OTC-26316-MS Variable Rotoconverter for Subsea Processing ...

OTC-26316-MS Variable Rotoconverter for Subsea Processing Applications

T. Normann, L. Krogstad Lien, M. Gabelloni and L. A. Müller, Aker Solutions, C. Pauchon and J. Pimentel, Total Copyright 2015, Offshore Technology Conference This paper was prepared for presentation at the Offshore Technology Conference Brasil held in Rio de Janeiro, Brazil, 27–29 October 2015. This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abstract The developments of subsea fields located at long tie-back distances, or fields located close to an existing facility with limited available topside space, require advanced power distribution technology. This may represent a major roadblock with existing electrical power solutions, so that some prospects may not be economical because of these constraints. This paper, based on studies carried out by Total and Aker Solutions over the last year, will present the application of a novel power transmission and conversion technology to significantly reduce the need for topside equipment and to extend the maximum power transfer of subsea cables to significantly longer distance from shore compared with existing technologies. The basis for this new technology is a Rotary Converter consisting of a lowfrequency subsea motor running a generator which can then supply high-frequency power (typically 50-200 Hz) to subsea pumps, compressors or other power-consuming equipment. The rotational speed is relatively low, which gives very limited wear and high robustness and the machine is hermetically sealed against surrounding seawater. It is supplied with low-frequency power (typically 50 / 60 Hz for shorter distances up to 100 km, and typically 10-30 Hz for longer step-outs) from a topside supply, significantly reducing the capacitive current in the cable and allowing more power and better system stability in long cables. This Rotary Converter (RC) technology is further explained in reference [1].

Figure 1: Typical subsea power system types and step-outs

In this paper, focus is on the Variable Rotary Converter (VRC). This device is basically the same as the rotary converter, but with a variable coupling in between. Through this, the generator can be controlled with variable speed and variable torque, and it allows for the VRC becoming an alternative to the more complex electronics-based subsea variable speed drive (VSD) technology. By eliminating the need for topside VSDs and associated equipment such as switchgear and transformers (which would have been the alternative to subsea speed control), significant weight and space reductions can be achieved on the topside facility. In addition, longer step-out can be achieved by adapting the VRC motor input fixed frequency to 10-30 Hz, in the same way as explained for the RC above. Moving the drive technology to the seabed also makes it possible to have only one supply cable to one or multiple subsea VRCs that run individual control of the subsea rotary boosting machinery. With topside VSDs, one cable for each subsea load is normally required.

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A proof of concept test for this technology has been performed, using a prototype 50 kW Rotary Converter unit, and results from this test will be shown, including start-up, transient tests, harmonics, dynamic load changes and thermal stability. Total and Aker Solutions have analyzed two specific cases of VRC based applications, one for subsea pumping at shorter step-out, demonstrating the system simplification benefits, and one for subsea compression demonstrating the long step-out capabilities of the technology. The model validated through the prototype 50 kW Rotary Converter proof of concept test has been applied to this case study and simulation results will be shown for the complete power distribution system. In general the new Subsea Rotary Converter technology can provide major benefits to the subsea processing industry, and specifically for high-power boosting and compression solutions, enabling CAPEX and OPEX reduction, longer step-out distance and better system stability and reliability.

Introduction and Technology Basics Variable Rotary Converter – VRC The patented subsea variable rotary converter (VRC) is a new product currently under development by Aker Solutions that will make it possible to transform a specific input frequency / voltage to another variable output frequency / voltage by using a motor and a generator with variable coupling in between. See Figure 2.

Figure 2: VRC transparent view

The VRC product is an evolution of the patented subsea rotary converter (RC) which consists of a motor and a generator mounted on a common shaft, giving a fixed transformation of the input frequency / voltage. The ratio of the input vs. output frequency is defined by adapting the rotational speed and the number of poles on the motor and the generator. For example if the RC has a motor with 6 poles and a input frequency of 16.7 Hz, the frequency can be stepped up to 50 Hz by having a generator with 18 poles, 3 times the poles on the motor. The number of poles and the rotational speed of the RC can be adapted to achieve any input / output frequency as long as it is practical with regards to stator slot design and winding insulation level which affects the physical size of the machine.

