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Title:
METHOD AND APPARATUS FOR CONNECTING PIPES TOGETHER
Document Type and Number:
WIPO Patent Application WO/2014/032704
Kind Code:
A1
Abstract:
A pipe connection assembly for connecting a first pipe to a second pipe (3), said assembly comprising: a third pipe (2) having a pipe end; said second pipe (3) having an outer surface, wherein said end of the third pipe (2) is arranged to be positioned in contact with said outer surface of the second pipe (3), for providing at least one area of contact therebetween; and electrical supply means for providing an electric current in the material of the second and third pipes to heat the second and third pipes at said area of contact, to join the second and third pipes together at said area of contact.

Inventors:
TORVESTAD JAN CHRISTIAN (NO)
APELAND KJELL EDVARD (NO)
BERGE JAN OLAV (NO)
LEITET STIG JOHNNY (NO)
Application Number:
PCT/EP2012/066710
Publication Date:
March 06, 2014
Filing Date:
August 28, 2012
Export Citation:
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Assignee:
STATOIL PETROLEUM AS (NO)
TORVESTAD JAN CHRISTIAN (NO)
APELAND KJELL EDVARD (NO)
BERGE JAN OLAV (NO)
LEITET STIG JOHNNY (NO)
International Classes:
F16L41/08; B23K11/00; B23K11/02; B23K11/18; B23K11/20
Domestic Patent References:
WO2008095286A12008-08-14
WO2003002902A12003-01-09
Foreign References:
US3412226A1968-11-19
US3849871A1974-11-26
US2374763A1945-05-01
US8028711B22011-10-04
DE3123718A11982-12-30
FR1488931A1967-07-13
US8028711B22011-10-04
Attorney, Agent or Firm:
WEAVER, Richard (Fletcher HouseHeatley Road,The Oxford Science Park, Oxford OX4 4GE, GB)
Download PDF:
Claims:
CLAIMS:

1 . A pipe connection assembly for connecting a first pipe to a second pipe, said assembly comprising:

a third pipe having a pipe end;

said second pipe having an outer surface, wherein said end of the third pipe is arranged to be positioned in contact with said outer surface of the second pipe, for providing at least one area of contact therebetween; and

electrical supply means for providing an electric current in the material of the second and third pipes to heat the second and third pipes at said area of contact, to join the second and third pipes together at said area of contact.

2. A pipe connection assembly as claimed in claim 1 , wherein said area of contact is less than a cross-sectional area of the wall of the third pipe.

3. A pipe connection assembly as claimed in claim 1 or 2, wherein the wall of the third pipe narrows to a point at the contact of the second and third pipes.

4. A pipe connection assembly as claimed in any preceding claim, wherein the said pipe end comprises a material comprising a nickel-chromium alloy.

5. A pipe connection assembly as claimed in any preceding claim, wherein the wall of the third pipe comprises first and second regions arranged to transmit electrical current through said regions, and is configured to provide greater electrical resistance in said second region than in said first region to produce heat at said area of contact contact.

6. A pipe connection assembly as claimed in claim 5, wherein said first region of the wall comprises a first material and said second region of the wall comprises a second material, wherein said first material is more resistive to the electric current than the second material.

7. A pipe connection assembly as claimed in claim 6, wherein said first and second regions are formed of said first and second materials respectively, and are arranged to transmit current in said materials of which the first and second regions are formed.

8. A pipe connection assembly as claimed in claim 6 or 7, wherein at least one of the first and second regions contains a third material arranged within the wall of the third pipe in said region, said third material being less resistive than either or both of the first and second materials.

9. A pipe connection assembly as claimed in any of claims 6 to 8, wherein said second material comprises an Inconel® alloy.

10. A pipe connection assembly as claimed in any of claims 6 to 9, wherein said first material comprises a metal. 1 1 . A pipe connection assembly as claimed in any of claims 5 to 10, wherein the wall of the third pipe has a greater cross-sectional area in the first region than in the second region.

12. A pipe connection assembly as claimed in any of claims 5 to 1 1 , wherein the wall of the third pipe has a greater thickness in the first region than in the second region.

13. A pipe connection assembly as claimed in any of claims 5 to 12, wherein said second region of the wall is closer to said pipe end than said first region.

14. A pipe connection assembly as claimed in any of claims 5 to 13, wherein said second region is located at or adjacent to or contains said end of the third pipe.

15. A pipe connection assembly as claimed in any preceding claim, wherein the wall of the third pipe tapers or narrows toward said pipe end.

