Method and device for induction heating of an inner pipe of an assembly of coaxial pipes

Abstract

A method of heating an inner pipe of a set of coaxial pipes, wherein the inner pipe is heated by induction using an electromagnetic induction coil (5) surrounding the outer pipe coaxially, the coil passing electrical power at a frequency lower than 100 Hz optimized for maximum energy efficiency of Joule effect heating of the inner pipe. A device (8) is also provided for induction heating an inner pipe of coaxial pipes, the device has a) an induction heater having at least one electromagnetic induction coil (5) coaxially surrounding the outer pipe of the coaxial pipes, and b) a raising device (9) for raising a portion (1-2) of coaxial pipes (1) above the sea bed (13) together with the induction coil(s) (5) surrounding it.

Claims

1. A method of heating an inner pipe of a set of coaxial steel pipes comprising an inner pipe and an outer pipe, wherein the inner pipe is heated by induction using an electromagnetic induction coil surrounding the outer pipe coaxially so as to maintain the temperature of the inner pipe at least 20° C. above the temperature of the water surrounding the outer pipe and to do so for a given duration, the coil passing electrical power at a frequency lower than 100 Hz optimized for maximum energy efficiency of Joule effect heating of the inner pipe.

2. The method according to claim 1, wherein the electromagnetic induction coil generates a magnetic field of strength of at least 0.1 teslas (T) with electrical power at an optimum frequency in the range 1 Hz to 10 Hz in order to obtain heating power of the inner pipe in the range 1 kW to 200 kW.

3. The method according to claim 1, wherein said pipes present the following characteristics: outside diameter of said inner pipe lies in the range 5 cm to 40 cm; outside diameter of said outer pipe lies in the range 7.5 cm to 50 cm; and the thicknesses of said inner and outer pipes lie in the range 10 mm to 30 mm.

4. The method according to claim 1, wherein the inner pipe is heated to 40° C., the inner and outer steel pipes presenting the thermal capacity characteristics of pipe steels and the thermal insulation of the annular space between the inner and outer pipes being such that the time required for cooling from 40° C. to 20° C. is at least 24 h.

5. The method according to claim 1, wherein a movable induction heater device is used, which includes at least one said electromagnetic induction coil surrounding the outer pipe coaxially, and said movable heater device is moved along said set of coaxial pipes.

6. The method according to claim 5, wherein, the inner pipe is heated to a temperature T2 with a movable induction heater device including a said electromagnetic induction coil surrounding the outer pipe coaxially, and said movable heater device is moved at a speed of at least n km/day so that the temperature of the inner pipe for a given length of pipe remains greater than T1 less than T2, where T2-T1 is greater than the temperature drop of the inner pipe over one day.

7. The method according to claim 5, wherein the following steps are performed: a) raising a portion of coaxial pipes resting on a sea bed, said pipe portion having at least one said induction coil arranged coaxially around the outer pipe; and b) moving a movable induction heater device that includes at least one said electromagnetic induction coil lengthwise relative to and along said coaxial pipes while simultaneously raising a new portion of said coaxial pipes.

8. The method according to claim 1, wherein a removable induction heater device is used that is suitable for being applied on coaxial pipes resting on the sea bottom, said device including a said electromagnetic induction coil comprising two independent semi-cylindrical coil portions suitable for being secured to each other in order to form a coaxial coil surrounding said outer pipe when raised above the sea bed.

9. A device for induction heating an inner pipe of coaxial pipes, the device comprising: induction heater means comprising at least one electromagnetic induction coil coaxially surrounding the outer pipe of the coaxial pipes; and raising means for raising a portion of coaxial pipes above a sea bed together with said induction coil(s) surrounding it.

10. The device according to claim 9, wherein it is a movable device, further comprising: motor-driven movement means suitable for enabling said induction coil(s) to move coaxially along the outer pipe of the coaxial pipes; and said raising means for raising a portion of coaxial pipes above the sea bed together with said induction coil(s) surrounding it and said motor-driven movement means.

