AU2006270578A1

AU2006270578A1 - System for supplying power to a flowline heating circuit

Info

Publication number: AU2006270578A1
Application number: AU2006270578A
Authority: AU
Inventors: Ole Johan Bjerknes; Svein Olaf Klementsen; Truls Norman
Original assignee: Aker Kvaerner Engineering and Technology AS
Current assignee: Aker Solutions AS
Priority date: 2005-07-15
Filing date: 2006-07-07
Publication date: 2007-01-25
Also published as: WO2007011230A1

Description

WO 2007/011230 PCT/N02006/000260 P2600PCOO POWER SUPPLY FIELD OF THE INVENTION 5 This invention is related in general to installations for oil and gas exploration and transport, for example on onshore and/or at subsea installations. In more detail the invention is related to a power supply system to be used with subsea installations, and to a power supply system for heating flowlines, possibly at subsea locations. 10 More specifically, the invention relates to a combined electrical power supply and flowline/pipeline heating system for a well flow line arrangement arranged in connection with a subsea installation. The flowline/pipeline heating allows for heating of the flowline in order to avoid or remove plugging effects in the flowline arising from the build-up of ice or hydrate plugs in the flowline 15 BACKGROUND OF THE INVENTION For a number of years work has been directed at finding methods of avoiding the effect of plugging in pipelines or flow lines due to the aggregation of condensed fluid material inside the pipeline by keeping the temperature of flowlines above critical temperature 20 values by means of electrical heating. A flowline heating method has been qualified and is being used on several fields in the North Sea [1]. Such a method requires supplying considerable power for the heating process to operate. Technically, there is thus a problem of supplying electric energy at increasing lengths of subsea flowlines in a manner which makes efficient use of the available electric power from a system point of 25 view. There exist several options using electric energy for heating of flowlines of which some are referred to as direct electrical heating (DEH), and some are referred to as skin-effect tracing systems (STS). Direct electrical heating of flowlines (DEH) is an alternative 30 method of heating the flowline which has been investigated recently, e.g. at SINTEF Energy Research. Such a heating method is based on applying an electrical current to a metallic conductor at the flowline, such as through the steel pipe itself. In such a system the steel pipe of the flowline will be part of an electric circuit. Electric current is WO 2007/011230 2 PCT/N02006/000260 supplied by en electric power generation and transmission system which supplies the current to the steel pipe using a cable based distribution network. An electric power generation and transmission system for supplying sufficient electric 5 power to electrically heat a flowline with a length in the range of several kilometers necessarily needs costly cables and high-power, high-voltage components. Typically, powers in the range of 1-10 MW would be required at a typical voltage of 5-50kV for such long flowlines, depending on the required W/m (Watt per meter). 10 In general three-phase systems are used to supply the levels of electric power required in such applications. The flowline circuit is conventionally coupled to a conventional three-phase power supply system using a an system of one or more transformers and high power electric cables and a symmetry control/adjustment network, the symmetry control/adjustment network being required to ensure that the load as seen by the three 15 phase power supply is symmetrical or almost symmetrical in all modes of operation in order to avoid unbalances disturbing the operation of equipment or components being supplied by the three-phase supply network. In the field of oil and gas exploration there are at present activity directed at developing 20 cost-effective subsea-solutions. In this field it is important that systems are simple to install and use, low on maintenance. And, of course, it is a primary economic goal to use technical solutions which provide an optimum cost-benefit relationship. From a system point of view it is also very often a requirement from a power generation 25 and transmission system to make available as much power as possible to electric devices arranged at the end of a flowline, typically being a part of a subsea well head. It is thus an objective of this invention to provide an improved, alternative flowline heating system which is simpler to install and less costly than present solutions, and 30 which may be expected to require less maintenance or adjustment during normal operation.

