ is open, [0172] flow regulation device 21′ in second connection line 11′ is open.

[0173] The flow regulation devices 42′, 42″ in the fluid lines 7′,7″ for heavier fluids may be open or closed dependent on the characteristics of the fluid from the well flow.

[0174] The fluid flow through the first compressor train and further through the second compressor train is indicated by the thick solid line in FIG. 4.

[0175] FIG. 5A shows an exemplary setup of two compressor trains in parallel after the first and second compressor train have been installed. The components forming part of the first and second compressor trains are similar to the setup described with reference to FIGS. 3 and 4.

[0176] Parallel operation of the first and second compressors 8′, 8″ in the first and second compressor trains, respectively, can be achieved by operating the following valves to be in a closed or an open position: [0177] flow regulation device 24′ in the first compressed fluid line 9′ is open, [0178] flow regulation device 20′ in the first connection line 10′ is closed, [0179] flow regulation device 22′ in the third connection line 12′ is open, [0180] flow regulation device 25′ upstream of the inlet 14′ to the second compressor 8″ is open, [0181] flow regulation device 24″ in the second compressed fluid line 9″ is open, and [0182] flow regulation device 21′ in second connection line 11′ is open, [0183] flow regulation device 20″ in the first connection line 10″ is closed, [0184] flow regulation device 21″ in the second connection line 11″ is closed, [0185] flow regulation device 22″ in the third connection line 12″ is closed.

[0186] The fluid flow through the first compressor train is indicated by the thick solid line in FIG. 5A. The fluid flow through the second compressor train is indicated by the dashed line in FIG. 5A.

[0187] FIG. 5B shows an alternative exemplary setup compared to the solution in FIG. 5A of two compressor trains in parallel after the first and second compressor train have been installed. The only difference between the solution in FIG. 5B compared to FIG. 5A is that there are fewer connection lines between the first compressor train and the second compressor train, as well as between the second compressor train and the third compressor train. This is achieved by connecting the first connection line 10′, 10″ to the third connection line 12′,12″ downstream of the flow regulation device 22′, 22″ in the third connection line 12′,12″. An additional flow regulation device 28′ is arranged in the inlet line to the second compressor 8″ on the opposite of the connection point of the first connection line 10′ compared to the flow regulation device 25′. In the setup in FIG. 5B, the same third fluid line 12′, 12″ can be used both for series and parallel operation of the first and second compressor trains.

[0188] FIG. 6 shows an exemplary setup of three compressor trains in parallel after the first, second and third compressor train have been installed. The second compressor train 13″ is supported partly by the first foundation structure 13′ and partly by the second foundation structure 13″. The third compressor train is supported partly by the second foundation structure 13′ and partly by the third foundation structure 13′″.

[0189] A third compressed fluid line 9′″ is connected to an outlet 15′″ of the third compressor 8′″ in one end thereof and to a common outlet 16 for the compressed fluid in the subsea system 1 in a second end thereof. The third compressed fluid line 9′″ comprises a third cooler 30′″. A third recycle line 40′″ with a third recycle valve 41′″ ensuring a minimum allowed flow in the third compressor 8′″ and the third cooler 30″ extends from first connection line 10″ upstream of the third compressor 8′″ to the third compressed fluid line 9″ downstream of the third cooler 30″. The third compressed fluid line 9′″ comprises a flow regulation device 24′″ downstream of the third cooler 30′″ and the connection to the third recycle line 40′″. A first connection line 10′″ for connecting the third compressor train with a fourth compressor train is connected to the third compressed fluid line 9′″ upstream of the flow regulation device 24′″ and downstream of the third cooler 30′″ and the connection to the third recycle line 40′″. The first connection line 10′″ comprises a flow regulation device 20′″ which is closed when no additional compressor trains are connected. A second connection line 11′″ for connecting the third compressor train with a fourth compressor train is connected to the third compressed fluid line 9′″ downstream of the flow regulation device 24′″ and upstream of the common outlet 16 for the compressed fluid in the subsea system 1. The second connection line 11′ comprises a flow regulation device 21′″ which is closed when no additional compressor trains are connected. A third connection line 12′″ for connecting the third compressor train with a fourth compressor train is connected to the fluid line 6′″ for lighter fluids. The third connection line 12′″ comprises a flow regulation device 22′″ which is closed when no additional compressor trains are connected.

