Field of the invention

The present invention relates to equipment located subsea far away from onshore or topside locations, particularly equipment for petroleum fluid pressure boosting requiring high alternating current electric power level or high direct current electric power level, but also other subsea equipment of various types. 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 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 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. 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 201 1 1233 and PCT/EP201 1/065797, 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 lower, and step-up to the actual operating frequency close to the subsea equipment. The technology of NO 201 1 1233 and PCT/EP201 1/065797 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 subsea step out length is about 150 km.

As discussed in NO 201 1 1233 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, 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. 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 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.

It is well known that long distance subsea power transmission could be solved by DC current because the problems of instability caused by the Ferranti effect, resonance etc., do not exist for DC transmission. For DC transmission the ohmic losses can be compensated by increased voltage and increasing conductor cross section area, since in the case of DC transmission, the whole conductor cross section area is utilised because there is no skin effect like for AC transmission, and increasing voltage decreases amperage. However, long DC subsea step out for high power level subsea loads is currently not a possible option because there is no existing equipment to use the high voltage DC power for loads of different kinds subsea, be it AC compressors or pumps rated at many MW power level or other high power level subsea loads. Currently, high power high voltage DC can not be used directly for DC motors at the voltage level required for step out transmission to high power subsea loads. Currently, subsea power electronic equipment that is reliable and able to transform the HVDC (High Voltage DC) down to usable AC or DC power characteristics do not exist. The introduction of solid state power electronic components has reduced the cost and improved reliability for power electronic systems dramatically, and it is an accepted truth of people skilled in the art that further improvements, also for subsea systems, will be based on improvements of the semiconductor solid state based power electronic components and systems.

In fact, the applicant and other major subsea technology developers have worked for over 20 years for developing feasible technology for long high voltage DC transmission, particularly for developing a HVDC (High Voltage Direct Currect) transformer for use subsea. Even though it seems achievable, the size and complexity of the design, based on power electronics, becomes a limiting factor. More specifically, the power electronics arrangement for a large, remote subsea field, with requirement for compression and possibly pumping, can become the size of a football field and the cost may be at a level making the subsea field development questionable from an economic point of view. But most critical, the reliability in practice becomes too low for the solution to be feasible because of the sheer complexity and size enhancing unreliability up to levels that can not be accepted. After more than 20 years of extensive research and development, and after investing millions in development, there is still no usable solution for HVDC subsea step out.

Several existing subsea petroleum fields, and many not yet found, are located more than 40 km from shore or platforms, some of which 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 40 km, preferably more than 150 km, such as 600 km, and the objective of the invention is to meet said demand. Summary of the invention

The invention provides a system for operation of electric power subsea loads supplied through a subsea step out cable, particularly high electric power subsea loads, distinctive in that the system comprises a high voltage direct current (HVDC) transmission subsea step out cable, the cable is connected to a HVDC source in a near end and to a HVDC motor in a far subsea end.

The loads comprises high electric power loads, as defined below, and the step out length can be or is over 40 km. The HVDC motor is a part of a subsea motor generator set or is coupled to a subsea pump or a subsea compressor.

HVDC is in this context DC voltage above 2 kV and high electric power loads are in this context loads of at least 1 MW maximum effect, such as effect in the range 5-25 MW. The source is usually a topside or onshore source, however, the source can also be a subsea source. The far end connected HVDC motor can be rated at lower voltage than the HVDC source since losses will occur and start up can be arranged always to be soft, controlled from the near end for example by an adjustable resistor. Preferably, the HVDC step out cable and HVDC subsea motor of the system of the invention, are rated to operate at a maximum voltage of 2-250 kV, such as 20-150 kV, at 1 , MW, 5, MW, 10 MW, 20 MW, 40 MW or even higher effect, and the subsea step out distance can be above 40 km, such as 50, 100, 150, 200, 400, 600 , 800 or 1600 km or above. Prior art AC power supply is limited to about 40 km subsea step out length at operation frequency for high power loads such as subsea compressors and pumps. Accordingly, the present invention is particularly relevant for subsea step out lengths above 40 km, for which no publicly known solution exist without a subsea VSD, and even more relevant for subsea step out lengths above 150 km where solutions based on subsea VSD are questionable.

The HVDC motor is preferably connected to a subsea compressor or pump, directly on the same shaft or by a coupling or a gear coupling such as a step up device, preferably a magnetic step up gear, or the HVDC motor is a part of a motor-generator set connected to a further load such as an AC subsea compressor or pump motor.

In a preferable embodiment the HVDC motor is part of a motor-generator set comprising a common shaft, for example an AC generator producing 6.6 kV or 1 1 kV power with a frequency of 150 Hz, the produced voltage from the generator is the voltage required by the electric loads (AC motors, UPS, control systems) that the generator supplies. Alternatively the HVDC motor is part of a DC motor-generator set.

Preferably the HVDC motor comprises stator windings with insulated high voltage cables instead of the traditional stator bars. This allows increasing the voltage to HVDC level that is above 2 kV, such as 10 kV, more preferably 15 kV, even more preferably above 20 kV such as 120 kV. Currently it is available HVAC motors with insulated windings for power level up to at least 150 kV, i.e. Motorformer of ABB. Even though it is challenging, there is no technical limitations for modifying DC motors to HVDC motors by using insulated windings as a general concept, and particularly by using XLPE insulated HVDC cable windings on the stator.

