GB2214118A

GB2214118A - Method and apparatus for welding a length of tube on to a pipeline

Info

Publication number: GB2214118A
Application number: GB8900657A
Authority: GB
Inventors: Jean-Clause Ancenys; Jacques Farenc
Original assignee: COMEX
Current assignee: COMEX
Priority date: 1988-01-19
Filing date: 1989-01-12
Publication date: 1989-08-31
Anticipated expiration: 2009-01-12
Also published as: GB2214118B; FR2625932B1; DK21189A; DK171784B1; NO177927B; NO890122D0; NL8900094A; NL194071C; DK21189D0; FR2625932A1; GB8900657D0; NO890122L; NL194071B; NO177927C

Abstract

The present invention provides a method and apparatus for welding a length of tube (3) onto a pipeline (1) in which the weld zone of the pipeline is preheated in order to obtain a welding temperature favourable to the welding operation by placing an annular inductor (4) around the welding zone (7) and around the length (3). The annular inductor comprises one or more layers of conductors fed with medium frequency current, the inductor (4) being disposed on an insulating mat (5) put into contact with the wall of the pipeline (1). Shielding (6) is disposed around the length of tube (3), the shielding being displaceable lengthwise on the length (3) enabling it to be positioned at a variable distance from the welding zone (7) in order to reduce the temperature in the length of tube (3). This counters the tube's inherent tendency to overheating due to its substantially coaxial position within the inductor (4) due to the fact that no fluid flow takes place inside the length of tube (3) which could reduce the temperature of the length. The invention is applicable to off-shore underwater work. <IMAGE>

Description

METHOD AND APPARATUS FOR WELDING A LENGTH OF TUBE ONTO A PIPELINE The present invention relates to a method and to apparatus for welding a length of tube onto a pipeline.

The technical field of the invention is welding work perfored under water, off-shore.

In order to weld onto a pipeline which is in service, the flow conditions of the fluid being transported, and essentially the type of fluid and its flow rate, often require the heating power used to be greater than that which can be provided by conventional electrical resistances.

In order to show that a method is capable of satisfying requirements suitable for obtaining a weld, operation is simulated under hyperbaric conditions and destructive tests are then performed on the weld.

Such operations are performed on land in a test bench constituting the subject matter of the Applicants' French patent application FR-87 17974.

Once it is observed that testing is positive, the welding operation on the underwater pipeline is then proceded with in situ on the seabed with the portion of the pipeline to be welded being isolated in a welding chamber.

In order to mitigate the lack of heating power provided by conventional electrical resistances for preheating the welding zone of the duct, it is common practice to use inductors around the zone to be preheated, with the inductors being disposed on an insulating mat, e.g. of glass fiber cloth.

Such inductors comprise one or more conductive layers powered with medium frequency current which generates eddy currents in the pipeline, thereby giving rise to a corresponding increase in temperature in the zone where welding is to be performed. If necessary, these conventional, specially designed conductors are cooled by a cooling fluid circuit in order to avoid overheating and damaging them.

A problem to be solved is the application of such a technique to welding a length of tube to a pipeline in service, i.e. to "tapping" the pipeline.

The substantially coaxial position of the length of tube and of the inductor causes currents to be induced in both masses of metal. However, a portion of the heat flux induced in the wall of the pipeline which is in service is dissipated by the fluid conveyed therein. In contrast, the eddy currents that occur in the length of tube in which no fluid is flowing have the effect of overheating the length to a temperature higher than its welding temperature.

The above-described situation is the same regardless of whether the welding operation takes place in a hyperbaric enclosure where testing is performed or In a welding chai#b#r where the length of tube is welded to the pipeline in service for the purpose of installing valves or other apparatus, or for installing a branch circuit.

In conventional manner, the tapping length of tube has a flange for enabling the work to be performed.

After the length of tube has been welded to the pipeline and prior to implementing the work outlined above, a hole is made entirely conventionally through the wall of the pipeline in the zone delimited by the length of tube.

Preferred implementations of the present invention provide a novel solution to the operation of welding a length of tube or "tap" to a pipeline whose welding zone is preheated by means of an eddy current inducing generator.

