Process of and apparatus for electric pipeline heating

Abstract: A process to clear pipelines carrying fluids which are viscous and/or rich in organic residues, utilizing heat generation through electric phenomena, whether or not combined with the injection of special fluids. The heating process of this invention includes basically the introduction of a heating element inside a pipeline carrying the fluid through a coupling spool which is intercalated in said fluid-carrying pipeline, whilst simultaneously applying to said heating element an electric current, so that the heat generated by said heating element is transmitted to the viscous fluid or to the compacted residual mass inside the fluid-carrying pipeline. This causes the detachment of the residues from the internal wall of the pipeline and reestablishment of the conditions for fluid flow.

Description: This invention refers to a process and apparatus to clear obstructed pipelines carrying fluids which are viscous and/or rich in organic residues, utilizing heat generation by electric phenomena, whether or not combined with fluid injection and/or steam generation. The invention refers also to mechanical devices allowing for the introduction of a heating element inside pipelines already installed, for instance, in onshore or offshore petroleum production fields. In offshore production fields, the flow lines which carry the production from the well to the platform, due to the characteristics of the crude oil and the low temperatures prevailing at the site where same are installed, usually at large depths where the water temperature can be around 5°C, are subject to severe obstruction problems due to residues which are found dissolved in that oil - paraffins, asphaltenes, resins, wax, silica, etc. It has been verified that those residues tend to accumulate in the line, even where their formation may have occurred earlier. Depending upon the fluid flow rate, those residues do not deposit immediately as the water temperature lowers. It is known that fluid flow can occur at temperatures below the so-called pour point, that is, the lowest temperature at which a fluid can flow under its own weight (Standard ASTM-D97). This means that, if pressure is applied to a fluid at a temperature below its pour point, it will flow. In practice, however, the application of pressure in the pipelines comes up against severe limitations as regards equipment, power and dimensions. As the pipelines become longer, the problem of obstruction grows worse, and, indeed, such obstructions extend for as long as some kilometres. It is well-known that obstructions caused by paraffinic residues can be reduced or even eliminated when the fluid which is being transported through the pipeline can be sufficiently heated, making it possible to unglue the residue from the pipeline walls, which makes its displacement easier. DE-A-3414284 discloses a coupling head to connect axially to an end of a conduit to be heated, wherein the heating is achieved by an elongate flexible electrical conductor allowing supply of electrical energy to allow the Joule effect to be used for heating the conduit along which the flexible conductor passes. There is no disclosure as to how any heating element can be introduced into and displaced along an existing line to be cleared. DE-A-1119070 discloses a separate heating cable to be located inside a line to be cleared. US Patent 4538682 describes a method to remove paraffinic residues which deposit in colder regions of the production string of a petroleum well located in a formation intersected by a water table. The method consists of running in the well, through the annulus formed between the production string and the casing, down to the level of the region where the deposit occurs, a coil or set of coils involving the obstructed interval of the string. The extremity of the coil is connected, at the surface, to a power supply with the purpose of heating the coil, which transfers heat to the production string and to the fluid inside same. The major disadvantage of this method is that heat is applied to a given region, not preventing the residues from,depositing in other points of the string where a new temperature differential occurs which allows for deposit formation. As a consequence, the coil has to be constantly displaced from one region to another. Our Brazilian Patent Application PI 8602278 describes a process for petroleum well deparaffination in which an electric current is applied to the production string of the well to promote the dissipation of heat to the fluid inside the string and prevent the occurrence of paraffin deposits at the string and at the pumping rod. In this case the fluid is maintained heated along the string, thus preventing new deposits. When the well is located at the seabottom, these methods become difficult to apply, since any intervention directly to the well implies laborious operations, rather time-consuming and of great risk, in addition to being very expensive. It must be recalled that it is the case of heating kilometers of piping located at large depths in the sea. In addition to those difficulties, in offshore installations the connections between the well and the platform are achieved by means of flexible pipelines, due to the handling facility. These flexible pipelines are formed basically by alternate layers of steel spirals intercalated by layers of an elastomer, usually a polyamide-base one. The internal steel layer is interlocked, whereas the outermost layer is constituted by a crossed shield designed to provide the pipeline with a stronger structural reinforcement. In view of said structure, those pipelines face a certain limitation as regards their application for transportation of heated fluids. This invention has the purpose of providing a process