Figure 3: 70 kVA / 50 kW RC Prototype with 6.6 kV motor

A key element of the RC is its robustness, which is achieved by having a low rotational speed. For instance, in the example above the rotational speed is only 334 rpm, which is approximately 10 times lower than for a subsea pump, and this

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increases the robustness of the machine as well as reduces the frictional losses in the machine significantly. The RC is suitable for long step-out transmission from a power source as the input frequency can be adapted to minimize the losses in the transmission cable. The transmission frequency, in the case of a long step-out RC application, must then be variable (e.g. 0-16.7 Hz) in order to have speed control of the subsea equipment. A 70 kVA 6.6 kV / 690 V RC prototype (see Figure 3) has been built and tested to prove the concept of using the RC as a frequency step-up device with variable input frequency. An extensive test program was carried out and all tests were successful, which will be shown later in this paper. To eliminate the need of variable transmission frequency the speed control can be placed locally subsea, close to the subsea equipment. By designing the VRC with a low frequency motor, it becomes applicable for long step-out transmission from a power source, enabling development of previously stranded fields. In the case of a brown-field application, the main benefit is the reduction in impact on topside facilities in the case of installation of an additional seabed pump, ESP or compressor, to extend the life time of the field. The VRC can be designed for the above mentioned applications by either using a low frequency motor (10-30 Hz) for long step-outs or a conventional 50 / 60 Hz motor for brown-field short to medium distance applications. An example of the VRC design process is given further. By adapting the number of poles of the motor and generator, the rotational speed of the shaft can be adapted, and the ratio of the input frequency vs. output frequency can be defined. For example, if the VRC has a an induction motor (IM) with 6 poles and the permanent magnet generator (PMSG) has 18 poles, the rotational speed of the shaft becomes approximately 334 rpm if 16.7 Hz is applied to the input terminals of the motor. Due to the ratio of the poles being 1:3, the output frequency of the generator then becomes three times higher, or 50 Hz at 100% speed transfer in the variable coupling. Similarly the output frequency will be 150 Hz if the input frequency is 50 Hz. And then the rotational speed increases to 1000 rpm. If for instance it is desirable to keep the rotational speed at 667 rpm with 50 Hz input, the number of poles must be increased to 12 on the motor and 36 on the generator. In this way, basically any input/output frequency and rotational speed can be achieved, as long as it is practical with regards to stator slot design, winding insulation level and other factors that affect the physical size of the machine. The motor may either be an induction machine or a permanent magnet design, whilst the generator can either be a synchronous machine or a permanent magnet solution. In order to have a robust design, the right combination of speed, power losses, length and / or diameter of the motor and generator has to be found. For the purpose of reducing wear and power losses in a liquid filled design the speed should be kept relatively low compared to subsea pumps. The formulas below show the relationship between speed, torque, power and viscous losses. According to equation number I power is dependent on the speed and produced torque of the motor. The motor torque is defined by equation number II, which shows that the length and air-gap diameter of the motor are determining the torque produced by the motor. The air-gap diameter has an influence on the produced torque in the power of 2, hence the power according to equation number I can be increased extensively with only a minor increase in motor air-gap diameter (Note, when increasing the air-gap diameter, the outer diameter of the motor will also increase). However for a liquid filled motor, friction losses between the outer surface of the rotor and the liquid are defined according to equation number III. From this equation it can be seen that the viscous losses are increasing by the power of 4 when increasing the diameter, and by the power of 3 when increasing the rotational speed of the motor. In other words, the VRC power and torque can be increased significantly with a limited diameter increase, according to I and II. Therefore by increasing the rotor diameter by, for instance, 3, the speed can be reduced to 1/32, or 1/9, keeping the same output power. As a consequence, the viscous losses in the air gap will be reduced to a positive overall size effect due to the combined effect of a small diameter increase and significant reduction of the rotational speed. I. II. III.