WO 2007/011230 PCT/N02006/000260 It is an additional objective of this invention to provide an alternative powers supply system for subsea installations, for example subsea installations including a flowline heating function. 5 It is yet an objective of this invention to provide an efficient system for supplying electric heating power along subsea flowlines connected to subsea installations, and to supply electric power to an electric load connected at a remote end of a long flowline. SUMMARY OF THE INVENTION 10 In order to meet the above objectives there is in a first aspect of the invention provided a system for providing electrical power to an electric load circuit, comprising a three-phase electrical power generation and power transmission part being coupled to an electric load. The invention is characteristic in that the three-phase generation and transmission part is connected to an electric load via a three-phase to two-phase 15 transformer, and the electric load is connected to the secondary side of said three-phase to two-phase transformer so as to form a substantially balanced electric load on said three-phase generation and transmission part. This power supply system is a fairly simple power system suitable for supplying high power to an electric load while demonstrating good load properties to a three-phase supply system. 20 In a preferable embodiment of the power system according to the first aspect of invention the three-phase power generation and transmission part is connected to a flowline/pipeline heating circuit forming the balanced electric load as well as to an end load located at the remote end of said system as seen from the power generation part. 25 Combining these two power supply requirements in one system helps to simplify the overall power supply system in oil and gas exploration arrangements utilizing long flowlines/pipelines to remote and often difficult to access locations. As an added benefit it is expected that the flowline/pipeline heating circuit (electric load) may be used to draw off reactive power developing along such a distributed power system, thereby 30 helping to optimize the available real power at the location of the end load. In a further preferable embodiment of the power system according to the first aspect of invention the three-phase to two-phase transformer is part of a subsea installation. The invention is particularly useful in subsea installations, as pipeline heating is often WO 2007/011230 PCT/N02006/000260 desirable to reduce or avoid plugging effects, and the invention also offers an alternative simple solution to provide electric power to an electric end load, for example components in an installation at the end of the pipeline. 5 In yet a preferable embodiment of the power system according to the first aspect of the invention the three-phase to two-phase transformer is part of an offshore topside or an onshore installation. In some applications it may be useful to locate the transformer at a topside or onshore location, in order to make it easier to replace or scale up or down the transformer being used. 10 In a yet further preferable embodiment of the power system according to the first aspect of the invention the electric load comprises a flowline heating circuit for enabling electric heating of the flowline, thereby reducing or preventing the plugging of the flow line by aggregation of condensed fluid material, as for example hydrates. The power 15 system according to the present invention is particularly useful for flowline heating, as adjacent, equal length sections of the flowline may easily form two components of the electric load which have equal or almost equal electrical impedance. In a still further preferable embodiment of the first aspect of the power system 20 according to the invention the end load comprises a subsea power distribution template for a subsea well head template. The end load may be a subsea power distribution template, whereby most of the power supply requirements for a subsea installation can be met by the present invention. Thus, additional power supply lines may not be required to the subsea installation, thus simplifying overall power supply system 25 requirements. In a second aspect of the present invention there is provided an electric power system for providing electrical power to a flowline heating circuit where a three-phase electrical power generation and power transmission system is coupled to the flowline heating 30 circuit, The power system is characteristic in that the three-phase generation and transmission system is connected to the flowline heating circuit via a three-phase to two-phase transformer, and the said flowline heating circuit is connected to the secondary side of said three-phase to two-phase transformer so as to form a balanced electric load on the three-phase system.

WO 2007/011230 5 PCT/N02006/000260 In a preferable embodiment of the power system according to the second aspect of the invention the said three-phase electrical power generation and power transmission system is also coupled to an end load, so as to also provide electric power to this end 5 load, whereby both the problem of flowline heating and the problem of supplying power to an installation at the end of a flowline are solved with the combined solution according to the present invention. In yet a preferable embodiment of the power system according to the second aspect of 10 the invention, the electric heating circuit comprises a part of the oil/gas transporting flowline. Typically, the flowline is made using electrically conducting materials, for example metals or combinations of metals, whereby a part of the flowline itself may be well suited as an electric heating element and/or an electrically conducting element of a flowline heating circuit. In addition an already installed flowline/pipeline may be well 15 suited for the addition of a heating circuit, provided the flowline/pipeline already is made, at least partly, from electrically conducting elements. In still a preferable embodiment of the power system according to the second aspect of the invention, the heating circuit includes a separate conductor mounted externally to 20 the oil/gas transporting part of the flowline. External conductors may be suitable for use on previously installed flowlines/pipelines where a heating circuit was not included at the planning stage of the installation. In yet a further preferable embodiment of the power system according to the second 25 aspect of the invention, the balanced electric load comprises two separate load elements of equal or near equal impedance. Normally, sections of flowline/pipeline of near equal length can be anticipated to provide sections of substantially equal, or nearly the same impedance characteristics. 30 In yet a further preferable embodiment of the power system according to the second aspect of the invention the end load forms a local power supply in or via a subsea power distribution template. Thus, the end load could be a variety of components or units connected at the template and where the termination of the power supply system (end load) effectively forms a power supply, for example for a subsea installation.