[0190] The remaining components forming part of the first and second compressor trains are similar to the setup described with reference to FIGS. 3, 4 and 5, except that there are additionally: [0191] a third compressor 8′″ and a third cooler 30′″, [0192] fluid lines 6′″, 7′″ for lighter fluids and heavier fluids, respectively, from the outlets 4,5 of the flow conditioning unit 3, [0193] a third connection line 12′″ connected to the fluid line 6′″ for lighter fluids for connecting the first compressor train with the fourth compressor train, the third connection line 12′″ comprises a flow regulation device 22′″, [0194] a flow regulation device 25″ upstream of the inlet 14′″ of the third compressor 8′″ at a position between the first and third connection lines' 10″, 12″ connection to the inlet 14′″ of the third compressor 8′″.

[0195] Parallel operation of the first, second and third compressors 8′, 8″, 8′″ in the first, second and third compressor trains, respectively, can be achieved by operating the following valves to be in a closed or an open position: [0196] flow regulation device 24′ in the first compressed fluid line 9′ is open, [0197] flow regulation device 20′ in the first connection line 10′ is closed, [0198] flow regulation device 22′ in the third connection line 12′ is open, [0199] flow regulation device 25′ upstream of the inlet 14′ to the second compressor 8″ is open, [0200] flow regulation device 24″ in the second compressed fluid line 9″ is open, and [0201] flow regulation device 21′ in second connection line 11′ is open, [0202] flow regulation device 20″ in first connection line 10″ is closed, [0203] flow regulation device 21″ in second connection line 11″ is open, [0204] flow regulation device 22″ in third connection line 12″ is open, [0205] flow regulation device 25″ upstream of the inlet 14′ to the third compressor 8″ is open, [0206] flow regulation device 24′″ in third compressed fluid line 9′″ is open, [0207] flow regulation device 20′″ in the first connection line 10′″ is closed, [0208] flow regulation device 21′″ in the second connection line 11′″ is closed, [0209] flow regulation device 22′″ in the third connection line 12′″ is closed.

[0210] The fluid flow through the first compressor train is indicated by the thick solid line in FIG. 5. The fluid flow through the second compressor train is indicated by the dashed line in FIG. 5.

[0211] The fluid flow through the first compressor train is indicated by the thick solid line in FIG. 6. The fluid flow through the second compressor train is indicated by the relatively shorter dashed line in FIG. 6. The fluid flow through the third compressor train is indicated by the relatively longer dashed line in FIG. 6.

[0212] FIG. 7 shows an exemplary embodiment where gas and liquid (water, oil and or condensate) are fed to the compressor trains in separate pipes based on the example setup in FIG. 5B. The fluid line 7″ for heavier liquids forms a fourth connection line and splits at the second compressor train. Flow regulation devices 29′, 29″ are arranged in heavy fluid line 7″ connected to inlet line upstream of second compressor 8″ and in the fourth connection line connected to subsequent compressor trains.

[0213] FIG. 8A shows an example where a pump 60 is connected to the outlet 5 for heavier fluids from flow conditioning unit 3 and the liquid entering the pump 60 is pumped out through the common outlet 16 of the system 1. The pump 60 may be an electric pump or can be operated by the gas in the system. In the latter, the pump 60 can for example be operated as disclosed in WO 2011051453 (Applicant: FMC Kongsberg AS).

[0214] FIG. 8B shows an example with one sole outlet from the flow conditioning unit, and where lighter and heavier fluids are mixed inside the flow conditioning unit before exiting through the common sole outlet. A flow regulation device 42′ can be arranged in the sole outlet, which flow regulation device can be operated to adjust the amount of mixed gas exiting the FCU and entering the compressor(s) 8′.

