Darrell A. Bacon
Enron Corp.
Houston
Pipeline-coating failures on the system operated by Enron Corp., Houston, have led to a number of rehabilitation projects in the past 24 months.
Using commercially available methods and technology, the company selected a rehabilitation approach suitable to each failure.
In some cases, stress-corrosion cracking (SCC) had developed. In others, general corrosion and pitting corrosion affected the serviceability of the pipe.
Excavating the pipelines allowed Enron to determine the cause of the coating failure in most cases. Severe soil stresses due to clay soils' shrinking and swelling caused by wetting and drying were factors in a number of the coating failures.
In other cases it was evident that coal-tar coatings had been "over cooked" and sometime backfilled before the coal tar had cooled sufficiently, as indicated by the dirt in direct contact with the pipe metal.
Some recent projects illustrate Enron's philosophy and procedures. Only rehabilitation as a result of external corrosion is discussed. Although a common cause of rehabilitation work, internal corrosion is best covered separately.
PROJECT IDENTIFICATION
Pipeline rehabilitation is extremely expensive and used when other methods to maintain safety and code compliance have proven unsatisfactory.
Candidates for rehabilitation work are determined by failure-history records, excessive cathodic protection and maintenance costs, stress-corrosion crack presence, electrical corrosion surveys, visual determination of coating condition, and regulatory audits.
Pipelines that have physical characteristics and environmental and operating histories similar to pipelines that have previously experienced deterioration may be investigated more vigorously to determine if rehabilitation is required.
Extensive planning is required successfully to undertake a rehabilitation project. Some of the factors considered include analysis of the market conditions and system requirements; resolution of gas-supply problems; environmental considerations, including permits; availability of equipment; right-of-way considerations; material selections; weather; and presence of SCC.
Numerous meetings are held during the planning stage to discuss the project parameters. When the project is finally turned over to the construction department, its progress is monitored by engineering and corrosion control to ensure that the desired results are achieved.
Using pictures and reports, the corrosion-control department documents the project extensively to build historical data for future reference.
Two approaches are used to rehabilitate deteriorated pipelines, depending on whether SCC is suspected or has been observed in the line.
Following are the steps Enron uses to rehabilitate a line containing SCC:
- Remove the line from service and hydrostatically test the pipe to about 110% of specified minimum yield strength (SMYS) to locate and eliminate severe SCC and arrest development of minor SCC.
- Excavate the line and place it on skids.
- Remove the deteriorated pipe coating
- Inspect the pipe surface for corrosion or other damage.
- Replace all failed or damaged pipe.
- Recoat the pipe.
- Tie-in and test the rehabilitated pipe.
Rehabilitating an out-of-service line free of SCC is identical to these procedures, except the 110% hydrostatic pressure test is eliminated.
The work may also be done while the line is in service. In this case, Steps 1 and 7 can be eliminated. A reduction in the operating pressure may be required, however, during the work.
After excavation, the pipelines are carefully inspected for damage, such as dents, gouges, seam splits, and corrosion. Areas of corrosion are evaluated by an "exact-area" method to determine their ability to withstand the maximum allowable operating pressure (MAOP) of the line.
METHODS, MATERIALS
Although some of Enron's rehabilitation work involves methods and materials more than 30 years old, new methods and materials are also investigated and used.
Enron prefers plant-applied, fusion-bonded-epoxy pipe coating for pipe replacement and for new pipelines. Coal-tar enamel applied by line-travel coating machines is the company's most used coating for pipeline rehabilitation.
Enron has also used tapes, coal-tar urethane, coal-tar epoxy, and wax coatings. Service temperatures are a primary consideration on the coating selected for a specific application.
Coal-tar epoxy and urethane have been used in high-temperature locations such as compressor-station piping.
The other coatings are used where service temperatures are generally less than 130 F.
Stripping the old coating from the pipe and cleaning the pipe surface use modern technology whenever possible. Hammers, knives, and cabling techniques for coating removal have given way to line-travel water blast, shot blast, sand blast, and grit-blasting equipment.
Enron has used all of these methods, depending on the requirements of the replacement coating for anchor pattern and surface cleanliness. Environmental concerns may also dictate the pipe-cleaning process if the old coating contains asbestos.
A further environmental problem has been encountered with pipelines that contain polychlorinated biphenyl (PCB) contamination from compressor oils.
And several rehabilitation projects have involved A. 0. Smith pipe, with the characteristic raised, square-seam weld (Fig. 1). The line-travel cleaning machines have proved very effective cleaners of the seam-weld area.
Attempts to remove original coating from this pipe by cabling were discontinued because cabling burrs over the edges of the bead, trapping coating beneath the burr.
Line-travel coating and priming machines are used to apply the coal-tar enamel coating, conjuring up dreams of a future that has effective coating applied by a cool, odorless method. The wax coating is applied at 275 F. with a "granny rag" technique reminiscent of the infancy of pipeline coating.
Pneumatic wrapping machines are used to put tape coatings on the pipe, and special spray equipment has been developed to mix and apply the two-component, coal-tar urethane coating.
PROJECTS
The following are descriptions of some of Enron's recent projects.
KANSAS LOCATIONS
The Bushton "C" line is 27 miles of 26-in. pipe located in Ellsworth County, Kan. in 1989, the line was removed from service, hydrotested to locate and eliminate the concern for serious SCC, excavated, cleaned and inspected, and recoated with coal-tar enamel.
The original coating was coal-tar enamel, which was removed with a line-travel, water-blast machine. The machine left the pipe surface adequately cleaned for the replacement coating, which was applied with a line-travel coating machine.
