The present invention relates to a system for electric heating of a pipeline and a method therefor.

Offshore oil and gas production is being pushed both into deeper waters, and pushed to extend further from or between operational facilities. As pipelines and flowlines used in such situations get longer and/or colder, the risk of hydrate plugging and wax formation increases, from an already known problem to expected greater problems.

The formation of hydrates and waxes in oil and gas pipelines can block such pipelines. Conventional methods of remediating the problem of hydrate or wax plugs in a pipeline can involve the use of chemical agents, blowdowns and pigging operations, all of which require at least temporary downtime or non-operation of the pipeline (for the transfer of oil and gas, etc.), along with associated costs for the cleaning operation.

An alternative form of remediation is direct electric heating of the pipeline, whereby a current is passed down the pipeline. Heat is produced by the electrical power loss from the current flowing through the pipeline, caused by a number of effects, including resistance and possibly eddy currents.

GB 2341442 A discloses a pipeline heating system in which a portion of the pipeline acts as a heating element, having a feeder cable and a return cable at opposite ends of the pipeline to provide the direct impedance heating. However, the feeder cable is outside the whole length of the pipeline portion, which requires separate installation, insulation and maintenance.

US 6,142,707 shows a system for direct electric heating of a pipe-in-pipe flow line having an outer carrier pipe and a concentric inner pipe, such that power passes along inner pipe and back along the outer pipe. However, even with cover around the outer pipe, a significant portion such as 40- 60% of the power supplied to the pipeline is still dissipated to the surrounding seawater, and cannot therefore be effective for heating of fluids transported by the pipeline.

It is an object of the present invention to provide an improved system for electrically heating a pipeline section.

Thus, according to one aspect of the present invention, there is provided a system for electric heating of at least one pipeline section comprising:

a first pipe-in-pipe flowline comprising a first electrically conductive inner pipe section: a second pipe-in-pipe flowline comprising a second electrically conductive inner pipe section; at least one electrical current pathway between the first and second inner pipes sections; a power input connected to the first inner pipe section; and a current return connected to the second inner pipe section to receive current from the power input, first inner pipe section and electrical pathway. In this way, there is no electrical current flowing along an outer pipe that would dissipate power to the surrounding seawater. The present invention therefore significantly increases the efficiency of the electrical heating of the pipeline section.

Pipe-in-pipe flowlines are well known in the art, and generally comprise an outer pipe and at least one inner pipe. Such flowlines are used to transport or convey fluids such as hydrocarbons, in particular oil and gas, between two or more static and/or moveable locations. Such flowlines are generally wholly or substantially under water or marine pipelines, and are also commonly termed "offshore" pipelines. Such flowlines may convey fluids between vessels, and/or be located at or near a seabed for the transportation of fluid from locations such as an oil head, in particular a remote oil head, to an underwater facility and/or to a riser towards sea level, and/or directly to an onshore facility.

Optionally, the system of the present invention can provide electric heating of one or more inner pipe sections of the first pipe-in-pipe flowline and one or more inner pipe sections of the second pipe-in-pipe flowline.

The first and/or second pipe-in-pipe flowline may comprise one or more electrically conductive inner pipe sections. The or each electrically conducive inner pipe section may extend any length along the first and/or second pipe-in-pipe flowline, such as partly, substantially or wholly, between one end and the other end of the first and/or second flowlines. This may be in particular where the first and second flowlines are concurrent, neighbouring or otherwise commonly located or laid.

The first electrically conductive inner pipe section and the second electrically inner pipe section may or may not extend the same distance; may or may not be parallel; and/or may or may not start and/or end at different locations along the first and second flowlines respectively.

Preferably, the first and second electrically conductive inner pipe sections extend longitudinally wholly or substantially together, generally in parallel.

Also preferably, the first and second electrically conductive inner pipe sections are the same or similar in relation to their construction and characteristics. That is, the first and second electrically conductive inner pipe sections are equivalent or corresponding pipe sections. This includes physical characteristics such as their weight, wall thickness, diameter, and other characteristics such as electrical resistance, etc., such that there is a uniform level of current through the first and second electrically conductive inner pipe sections, in particular to avoid overheating or underheating of one inner pipe section compared to the other inner pipe section.

The first and second pipe-in-pipe flowlines may also have the same or similar characteristics.

