AU2012329632B2

AU2012329632B2 - System for very long step-out transmission of electric AC power

Info

Publication number: AU2012329632B2
Application number: AU2012329632A
Authority: AU
Inventors: Kjartan PEDERSEN; Kjell Olav Stinessen
Original assignee: Aker Solutions AS
Current assignee: Aker Solutions AS
Priority date: 2011-10-26
Filing date: 2012-10-26
Publication date: 2017-04-06
Anticipated expiration: 2032-10-26
Also published as: NO20111448A1; AU2012329632A1; WO2013062422A1; BR112014009898A2; RU2014119541A; CN103959588A; RU2618517C2; NO333443B1; CN103959588B

Abstract

System for operation of electric power subsea loads or loads supplied through an alternating current subsea crossing cable, particularly high electric power subsea loads. The system is distinctive in that it comprises an alternating current subsea step out cable or subsea crossing cable divided into at least two subsea cable segments connected in series, each segment has a length feasible for stable operation, the connection between the segments comprises a motor-generator set where the motor part is not electrically connected to the generator part, the motor-generator set is arranged in at least one housing.

Description

2012329632 17 Feb 2017

P4080PC00-EH

SYSTEM FOR VERY LONG STEP-OUT TRANSMISSION OF ELECTRIC AC POWER 5 Field of the invention

The present invention relates to equipment located subsea far away from onshore or topside locations, particularly equipment for fluid pressure boosting requiring high alternating current electric power level, but also other equipment of various types. 10

Background of the invention and prior art

Operation of electric loads subsea is challenging. Water ingress must be prevented and often very high pressures must be coped with. In addition to expected electrical effects like high loss and very high capacitance in or near 15 the quite electrolytic seawater, also the Ferranti effect and electric resonance effects, transients and ripples associated with power electronics must be under control.

The problems are enhanced at increasing power level, therefore operation of 20 subsea equipment requiring a high level of electric power, such as subsea pressure boosting equipment like subsea compressors, subsea multiphase pumps and subsea pumps, is most challenging. The problems increase with increasing subsea step out cable length, increasing voltage, increasing frequency and increasing capacitance. 25

So far subsea step out lengths of about 40 km is feasible for power level of about 20 MW, transmission frequency of 100-200 Hz and a voltage level of about 100 kV, feasible for operation of subsea compressors without too high ohmic losses or electrical instability. However, the patent applications NO 30 20111233 and PCT/EP2011/065797, both in the name of the applicant, provides technology that can work at subsea step out lengths up to about 150-200 km for subsea high power loads like compressors and multiphase pumps. This is achieved by comparatively low transmission frequency, about 50-60 Hz, and step-up of the actual operating frequency close to the subsea equipment.

8736390 1 (GHMatters) P96829.AU 2 2012329632 17 Feb 2017

The technology of NO 20111233 and PCT/EP2011/065797 involves that the subsea power electronic control installation, the subsea VSD, which is big, expensive and in practice unreliable, is replaced by other technology.

Depending on operating parameters and the load, the maximum achievable 5 subsea step out length is about 150 km.

As discussed in NO 20111233 and prescribed in WO 2009/015670 (Siemens), an option is to use a subsea VSD-variable speed drive (also called variable frequency drive, VFD, and other terms) at the cable far end, but this is complex, 10 expensive and surprisingly it is also unreliable. The reason for the lack of reliability of a subsea VSD, despite each of its components being of top quality, is assumed to be the large complexity and number of components, resulting in that a very small risk of failure for each component over many components is enhanced up to a significant risk of failure. 15

None of the existing solutions mentioned above is estimated to be able to deliver high power, that is up to many MW of alternating current power, such as 150 MW, at distances above about 150 km without the above mentioned effects deteriorating the power supply. Several effects and factors limit the length, such 20 as the size of transformers and the minimum feasible voltage and frequency. Increasing dimensions of equipment may enhances problems, for example may increasing the conductor cross section area increase the capacitance and the Ferranti effect, destroying insulation and making the system unstable. 25 Some existing subsea petroleum fields, and many not yet found, are located more than 150 km from shore or platforms. A demand exists for even longer subsea step out lengths, which in this context means possible lengths of more than 150 km, such as 600 km. 30 Summary of the invention

According to a first aspect, the invention provides a system for operation of electric power subsea loads or loads supplied through an alternating current subsea crossing cable, particularly high electric power subsea loads. The system comprises:

