METHOD FOR DETERMINING THE AC LINEAR ELECTRICAL RESISTANCE OF A STEEL PIPE AND DEVICE FOR IMPLEMENTING SAID METHOD

Abstract

The invention relates to a method for determining the AC linear electrical resistance of a steel pipe, comprising the steps of generating an induced current in a portion (4) of the pipe by means of an induction coil (6) centered on a longitudinal axis (XX) of the pipe and traversed by an alternating current, the coil being housed in a ferromagnetic cylinder head (8) to confine the magnetic field over a predefined surface of the pipe portion, measuring the active power dissipated by the pipe portion subjected to the magnetic field, measuring the amplitude of the produced magnetic field, and determining the AC linear electrical resistance of the pipe portion from the measurements of the dissipated active power and the amplitude of the induced magnetic field. The invention also relates to a device (2) for implementing said method.

Description

The invention relates to the general field of characterizing steel pipes used for transporting fluids, particularly oil and gas. More specifically, it relates to a method for determining the alternating current linear electrical resistance of a steel pipe. Preliminary technique In a single offshore hydrocarbon production field, it is common to operate several wells that may be separated from each other by several kilometers, even tens of kilometers. The fluids from these different wells can be collected via subsea metallic pipelines (usually made of steel) laid on the seabed and transferred through bottom / surface connecting pipes to a surface facility, such as a platform, a vessel, or an onshore collection point, which will collect the fluids for storage (and possibly for treatment). Fluids from production wells tend to cool rapidly as they travel through the many kilometers of subsea pipelines or during production shutdowns. However, if no measures are taken to maintain a minimum threshold temperature within these pipelines, there is a significant risk that gas molecules, particularly methane, contained in the transported fluids will combine with water molecules to form hydrate crystals at low temperatures. These crystals can adhere to the pipeline walls, agglomerate, and lead to the formation of plugs capable of blocking fluid flow. Similarly, the solubility of high-molecular-weight compounds, such as paraffins or asphaltenes, in oil decreases as the temperature drops, resulting in solid deposits that can also obstruct flow. In the case of subsea pipelines known as "single-lined" subsea pipelines, an active heating solution is typically used. The most common solution involves heating the subsea pipeline by applying an alternating electric current directly to the inner steel lining of the pipeline. This lining is connected to each end of the pipeline by an electrical cable. The inner lining is not electrically insulated from the seawater in which the pipeline is submerged, resulting in strong currents that circulate through the seawater and significantly reduce the effectiveness of the heating solution. In the case of underwater pipelines called "double-lined" underwater pipelines of the "pipe-in-pipe" or PIP type, in which an inner lining carries the fluids and an outer lining coaxial with the inner lining is in contact with the seawater, it is also known because it heats the inner lining of the pipe by applying an alternating electric current directly onto the inner steel lining of the pipe, using the outer steel lining as a conductor for the return path CAnfrnn / eznz / E / YiAi of the electric current. Therefore, the alternating electric current passing through the inner lining allows it to be heated by the Joule effect. More specifically, the heating of the inner lining occurs due to the Joule effect caused by the current passing through it; the heat produced is largely transferred to the fluids of the inner lining, since heat losses through the insulation filling the annular space between the inner and outer linings are relatively small. This electrical heating solution is called “Direct Electric Heating” (or DEH). Regardless of the pipe type (single- or double-walled), it is essential to ensure before commissioning that the operating linear heating output will be sufficiently uniform throughout the pipe. Since the heating output of a pipe section is equal to its AC electrical resistance multiplied by the square of the effective current passing through it, an out-of-tolerance AC linear electrical resistance in one section of the pipe will result in an out-of-tolerance heating output under operating conditions. The AC linear electrical resistance of a steel pipe depends on many factors and can vary from one pipe section to another, especially within the same manufacturing batch. Furthermore, it seemed crucial to be able to determine the AC linear electrical resistance upon receipt of batches of pipe sections. To this end, one of the known methods for measuring the AC linear electrical resistance of pipe sections is based on applying direct current to both ends of each pipe section using a dedicated electronic power supply. Reference can be made in particular to the publication entitled “Electromagnetic Modelling for Electrical Heating of Pipelines” published in 2007 by