to clear pipelines carrying viscous fluids rich in organic residues, particularly pipelines located at the seabottom, upon the application of heat, generated as from electric phenomena, utilizing heating elements, whether or not combined with the use of special fluids. The process of this invention is characterised by the features of claim 1. The apparatus of this invention is characterised by the features of claim 6. The preambles of claims 1 and 6 are based on the disclosure of DE-A-3414284. During the introduction of the heating element inside the pipeline carrying viscous fluids rich in organic residues, the heating element passes through the region obstructed by said organic residues, transmitting to them sufficient heat to release those residues from the pipeline walls and reestablishing the conditions required for fluid flow. The heating element to be utilized in the process consists of a heating tube formed by the set of one or more ferromagnetic conductors electrically isolated, and which also has a ferromagnetic external layer, the extremities of the set of conductors being connected among themselves or to the external layer. For the introduction of said heating element in the fluid-carrying pipelines, a coupling spool is utilized, especially designed for this process, which includes a cylindrical body provided in its extremities with adaptors, with the purpose of allowing its connection to the fluid-carrying pipeline, and having a lateral opening in which a guide-channel is installed. The guide-channel penetrates partially into the cylindrical body of the connector body, so as to orient and centralize the penetration of the heating element inside said fluid-carrying pipeline. With the purpose of making easy the understanding of the invention, we shall hereafter describe it in detail and by way of example with reference to the accompanying drawings, wherein:- FIGURES 1 to 1E show in side and section views various constructional alternatives of a heating element embodying the invention; FIGURE 2 shows a longitudinal section of a coupling spool for a pipeline with introduction of the heating element into said pipeline; and FIGURE 3 represents schematically a part of an offshore petroleum system. Detailed description of the invention This invention is based on heat generation as from electric phenomena, obtained through the application of an electric current to a conductor. As an electric current is applied through a conductor, a certain quantity of energy is transformed into heat, which may be represented by the equation Q = RI², in which Q represents the heat generated, R is the resistance of the material of which the conductor is made, and I is the intensity of the electric current which flows through the conductor. That phenomenon,based on the resistive effect of the electric current, is known as Joule Effect. Another way of obtaining heat by means of electric current is based on the inductive effect. If we take a duct formed by an internal conductor coated by an insulating material and by an external carcass of a ferromagnetic material, the circulation of alternating current through the internal conductor of the duct induces a variable magnetic flow in its coating, generating voltages in same, which promotes the circulation of a current in the coating. Said voltages follow Faraday's Law: e = - N d⊘ / dt where e represents the electric voltage, ⊘ is the magnetic flow, and t is the time. The energy originating from said current circulation is dissipated in the form of heat in the coating itself. Another phenomenon which occurs also is the so-called skin effect , which makes the distribution of the current non uniform in a crosssection of the electric conductor. This is due to the fact of the electromotive forces induced at the center of the conducting material being larger than the forces developed at the periphery, forcing the electric current which is circulating to migrate towards the periphery of the material. Thus, the effective area for the passage of current becomes smaller, increasing the effective resistance of the circuit, making it possible to generate a larger quantity of heat per unit of circulating current. The thickness of this area is defined so that the value of the circulating current be equal to l/ e of the value of the current at the periphery of the conducting material, and may be calculated by the equation: δ = 1 / [ πσµf ] 1/2 where δ is the thickness or depth of the skin effect, σ represents the conductivity, µ is the permeability of the conducting material and f is the frequency of the electric current. It may be observed that, the higher the frequency, the larger the skin effect, due to the reduction in the thickness of the layer through which the electric current will circulate. This invention is based on the combination of those effects above described to generate heat to the environment, taking advantage of that heat generated for application to the process of clearing pipelines carrying fluids which are viscous and/or rich in organic residues. As shown on Figure 1, the heating element (1) utilized in the process of the invention includes a heating tube (2) formed by at least one ferromagnetic conductor (3), selected from iron, steel or alloys, electrically isolated from its external coating (4) by means of ceramic material, PTFE or any other adequate material (5); said conductor (3) being connected to the external coating (4), as shown on Figure 1A or, if more than one conductor is are used, connected among themselves as shown on Figure 1B. The external coating (4) is of the corrugated or interlocked type (made also of a ferromagnetic material), which provides it with tensile stress and flexibility, allowing for an easier handling at the time of its introduction in the fluid-carrying pipeline, as