8736390 1 (GHMatters) P96829.AU 3 2012329632 17 Feb 2017 an alternating current subsea step out cable or subsea crossing cable divided into at least two subsea cable segments connected in series, each segment having a length feasible for stable operation, wherein the connection between the segments comprises a motor-5 generator set where a coupling between a motor part and a generator part of the set is a mechanical, hydraulic or magnetic coupling, and the motor part is not electrically connected to the generator part, wherein the motor-generator set is arranged in at least one housing. 10 The term that the connection between the segments comprises a motor- generator set where the motor part is not electrically connected to the generator part, means that the power is generated by the generator for each new cable segment connected in series but electrical problems as discussed above and below are in substance not transferred from one step out cable segment to the 15 next. The step out cable segment toward the onshore or topside end, the near end or nearer step out cable segment, is connected to the motor and drives the motor of the motor-generator set. The step out cable segment further away from the near end onshore or topsides, the far end or further out step out cable segment, is connected to the generator and receives the power generated by 20 the motor-generator set between said segments. The near end and the far end cable segments are not electrically connected, meaning that Ferranti effect-induced voltage peaks, resonances and other transients or electric problems are not transferred from one cable segment to the next. There is in substance no electrical coupling between the segments connected in series, they can be 25 said to be electrically isolated or electrically insulated. The motor-generator set does not necessarily comprise any material for electric insulation between the motor and generator, since the coupling between the motor and generator is mechanical, hydraulical and/or magnetic. The motor-generator set provides inductance to the system, which is an important feature for stability. The subsea 30 step out cable or subsea crossing can be divided into two, three, four or more step out cable segments connected in series. With the term subsea step out cable it is meant a subsea cable delivering power to subsea loads, like subsea compressors and pumps. With the term subsea crossing cable it is meant a cable where both the near end and the far end are located topsides or onshore,

8736390 1 (GHMatters) P96829.AU 4 2012329632 17 Feb 2017 but the cable has a long subsea distance, which subsea distance may exceed the maximum distance up to now considered feasible for alternating current supply, as explained above. The subsea crossing cable can be used to cross an ocean or fjord stretch. The term each segment has a length feasible for stable 5 operation, means in the context of the invention stable operation at the prevailing operating conditions or the design conditions. The motor-generator set is preferably of a rotating type, inter alia because of good efficiency, but can alternatively be of another type, such as a linear type motor-generator set. The subsea electric power step out cable or subsea crossing cable must be divided 10 into segments of length so that each segment can be operated with stability, for example maximum 100, 150, 200 or 250 km for each segment for typical subsea pressure boosting equipment, dependent upon power level and frequency and other factors as herein discussed or mentioned. For crossing cables connecting grids, the voltage and power can be much higher than for 15 subsea loads, and maximum length for stable operation can be as low as about 10 km, but more typical 20, 50 or 100 km. The maximum segment length is also dependent on the redundancy required for the application, for example dividing the power level by having two or more parallel cables for critical applications open up for longer segments. The at least one housing of the motor-generator 20 set isolates the motor and generator from the surrounding water, said housing is preferably a pressure housing at least rated to ambient pressure if it is gas filled and it is preferably a low pressure rated pressure housing, termed an enclosure, if it is liquid filled and pressure compensated to ambient seawater pressure. If the liquid is oil it can also provide electrical insulation, lubrication of bearings 25 and cooling of motor and generator.

Preferably the motor-generator set comprises a common shaft. In this context, the term “common shaft” also includes any coupled shafts or assembled shafts comprising parts or sections that are connected or coupled by a rigid, flexible or 30 other coupling so that the individual shafts of motor and generator rotate with the same speed. A motor-generator set with a common shaft can conveniently be arranged in one pressure housing, with fewer bearings and connectors. The motor generator frequency ratio can be 1 :1 or with stepping up or down of the frequency by selecting pole numbers accordingly. The at least one pressure

8736390 1 (GHMatters) P96829.AU 5 2012329632 17 Feb 2017 housing may comprise a pressure compensator. The motor-generator set may comprise a mechanical gear but this is not preferable because of high friction, lower reliability and increased utility system requirements. The motor-generator set may comprise a hydraulic coupling or gear connecting the motor and 5 generator. In principle, any means connecting the motor and generator without transferring the electrical anomalies from one cable segment to the next can be used. The motor-generator set can be arranged in a common pressure housing or in separate pressure housings. Reference is made to NO 20111233 for more detailed information on rotating motor-generator sets, and the contents of NO 10 20111233 is herein incorporated by reference. According to NO 20111233 the motor-generator set is used only at the subsea step out cable far end, however according to embodiments of the present invention it is used in between serial connected subsea step out cable segments in order to prolong the maximum subsea step out cable length for subsea alternating current loads. 15