WO 2007/011230 6 PCT/N02006/000260 In a still further preferable embodiment of the power system according to a second aspect of the invention the local power supply is connected to any one or more devices from a group of standard electric subsea devices, said group of devices including 5 subsea control systems, electric compressors, electric pumps, water pumps, frequency converters, AC-motor drives, etc. The list is not intended to be exhaustive, but to illustrate a number of typical units which could be powered in the power system according to the present invention. 10 In a still preferable embodiment of the power system according to the second aspect of the invention the heating circuits are arranged so as to draw reactive power from the three-phase power supply in order to optimize the available electric power at the end load. Thus the distributed loading of the pipeline/flowline heating circuit is well suited to burn off reactive power, whose build up is often a problem along extended distributed 15 power systems. Thereby, the overall power characteristics of the system, in particular available real power for the end load, can be optimized by controlling the amount of power drawn by the heating circuits. In order to explain in more detail how the objectives stated above and other advantages 20 of the invention are achieved the following detailed description includes references to the appended drawings, in which BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates schematically the main components of a flowline heating 25 system according to the invention. Figure 2 shows a more detailed diagram of a first embodiment of a flowline heating system according to the invention. Figure 3 shows a more detailed diagram of a second embodiment of a flowline heating system according to the invention. 30 Figure 4A-C illustrates diagrammatically a voltage vector and phase diagrams of the voltage applied to the flowline in a system as shown on Figure 3. The total voltage between electrical end connections on the flowline is illustrated. Figure 5A-C illustrates three alternative ways of utilizing a three-to-two phase transformer in a pipe-line heating system according to the invention WO 2007/011230 PCT/N02006/000260 wherein the two-phase transformer windings are coupled in a serial manner to provide a sum voltage C being applied only to the end points of the flowline to be heated. Figure 6A-B illustrates an example embodiment of the invention where two three-to 5 two phase transformers are used, Figure 6A illustrating a transformer coupling providing two outputs, and Figure 6B showing two separate well stream pipeline elements, connected to the transformer output of Figure 6A. Figure 7A-B illustrates another embodiment of the invention where two three-to-two 10 phase transformers are connected together in a transformer coupling in Figure 7A, and Figure 7B shows how the transformer output can be connected to two consecutive sections of a well stream pipeline. Figur 8 illustrates an electric load connected to a subsea well head end of a well stream flowline electric power supply, the flowline being connected to an 15 oil and gas processing plant at the opposite end. DETAILED DESCRIPTION OF THE INVENTION These inventors realized that it is possible to solve the problem of electric heating of 20 flowlines in a manner which gives a system which is simpler and easier to maintain as compared with known prior art solutions, due to the fact that the new solution does not require a separate network for symmetry control or adjustment. In this description the term flowline is meant comprise several types of flowline such as e.g. a pipeline, a trunk line, an umbilical, or alternatively, combinations of these or equivalent flowlines or 25 flowline elements. These inventors also realized that it is possible to solve the problem of electric heating of flowlines and the problem of supplying electric power to a remote subsea flowline installation in a new and efficient manner with a combination solution as describes in 30 the invention as described herein. Due to the use of a special transformer coupling, the new solution does not require a separate network for symmetry control or adjustment. In this description the term flowline is meant to include several types of flowline such as e.g. a pipeline, a trunk line, an umbilical, or alternatively, combinations of these or equivalent flowlines or flowline elements.