16. A pipe connection assembly as claimed in any preceding claim, wherein said pipe end defines an edge at which outer and inner surfaces of the wall of the third pipe meet, said edge arranged to contact the outer surface of the second pipe.

17. A pipe connection assembly as claimed in any preceding claim, comprising a wire for delivering current to the end of the third pipe.

18. A pipe connection assembly as claimed in any preceding claim, wherein the wall of the second pipe has greater thickness than the wall of the third pipe at or adjacent to the area of contact.

19. A pipe connection assembly as claimed in any preceding claim, wherein the internal diameter of the third pipe is less than the internal diameter of the second pipe.

20. A pipe connection assembly as claimed in any preceding claim, wherein the internal diameter of the first pipe is in the range of 1 to 10 inches.

21 . A pipe connection assembly as claimed in any preceding claim, wherein said electrical supply means is arranged to provide current in the pipe with a peak amplitude in the range of 3 to 50 kiloamps.

22. A pipe connection assembly as claimed in any preceding claim, wherein said electrical supply means is arranged to provide a current in the pipe for a period in the range of 5 to 10 seconds.

23. A pipe connection assembly as claimed in any preceding claim, further comprising a collar attached to the second pipe, and arranged to receive said end of the third pipe.

24. A pipe connection assembly as claimed in any preceding claim, wherein the third pipe has a flange and the assembly further comprises tensioning means arranged to be connected to the flange for applying a continuous tensioning force to press the third pipe against the second pipe for joining the first and second pipes together.

25. A pipe connection assembly as claimed in any preceding claim, wherein the second and third pipes have respective longitudinal axes, and said third pipe is arranged to be positioned with respect to the second pipe with said axes at a right angle or an acute angle to each other.

26. A pipe connection assembly as claimed in any preceding claim, wherein the third pipe is positioned so as to extend from the second pipe in a direction away from the outer surface of the second pipe. 27. A pipe connection assembly as claimed in any preceding claim, wherein the second pipe contains fluid from a well.

28. A pipe connection assembly as claimed in claim 27, wherein said fluid comprises a hydrocarbon fluid.

29. A pipe connection assembly as claimed in any preceding claim, being a subsea assembly.

30. A method of joining a first pipe to a second pipe, the method comprising the steps of:

a. positioning an end of said first pipe in contact with an outer surface of said second pipe to provide at least one area of contact therebetween; and

b. applying an electrical current through the material of the first and second pipes and through said area of contact to produce heat at said area of contact, so as to join the first and second pipes together at said area of contact.

31 . A method as claimed in claim 30, which further comprises forming a wall of the first pipe, said wall having a cross-sectional area greater than said area of contact. 32. A method as claimed in claim 30 or 31 , which further comprises forming a wall of the pipe that narrows to a point at the contact of the first and second pipes.

33. A method as claimed in any of claims 30 to 32, wherein said area of contact provides a continuous ring of contact, and step b is performed to join said first and second pipe walls to form a gas-tight seal therebetween.

34. A method as claimed in any of claims 30 to 33 which further comprises applying a continuous force to press said end of the first pipe into contact with said outer surface of the second pipe during the process of step b

35. A method as claimed in any of claims 30 to 34 which further comprises forming an aperture through a wall of said second pipe, said aperture providing a fluid connection between an inside of said second pipe and an inside of the first pipe. 36. A method as claimed in claim 35, wherein the step of forming said aperture comprises machining out material of the wall of the second pipe.

37. A method as claimed in any of claims 30 to 36 which comprises using a pipe connection assembly as claimed in any of claims 1 to 29, wherein:

the third pipe of said assembly is the first pipe of said method; and

the second pipe to which said assembly is applied is the second pipe of said method.

A method as claimed in any of claims 30 to 37 performed subsea.

Description:
Method and apparatus for connecting pipes together

The present invention relates to pipes, in particular pipe connection techniques. Background

Pipes are widely used to transport fluids from one location to another. In hydrocarbon production, pipelines are used to transport fluids from a well, often times long distances and in many instances subsea, for example using a pipeline installed on the seabed.

It can be desirable to connect a new pipeline to an existing pipeline, for example to form a branch for directing fluid from the existing pipeline through the branch pipe to a separate facility. Adding a branch may also enable satellite wells to be fed into (unprepared) pipelines and thus exploit existing infrastructure. Some retrofit connections can be performed remotely, on pipelines at great depths.