Preferably each of the motor and generator comprises more than 2 poles, such as 4, 6,8,12 or 24 poles, in order to reduce the speed of rotation and thereby the frictional losses. The friction can also be reduced by increasing the gap between the rotor and stator. However, there will be a feasible number of poles 20 and feasible gaps dependent on many factors such as size, power factor, length of cable segments, power level and other factors, for each specific case and each motor-generator set. It is a question of optimization to find the best solution in each case. 25 In a preferable embodiment the motor-generator set in the system of the invention comprises a high voltage electric motor, e.g. a Motorformer (ABB), and/or a high voltage generator, e.g. a Powerformer (Alstom), both of which indicatively can have a voltage range of 30-150 kV and power range of 40-200 MW. This may eliminate transformers and reduce the electrical losses further. In 30 high voltage motors and generators high voltage cables like the high voltage cables used for power transmission are used instead of the traditional stator bars or stator coils, allowing high power, particularly high voltage power directly through the windings. However, a significant development work must be undertaken in order to marinise high voltage motors and generators.

8736390_1 (GHMatters) P96829.AU 6 2012329632 17 Feb 2017

In an embodiment the system comprises motor-generator sets with contactless magnetic bearings. Magnetic bearings will reduce losses due to friction, also because a gas filled pressure housing can be used. This is described in further detail in the patent application NO 20110396, in the name of the applicant, to 5 which reference is made, and the contents of said publication is herein incorporated by reference.

The system comprises one or more of the loads: a subsea compressor, a subsea multiphase pump, a subsea pump, a control system, a heat tracing 10 system, a valve actuator, a processing facility, an uninterruptible power supply, a rectifier, an onshore or topside grid point or load at the far end of the cable, and other loads. The system according to an embodiment of the invention can provide the main power supply or contingency power supply for each and every one of the mentioned equipment and systems, in different numbers, simplifying 15 the overall system. For example, using electric power generated by the generators of the system for operating a heat tracing system arranged along a subsea pipeline, may eliminate methanol or glycol systems for hydrate inhibition, which will be a very favorable feature for remote subsea field developments for which the step out cable and pipelines can be arranged more 20 or less in parallel, with power take out from generators along the pipeline.

According to a second aspect, the invention also provides a method of arranging a system for operation of electric power subsea loads or other loads supplied through an alternating current subsea crossing cable, particularly high 25 electric power subsea loads. The method comprises: dividing a subsea electric power step out cable or subsea crossing cable into at least two subsea cable segments connected in series, each segment having a length feasible for stable operation, by arranging a motor-generator set between the segments, wherein a coupling between a motor part and a 30 generator part of the set is a mechanical, hydraulic or magnet coupling and the motor part is not electrically connected to the generator part, and arranging the motor-generator set in at least one housing.

8736390 1 (GHMatters) P96829.AU 7 2012329632 17 Feb 2017

Also, according to a third aspect, the invention provides use of a subsea operable electric motor-generator set, for dividing an alternating current subsea electric power step out cable or an alternating current subsea crossing cable into subsea cable segments of lengths that each can be operated with stability, 5 wherein a coupling between a motor part and a generator part is a mechanical, hydraulic or magnetic coupling and the motor part is not electrically connected to the generator part, wherein the motor-generator set is arranged in at least one housing. 10 According to a fourth aspect, the invention also provides a subsea step out or subsea crossing cable for alternating current, distinctive in that the cable is divided into at least two subsea cable segments connected in series, each segment having a length feasible for stable operation, wherein the connection between the segments comprises a motor-generator set where a coupling 15 between a motor part and a generator part of the set is a mechanical, hydraulic or magnetic coupling and the motor part is not electrically connected to the generator part, and the motor- generator set is arranged in at least one housing. Each segment can be for example maximum 10, 20, 50, 100, 150, 200 or 250 km, dependent upon power level and frequency and other factors as herein 20 discussed or mentioned.

In addition, a fifth aspect of the invention provides a system for operation of electric power subsea loads or loads supplied through a subsea crossing cable, particularly high electric power subsea loads; and flow assurance for transport 25 of hydrocarbons or other fluid through a subsea pipeline.