The load is preferably arranged in a gas filled pressure housing or a pressure compensated oil filled housing. Loads with active magnetic bearings in a gas filled pressure housing provides high efficiency, and represents a preferable embodiment.

In a preferable embodiment the system comprises a control at a near end topsides or onshore. This can have many embodiments, of which the simplest is a variable resistor device useful for controlling the speed of the connected loads at the near end from topsides or onshore.

In addition or alternatively the system of the invention comprises a local control device at each subsea load. The combined high voltage DC motor and an AC or DC generator on the same shaft is one such local control device, in the context of this invention called "DC Transformer". The speed of the DC motor can be regulated by known methods (ref. NO 201 1 1235), and because the generator speed will vary accordingly and by doing this the subsea DC Transformer can also have the function similar to that of a subsea VSD. If one DC Transformer is arranged per compressor motor, the speed of each compressor can be varied individually. A DC transformer with DC generator can be used to transform the DC voltage level down to a lower level more feasible for a further DC load, providing higher amperage, and similarly for an AC load if the generator is an AC generator. In a preferable embodiment the system of the invention comprises a single or two or more in substance parallel HVDC subsea step out cables, each cable is connected to one or a number of loads, the cables and loads are dimensioned so that at the maximum workload of the loads the far end cable voltage equals the maximum voltage level allowable for the loads whilst the near cable end voltages equal the far cable end voltage plus ohmic loss in the respective cables and in the near end the respective cables comprises a means for voltage control, such as an adjustable resistor or other known devices. Thereby, DC loads of lower voltage rating than the HVDC subsea step out cable can be used. Starting from 0 or low voltage, the near end voltage of the source can be adjusted up to higher voltage than allowable for the loads when operating the HVDC motor loads at full or high speed. If one or more loads are not in operation, the voltage can be adjusted down at the near cable end so as not to exceed the maximum allowable DC motor voltage at the far end of the step out cable. The above-mentioned operational method steps represent an

embodiment of the present invention.

Preferably, the system comprises one or more of the loads: a subsea

compressor, a subsea multiphase pump, a subsea pump, a subsea control system, a subsea heat tracing system, a subsea valve actuator, a subsea processing facility, a subsea uninterruptible power supply, a subsea DC transformer and a subsea inverter.

The invention also provides a method of arranging a system for operation of high electric power subsea loads, distinctive by arranging a subsea electric HVDC power step out cable, connecting a HVDC source at a near end and a subsea HVDC motor at a far subsea end. The method preferably comprises to operate the HVDC step out cable and HVDC subsea motor of the system of the invention, up to a maximum voltage of 2-250 kV, such as 20-150 kV, at 40 MW or higher effect, at a subsea step out distance that can have length above 40 km, such as 50, 100, 150, 200, 400, 600, 800 or 1600 km. The HVDC motor is connected to a subsea compressor or pump, directly on the same shaft or by a gear coupling such as a step up device, or the HVDC motor is a part of a motor- generator set connected to a further load such as an AC subsea compressor or pump motor.

The invention also provides use of a system of the invention, for operating high power level subsea loads at subsea step out distances that can be longer than 40 km of length.

Figures

The invention is illustrated with two figures, namely

Figure 1 , illustrating a system of the invention, and

Figure 2, illustrating another system of the invention.

Detailed description

Reference is made to Figure 1 , illustrating an embodiment of a system 4 of the invention, more specifically a surface HVDC source 1 , a subsea HVDC step out cable 2 of 50 kV, at power level of 40 MW or higher, the cable subsea step out length is above 150 km. The cable is connected directly to four subsea loads 3 at the cable far end. The system 4 in the illustrated embodiment comprises the HVDC source 1 , the HVDC subsea step out cable 2 and the loads 3 which all includes HVDC motors connected to the step out cable. The loads are DC high voltage compressor motors, each of 10 MW power level. 6,6 kV voltage, all motors are controllable as a group from the near end by a variable resistor or other known means, such as an electronic speed controller.

If there is only one DC motor, the speed of the motor is conveniently controlled up and down by varying the DC voltage from surface up and down. If there are more motors, the speed level of all motors can be varied by varying the surface voltage, and additionally the speed of each motor can be controlled by local individual speed control of each motor by known method, e.g. voltage control to the individual motor by variable resistor, rheostats, or electronic speed control, by known methods for parallel, series or compound DC motors.

The high voltage DC motors can be of the known traditional type, but modified with insulated stator windings, such as stators wound with high voltage DC cable with XLPE (cross linked polyethylene) insulation or other feasible insulation, and optional additional cooling, and optionally the motors can have permanent magnet rotor.

Figure 2 illustrates another embodiment of a system 4 of the invention, with loads 5 as four DC transformers. In other words, for the illustrated embodiment, for each load a HVDC motor drives a 6,6 kV AC generator arranged on the same shaft, the AC generator is connected to an AC compressor motor 6 of 10 MW and 6,6 kV.

The invention makes it possible to transmit electric power over very long distances. There will in principle not be any technical limitation of stable subsea power transmission length, however in practice a possible limitation may be a practical or economical copper cross section area for control of ohmic power loss. With a system of the invention, the source can be a HVDC source in northern Norway and the loads can be subsea compressors and pumps arranged subsea at the seabed at the North Pole, under the arctic ice cap, supplied by a HVDC cable connected to the source. The system of the invention may include any feature as described or illustrated in this document, in any operative combination, each operative combination is an embodiment of the invention. The methods of the invention may include any feature or step as described or illustrated in this document, in any operative combination, each operative combination is an embodiment of the invention.