Method for repairing local damage to pipelines by applying cladding with an interposed protective sheath

Abstract: A method for repairing local damage (2) to pipelines (1) by applying a cylindrical cladding consisting of two cylindrical half-shells (3,4) welded together to create an interspace, which is sealed by applying gaskets forced against the circular edges Of the cladding, after which resin is injected into the interspace at high pressure.

Description: This invention relates to the repair of fluid transporting pipelines such as water, gas, oil or other pipelines for civil and industrial use. These pipelines are generally formed by welding together lengths of metal pipe, the continuous pipeline assembled in this manner being laid in its final seat, generally consisting of a sufficiently deep trench, and then covered in order to recreate the pre-existing environment and hence not hinder human activity on its surface. The assembled continuous pipeline is provided with protection against wet corrosion, as the environment in which the underground pipeline lies is very aggressive towards ferrous materials, this protection generally being in the form of protective coverings and cathodic protection with an applied current, and being provided with a monitoring circuit and frequently inspected. The integrity and preservation of the pipeline during its entire operating life are very important, not only because of its high construction cost, but also to prevent danger, pollution or serious disturbance to users. This protection is often insufficient to protect an underground pipeline for the required time for the following reasons. The covering materials are not free of a certain porosity and a certain degradation with time, and during the application of the covering and the laying and burying of the pipeline slight damage to and/or separation of the covering can occur, and which can subsequently trigger corrosion phenomena even after a considerable time. Natural phenomena, such as earthquakes, collapsing and landslips, or accidental events deriving from human activity on the surface, can cause local damage to the pipeline. Various methods for repairing the damaged portions have been proposed and practised in the known art. The most radical repair method consists of replacing the damaged portion by removing the damaged metal pipe piece and inserting a new pipe piece in its place, so basically repeating the pipeline construction procedure. This method has considerable drawbacks because it is very difficult and costly, and because it puts the entire pipeline out of operation for a prolonged time. In a less radical but rather questionable method it has been proposed to make the repair by applying a reinforcement to the outside by welding on a piece of metal plate of the same shape as the damaged part or by directly inserting new material into the damaged region to restore the thickness of the pipeline. Another handy repair method of more widespread use is to apply two cylindrical steel half-shells to the outside of the damaged portion of the pipeline and weld them together along their generators, to hence create a metal shell about the damaged portion. The interspace between the pipeline and the two half-shells, these being known as cladding, is sealed at its two ends with a cement mix. After waiting for the cement mix to set, a resin comprising two or more components is injected into the interspace of circular ring cross-section created between the pipeline and the welded cladding, and is allowed to harden within the interspace to produce a sheath which creates continuity between the inner damaged pipe piece and the outer shell, so that the force due to the pressure within the pipeline is discharged onto the shell. This method has considerable drawbacks due both to the waiting time for the cement to set and to the fact that having used a cement mix for sealing, the resin components cannot be injected at high pressure. If the resin were to be injected at high pressure, the cement seal would not be able to remain in position and would be pushed outwards to allow the resin to escape. Injecting the resin at insufficient pressure can result in incomplete filling of the interspace and in addition this method cannot be relied on to produce continuity between the damaged pipe piece and the outer shell because of the fact that resins generally shrink during polymerization and hardening, even if inert fillers have been added, with the result that the force deriving from the pressure within the pipeline is not completely discharged onto the shell. This discontinuity between the shell and resin sheath is due to this shrinkage phenomenon. Another method in which a resin is injected in the interspace between two half shells and the pipeline is known from GB-A-2 080 475. The pipeline repair method according to the present invention is still based on the principle of using outer cladding not welded to the pipeline, but avoids the problems arising from the discontinuities between the pipeline and the cladding which occur in the known art. These problems are solved by a method as defined in claim 1. The advantages of the pipeline repair method of the present invention will be more apparent from the description of a typical embodiment thereof given hereinafter by way of non-limiting example with reference to Figures 1 to 4. The pipeline 1 comprising local damage 2, such as a corrosion crater due to infiltration below the protective covering, is exposed by removing the ground around it, and its protection covering is removed to the extent sufficient for the repair. The pipeline surface is cleaned, degreased and sanded to achieve bare metal in order to ensure good resin adhesion. The two cylindrical steel half-shells are then positioned, preferably