The system is distinctive in that it comprises: a subsea step out cable or subsea crossing cable of one segment or divided into two or more subsea cable segments connected in series, each segment having a length feasible for stable operation, 30 a motor-generator set arranged so as to connect cable segments, wherein a coupling between a motor part and a generator part of the set is a mechanical, hydraulic or magnetic coupling, and the motor part is not electrically connected to the generator part, wherein the motor-generator set is arranged in at least one housing, wherein at a far end of the cable there is

8736390 1 (GHMatters) P96829.AU 8 2012329632 17 Feb 2017 provided: a further motor-generator set; a static frequency step up device or a variable speed drive; or an inverter, and equipment for electric heating, operatively arranged from said subsea cable to the pipeline in order to heat the pipeline. 5

The equipment for electric heating is for direct electric heating, heat tracing or other electric heating, alone or in combination, and coupled from the generator at the end of the subsea step out cable, and optionally to further generators along the subsea cable, or coupled from static frequency step up devices or 10 variable speed drives, for heating the pipeline in a situation where flow through the pipeline has been shut down.

Preferably the loads comprises subsea pumps or compressors or both, arranged so as to boost the fluid pressure for transport of hydrocarbon fluids 15 through the subsea pipeline, powered through the subsea step out cable comprising one or several segments, but in a situation of shut down of the subsea pipeline the subsea cable is used to power electric heating of the subsea pipeline in order to avoid hydrate formation. 20 More specifically, the subsea step out cable is one of: an alternating current subsea step out cable, in one or more segments, each segment has a length feasible for operation of the operatively connected loads at the frequency and power level as operated in the cable, a motor-generator set is arranged at the end and between cable segments, the motor 25 generator set in the cable far end is a frequency step up device or a rectifier, equipment for electric heating of the pipeline is powered from at least one of the generators, an alternating current subsea step out cable, in one or more segments, each segment has a length feasible for operation of the operatively connected 30 loads at the frequency and power level as operated in the cable, a static frequency step up device or a variable speed drive is arranged at the far end of the cable and optionally for connecting cable segments, equipment for electric heating of the pipeline is powered from at least one of the step up devices or variable speed drives, or

8736390 1 (GHMatters) P96829.AU 9 2012329632 17 Feb 2017 a direct current subsea step out cable, in one or more segments, each segment has a length feasible for operation of the operatively connected loads at the voltage and current level as operated in the cable, a inverter is arranged at the far end of the cable and optionally for connecting cable segments, each 5 inverter is one of: a motor generator set or a static inverter, equipment for electric heating of the pipeline is powered from at least one of the inverters.

As well known to persons skilled in the art, it is in a situation of shutdown the pipeline contents cool down and hydrates are likely to be formed. But in a 10 shutdown situation, the pumps and compressors are inactive and the subsea cable can be used to power electric heating of the pipeline, directly or via rectifiers or other equipment as found required. Equipment for direct electric heating and heat tracing are available commercially and equipment for arranging between the generators and the equipment for electric heating, if 15 required, is commercially available or can be designed for the purpose using previously known technology and good subsea engineering skills.

Figures

Embodiments of the invention is illustrated with two figures, namely 20 Figure 1, illustrating a motor-generator set feasible for use in the system according to an embodiment of the invention, and

Figure 2, illustrating an embodiment of a system according to an embodiment of the invention. 25 Detailed description

Reference is made to Figure 1, illustrating a motor-generator set in an enclosure that is pressure compensated to ambient seawater pressure. More specifically, Figure 1 illustrates a motor-generator set 2 for use in a system according to an embodiment of the invention. The motor generator set 2 30 comprises a motor part 3 and a generator part 4, connected together by a common shaft 5. Penetrators 6 for power in are illustrated, and penetrators 7 for power out are illustrated, the penetrators are the electrical connections through the housing 9 of the motor generator set 2. The housing of according to an embodiment of the invention is not a pressure housing, in the illustrated

8736390 1 (GHMatters) P96829.AU 10 2012329632 17 Feb 2017 embodiment it can be a rather thin walled enclosure since the illustrated embodiment thereof is filled with oil or other liquid, with a pressure compensator for pressure compensation to ambient pressure. More specifically, a pressure balancing unit 8, also called a pressure compensator, and an oil cooler 10, are 5 illustrated. Gas filled housings must however be pressure housings. The penetrators 6 for power in are connected to the motor and the subsea step up cable nearer topside or onshore. The penetrators 7 for power out are connected to the generator and the subsea step out cable segment further away from topside or onshore, that is closer to the subsea loads. There is in substance no 10 electrical connection or coupling between the power in and out from the motor-generator set, the penetrators 6 for power in can be said to be electrically isolated or insulated from the penetrators 7 for power out. A motor-generator set functioning as a transmission stabilizer in a subsea step 15 out or subsea crossing cable, as prescribed according to an embodiment of the present invention, will typically have equal numbers of poles in motor and generator, the illustrated motor-generator set has a 6-pole motor and a 6-pole generator. The result is that a power frequency in of 25 Hz results in a power frequency out of 25 Hz, i.e. equal in and out frequencies. In this example the 20 rotating speed will be only 500 rpm, which is beneficial with respect frictional losses.

