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

Abstract

A method for determining the linear electrical resistance in AC mode of a steel pipeline, including the steps of generating in a portion of the pipeline an induced current by means of an induction coil centered on a longitudinal axis of the pipeline and traversed by an AC current. The coil is housed in a yoke made of ferromagnetic material in order to confine the magnetic field to a predefined surface of the pipeline portion, measuring the active power dissipated by the pipeline portion subjected to the magnetic field, measuring the amplitude of the produced magnetic field, and determining the linear electrical resistance in AC mode of the pipeline portion from the measurements of the dissipated active power and the amplitude of the induced magnetic field. A device is provided for implementing such a method.

Claims

1.-13. (canceled)

14. A method for determining the linear electrical resistance in AC mode of a steel pipeline, comprising the steps of: a) generating in a portion of the pipeline 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 pipeline and traversed by an AC current, the induction coil being housed in a yoke made of ferromagnetic material in order to confine the magnetic field to a predefined surface of the pipeline portion; b) measuring the active power dissipated by the pipeline portion subjected to the magnetic field; c) determining the amplitude of the produced magnetic field; and d) determining the linear electrical resistance in AC mode of the pipeline portion from the measurements of the dissipated active power and the amplitude of the induced magnetic field, the linear electrical resistance in AC mode of the pipeline portion being determined in step d) by the equation: RAC_DEH=(2×Πheat_JIMP)/(π×OD×HJIMP)2; where RAC_DEH is the electrical resistance in AC mode, Πheat_JIMP is the active power dissipated per unit length of the pipeline, OD is the diameter of the pipeline, and HJIMP is the amplitude of the produced magnetic field.

15. The method according to claim 14, wherein the frequency of the magnetic field produced in step a) varies in order to determine the linear electrical resistance in AC mode of the pipeline portion at different frequencies.

16. The method according to claim 14, wherein steps a) to d) are repeated over the entire length of the pipeline by moving the induction coil along the pipeline.

17. The method according to claim 14, wherein the induction coil of step a) is disposed inside the pipeline.

18. The method according to claim 14, wherein the induction coil of step a) is disposed outside the pipeline.

19. The method according to claim 14, wherein the frequency of the magnetic field produced in step a) is comprised between 5 Hz and 10 kHz.

20. A device for implementing the method according to claim 14, comprising a subsea pipeline made of steel and intended to transport fluids such as oil and gas, an induction coil intended to be centered on a longitudinal axis of the pipeline and to be traversed by an AC current, a yoke made of ferromagnetic material inside which the induction coil is mounted in order to confine the magnetic field to a predefined surface of the pipeline portion, and an apparatus for measuring the active power connected to the terminals of the induction coil to measure the active power dissipated by the pipeline portion subjected to the magnetic field.

21. The device according to claim 20, wherein the yoke comprises a tubular body concentric with the induction coil which terminates at each end in an annular collar delimiting an air gap with the portion of the pipeline.

22. The device according to claim 20, wherein the induction coil is made by winding of a conductive wire with a variable pitch over the length of said induction coil.

23. The device according to claim 20, wherein the induction coil is made by winding of conductive wires in several layers over all or part of its length.

24. The device according to claim 20, further comprising means for minimizing the influence of the edge effects on the quality of the measurements.

25. The device according to claim 24, comprising plates made of ferromagnetic material which are able to radially slide on each collar of the yoke in order to come into contact with the surface of the pipeline portion to minimize the influence of the edge effects on the quality of the measurements.

26. The device according to claim 24, comprising laminated flexible blades made of ferromagnetic material which are positioned in a star configuration around the pipeline and at each end of the tubular body of the induction coil in order to come into contact with the surface of the pipeline portion to minimize the influence of the edge effects on the quality of the measurements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic view of an example of a device for the implementation of the method according to the invention.

[0029] FIG. 2 is a schematic view of another example of a device for the implementation of the method according to the invention.

[0030] FIG. 3 represents an example of a distribution of the magnetic field produced during the method according to the invention in the induction coil and in a pipeline portion.