The present invention provides a method of welding a length of tube onto a pipeline in which the weld zone of the pipeline is preheated in order to obtain a welding temperature favorable to the welding operation by placing an annular inductor around the welding zone and around the length, the annular inductor comprising one or more layers of conductors fed with medium frequency current, the inductor being disposed on an insulating mat put into contact with the wall of the pipeline, wherein the length of tube is surrounded by shielding displaceable along the length of the tube in order to enable the shielding to be placed closer to or further away from the welding zone in order to reduce the temperature in the length of tube given the inherent overheating therein due to its substantially coaxial position inside the inductor and the fact that no flow takes place inside the length of a fluid suitable for reducing the temperature of the length.

In one particular implementation of the method, a flow of cooling fluid is established between the shielding and the length of tube in order to adjust the temperature of the length.

In another implementation, a flow of cooling fluid is set up inside the shielding in order to adjust the temperature of the length of tube, and the shielding is insulated by means of an electrically insulating material.

In the method, during which a welding operation is performed on a portion of pipeline disposed inside a hyperbaric enclosure or on a port w underwater pipeline which is isolated in a welding chamber, with the inductor being fed with medium frequency current by two electric cable lines connected via a feedthrough through the wall of the enclosure or of the welding chamber to two electric cable feed lines disposed outside the enclosure or the chamber, the inside and outside cables of each of the lines are connected via two strips which are coaxial in order to cancel the magnetic fields induced by the feedthrough, thereby preventing the wall of the enclosure or of the chamber from being heated.

The invention also provides apparatus for welding a length of tube onto a pipeline in which the weld zone of the pipeline is preheated in order to obtain a welding temperature favorable to the welding operation by placing an annular inductor around the welding zone and around the length, the annular inductor comprising one or more layers of conductors fed with medium frequency current, the inductor being disposed on an insulating mat put into contact with the wall of the pipeline, the apparatus comprising shielding disposed around the length of tube, the shielding including means for enabling it to be displaced lengthwise on the length and enabling it to be positioned at a variable distance from the welding zone in order to reduce the temperature in the length of tube, given the tube's inherent tendency to overheating due to its substantially coaxial position within the inductor due to the fact that no fluid flow takes place inside the length of tube which could reduce the temperature of the length.

In one embodiment, shielding is constituted by a metal sleeve made of a substance which is a good conductor of electricity, the sleeve being disposed substantially coaxially about the length of tube and being maintained at a small distance from the outside face thereof by means inserted between the length of tube and the sleeve in order to reserve an annular space therebetween.

Said id means inserted between the length of tube and the sleeve may be O-flngs.

The sleeve may include connectors opening out into the annular space closed off by the O-rings in order to connect the assembled sleeve and length of tube to a cooling fluid flow circuit in order to adjust the temperature of the length to the welding temperature.

In another embodiment, the shielding is constituted by a double-walled sleeve made of a material which is a good conductor of electricity, the walls delimiting an annular space therebetween which is closed at the ends of the sleeve, with the inside wall being put into contact with the length of tube via a sheet of electrically insulating material. In this embodiment, the sleeve includes connectors opening out into the inside of the double wall in order to connect the sleeve to a cooling fluid circuit in order to adjust the temperature of the length of tube to the welding temperature.

In another embodiment, the shielding is constituted by a winding of pipe having contacting turns closely surrounding the length of tube with an interposed sheet of electrically insulating material.

In this embodiment, the shielding is constituted by a winding of electrically insulated pipe, with contacting turns which closely surround the length of tube.

The two ends of the winding are connected to a cooling fluid circuit for adjusting the temperature of the length of tube to the welding temperature.

In an embodiment, the shielding is cylindrical and if it is constituted by a winding of pipe, the winding is helical and the turns touch one another.

In another embodiment, the shielding is cylindrical and its bottom portion follows the outline of the welding zone and runs parallel thereto.

In the apparatus of the invention in which the welding operation is performed on a length of pipeline disposed inside a hyperbaric enclosure or on a length of underwater pipeline which is isolated inside a welding chamber, and in which the inductor is fed with medium fxetiuency current via two electric cable lines connected via a feedthrough in the wall of the enclosure or of the welding chamber to two electric cable power lines situated outside the enclosure or the chamber, the feedthrough is constituted by two strips which are coaxial, with one of the strips being in the form of a tubular element surrounding the other strip which is in the form of a core, the through tube and core of the feedthrough each being connected to one of the cable lines feeding the inductor, the disposition having the effect of cancelling the magnetic fields induced in the feedthrough and consequently of preventing the wall of the enclosure or of the chamber from heating.

The tubular element and the core are embedded in insulating material constituting the feedthrough.

The result of the invention is to implement a novel solution for welding a length of tube onto a pipeline in which the welding zone is preheated by induced current.