the International Society of Offshore and Polar Engineers (ISOPE-1-07-378). However, this method has several drawbacks, including a relatively long measurement cycle time (approximately 15 minutes for a 12-meter-long pipe section). Furthermore, this method requires straightening the pipe section at the connection points to the electronic power source. Additionally, the equipment needed for this measurement (particularly the power source) is quite bulky. Description of the invention The object of the invention is, therefore, to propose a method for determining the alternating linear electrical resistance of a steel pipe that does not present the aforementioned drawbacks. According to the invention, this objective is achieved by means of a method for determining the alternating linear electrical resistance of a steel pipe, comprising the steps of: a) generating in a portion of the pipe an induced current resulting from the production of a magnetic field at a predefined frequency by means of an induction coil centered on a longitudinal axis of the pipe and traversed by an alternating current, the induction coil being housed in a cylinder head of ferromagnetic material to confine the magnetic field to a surface CRnbnn / Qznz / B / YiAi predefined pipe portion; b) measure the active power dissipated by the portion of pipe subjected to the magnetic field; c) determine the amplitude of the magnetic field produced; and d) Determine the alternating-mode linear electrical resistance of the pipe portion from measurements of the dissipated active power and the amplitude of the induced magnetic field. The method according to the invention is notable because it provides for the determination of the alternating current (AC) linear electrical resistance of a pipe section by generating a magnetic field at a predetermined frequency capable of inducing a current within the pipe. Specifically, it is possible to perform a local measurement of the AC linear electrical resistance or to scan the entire pipe by moving the induction coil along its length. In this case, the time required to determine the AC linear electrical resistance can be considerably reduced compared to the known prior art method (typically 5 minutes for a 12-meter-long pipe). Furthermore, the method according to the invention does not require any rectification (or other operation) of the pipe, and the equipment necessary for its application occupies very little space. In addition, the method according to the invention allows for the measurement of AC linear electrical resistance on both the outer and inner surfaces of the pipe. Advantageously, the AC linear electrical resistance of the pipe portion is determined in step d) by the equation: Rac_deh= (2 x ncaior_jiMp) / (π x OD x Hjimp)2; where Rac_deh is the AC linear electrical resistance, nCaior_jiMP is the active power dissipated per unit length of the pipe, OD is the pipe diameter, and Hjimp is the amplitude of the magnetic field produced. Furthermore, it is advantageous that the frequency of the magnetic field produced in step a) can be continuously varied to determine the alternating linear electrical resistance of the pipe portion at different frequencies. According to one application, steps a) and ad) are repeated along the entire length of the pipe by moving the induction coil along the length of the pipe. The induction coil from step a) can be placed inside the pipe to determine the AC linear electrical resistance of the pipe's inner face. Alternatively, it can be placed outside the pipe to determine the AC linear electrical resistance of the pipe's outer face. The frequency of the magnetic field produced in step a) can be between 5 Hz and 10 kHz. The invention also relates to a device for implementing the method as defined above, comprising an induction coil intended to be centered on a longitudinal axis of the pipe and traversed by an alternating current, and a ferromagnetic cylinder head within which the induction coil is mounted to confine the magnetic field to a predefined surface of the pipe portion. The cylinder head may comprise a tubular body concentric with the induction coil that terminates at each end in an annular collar that delimits an air gap with the portion of the pipe. ΓΑηΐτηη / Γζηζ / Ε / γίΛΐ The induction coil can be manufactured by winding a conductor wire of variable pitch along its length to compensate for edge effects. Alternatively, the induction coil can be made by winding conductor wires in several layers along all or part of its length. Preferably, the device further comprises means for minimizing the influence of edge effects on the quality of the measurements. In this case, the device may comprise plates made of ferromagnetic material that are capable of sliding radially on each collar of the cylinder head to make contact with the surface of the pipe portion, thereby minimizing the influence of edge effects on the quality of the measurements. Alternatively, the device may comprise flexible, laminated blades made of ferromagnetic material that are positioned in a star configuration around the pipe and at each end of the tubular body of the induction coil to make contact with the surface of the pipe portion, thereby minimizing the influence of edge effects on the quality of the measurements. Brief description