[0031] FIG. 4 represents an example of a distribution of the active power density dissipated in the induction coil and in a pipeline portion subjected to the method according to the invention.

[0032] FIG. 5 is a curve showing an example of a linear electrical resistance in AC mode as a function of the amplitude of the magnetic field of the outer face of a pipeline portion subjected to the method according to the invention.

[0033] FIG. 6 is a curve showing an example of a linear electrical resistance in AC mode as a function of the amplitude of the magnetic field of the inner face of a pipeline portion subjected to the method according to the invention.

[0034] FIG. 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 the edge effects on the quality of the measurement.

[0035] FIG. 7B is a cross-sectional view of the device of FIG. 7A.

[0036] FIG. 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 the edge effects on the quality of the measurement.

[0037] FIG. 8B is a cross-sectional view of the device of FIG. 8A.

[0038] FIG. 9 represents a variant of the system of FIGS. 8A and 8B to minimize the influence of the edge effects on the quality of the measurement.

[0039] FIG. 10 represents another variant of the system of FIGS. 8A and 8B to minimize the influence of the edge effects on the quality of the measurement.

[0040] FIG. 11 represents yet another variant of the system of FIGS. 8A and 8B to minimize the influence of the edge effects on the quality of the measurement.

[0041] FIG. 12 represents yet another variant of the system of FIGS. 8A and 8B to minimize the influence of the edge effects on the quality of the measurement.

DESCRIPTION OF THE EMBODIMENTS

[0042] The method according to the invention applies to any subsea pipeline (single-shell or double-shell pipeline) made of steel and intended to transport fluids such as oil and gas.

[0043] The method according to the invention applies more particularly to subsea pipelines made of steel (in particular but not exclusively carbon steel) which are subjected to an electrical heating of the “Direct Electrical Heating” (or DEH) type.

[0044] This type of electrical heating consists in applying an AC electric current to the shell to be heated. The heating of the shell is produced by the Joule effect by the current passing therethrough, the heat produced being largely transmitted to the fluids circulating in the shell.

[0045] The method according to the invention aims to determine the linear electrical resistance in AC mode (AC) of such a pipeline by means of a device such as the one represented in FIGS. 1 and 2.

[0046] In these two embodiments, the device is formed by an assembly of an induction coil (or solenoid) intended to be powered by an AC current and of a yoke made of ferromagnetic material.

[0047] In the embodiment of FIG. 1, the device 2 according to the invention is disposed around a pipeline portion 4 (which can be the inner shell of a double-shell pipeline or the shell in the case of a single-shell pipeline) so as to determine the linear AC electrical resistance of the outer face of the pipeline portion.

[0048] The device comprises an induction coil 6 which is centered on the longitudinal axis X-X of the pipeline 4, and a yoke 8 made of ferromagnetic material inside which the induction coil 6 is mounted and which allows confining the field magnetic to a predefined surface of the pipeline portion.

[0049] More specifically, the yoke 8 comprises a tubular body 8a which is centered on the longitudinal axis X-X of the pipeline 4 and which ends at each longitudinal end in an annular collar (or cheek) 8b turned inwards. These collars 8b are positioned facing the outer face of the pipeline portion in order to confine the magnetic field produced by the induction coil in a delimited and predefined annular space, particularly over a precise length of the pipeline.

[0050] The yoke 8 is made of a ferromagnetic material, such as for example ferrite, or of laminated electric steel.

[0051] As for the induction coil 6, it is made by winding of a conductive wire, for example copper or aluminum, this winding can be single-layered or multilayered.

[0052] Moreover, in order to minimize the edge effects, the winding of the conductive wire can have a pitch that varies over the length of said induction coil with a greater conductive wire density at the two longitudinal ends of the tubular body 8a of the yoke than at its center. Alternatively, to obtain the same effect, the conductive wire winding can be done in several layers at these two longitudinal ends and be single-layered in the center.

[0053] In addition, the induction coil 6 is connected to an AC current source (not represented in the figures).