WO 2007/011230 8 PCT/N02006/000260 This is achieved by combining a two-phase dedicated load interface with three-phase power generation and general distribution for the flowline heating. Use of three-to-two phase power conversion is very rarely used today, except in servomotor applications 5 [2]. One textbook states "The Scott connection is a special type of T-connection used to transform three-phase power to two-phase power for operation of electric furnaces and motors" [3]. As an example, most Japanese AC electrified trains today use a three-phase to two-phase transformation system in order to supply power to the electric motors [4]. 10 Three-to-two phase conversions with a balanced three phase draw may be obtained by using a Scott-T connection for the power feed system interface. The Scott connection was presented by Charles F. Scott in 1894. He pointed out that a combination of a three phase system for power transmission and a two-phase system for power distribution might give a total system securing advantages from both systems [5]. 15 This three-to-two phase transformation requires a balanced load on the two phase side in order for to transfer the load symmetry to the three phase side. Two-phase load balance can be obtained with two equal load elements or by interconnecting the two phase voltage vectors and then use the vector sum to interface a single phase load. 20 These inventors are aware that the Scott-T connection is still in use today in some electric railway systems and some electrically powered furnaces. Such solutions, the details of which are not all known to the present inventors, should probably be considered as the closest prior art as related to the present invention. In these known 25 applications two sets of low voltage elements in furnaces are a typical balanced two phase load configuration and electric railway systems are examples of a single-phase load configurations. Despite the fact that three-to-two phase power systems have not found very many 30 applications, these inventors have found that when using a using a three-to-two phase converter with power supply circuits intended to supply electric power to subsea installations as detailed in this description, and in particular to one or more flowline heating elements, a number of advantages are obtained. Transformers connected for three-phase to two-phase power conversion can make it sensible to use high WO 2007/011230 PCT/N02006/000260 9 transmission voltage from a three-phase power system even for supplying a distribution network that is unbalanced. The objectives of the invention are in principle achieved, as illustrated in Figure 1, by 5 connecting a three-phase power source 1 via a three-to-two phase power conversion unit 2 to a subsea located power consuming element 4, such as e.g. an electrically conducting part of an oil/gas flowline. Typically, a distribution cable 3 may be used to distribute the two-phase power to the subsea located electric load 4. 10 In Figure 2 there is shown a more detailed illustration of a first preferred embodiment of an electric flowline heating system according to the invention. In Figure 2 it is illustrated how a first conductor 5A of a power "piggy-back" cable 5A, 5B is connected to a first conductor 3A of the distribution cable 3. This first conductor section 5A runs to one first end the flowline 4 to be heated, at which end the conductor 5A is electrically 15 connected to the steel of the flowline 4. A second conductor 3B of the distribution cable 3 is connected to a second conductor 5B of the power "piggy-back" cable 5A, 5B. This second conductor 5B runs to a second end opposite said first end of the flowline 4 to be heated. At this opposite end the second conductor 5B is also connected to the steel of the flowline. 20 In Figure 2 it is illustrated how, in one embodiment of the invention, the three-to-two phase conversion unit can be a Scott-T connected transformer 2 [6]. The secondary side of the transformer 2B1, 2B2 is split in two serially connected windings, where one end of each winding together forms two outputs OUT1, OUT2, while a third optional output 25 OUT3 is taken from a point between the two secondary windings. One of the two outputs at the secondary side of the Scott-T connected transformer is connected to a conductor of the distribution cable 3 being connected to one end of the flowline 4 to be heated, and the other of the two outputs are connected to the conductor in the distribution cable 3 being connected to the opposite end of the flowline 4 to be heated. 30 Optionally, the third output may be connected to a third conductor 3C of the distribution cable 3. At the flowline 3 the third conductor 3C may be connected to the steel of the flowline roughly midway between the connection point of the first conductor 3A and the second conductor 3B on the flowline 4. At the primary side of the transformer 2 a three-phase power supply is shown to be connected in a delta system, however on the WO 2007/011230 10 PCT/N02006/000260 primary side a Y or Z connection, could also be used. As the third conductor during normal operation in a DEH-system, depending on the winding connection, might carry only a smaller current, this third conductor could have a reduced cross section, as compared to the other two conductors. 5 In Figure 3 there is illustrated a second embodiment of a flowline heating system according to the invention, where the primary side of the transformer 2 is delta connected in a conventional manner. Using a split construction, each of the three transformer secondary windings is divided in two, whereby access to a center point of 10 each winding is provided. The three secondary windings are thereby effectively split into six separate windings. In Figure 3 the six secondary windings are connected in a serial fashion. The center point of the center winding in the series connection, is taken as a third output of the transformer secondary. The three outputs of the transformer are connected to the flowline in the same manner as in Figure 2. 15 The transformer rating can be reduced if reactive power compensation capacitors are connected to the transformer secondary outputs, each capacitor being connected between the third output and the first and second outputs, respectively, if they all are available. Subsea installations should preferably be without capacitors installed subsea. 20 These inventors have realized that direct electric heating (DEH) of long flow-lines has a nature that is quite compatible with two-phase power systems where the phase angle is close to 90' or preferably more. This opens up for a renaissance for the Scott-T connection and other three-to-two phase transformer connections with more than 900 25 phase angle, as power generation and distribution since the 20t century in practice has been totally dominated by three-phase systems. With a three-to-two phase transformer in a flowline heating system according to the present invention it will be possible to supply a high transmission voltage to an 30 unbalanced distribution system from a three-phase supply by directly connecting the unbalanced system to the three-phase supply via e.g. a Scott-T transformer connection. Such a three-to-two phase transformer connection provides a two-phase DEH flowline interface suitable for voltage level control subsea along a flowline. Without the WO 2007/011230 11 PCT/N02006/000260 requirement for a topside symmetrization network the required space is reduced, weight is reduced and the utilization of the area on offshore surface installations is improved. If a divided flowline results in an unsymmetrical load, then some of the unsymmetrical part of the current can flow in the "neutral" phase from the midsection of the flowline. 5 The direct load balancing achieved using the three-to-two phase transformer in accordance with this invention, simplifies the total system design because it works without an LCR symmetrization network or symmetrization by power electronics. Hence, qualification of subsea installations is expected to be more easily achieved. 10 Traditional three-phase electrical power transmission through a relatively small cable to a suitable subsea location for electrical distribution to a direct electrical heating systems, a subsea pump, and/or compressor drives can reduce the number of cable runs along the flowline path and enable more efficient use of vessels for cable and flowline 15 installations subsea. Local subsea control voltage distribution via three-to-two phase transformers is expected to be compatible with most standard equipment normally installed on subsea templates. The normal redundancy level and interface can then be maintained for single 20 phase consumers with three-phase power transmission through umbilical(s) bridging the long distance back to the onshore or topside installation. The principle of the flowline heating system according to the present invention is thus particularly suitable for subsea power distribution over longer distances. A main 25 advantage versus existing alternative solutions is the simplicity of the solution and the elimination of the need for a symmetrization network for balancing large single phase loads in three phase power systems. Subsea installations of active inverters or LCR symmetrization networks may be found to require retuning after installation, which is a significant risk element as compared to the simple design of a power transformer. 30 Another subsea application of an electric power system according to the invention, using a three-to-two phase transformer according to the principle of the present invention, is in control voltage distribution in subsea installations, i.e. distribution of WO 2007/011230 12 PCT/N02006/000260 voltage to "Subsea Control Modules" from a "Subsea Control Distribution Unit" after a long three-phase transmission step-out. As an aid to understanding this invention, Figures 4A-C illustrate vector and phase 5 diagrams for the total voltage applied to the flowline 4, as a sum of the voltages from the two phase voltage output of the transformer in the embodiment of the invention illustrated on Figure 3. Figure 5A shows one example of how a three-to-two phase transformer 2 may be used 10 in a manner similar to a Scalene Scott connection [4] in direct heating circuit which generates a sum voltage C being applied between end points 4A,4B of the flowline 4 to be heated. The primary windings 2A1,2A2,2A3 are connected to the three phase supply lines Ll,L2,L3 in a conventional delta-connection, while the secondary windings 2B1,2B2,2B3,2B4,2B5,2B6 are connected in a series arrangement as illustrated to 15 generate the sum voltage C. In this case, the midpoint M is not utilized, making the output a two-conductor cable 3 to the pipeline. A first conductor 3A terminates in a first connection point 4A at one end of the pipeline segment to be heated. A second conductor 3B is connected to a piggy-back cable 5 running along the pipeline to a second connection point 4B some distance away from the first connection point 4A 20 along the pipeline. A reactance compensating capacitor C2 can be connected between the two output lines of the three-to-two phase transformer. Figure 5B and Figure 5C illustrates alternative three-to-two phase transformer configurations which could replace the one in Figure 5A. Hence, there is not only one 25 three-to-two phase transformer which will be applicable to this invention, but several configurations may be envisaged. Figure 6A illustrates how a double three-to-two phase transformer 2 may be connected to provide two voltage outputs, each output being connected to one pipeline segment to 30 be heated, as illustrated in more detail in Figure 6B. Figure 6B shows two separate well stream pipelines 4, the first pipeline having a direct electric heating circuit, 4, 4A,4B,5 being supplied with electric power via a pair of conductors 3A1,3B1 being connected to a first output of the two-phase side of the three-two phase transformer 2 as shown on Figure 6A. The second pipeline has a direct heating electric circuit 4,4A,4B,5 being WO 2007/011230 13 PCT/N02006/000260 supplied with electric power via a pair of conductors 3A2,3B2 being connected to a second output of the two-phase side of the three-two phase transformer 2 as illustrated on Figure 6A. The double three-to-two phase transformer output is by some textbooks classified as a four phase system. 5 Figure 7A and Figure 7B illustrate how a double three-to-two phase transformer 2-1,2 2 could be connected via a power supply cable 3 to two sections of a single well stream pipeline 4, using three contact points 7A,7B,7C on the pipeline 4, a first contact point 7A being coupled via said power supply cable 3 to a first output of the double three-to 10 two phase transformer, a second contact point 7B being coupled via said power supply cable 3 to a second output of the double three-to-two phase transformer 2-1,2-2. A third contact point 7C is common to the heating circuit of both of the two sections of the pipeline 4, and is connected to a common output of the three-to-two phase transformer 2-1,2-2. Typically, the first and second contact points 7A,7B will be at the end of a 15 section of the pipeline 4 and the third contact point 7C will be somewhere between the two end contact points 7A,7B, preferably roughly midway between the end contact points 7A,7B, particularly in the case of a pipeline 4 of longitudinally homogeneous cross section, as illustrated on Figure 7B. 20 A series arrangement of heating circuits along a flowline 4 is feasible, as illustrated in Figure 8. Three heating sections 41, 42 and 43 are connected separately to one each of three 3-to-2 phase transformers 21, 22 and 23, respectively. Each of the three 3-to-2 transformers are connected in parallel manner, possibly stepped out via three delta-star transformers 211, 212 and 213, from a main three-phase supply 30. In Figure 3 only three 25 heating circuits are shown for three consecutive sections along the flowline, however a flowline heating system could in principle comprise any number of heating sections. On Figure 8 a first heating section 41, is on an onshore part of the flowline, while the other heating sections 41, 42, etc. are at subsea locations. 30 In another aspect of the invention, illustrated by the example embodiment on Figure 8, the power supply system to the flowline heating also functions as a power system for transferring electric power to a subsea end load 30. These inventors have realized that a combined system for flowline heating 21,2 and end load 30 as described in this disclosure provides an additional beneficial combined effect in that the flowline heating WO 2007/011230 14 PCT/N02006/000260 may be used to draw off some reactive power along the power line extending to a subsea installation at the end of the pipeline thereby improving the available real power at the subsea installation. At the subsea installation, which for example is a subsea well head template, the three-phase power cable running along the well stream flowline is 5 connected to a local end load 30 which comprises a subsea power supply which can be wired up so as to supply power to a number of electric remote subsea devices, such as pre-compressors, condensate pumps, water pumps, etc. The electric subsea devices could be placed at the end of the pipeline or in principle anywhere else near the end of the pipeline and near the end of the three-phase power cable. The dominant electric load 10 of the subsea remote end installation 30 typically comprises frequency converters and/or AC motor drives. Due to this typical dominant load the local power supply should be of a standard three phase solution, whereas generation of local control voltage and/or further DEH on smaller tie-ins can be realized via three-two phase transformers connected together with other end consumers via a switchgear assembly, for example 15 such as has been proposed for the Ormen Lange installation [7], for typical remote end subsea power distribution to various electric drives, etc. It is an advantageous result that the pipeline heating circuit may draw off reactive power along the pipeline towards the remote end subsea power supply and/or installation 30, 20 thereby maximizing real power available at the remote subsea power supply and/or installation 30, in particular, the pipeline heating circuits may be provided with a controllable switching device 51, said switching device 51 being a part of a control system including functionality for turning these heating circuits on and off or for varying the power drawn by the heating circuits. Such a control system preferably 25 comprises a control unit 50 associated with the electric power source 1, control signal transmission devices for sending and/or receiving control signals or monitoring signals to local control circuits associated with each heating circuit 21-1, 21-2, and 21-3 and/or with the remote end subsea power supply and/or installation 30. As an example, the controllable switching device 51 could be a controllable step-by-step switch which 30 could be used to controllably regulate the amount of electric power consumed drawn by each pipeline heating circuit. In this case a control unit is coupled to each step-by-step switch. The control unit could be a central control unit located on-shore or it could be a distributed control unit, distributed between one or more control and/or computing devices cooperatively arranged in a distributed fashion in the power supply system.