The system of the present invention can provide for electric heating of a pipeline section along any part or length of at least the first pipe-in-pipe flowline. Optionally, the system of the present invention can provide for electric heating of a pipeline section along any part or length of the first pipe-in-pipe flowline and any part or length of the second pipe-in-pipe flowline. Indeed, the system of the present invention can provide electric heating to a pipeline section or pipeline sections as one or more parts along the length of the first pipe-in-pipe flowline, and/or to a pipeline section or pipeline sections as one or more parts along the length of the second pipe-in-pipe flowline, or any single or multiple shorter sections thereof. The first and second pipe-in-pipe flowlines may comprise one or more electrically conductive inner pipe sections not intended to be electrically heated, and/or one or more non-electrically conductive inner pipe sections.

Preferably, there is at least some insulation between the inner pipe sections and the outer pipes of the first and second pipe-in-pipe flowlines, usually in the annular gap(s) thereinbetween, to prevent and/or minimize the formation of electrical discharges between the inner pipe sections and the outer pipes of such pipelines.

Thus, in one embodiment of the present invention, electrical insulation is located around the first and/or second inner pipe sections, preferably both sections. The electrical insulation may comprise any suitable type of electrical insulation known in the art, generally intended to eliminate and/or minimise dissipation of the electrical current passing through the relevant part of the first and second inner pipe sections. The electrical insulation may comprise one or more coats and/or liners and/or coatings, and/or one or more thicker layers.

The system may further include thermal insulation around the first and/or second inner pipe sections, preferably both. Suitable thermal insulation includes various foams, in particular, low density foams known in the art.

In a preferred embodiment of the present invention, the electrical insulation wholly or substantially also comprises the thermal insulation.

According to another embodiment of the present invention, the first and second flowlines comprise one or more power connectors able to provide an electrical pathway from outside the flowline to the inner pipe sections. Such power connectors may provide an electrical pathway for the connection of the power input to the first inner pipe section, and/or for the current return connect to the second inner pipe section, and/or to provide the at least one electrical current pathway between the first and second inner pipe sections.

In one embodiment of the present invention, the first flowline and/or second flowline comprise two or more power connectors. In this way, it may be possible to vary the location and/or inner pipe section lengths for the electric heating. For example, the first flowline and/or second flowline may comprise a series of power connectors, (said series being regularly, periodically or irregularly distributed along the length of the flowline), each able to provide an electrical pathway to different locations along the length of at least the first flowline so as to electrically heat a different section and/or section length of the first flowline.

In a preferred embodiment, the power connectors are spaced approximately 1 km apart.

In another embodiment, the power connectors can be provided and/or installed during rigid pipeline fabrication for a reeled pipeline. For example, the power connectors could be installed at each end of pre-assembled pipe sections of 1 km length prior to their welding and their spooling on to a storage reel of a pipelay vessel.

In a further embodiment of the present invention, the first flowline comprises at least two or at least three power connectors, and the second flowline comprises at least two or at least three power connectors, said power connectors being wholly or substantially opposite each other to create a parallel electric heating arrangement in the two flowlines.

The system of the present invention may also comprise two or more electrical current pathways between the first and second inner pipe sections.

The first flowline and/or second flowline may be wholly or substantially continuously electrically conductive, and/or include one or more electrically conductive breaks such as by the use of one or more breaks in the inner pipes and/or the use of one or more intervening insulators or isolators.

According to another embodiment of the present invention, current from the power input passes unidirectionally along the first inner pipe section. Such unidirectional current flow may be due to the power input being provided at the beginning of the first flowline, and/or by the use of one or more breaks and/or insulators in the first flowline able to provide a defined electrically conductive start for the first inner pipe section.

Similarly, the system may provide a unidirectional electrical current pathway along the second inner pipe section to the current return, possibly due to the current return being provided at the end of the second flowline, and/or by the use of one or more breaks and/or insulators in the second flowline able to provide a defined electrically conductive end for the second inner pipe section.

Alternatively, the system of the present invention allows current from the power input to pass bidirectionally along the first inner pipe section, and/or bidirectionally along the second inner pipe section. The power input of the system is able to provide power to the first inner pipe section. Such power could be provided from a suitable power source. The power source may be any suitable apparatus, unit or device, such as an electrical generator or similar known in the art. Electric heating of pipelines can be provided both by AC and DC sources, although AC power is generally more advantageous for transmitting power along a longer pathway. Electric power is generally provided at a high voltage such as several kilovolts and commonly at a high current.

