Technical Field

[0001] The present invention relates to a power distribution system. In particular, the presented power distribution system is suitable for distribution of power between different offshore locations, or offshore locations and onshore locations. Typically, with the system, electric power may be supplied from a first location, which may be a topside or onshore location, to a subsea location. The system may, however, also be applied with two onshore locations.

Background Art

[0002] There are known various configurations for power supply systems that are adapted for powering remote, electrical high-power consumers on the seabed. A typical application may be powering an electric motor that runs a compressor or a pump for transporting produced hydrocarbons through a steel pipeline. Such power supply systems must be reliable since a repair or replacement of malfunctioning equipment on the seabed is both time-consuming and expensive.

[0003] Various power supply systems have been disclosed, which are interchangeable between a 3-phase motor powering mode, and a heating mode for heating of a subsea pipeline. A common heating solution is known as direct electric heating (DEH), where current is carried in a cable adjacent the pipeline and enters the end portion of the pipeline so that the pipeline is heated due to the current flow inside it.

[0004] It is also known to heat a subsea pipeline by means of induction, wherein varying electromagnetic fields are used to heat the pipeline.

[0005] Publication US6617556 describes a system with a topside 3-phase power supply and a subsea cable leading to a wellhead platform. The system can be switched between a heating mode and a powering mode. In the heating mode, electric power is used to heat a subsea pipeline arranged adjacent the subsea cable. In the power mode, the subsea cable conveys electric power to subsea loads. The system comprises power switches at both ends of the electric cable.

[0006] EP1836375 also presents a system having a topside power supply that can be switched between a pipeline heating mode, and a motor powering mode. In the motor powering mode, a frequency converter connected to a three-phase power source delivers power, through a subsea cable, to a subsea motor. The subsea cable is arranged adjacent a subsea pipeline. In a heating mode, a symmetry and power factor compensation unit connects to the three-phase power source and delivers, through the subsea cable, electric current into the pipeline (DEH - direct electric heating). Topside switches and a subsea switch are used to select between the modes. [0007] Publication EP2964993 relates to an electric power supply system configured for providing a single-phase supply for DEH and 3-phase supply for an electric motor. A switching section is provided for interchanging between the single-phase mode and the 3-phase mode.

[0008] With the growth of renewable power production, typically from wind turbines, PV systems or wave power plants, electrical power distribution systems are developed further.

[0009] As a background for the present disclosure, reference is made to Norwegian patent application NO20190706, which was published on December 8, 2020. Various techniques for power distribution, similar or corresponding to the techniques discussed in the present application, are discussed in said application.

[0010] An object of the present invention may be to provide an electric power distribution system which is less exposed for malfunctioning.

[0011 ] Another object of the present invention may be to provide an electric power distribution system which is less expensive, while still meeting the needs of an operator.

[0012] A further object of the present invention may be to provide a versatile power distribution system configured for simultaneous import and export of electric power.

[0013] Still a further object may be to provide a power distribution system where different power transmissions can be transmitted through the same cable arrangement.

Summary of invention

[0014] According to a first aspect of the present invention, there is provided a power distribution system according to claim 1 .

[0015] In some embodiments, the system comprises a swivel with electric slip rings for transmission of power through the turret of a ship, such as an FPSO.

[0016] As the skilled reader will appreciate, the said cable arrangement, which connects the first distribution lines and the second distribution lines, may be one of more components that establish that connection. I.e. , the cable arrangement may be directly or indirectly connected to said distribution lines. For instance, the said cable arrangement may be one of more cables.

[0017] According to a second aspect of the invention there is provided a method according to claim 8.

[0018] Furthermore, according to a third aspect of the invention, there is provided a power distribution system according to claim 9.

[0019] The common transformer flux path, shared by two or more windings, will typically be in one common transformer leg. In some embodiments though, the common flux path may be in different transformer legs, which constitute a common flux path.

[0020] According to a fourth aspect of the invention, there is provide another power distribution system according to claim 10.

[0021] Moreover, a direct electrical heating system (DEH system) according to claim 14 is also provided.