[0054] Finally, the device 2 according to the invention can include means for moving over the entire length of the pipeline in order to perform a measurement of the AC electrical resistance of the entire pipeline. These means, not represented in the figure, can comprise plastic pads or rollers disposed on the external face of the two collars 8b with the aim of centering the device on the pipeline while allowing an axial translation without excessive friction.

[0055] In the embodiment of FIG. 2, the device 2′ according to the invention is disposed inside a pipeline portion 4 (which can be the outer shell of a double-shell pipeline) so as to determine the AC electrical resistance of the inner face of the pipeline portion.

[0056] Compared to the embodiment of FIG. 1, it results from this disposition that the yoke 8′ comprises a tubular body 8′a which ends at each longitudinal end in collars 8′b turned outwards. These collars 8b are positioned facing the inner face of the pipeline portion and confine the magnetic field produced by the induction coil in a delimited and predefined annular space, particularly over a precise length of the pipeline.

[0057] As for the induction coil 6′, it is similar to that of the embodiment of FIG. 1.

[0058] The method according to the invention is implemented by such a device 2, 2′ and provides for the following steps.

[0059] The induction coil 6, 6′ of the device is supplied with AC current at a predetermined frequency f. The power supply of the induction coil generates a magnetic field at a predefined frequency, this magnetic field inducing a current in the thickness of the pipeline portion subjected to the device according to the invention (on its inner face or its outer face).

[0060] More specifically, when the pipeline is excited by an induction coil JIMP of length lg.sub.JIMP and including n turns each traversed by a current i, an orbital current I.sub.JIMP develops by induction at the surface of the pipeline in a pipeline portion of length lg.sub.JIMP and of skin thickness δ.sub.JIMP given by the following equation:

[00001]$\begin{array}{cc}{\delta }_{\mathrm{JIMP}}=\sqrt{\frac{\rho }{\pi .f.\mu }}& \left[\mathrm{Math}.1\right]\end{array}$

[0061] In this equation, f is the frequency of the AC current, ρ is the electrical resistivity of the material of the pipeline, and μ is the magnetic permeability of the material of the pipeline.

[0062] The amplitude of the magnetic field H.sub.JIMP under the induction coil is theoretically constant and is given by the following equation:

[00002]$\begin{array}{cc}{H}_{\mathrm{JIMP}}=\frac{n.Math.i.Math.\sqrt{2}}{{\lg }_{\mathrm{JIMP}}}=\frac{I_{\mathrm{JIM}}.Math.\sqrt{2}}{{\lg }_{\mathrm{JIMP}}}& \left[\mathrm{Math}.2\right]\end{array}$

[0063] The active power P.sub.heat_JIMP dissipated by the pipeline portion subjected to the magnetic field is given by the following equation:

[00003]$\begin{array}{cc}{P}_{\mathrm{heat}\_\mathrm{JIMP}}=\rho .Math.\frac{\pi .Math.\mathrm{OD}}{{\lg }_{\mathrm{JIMP}}.Math.{\delta }_{\mathrm{JIMP}}}.Math.I_{\mathrm{JIM}}^2=\rho .Math.\frac{\pi .Math.\mathrm{OD}}{{\lg }_{\mathrm{JIMP}}.Math.{\delta }_{\mathrm{JIMP}}}.Math.{\left(H_{\mathrm{JIMP}}.Math.\frac{{\lg }_{\mathrm{JIMP}}}{\sqrt{2}}\right)}^2& \left[\mathrm{Math}.3\right]\end{array}$

[0064] In this equation, OD is the diameter of the pipeline (i.e. the outside diameter of the pipeline if the device is disposed around the pipeline or the inside diameter of the pipeline if the device is disposed inside the pipeline).