The current return of the system is able to receive current from the second inner pipe section. It is possible for the current return to subsequently use any power remaining from the second inner pipe section, and/or to be a power sink. Preferably, the current return is connected to the power input, thereby creating a complete circuit around the first and second inner pipe sections.

Suitable currents and voltages for the present invention vary as a function of the power need per metre length of pipe section, and possibly the pipe- in-pipe flowline length. By way of example only, the current could be in the range 500-1500 A and the voltage in the range 3-6 KV (with generally more voltage with longer pipe length). The frequency could be between 50 and 60 KV.

The power input may be directly connected to the first inner pipe section, for example using direct connection to one end of the first flowline.

Preferably, the power input is connected to the first inner pipe section via a power connector in the flowline able to provide an electrical current pathway between the outside of the first flowline and the first inner pipe section. Similarly, the current return may be directly connected to the second inner pipe section, for example using direct connection to one end of the second flowline. Preferably, the current return is connected to the second inner pipe section via a power connector in the flowline able to provide an electrical current pathway between the second inner pipe section and the outside of the second flowline.

In one embodiment of the present invention, the power input and current return are attached to one or more power umbilicals. The power umbilicals are able to provide an electrical current pathway to the power input, and away from the current return.

The or each power umbilical is able to allow the location of the power input into the first inner pipe section, and/or the location of the current return from the second inner pipe section, to be any suitable distance from a power source.

Preferably, where the power input and current return are attached to two separate power umbilicals, the power umbilicals are concurrent from a suitable facility until the locations of the power input and current return. In this way, a power input umbilical and a current return umbilical can be provided in a single umbilical housing until dividing or splitting near the first and second inner pipe sections. The splitting could be provided in a splitter box near the first and second inner pipe sections.

The present invention provides electric heating, in particular direct electric heating, to at least one pipeline section of a first pipe-in-pipe flowline. Preferably, the first flowline is a fluid flowline, more preferably being an oil and/or gas pipeline. The types and ranges of hydrocarbons passable along oil and/or gas pipelines are well known in the art, and the degree to which such pipelines require heating in order to prevent and/or minimise plug formations, for example hydrate and wax plugs, is well known to those skilled in the art.

The second flowline may also be a fluid flowline, such as an oil and/or gas pipeline, such that the system can provide two pipe-in-pipe flowlines able to convey or transfer a fluid such as oil and/or gas concurrently. Such flowlines may have a smaller diameter than a single pipe-in-pipe flowline presently used, such as using two 8" flowlines instead of a single 12" pipe- in-pipe flowline.

Additionally and/or alternatively, the second flowline may be regularly, periodically and/or irregularly used as a service flowline for servicing the first flowline and/or one or more other flowlines. A service flowline could be used for the conveyance, transfer or passage of one or more other fluids and/or apparatus, units or devices, such as a pig, whose action is to service or maintain, etc another flowline, such as the first flowline. Thus, the second flowline could be a maintenance flowline.

Preferably, the first and second flowlines are marine flowlines, preferably offshore flowlines.

According to another embodiment of the present invention, there is provided a method of electrically heating at least one pipeline section in a pipe-pin-pipe flowline comprising at least the steps of:

passing a current from a power input along a first electrically conductive inner pipe section of a first pipe-in-pipe flowline; passing the current from the first inner pipe section along at least one electrical current pathway from the first inner pipe section to a second electrically conductive inner pipe section of a second pipe-in-pipe flowline; and passing current along the second electrically conductive inner pipe section to a current return.

The method of electrically heating as described herein preferably uses a system as herein defined.

The present invention encompasses all combinations of various embodiments or aspects of the invention described herein. It is understood that any and all embodiments of the present invention may be taken in conjunction with any other embodiment to describe additional embodiments of the present invention. Furthermore, any elements of an embodiment may be combined with any and all other elements of any of the embodiments to describe additional embodiments.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying diagrammatic drawings in which:

Figure 1 shows a schematic illustration of a first system according to one embodiment of the present invention;

Figure 2 shows a cross-section of a second system according to a second embodiment of the present invention;

Figure 3a shows a cross-section of a third system according to a third embodiment of the present invention; and Figure 3b shows a schematic representation of the electrical current pathways in the third system shown in Figure 3a.