of the drawings Figure 1 is a schematic view of an example of a device for implementing the method according to the invention. Figure 2 is a schematic view of another example of a device for implementing the method according to the invention. Figure 3 represents an example of the distribution of the magnetic field produced during the method according to the invention in the induction coil and in a portion of pipe. Figure 4 represents an example of the distribution of the active power density dissipated in the induction coil and in a portion of pipe subjected to the method according to the invention. Figure 5 is a curve showing an example of alternating linear electrical resistance as a function of the magnetic field amplitude of the outer face of a pipe portion subjected to the method according to the invention. Figure 6 is a curve showing an example of alternating linear electrical resistance as a function of the magnetic field amplitude of the inner face of a pipe portion subjected to the method according to the invention. Figure 7A represents in longitudinal section a variant of the device according to the invention provided with an example of a system to minimize the influence of edge effects on the quality of the measurement. Figure 7B is a cross-sectional view of the device in Figure 7A. Figure 8A represents in longitudinal section another variant of the device according to the invention provided with another example of a system to minimize the influence of edge effects on the quality of the measurement. CAnfrnn / eznz / E / YiAi Figure 8B is a cross-sectional view of the device in Figure 8A. Figure 9 represents a variant of the system in Figures 8A and 8B to minimize the influence of edge effects on measurement quality. Figure 10 represents another variant of the system in Figures 8A and 8B to minimize the influence of edge effects on measurement quality. Figure 11 represents yet another variant of the system in Figures 8A and 8B to minimize the influence of edge effects on measurement quality. Figure 12 represents yet another variant of the system in Figures 8A and 8B to minimize the influence of edge effects on measurement quality. Description of the modalities The method according to the invention applies to any underwater pipeline (single or double casing pipeline) made of steel and intended to transport fluids such as oil and gas. The method according to the invention applies more particularly to underwater steel pipes (in particular, but not exclusively, carbon steel) that are subjected to electrical heating of the “direct electric heating” (or DEH) type. This type of electrical heating involves applying an alternating electric current to the lining that is to be heated. The lining heats up due to the Joule effect caused by the current passing through it, with much of the heat being transferred to the fluids circulating within the lining. The method according to the invention aims to determine the linear electrical resistance in alternating current (AC) mode of a pipe of this type using a device such as that shown in figures 1 and 2. In these two modalities, the device is formed by an assembly of an induction coil (or solenoid) intended to be powered by an alternating current from a cylinder head made of ferromagnetic material. In the embodiment of Figure 1, the device 2 according to the invention is placed around a portion of pipe 4 (which may be the inner lining of a double-lined pipe or the lining in the case of a single-lined pipe) to determine the AC linear electrical resistance of the outer face of the pipe portion. The device comprises an induction coil 6 that is centered on the longitudinal axis XX of the pipe 4, and a cylinder head 8 made of ferromagnetic material within which the induction coil 6 is mounted and which allows the magnetic field to be confined to a predefined surface of the pipe portion. More specifically, the cylinder head 8 comprises a tubular body 8a that is centered on the longitudinal axis XX of the pipe 4 and terminates at each longitudinal end in an inwardly oriented annular collar (or flange) 8b. These collars 8b are positioned opposite the outer face of the pipe portion to confine the magnetic field produced by the induction coil within a delimited and predefined annular space, particularly over a precise length of the pipe. Cylinder head 8 is made of a ferromagnetic material, such as ferrite or of CAnfrnn / eznz / E / YiAi > GC c rolled electrical steel. * As for the induction coil 6, it is made by winding a conductive wire, for example, copper or aluminum; this winding can be single-layer or multi-layer. Furthermore, to minimize edge effects, the winding of the conductor wire can have a variable pitch along the induction coil, with a higher conductor wire density at the two longitudinal ends of the tubular body 8a of the cylinder head than at its center. Alternatively, to achieve the same effect, the conductor wire winding can be made in several layers at these two longitudinal ends and a single layer at the center. In addition, the induction coil 6 is connected to an AC power outlet (not shown in the figures). Finally, the device 2 according to the invention may include means for moving along the entire length of the pipe in order to perform an AC electrical resistance measurement of the