DOUBLE-WRAP TAPE SYSTEM ADOPTED FOR IN SITU RECOATING

Joseph L. Pikas
Transcontinental Gas Pipe Line Corp.
Houston

Use of a double-wrap tape system has proved the best choice for Transcontinental Gas Pipe Line Corp. (Transco) for in situ coating on large or small diameter gas lines in service.

Past application problems with polyurethane and coal-tar epoxy coatings led Transco to conclude that these systems were not cost effective for its mode of operation.

In situ recoating with a double-wrap tape system, the company found, allows coating to be applied consistently and at a low cost while the pipeline remains in service.

TRANSCO OPERATIONS

Degradation of coating resulting in external corrosion or leaks is a major problem in the U.S. today as many pipelines are approaching 40-50 years of age. Over the past few years, there has been general recognition of this problem and the importance of recoating.

The choices of methods (taking the line out of service from valve section to valve section) and materials (limited by application techniques and equipment) have strained the economics of the situation, however, particularly because all of the coating may not have failed.

Transco's recoating program employs in situ methods for the recoating of large diameter pipelines. The company operates some 10,000 miles of interstate pipeline which transports gas from the Texas Gulf Coast to a multistate market area along the Gulf Coast and eastern seaboard to New York City (Fig. 1).

There are 36 compressor stations located along the pipeline at 35 to 80-mile intervals between Texas and New Jersey. The main line operates at between 500 and 800 psi.

The Gulf area off the Louisiana and Texas coasts supplies most of the gas for the pipeline. Onshore fields in Texas, Louisiana, and Mississippi, along with Canadian and Mexican gas make up the remainder of the gas supply.

Since laying the first pipeline in 1949, Transco has operated an effective corrosion-control program through the use of coatings and cathodic protection.

Coatings, as the first line of defense in providing corrosion protection, are intended to form a continuous film of an electrically insulating material over a metallic surface to isolate the metal from direct contact with the surrounding soil or electrolyte (Fig. 2).

Fig. 3 shows how coating degradation had taken place at one location since the initial coating was applied. Several factors that may have contributed to this example are soil stresses, excessive operating temperatures, hydrostatic testing, poor application, and time.

A poorly coated line or line with deteriorated coat becomes difficult or sometimes impossible to be cathodically protected. The coated line can become disbanded, thus acting as a shield to the pipeline and restricting protective cathodic currents.

If the coating deterioration is intermittent along the pipeline, it then becomes uneconomical and ineffectual to install rectifiers and groundbeds at these sporadic holiday areas in order to restore the potentials to protective levels. Therefore, it cannot be overemphasized that a good coating is the better way to achieve protection.

An added feature of a wellcoated system is that it can also protect it from microbiologically influenced corrosion (MIC) which recently is being given more attention by the pipeline industry.

EXISTING COATINGS

Transco's system consists of these coatings (with ages of each coating system) given in Table 1:

Coal-tar enamel. Main Line "A," the oldest line (41 years), has a coal-tar enamel coating system. These coatings have been found to be in good to excellent condition because coal tar exhibits many desirable properties that establish a permanent barrier between the structure and its immediate environment.
However, problems existed immediately downstream of compressor stations where temperature was a problem at one time; also, at tie-ins where poor application conditions were found.
It should be noted that this is the only line on which Transco has experienced stress-corrosion cracking (SCC). Most of the SCC has been found immediately downstream of the compressor stations where temperature was one of the main contributing factors.
Through hydrostatic testing and replacement programs, most of the pipe has been replaced that has experienced the detrimental effects of SCC.
Asphalt enamel. Main Lines "B," "C," and portions of "D" (Table 1) all have asphalt-enamel coating systems.
It is suspected that water absorption, soil stress, and inadvertently high cathodic protection (CP) potentials at some locations with the combination of hydrostatic test forces are the major factors in causing high disbandment rates on asphalt-enamel coating systems.
This has made it difficult to maintain adequate cathodic protection as a result of shielding from coating disbondment or coating deterioration.
Asphalt enamels in the mid-1950s were sold at a significant cost reduction per ton compared to coal-tar enamels and were touted as an equal to coal-tar enamels. However, this proved not to be the case in the long run.
Fusion-bonded epoxy (FBE). Transco has been using FBE coatings for the last 16 years. Since the introduction of FBE coatings, no major problems have been found except for intermittent cathodic blisters.
In addition, little or no corrosion has been found in these blistered areas.
Fusion-bonded polyethylene (FBP). In the last 6 years, Transco has used FBP, also called the Sintering coating systems and Mapec coating systems. These systems are mainly from Europe and are beginning to gain acceptance in the U.S.

The Sintering coating system is the application of powdered polyethylene sifted or cast onto a 300 C. preheated pipe to attain a minimum thickness of 118 mils.