Brief description of drawings

[0022] While various features of the invention have been discussed in general terms above, some more detailed examples of embodiment are given in the following with reference to the drawings, in which

Fig. 1 depicts a principle overview of an embodiment of the invention, where an FPSO is importing power from shore while powering various subsea consumers;

Fig. 2 is a schematic illustration an embodiment of the invention, showing winding of a first and a second transformer;

Fig. 3 is another schematic illustration of an embodiment of the invention, where the power distribution system is configured to transmit two three-phase distributions;

Fig. 4 depicts the inventive idea of the invention with a schematic illustration;

Fig. 5 illustrates the inventive idea with a simplified electric diagram;

Fig. 6 depicts another possible embodiment of the invention;

Fig. 7 depicts an alternative embodiment of the system shown in Fig. 6; and

Fig. 8 depicts a DEH system configured for heating two distinct steel pipelines or two parts of one steel pipeline.

Detailed description of the invention

[0023] Fig. 1 depicts an example of application of the power distribution system according to the present invention. An FPSO 1 (floating production storage and offloading) is arranged on the sea surface 3. The FPSO 1 is arranged at a first location 10. In the shown embodiment, the first location 10 is an offshore location. However, the first location may in other embodiments be an onshore location.

[0024] A second location 20 is at the seabed. The second location could however be elsewhere, such as onshore or at the sea surface. [0025] A riser extends between the seabed and the surface structure 1 . The riser comprises a cable arrangement 29, through which power can be imported to or distributed from the FPSO 1 .

[0026] In the embodiment shown in Fig. 1 , a steel pipeline 31 extends along the seabed. Typically, the steel pipeline 31 will carry produced hydrocarbons originating from a subsea well (not shown). As is well known to the skilled person in the art, such pipelines may need to be heated to prevent formation of wax and hydrates inside the pipeline. Such heating may typically be done with direct electric heating (DEH), which is well known in the art. Thus, the FPSO 1 may deliver power for heating of the pipeline 31 .

[0027] The power distribution system comprises a first transformer 125 and a second transformer 25. In the shown embodiment, the first transformer 125 is arranged on the shown FPSO 1 , while the second transformer 25 is arranged at the seabed. It will be understood, however, that their locations may be different for other embodiments.

[0028] In the shown example, power from the first transformer 125 is transferred through a swivel 5 on the FPSO 1 (typically in the FPSO turret). Since the swivel 5 has a limited number of slip rings (not shown), through which electric power can be transmitted, it is preferable to exploit the available slip rings as best as possible.

[0029] It will be understood, that in other embodiments, such as without an FPSO or without a swivel, the invention will still be advantageous. For instance, as a result of the invention, one may exploit a cable arrangement having less conductors than what would be needed with power distribution systems according to the prior art.

[0030] Still referring to Fig. 1 , the FPSO 1 receives power from a PfS arrangement 7 (Power from Shore). The PfS arrangement 7 is arranged at sea, as indicated. The FPSO 1 has a power grid 8.

[0031] Hence, power is supplied from the PfS arrangement 7, through the second transformer 25, further through the cable arrangement 29 to the first transformer 125, and to the power grid 8 of the FPSO.

[0032] Furthermore, the FPSO 1 powers various equipment subsea. The FPSO 1 comprises a symmetrisation unit 135. Power from the symmetrisation unit 135 is transmitted via the first transformer 125, through the cable arrangement 29, and to the second transformer 25. From the second transformer 25, this power, i.e. , this current, is connected to a steel pipeline 31 on the seabed. One line 235X1 connects the second transformer 25 to one of said ends, while another line 235X2 connects the second transformer 25 to a more remote end of the pipeline.

[0033] Furthermore, in this embodiment, the second transformer 25 further comprises auxiliary windings 225 connected to auxiliary primary lines 223. The auxiliary primary lines 223 connect to a subsea switchboard 11 . From the subsea switchboard 11 extends a power line 13a that powers a subsea motor 15. [0034] Also extended from the subsea switchboard 11 is another power line 13b, which extends to a subsea compression arrangement 17, configured for compression of natural gas.

[0035] A further power line 13c extends between the subsea switchboard 11 to an offshore wind turbine facility 19. The wind turbine facility 19 can thus provide power to the FPSO 1 .

[0036] Fig. 2 depicts the first transformer 125, the second transformer 125, and the cable arrangement 29 between them. Also shown in Fig. 2 is how the pipeline 31 is connected to the second transformer 25 for heating of the pipeline.

[0037] On the left-hand side of Fig. 2, there is shown two power supplies connected to the first transformer 125, hence not corresponding to the set-up shown in Fig. 1 . This is because in the embodiment shown in Fig. 1 , as discussed above, the PfS arrangement 7 provides power to the power grid 8 on the FPSO 1 . On the right-hand side, there is shown two power consumers in the form of the pipeline 31 and the subsea switchboard 11. It will be understood, however, that instead of two power consumers on the right-hand side, there could be one or two power supplies. Correspondingly, on the left-hand side of the first transformer 125, there could be two power consumers.