𝑃=𝑇∙

2∙𝜋∙𝑛

P = Power [W], T = torque [Nm], n = rotational speed (rpm)

60

𝑇 = 𝑘1 ∙ 𝐷2 ∙ 𝐿

𝑃𝑣 =

1

32

𝑘2 ∙ (

2∙𝜋∙𝑛 3 ) 60

D = Diameter of air gap [mm], L = Active axial length of rotor [mm], k1 = a constant ∙ 𝐷4 ∙ 𝐿

Pv = viscous losses [W], k2 = a constant

With the right combination of parameters for these equations it is possible to design a VRC with a horizontal shaft design that can fit into a standard installation vessel 6.0 m x 6.0 m moon pool up to an 8 MVA rating. This is very compact, for instance compared to subsea variable speed drives (VSD) or pumps with the same power rating. The VRC unit can be gas-filled, typically for high power applications, such as when driving compressors, or liquid-filled for pumps and ESP applications with less power requirement. High power application requires a larger and faster spinning VRC, so in order to reduce the frictional losses in the machine, a nitrogen-filled design should be chosen. As this paper is focused on typical pump applications for short step-out from existing facilities, the gas-filled design will not be described further in this paper. The liquid filled VRC can operate at any depth as it is based on a pressure compensated design and hermetically sealed. Cooling of the unit is done by passive cooling through the enclosure wall or with an additional passive external cooler loop when needed. The internal liquid is circulated by a shaft impeller on the motor / generator shaft. The bearings are lubricated

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by the liquid in the VRC and no separate lubrication system is needed. The variable coupling in the VRC allows a constant frequency on the motor input, whilst the frequency and torque on the generator output can be controlled from 0-100 percent. This variable coupling may either be a hydrodynamic solution or a magnetic design. The magnetic coupling, which is based on permanent magnet technology, transmits power across an air gap, between the conductor rotor and the magnet rotor (ref. Figure 4), which means that the input and output shafts are completely disconnected from each other. The output speed is adjusted by varying the air gap between the conductor rotor and the magnet assembly rotor. The spacing is controlled by an actuator mechanism, which moves the magnet assembly in the axial direction away or towards the conductor rotor. The distance between the magnet assembly and the conductor rotor controls the strength of the magnetic coupling, which controls the slip and speed output of the coupling.

Figure 4: Magnetic Coupling (left) and Torque Converter (right)