In order to connect the new pipeline, a "hot tap" connection operation may be performed. "Hot tapping" involves forming an aperture in the wall of the existing parent pipeline and fitting the branch pipe so that fluid communication is established with the parent pipe through the aperture.

Such an operation can be performed whilst the parent pipeline is pressurised and contains production fluid. That is, the pipeline does not need to be shut down in order to create the hot tap connection.

One way to connect a branch pipe is using a tee which is "retro fitted" to the parent pipe. Such a tee may comprise a sleeve that may be clamped and tensioned around the existing parent pipe to grip the pipe. The tee may have a tubular collar to receive and position a branch pipe section with respect to the parent pipe perpendicularly or at some other angle, in order to facilitate joining the branch pipe section to the parent pipe.

Examples of operations for hot tapping are described in the published patent documents WO 03/002902 and US 802871 1 . Use of a retro-fit tee is described in WO 03/002902. Retro-fit tees are typically installed either by divers or remotely.

An end of a section of the branch pipe may be placed against the outer surface of the parent pipe with the assistance of the tee. A gas-tight seal of the branch pipe to the parent pipe is sought in order to provide a successful branch connection, to prevent gas from leaking from inside the pipe to the pipe environment at the connection point.

Various connection techniques are known for providing a gas tight join of a section of branch pipe to a parent pipe, for example:

i) diver assisted welding by divers in a habitat;

ii) remote welding inside the branch pipe section using a remote-operated welding machine;

iii) using elastomer or other (e.g. graphite) seals on the end of the branch pipe section; or

iv) applying a two-component glue or grout to the contacting components, providing a so-called "grouted tee" connection.

There are drawbacks associated with these techniques.

Diver assisted welding depends on divers. However, diving beyond a certain depth may not be possible without sustaining bodily injury. It is expected that diving beyond 600 m causes tissue damage to humans, and may therefore represent the theoretical limit of diver depth. On the Norwegian continental shelf, rules limit divers to operating in water depths of no more than 180 m or whilst in other territories a 360m limit may apply. A diver assisted approach is also normally costly as it requires diver qualification and diver vessels. Deploying divers to work on live pipelines may also be a safety risk. Remote welding machines are designed to be inserted and used inside the branch pipe. Therefore, these machines are limited to use with branch pipes that are large enough to accommodate the machines. It may therefore be impractical to connect small-diameter branch pipes, for example with an internal diameter of less than 8", using this approach. Elastomeric seals are sensitive to temperature and can degrade over time. The use and performance of seals over time is unproven.

Grouted connections typically involve using a two-component filler (epoxy or grout) that is injected in the dead-space between the branch pipe and the tee. This can make it difficult to control the coverage of the glue. There are also some uncertainties regarding the durability over time, such as risk of cracking etc.

Summary of the invention

According to a first aspect of the invention there is provided a method of joining a first pipe to a second pipe, as set out in the claims appended hereto.

According to a second aspect of the invention there is provided a pipe connection assembly for connecting a first pipe to a second pipe, as set out in the claims appended hereto.

Each and any of the above aspects may include further features, as set out in the claims appended hereto or in the present description.

It will be appreciated that features mentioned in relation to any of the above aspects, whether in the claims or in the description, may be combined between the different aspects in any appropriate combination. Drawings and description of the invention

There will now be described by way of example only, embodiments of the invention, with reference to the accompanying drawings, of which:

Figure 1 is a perspective view of apparatus for joining two pipes according to an embodiment of the invention; and

Figure 2 is a close up perspective view of the apparatus of Figure 1 . With reference firstly to Figure 1 , apparatus 1 for joining a branch pipe section 2 to an existing, parent pipe section 3, is shown.

The apparatus 1 includes a retrofit tee 4 which comprises a sleeve 5 which is fitted around the parent pipe section 3. The tee 4 further comprises a collar 6 which extends radially outward from the sleeve portion and/or parent pipe section 3. The collar 6 comprises a tubular receptacle defining a space into which the branch pipe section 2 is received. The collar 6 may act as a guide for the branch pipe section 2 as it is received therein to help position the branch section 2 appropriately with respect to the parent pipe section 3. In this example, the receptacle of the collar 6 defines a central, longitudinal axis 7 extending therethrough, along a length thereof. The sleeve portion has similarly a longitudinal axis 8 extending therethrough. The collar 6 is connected to the sleeve portion 5 and is arranged in this example with the longitudinal axis 7 of the collar and the longitudinal axis 8 of the sleeve portion perpendicular to one another. Thus, the tee 4 seen in this example is arranged to join the branch pipe section 2 to the parent pipe section 3 at right angles to each other. It will be appreciated that the collar may in other embodiments be oriented differently, for example with the respective axes 7, 8 intersecting at an acute angle, to provide a corresponding connection of a branch pipe section 2 at an acute angle to the parent pipe section 3.