Implementations and embodiments of the invention are described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagram showing the implementation of the method and the application of the apparatus of the invention to a length of pipeline placed in a hyperbaric enclosure; Figure 2 is a diagram showing the implementation of the method and the application of the apparatus of the invention on an underwater pipeline whose portion to be welded to the length of tube is isolated in a welding chamber; Figures 3 to 5 are partial section views of the welding zone of a pipeline to which a length of tube is to be fixed, showing three different embodiments of the shielding; Figure 6 is a fragmentary perspective view showing the various components of a conductor used for constituting the inductor applied to preheating the welding zone;; Figure 7 is a section view through a connector for feeding the inductor with medium frequency current; and Figure 8 is a section of a feedthrough in accordance with the invention for feeding the preheating inductor with medium frequency currerlt.

Reference is made initially to Figures 1 and 3 of the drawings which show a portion of a pipeline 1 disposed in a hyperbaric enclosure 2. The aim to be achieved is to weld a length of tube 3, or "tap", to the portion 1 and extending orthogonally thereto.

In order to avoid microcracking and cracking due to hydrogen during welding, which cracking takes place at temperatures of less than 100 C, the welding zone is preheated, with preheating also having the effect of facilitating the diffusion of hydrogen in the welded zone.

By slowing the rate at which the welded zone cools down, preheating also serves to reduce the hardness of the metal at the weld or in the zone affected by the heat, and thereby makes the weld less sensitive to the propagation of microcracks or other defects.

In order to obtain a preheat temperature which favors welding, an annular inductor 4 is placed around the welding zone, and thus around the length of tube 3. The annular inductor 4 comprises one or more conductor layers, e.g. two layers (Figure 3), which are fed with medium frequency current.

Said inductor 4 is disposed on an insulating mat 5 put into contact with the wall of the portion of pipeline 1 and fixed thereto by any conventional means, e.g. by means of a girth strap, preferably made of insulating material. The mat 5 may be made of glass fiber cloth, for example.

The conductors constituting the inductor 4 are of the hollow type and an example 4a is shown in Figure 6 of the drawings and comprises, from the inside going outwards, a brass or copper pipe 4a1, three superposed layers of copper braid 4a2, and an insulating protective sheath 4a3, e. g. made of glass fiber.

Such conductors 4a are designed to convey a flow of cooling fluid, e.g. water, in order to control the temperature of the conductor 4a.

Since the preheat temperature for the pipeline in service is about i5O for example, and rj1##fl he substaiitially coaxial situation of the length of tube 3 and of the inductor 4, the fact that the length 3 has no fluid flowing therealong which could reduce its temperature, means that it is observed that the length of tube 3 overheats while the preheat temperature of the pipeline in service is limited by the flow of fluid conveyed therealong.

In order to avoid this overheating, the length of tube 3 is surrounded by shielding 6 designed to be displaced along the length 3 in order to take up a position at a variable distance from the welding zone 7 in order to cause the temperature in the length of tube to be brought to the design temperature for welding purposes.

In the embodiment shown in Figure 3, in particular, the length of tube 3 is surrounded by a metal sleeve 8 made of a material which is a good conductor of electricity. For example, the sleeve may be made of copper sheet and it is disposed substantially coaxially around the length of tube 3 while being maintained at a small distance from the outside face thereof, e.g. by means of O-rings 9, so as to form an annular space 10 between the tube 3 and the sleeve 8, with the ends of the annular space being closed by the sealing rings 9.

In order to control the temperature of the length of tube 3 as well as possible, a flow of fluid, e.g. water, is established in the annular space 10. The sleeve 8 receives two tube connectors 11 which open out into the annular space and which are connected to respective fluid flow "go" and "return" pipes 12 and 13.

In another embodiment of the shielding 6, a double-walled sleeve 14 is used (see Figure 4).

This sleeve thus comprises two walls 14a and 14b which are coaxial with the length of tube 3 and which are closed at their ends by additional walls 14c and 14d in order to delimit an annular space 15. The wall 14b is applied against the tube 3 by means of a sheet 141 of electrically insulating material.

In some circumstances, connectors 16 are fixed to the outside wall 14a and open out into the annular space 15. These connectors are connected like the connectors 11 of Figure 1 mentioned above to fluid flow "go" and "tetl.'rn" pipes 12 and 13, e.g. for conveying water.

Another embodiment of the shielding (6) is shown in Figure 5.