11. The device according to claim 10, wherein the motor-driven movement means comprise crawler devices suitable for bearing against the outer pipe and thus for moving along the outer pipe when they are actuated, said coils sliding coaxially relative to said outer pipe.

12. The device according to claim 10, wherein said raising means comprise rollers supporting the outer pipe and suitable for sliding relative to said outer pipe when said motor-driven movement means are actuated; said rollers, said coils, and said crawlers being suspended from buoys.

13. The device according to claim 9, wherein the raising means comprise buoys that can be ballasted or deballasted in controlled manner.

14. The device according to claim 9, wherein said raising means and said coils, and said motor-driven movement means, are suitable for being secured to one another and supported by a common support structure.

15. The device according to claim 9, wherein said induction heater device is removable and is suitable for being applied on coaxial pipes resting on the sea bottom, the device including a said electromagnetic induction coil comprising two independent semi-cylindrical coil portions suitable for being secured to each other in order to form a coaxial coil surrounding said outer pipe.

16. The device according to claim 15, wherein the two independent semi-cylindrical coil portions are mounted so as to be suitable for being moved relative to each other in translation transversely so as to enable them to be installed on a pipe that is raised above the sea bed so as to form a coaxial coil surrounding said outer pipe.

17. A method of heating an inner pipe of a set of coaxial steel pipes comprising an inner pipe and an outer pipe, wherein the inner pipe is heated by induction using an electromagnetic induction coil surrounding the outer pipe coaxially to 40° C., the coil passing electrical power at a frequency lower than 100 Hz optimized for maximum energy efficiency of Joule effect heating of the inner pipe, the inner and outer steel pipes presenting the thermal capacity characteristics of pipe steels and the thermal insulation of the annular space between the inner and outer pipes being such that the time required for cooling from 40° C. to 20° C. is at least 24 h.

18. A method of heating an inner pipe of a set of coaxial steel pipes comprising an inner pipe and an outer pipe, wherein the inner pipe is heated by a movable induction heater which includes at least one electromagnetic induction coil surrounding the outer pipe coaxially, the heater movable along the set of coaxial pipes, the coil passing electrical power at a frequency lower than 100 Hz optimized for maximum energy efficiency of Joule effect heating of the inner pipe, wherein the inner pipe is heated to a temperature T2 with the movable induction heater device, said movable heater device being moved at a speed of at least n km/day so that the temperature of the inner pipe for a given length of pipe remains greater than T1 less than T2, where T2-T1 is greater than the temperature drop of the inner pipe over one day.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention appear from the following description given with reference to the accompanying drawings, which show an embodiment having no limiting character. In the figures:

(2) FIG. 1 is a diagrammatic cross-section view of an undersea pipe of PIP type;

(3) FIG. 1A is a graph plotting Joule effect losses in the inner tube as a function of the frequency of the electrical power in a coaxial coil surrounding the outer tube of the PIP type pipe;

(4) FIG. 1B is a graph showing the Joule effect losses in the outer tube as a function of the frequency of the electrical power in a coaxial coil surrounding the outer tube of the PIP type pipe;

(5) FIG. 1C is a graph plotting energy efficiency as a function of the frequency of the electrical power in a coaxial coil surrounding the outer tube of the PIP type pipe;

(6) FIG. 2 is a diagrammatic view of an induction heater installation suitable for being moved along the PIP type pipe;

(7) FIG. 2A shows a variant of the movable induction heater device;

(8) FIG. 3 is a diagrammatic view of a coil suitable for forming a removable induction heater installation that is suitable for being fitted around a PIP type pipe; and

(9) FIG. 3A shows a variant of the movable induction heater device having a removable coil 5.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Example 1

(10) There follows a description of an example of a movable system for applying induction heating locally to a PIP type pipe 1 made of ASTM A512 type steel, which system is capable of heating a length of about 3 km of pipeline to 40° C. per day. The pipeline has a 12″ inner tube full of water. The fluid contained in the pipeline generally has heat capacity greater than that of water. This example is given by way of illustration.