The branch pipe section 2 has a first end portion 9 in contact with an outer surface 10 of the parent pipe 3. The branch section has a second end portion 19 comprising an outward protruding flange 1 1. The collar 6 has a collar flange 12 extending around and radially outward from the collar 6. Tension bolts 13 are provided through bolt holes in the respective flanges 1 1 and 12. The bolts are used to apply a tensioning force in the direction of the axis 7 to the flanges 1 1 , 12 to press the branch pipe section firmly against the parent pipe.

With further reference now to Figure 2, the apparatus 1 in the region of contact between the branch pipe section 2 and the parent pipe section 3 can be seen in more detail.

As can be seen, the branch pipe section 3 comprises a pipe wall 20 having a main wall portion 16 which provides mechanical support. The flange 1 1 is formed as an extension of the main wall portion. The main wall portion 16 is arranged to conduct electrical current, and may be formed of an electrically conductive material for carrying an electrical current therein to the end portion 9 of the branch pipe section. Inside of the main wall portion, the pipe wall has a tubular insert 14. The insert is securely fixed to the main wall portion, such that the pipe section 3 including the main wall portion 16 and insert 14 can be considered an integral component. The insert provides an inner surface of the branch pipe section, the surface defining a space inside the branch section 2. The insert 14 in this example is formed from a metal such as Inconel®, which provides a greater electrical resistance than the main wall portion. The insert protrudes slightly beyond the end 22 of the main wall portion 16 defining surfaces that meet along a sharp, thin edge 15 which provides in this case a line of contact against the outer surface 10 of the parent pipe 3. The line of contact, as indicated by the figure may take the form of a closed line or ring defining a saddle shape. The protruding part 21 of the insert 14 has a thickness in this protruding region that tapers or narrows toward the edge or to a point on the edge, at the area of contact between the branch and parent pipes. As can be seen, the thickness of the main wall portion 16, and of the pipe wall 20 overall, also tapers or has a reduced thickness toward the edge in the end portion 9.

Outside of the main wall portion, the pipe wall includes an insulating layer 17, formed of an electrically insulative material, which is used to insulate the retrofit tee from current conducted in the main wall portion 16.

It may also be noted that a small gap 23 may be present between the tee and an outer surface of the branch pipe wall.

It can also be noted, as seen in Figure 1 , that a layer of insulative material 18 is provided between the collar and the tension bolts 13 to keep the tee insulated.

It will be appreciated that the pipe wall of the branch pipe may in other embodiments have a different structure. For example, a resistive Inconel® structure could be provided on the end of the current conducting main wall portion or on the outside of the main wall portion.

After installing the retrofit tee 4 and arranging the branch pipe section 2 against the parent pipe section 3 as described above, the branch pipe section is joined to the outer surface 10 of the parent pipe section 3 at the area of contact therebetween, using a resistor welding technique. More specifically, an electrical current is provided in the material of the wall of the branch pipe section 2 which due to the electrical resistance provided particularly by the Inconel® insert 14 at the area of contact 18 generates a significant amount of heat in that area. The heat causes the material of the branch pipe wall 20 and the wall of the parent pipe 3 to soften or melt. The application of pressure using the tensioning bolts together with the generation of heat joins and seals the branch and parent pipe sections 2, 3 together at the contact point. The branch and parent pipe sections are thereby welded together. By welding in this way, a gas-tight seal is provided.

The parent pipe may have a wall thickness typically of around 20-25 mm. The branch pipe should be relatively thin, for example only 5 mm, at the bottom end at the "knife" edge 15 interface with the parent pipe. Away from the interface or tapered portion, the wall thickness of the branch pipe may be in excess of 30 mm.

The parent pipe is typically made of carbon steel (for example X65 or X70), stainless steel, or any type of electrically conductive material with a suitable melting point.