This is constituted, for example, by a winding of copper pipe 17 having turns which touch one another and which closely surround the length 3 with an interposed sheet of electrically insulated material 141. In a variant embodiment, and based on the same principle, the shielding 6 is constituted by a winding of insulated pipe 17 with turns that touch one another.

In this embodiment, the two ends of the winding are electrically interconnected.

If necessary, the two ends 17a and 17b of the winding are connected e.g. by means of the pipes 12 and 13 of the abovementioned fluid flow circuit having water, for example, flowing therealong.

In one kind of embodiment, the shielding of Figures 3 to 4 is cylindrical and its end edges are substantially parallel to each other as shown to the~right of Figure 5.

In another kind of embodiment, the shielding in the figures is cylindrical and its bottom portion adopts the outline of the weld zone and runs parallel thereto, as shown in Figure 1 and in the righthand sides of Figures 3 and 4.

Figures 1, 7, and 8 show the means used for feeding the inductor 4 with medium frequency current via two electric cable lines connected via a feedthrough 18 provided in the wall of the hyperbaric enclosure (Figure 1). Power is supplied, for example, via an external umbilical cord 19 connected to a low frequency current generator and including, in particular, two cable lines 19a and 19b whose ends have connection terminals 19a1 and 19by. Within the enclosure, the conductors 4a of the inductor 4 are each provided with a connector 20 (Figure 7) for enabling the conductors 4a to be connected to two electric cable lines 21 and 22 having connection terminals 21a and 21b at their ends. The connectors 20 are placed inside insulating sheaths 20a.

In accordance with the invention and as shown in Figure 8, the feedthrough comprises two coaxial strips 18a and 18b one of which (18a) is in the form of a tubular component and the other of which (18b) is in the form of a core. One of the outside lines 19a has its terminal l9a1 connected to the end of the tubular element 18a that projects outside the hyperbaric enclosure 2. The other end of the tubular element 18a extending into the enclosure 2 receives the terminal 21a of the line 21.

The other outside line 19b has its terminal 19b1 connected to that end of the core 18b which extends outside the enclosure 2.

The other end of the core l9b extending inside the hyperbaric enclosure 2 is connected to the terminal 22a on the line 22.

This design has the effect of cancelling the magnetic field induced by the feedthrough and thus of preventing the wall of the hyperbaric enclosure 2 from heating.

The tubular element 18a and its core 18b are preferably embedded in insulating material 23 in order to insulate them electrically from the wall of the enclosure 2.

Steps formed in the peripheries of the terminals 18a and 18b ensure that they are properly held in the insulating material.

Cooling fluid flow is shown in Figure 1 of the drawings.

The bottom of the hyperbaric enclosure 2 constitutes a water retention zone, given that water is the cooling fluid, and it is caused to flow, in the present example, both in the inductor 4 and in the shielding 6.

The water is pumped by pumps 24 and 25 and returns to the bottom of the enclosure via the "return" pipes 12 and 4b.

Reference is finally made to Figure 2 which shows a diver 26 performing a welding operation on an underwater pipeline 27, with a portion 27a thereof on which the length of tube 3 is being welded being isolated inside a welding chamber 28.

Welding on the seabed is performed under practically the same conditions and using the same equipment as is used inside the hyperbaric chamber 2. As a result corresponding items are given the same reference symbols.

In order to provide the pumping required to cause the cooling fluid to flow through the inductor 4 and the shielding 6, the pipes 4b, 12, and 29 are plunged into seawater 30 and seawater is pumped in or out to flow through the inductor and through the shielding Welding may be performed, for example, by means of a TIG head 31 powered from a diver's welding station 32.

Claims

1/ A method of welding a length of tube onto a pipeline in which the weld zone of the pipeline is preheated in order to obtain a welding temperature favorable to the welding operation by placing an annular inductor around the welding zone and around the length, the annular inductor comprising one or more layers of conductors fed with medium frequency current, the inductor being disposed on an insulating mat put into contact with the wall of the pipeline, wherein the length of tube is surrounded by shielding displaceable along the length of the albe in order to enable the shielding to be placed closer to or further away from the welding zone in order to reduce the temperature in the length of tube given the inherent overheating therein due to its substantially coaxial position inside the inductor and the fact that no flow takes place inside the length of a fluid suitable for reducing the temperature of the length.

2/ A method according to claim 1, wherein a flow of cooling fluid is established between the shielding and the length of tube in order to adjust the temperature of the length.

3/ A method according to claim 1, wherein a flow of cooling fluid is set up inside the shielding in order to adjust the temperature of the length of tube, and the shielding is insulated by means of an electrically insulating material.