[0065] It is deduced that the skin thickness δ.sub.JIMP of the orbital current at the surface of the pipeline is given by the equation:

[00004]$\begin{array}{cc}{\delta }_{\mathrm{JIMP}}=\frac{\pi .Math.\mathrm{OD}.Math.\rho .Math.H_{\mathrm{JIMP}}^2}{2.Math.{.Math.}_{\mathrm{heat}\_\mathrm{JIMP}}}& \left[\mathrm{Math}.4\right]\end{array}$

[0066] In this equation, Π.sub.heat_JIMP is the linear active power in the pipeline.

[0067] Moreover, 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 which develops by the induction coil is the same as that of an axial AC current used to heat a pipeline according to the DEH (direct electrical heating) method.

[0068] However, for a pipeline heated by the DEH method, it has been established that the linear electrical resistance in AC current R.sub.AC_DEH is given by the following equation:

[00005]$\begin{array}{cc}{R}_{\mathrm{AC}\_\mathrm{DEH}}=\frac{\rho }{\pi .\mathrm{OD}.{\delta }_{\mathrm{DEH}}}& \left[\mathrm{Math}.5\right]\end{array}$

[0069] Also, by substituting the skin thickness value δ.sub.JIMP calculated previously in the expression above of the value of the linear electrical resistance in AC current R.sub.AC_DEH, the following equation is obtained:

[00006]$\begin{array}{cc}{R}_{\mathrm{AC}\_\mathrm{DEH}}=\frac{\rho }{\pi .\mathrm{OD}.{\delta }_{\mathrm{DEH}}}=\frac{\rho }{\pi .\mathrm{OD}.{\delta }_{\mathrm{JIMP}}}=\frac{2.{.Math.}_{\mathrm{heat}\_\mathrm{JIMP}}}{{\left(\pi .\mathrm{OD}.H_{\mathrm{JIMP}}\right)}^2}& \left[\mathrm{Math}.6\right]\end{array}$

[0070] The linear electrical resistance in AC current R.sub.AC_DEH of the pipeline portion subjected to the magnetic field H.sub.JIMP is then given by the following equation:

[00007]$\begin{array}{cc}{R}_{\mathrm{AC}\_\mathrm{DEH}}=\frac{2.{.Math.}_{\mathrm{heat}\_\mathrm{JIMP}}}{{\left(\pi .\mathrm{OD}.H_{\mathrm{JIMP}}\right)}^2}& \left[\mathrm{Math}.7\right]\end{array}$

[0071] In this equation, Π.sub.heat_JIMP is the linear active power in the pipeline and H.sub.JIMP is the amplitude of the magnetic field in the annular space comprised between the induction coil and the pipeline.

[0072] FIG. 3 shows an example of a distribution of the amplitude of the magnetic field H (measured in Weber) produced during the method according to the invention in a pipeline portion 4 and in the induction coil 6 of the device according to the invention, this magnetic field having been generated by means of a device according to the invention as described above.

[0073] The inventors have found that the smaller the distance between the collars 8b of the yoke 8 and the (inner or outer) face of the pipeline portion 4, the more effective the confinement of the magnetic field on the surface of the pipeline portion. Thus, in the example of FIG. 3 in which the air gap is close to 0, the amplitude of the magnetic field H is relatively homogeneous over the entire surface of the pipeline portion comprised between the two collars of the yoke of the device.

[0074] As described previously, in order to minimize any edge effects due to the clearance between the collars of the yoke and the pipeline portion, the winding of the conductive wire making up the induction coil of the device may have a greater density at the level of the two collars of the yoke of the device.

[0075] FIG. 4 represents an example of a distribution of the active power density P.sub.heat dissipated (measured in W/m.sup.3) in the induction coil and in a pipeline portion subjected to the method according to the invention. This active power can be measured by means of an apparatus for measuring the active power, for example a network analyzer connected to the terminals of the induction coil.

[0076] Here again, it is observed that the dissipated active power is essentially concentrated in the pipeline portion subjected to the device according to the invention.

[0077] From these measurements, the method according to the invention provides for determining by calculation (see equation 6) the linear electrical resistance in AC mode R.sub.AC_JIMP of the pipeline portion.