When joining the pipes, a high electrical current of several thousand amperes in a few seconds may be led through the branch pipe 2 into the parent pipe 3 through the area of contact there between. A squeeze between the branch and parent pipe sections is maintained by the tensioning bolts. The electrical current induces heat due to electrical resistance in the current-conducting closed loop defined by at least the branch and parent pipe sections across the contact point. The current amplitude and duration may be adapted for various branch sizes, and/or for various types of parent pipe (material, wall-thickness, pressure rating etc). A typical current surge could be 5-10 seconds in duration with a 3000-15000 Ampere current peak. It may be an advantage to have a short and large peak in order to keep the heat concentrated at the intended location (in the interface between the knife edge and surface of the mother pipe). The amplitude and duration of the current peak must contain enough energy to heat up the branch and parent pipe at the interface to approximately 50 to 80% of the melting point to achieve a proper bonding (corresponds to 675°C - 1080°C for Inconel ® 625). A "long" heating time (for example in excess of 30 seconds) allows for more heat distribution to larger sections of the parent-pipe and/or branch or tee which may destroy the integrity of the pipe. In this case, a short and massive energy peak may be sought which can lead to more local and concentrated heat development. The resistance is determined 1 ) by the material properties (more or less conductive/resistive properties) and 2) by the cross section of the current carrying parts. The "knife edge" may be made of a strong alloy (e.g. Inconel® or similar). The edge has a small and thin cross section and will therefore have more current per area resulting in more local heat relative to the remaining parts of the construction. The current-carrying parts desirably provide a sufficiently low resistance (as a result of material properties and size of cross-section) to avoid being overheated by applying a current that is sufficiently high to bond or join the edge of the branch pipe to the parent pipe. Using materials with lower electric resistance properties in the current carrying parts of the wall (except from the knife edge) may be an advantage to minimize the required electric energy surge and to produce less heat in the support structure. Having different material properties (lower resistance) may not be a requirement because it can be compensated by having a significantly larger cross section. Thus, it will be appreciated that the pipe may have a single material wall in some embodiments. It can however be an advantage to have different material properties. Having internal cavities inside an upper portion of the branch wall for example filled with e.g. copper can be a way of minimizing electrical resistance (and make a portion of the wall more "conducting") in the area of the cavity. Another method is to use temporary installation tool, for example located inside the branch, to lead current, for example through wires, to the end portion 9 of the branch pipe.

It will therefore be appreciated that the terms resistive and conductive do not merely refer to the material properties, but also the nature of the structure. The structure and properties may be selected according to where in the construction the energy is intended or desired to produce heat. The voltage drop is highest at the thin mechanical interface (or contact point or line) between branch and the parent pipe 3. The greatest heat generation in the pipe sections therefore occurs at the interface. The amplitude and duration of the current surge can be adapted in such a way that it melts, and thus welds, the end of the branch section into the outer-layer of the parent pipe section without damaging the integrity of the parent pipe section 3. For example, the branch pipe section 2 may be joined such that the internal diameter of the parent pipe section 3 is unaffected. It will be appreciated that the appropriate amplitude and duration of current may depend on the structure and materials of the branch and parent pipe sections 2, 3. Thus, the material and shape of the edge 15 at the first end of the branch pipe may be selected to produce an appropriate amount of heating in order to join the branch pipe section 2 without affecting the integrity of the parent pipe section 3. The contacting edge may be formed of metal or a metal alloy, or for example a nickel-containing or nickel-chromium alloy such as an Inconel® alloy, for example Inconel® 625 or other Inconel® alloy as marketed by Special Metals Corporation (www.specialmetals.com). In principle any kind of insulating material may be used for the electrical insulation that can withstand water pressure and some heat during installation. Examples of such materials for the layers 17, 18 may be polytetrafluoroethylene (PTFE) which withstands 325°C for 10 minutes) or a polyamide such as Kapton® which withstands 500°C for 10 minutes. It is not expected that the heat from the pipe bonding/interface area will reach the insulating material fast enough to cause to degrade it during the heating (and current conducting) period. The insulating material is only required during the fusion/bonding. Potential degradation of the insulating material after welding is not a particular concern. The electrical resistance and heating characteristics of different materials can be readily tested. For example, an electrical current may be supplied to a material to be tested, and a change in temperature over time and for different current values may be measured. Alternatively, characteristics for many materials may be available from commercial providers of resistor materials.

The thin edge 15 formed at the end of the branch section, as mentioned above, may be formed of Inconel®. This material has a property of providing increased electrical resistance with rising temperature. Other materials with a similar property may also be used. This means that most current will pass through the coolest parts in the branch- parent pipe interface since the electrical resistance is smaller in these parts. This provides a "self-regulating effect" by distributing the most electric energy - and thus heat - to the coldest sections of the interface (or point or line of contact). This physical property can therefore contribute to distribute heat evenly. The wall 20 of the branch pipe section 2 is configured to produce a higher electrical resistance in the end portion 9, for instance at or near the edge 15. Where the insert tapers toward the edge 15, the end portion 9 of the pipe wall 20 has a thickness that is less than in other sections of the wall 20 that are further away from the contact point with the parent pipe 3. The edge 15 marks the thinnest section of the wall, and thus provides high electrical resistance and heat generation in this section.