described later on. The internal conductors may be combined in several ways and in the amount desired, and may present a circular, elliptical or any other adequate section. Figure 1 shows some of those possibilities, but does not limit the invention. Another alternative, shown in Figure 1E, consists of replacing one of the electric conductors by a fluid-carrying pipe (6), which may be utilized for introduction of water or any special fluid as, for instance, solvents, to help dissolving the residue. The terminal end of the heating element, not shown on the Figures, includes a piece which is adjusted to the pipeline extremity, protecting the electric connections and shaped so as to make easy its penetration in the mass of residues stuck inside the pipeline. Where the utilization of a solvent-carrying pipe is anticipated, the terminal end shall be provided with one or more orifices allowing for the outlet of the solvent. It must be pointed out that other possibilities not shown or described herein, but which may be easily inferred by the experts in the subject, shall be covered by the scope of this invention. Figure 2 shows, in a longitudinal section, a pipeline coupling spool which is of fundamental importance for the good performance of the process of the invention. The connector body (7) includes a cylindrical body (8) with diameter equal to that of the pipeline to be coupled with it, provided with adaptors (9, 9A) in its extremities for easy connection between the pipelines and with an opening (10) in its lateral wall. Through this lateral opening (10) a guide-channel (11) is introduced, which penetrates inside the cylindrical body (8) of the connector body (7) and is attached to said lateral opening (10), fully sealing said opening with the purpose of preventing the fluid from coming out to the environment. The guide-channel (11) allows for the centralized introduction of the heating element (1) inside the obstructed pipeline and its inclination is calculated so as to make possible the slippage of said heating element into the connector body (7) and therefrom to the obstructed pipeline, orienting its displacement. The portion of the channel (11A) which extends outside the cylindrical body of the connector body is provided with mechanical means (12), such as bearings and/or gears, which help the introduction of the heating element (1) into the pipeline. In this portion (11A), the interior of the channel is provided with means for dynamic sealing, not shown on the Figure, to ensure fluid tightness. For offshore petroleum production, the choice of the production system takes into consideration parameters such as the number of producing wells, the field size and behavior, the recovery method which shall be utilized, the water depth, etc. Figure 3 shows schematically, in a rather simplified way, a production system in which the production from the wells is carried by individual lines (A, B, ... Z) to a subsea production manifold (13), which is set at the seabottom, and therefrom to the platform (14), through one single line (15) of larger diameter. Alternatively, depending upon the number of producing wells, the individual lines may be directly connected to the platform. The process of this invention includes the possibility of intercalating one or several connector bodies in the individual lines, or in the single line which is connected to the platform, and through them introducing the heating element inside those lines. As the heat generated by the heating element is transmitted to the fluid inside the fluid-carrying pipeline, the viscosity of said fluid is altered, making easy the penetration and the displacement of the heating element, or heating pipe as described above, into the residual mass contained in side the pipeline. The electric pipeline heating process of this invention, consists of disconnecting, at predefined points, the fluid-carrying pipelines (15, A, B, ... Z) - or production lines - which are obstructed, intercalating at these points a pipeline connector body (7) provided with a lateral opening (10), introducing through this lateral opening (10) a heating element (1), which is constituted by a heating pipe (2), making the heating element penetrate and advance through said fluid-carrying pipelines and applying to said heating element (1), simultaneously with its introduction into said connector body (7), an electric current originating from a generation source located at the surface, so that the heat generated by said heating element (1) is transmitted to the viscous fluid or to the compacted residual mass inside the carrying pipelines to modify its physical characteristics until at least the residual mass detaches itself from the inner surface of the carrying pipeline and restablish es the conditions for fluid flow. Once the flow is restablished, the temperature of the fluid may be con trolled by periodically disconnecting the passage of electric current. The shape of the terminal end of the heating pipe contributes significantly towards the penetration of the heating pipe into the residual mass, at the beginning of the process, when the temperature differential is still large and the residual mass is consolidated. One advantage of the process of the invention is that the heating element maintains the fluid heated along the whole extent of the carrying pipeline already traversed, preventing the fluid from solidifying after its passage, thus ensuring the conditions for flow regularization in the carrying pipeline. Another advantage of the process is that the fluid temperature may be controlled, so as not to affect the material of the fluid-carrying pipeline, which, as mentioned previously, consists of a flexible pipe, the structure of which may be damaged by excess temperature. Still another advantage is that the heating