Referring to the drawings, Figure 1 shows a first system 2 according to a first embodiment of the present invention, comprising a first pipe-in-pipe flowline A1 and a second pipe-in-pipe flowline B1.

The first flowline A1 may be, by way of example only, an oil pipeline able to pass oil from an underwater facility 4 on the seabed 5, such as a well head or processing unit, and a vessel 6, such as a seagoing oil transporter, and/or floating oil processing plant. The provision of flowlines between such facilities and vessels, etc. is well known in the art, and the arrangement of the first flowline A1 having a seabed parallel portion and a riser portion is shown for illustrative purposes only.

The second flowline B1 may also be an oil pipeline, and for the purposes of illustration only, is shown to have a section parallel to the first flowline A1 and then to extend further along the seabed 5, such as to another underwater facility such as a remote wellhead.

The first flowline A1 comprises a first electrically conductive inner pipe section 10, extending between a power input 12 connected to the first inner pipe section 10, and a power lead 14.

The second flowline B1 comprises a second electrically conductive inner pipe section 20 between a current return 22 and the power lead 14.

The power lead 14 provides an electrical current pathway between the first and second inner pipe sections 10, 20. Figure 1 shows the first and second flowlines A1 , B1 laid or extending in parallel for a period prior to diverging after the power lead 14. Figure 1 also shows the length of the first inner pipe section 10 being a different, although substantially the same, length as the second pipe section 20.

In use, a power source 24, shown by way of example only as located with the underwater facility 4, is able to provide power along a first power umbilical 26 to the power input 12, from which power is provided to the first inner pipe section 10. As power passes along the first inner pipe section 10, the electrical resistance of the first inner pipe section 10 and/or other effects such as eddy currents, creates heat energy in the first inner pipe section 10, which is able to maintain and/or increase and/or reduce the decrease in the temperature of oil passing along the first flowline A1.

At the end of the first inner pipe section 10, power continues through the power lead 14 to the second inner pipe section 20, and then along the second inner pipe section 20 to: the current return 22, through the second power umbilical 28 and return to the power source 24.

The power provided by the power source is able to heat at least one pipeline section being the first inner pipe section 10. Preferably, sufficient power is provided that the system 2 shown in Figure 1 is also able to electrically heat the second inner pipe section 20, preferably when the second flowline 20 is also conveying or transferring a fluid such as oil and/or gas therethrough.

Each of the first and second flowlines A1 , B1 comprises an outer pipe (not shown in Figure 1), and optionally one or more other inner pipes. Figure 2 shows a second system 30 according to a second embodiment of the present invention, comprising a first pipe-in-pipe flowline A2 and a second pipe-in-pipe flowline B2. The first flowline A2 comprises a first electrically conductive inner pipe section 32 and a first outer pipe 34. Between the first inner pipe section 32 and the outer pipe 34 is insulation 36. The insulation 36 is preferably able to provide both electrical insulation and thermal insulation between the inner pipe section 32 and the outer pipe 34. The electrical insulation reduces dissipation of power from the first inner pipe section 32 to the outer pipe 34, and the thermal insulation heats maintain heat in the first inner pipe section 32, and helps insulate the first inner pipe section 32 from the marine environment temperature in a manner known in the art.

The second flowline B2 comprises a second electrically conductive inner pipe section 40 and a second outer pipe 42 having a second insulation 44 thereinbetween in a manner as described for the first insulation 36 hereinabove.

Figure 2 also shows a power input 46 and a current return 48 having an electrical pathway 50 thereinbetween.

The second system 30 shown in Figure 2 may relate with the first system 2 shown in Figure 1. The lengths of the first and second flowlines A2 and B2 are diagrammatic only, and may represent pipelines having an extensive length from several meters to several hundred meters or kilometres and further.

Figure 2 shows the power input 46 having a schematic connection to the first inner pipe section 32 so as to provide power therealong from the power input connection 46 to a second power lead 52. The second power lead 52 is able to provide an electrical current pathway between the first and second inner pipe sections 32, 40.

Similarly, Figure 2 also shows a schematic electrical pathway from the second inner pipe section 40 to the current return 48.