[0078] FIGS. 5 and 6 are examples of curves representing linear electrical resistances in AC mode (in μΩ/m) as a function of the amplitude of the magnetic field (in A/m), on the one hand for a device positioned outside the pipeline (FIG. 5—corresponding to the configuration of the embodiment of FIG. 2), and on the other hand for a device positioned inside the pipeline (FIG. 6—corresponding to the configuration of the embodiment of FIG. 1).

[0079] It will be noted that the determination of the linear electrical resistance in AC mode according to the method in accordance with the invention can be carried out over the entire length of a pipeline by moving the device along the longitudinal axis of the pipeline and by repeating the previously described steps.

[0080] It should also be noted that the linear electrical resistance in AC mode can be determined as a function of the skin depth by varying the frequency of the electric current supplying 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 the pipeline from 0.1 mm to 20 mm of skin depth.

[0081] It will also be noted that the amplitude of the magnetic field H.sub.JIMP under the induction coil which is necessary to determine the linear electrical resistance in AC mode R.sub.AC_JIMP of the pipeline portion can be measured by a probe or deduced from a measurement of the current i in the induction coil using the following equation: H.sub.JIMP=n×i/lg where n is the number of turns of the induction coil and lg is its length.

[0082] Moreover, to minimize the influence of the edge effects on the quality of the measurement, the inventors have proposed several possible systems for closing the clearance that necessarily exists between the collars of the yoke and the pipeline portion whose dimensional tolerances may be wide. Indeed, this clearance degrades the confinement of the magnetic field in the air gap, and disturbs the magnetic flux circulating in the yoke and the pipeline.

[0083] FIGS. 7A and 7B represent one exemplary embodiment of a system to minimize the influence of the edge effects on the quality of the measurement. In this example, this system is based on the use of plates 10 made of ferromagnetic material which are able to slide radially on each collar 8b of the yoke 8 in order to come into contact with the surface (external surface here) of the pipeline portion. In this way, these plates 10 form a field bridge above the clearance existing between the collars of the yoke and the pipeline portion.

[0084] More specifically, the plates 10, for example twelve in number, form a ring when they are placed end to end about the axis X-X of the pipeline (see FIG. 7B). These plates are housed in an annular groove 12 delimited between the respective collars 8b of the yoke and annular flanges 14 (for example made of plastic) assembled against these collars. An O-ring 16 positioned around the plates 12 allows tensioning them in order to cause them to radially bear against the surface of the pipeline.

[0085] Another exemplary embodiment of this system to minimize the influence of the edge effects represented by FIGS. 8A and 8B consists of the use of laminated flexible blades 18 made of ferromagnetic material which replace the collars of the yoke.

[0086] As represented in FIGS. 8A and 8B, these flexible blades 18 are positioned at each end of the tubular body 8a of the induction coil and in a star configuration around the pipeline in order to radially bear on its surface. The greater or lesser flexion of the blades allows absorbing the variable clearance as a function of the tolerances of the pipeline.

[0087] In one variant of this system represented in FIG. 9, the blades can be replaced by a ring 20 (or ring segments) made of ferromagnetic material which is inflatable. More specifically, as represented in the lower part of FIG. 9, the ring 20 is able to inflate in order to radially bear against the surface of the pipeline 4.

[0088] In another variant of this system represented in FIG. 10, the blades are replaced by a ring 22 (or ring segments) made of ferromagnetic material which is expandable. As represented in the lower part of FIG. 10, the ring 22 is able to expand in order to radially bear against the surface of the pipeline 4.

[0089] In yet another variant of this system represented by FIG. 11, the blades are replaced by a ring 24 (or ring segments) made of ferromagnetic material which is hollow and flexible. As represented in the lower part of FIG. 11, the ring 24 is able to be deployed in order to radially bear against the surface of the pipeline 4.

[0090] In yet another variant of this system represented by FIG. 12, the blades are replaced by a ring 26 (or ring segments) made of ferromagnetic material which is corrugated and flexible. As represented in the lower part of FIG. 12, the ring 26 is able to be deployed in order to radially bear against the surface of the pipeline 4.