(11) It is desired to heat the inner pipe 1a to 40° C. above the temperature of the sea water in less than 10 minutes (min) over a distance of the order of 10 meters (m), thus making it possible to travel approximately 1.5 kilometers per day per machine.

(12) The PIP comprises an outer pipe 1c, annular thermal insulation 1b based on aerogel, and an inner pipe 1a as shown in FIG. 1. The time required for the inner tube to cool from 40° C. to 20° C. is 24 h.

(13) Two machines make it possible to reestablish the entire system in the event of total failure of the electrical heat tracing system.

(14) The dimensions of the inner and outer pipes used are given below in Table 1 where ID=inside diameter and OD=outside diameter.

(15) TABLE-US-00001 TABLE 1 dimensions of the PIP PIP dimensions Inner pipe ID (mm) 279.4 OD (mm) 323 Outer pipe ID (mm) 412.6 OD (mm) 445.4 Coil ID (mm) 460 OD (mm) 480

(16) The energy that is useful for heating is the energy given off by the Joule effect in the inner tube. The inner tube is sufficiently well insulated to ensure that the energy given off by conduction is negligible throughout the entire time required for the machine to heat the 10 m long portion of pipeline. All of the energy given off by the Joule effect in the outer pipe or in the induction coil is considered as being lost. The constants for the materials used are as given in Tables 2 and 3 below.

(17) TABLE-US-00002 TABLE 2 thermal properties used PIP heat capacity Density (kg/m.sup.3) Cp (J/kg/K) Steel 7850 470 Fluid 1000 4185

(18) The inner pipe is insulated from sea water with thermal insulation presenting a U-value of about 1 watt per square meter per kelvin (Wm.sup.−2K.sup.−1).

(19) TABLE-US-00003 TABLE 3 electromagnetic properties of the materials used Electrical Electrical Resistivity properties conductivity (S/m) (Ω .Math. m) Copper (turns) 5.80E+07 1.72E−08 Steel (inner tube) 2.00E+06 5.00E−07

(20) In FIG. 1C, energy efficiency is calculated as a function of frequency by calculating the ratio of the total power given off by the Joule effect in the inner tube over the total power delivered.

(21) The curves of FIGS. 1A to 1C show that it is possible to perform induction heating of the inner pipe of this pipe-in-pipe providing electrical power is delivered to the coil surrounding the outer pipe at the specific frequency for maximum efficiency of 25% (which is about 4 Hz) corresponding to useful power for Joule effect heating of 25 kW per meter of coil, i.e. total power of 10×25=250 kW. This value makes the project feasible since the power needed to heat a 10 m long pipe string in 10 min is less than 250 kW.

Example 2

Movable Induction Heater Device

(22) FIG. 2 shows a movable induction heater device 8 capable of heating up to 10 km of pipeline per day, depending on the pipe diameters, on the power available, and on the required heating power.

(23) When restarting the pipeline, each movable device 8 begins by heating the PIP type pipe 1 to +40° C. above the surrounding temperature in order to destroy hydrate plugs.

(24) The movable induction heater device 8 may be pre-installed when laying the PIP type pipe 1, or it may be installed on the pipe 1 resting on the sea bottom, as described in Example 3.

(25) In operation, the entire movable device 8 is raised above the sea bed 13 by controlled deballasting of buoys 3, 4 as described below.