Reference is made to Figure 2, illustrating a system 1 according to an embodiment of the invention. The system comprises a subsea step out cable 11 25 divided into three segments 11s, the segments are connected in series by motor-generator sets 2. The illustrated subsea step out cable has a near end 11 n topsides on a platform or onshore. A VSD, a variable speed drive 12, is arranged to the cable at the near end in order to control power frequency fed into the cable at the near end and a transformer downstream to the VSD 30 controls the voltage fed into it. In the illustrated embodiment the loads are compressors. In other embodiments the subsea step out cable can be connected directly to the grid 15 and therefore have constant transmission frequency of 50 or 60 Hz or it can be connected to other power sources or to other means than a VSD for control of frequency and power level, e.g. a diesel

8736390 1 (GHMatters) P96829.AU 11 2012329632 17 Feb 2017 aggregate. In the illustrated embodiment the subsea step out cable is 600 km long as it comprises three 200 km subsea step out cable segments 11 s connected in series. The cable receives 150 MW power from surface, at the near end, the power can be delivered to two subsea compressor loads 13 5 positioned 600 km away, the operation transmission frequency through the cable is for example 25 Hz. At the power level mentioned, the maximum stable cable segment distance is estimated to be about 200 km. At higher power loads or higher frequencies the maximum segment distance is reduced. Reducing the load and frequency increases the maximum distance, however, as discussed 10 above many factors are significant, for which reason generalisation on maximum distance is not advisable as it should be considered in each case. On either side of the motor-generator sets between cable segments, and between the cable and one of the loads and on either sides of the surface VSD, transformers are arranged in order to transform the power characteristics into 15 parameters feasible for operation of the motor-generator sets, the VSD and the connected loads. The loads are compressors 13 with high speed motor, and therefore the motor-generator sets connected to the compressors are step up devices 14. Said step up gears or devices have larger generator pole number than motor pole number, the pole number ratios correspond to the frequency or 20 rotational speed step up ratios. In the example it is assumed that the lengths of cable segments 11 s, frequency and voltage is the same from the near end and into the transformer at the far end. This is most likely the best solution, but it has to be noted that these three parameters can be selected different if estimated to be beneficial for technical or economical reasons. 25

Several of the illustrated features can be varied. The type and design of motor-generator set can be varied freely as long as the electrical problems described above are not transferred from one cable segment to the next cable segment connected in series. High voltage motors and high voltage generators can be 30 used as motor-generator set components, eliminating transformers or reducing the rating of transformers. The systems of the invention can comprise any features as described or illustrated in this document in any operative combination, each such combination is an embodiment of the present invention. The method of the invention can comprise any features or steps as described or

8736390 1 (GHMatters) P96829.AU 12 2012329632 17 Feb 2017 illustrated in this document in any operative combination, each such combination is an embodiment of the present invention. The subsea step out cable or subsea crossing cable of the invention can comprise any features as described or illustrated in this document in any operative combination, each 5 such combination is an embodiment of the present invention.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or 10 “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such 15 reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