entire pipe. These means, not shown in the figure, may comprise plastic pads or rollers arranged on the outer face of the two collars 8b in order to center the device on the pipe while simultaneously allowing axial translation without excessive friction. In the embodiment of Figure 2, the device 2' according to the invention is placed inside a portion of pipe 4 (which may be the outer lining of a double-lined pipe) to determine the AC electrical resistance of the inner face of the pipe portion. Compared to the embodiment in Figure 1, this arrangement results in the cylinder head 8' comprising a tubular body 8'a terminating at each longitudinal end in outwardly oriented collars 8'b. These collars 8b are positioned opposite the inner face of the pipe portion and confine the magnetic field produced by the induction coil within a delimited and predefined annular space, specifically over a precise length of the pipe. As for the induction coil 6', it is similar to that of the mode in figure 1. The method according to the invention is implemented by means of such a device 2, 2' and provides for the following steps. The induction coil 6, 6' of the device is powered by alternating current at a predetermined frequency f. The power supply to the induction coil generates a magnetic field at a predefined frequency; this magnetic field induces a current in the thickness of the portion of pipe subjected to the device according to the invention (on its inner or outer face). More specifically, when the pipe is excited by an induction coil JIMP of length IgjiMP and which includes n turns each traversed by a current i, an orbital current Ijimp is developed by induction on the surface of the pipe in a portion of pipe of length IgjiMP and skin thickness ójimp given by the following equation: Mathematical equation 1 SjIMP — l~y~ In this equation, f is the frequency of the alternating current, p is the electrical resistivity of the pipe material, and μ is the magnetic permeability of the pipe material. The amplitude of the magnetic field Hjimp under the induction coil is theoretically constant and is given by the following equation: Mathematical equation 2 ΓΑηΐτηη / Γζηζ / Ε / γίΛΐ ni V2 ^ImV2 hjdíp ~ 7“ ~ “Γ -JIMP -JIMP The active power Pcaior_jiMP dissipated by the portion of pipe subjected to the magnetic field is given by the following equation: Mathematical equation 3 p •“heat π OD 2 π ΟΟ ; -JIMP JB1P=P'7 71JIM=PX ' ·HJIMP 7T~ - JIMPÓJIMP¡?JIMPÓJIMP V2 In this equation, OD is the pipe diameter (i.e., the external diameter of the pipe if the device is arranged around the pipe, or the internal diameter of the pipe if the device is arranged inside the pipe). It follows that the thickness of the orbital current skin on the pipe surface is given by the equation: Mathematical equation 4 π OD p Hjj^jpÓJBIP—“Ti -1,calor_JI\IP In this equation, nCaior_jiMP is the linear active power in the pipe. Furthermore, the inventors have observed that, in the case of homogeneous and linear materials as in the case of non-linear ferromagnetic materials (such as carbon steel), for magnetic fields of the same amplitude, the skin depth of the orbital current developed by the induction coil is the same as that of an axial alternating current used to heat a pipe according to the DEH method (or direct electric heating). However, for a pipe heated by the DEH method, it has been established that the linear electrical resistance in alternating current Rac_deh is given by the following equation: Mathematical equation 5 R -P RAC.DEH-n0D.8DEH Likewise, substituting the previously calculated skin thickness value ójimp into the previous expression for the linear electrical resistance value in alternating current Rac_deh, the following equation is obtained: Mathematical equation 6 _ P _ P _ 2.nca|Or_jIMPn.OD. 6dehk.0D.8 / ¡mp(Tt.OD.Hpyfp)The linear electrical resistance in the alternating current Racjdeh of the portion of pipe subjected to the magnetic field Hjimp is given by the following equation: Mathematical equation 7 2. üheatJIMPRac-deh =(v.0D.H~,MPr In this equation, nCaior_jiMP is the linear active power in the pipe and Hjimp is the amplitude of the magnetic field in the annular space between the induction coil and the pipe. Figure 3 shows an example of the distribution of the amplitude of the magnetic field H (measured in Weber) produced during the method according to the invention in a portion of pipe 4 and in the induction coil 6 of the device according to the invention, this magnetic field having been generated by a device of the invention as described above. The inventors have discovered that the smaller the distance between the collars 8b of the cylinder head 8 and the face (inner or outer) of the pipe portion 4, the more effectively the magnetic field is confined to the surface of the pipe portion. Therefore, in the example in Figure 3, where the air gap is close to 0, the amplitude of the magnetic field H is relatively homogeneous across the entire surface of the pipe portion between the two collars of the device's cylinder head. As described above, in order to minimize any edge effect due to the clearance between the cylinder head collars and the pipe portion, the winding of the conductive wire that makes up the induction coil of the device can have a higher density at the