element, since it has the shape of a pipe, may be manufactured in standard segments, which, according to the particulars of the situation, may be coupled among themselves until the required length is reached. Therefore, no additional expense is required for its handling, since the conventional equipment utilized in the branch and usually available at the operation site may be used. In case the extent of the carrying pipeline to be cleared is too large, as from a pre-defined point, it is possible to introduce at least two of those heating elements in opposite directions, it being sufficient for that to utilize two connector bodies with the guide-channels oriented towards opposite directions. Depending upon the nature of the compacted residual mass in the pipeline, it may be required to use special fluids, such as steam or solvents, to facilitate the penetration of the heating element or even to dissolve the residual mass. In those cases the alternative shown on Figure 1E may be utilized, which includes a fluid-carrying pipe (6) which makes it possible to inject special fluids directly at the desired point. In case water injection is utilized, the temperature developed by the heating pipe (2) may be sufficient to generate steam, which on its turn may be used as an agent to make easy the penetration of the pipe into the residual mass and also to carry the residues already disaggregated. A preliminary laboratory test, in conditions of natural air convection and ambient temperature of 22°C, showed that it is possible to obtain temperatures around 54°C in the external coating of the heating pipe, by applying a 40 A, 60-Hz electric current to a heating pipe formed by an internal iron conductor with 0.47 cm (3/16″) diameter, with internal ceramic isolators and corrugated external coating with 2.54 cm (1″) diameter, for nearly 60 minutes. It becomes clear to the experts in the subject that this process applies as well to onshore pipelines, as oil pipelines for instance, or even to onshore wells. In case of new installations of production systems in regions where the fluid is known to present the characteristics described previously, the heating element may be previously installed, at the time of installation of said new production system.

Claims: A process of unclogging an obstructed pipeline which carries a fluid which is viscous and/or which is rich in organic residues, characterised by disconnecting the obstructed fluid-carrying pipeline sections (15, A, B, ... Z) at pre-defined points; intercalating at these points a pipeline connector body (7) provided with an opening (10); introducing through this opening (10) a heating element (1) to penetrate said fluid-carrying pipeline sections and to advance through the same; and applying to said heating element (1), as it is introduced into said connector body (7), an electric current generating heat which is transmitted to the viscous fluid or to the compacted residual mass inside said fluid-carrying pipeline in order to modify its physical characteristics, whereby under the effect of the heat generated the residual mass detaches itself from the internal surface of said fluid-carrying pipeline to reestablish the conditions for fluid flow. A process according to claim 1, wherein the fluid temperature inside the fluid-carrying pipeline is controlled through the intermittent passage of an electric current through the heating element. A process according to claim 1 or 2, including releasing through the heating element (1) into said pipeline fluids (6) acting to release the compacted residual mass in the pipeline. Apparatus for carrying out the process of claim 1, said apparatus comprising: a connector body (7) adapted to be inserted between two successive lengths of pipeline and provided with a lateral opening (10), a heating element (1) adapted to be inserted into the pipeline through said lateral opening (10), and means for supplying electric current to said heating element (1), the apparatus being characterized by: a guide channel (11) which sealingly penetrates into said connector body (7) through said lateral opening (10), means for effecting dynamic sealing between the guide channel (11) and the heating element (1), and means for helping the introduction of the heating element into the pipeline Apparatus according to claim 4, wherein the heating element also includes a conduit (6) for feeding of a hot fluid or a solvent therethrough. Apparatus according to claim 4 or 5, wherein the heating element is arranged to be electrically heated resistively and/or inductively. Apparatus according to any of claims 4 to 6, wherein said heating element (1) comprises a heating pipe (2) formed by at least one ferromagnetic conductor (3), electrically isolated from its external coating (4), also made of a ferro-magnetic material, and in which the ends of said internal conductors (3) are connected to each other or to the external coating (4). Apparatus according to any of claims 4 to 7, wherein the connector body (7) comprises a cylindrical body (8) with a diameter equal to that of the pipeline and provided at its ends with end adaptors (9, 9A) for connection between pipelines and wherein the guide-channel (11) which penetrates through said opening (10) into said cylindrical body (8) extends partially outside the cylindrical body (8) and its outwardly projecting portion is provided with means for dynamic sealing and with mechanical means (12), such as bearings and/or gears, which orients, centralizes and helps to introduce said heating element (1) inside the pipeline.

Inventors: GONCALVES CARLOS ALBERTO DE CA, GONCALVES, CARLOS ALBERTO DE CASTRO
Applicants: PETROLEO BRASILEIRO SA, PETROLEO BRASILEIRO S.A. - PETROBRAS
Application year: 1991
Main classification: E21B36
IPC codes: H05B6, H05B3, F16L53, E21B36, E21B17, B08B9