Embodiments of the invention relate to the field of heat tracing systems. More particularly, embodiments of the invention relate to a skin effect heating system and method having improved heat transfer characteristics and an associated support configuration.
Heating systems are employed to facilitate the extraction of oil, gas and similar media from subterranean environments. For example, heating systems are used to prevent production losses resulting from paraffin deposits and hydrate formation in the extraction production tube as well as improving production of heavy oils by lowering the viscosity to provide better flow applications. One way to facilitate the heating of production pipes through which the media, such as oil, is extracted is to employ a heat tracing system. Electrical heat tracing systems are typically used in various industries including oil and gas, but may also be used in power, food and beverage, chemical and water industries. In these systems, a heating cable is connected or wrapped around a production or process pipe and power is supplied to the cable to form a heat tracing circuit.
One type of pipe employed in heat tracing systems is a skin effect heat tracing pipe. Skin effect heat tracing pipes are preferred in many different pipeline environments, including downhole or wellbore heating associated with oil extraction. When this type of pipe is employed, the inner surface of a ferromagnetic pipe or tube is electrically energized (AC voltage) and an insulated, non-ferromagnetic return conductor is used to complete the circuit. The inner surface of the pipe carries full current and heats up, but the outer surface remains at ground potential. The path of the circuit current is pulled to the inner surface of the heat tube by both the skin effect and the proximity effect between the heat tube and the conductor. The skin effect circuit impedance is mainly resistive, thereby generating heat in the tube wall and, to a lesser extent, in the insulated conductor. Additional heat transfer results from eddy currents induced in the tube wall by the current flow through the conductor. These eddy currents are the result of the changing magnetic field due to variations of the field over time which causes a current within the conductor. In this manner, the skin effect pipes are in contact with the outer surface of the delivery conduit and thermal conduction is used to transfer the heat from the skin effect pipe to the delivery conduit and consequently to the process media.
The size and depth of the skin effect heating system depends on the length of the circuit within the subterranean application, the power output of the circuit, the tube and conductor size as well as the process media pipe temperature. All of these factors contribute to the efficiency and effectiveness of the heating system. However, a drawback associated with these systems is that the heat transfer from the conductor to the conduit or tube results in high conductor temperatures, thereby limiting the overall power supplied by the heat tracing system. In addition, the mechanical tension on the heat tracing cable within the conduit or tube, which increases with subterranean depth due to gravitational forces, may compromise the integrity of the heating cable. Thus, there is a need for a skin effect heating system which provides improved heat transfer to the tube or conduit from the conductor and a tension management system which maintains the integrity of the cable within the wellbore.
Exemplary embodiments of the present invention are directed to a skin effect heat tracing system with improved heat transfer characteristics and an associated support configuration. In an exemplary embodiment, the skin effect heat tracing cable is positioned along a production pipe carrying process media and includes a heating tube, a conductor, an insulating jacket and a dielectric fluid. The heating tube is in contact with the production pipe to transfer heat thereto. The conductor is disposed within the heating tube and is connected to a power supply to supply current to the conductor. The power supply is also connected to the heating tube to complete a heat tracing circuit with the conductor. The insulating layer or insulating jacket is positioned around the conductor and the dielectric fluid is disposed between the heating tube and the insulating layer.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout.
Conductor 14 is surrounded by insulating layer 16 having a thickness, for example of from about 0.060″ to about 0.120″ and more preferably from 0.080″ to about 0.100″ and capable of withstanding temperatures from about >100° C. Insulating layer 16 may be comprised of, for example, a cross linked polyethelene formulation, fluoropolymers and the like. For example, polyethylene formulations are particularly applicable for higher voltage applications, whereas fluoropolymers are particularly useful for high temperature applications.
Conduit or tube 12 may be, for example, a coiled ferromagnetic steel tube and is non-porous to contain dielectric fluid 18. Conduit 12 may also be any ferromagnetic heatable encasement configuration such as steel pipe, coiled tube, roll formed tube, etc., which is capable of withstanding elevated temperatures found in wellbore applications. The diameter size of conduit 12 is dependent on the particular wellbore application, but may be, for example, from about 3″ outer diameter (O.D.) to about 0.5″ O.D. and preferably from about 2″ O.D. to about 1″ O.D. Conduit 12 has an inner wall surface 12a and a wall thickness from about 0.1″ to about 0.5″.
A dielectric fluid layer 18 is disposed between insulating layer 16 and conduit 12. In particular, dielectric fluid is filled into cable 10 between insulating layer 16 and the inner wall 12a of tube 12. Dielectric layer 18 wraps around conductor 14 and is used to reduce the gravitational tension and/or compression loads in the conductor 14. The gravitational tension is reduced by decreasing the weight of the heating cable and thus the gravitational forces applied to the cable positioned within the wellbore. In addition, dielectric fluid layer 18 also improves the heat transfer characteristic from conductor 14 to conduit 12, while improving the dielectric capabilities to the insulation 16. Dielectric fluid 18 improves the heat transfer characteristic by eliminating air from around insulation layer 16. This minimizes the risk of partial discharge (PD) which is a particular concern for fluoroploymer insulations. In this manner, by adding dielectric fluid 18, an increase voltage may be employed for use with high temperature insulating layer 16. Representative dielectric fluids 18 may include, for example, mineral oils, organic based transformer oils and similar materials capable of providing sufficient dielectric strength and thermal stability to further electrically insulate the conductor 14 from tube or conduit 12. Representative dielectric fluids include SHELL DIALA®Oil HFX sold by Shell Oil Company of Houston, Tex. (USA).
While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US08/77347 | 9/23/2008 | WO | 00 | 4/18/2011 |
Number | Date | Country | |
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Parent | 60975196 | Sep 2007 | US |
Child | 13121296 | US |