[0038] Three first primary windings 134a, 134b, 134c are wound about respective transformer legs of the first transformer 125. These windings are configured with a delta-configuration and connect to a three-phase power-supply. On the same transformer legs there are wound pairs of first secondary windings TWA1 , TWA2, TWB1 , TWB2, TWC1 , TWC2. As shown, one pair of windings are wound about respective transformer legs. Furthermore, the windings of each pair are wound in the same direction. Each first secondary winding TWA1 , TWA2, TWB1 , TWB2, TWC1 , TWC2 connects to a respective first secondary line 135A1 , 135B1 , 135C1 , 135A2, 135B2, 135C2. In this embodiment, the first secondary lines connect to a 1 -phase power supply. Thus, as shown, three first secondary lines 135A1 , 135B1 , 135C1 connect to one node of the 1 -phase power supply, while the other three connect to the other node. The first secondary lines are divided into two sets 135X1 , 135X2 of first secondary lines.

[0039] Since the current through one set 135X1 of first secondary lines 135A1 , 135B1 , 135C1 is opposite to the current through the other set 135X2 of first secondary lines 135A2, 135B2, 135C2, and the first secondary windings TWA1 , TWA2, TWB1 , TWB2, TWC1 , TWC2 are wound in the same direction and same number of turns, the induced flux in the transformer legs are cancelled out. I.e. the current from the 1 -phased power supply does not induce flux in the first transformer 125.

[0040] The current from the 1 -phase power supply flows further through the conductors of the cable arrangement 29, and into second secondary windings SWA1 , SWA2, SWB1 , SWB2, SWC1 , SWC2 of the second transformer 25. As the skilled reader will appreciate, the shown first transformer 125 corresponds to the second transformer 25. Thus, the current from the 1 -phase power supplies flows through the second transformer 25 and into the steel pipeline 31 .

[0041] The current flowing from the 3-phase power supply, on the left-hand side of the first transformer 125, will, however, induce flux in the transformer legs. This flux will induce current in the first secondary windings TWA1 , TWA2, TWB1 , TWB2, TWC1 , TWC2 of the first transformer 125. This current will also induce a flux in the second transformer 25 at the second location 20. Furthermore, this induced flux will induce electric current in second primary windings 34a, 34b, 34c of the second transformer. Thus, the power from the 3-phase power supply will power the switchboard 11 connected to second primary lines 23a, 23b, 23c of the second transformer 25.

[0042] Also schematically depicted in Fig. 2 is the swivel 5 discussed with reference to Fig. 1 above. It will be appreciated that the power distribution system naturally would function without the swivel 5.

[0043] In the embodiment shown in Fig. 2, the cable arrangement 29 comprises six conductors. Through these six conductors, one is able to independently distribute electric power between a 3-phase power supply and a 3-phase power consumer, and a 1 -phase power supply and a 1 -phase consumer. Notably, the direction of power through the power distribution system is arbitrary. I.e. the 3-phase power could for instance be transmitted in the direction from the second transformer 25 to the first transformer 125, while the 1 -phase power could be transmitted the opposite direction.

[0044] Fig. 3 depicts another embodiment. The functional principle is however the same as the principle of the embodiment shown in Fig. 2.

[0045] The first transformer 125 comprises three first primary lines 133a, 133b, 133c, configured to transmit 3-phase power. Furthermore, the first transformer 125 comprises three first secondary lines SL1 , SL2, SL3, also configured to transmit 3- phase power.

[0046] In the shown embodiment, the first secondary lines SL1 , SL2, SL3 connect to a variable speed driver (VSD I VFD) 14. The VSD 14 and the first primary lines are connected to a grid 8.

[0047] Each first secondary line SL1 , SL2, SL3 connect to three first secondary windings TWA1 , TWA2, TWA3, TWB1 , TWB2, TWB3, TWC1 , TWC2, TWC3, of which one winding is attached on respective transformer legs as shown in Fig. 3. Since the first secondary windings are wound in the same direction and with the same number of turns, the 3-phase current flowing through the respective first secondary line SL1 , SL2, SL3 will induce fluxes in the transformer legs that cancel each other out. This corresponds to the embodiment shown in Fig. 2. Thus, 3-phase power is transmitted between the VSD 14 and a motor 15 that is connected to second secondary lines SL11 , SL12, SL13 of the second transformer 25.