The hydrodynamic coupling is liquid filled and regulates the output speed by restricting the liquid flow in the coupling with guide wanes, based on the Torque Converter (TC) principle (ref. Figure 4). The liquid level is constant in the TC Coupling, which makes it applicable for an oil-filled VRC. From the pump wheel (red in Figure 4) the oil flows to the turbine wheel (blue in Figure 4) through the guide vanes (green in Figure 4, ref. [2]) back to the pump wheel to complete the circuit. Note that “pump” in this context is not the load machine / pump, but the name of the turbine half sitting on the VRC motor side. Similarly the “turbine” sits on the generator side. By regulating the opening through the guide vanes, from 0 - 100 percent, the flow in the coupling is regulated, hence also the output speed and torque. The Torque Converter operates in a closed loop inside the VRC. It does not require a separate continuous supply of coupling oil through the umbilical (which is the case for a lubricator and barrier fluid oil system in a subsea pump). The liquid used in the coupling is the same as found elsewhere in the VRC, i.e. no separate liquid system for the hydraulic coupling. The liquid is circulated through the VRC motor and generator for cooling of the electrical machines and through the TC where it serves as working fluid, then to an external heat exchanger cooling coil in the same way as subsea pumps. In some cases, passive cooling will be sufficient. Since the rotational speed typically is 5 - 20 percent of what normally is used for pumps and compressors, degradation and pollution of the oil becomes significantly lower in case of an oil-filled design. An alternative to the oil is a water / glycol mix. By implementing the variable coupling, the VRC essentially works like a subsea VSD, but it has the advantage that it contains fewer components than a typical subsea VSD. This is expected to have a positive impact on the reliability of the unit, which is of high importance subsea due to costly marine operations in cases of repair or replacement needs. Typical onshore / offshore torque converters have proven mean time between failure (MTBF) of 48 years or more [3], in configurations that are even more complex than what is required for a subsea VRC. It is also possible to adapt equal or different voltage on the motor and generator. The VRC motor can for instance have 11 kV on the motor and 4.16 kV on the generator, which could be a relevant design for subsea pumps or electric submersible pumps (ESPs), in order to eliminate the need for subsea transformers and topside VSD up to a 30 - 40 km step-out. This solution also requires minimal impact to the topside facility as the transmission voltage can be adapted to direct connection to the topside main busbar. A subsea VSD typically does not have high voltage (HV) power electronics above 7.2 kV rectifier input rating, and would require an additional transformer that would add weight and space requirements, as well as electrical interconnections which often is a large cost driver on the subsea equipment cost. The VRC also has the advantage that it never needs more HV connections than a subsea VSD, as it only has three phases into the motor and three phases out of the generator. And a subsea VSD normally needs HV connections from the input transformer to the power electronics rectifiers, whether the transformer is integrated or standalone. In addition a subsea VSD needs more control power and communication signal interconnections.

Business Drivers

The VRC product has the potential to simplify subsea boosting and processing power system architectures, and by this reduce total investment cost substantially. Figure 5 illustrates a conventional topside VSD based system for 4 pumps to the left, where multiple cables and a lot of topside equipment are needed. This also includes topside output switchgear, which are needed when multiple power cables are combined in a power umbilical. In case of maintenance or service on one VSD

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system the other systems can continue to run in a safe manner with this switchgear, as the induced voltage due to electromagnetic crosstalk is kept out of the system being serviced.

Figure 5: System simplification with subsea VRCs – no need for multiple subsea cables, and significant reduction in topside equipment

To the right hand side the architecture for the VRC based system is shown. Here there is no need for topside grounding switchgear, as only one cable is needed subsea (no electromagnetic crosstalk). Each VRC controls its own pump. With such a solution the total power system cost can be reduced by up to 50% and the need for topside equipment by up to 90%, or more, depending of number of subsea consumers. The offshore modification work scope and complexity is also significantly reduced. For a subsea tie back with 4 subsea pumps, the topside equipment weight (including PCM (power and control module) building with VSDs, HVAC, transformers and switchgear) can be reduced drastically, as the only equipment needed will be the topside pump control cabinets (if the main 11 kV feeder switchgear is in existing rooms onboard already). Arguably, the above comparison could have been made with subsea switchgear and a subsea VSD for each consumer. However, the choice is then between complex subsea power electronics (with input transformers) and rather robust low speed rotary machines. A further important feature about the VRC solution is that it is very compact. This means lower installation cost compared to larger alternatives, and lower maintenance risk due to lower complexity. Figure 6 shows the comparison of the existing 16 MVA subsea VSD design for 12.5 MW subsea compressors (similar to what is used in the Ormen Lange subsea compression pilot project) and a corresponding 16 MVA subsea VRC. As can be seen, the size and weight difference is significant; it is expected to be the same typical weight / size reduction potential of 50 - 70 percent for future subsea VSD solutions. This is explained by the fact that an electric motor (and corresponding generator in the VRC) simply represents the most compact power density that is possible when comparing subsea electric machines with subsea power electronics (due to the number of electronic components, insulating distances, cooling etc), and it also is explained by the fact that electric machines can be efficiently cooled by a simple impeller on the shaft (when needed). On the other hand, a subsea VSD (typically 90 to 95 percent efficiency) normally will have lower power losses compared with a VRC (typically 80 to 90 percent efficiency), but the effect of this on the total weight, size and cost difference is limited, although some more power is required for a VRC serving a given subsea load compared to the subsea VSD. This effect is however smaller with longer step-outs, as low frequency AC transmission can be used for the VRC, whilst current subsea VSD solutions require 50 or 60 Hz input. When using low frequency (10-30 Hz) the transmission losses are reduced, and at a certain step-out, this will outweigh the subsea VSD efficiency advantages at more moderate step-outs.

