NO20171503A1 - Power supply of electric subsea motors - Google Patents

Description

Power supply of electric subsea motors

Technical Field

[0001] The present invention relates to electric power supply of large electric motors arranged on the seabed. A typical application is power supplying the electric motors that are used for running large mechanical loads, such as fluid pumps and compressors at a subsea facility.

Background Art

[0002] There exist many different solutions for powering large subsea motors on the seabed. For instance, in international application publication WO2007055593, one power supply cable is extended from an onshore facility to a subsea power distribution facility. At the subsea location, several VSDs (variable speed drive) are allocated to drive their separate electric motors, so that each VSD connects to one single motor. The VSDs are powered with a constant frequency in the subsea power distribution facility.

[0003] A similar solution is disclosed in the paper" Technical Status and Development Needs for Subsea Gas Compression", Erik Baggerud, et.al., OTC 2007.

[0004] WO2013039404 also describes solutions where the speed of subsea motors are controlled with a VSD. Here, a single VSD controls a plurality of motors simultaneously. Alternatively, a number of VSDs are dedicated to a number of motors, so that the motor speeds can be controlled separately.

[0005] WO2015170991 also relates inter alia to power supply of large electric motors. Here, a number of solutions are presented, wherein the speed of a large electric subsea motor is controlled by operating a topside VSD.

[0006] Particularly when operating large electric subsea motors that are arranged remote from an electric power source / grid, a large cost is related to the long cables extending between the subsea location and the power source. An object of the present invention may be to present a subsea power supply that reduces such cost.

[0007] In addition to the cost of the cable itself, a significant cost is associated to the installation of such subsea cables. Thus, an object of the present invention may be to reduce the installation cost of a subsea power supply assembly.

[0008] In some cases, typically where the topside equipment is installed on a floating ship, the power supply involves the use of a turret. In such cases, one may want to reduce the number of electrical conductors or electric paths that are extended between the equipment on the ship and the subsea equipment. Such electrical connections are typically provided with a collector ring or slip ring in association with the turret. An object of the present invention may thus be to provide a subsea power supply that reduces the number of necessary electric connections through the turret.

[0009] For many offshore installations, the available space on a topside installation may be scarce. Of this reason, it may be advantageous if one may arrange more equipment on the seabed instead of installing it on the topside installation, such as on a ship or other installation. Consequently, an object of the present invention may be to provide a subsea power supply assembly, where the amount of topside equipment is reduced.

Summary of invention

[0010] According to the present invention, there is provided an electric power supply assembly comprising a first electric motor and a second electric motor, which are arranged at a subsea location and which are in the power range of above 0,5 MW. The assembly has a first variable speed drive arranged at a topside location, and a step-out cable is in a position between the topside location and the subsea location. The step-out cable is powered by the first variable speed drive. A second variable speed drive is arranged at the subsea location. The first electric motor and the second variable speed drive are arranged to receive power from the step-out cable, at a frequency equal to the output frequency of the first variable speed drive. According to the invention, the second electric motor is arranged to receive power from the second variable speed drive.

[0011] Thus, the operator may run the first electric motor and the second electric motor independently with the first and second variable speed drive, respectively. Furthermore, with the assembly, the operator is able to control the speed of the first electric motor by operation of the topside (first) VSD, while controlling the speed of the second electric motor by operation of the subsea (second) VSD.

[0012] By the term topside location is meant any position at the sea surface, such as on a floating installation, or an onshore position.

[0013] In some embodiments, the assembly according to the invention may further comprise a step-down transformer that is arranged between the step-out cable and the first electric motor. In such embodiments, the second variable speed drive is connected to receive electric power from the step-down transformer. Typically, the second VSD may then be connected to a secondary winding of the step-down transformer.

[0014] In embodiments comprising a step-down transformer arranged at the subsea location, there may typically be arranged a step-up transformer between the step-out cable and the first VSD.

[0015] The assembly according to the invention may in some embodiments further comprise more than one second electric motor. In such embodiments, each second electric motor can be connected to a respective second variable speed drive. Such second VSDs will, according to the invention, receive electric power from the step-out cable, which power is of frequency equal to the output frequency of the first VSD at the topside location.

[0016] In some embodiments of the invention, the electric power supply assembly comprises a turret collector ring, which is arranged between the step-out cable and the first variable speed drive. Such a collector ring can typically be a part of or in association with a ship's turret, and be configured to transmit electric power from the first VSD arranged on the ship and to step-out cable.