WO 2007/011230 15 PCT/N02006/000260 In one embodiment of the invention use of three-to-four phase transformers as replacement for each pair of transformers 21-1,21-2 on Figure 8 could simplify the subsea layout by reducing the number of transformers used for the DEH along the 5 pipeline. On Figure 8 the two transformers 21-1,21-2 can for example be replaced by a single three-to-four phase transformer performing a three-to-four phase power transformation with magnetic core balance since the sum of all four-phases are zero. In a manner similar to that shown on Figure 7A and Figure 7B, it should also be 10 possible to use the two two-phase systems that can be derived from a four-phase system by using the four phase winding sets of the transformer connected to give two separate Scalene Scott connected outputs from the two winding sets each. These could be arranged similar to the Scalene Scott connections described for example in ref [4]. 15 There are thus a number of subsea systems which could benefit from the present invention. A list of some typical systems is as follows: e One single flowline with a DEH installation can be supplied with electric power from a power supply on an offshore installation. 20 e One single flowline with a DEH installation can be supplied from shore and or from another offshore installation via a subsea three-phase to two-phase transformer located at or adjacent the flowline section to be heated. * Electric cables can connect several subsea installations for DEH and other subsea control or operational purposes. A subsea grid of cables may have one or 25 two connections to mainland equipment for similar purposes. " Subsea installations, e.g. pumps and compressors, may be supplied from shore or from surface offshore installation(s). A power distribution grid may be connected to one template and continue to a next template. The power supply system may include connections to several templates. If a series of DEH 30 installations are distributed evenly along a very long flowline, then they could be used to draw off reactive power and hence enable ac-power to supply power far beyond the critical cable lengths for high voltage ac-power that normally limit power transmission via ac-cables.