The Mapec coating, also called the 3 Coat System, consists of the application of a 2-mil duraplastic epoxy primer, followed by an 8-mil polyethylene co-polymer adhesive extruded around the pipe followed by 118-mil polyethylene extruded spirally over the adhesive to a 150 C. preheated pipe.

One quality of this system is that it exhibits high dielectric resistance.

On a Transco 30-in., 125-mile line installed approximately 3 years ago in Alabama, it was found that the current requirements for this pipeline were only 500 ma for the entire length of the system.

OLD RECOAT PROGRAM

Transco started a recoating program in 1971 with cold-applied, polyethylene-backed, single-wrap tapes.

Several hundred feet were applied the first year as compared to several thousand feet applied later in 1975. All these tapes were hand applied by the local pipeline crews.

The old coating was removed with hammers and scrapers, the pipe was cleaned to a commercial blast surface (NACE 3/SSPC-SP6), a primer was applied, and the tape was single wrapped with a 50% overlap.

Because of inconsistent manual application techniques and incorrect use of the tape materials, problems such as soil stresses and water penetration soon developed. A redirection by Transco in the use of tapes has been taken and will be discussed later.

In 1976, application of a hot-applied, coal-tar pitch tape was introduced into the pipeline recoating system. This tape consisted of a plasticized material with a synthetic or cotton fabric woven into the system with an outer wrap of Mylar.

Initially, these tapes were hand applied by local pipeline crews and later by contract labor. Again, several hundred feet were applied in 1976 compared to more than 100,000 ft in 1986.

The old coating was removed with hammers and scrapers, the pipe was cleaned to a commercial blast surface (NACE 3/SSPC-SP6), and a primer applied. The tape was then "cigarette-wrapped," applying heat both to the material and the pipe surface to form a bonded seamless coating. A minimum of an inch overlap was maintained.

This coating system proved to be unsuccessful, however. Application was somewhat difficult, quality control of thickness by the manufacturer was inconsistent, water penetration was often evident after 2 weeks in the ground, soil stress took its toll, and poor adhesion occurred in these areas.

In short, this coating system failed and is continuing to fail. In fact, after 3-4 years in service, Transco is having to remove it and recoat with a new system.

NEW COATINGS

In 1987, several new coatings were tried on the pipeline system: a polyurethane 100% solids coating, a coal-tar epoxy, and a double-wrap, 50-mil polyethylene.

Polyurethane coatings generally exhibit good abrasion resistance and good adhesion, and they are resistant to many environmental contaminants. Laboratory and field tests were conducted on this coating material at approximately the same time.

Based on the laboratory test data, the coating did not perform well at 130 F. with 1.5 v in a cathodic-disbondment test. A field test was conducted on 500 ft of 30-in. OD line to evaluate this coating material and application.

It was found that, even with experienced personnel, this coating was difficult to apply because of the sophisticated application procedures and equipment required. In addition, these coatings required a white metal-blast surface with a 4-5 mil anchor pattern for optimum results.

The economics of the material and application are very high when it is being applied in the field.

Coal-tar epoxy coatings generally display good resistance to soil stresses and to cathodic disbandment because they have high adhesion and exhibit little blistering at elevated temperatures. They are excellent for immersion service.

Field application of coal-tar epoxy requires experienced personnel, however, using sophisticated procedures and equipment. These coatings are less tolerant in application than tapes, particularly to environmental contamination.

Up to 5 days are generally required completely to cure and harden the coating prior to handling and backfilling. A near-white abrasive blast or white-metal abrasive blast surface is required with a minimum of 2-5 mils anchor pattern.

Again, costs are high when this coating is being applied in the field. On one job in Louisiana, a 60% coating material loss was experienced. This occurred primarily from inexperience of the coating applicators in handling these materials.

Loss of material to over-spray was high, and it was difficult to achieve uniform coating thickness due to constraints in the ditch.

Total cost for applying this material on a 30-in. OD line on relatively flat terrain ran up to $65/ft including coating material at $10/ft. This was unacceptable because of the high field-application costs.

Even with experienced applicators, these costs would be prohibitive because of the labor required and difficult conditions in the ditch.

Coal-tar epoxy coatings have their place in the field such as recoating short sections of pipelines, piping immediately downstream of compressor station, block valves, and ground-level riser piping.

DOUBLE-WRAP SYSTEMS

The third type of coating material was the polyethylene tape system double wrapped to 50 mils. This system was field applied on a 24-in. OD operating line in the Edna, Tex., area primarily to test for the effect of soil stresses.

The manufacturer of this coating was conducting similar soil stress tests at a site near Victoria, Tex., long before Transco started considering use of this coating system.

After 2 years of field testing, the decision was made to use the double-wrap polyethylene tape system for recoating after verifying the results from field test and laboratory tests. In addition to passing these tests, the coating was economical, and application was easier and consistent compared with the coal-tar epoxy and polyurethane coating problems discussed earlier.