(26) The main elements of the mobile induction heater device 8 are as follows:

(27) a rigid support structure 7;

(28) two induction coils 5, 5-1 and 5-2, that are spaced apart in the longitudinal direction of the pipe, being arranged coaxially around the outer pipe and serving to generate the varying magnetic field that is needed for heating the inner pipe, the coils 5 being secured to said support structure 7;

(29) first buoys 3, 3-1 and 3-2 respectively downstream from the first coil 5-1 and upstream from the second coil 5-2, the first buoys 3 being secured to said support structure 7 and being suitable for raising said support structure 7 and the pipe 1;

(30) second buoys 4 of Trelleborg subsea buoyancy type that are secured to the support structure 7 and that serve to control the controlled deballasting for raising the support structure 7 off the sea bed 13;

(31) rollers 2 arranged against the underface of the outer pipe of the pipeline 1 and suspended from the first buoy 3 via ties 3a, the rollers 2 supporting the pipeline and being suitable for enabling movement relative to the pipeline 1 by relative sliding of the pipe 1 on the rollers 2 when the pipe 1 and the support structure 7 are raised by means of the first buoys 3, the movable device 8 being moved in translation relative to the pipeline 1; and

(32) a crawler device 6 supported by the support structure 7 and serving to move the movable device 8 along the PIP type pipe 1, with upper and lower crawler tracks 6a and 6b suitable for clamping the pipe between them by moving radially relative to the pipe using relative radial movement means 6′.

(33) The support structure 7 thus supports the coils 5 and the crawler device 6 directly, and it supports the pipe 1 indirectly via the rollers 2.

(34) The crawler device 6 is of the hydraulically powered type tensioner type (Huisman, 4C Offshore).

(35) A transformer on the ship 11 at the surface 12 serves to transform three-phase alternating current (AC) electrical power at 400 volts (V) and 50 Hz on board the ship 11 at the surface 12 into equivalent power at a voltage higher than 10 kilovolts (kV) or more depending on the distance between the ship and the machine 8 so as to be transported with little power loss. This transport of electricity may be performed by means of an umbilical, generally a cable filled with oil that is a good electrical insulation. The machine 8 needs to have an AC source placed in a leakproof environment for generating single-phase AC in the frequency range 0.1 Hz to 50 Hz so to enable the induction coil to operate in optimum manner. The machine 8 is connected to the ship via an umbilical 10 comprising an undersea cable delivering both electrical and hydraulic power for operating the induction module and the movement module.

(36) The method of using the movable induction heater device 8 thus comprises the following steps:

(37) raising a longitudinal portion 1-2 of pipe 1 that takes on a hump-back shape with two pipe portions 1-1 and 1-3 resting on the sea bed 13 on either side of the portion 1-2 that forms a hump raised above the sea bed 13;

(38) applying the crawlers 6 against the pipe 1;

(39) applying the coils 5 around the pipe 1; and

(40) actuating the crawlers 6 so as to cause the entire heater device 8 (comprising the structure 7 and the elements 2, 3, 4, 5, and 6 that are secured thereto) to move relative to the pipe along the pipe while simultaneously shifting the raised hump-back portion along the pipe 1.

(41) In a variant, shown in FIG. 2A, the support structure 7 has rigid rods 7a, 7b that support and connect securely together the buoys 3-1 and 3-2. Each of the two buoys 3-1, 3-2 supports a tie 3a having a plurality of small rollers 2 mounted as a string on the tie 3a and located against the underface of the pipe 1. The crawler track device 6 arranged between the two coils 5-1 and 5-2 has endless loop type crawler tracks suitable for being pressed against the pipe by an actuator 6′ (not shown), i.e.: two top crawler tracks 6a1, 6a2 and two bottom crawler tracks 6b1 and 6b2. The support structure 7 supports the two coils 5 via support modules 5′. The second buoys 4 support the crawler track device 6 via rigid vertical rods 4a secured at 4b to the horizontal rigid rod 7a of the support structure 7.

(42) The coils 5 are constituted by single-layer or double-layer coils of the required power, having turns in the form of copper tubes that are cooled, e.g. by water. The coils are placed in a chamber filled with a specially designed fluid that does not come into contact with sea water.

(43) In Examples 2A and 2B below, the pipes of the PIP type pipe were made of the materials of Example 1 and they were configured to take 24 h for cooling down from +40° C. to 20° C. with inner and outer pipes made of steel having thermal capacity of 1 W/m.sup.2/K in order to process a PIP type pipe having a length of 12 km in 24 h. The inner pipes were full of water.