[0017] The first electric motor and the second electric motor may in some embodiments be arranged to drive respectively a compressor and a pump of a subsea gas compression facility. Or oppositely, the first and second electric motor may be arranged to drive a pump and a compressor of a subsea gas compression facility.

[0018] In some embodiments according to the present invention, the step-out cable can be more than 20 km long, or even more than 50 km long, and the number of sets of three-phase power supply conductors in the step-out cable can be one or in some embodiments two.

Brief description of drawings

[0019] While the present invention has been described in general terms above, a more detailed example of embodiment will be given in the following with reference to the drawings, in which

Fig. 1 depicts a schematic diagram of an embodiment håving two subsea electric motors that can be operated separately; Fig. 2 depicts an embodiment similar to the embodiment of Fig. 1, however including a plurality of second electric motors; Fig. 3 depicts an embodiment with a step-out cable that has one set of three electric conductors, configured for transmission of three phase electric power; and Fig. 4 depicts an embodiment with a step-out cable that has two sets of three electric conductors, configured for transmission of two sets of three phase electric power.

Detailed description of the invention

[0020] Fig. 1 illustrates a simplified example embodiment of an electric power supply assembly according to the present invention. In this diagram, several components have been removed for the sake of simplicity.

[0021] In this embodiment, two large electric motors are arranged at a subsea location 10, in association with a subsea gas compression facility (not shown). A first electric motor 1 is configured to drive a gas compressor (not shown). A second electric motor 3 is configured to drive a pump (not shown). In other embodiments according to the invention, the second electric motor could drive the gas compressor, while the first electric motor could drive the pump.

[0022] At a topside location 20, such as on a floating surface installation or an onshore installation, a first VSD (variable speed drive) 5 is arranged. The first VSD 5 receives electric power from a power grid 7 at a fixed frequency.

[0023] Between the topside location 20 and the subsea location 10, extends a step-out cable 9. The step-out cable 9 can be several kilometers long, for instance more than 20 km or more than 50 km.

[0024] Electric power is provided from the first VSD 5 to the first electric motor 1, through the step-out cable 9. It comprises three conductors for transmission of three phase electric power. The step-out cable 9 has a topside end 11 at the topside location 20 and a subsea end 13 at the subsea location 10. As the skilled person will appreciate, the ends of the step-out cable 9 itself may be arranged in other positions than shown in Fig. 1. For instance, there may be a subsea junction box (not shown) on the seabed, to which the step-out cable connects. Moreover, the step-out cable may be assembled by a number of cable segments that are connected.

[0025] At the subsea location 10, the step-out cable 9 connects to a subsea step-down transformer 15. Between the first VSD 5 and the step-out cable 9, there is a step-up transformer 14.

[0026] The first electric motor 1 is connected to the step-down transformer 15. A power switch /circuit breaker 17 is arranged between the first electric motor and the stepdown transformer 15.

[0027] To control the speed of the first electric motor 1, the operator controls the output from the first VSD 5, arranged topside. Thus, the frequency of the electric power delivered to the first electric motor 1 at the output of the stepdown transformer 15, is the same frequency as on the output of the first VSD 5.

[0028] The step-down transformer 15 and the power switch / circuit breaker 17 are arranged within a pressure compensated enclosure 18.

[0029] The stepdown transformer 15 comprises an auxiliary winding 19. The second electric motor 3 receives electric power through this auxiliary winding 19. Notably, a second VSD (variable speed drive) 21 is arranged between the second electric motor 3 and the auxiliary winding 19. Thus, the frequency of the electric power delivered to the second VSD 21 is equal to the frequency delivered to the first electric motor 1.

[0030] With the second VSD 21 interposed between the second electric motor 3 and the step-down transformer 15, the operator may run the second electric motor 3 with a speed that is independent of the speed of the first electric motor 1. That is, the operator controls the speed of the second electric motor 3 by controlling the output frequency of the second VSD 21.

[0031 ] With the electric power supply assembly shown in Fig. 1, the number of

VSDs is the same as the number of subsea electric motors, wherein the speed of the electric motors are controlled by controlling the VSDs. Notably, one VSD is arranged topside and feeds the step-out cable 9. The other VSD is arranged subsea and is fed through the step-out cable 9. Contrary to known solutions, with such an electric power supply assembly, one can independently operate two subsea motors, while håving only one step-out cable and one subsea VSD. That is, one can supply power two large (power range of above 0,5 MW) three-phase subsea electric motors with one three phase power cable (step-out cable 9) håving only three electric conductors.