The first and second systems 2, 30 show in Figures 1 and 2 represent embodiments having unidirectional current flow along the first inner pipe sections 10, 32, and unidirectional current flow along the second inner pipe sections 20, 40, and whose circuits are completed by the first and second power leads 40, 52 and connection of the current returns, 22, 48 and power inputs 12, 46. Such unidirectional current flows may be achievable by the use of an electrically conductive break or other insulation in the first inner pipe sections 10, 32 prior to the power inputs 12, 46. Similarly, unidirectional current flows in the second inner pipe sections 20, 40 may be achievable by providing electrically conductive breaks and/or insulators in the second inner pipe sections 20, 40 to create non-conducting sides at the connections with the first and second power leads 14, 52.

Figure 3a shows a third system 60 according to a third embodiment of the present invention. In the third system 60, there is provided a first pipe-in- pipe flowline A3 and a second pipe-in-pipe flowline B3.

The first flowline A3 in Figure 3a comprises a first electrically conductive inner pipe section 62 having three ring or spring connectors 64a, b located thereon. Each ring connector 64a, b is connected to a respective power connector 66a, b which passes through a first outer pipe 68 of the first flowline A3. The second flowline B3 in Figure 3a comprises a second electrically conductive inner pipe section 70 having three ring or spring connectors 72a, b located thereon. Each ring connector 72a, b is connected to a respective power connector 74a, b which passes through a second outer pipe 76 of the second flowline B3.

The ring connectors 64, 72 may be dedicated point connectors such as spring coils, or elongate connectors which at least partly extend around the first inner pipe sections 62, 70 in the form of a ring and/or collar surrounding the inner pipe sections 62, 70.

Figure 3a also shows a power input 80 and a current return 82, both being at least partly within a splitter box 84. Power to the power input 80 is provided along a first power umbilical 86, and current from the current return 82 passes along a second umbilical 88. The first and second umbilicals 86 and 88 can be provided along a single umbilical line 90 from a suitable apparatus, unit or facility, such as a power generator onshore or on a nearby vessel, to the splitter box 84.

In use, power is provided from a power source 94 (schematically represented in Figure 3b) along the first power umbilical 86 to the power input 80, and then through the centre power connector 66a to the centre ring connector 64a on the first inner pipe section 62. The power and current from the central ring connector 64a passes bidirectionally along the first inner pipe section 62 to the two end ring connectors 64b, thus providing electric heating of the first inner pipe section 62 thereinbetween as described hereinabove.

Thereafter, current from the two end ring connectors 64b passes through the two end power connectors 66b and through suitable power flying leads 92 between the end power connectors 66b in the outer pipe 68 of the first flowline A3 to the two end power connectors 74b in the outer pipe 76 of the second flowline B3. Power flying leads are known in the art, and are protected and/or insulated from the surrounding environment, usually a marine environment, so as to prevent and/or minimise any power dissipation therefrom.

From the two end second power conductors 74b, current passes along electrical pathways to the end ring connectors 72b along the second inner pipe section 70, and then along the second inner pipe section 70 itself to the centre ring connector 72a. The current then passes via an electrical pathway to the centre power connector 74a, to the current return 82 and to the second umbilical 88, optionally back to the power source 94 to complete the electrical circuit.

Figure 3b shows a schematic representation of the electrical pathways shown in Figure 3a.

The present invention provides a system for heating at least one pipeline section which is advantageously more efficient than conventional systems, so that a higher amount of supplied energy is dedicated to the heating of the pipeline section. There is no current circulating in an outer pipe of a pipe-in-pipe flowline in the present invention. As such, loss of energy by dissipation from an outer pipe to the surrounding environment, in particular surrounding seawater, is avoided.

Another advantage of the present invention is the use of multiple power connections as shown in Figure 3a above, which provide flexibility regarding the location and/or length of the at least one pipeline section to be heated. The system may also comprise more than one power input and/or more than one current return, so as to increase the flexibility concerning the electrical pathways through the first and second pipe - sections. For example, more than one splitter box as shown in Figure 3a could be positioned alongside the first and second flowlines, in particular on the seabed, for providing power to different locations along the first inner pipe section, and/or to two different inner pipe sections of the first flowline.

Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined herein. Although the invention has been described in connection with specific preferred embodiments it should be understood that the invention as defined herein should not be unduly limited to such specific embodiments.