[0215] FIG. 9A shows an exemplary installation sequence for the first foundation structure 13′. The first foundation structure 13′ is lowered to the seabed using a wire 45. Three installation wires 46′, 46″, 46′″ are connected to the wire 45 in one end and to wire connection points 47′, 47″, 47′″ on the first foundation structure 13′, respectively. When the first foundation structure 13′ is positioned at the seabed (not shown), levelling of the first installation structure 13′ may be performed using a foundation element in the form of mudmat(s) and/or piles. In the example of FIG. 9A three mudmats 48′, 48″, 48′″ are shown. Once the first foundation structure 13′ has been levelled, piling of the first foundation structure 13′ to the seabed can be performed using one or more piles 49. The first foundation structure 13′ comprises a connection interface 50′ for connection to a second foundation structure (see FIG. 9B). The connection interface 50′ in FIG. 9A is in the form of a connection enabling a pivotally connection to the second foundation structure.

[0216] FIG. 9B shows an exemplary installation sequence for the second foundation structure 13″ to the first foundation structure 13′. Any consecutive foundation structures (not shown) have the same interfaces for connection to the previous foundation structure and to any consecutive foundation structures. The first foundation structure 13′ has been installed on the seabed. The second foundation structure 13″ is hung off on the first foundation structure 13′ by a complementary connection interface 51′ on the second foundation structure 13″ which mates with the connection interface 50′ of the first foundation structure 13′. When connected, the connection interface 50′ and the complementary connection interface 51′ form a pivot connection between the first and second foundation structures 13′, 13″ such that the second foundation structure 13′ can be pivoted relative the first foundation structure 13′ and which supports the second foundation structure 13″ when positioned on the seabed. When the second foundation structure 13″ is positioned on the seabed, levelling can be performed using one or more mudmats 48″ “. If further support is required after levelling, piling of the second foundation structure 13” to the seabed can be performed using one or more piles 49 (not shown in FIG. 9B, see FIG. 9A). The second foundation structure 13″ comprises a connection interface 50″ for connection to a third foundation structure (not shown).

[0217] FIG. 9C shows an example of the connection between two consecutive foundation structures, i.e. the connection formed between the connection interface 50′, 50″ of a nth foundation structure 13′, 13″, 13′″ and a complementary connection interface 51′, 51″ of a n+1 foundation structure 13″, 13″. For the ease of explanation of the Figure, the reference numerals in FIG. 9C identify the connection between the first and second foundation structures 13′,13″, however the connection will be similar between the second and third foundation structure and between the third and fourth foundation structure, . . . , etc.

[0218] FIG. 10A is a perspective view of is a perspective view of a subsea system 1 comprising a first foundation structure 13′ installed subsea with a first compressor train 8′ and a second compressor train 8″ installed thereon. The first and second compressor trains 8′,8″ share a common manifold 70. The features of the first foundation structure 13′ are similar to the embodiment disclosed on FIG. 9A.

[0219] FIG. 10B is a perspective view of the subsea system 1 of FIG. 10A, as well as a second foundation structure 13″ connected to the first foundation structure 13′ via connection interfaces 50′ on the first foundation structure 13′ and complementary connection interfaces 51′ on the second foundation structure 13″. The second foundation structure 13″ comprises a mudmat 48″″ in the end facing away from the connection to the first foundation structure 13′. The second foundation structure 13″ is shown without any equipment mounted thereon.

[0220] FIG. 10C is a is a perspective view of the subsea system 1 of FIGS. 10A and 10B and a second foundation structure 13″ connected to the first foundation structure 13′ and where a third compressor train 8′″ and a fourth compressor train 8″″ are installed on the second foundation structure 13″. Each of the compressor trains 8′,8″,8′″,8″″ on the first and second foundation structures 13′,13″ are connected to a cooler 30′,30″,30′″,30″″, respectively. The features of the first foundation structure 13′ and the second foundation structure 13″ of FIGS. 10B and 10C are similar to the embodiment disclosed on FIGS. 9B and 9C.