Coal-tar epoxy was applied to the compressor-station piping to meet the temperature conditions.
Weather conditions extended the curing time of the epoxy for 2 weeks, much to the consternation of the contractor and the operating personnel.
Inspection of the coating a year later indicated that it was well cured and strongly bonded to the pipe.
Prior to the recoating, 15 rectifiers were connected to this 27 miles of line and were not completely providing required levels of cathodic protection. Two rectifiers were connected to the line after backfilling and are now providing very satisfactory levels of protection with very low amperage.
Were not this line adjacent to lines in need of recoating, neither of the current rectifiers would be needed.
The Bushton "D" line is 27 miles of 30-in. pipe, also located in Ellsworth County, Kan.
Last year, it was removed from service, excavated, cleaned, and recoated with coal-tar enamel, with a line-travel coating machine.
The initial hydrotest was eliminated because SCC was not suspected on this line. Because the original coating was also coal-tar enamel, the pipe surface was again cleaned with water-blast equipment.
Visual inspection of the pipe prior to recoating found numerous areas of severe corrosion, resulting in the removal and replacement of the corroded pipe.
The compressor-station piping was sand blasted to provide a surface that would accept hand-held, spray-applied, coal-tar urethane coating. This coating requires expensive, specialized equipment to apply, but it sets up rapidly.
Peel tests when the pipe was hot, however, indicated a possible bonding problem. Current-requirement surveys made after the line was backfilled in August 1990 proved that the recoating was successful.
The presence of connections to another line that also needs to be recoated, however, is causing some problems in balancing the rectifier outputs to optimal levels. Planned recoating work will eliminate this problem.
GULF COAST; NEBRASKA
Also in 1990, along Florida Gas Transmission's (FGT) 24-in. line near Zachary, La., approximately 12.4 miles of pipe with a badly deteriorated tape coating were cleaned and recoated with coal-tar enamel applied by a line-travel machine.
The tape, which was applied in 1981, had disbanded in many locations. Pipe cleaning was done by a combination of cabling and line-travel, water-blast machine, followed by a line-travel, sand-blast machine (Fig. 2).
Prior to a decision on the cleaning method for this project, tests were run by cleaning a section of the line with water blast and another section with water blast and shot blast.
Cathodic-disbondment tests indicated that the performance of the coal-tar coating was superior on the section of line cleaned by the water blast followed by the shot blast. Enron believes that the shot blast removed all traces of the tape residue, contributing to the better bond of the coal tar.
Sand blasting was substituted for the shot blast on the job, but the same results as achieved with the shot blast were expected.
This line was constructed with A.O. Smith pipe, with its raised, square longitudinal weld seam. Because cabling to remove the original coating frequently burnished over the edges of the weld seam, the ability of any cleaning method adequately to clean the seam was doubted.
The experience with cabling on this line and the Bushton "D" line, which also had A.O. Smith pipe, has led Enron to recommend that cabling not be used on A.O. Smith pipe.
In another 1990 FGT wax-recoating project, about 20 miles of 20 and 22-in. pipe located along Texas' Gulf Coast and originally coated with coal-tar enamel, was surveyed to locate areas of bad coating.
That the replacement of the deteriorated coating had to be done with the line in service eliminated the use of line-travel cleaning and coating machines.
The heavy clay soils along the line eliminated tape as a replacement coating, and some recent poor performance of polyester, flake-filled coatings precluded them as well.
Wax seemed to be the best compromise. Its low service temperature, however, required selection of another coating for use near compressor stations.
When excavation revealed less extensive coating damage than originally estimated, the length of the project was reduced.
The wax was heated in a large kettle and delivered to the pipeline in the ditch by cans with pouring spouts. The wax was poured from the can onto the top of the pipe and spread along the lower part of the pipe by the "granny rag" method with butcher paper (Fig. 3).
A layer of wax-saturated tape was spirally wound over the first wax coat to build thickness and mechanical strength. Then, a second coat of wax was poured and granny ragged over the tape.
Upon hardening, which is almost immediate, the wax was "jeeped" (inspected), "holidays" repaired where necessary, and the line backfilled. Extra care was taken to prevent large dirt clods from falling on the pipe during backfilling, as the wax remains soft and is easy to penetrate.
In 1989, a section of the 20-in. Hooper "B" line 6 miles north of Palmyra, Neb., was recoated. Excessive current requirements indicated poor coating, which was verified by further investigation.
The pipeline was removed from service, excavated, placed on skids, and the original coal-tar coating removed by a line-travel, water-blast machine. After cleaning and inspection, the pipe was recoated by line-travel, coal-tar method, tested, and backfilled.
This project was identified early enough so that the pipe was in very good condition, and there were very few cases in which the pipe had to be replaced.
A line of gouges was found on the pipe that apparently came from the coating machine that applied the original coating. Some of these gouges were severe enough to require sanding or grinding to prevent them from penetrating the replacement coating.
REPLACE VS. REHAB
Post-project reviews on some of the projects indicated that the pipelines could have been replaced more economically than rehabilitated.
The construction contracts were generally bid with a lump sum for the excavation, testing, coating removal, and recoating. However, a unit price was used for all cutouts of defective pipe.
Projects that had pipe in worse condition than originally estimated revealed that the large number of cutouts quickly caused the rehabilitation cost to exceed the replacement cost.
It is advisable carefully to determine the pipe condition prior to a decision on whether to replace or rehabilitate a pipeline.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