Furthermore, power is transmitted between the PfS arrangement 7 and the three first primary lines 133a, 133b, 133c, on the same conductors in the cable arrangement 29. As indicated in Fig. 3, the cable arrangement 29 comprises nine conductors.

[0048] The electric power distribution system according to the invention will be beneficial in particular in combination with renewable power production. Since the sun does not always shine and the wind does not always blow, the flexibility offered with the disclosed system facilitates adjustment of power flows in desired directions.

[0049] The power distribution system may also be employed for powering hydrogen production during excessive renewable power production.

[0050] Fig. 4 schematically depicts, with a simplified diagram, the advantage of the invention. Electric end points P, Q of a first pair of end points are distributed on separate sides of the power distribution system. Electric end points R, S of a second pair of end points are also distributed on separate sides of the power distribution system. These end points may be electric power supplies or electric power consumers. Through the same conductors of the cable arrangement 29, electric power between the end points of the respective pair can be optionally directed.

[0051] Fig. 5 illustrates the inventive idea with a simplified electric diagram.

[0052] Fig. 6 depicts an embodiment of a power distribution system, which has several similarities to the diagram shown in Fig. 1 . A first power supply 131 , in the form of a variable speed drive (VSD) and a second power supply 133, also in the form of a VSD, are arranged. Moreover, a third power supply 135 is arranged and is in form of the symmetrisation unit, which was also shown in Fig. 1.

[0053] A primary first transformer 125a connects to the first power supply 131 , and a secondary first transformer 125b connects to the second power supply 133.

[0054] To the symmetrisation unit, which in the shown embodiment constitutes a third power supply 135, there is connected a switching arrangement 135Z. The switching arrangement 135Z is configured to selectively connect the third power supply 135 to the primary first transformer 125, such as in Fig. 1 , and/or to the secondary first transformer 125b, or to none of them. It will be understood that, in the shown embodiment, the third power supply 135 will then connect to the first transformer 125a, 125b in the manner shown and discussed with reference to Fig. 2 above, i.e. to the neutral point of the star-connected winding.

[0055] Still referring to Fig. 6, the primary first transformer 125a can typically have a Dyn5yn11 configuration, with six lines exiting the transformer, corresponding to the first transformer 125 shown in Fig. 2. It will be noted though, that the winding directions are changed, i.e. the winding directions are typically changed from Dyn5yn5 configuration (Fig. 2) to Dyn5yn11 configuration (Fig. 6). Furthermore, the secondary first transformer 125b can typically have a Dxn configuration, with four lines exiting the transformer. Hence, exiting the primary and secondary first transformers 125a, 125b are ten lines. These ten lines are assembled in a first cable arrangement 29a. The first cable arrangement 29a thus comprises ten conductors, as shown with the inserted cross section view in Fig. 6. [0056] The first cable arrangement 29a is thus capable of transmitting electric power from the first, second, and third power supplies 131 , 133, 135.

[0057] As indicated in Fig. 6, the first cable arrangement 29a extends along a first steel pipeline 31a. It will, however, be understood that the disclosed power distribution system can also be without said steel pipeline.

[0058] A primary second transformer 25a, which typically can have a Dyn5yn11 or YN5YN1 1d0 configuration, connects to the six lines that exit the primary first transformer 125a. Similar to the embodiment shown in Fig. 1 , the primary second transformer 25a can deliver power for operation of a first electrical load, such as the first electric motor 15a. This power is delivered by the first power supply 131 , being a VSD in the present example.

[0059] Moreover, with the same technique as discussed above, electric power from the third power supply 135 can be tapped from the primary second transformer 25a. This power can, as an example, be used for direct electrical heating of the first steel pipeline 31a. Notably, by means of the switching arrangement 135Z, the operator is enabled to choose if the first steel pipeline 31a shall be heated.