Other parts of the wall 20 and/or insert 14 are thicker, and provide less resistance. In the example of Figures 1 and 2, the main wall portion acts as a conductor to facilitate directing current to the resistive parts in the wall and/or contact point 18. The main wall portion may thus be formed from a highly conductive material such as a metal. In other embodiments, the wall may use one or more integrated electrical conductors, for example wires that are built into the wall, to direct current to the resistive parts in the wall and/or contact point 18. This configuration prevents heat build up in parts of the wall where heating is not required. The wall 20 and/or conductors may therefore be configured in such a way that current is optimally distributed inside the branch.

It can be noted that a high current can be achieved by using one or more capacitors that are connected to the branch pipe. The capacitors are charged from an electrical power supply and then discharged to provide the electrical current in the wall of the branch pipe.

In a subsea operation, an umbilical cable may extend between the capacitors to a surface vessel, and electrical power may be provided through the umbilical to charge the capacitors. A subsea control system can be used to ensure that the charging current is within the specifications for the umbilical. The control system may then adjust the amplitude and duration of the current surge in a safe manner in accordance with the specific setup of the retrofit-tee, branch and parent pipe specifications to join the pipes together.

After applying the current to the pipe and the current is removed, the pipe sections in the region of the join will cool down. Further hot-tapping procedures may then be performed such as forming an aperture through the wall of the parent pipe, for example by milling or grinding out the material of the wall of the parent pipe inside the branch pipe section to fluidly connect the insides of the pipe sections 2, 3. The tensioning bolts 13 may be removed from the flanges 1 1 , 12 and further branch pipe sections may be connected to the section 2, for example using the flanges from which the bolts are removed as connection means, to form a branch pipeline leading to a separate facility. A valve may be connected to the branch section 2 to control fluid flow into and pressure communication with the branch section.

The parent pipe section may contain a fluid from a well. The connection procedure described may be performed whilst the fluid contained in the pipeline is being transported through the pipeline to another location, i.e. while the pipeline is in "live" operation. The fluid contained in the parent pipe may include a hydrocarbon fluid, for example oil or hydrocarbon gas.

Oxidation of steel may take place by heating to around 700-800°C, depending on the environment of the parent and branch pipe where the join is performed. An option to avoid oxidation may be to create an inert atmosphere adjacent to the pipe during heating or welding. For example, water may be purged away from the weld area using an inert gas such as Argon or Helium. In addition, it may be useful to flush the surfaces to be joined with pure hydrogen prior to heating/bonding, in order to remove (or reduce the amount of) any existing oxides in the surface. Removing oxides prior to welding may increase the quality and/or strength of the weld. The branch pipe section may comprise a socket or connecting tube for receiving or connecting a further pipe section for building a branch line.

The apparatus described may be deployed subsea, that is, anywhere underneath the sea surface, for example at or near the seabed. Thus, the pipe sections can be joined in the manner described in a subsea setting.

It will be appreciated that the current may be delivered in the opposite sense, from the parent pipe and into the branch pipe. The heat distribution should be the same regardless of the direction of the current. There may however be other effects either on a metallurgic micro level in the bonding area or more "global" galvanic effects (corrosion induced by voltage on pipe). Alternating or direct current may be used.

The technique for joining the branch and parent pipes described above has a number of advantages.

It can be used to connect a branch to a parent pipe where welding machines cannot be applied inside the branch due to limited pipe diameter. The present technique can be used with branches having an internal diameter of 1 " to 10" or larger.

The technique provides a way of making permanent, durable gas tight metal seals in a very fast and cost efficient manner on small dimensional branches without the need for manned intervention. It is suited to forming hot tap branch connections at depths beyond divers.

The technique may also be cheaper, faster and safer than existing diver assisted welding techniques. The joining operation can be applied subsea, at all depths, and be controlled remotely.

It will be appreciated that the term "subsea" should be understood to include usage in land locked or partially land locked seas, such as lakes, fjords or estuarine channels, in addition to open seas and oceans whether containing salt water or fresh water, or mixtures thereof.

Various modifications and improvements may be made without departing from the scope of the invention herein described.