WO 2007/011230 16 PCT/N02006/000260 * Control voltage power supply to a remote installation (typically subsea) where Using three-phase power transmission and two-phase local distribution to provide a control voltage power supply to single phase loads yields a simplified configuration which also is expected to give enhanced performance and 5 reliability in such systems. To conclude, the present invention provides a flowline heating system which gives a balanced load on the three-phase power supply. Further, the need for a complex symmetrization system to meet the requirements from the local or mainland power grid 10 is eliminated. This is especially important when power is supplied from a relatively small power grid with local generation, as for an offshore oil & gas platform. Also the use of a three-to-two phase transformer provides the added benefit that a suitable voltage or combination of voltage vectors is provided which enables voltage 15 control for selected parts of the load, i.e. for DEH these could be the different sections of the flowline. The subsea located power consumer (electric load) 4 may comprise one or more temporarily or permanently installed components. The power generation and 20 transmission part 1 may in the same manner comprise one or more permanently or temporarily installed components. For example, a surface vessel adapted for handling cables and other equipment for supplying electric power could be used together with a riser cable, for example as described in International Patent Application PCT/NOOO/000 177, as a power generation and transmission part. However any other 25 power supply equipment designed for supplying power to subsea devices known to a person skilled in the art could in principle be used as the power generation and transmission part 1 of this invention. Although this description has referred to the DEH arrangement any person skilled in the 30 art will understand, having benefit from this description, that the subsea electric load circuit 30 or flowline heating circuit may take on a number of forms and variations without departing from the scope of the invention as defined in the appended claims.