centered about the damage 2. The inner diameter of the two half-shells is slightly greater than the outer diameter of the pipeline, indicatively by 20-50 mm, to enable a resin sheath of 10-25 mm thickness to be created. The inner surfaces of the two half-shells or cladding are treated in the same way as the outer surface of the pipeline to obtain the same adhesion between the resin and the metal. The two half-shells are then joined together along their edges to create a cylindrical cup about the pipe. The joint can be of removable mechanical type, for example using studs and nuts, but in its more common implementation, which is described herein by way of nonlimiting example, a permanent welded joint is used. The lower half-shell 3 is supported by supports, not shown in the figures, and the upper half-shell 4 is positioned with the horizontal edges to be welded together in mutual contact. These edges are shaped to receive a weld seam using welding metal. The two half-shells are then welded together to form the cylindrical metal cup about the pipe. The interspace is then cleaned with a compressed air blast. This preparation is common to the stated method of the known art. According to a preferred embodiment of the invention, threaded holes 5 are distributed over the surface of the two half-shells to allow the insertion of bolts 6 for centering the cup about the pipe, in order to obtain an interspace between the cup and pipeline which is of uniform thickness. Two circular half-rings 7 are welded to the interior of the half-shells 3 and 4 in correspondence with the ends of the two half-shells, as shown in the detailed view of Figure 2. Their outer face 8 is inclined as a cone with its vertex towards the interior of the cup formed by the two half-shells 3 and 4. At both ends of the cup, against the ring formed by the two half-rings 7, there is provided a gasket 9 in which at least that surface facing said ring is inclined conically to match the surface 8. It is preferably prepared from a linear section bar of rubber or another elastic material, which is cut to size and wrapped around the pipeline 1, then joined together at its ends with adhesive and pushed into contact with 8. To improve the gasket seal, a thin layer of resin can be spread over the surface of the pipeline 1 where the gasket is positioned. Two flanges 10, shown in detail in Figures 2 and 3 and having a slightly greater diameter than the pipeline 1, are placed against the ends of the cup. The two flanges each consist of two half-flanges 11 which are closed in the manner of jaws about the pipeline and connected together by bolts passing through the end holes 12 and locked with a nut and locking nut. The two outer flanges 10 are provided with a plurality of holes 14 in their projecting edge 15 for the insertion of threaded tie bars 16. They are preferably provided with a recess 17 of circular ring shape in their inner face in a position corresponding with the edge of the cup, and with a centering ring 18 on the opposite face. After the two flanges 10 have been mounted they are forced to clamp the two gaskets 9 by tightening the nuts 19 screwed onto the bars 16, preferably using dynamometric spanners to balance the pulling forces so that the flange is always maintained perpendicular to the axis of the pipeline. The tightening action compresses the gaskets 9 both against the surface 8 and against the pipeline 1 by the effect of the inclined lead-in. The seal is improved by the said possible interposing of a thin layer of resin between the pipeline and the gasket and which, after tightening, sets against the pipeline and the inner face of the gasket. Figure 4 shows an alternative embodiment of the tightening device for the gaskets 9, which may be preferable for repairing pipeline portions requiring longer cups, or curved pipeline portions. In this embodiment the half-shells 3 and 4 are provided near their ends with semicircular backing flanges 20 comprising through holes corresponding with those of the outer flanges 10. Between the flange and backing flange there are provided shorter threaded bars, on which the tightening nuts are screwed. After tightening, the centering bolts 6 are removed, the holes 5 are closed with threaded plugs and the cup is ready for injection of the resin sheath. Before injection, it should be checked, for example with compressed air of 0.5-1.5 MPa pressure, that the interspace is sealed, and if leakages occur the threaded bars 16 should be further tightened to further compress the gaskets. In the lowest part of the lower half-shell 3 there is a valved connector 21, screwed for example into a threaded hole analogous to the holes 5, for connecting the resin mixture injection pump, not shown in the figures, which is provided for example with a number of vessels containing the resin components which are to be mixed together at the moment of injection. In the highest part of the upper half-shell 4 there is a valved connector 22 for venting the air expelled by the injected resin. The valved connectors 21 and 22 can also be used for the preliminary interspace seal tests. The resin injection, which constitutes one of the characterising parts of the method according to the invention, comprises the following stages. Various types of commercially available resins can be used for the purpose. By way of example, the following commercially available resin types can be used: liquid epoxy resins used in combination with a preferably amine-type hardener, or acrylic, vinyl or allyl liquid monomers used in combination with a polymerization catalyst able to generate free radicals, and normally chosen from organic percarbonates, peroxides and hydroperoxides, polymerization being effected within the interspace. According to a preferred embodiment of the invention, to