[0221] The invention is now explained with reference to non-limiting embodiments. However, a skilled person will understand that there may be made alternations and modifications to the embodiment that are within the scope of the invention as defined in the attached claims. For example, if the well flow is mainly liquid, pump trains may be set up in a similar manner as the compressor trains as described herein.

TABLE-US-00001 List of references:  1 Subsea system  2 Well flow line  3 Flow conditioning unit (FCU)/ slug catcher/ separator  4 First outlet (FCU) / Light fluid outlet  5 Second outlet (FCU) / Heavy fluid outlet  6’ Fluid line/ light fluid line between light fluid outlet and first compressor  6’’ Fluid line/ light fluid line between light fluid outlet and second compressor  6’’’ Fluid line/ light fluid line between outlet and third compressor  7’ Fluid line/ heavy fluid line between heavy fluid outlet and first compressor  7’’ Fluid line/ heavy fluid line between heavy fluid outlet and second compressor 8’, 8’’, 8’’’ First compressor  8’’ Second compressor  8’’’ Third compressor  9’ First, second, third compressed fluid line  9’’ Second compressed fluid line  9’’’ Third compressed fluid line 10’ First connection line between first compressor train and second compressor train 10’’ First connection line between second compressor train and third compressor train 10’’’ First connection line between third compressor train and fourth compressor train 11’ Second connection line between first compressor train and second compressor train 11’’ Second connection line between second compressor train and third compressor train 11’’’ Second connection line between third compressor train and fourth compressor train 12’ Third connection line between first compressor train and second compressor train 12’’ Third connection line between first compressor train and third compressor train 12’’’ Third connection line between first compressor train and fourth compressor train 13’ First foundation structure 13’’ Second foundation structure 13’’’ Third foundation structure 14’ Inlet first compressor 14’’ Inlet second compressor 14’’’ Inlet third compressor 15’ Outlet first compressor 15’’ Outlet second compressor 15’’’ Outlet third compressor 16 Common outlet for the compressed fluid in the subsea system 17’, 17’’, 17’’’, 17’’’ Foundation elements first foundation structure 18’, 18’’ Foundation elements second foundation structure 19’, 19’’ First and second variable speed drive (VSD) 20’ Flow regulation device in first connection line between first and second compressor trains 20’’ Flow regulation device in first connection line between second and third compressor trains 20’’ Flow regulation device in first connection line between third and fourth compressor trains 21’ Flow regulation device in second connection line between first and second compressor trains 21’’ Flow regulation device in second connection line between second and third compressor trains 21’’’ Flow regulation device in second connection line between third and fourth compressor trains 22’ Flow regulation device in third connection line between first and second compressor trains 22’’ Flow regulation device in third connection line between first and third compressor trains 22’’’ Flow regulation device in third connection line between first and fourth compressor trains 23 Transformator (Trafo) 24’ Flow regulation device in first compressed fluid line 24’’ Flow regulation device in second compressed fluid line 24’’’ Flow regulation device in third compressed fluid line 25’ Flow regulation device in inlet line to second compressor 25’’ Flow regulation device in inlet line to third compressor 26 Electric cable 27’, 27’’ Required pipes for each compressor train 28’ Flow regulation device in inlet line to second compressor 29’ Flow regulation device in heavy fluid line 29’’ Flow regulation device in heavy fluid line 30’, 30’’, 30’’ First, second, third cooler 40’, 40’’, 40’’’ First, second, third recycle line 41’, 41’, 41’’’ First, second, third anti-surge valve in recycle lines 42’ Flow regulation device between second outlet of FCU and first compressor 42’’ Flow regulation device between second outlet of FCU and second compressor 43 Flow regulation device in well flow line upstream FCU 44 Inlet FCU 45 Wire 46’, 46’’, 46’’’ Installation wires 47’, 47’’, 47’’’ Wire connection points 48’, 48’’, 48’’’, 48’’’’ Mudmat(s) 49 pile 50’ Connection interface on first foundation structure 50’’ Connection interface on second foundation structure 51’, 51’’ Complementary connection interface 60 Pump 70 manifold A First subsea component (prior art) B Second subsea component (prior art) C Spool with liquid trap