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Figure 6: Comparison of the existing Aker Solutions 16 MVA Subsea VSD Module and the corresponding 16 MVA Subsea VRC Module design

Prototype Development and Proof of Concept. In 2012 a proof of concept test program, funded by Aker Solutions, to qualify the small scale 70 kVA Rotary Converter to Technology Readiness Level (TRL) 4 was initiated (ref. API RP17N and [4]). TRL 4 means that the unit is tested in its right environment, as well as function and performance, as a prototype. The intention was to have a large enough machine to cover a kW rating that can be translated and scaled into a larger machine in the MW range, and thus have a solution that covers the MW range to TRL 2 (validated concept).

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Figure 7: 70 kVA Rotary Converter Proof of Concept test set-up

Figure 7 shows the test set-up with the RC prototype in a 12 m deep test pit (water filled during the test), where a 50 kW pump was used as a centrifugal boosting load. With this set-up it was possible to replicate similar operational conditions to those that occur for subsea ESPs and other pumps, for example during start-up, transient behavior (e.g., slug and corresponding fast load changes / interruptions), load trip, etc.

Figure 8: 70 kVA Rotary Converter voltage (left) and current (right) waveforms – VSD / 6.6kV RC motor side in the upper curves, 690V load machine / RC generator in the lower curves

These tests were very successful and exceeded expectations. Among key results, it was clearly demonstrated that the RC works like an efficient sine filter, both in dampening harmonics from the VSD, and also from load to source.

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This is shown in Figure 8, where the upper curves are for the RC motor side, and the lower curve is for RC generator side. The voltage ripples from the VSD are eliminated, which is advantageous for the load machine insulation system. Another important case tests normal start-up ramping with the VSD and power flow in the PM motor and PM generator, as well as speed on the load machine (ref. Figure 9). As can be observed from the start-up curve, the PM motor power first increases to overcome the breakaway torque, which in turn increases the power of the PM generator, and then the load pump starts a few seconds after being magnetized by the generator (at 77 seconds the machines are ramped down in two steps, and stops again after 2 min). In order to demonstrate the dynamic capabilities of the RC, transient tests with sudden load increase and sudden loss

Figure 9: RC and load pump start-up

of load has been tested by quickly operating a valve that controls the pump flow. An example with a 30 percent drop in load torque in less than 2 sec was run in order to simulate a multiphase pump that mainly pumps liquid, and suddenly receives a larger gas volume throughput. The results of this transient test, shown in Figure 10, clearly show that the stability in voltage and current are very good, stabilizing quickly after the transient event. There is only a 2 percent increase in the voltage during this transient event. Motor and generator phase current

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Ch19 | IT-04 | RC current, motor phase L1[01] Ch21 | IT-06 | RC current, motor phase L3[01]

RMS_U_Motor_L1-L2[01]

Ch20 | IT-05 | RC current, motor phase L2[01]

6020

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6000

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5980

4

[A]

2

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5940

-2 -4

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-8

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Ch16 | IT-01 | RC current, generator phase L1[01] Ch18 | IT-03 | RC current, generator phase L3[01]

RMS_U_Gen_L1-L2[01]

Ch17 | IT-02 | RC current, generator phase L2[01] 685

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[A]

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Figure 10: Load reduction in 2 sec from 100 % to 70 % - current (left) and voltage (right) stability subsea control system loss of 50% power