8736390 1 (GHMatters) P96829.AU

Claims

Claims defining the invention are as follows; 1. A system for operation of electric power subsea loads or loads supplied through an alternating current subsea crossing cable, the system comprising an alternating current subsea step out cable or subsea crossing cable divided into at least two subsea cable segments connected in series, each segment having a length feasible for stable operation, wherein the connection between the segments comprises a motor-generator set where a coupling between a motor part and a generator part of the set is a mechanical, hydraulic or magnetic coupling, and the motor part is not electrically connected to the generator part, wherein the motor-generator set is arranged in at least one housing.
2. The system according to claim 1, wherein the motor-generator set comprises a common shaft.
3. The system according to claim 1 or 2, wherein each of the motor part and the generator part comprises more than 2 poles.
4. The system according to any of claim 1-3, wherein the motor-generator set comprises magnetic bearings.
5. The system according to any one of claim 1 -4, wherein the loads comprise one or more of: a subsea compressor, a subsea multiphase pump, a subsea pump, a control system, a heat tracing system, a valve actuator, a processing facility, an uninterruptible power supply, a rectifier, an onshore or topside grid point or load at a far end of the cable.
6. The system according to any one of claim 1 -5, wherein the system comprises more than two subsea step out cable segments connected in series, each segment having a length within the maximum length for stable power supply at the operation power level and frequency for the segment.
7. A method of arranging a system for operation of electric power subsea loads or other loads supplied through an alternating current subsea crossing cable, the method comprising dividing a subsea electric power step out cable or subsea crossing cable into at least two subsea cable segments connected in series, each segment having a length feasible for stable operation, by arranging a motor-generator set between the segments, wherein a coupling between a motor part and a generator part of the set is a mechanical, hydraulic or magnet coupling and the motor part is not electrically connected to the generator part, and arranging the motor-generator set in at least one housing.
8. Use of a subsea operable electric motor-generator set, for dividing an alternating current subsea electric power step out cable or an alternating current subsea crossing cable into segment of lengths that each can be operated with stability, wherein a coupling between a motor part and a generator part is a mechanical, hydraulic or magnetic coupling and the motor part is not electrically connected to the generator part, wherein the motor-generator set is arranged in at least one housing
9. A subsea step out or subsea crossing cable for alternating current, divided into at least two subsea cable segments connected in series, each segment having a length feasible for stable operation, wherein the connection between the segments comprises a motor-generator set where a coupling between a motor part and a generator part of the set is a mechanical, hydraulic or magnetic coupling and the motor part is not electrically connected to the generator part, and the motor-generator set is arranged in at least one housing.
10. The subsea cable according to claim 9, wherein the motor-generator set comprises magnetic bearings.
11. A system for operation of electric power subsea loads or loads supplied through a subsea crossing cable, and flow assurance for transport of hydrocarbons or other fluid through a subsea pipeline, the system comprising a subsea step out cable or subsea crossing cable of one segment or divided into two or more subsea cable segments connected in series, each segment having a length feasible for stable operation, a motor-generator set arranged so as to connect cable segments, wherein a coupling between a motor part and a generator part of the set is a mechanical, hydraulic or magnetic coupling, and the motor part is not electrically connected to the generator part, wherein the motor-generator set is arranged in at least one housing, wherein at a far end of the cable there is provided: a further motor-generator set; a static frequency step up device or a variable speed drive; or an inverter, and equipment for electric heating, operatively arranged from said subsea cable to the pipeline in order to heat the pipeline.
12. The system according to claim 11, wherein the equipment for electric heating is for direct electric heating, heat tracing or other electric heating, alone or in combination, is coupled to the generator part of the motor-generator set at the far end of the subsea step out cable, or coupled to the static frequency step up devices or variable speed drive, or coupled to the inverter, for heating the pipeline in a situation where flow through the pipeline has been shut down.
13. The system according to claim 11 or 12, wherein the loads comprises subsea pumps or compressors or both, arranged so as to boost the fluid pressure for transport of hydrocarbon fluids through the subsea pipeline, powered through the subsea step out cable comprising one or several segments, but in a situation of shut down of the subsea pipeline the subsea cable is used to power electric heating of the subsea pipeline in order to avoid hydrate formation.
14. The system according to claim 11, 12 or 13, wherein: if the system comprises the further motor-generator set at the far end of the cable, the subsea step out cable is an alternating current subsea step out cable, in one or more segments, each segment having a length feasible for operation of the operatively connected loads at the frequency and power level as operated in the cable, wherein the motor-generator set at the cable far end is a frequency step up device or a rectifier, wherein equipment for electric heating of the pipeline is powered from a generator part of the motor-generator set at the far end, if the system comprises the static frequency step up device or variable speed drive at the far end of the cable, the subsea step out cable is an alternating current subsea step out cable, in one or more segments, each segment having a length feasible for operation of the operatively connected loads at the frequency and power level as operated in the cable, wherein equipment for electric heating of the pipeline is powered from at least one of the step up devices or variable speed drives, or if the system comprises the inverter at the far end of the cable, the subsea step out cable is a direct current subsea step out cable, in one or more segments, each segment having a length feasible for operation of the operatively connected loads at the voltage and current level as operated in the cable, wherein the inverter is one of a motor generator set and a static inverter, wherein equipment for electric heating of the pipeline is powered from the inverter.