[0060] The four lines exiting the secondary first transformer 125b, and which extends through the first cable arrangement 29a, extend further as a second cable arrangement 29b comprising four conductors. In the shown embodiment, the second cable arrangement 29b extends along a second steel pipeline 31 b. It will be understood that the first and second pipelines 31a, 31 b typically can transfer the same fluid, such as hydrocarbons. Moreover, in some embodiments, the first and second pipelines 31a, 31 b can be different lengths of

[0061 ] At a far end of the second cable arrangement 29b, the four lines exit the cable and are connected to a secondary second transformer 25b. The secondary second transformer 25b can typically have an XNd configuration. Through these four lines, the second power supply 133 is enabled to run a second electric load, such as the second electric motor 15b. As shown in Fig. 6, the second electric motor 15b connects to the secondary second transformer 25b. The second electric motor 15b can for instance run a subsea pump, compressor or the like.

[0062] While the first electric motor 15a and the second electric motor 15b can be considered a first and second electric load, the first pipeline 31a and the second pipeline 31 b can then be said to constitute a third and a fourth electric load. Consequently, by means of the three power supplies 131 , 133, 135, the operator, with the shown power distribution system, to selectively power-supply any of the four electric loads 15a, 15b, 31 a, 31 b with the three power supplies 131 , 133, 135. While the electric loads in the present example are constituted by electric motors and steel pipelines, it will be appreciated that the electric loads can be of other types.

[0063] Fig. 7 depicts an embodiment, wherein a third steel pipeline 31 with DEH heating has been added to the power distribution system shown in Fig. 6. Only a portion of the items shown in Fig. 6 have been included in Fig. 7. One will recognize the second steel pipeline 31 b and the secondary second transformer 25b from Fig. 6. The secondary second transformer 25b now delivers electric power for heating both the second and third steel pipeline 31 b, 31c.

[0064] The electric power for heating of the third steel pipeline 31 c is transferred through a third cable arrangement 29c. The while the first cable arrangement 29a comprises ten conductors, and the second cable arrangement 29b comprises four conductors, the third cable arrangement 29c can be a single conductor cable. Typically, the third cable arrangement 29, as well as the first and second cable arrangements 29a, 29b, can be piggybacked onto the respective steel pipelines 31c, 31a, 31 b.

[0065] Still referring to Fig. 7, while the second secondary transformer 25 shown in Fig. 6 was connected to the second electric motor 15b, in this embodiment it is connected to a subsea switchboard 11 .

[0066] Fig. 8 depicts an assembly that may be a part of the power distribution system according to the invention. Two separate auxiliary steel pipelines, i.e a first auxiliary steel pipeline 31 p and a second auxiliary steel pipeline 31 q, are heated with power from the primary second transformer 25a. A first heating line 201 p, which is for heating of the first auxiliary steel pipeline 31 p, connects to one of the neutral points of the star-connected winding. Moreover, a second heating line 201 q, which is for heating of the second auxiliary pipeline 31 q, connects to the other of the neutral points of the star-connected winding (of the primary second transformer 25a). Thus, current from the third power supply 135 (see Fig. 6) is split in two, such that half of the current is directed into one of the respective two auxiliary steel pipelines 31 p, 31q.

[0067] The embodiment disclosed in Fig. 8 can be particularly useful for steel pipelines having smaller diameters which thus requires less power for heating.

[0068] Reference is again made to Fig. 6. As briefly discussed above, the third power supply 135, which in the current embodiment is in form of a symmetrisation unit, can be made to supply power to heat only the first steel pipeline 31a, only the second steel pipeline 31 b, or both, or none of them. This is enabled with the switching arrangement 135Z.

[0069] The symmetrisation unit 135 has a first output 135f and a second output 135s. If the first steel pipeline 31a shall be heated, current is transferred through the return line 135R, which then is connected to the second output 135s. Current through the first steel pipeline 31a can be provided from the primary first transformer 125a, via the primary second transformer 25a. Notably, however, current through the first steel pipeline 31a can also, instead or in addition, be provided through the secondary first transformer 125b, via the secondary second transformer 25b and via the second steel pipeline 31b. As illustrated in Fig. 6, the adjacent ends of the first and second steel pipelines 31a, 31b are interconnected with a line. Consequently, the operator has a means for selecting the heating power applied to the first steel pipeline 31a. [0070] In situations wherein the first steel pipeline 31a is not heated, but wherein the second steel pipeline 31 b is heated, the return line 135R will not be connected. Then, the second output 135s can connect to the secondary first transformer 125b, and the first output 135f connects to the primary first transformer 125a.

[0071 ] Notably, the heating of the respective pipelines can occur without affecting the operation of the first and second motors 15a, 15b.

[0072] It will be understood, that with embodiments as shown in Fig. 7, heating of the second and the third steel piplines 31 b, 31c occurs simultaneously.