JPS6051955B2 - Double pipe end treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The disclosed technology belongs to the field of technology for preventing stress corrosion cracking at the joint ends of double pipes for transporting corrosive fluids, such as oil pipes and oil country tubular goods.

Therefore, the present invention provides a method for preventing stress corrosion cracking by stacking double pipes such as oil pipes and oil country tubular goods with a corrosion-resistant lining pipe as the inner pipe and applying compressive residual stress to the outer pipe. The present invention relates to a processing method in which compressive stress is applied in the axial direction to the end of the connection part, and in particular, the end of the inner pipe to which the compressive residual stress is applied is applied to the end of the inner pipe. A pressing force is applied to the outer tube to bring it into a relatively pushed state, and while this state is maintained, the inner tube end is welded to the inner bottom for a fixed seal, an axial compressive stress is applied to the inner tube end, and then the coupling Alternatively, the present invention relates to a method for treating the ends of double pipes in which a predetermined process is performed for joint welding or the like.

As is well known, pipe bodies used for transporting corrosive fluids, such as oil country tubular goods or oil transmission pipes, are directly connected to the flow of the corrosive fluid, and have corrosion-resistant inner pipes such as stainless steel lined pipes. A double tube consisting of an outer tube such as a carbon steel tube that provides strength such as pressure resistance is used.
Therefore, in this type of oil transmission pipe, it is often necessary to transport the oil under pressure to compensate for the inevitable pressure loss caused by long-distance transport, and stress corrosion cracks may form when stainless steel or other materials are used for the inner pipe material. There are some disadvantages.

To deal with this, the applicant utilizes the difference in descending stress between the inner and outer tubes to expand the tube beyond the elastic limit strain, and then removes the expansion force to generate compressive residual stress in the inner tube. We have developed and introduced a technology that prevents stress corrosion cracking by preventing the generation of tensile stress exceeding the critical tensile stress for stress corrosion cracking in the inner pipe even when medium tensile stress is applied.

However, even though compressive residual stress is generated in the general part of the internal pipe, the length of the unit pipe is limited due to manufacturing process constraints, and cutting or seal welding is required at the pipe end. Because of the extent to which this occurs, the axial compressive stress is essentially zero at the ends.

Therefore, even if the pipe is connected by threading a coupling or by forming a weld joint groove, there is a drawback that the stress corrosion cracking phenomenon described above cannot be prevented at the connected portion.

In addition, couplings have the disadvantage of being integrally molded with corrosion-resistant material up to the fluid flow surface, which increases costs and does not provide sufficient strength.On the other hand, welded joints suffer from distortion due to heat input at both ends of the inner and outer pipes. There were also some difficulties that caused differences.

In view of the above-mentioned problem of stress corrosion cracking at the end of the corrosion-resistant double pipe, the object of the present invention is to Pressure is applied to create a pushed state against the outer pipe, and in this state, the inner and outer pipe ends are fixed and sealed by internal welding using a corrosion-resistant material of the same quality as the inner pipe material, and the inner and outer pipe ends are also sealed. This is an excellent second design that applies compressive stress in the axial direction to the inner tube to perform coupling threading to the tube end or bevelling of welded joints, and prevents stress corrosion cracking from occurring throughout the entire area during operation. The present invention aims to provide a method for treating the ends of heavy pipes.

Next, embodiments of the present invention in accordance with the above object will be described below with reference to the drawings.

As shown in Fig. 1, a double pipe 1 for oil country tubular goods has an inner pipe 2 as a stainless steel corrosion-resistant lining pipe layered on a carbon steel outer pipe 3, and as described above, the inner pipe 2 is bonded by a prescribed heat expansion method, and the inner pipe 2 is left under compression. A stress of 4 is applied.

The inner tube 2 has a longer tube end 2'' with respect to the outer tube 3. As mentioned above, as shown in the figure, the compressive residual stress 4 is elongated at the tube end. has been reduced to zero.

As shown in FIG. 2, the unit double pipe 1 manufactured and formed in this way is fitted with an appropriate hydraulic jack 5 as a pushing jig at one end, and then inserted into the pipe end 2'' of the inner pipe 2. Axial pressure force F
is applied, and the other end is fixed appropriately. By doing so, the inner tube 2 is pushed into the outer tube 3, and therefore, the axial compressive stress 4 is formed almost uniformly over the entire length of the tube at the tube end, as shown in the figure. In this case, the compressive stress should be approximately equal to the compressive stress caused by the above compressive stress and the additional pressing force. In order to do this, the pressing force F may be determined by appropriate theoretical calculations, experiments, etc. Then, while maintaining the posture in which the compressive stress 4 is formed over the entire length region, the inner pipe end 2'' and the outer pipe 3 are pushed in a set length and shortened as shown in Fig. 3. A build-up weld 6 is formed using the same corrosion-resistant stainless steel material as the inner tube 2 to fix the ends of both tubes together and seal them.