WO 2007/011230 17 PCT/N02006/000260 REFERENCES: [1] "Integrated Production Umbilical (IPUTM): Heated Flowline Technology for Satellite Tie-Back at Norne ", paper DOT 2002, by 0. Heggdal and E. 5 Ulland, Deep Offshore Technology Conference for Deep Water Oil Exploration and Drilling, November 13-15 2002, New Orleans, Louisiana, USA. [2] "Electrical Power Technology", by Theodore Wildi, published by John 10 Wiley & Sons, New York, 1981, pages 236-7. [3] The Electrical Engineering Handbook, Ed. Richard C. Dorf, Chapter 1 Passive Components, Section 1.3 Transformers, by M. Pecht, P. Lall, G. Ballou, C. Sankaran, and N. Angelopoulos; Published: CRC Press LLC, 15 2000, Boca Raton. [4] Principle Theory of Single Phase Feeding Power Conditioner for AC traction, by Tetsuo Uzuka, Yoshifumi Mochinaga, Shin-ichi Hase; http://www.rtri.or.jp/infoce/wcrr97/C135/C135.html 20 [5] Charles F Scott 1864-1944; IEEE History Center, Legacies; http://www.ieee.org/organizations/history center/legacies/scott.html. [6] Scott Transformer (Product Data from L/C Magnetics of Anaheim, CA, 25 USA); www.lcmagnetics.com/scott.htm [7] Subsea Feasibility Studies (Ormen Lange included), presented at the IAS Chapter Meeeting (IEEE, Norway Section, Industry Applications Society Chapter), Stavanger Norway, April 19, 2005. 30