level of the two cylinder head collars of the device. Figure 4 shows an example of the distribution of the dissipated active power density Pcaior (measured in W / m³) in the induction coil and in a section of pipe subjected to the method according to the invention. This active power can be measured using an active power measuring device, for example, a network analyzer connected to the terminals of the induction coil. Likewise, in this case it is observed that the dissipated active power is essentially concentrated in CAnfrnn / eznz / E / YiAi the portion of pipe subjected to the device according to the invention. Based on these measurements, the method according to the invention allows the linear electrical resistance Rac_jimp of the pipe portion to be determined by calculation (see equation 6). Figures 5 and 6 are examples of curves representing alternating linear electrical resistances (in μΩ / m) as a function of the magnetic field amplitude (in A / m), on the one hand, for a device placed outside the pipe (Figure 5 - corresponding to the n-mode configuration of Figure 2), and on the other hand for a device placed inside the pipe (Figure 6 - corresponding to the mode configuration of Figure 1). It should be noted that the determination of the linear electrical resistance in alternating mode according to the method of the invention can be carried out over the entire length of a pipe by moving the device along the longitudinal axis of the pipe and repeating the steps described above. It should also be noted that the linear electrical resistance in AC mode can be determined based on skin depth by varying the frequency of the electrical current powering the induction coil. For example, by varying this frequency from 5 Hz to 10 kHz, it is possible to determine the linear electrical resistance of a pipe from 0.1 mm to 20 mm of skin depth. It will also be observed that the amplitude of the magnetic field Hjimp under the induction coil that is necessary to determine the alternating linear electrical resistance Rac_jimp of the pipe portion can be measured by a probe or deduced from a measurement of the current i in the induction coil using the following equation: Hjimp = nxi / Ig where n is the number of turns of the induction coil and Ig its length. Furthermore, to minimize the influence of edge effects on measurement quality, the inventors have proposed several possible systems for closing the gap that necessarily exists between the cylinder head collars and the section of pipe whose dimensional tolerances may be wide. This gap degrades the confinement of the magnetic field in the air gap and disrupts the magnetic flux circulating through the cylinder head and the pipe. Figures 7A and 7B illustrate an exemplary system for minimizing the influence of edge effects on measurement quality. In this example, the system relies on the use of ferromagnetic plates 10 that can be slid radially over each collar 8b of the cylinder head 8 to make contact with the surface (here, the external surface) of the pipe section. In this way, these plates 10 create a field bridge across the gap between the cylinder head collars and the pipe section. More specifically, the plates 10, for example twelve in number, form a ring when placed end to end around the XX axis of the pipe (see Figure 7B). These plates are housed in an annular groove 12 delimited between the respective collars 8b of the cylinder head and the annular flanks 14 (for example, made of plastic) assembled against these collars. An O-ring 16 placed around the plates 12 allows them to be tensioned so that they radially support the pipe surface. CAnfrnn / eznz / E / YiAi > N α c C Another exemplary form of this system to minimize the influence of edge effects* represented by Figures 8A and 8B consists of the use of flexible sheets 18 of ferromagnetic material £ that replace the cylinder head collars. As shown in Figures 8A and 8B, these flexible blades 18 are located at each end of the tubular body 8a of the induction coil and arranged in a star configuration around the pipe to radially support its surface. The degree of blade deflection allows for the absorption of varying clearances depending on the pipe tolerances. In a variant of this system shown in Figure 9, the blades can be replaced by a ring 20 (or ring segments) made of unreliable ferromagnetic material. More specifically, as shown in the lower part of Figure 9, the ring 20 is inflatable to bear radially against the surface of pipe 4. In another variant of this system, shown in Figure 10, the blades are replaced by an expandable ferromagnetic ring 22 (or ring segments). As shown in the lower part of Figure 10, the ring 22 is able to expand to bear radially against the surface of the pipe 4. In another variant of this system, shown in Figure 11, the blades are replaced by a ring 24 (or ring segments) made of hollow, flexible ferromagnetic material. As shown in the lower part of Figure 11, the ring 24 can be deployed to rest radially against the surface of the pipe 4. In another variant of this system, shown in Figure 12, the blades are replaced by a ring 26 (or ring segments) made of flexible, corrugated ferromagnetic material. As shown in the lower part of Figure 12, the ring 26 can be deployed to rest radially against the surface of the pipe 4.