4/ A method according to any one of claims 1 to 3, in which a welding operation is performed on a portion of pipeline disposed inside a hyperbaric enclosure or on a portion of underwater pipeline which is isolated in a welding chamber, with the inductor being fed with medium frequency current by two electric cable lines connected via a feedthrough through the wall of the enclosure or of the welding chamber to two electric cable feed lines disposed outside the enclosure or the chamber, wherein the inside and outside cables of each of the lines are connected via two strips which are coaxial in order to cancel the magnetic fields induced by the feedthrough, thereby preventing the wall of the enclosure or of the chamber from being heated.

5/ Apparatus for welding a length of tube onto a pipeline in which the weld zone of the pipeline is preheated in order to obtain a welding temperature favorable to the welding operation by placing an annular inductor around the welding zone and around the length, the annular inductor comprising one or more layers of conductors fed with medium frequency current, the inductor being disposed on an insulating r.#t put into contact with the wall of the pipeline, the apparatus comprising shielding disposed around the length of tube, the shielding including means for enabling it to be displaced lengthwise on the length and enabling it to be positioned at a variable distance from the welding zone in order to reduce the temperature in the length of tube, given the tube's inherent tendency to overheating due to its substantially coaxial position within the inductor due to the fact that no fluid flow takes place inside the length of tube which could reduce the temperature of the length.

6/ Apparatus according to claim 5, wherein the shielding is constituted by a metal sleeve made of a substance which is a good conductor of electricity, the sleeve being disposed substantially coaxially about the length of tube and being maintained at a small distance from the outside face thereof by means inserted between the length of tube and the sleeve in order to reserve an annular space therebetween.

7/ Apparatus according to claim 6, wherein the means inserted between the length of tube and the sleeve are O-rings.

8/ Apparatus according to claim 7, wherein the sleeve includes connectors opening out into the annular space closed off by the O-rings in order to connect the assembled sleeve and length of tube to a cooling fluid flow circuit in order to adjust the temperature of the length to the welding temperature.

9/ Apparatus according to claim 5, wherein the shielding is constituted by a double-walled sleeve made of a material which is a good conductor of electricity, the walls delimiting an annular space therebetween which is closed at the ends of the sleeve, with the inside wall being put into contact with the length of tube via a sheet of electrically insulating material.

10/ Apparatus according to claim 9, wherein the sleeve includes connectors opening out into the inside of the double wall in order to connect the sleeve to a cooling fluid circuit in order to adjust the temperature of the length of tube to the welding temperature.

11/ Apparatus according to claim 5, wherein the shielding is constituted by a winding of pipe having contacting turns closely surrounding the length of tube with an interposed via a sheet of electrically insulating material.

12/ Apparatus according to claim 5, wherein the shielding is constituted by a winding of electrically insulated pipe, with contacting turns which closely surround the length of tube.

13/ Apparatus according to claim 11 or 12, wherein the two ends of the winding are connected to a cooling fluid circuit for adjusting the temperature of the length of tube to the welding temperature.

14/ Apparatus according to any one of claims 5 to 13, wherein the shielding is cylindrical.

15/ Apparatus according to any one of claims 5 to 10, wherein the shielding is cylindrical and wherein its bottom portion follows the outline of the welding zone and runs parallel thereto.

16/ Apparatus according to any one of claims 5 to 15, in which the welding operation is performed on a length of pipeline disposed inside a hyperbaric enclosure or on a length of underwater pipeline which is isolated inside a welding chamber, and in which the inductor is fed with medium frequency current via two electric cable lines connected via a feedthrough in the wall of the enclosure or of the welding chamber to two electric cable power lines situated outside the enclosure or the chamber, wherein the feedthrough is constituted by two strips which are coaxial, with one of the strips being in the form of a tubular element surrounding the other strip which is in the form of a core, the through tube and core of the feedthrough each being connected to one of the cable lines feeding the inductor, the disposition having the effer#t of cancelling the magnetic fields induced in the feedthrough and consequently of preventing the wall of the enclosure or of the chamber from heating.

17/ Apparatus according to claim 16, wherein the tubular element and the core are embedded in insulating material constituting the feedthrough.

18/ A method of welding a length of tube onto a pipeline substantially as herein described with reference to the accompanying drawings.

19/ Apparatus for welding a length of tube onto a pipeline substantially as herein described with reference to and as illustrated in the accompanying drawings.