Time

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Area of Application 1 – Subsea VRC to eliminate topside Equipment. In this case study the target has been to identify how subsea FPSO main switchboard: 11kV / 50Hz VRCs can be used to simplify a system with 2 subsea pumping stations, with two 2.5 MW pumps each (total of 4 pumps). The main objective is to reduce the topside equipment, and free more space Topside and weight compared with the power & control module used today Subsea to house the power equipment, which weighs approx. 520 metric tons. Each pump is supplied from an individual topside VSD system (total of 4 systems) supplying power to the subsea pump station via a Power Umbilical power umbilical, one for each station. The FPSOs main switchboard 18km is at 11 kV / 50Hz, which means that the topside VSDs have both 2x 3x1x150mm2 step-down transformers, as well as step-up transformers to ensure 12/20(24)kV that the subsea motors receive their rated 6.6 kV. Step-out is 18 km, and there is a limit on maximum 240 mm2 power cable size. By introducing the subsea VRCs instead it is possible to go with 11kV directly from the main switchboard onboard and directly into the power umbilical (with 2x three power cables) to each station. Then a subsea transformer (not shown) can be used to step down the M M M M Variable voltage to 6.6kV for the VRC motor, or it is also possible to qualify / RotoConverter™ use an 11kV VRC motor to even eliminate the subsea transformers V V V V 4MVA / 2.5MW as well. The VRC in turn supplies the 2.5 MW pumps, as shown in 12kV / 6.6kV G G G G Figure 11. It would be possible to reduce the number of cables to each subsea station to only 3 (from 6), but that would require subsea switchgear, which has not been considered here, since there are Pump existing power cables installed. M M M M 2.5MW One of the main challenges with such a system is the direct on6.6kV line (DOL) starting of the subsea VRC motors (by using a circuit 60Hz / 3600rpm P P P P breaker combined with over-load protection for each subsea VRC), as this results in a voltage drop (also through the long cable) during Pump Station 1 Pump Station 2 the seconds of start-up progression, leading to reduced torque on the motor. If this torque becomes too low, the machine will not start. However, starting motors on topside platforms and FPSOs is Figure 11: Single line diagram of application area 1 commonly done, also for relatively large machines. As long as the power system is designed for it, and the minimum short-circuit power is sufficient, this is an efficient starting method. One key difference between the topside motors and a subsea VRC is that the VRC coupling can be inactive, so that the moment of inertia is significantly lower than if the generator and load machine was connected to the VRC motor shaft. Also, since there is no load on the VRC motor during this start-up, the starting becomes easier and faster. When the VRC motor reaches nominal speed, the generator and load machine can be started with the variable coupling, working as a soft-starter which does not have any impact on the topside voltage. Figure 12 shows a simulation model of the system with one of the two subsea pump stations (not relevant to simulate both stations as they are identical), with 2 subsea pumps supplied by individual subsea VRCs and feeders from topsides. In addition, a topside 5 MW pump is added into the analysis. This is included in the model to demonstrate that DOL starting of subsea VRCs is less stressful for the topside system than DOL of large topside machines. To have a conservative model, subsea transformers are also included, although 11kV motor (to be qualified, not available for subsea applications yet) is an aim for the VRC development. A DOL start-up sequence is run in Figure 13. The topside motor is included for comparison. As can be seen, all machines are easily started. The subsea VRC motors have a voltage drop of up to 50 % during this sequence, which means that the available torque is around 25 % of rated. As this is a horizontal machine, this is well above the required breakaway torque (where the generator is not connected). If enough torque at this step-out of 18 km was a challenge, the VRC motor can relatively easily be changed in dimensions to accommodate this. At this particular short-circuit power (500 MVA) the topside main switchboard voltage dip is only approx 5 %, which is well within the IEC 61000-2-4 requirements. The analytical results show that for short or moderate step-outs up to a few 10’s of kilometers, subsea VRCs at conventional input frequency of 50 or 60 Hz can be easily started, and system stability kept within design limits. This ensures a significant simplification of the power system architecture, eliminating all the topside equipment needed in conventional systems with topside VSDs. A total of 520 metric tons can be eliminated in this example, including 4 VSDs, 4 VSD input transformers and 4 step-up transformers, including auxiliary systems like HVAC etc. With such a minimized topside footprint, effectively only being the pump control cabinets left topside, a system with subsea VRCs can significantly reduce the total project development CAPEX. .