Claims

1. A method for determining the alternating linear electrical resistance of a steel pipe, comprising the steps of: a) generating in a portion (4) of the pipe an induced current resulting from the production of a magnetic field at a predefined frequency by means of an induction coil (6) centered on a longitudinal axis (XX) of the pipe and traversed by an alternating current, the induction coil being housed in a cylinder head (8) of ferromagnetic material to confine the magnetic field to a predefined surface of the pipe portion; b) measuring the active power (Pcaior) dissipated by the pipe portion subjected to the magnetic field; c) determining the amplitude (H) of the magnetic field produced;(d) Determine the AC linear electrical resistance of the pipe portion from measurements of the dissipated active power and the amplitude of the induced magnetic field, determining the AC linear electrical resistance of the pipe portion in step (d) using the equation: Rac_deh = (2 x flcaior jiMp) / (zr x OD x Hjimp)2; where Rac_deh is the AC electrical resistance, IIheat_jiMP is the active power dissipated per unit length of the pipe, OD is the diameter of the pipe and Hjimp is the amplitude of the magnetic field produced.

2. The method according to claim 1, wherein the frequency of the magnetic field produced in step a) varies to determine the alternating linear electrical resistance of the pipe portion at different frequencies.

3. The method according to any of claims 1 and 2, wherein steps a) and ad) are repeated over the entire length of the pipe by moving the induction coil along the pipe.

4. The method according to any of claims 1 to 3, wherein the induction coil of step a) is placed inside the pipe.

5. The method according to any of claims 1 to 3, wherein the induction coil of step a) is placed outside the pipe.

6. The method according to any of claims 1 to 5, wherein the frequency of the magnetic field produced in step a) is between 5 Hz and 10 kHz.

7. A device (2; 2j) for applying the method according to any one of claims 1 to 6, comprising a steel underwater pipe (4) intended to transport fluids such as oil and gas, an induction coil (6; 6j) intended to be centered on a longitudinal axis (XX) of the pipe (4) and traversed by an alternating current, a cylinder head (8; 8j) of ferromagnetic material within which the induction coil is mounted to confine the magnetic field to a predefined surface of the pipe portion, and an active power measuring apparatus connected to the terminals of the induction coil to measure the active power (Pcaior) dissipated by the pipe portion subjected to the magnetic field.

8. The device according to claim 7, wherein the cylinder head (8; 8j) comprises a tubular body (8a; 8'a) concentric with the induction coil, terminating at each end in an annular collar (8b; 8'b) that delimits an air gap with the portion of the pipe.

9. The device according to any of claims 7 and 8, wherein the induction coil is made by winding a conductive wire with a variable pitch along said induction coil.

10. The device according to any of claims 7 and 8, wherein the induction coil is manufactured by winding conductive wires in several layers along all or part of its length.

11. The device according to any of claims 7 to 10; further comprising means (10; 18; 20; 22; 24; 26) for minimizing the influence of edge effects on the quality of measurements.

12. The device according to claim 11, comprising plates (10) of ferromagnetic material that can be slid radially over each collar (8b) of the cylinder head (8) to come into contact with the surface of the pipe portion to minimize the influence of edge effects on the quality of measurements.

13. The device according to claim 11 comprising laminated flexible blades (18) made of ferromagnetic material arranged in a star configuration around the pipe and at each end of the tubular body of the induction coil in order to make contact with the surface of the pipe portion to minimize the influence of edge effects on the quality of measurements.

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