Additional advantages of recoating with a double-wrap tape system are that it is nontoxic and environmentally safe, employs no dangerous open flames, requires no special cure techniques, has a wide application-temperature range, allows immediate holiday inspection, is compatible with most pipeline coatings, yields better dielectric resistance, and is chemically stable.

This coating material and system appear to be the answer to recoating in situ where coating deterioration has taken place over long sections of pipe.

Typical overall costs for a 30-in. OD line run approximately $30/ft including material costs of about $4.50/ft. These latter are about 15% of the total project, generally.

Small-diameter lines run proportionately lower in cost/foot. Hilly terrain, rock conditions, and congested or paved areas can increase costs.

Because of the polyurethane and coal-tar epoxy application problems, environmental constraints, and economic problems experienced on previous jobs, the double-wrap polyethylene tapes present a good alternative.

Based on problems with the manual application of tape, discussed briefly earlier, Transco needed a "wrapster" that could apply the coating within the confines of the ditch and maintain constant tension. The result in consistent high quality would enhance the life of the coating.

IN SITU METHODS, PROCESS

Two in situ methods of tape application used on the Transco system are hand-assisted wrapsters and power-assisted wrapsters.

The first uses a hand-assisted wrapster which is pushed and pulled around the pipe by as many as four or five workers per machine.

The second is a modification of a hand-assisted wrapster machine powered by an air motor.

With cooperation between a tape company and a coating-machine fabricator, a hand-assisted wrapster was built for Transco (Fig. 4) that met the company's criteria for constant tension, uniform overlaps, no wrinkles, and work within a 12-in. clearance underneath the pipe.

Although this machine met expectations, it was felt that the coating rate could be increased if the coating machine were made power-assisted.

This did indeed increase the rate of recoating and reduced the laborious work required by the hand-assisted model.

Both machines are currently used on Transco's recoating jobs today, depending on the length of taping required. However, the hand-assisted machine is primarily used where the pipe is skidded in support areas and on short sections of lines.

The power-assisted machine is used for the majority of recoating where long sections of lines are involved.

The first step in the recoating process is to excavate the pipeline completely to provide an adequate work space and a minimum clearance of 12 in. between the bottom of pipe and ditch floor and sides.

Earthen plugs are left in the pipeline at predetermined intervals, and skids are placed in the open ditch to prevent any movement of the pipe.

A crew using hammers and scrapers removes the old coating. This method is very effective because the old coating is generally not bonded to the pipe surface. With an abrasive blast from standard blast equipment, the pipe is then cleaned to a commercial-blast surface specification (NACE 3/SSPCSP6).

In addition to these methods of cleaning the surface of the pipe, Transco has also been using a machine which removes the old coating and prepares the surface, all in one operation. It uses rotating carbide-tip drums that move in an oscillating motion to remove the coating with a unique anchor pattern. This line-travel cleaning machine can be used either in situ or over the ditch.

After the pipe is cleaned and completely inspected for any corrosion or defects, a primer of 1 mil dry-film thickness (DFT) is applied by roller or spray application. The primer provides a bonding surface for the subsequent tape coatings.

Next, a 4-in., 35-mil thick butyl-backed tape is manually applied over all longitudinal and girth-weld seam areas. This tape material provides a smooth and uniform surface to prevent bridging of the subsequent layers of tape coating and moisture accumulating next to the weld seam.

The tape wrapster is manually placed around the pipe, adjustments are made, and the air motor is then attached.

Tape rolls are loaded to the tension-brake spindles. Maintaining tension is important because it is very critical to have good adhesion to the pipe surface to ensure a good bond.

With the air-assisted power wrapster, the first full layer of tape is applied which consists of a 25-mil thick black polyethylene material. This layer of tape provides the initial corrosion barrier.

A holiday detector is run immediately behind the wrapster to find any defects in the first layer of tape.

The wrapster is manually removed from the pipe within the confines of the ditch and reinstalled in order to wrap the final layer of tape as shown in Fig. 5.

This material provides the mechanical barrier against the backfill material and other external forces such as soil stress.

In 1989, a modification of the original power-assisted model was made which applies both the inner and outer tape wraps simultaneously. Results to date have been excellent.

With no curing required, backfilling occurs immediately after the final coat is applied, as shown in Figs. 6 and 7.

BIBLIOGRAPHY

Appleman, B. R., "Tape Systems for Pipeline Protection," Journal of Coatings and Linings, Pittsburgh, July 1987.

Noonan, J. R., and Thomas, S. J., Application of Polyethylene Tape Coating for Use on Reconditioning/Rehabilitation of Pipelines, Polyken Technologies, Boston, 1987.

Peabody, A. W., Control of Pipeline Corrosion, NACE, Houston, 1967.