Application Example 2a

(44) Machine 8 pre-installed on the tube;

(45) Inner pipe: outside diameter 4″ and thickness 10 mm;

(46) Outer pipe: outside diameter 8″ and thickness 10 mm;

(47) Induction coils: two 1 m coils 5, serving to generate a magnetic field of 0.1 (T) approximately, and each serving to generate a useful power of 50 kW;

(48) 400 kW AC source at a frequency of 4 Hz and a useful power of 100 kW; and

(49) Travel speed of the device 8: 5.6 km/day.

Application Example 2B

(50) Machine 8 pre-installed on the tube;

(51) Inner pipe: outside diameter 12″ and thickness 20 mm;

(52) Outer pipe: outside diameter 16″ and thickness 15 mm;

(53) Induction coils: two 2 m coils 5, serving to generate a magnetic field of 0.1 (T) approximately, and each serving to generate a useful power of 100 kW;

(54) 800 kW AC source at a frequency of 2 Hz and a useful power of 200 kW; and

(55) Travel speed of the device 8: 1.5 km/day.

Example 3

Removable Induction Heater Device

(56) This is a device that can be fitted locally on a pipe resting on the sea bottom in order to perform local heating, but it could equally well be used in the context of a movable device as described in Example 2 and shown in FIG. 3A.

(57) The induction coil 5 shown diagrammatically in FIG. 3 comprises two symmetrical semi-cylindrical portions 5a and 5b that are suitable for surrounding the PIP type pipe and that are connected together by means of electrical connectors 5c at the ends of the half-turns 5d that they support.

(58) These semi-cylindrical half-coils were prepared as described below.

(59) Copper tubes were used of round section of about 1000 mm.sup.2 or more for passing currents of the order of 10,000 amps (A) so as to have only a single layer of conductive tubes for connecting together.

(60) The two portions 5a and 5b were connected together by flat connectors 5c or by connectors that engage one in another and of larger section. The water cooling circuits may be independent for the two portions of the coil.

(61) The fabrication steps comprise:

(62) bending the copper tube into standard coil turns;

(63) cutting the resulting coil in an axial plane; and

(64) welding a flat connector 5c to each end of each half-turn 5d.

(65) In the movable device of FIG. 3A, the two half-coils 5a and 5b are mounted so as to be capable of moving in translation relative to each other on a transverse structure 5′ under the horizontal beam 7a of the support structure 7.

(66) It is thus possible to mount the coils 5 around the pipe 1 after the pipe 1 has been raised by the buoys 3 and 4 and without requiring the coils to be pre-installed.

(67) More precisely in FIG. 3A, the support structure 7 is made up of three portions 7a, 7b-1, and 7b-2 that are initially independent and that are secured to one another prior to operation.

(68) Vertical beams 7b-1 and 7b-2 are secured respectively to each of the buoys 3-1 and 3-2. A horizontal beam 7a secured to the second buoys 4 supports the coils 5 and the crawlers 6.

(69) Initially, the buoys 3-1 and 3-2 together with the beams 7b-1 and 7b-2 are used so as to raise the portion 1-2 of pipe 1. To do this, they are positioned spaced apart along and above the pipe by using a remotely operated vehicle (ROV) (not shown) and the ties 3a supported by the buoys 3 are arranged around the pipe 1 with the rollers 2 against the underface of the pipe.

(70) Thereafter, the horizontal central beam 7a with its buoys 4 is positioned between the vertical beams 7b-1 and 7b-2 so as to position the coils 5 and the crawlers 6 around the pipe 1. For this purpose, and once in position, the half-coils 5a and 5b are moved in translation as to form coils 5 that are arranged coaxially around the pipe 1, and the crawler tracks 6a and 6b are actuated radially by means of the actuator 6′ so that they press against the pipe 1, and the beams 7b-1 and 7b-2 are fastened to the central beam 7a so as to form a common support structure 7.