limit shrinkage or improve the mechanical characteristics of the formed polymer sheath, finely divided inert inorganic materials such as powdered marble can be added to the component mixtures. At the commencement of the injection operation the injection pump vessels are filled with the products for preparing the resin mixture, and said pump is connected to the lower connector 21. For field use, the injection pump is normally operated by compressed air generated by a motor-driven compressor available in the mobile repair shop. The upper vent valve 22 is opened and the pump started to commence injection of the resin mixture. When the resin begins to escape from the upper valve 22, this valve is closed. Resin injection is continued until the pressure of the pressure gauge on the pump or sealed cup reaches a substantial gauge pressure, indicatively of the order of 20-60 bar and preferably between 30 and 45 bar. This pressure is very important for the correct formation of the resin sheath. By this means, in addition to achieving complete filling of the interspace, the cup wall is put under substantial tension and the pipeline wall under compression. During resin setting and consolidation, its shrinkage is adequately compensated by the release of the cup tension stresses and pipeline compression stresses, so allowing these parts to return to their original configuration and maintaining the coaxial metal surfaces of the cup and pipeline always in contact with the resin sheath being consolidated, so that when hardening is complete there is perfect continuity between the outer cup and the pipeline. After reaching the required gauge pressure, the injection pump is halted and the lower valve 21 closed. The pump is disconnected and cleaned, and the resin left to harden for the prescribed time. After the resin has hardened the connectors can be removed, closing the holes with threaded plugs, and the flanges 10 and tie bars 16 can also be dismantled and removed, leaving on the repaired pipeline 1 the cup formed from the welded half-shells 3 and 4 and the gasket 9, which is retained by the resin. New protection covering is applied to the pipeline, which can then be reburied. The applied cup must be connected to the pipeline cathodic protection system. According to a preferred embodiment of the invention connection bolts between the cup and pipeline are used, these being inserted through and left in some of the centering holes 5 before injecting the resin. That end of the bolt in contact with the pipeline is fitted with a terminal providing good electrical connection, such as an aluminium plate. A further advantage of the invention is that the pipeline can be repaired in a short time, without having to interrupt its normal operation and without having to weld to the actual pipeline.

Claims: A method for repairing local damage to pipelines by applying cladding or cups with a protective sheath interposed, said cups being formed from two cylindrical half-shells 3 and 4 applied to the outside of the pipeline and then joined together along their edges to form a cylindrical cup about the pipe, and by positioning a circular gasket 9 at each of the two ends of the cup in correspondence with a circular projection 7 provided on the inside of the cup ends, said gasket being formed preferably with an inclined bearing surface matching the surface of the projection 7, the two gaskets then being compressed against the projections 7 and against the outer wall of the pipeline 1 by tie bars 16 applied to flanges 10 positioned at the two ends of the cup, to form a pressure-tight interspace about the damaged portion of the pipeline, a resin mixture then being injected into said interspace to completely fill it, the evacuated air being expelled through a vent valve, the interspace vent valve then being closed and injection of the resin mixture continued until a substantial gauge pressure is obtained within said interspace to put the cup in a state of substantial tension and the pipeline in a state of compression, the injection valve being finally closed and the injected resin left to harden, the release of the stresses induced in the cup and pipeline walls enabling resin shrinkage during hardening to be compensated, to obtain continuity between the cup, the hardened resin sheath and the pipeline. A method for repairing local damage to pipelines as claimed in claim 1, characterised in that the injection gauge pressure is between 20 and 60 bar and preferably between 30 and 45 bar. A method for repairing local damage to pipelines as claimed in claim 1, characterised in that the gaskets 9 are compressed by tightening with threaded bars 16 which connect together two terminal flanges 10 at the two ends of the cup. A method for repairing local damage to pipelines as claimed in claim 1, characterised in that the gaskets 9 are compressed by tightening with threaded bars 16 which connect each of the two terminal flanges 10 at the two ends of the cup to backing flanges 20 provided on the corresponding ends of the cups. A method for repairing local damage to pipelines as claimed in one or more of the preceding claims, characterised in that the resin used for the injection is an epoxy resin used with a hardener, preferably of amine type. A method for repairing local damage to pipelines as claimed in one or more of claims 1 to 5, characterised in that the resin used for the injection is polymerized within the interspace and consists of acrylic, vinyl or allyl liquid monomers used with a catalyst able to generate free radicals and consisting of organic percarbonates, peroxides or hydroperoxides.

Inventors: GOGLIO ALFEO, PISTONE VALENTINO, GOGLIO, ALFEO, PISTONE, VALENTINO
Applicants: SNAM SPA, SNAM S.P.A.
Application year: 1993
Main classification: F16L55
IPC codes: F16L55