Therefore, by fixing the inner and outer tubes 2 and 3 through the overlay welding 6, uniform distribution of the compressive stress 4 over the entire length is ensured, and in this state, the hydraulic jack 5 attached to the tube end is released. The end of the pipe is machined for coupling joining as shown in Fig. 4, and an overhang part 1 of a protruding shape is formed between the inner pipe 2 and the overlay weld at the tip of the end. A tapered male thread 8 is machined on the outer surface.

Note that a joint undercut is machined at the other corresponding tube end.

Of course, the coupling is separately formed and manufactured in a predetermined manner.

The embodiment shown in FIG. 5 shows connection by coupling, but the overhang part 7 and undercut part 7'' in the stainless steel build-up part 6 are connected to the unit double pipes 1, 1 formed relative to each other, and the taper of the overhang part 7 and the undercut part 7'' is When each part of the shaped male screw parts 8, 8 and the female screw part 10 of the carbon steel coupling 9 are screwed together by relative rotation, the overhang part 7 and the undercut 7'' are engaged and in close contact, and the inner tubes 2, 2 are screwed together. and the inner filling part 6,
6, it is completely sealed.

Therefore, the coupling 9 does not require corrosion resistance, and only needs to be designed for strength.

Further, the embodiment shown in Figs. 6 and 7 is a joint connection mode of an oil pipe vibrating line, and the axial compressive stress 4 up to Fig. 5 is
After applying this to the pipe end of the inner pipe 2 of the unit double pipe 1 to form the overlay weld 6, the groove 11 of the butt weld is machined to the pipe end as shown in FIG. As shown in FIG. 7, a welded joint 12 is formed to join and connect both unit double pipes 1,1.

Even in this embodiment, as is clear from the drawing, since the outer tubes 3, 3 are sealed by the stainless steel inner tubes 2, 2 and the stainless steel build-up parts 6, 6 at the connecting portion, corrosion resistance is sufficiently ensured. .

It goes without saying that the embodiments of the present invention are not limited to the above-mentioned embodiments; for example, compressive stress may be applied by pushing the inner tube end inward from the outer tube end by appropriate design. Various embodiments are possible.

As described above, according to the present invention, compressive residual stress is applied to the inner pipe of a double pipe having a corrosion-resistant lining pipe to prevent stress corrosion cracking, and the stress corrosion cracking is also applied to the end of the joint pipe. In a pipe end treatment method that applies compressive stress as a measure to prevent stress corrosion cracking, the inner pipe is retracted from the outer pipe at the pipe end,
The inner and outer tube ends are overlay welded with a corrosion-resistant material while applying compressive stress to seal and fix them.Finally, mechanical processing for coupling or butt welding is performed. By doing this, basically, even if compressive residual stress to prevent stress corrosion and cracking is applied to unit heavy pipes, there is no axial compressive stress at the end of the pipe, but compressive stress is applied over the entire length. Therefore, during operation, stress corrosion cracking can be prevented even at joints and connections, which is an excellent effect.

Since the inner and outer tubes are internally welded with a corrosion-resistant material at the ends of the tube, not only is there no risk of corrosion due to the intrusion of corrosive fluids, but the shear area between the inner and outer tubes is also expanded. This has the advantage of maintaining the compressive stress as described above, and furthermore, there is no need to use a corrosion-resistant material in the coupling, which reduces costs and increases strength. .

In addition, since the tube ends are made of the same corrosion-resistant material, there are advantages that the rigidity is increased, the process is easy, and the on-site welding is easy.

[Brief explanation of the drawing]

The drawings show examples of the present invention, and FIGS. 1 to 4 are illustrations of the processing process of one embodiment, FIG. 5 is an illustration of the joining mode, and FIG. 6 is an illustration of the processing of another embodiment. FIG. 7 is an explanatory diagram of a joining mode. 3...Outer pipe, 2...Inner pipe, 4...
...Compressive residual stress (compressive stress), 1...Double pipe, 2''...End, 6...Overlay welding, 7,
7″, 11… processing.

Claims (1)

[Claims] 1. In a treatment method in which compressive residual stress is applied to the inner tube relative to the outer tube, and axial compressive stress is applied to the end of the double tube formed, the inner tube end of the double tube to which compressive residual stress is applied is A double pipe end treatment method, characterized in that the ends of the inner and outer pipes are welded overlay in a state in which the inner and outer pipe ends are retracted from the outer pipe, the ends of the inner and outer pipes are fixedly sealed, and compressive stress is applied in the axial direction. .