This paper discusses the procedure for rehabilitating large-diameter pipes in stationary in-plant conditions. The procedure was developed to rehabilitate steel pipe, after field operation or long-term storage, for further use in pipelines and other industrial and construction purposes.
All pipelines go through a number of specific life-cycle stages, the last of which is characterised by a growing number of failures and consequently by a larger volume of repair operations. Meanwhile, reliability and security decrease and risk of damage increases. As a result, at a certain point the operation of a pipeline becomes uneconomical and worn-out sections must be decommissioned and replaced with new lines.
Decommissioned pipeline can be used for the transportation of other products in safer operating conditions, otherwise – if there is no way to provide the pipeline security after decommissioning – then it should be removed. The re-use of dismantled pipes is economically effective, with the cost of rehabilitated pipes calculated to be as much as half the cost of similar new pipes with equal operating characteristics.
After a pipeline has been removed from a trench and cut up, a decision must be made on whether to scrap (re-melt) the pipes or to use them again. Through experience, the authors of this paper have achieved practical knowledge in a specific method of rehabilitation and re-use of decommissioned pipelines, which has proved to be profitable.
To organise the rehabilitation and pre-treatment of the dismantled pipes at sufficient volumes, a number of tasks have to be solved effectively, taking into account the peculiarity of the problem including:
- Non-destructive testing (NDT) of dismantled and rehabilitated pipes in order to detect the flaws
- Selection of the most efficient procedures of pipe rehabilitation
- Identification of allowable operational conditions for pipes after rehabilitation.
Inspection and repair
After cleaning procedures, pipes are transferred to the area for inspection and repair, where – depending on the requirements of the operating company – different inspection procedures are used. The body of the pipe as well as any longitudinal, spiral, or girth-weld seam is usually completely inspected. Metal thinning of the walls due to corrosion and defects such as gouges, dents, microbially induced corrosion (MIC), and stress-corrosion cracks are also inspected.
Stress-corrosion cracks less than 10 per cent of the wall thickness can be checked using eddy currents, and ultrasonic (UT) and magnetic inspections can also be used to determine stress-corrosion cracking and metallurgical defects. When necessary, it is possible to determine hardness and chemical composition of the pipe metal.
The entire body of the pipe is then inspected using magnetic flux leakage (MFL) technology, which is used differently from conventional in-line inspections (ILI). The equipment used is similar to that utilised to inspect tank bottoms for defects.
After the body of the pipe has been inspected, weld seams – longitudinal, spiral, and girth – are inspected for weld defects.
All of the results are entered into the program for monitoring and control of each pipe, and any defect areas are marked on the pipe surface. Pipes considered unusable for line pipe are then transported to the storage area to be used in construction projects.
Joints that can be used as line pipe are transported to the bevelling area where oxyacetylene units cut off the ends of the pipe. Some joints may have defects that need to be cut out, using oxyacetylene cutting rigs, before pipe sections less than 8 m long are transported to the welding area. In this area short sections are welded together creating joints 8-12 m long.
Once the pipe ends have been cut off, the ends are faced. Two pipe facing machines are set up identically and are used to apply a standard pipe mill bevel to the ends of the pipe.
The pipe ends to be faced are very likely to be out-of-round. Only the end 100 mm of new pipe is actually controlled for roundness. Fixed plate style pipe facing equipment is not suitable for facing out-of-round pipe. The pipe facing equipment supplied included an internal diameter tracking tool module for bevelling out-of-round pipe which can accommodate up to
32 mm of ovality.
Repaired pipes are then transported to the intermediate storage area for further coating application.
The coating application is performed in a separate building. Pipes are transported to the incoming racks and are visually checked, before they are transported to the blast line conveyor which helically transfers the pipe through the blast line.
Pipes are first transported through the initial heating station, where their temperature is raised to 50°. The initial heating removes any moisture from the pipe surface, to increase the quality of blasting and to prevent corrosion after blasting.
The pipe is then transported through the blast machine where mechanical blast wheels hurl shot and grit at the pipe surface. The spent shot and grit is collected, cleaned, recycled through the system and reused.
After blasting, the pipe is ejected on to intermediate racks where any remaining shot and grit inside the pipe is blown out. The pipe is then forwarded to the inspection station, where the blasting quality is checked. When necessary, rejected pipes can be returned for repeated blasting. Acceptable pipes are then transported to the coating line.
Prior to the application of a three-layer coating, the pipe undergoes a chromate treatment, which improves adhesion and resistance to water penetration, and is applied as a solution. To accelerate the drying of the chromate layer, the pipe temperature is increased to 80°.
The chromate solution dries in 20-40 seconds before the pipe is forwarded for coating application. An induction heating system increases the pipe temperature to 180-220° which can be increased dependent on the requirements of the coating materials being applied.
The three-layer coating process consisted of the following operations:
- FBE powder application
- 10-40 seconds later, adhesion layer is applied
- Immediately after, the main layer of polyethylene (PE) is extruded on the pipe
- Finally, a pressure roller is used to compensate for the thickness in the weld seam area.
The pipe is then conveyed to a water cooling system, where the pipe temperature is reduced to 60°. At the end of the conveyor the pipes were ejected on to intermediate racks where 150-250 mm of coating is removed from the pipe ends. The quality of the new coating is checked at the inspection station. After inspection, pipe markings are applied, before the pipes are transported out of the facility, of exterior storage, or transported to the internal pipe coating facility.
Some pipes required an internal coating, which is usually a two-component polyurethane or epoxy coating. The internal surface preparation for large diameter pipe is accomplished by moving a mechanical wheel blast head through the pipe on a motorised lance. Shot and grit cleans the surface of the pipe and provides the desired anchor pattern for the coating material.
After blasting, the shot and grit remaining in the pipe is blown out using compressed air. The dust and debris are separated from the blast material, and the blast material is then recirculated.
After blasting, the pipes are transported to the coating application station. Conventional two component metering systems supply the coating material to the spray gun, which moves through the pipe on a motorised lance while the pipe is being rotated. Once coated, the pipes are moved to the curing area where the curing of the coating is accelerated using hot air. The curing system consists of supports, which lift up the pipe and placed it on a chain transporter. The chain transporter indexes the pipe through the heating chamber at a controlled rate. The curing of the coating is accelerated by blowing heated air through the pipe, with air temperatures of 60-70° and a curing time of 40-60 minutes depending on the applied coating.
The production rate of the line is about 4-6 pipes per hour (50-70 m per hour) for pipe with a diameter of 1,020-1,420 mm (40-48 inches). The rehabilitated pipes meet the highest standards, for example, JSC Gazprom and JSC Transneft GOST R 51164-98 the technical requirements for coated pipes, and similar main international standards.
In spite of the relatively high initial investments in such facilities, reconditioning old pipes has proved to be a very profitable operation.
1 Incal Pipeline Rehabilitation
2 Selmers Technology B.V.
3 Selrus Rehabilitation
This article is an extract from Steel pipe rehabilitation for further use after field operation or long-term storage, a paper that was presented at the Fixing Pipelines Problems event in Berlin, Germany in 2014. The full version of this paper was published in the Journal of Pipeline Engineering. To request a copy of the full article, or to subscribe to the journal, visit the JPE website.
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