Claims

1. System for providing electrical power to an electric load circuit, comprising a three-phase electrical power generation and power transmission part (1) being coupled 5 to an electric load (4), c h a r a c t e r i z e d i n t h a t the three-phase generation and transmission part (1) is connected to an electric load (4) via a three-phase to two-phase transformer (2), and the electric load (4) is connected to the secondary side of said three-phase to two-phase transformer (2) so as to form a substantially balanced electric load on said three-phase 10 generation and transmission part (1).

2. System according to claim 1, wherein said three-phase power generation and transmission part (1) is connected to an end load (30) located at the remote end of said system (1) as seen from the power generation part (1). 15

3. System according to claim 1, wherein the three-phase to two-phase transformer is part of a subsea installation.

4. System according to claim 1, wherein the three-phase to two-phase transformer is 20 part of an offshore topside or an onshore installation.

5. System according to claim 1, wherein the electric load (4) comprises a flowline heating circuit (21-1, 2-1,4-1) for enabling electric heating of the flowline, thereby reducing or preventing the plugging of the flow line by aggregation of condensed fluid 25 material, as for example hydrates.

6. System according to claim 1, wherein the end load (30) comprises a subsea power distribution template for a subsea well head template. 30 7. Electric power system for providing electrical power to a flowline heating circuit (21 1,21-2,21-3, 2-1,2-2,2-3, 4-1,4-2,4-3), where a three-phase electrical power generation and power transmission system (1) is coupled to the flowline heating circuit (21-1, 21-2,

21-3, 2-1,2-2,2-3, 4-1,4-2,4-3), WO 2007/011230 19 PCT/N02006/000260 c h a r a c t e r i z e d i n t h a t the three-phase generation and transmission system (1) is connected to the flowline heating circuit (21-1, 21-2,21-3, 2-1,2-2,2-3, 4-1,4-2,4 3) via a three-phase to two-phase transformer (2), and the said flowline heating circuit (21-1, 21-2,21-3,2-1,2-2,2-3, 4-1,4-2,4-3) is connected to the secondary side of said 5 three-phase to two-phase transformer (2) so as to form a balanced electric load on the three-phase system. 8. Power system according to claim 7, wherein said three-phase electrical power generation and power transmission system (1) is also coupled to an end load (30), so as 10 to also provide electric power to said end load (30). 9. Power system according to claim 7, wherein the electric heating circuit comprises a part of the oil/gas transporting flowline (4-1,4-2,4-3). 15 10. Power system according to claim 7, wherein the heating circuit includes a separate conductor mounted externally to the oil/gas transporting part of the flowline (4-1,4-2,4 3). 11. Power system according to any of the claims 7-10, wherein the balanced electric 20 load comprises two separate load elements of equal or near equal impedance. 12. Power system according to any of the claims 7-10, wherein the end load (30) forms a local power supply in or via a subsea power distribution template. 25 13. Power system according to any of the claims 7-12, where the local power supply is connected to any one or more devices from a group of standard electric subsea devices, said group of devices including electric compressors, electric pumps, water pumps, frequency converters, AC-motor drives, etc. 30 14. System according to any of the claims 7-13, wherein the heating circuits (4-1,4-2,4 3) are arranged so as to draw reactive power from the three-phase power supply in order to optimize the available electric power at the end load (30).