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Figure 12: Load flow for one pump station with 2 VRCs and 2 pumps (model includes topside motor for comparison of start-up)

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Figure 13: Start-up of Subsea VRC Motors and topside motor. Upper left: voltage on main switchboard (brown) and on VRCs (green, red). Upper right: motor currents. Lower left: speed. Lower right: motor power (MW and MVAr)

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Area of Application 2 – Subsea VRC for Long Step-out. A single line diagram of this VRC application study case is shown in Figure 14. Long step-out power transmission theory is explained in [1]. Topside/onshore reactive power compensators are optional equipment, as this depends upon cable length, transmission voltage level, and frequency. All subsea VRC motors are direct online started in a particular sequence, and the VRC motor runs in no-load when the variable coupling is adjusted for ramping up the generator and load motor. Figure 15(a) shows the start-up sequence, and Figure 15(b) shows the voltage drop at different locations during start-up. When the second VRC motor has reached nominal speed, the onshore tap-changer will trigger, and changes its position to the optimal voltage topside and subsea. Then the third and fourth VRC starts before the on-load tap changer adjusts the voltage again. The voltage drop during this start-up sequence is only 1 percent at the onshore terminal, and is up to 16 percent at the subsea circuit breaker main bus, both which are within IEC 61000-2-4. For the next simulation case it is assumed that three subsea VRC modules are running at nominal speed and power. The fourth VRC is started in idle mode (generator is in standstill). This case is considered as the most conservative case with respect to start-up. The detailed simulation results are presented in Figure 16. Note that some additional negligible slip / change in speed in the other loaded machines occur as a result of the start-up of the fourth motor due to the associated voltage drop.

Onshore/Topside supply: 22kV / 16.7Hz

50MVA 22/105kV 16.7Hz

Topside Subsea

STATCOM

Power Cable 300km 72.5/132(145)kV 3x1x300mm2 Transformer 40MVA 96/6.6kV 16.7Hz Circuit Breaker Unit

VRC – Variable RotoConverter™ 9MVA 6.6kV 20Hz in 0-6.6kV / 0-120Hz out Compressors 6.6kV 6.0MW 120Hz

6.6kV/16.7Hz

M

M

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Figure 14: Single line diagram of application area 2

(a)

(b)

Figure 15 Start-up scenario of 4x7.8MW RC motors, rotor speed (a) and system voltages (b)

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(a)

(b)

(c) (d) Figure 16: System voltages (a), rotor speed in VRC motors (b), electrical shaft torque (c) and active and reactive power balance

A contingency case in which all four full loaded machines suddenly simultaneously trip is simulated and shown in Figure 17. The voltage peak transient at the topside supply bus is approximately 1.19 p.u. in 1 ms. This result is also within the standard requirements for topside supply transient voltages.

(a)

(b)

Figure 17: Full load trip scenario, system voltages (a) and RC motor speed and torque variables (b) during loss of power

Detailed dynamic and transient power system analysis studies show that in systems with one cable from shore / topside to a subsea pump / compression station with 2 - 8 (or more) VRCs / boosting machines, the VRCs can be easily started even with up to 300 km cables (and longer). The VRC can have low frequency input for a long step-out, but it can also use 50 / 60Hz input for more moderate step-out

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distances (e.g., 10, 20 or 50 km, where the alternative may be subsea VSDs), as explained in the Application Area 1 example. In such cases low frequency AC transmission is not needed. The simulation models have been verified against 70 kVA Rotary Converter prototype testing, and the simulations and testing have thus proven the concept. The combined test and simulation results are particularly promising for the next development phases for a full scale prototype.