[0032] Known VSDs for driving large subsea electric motors are configured to tolerate some frequency and voltage deviation from their rated values. With the assembly according to the present invention, the second VSD 21 may be adapted to tolerate even more deviation / variation in voltage and/or frequency, than known VSD's. Such adaptation may be provided by the skilled person, such as by providing a suitable size of the capacitor on the power receiving side of the VSD.

[0033] Fig. 2 is a schematic diagram similar to Fig. 1, however depicting an alternative embodiment of the present invention. In this embodiment, two second motors 21 are arranged on the subsea location 10. Both the second motors 21 are operated through a designated second VSD 21, which connect to the auxiliary winding 19 of the step-down transformer 15.

[0034] Instead of providing power to both second motors 3 through the same auxiliary winding 19, one could arrange more auxiliary windings in the step-down transformer 15, so that each second motor 3 would be fed via a dedicated auxiliary winding.

[0035] Alternatively, in the embodiment shown in Fig. 1 and in Fig. 2, one could arrange one step-down transformer in association with each separate first or second electric motor 1, 3. The step-down transformers would then not need any auxiliary windings.

[0036] While the embodiment shown in Fig. 2 involves two second subsea electric motors, one could also include more than two such second motors and hence more than two second VSDs to operate them.

[0037] In some embodiments, one may also provide a subsea power supply assembly according to the invention without the step-up transformer 14 and step-down transformer 15. Such embodiments may for instance be relevant with step-out lengths that are fairly short, e.g. less than 20 km, and with a relatively high voltage, e.g. 11 kV.

[0038] In Fig. 2, a slip ring assembly 23 of a not shown turret is shown at the topside location 20. Such a turret is typically a part of a ship, from which the power supply cable, such as the step-out cable 9, extends to the subsea location, where the first and second electric motors 1, 3 are positioned.

[0039] Fig. 3 schematically depicts a cross section of the step-out cable 9 shown in Fig. 1 and fig. 2. In the embodiment shown in Fig. 3, the step-out cable 9 has three conductors, configured to transmit three phase electric power to the electric motors 1, 3.

[0040] Fig. 4 depicts another embodiment, where the step-out cable 9 has two sets of three phase conductors. In such embodiments, two first VSDs 5 may connect to the respective sets of three phase conductors. The two first VSDs 5 may then control two first electric motors 1, and two or more second electric motors 3 may receive electric power from the respective first VSDs 5, via the step-out cable 9 and second VSDs 21.

Claims (6)

1. Electric power supply assembly comprising - a first electric motor (1) and a second electric motor (3), which are both arranged at a subsea location (10) and which both are in the power range of above 0,5 MW; - a first variable speed drive (5) arranged at a topside location (20); - a step-out cable (9) positioned between the topside location (20) and the subsea location (10), the step-out cable being configured to be powered by the first variable speed drive; - a second variable speed drive (21) arranged at the subsea location (10); wherein the first electric motor (1) and the second variable speed drive (21) are arranged to receive power from the step-out cable (9), at a frequency equal to the output frequency of the first variable speed drive, and wherein the second electric motor (3) is arranged to receive power from the second variable speed drive (21).

2. Electric power supply assembly according to claim 1, wherein it further comprises a step-down transformer (15) arranged between the step-out cable (9) and the first electric motor (1), and wherein the second variable speed drive (21) is connected to receive electric power from the step-down transformer (15).

3. Electric power supply assembly according to claim 1 or claim 2, further comprising more than one second electric motors (3), wherein each second electric motors are connected to a respective second variable speed drive (21).

4. Electric power supply assembly according to one of the preceding claims, further comprising a turret slip ring assembly (23) arranged between the step-out cable (9) and the first variable speed drive (5).

5. Electric power supply assembly according to one of the preceding claims, wherein the first electric motor (1) and the second electric motor (3) are arranged to drive respectively a compressor and a pump of a subsea gas compression facility, or vice versa.

6. Electric power supply assembly according to one of the preceding claims, wherein the step-out cable (9) is more than 20 km long and wherein the number of sets of three-phase power supply conductors in the step-out cable (9) is one or two.