Conclusions In the current market conditions, characterized by uncertainty and volatile oil prices, the oil industry is focusing more and more on increasing hydrocarbons recovery from existing fields, at the same time minimizing related CAPEX and utilizing robust and reliable solutions. From this perspective, the Subsea Rotary Converter technology is a key enabler for subsea boosting projects targeting a maximized hydrocarbon recovery, like seabed pump systems, or down-hole and mudline ESP applications. Specifically for most brown-field subsea ESP projects, due to the overall low project CAPEX, it will be difficult to justify conventional subsea VSD equipment. Therefore a low cost alternative like the Subsea Variable Rotary Converter would significantly contribute in making a profitable business case. In addition, this technology provides significant topside weight and space savings, which are fundamental for scenarios with several subsea pumps or ESP powered from the same platform. It also removes the requirement for an expensive offshore heavy lifting campaign with all associated risks. Regarding long step-out applications, the Subsea Variable Rotary Converter technology can leverage low frequency AC transmission to extend the maximum reach-out and power transfer of subsea cables compared with existing technologies. This could allow putting in production fields that are currently not economically feasible. In this paper the current development status of the Subsea Variable Rotary Converter technology has been presented, together with case studies demonstrating the feasibility and benefits for both short and long step-out areas of application. The next step in this development is to qualify a full size Subsea Variable Rotary Converter covering ESP applications (typically up to 1.5 MW duty) and seabed pumps (in the range 2.5 MW to 4 MW shaft power), and to test this in a real pilot application. Such a subsea module can either be integrated into an existing subsea boosting project, or be integrated in a future ESP, pump or compressor delivery project.

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Acknowledgements The development of the subsea 70 kVA Rotary Converter prototype (2012-2014) has been carried out together with Smart Motor / Rolls Royce Marine (Trondheim, Norway), who have constructed the machine based on Aker Solutions’ design specifications, and supported proof of concept testing at Aker Solutions Test and Manufacturing Centre (Tranby, Norway). They have also participated in studies of an 8 MVA design of an RC and a VRC for subsea ESPs, other pumps, and compressors requiring up to 6 MW shaft power. VOITH (Crailsheim, Germany) has been involved in studies to implement the hydrodynamic coupling for this larger VRC, as well as to determine its interface with the motor and generator in one sealed subsea enclosure. Furthermore, SINTEF Energy Reseach (Trondheim, Norway) has supported via third party evaluations of the prototype, as well as long step-out power system design / performance verifications. TOTAL has been involved over the last year in various system and RC / VRC equipment studies, both for long step-out and for cases where the aim has been to eliminate topside equipment and reduce cost and complexity.

References [1]. [2]. [3]. [4].

OTC-25730-MS Rotary Converter for Long Step-out Subsea Power Supply, 2015 VOITH Torque Converter: http://voith.com/en/218_e_cr228_en_voith-torque-converter.pdf VOITH Vorecon reliability data: http://www.voith.com/en/press/press-releases-99_44597.html DNV-RP-A203 Recommended Practice, 2001, DNV-GL

Abbreviations AC CAPEX DOL ESP FPSO HV HVDC IM LFAC MTBF OPEX PCM PM PMSG PMSM RC RLC SC TC TRL VRC VSD

Alternating current Capital expenditures Direct on-line Electrical submersible pump Floating production, storage and off-loading vessel High voltage High voltage direct current Induction motor Low frequency AC Mean time between failures Operational expenditures Power and control module (container/building) Permanent magnet Permanent magnet synchronous generator Permanent magnet synchronous motor Rotary Converter(rotary converter) Resistance, inductance and capacitance (circuit) Scoop coupling Torque converter Technology readiness level (ref. DNV-GL) Variable Rotary Converter(variable rotary converter) Variable speed drive (based on power electronics)