Methods of Application

In document Chapter 24 Pipeline Coatings (Page 22-43)

Some coatings are applied only in a stationary coatings facility; some may be applied either in a plant or in the field, while others are applied only in the field. The following table illustrates how various pipe coatings usually are applied.

Table 24.1

Pipe coating Plant application Field Application

FBE Yes Yes

Coal-tar urethane Yes Yes

(Mainline/joints or re-hab)

Shrink-fit sleeves No Yes

(Field joint/rehab)

Plant Coating Application—General Sequence

The pipe coating operation in a stationary coatings plant, with noted exceptions, follows a basic sequence, and the plant lay out is much the same for different generic coating types. Generally, the operation, which allows for inspection hold points, is approximately as follows:

• Pipe to storage

− Log in pipe sections, pre-inspection for damage

• Pipe delivery to start of coating operation

− Pre-inspection

• Pre-cleaning

• Abrasive blast cleaning

− Inspection

− Mechanical repairs

• Priming (if required by the specifications)

• Coating and wrapping

• Inspection

− Visual

− Holiday inspection

− DFT measurements

• Coating rework or loading for storage or for delivery to pipeline right-of-way

Plant Coating Operation by Generic Coating Types

Coal-Tar Enamel

The starting point for the manufacture of coal-tar enamel is coal-tar pitch, which is the stable residue from the coking process in a steel mill. In this process, coal undergoes controlled destructive distillation at 1,100°C (2,000°F) to produce high-purity coke (carbon) for use in the reduction of iron ore. By products of the coking ovens are coal-tar pitch, a semi-solid of high density, and lighter fractions such as benzene, xylol, toluol, etc.

Coatings derived from coal-tar pitch—such as coal-tar cutbacks, coal-tar epoxy, and coal-tar enamel—contain a high ratio of aromatic compounds, characterized by the stable diamond-shaped (hexagonal) ring structure.

Coatings based on this aromatic ring structure usually:

• Are very waterproof; they exhibit excellent resistance to moisture absorption and moisture vapor transmission

• Exhibit relatively poor resistance to aromatic solvents (toluol, xylol, etc.)

• Exhibit poor resistance to sunlight; with time, they crack

• Exhibit excellent bond to steel

The coating, coal-tar enamel, is a thermoplastic, semi-solid material, which is manufactured by digesting (cooking) a mixture of coal-tar pitch, selected pulverized coal, and coal-tar oils, and then adding inert fillers. These fillers or bulking agents, such as talc or powdered slate, provide mechanical strength and heat resistance to the enamel.

• Enamel coating operations

In the coating plant, coal-tar enamel is heated to 230 to 245°C (450 to 490°F), and applied in molten form to a DFT of about 2,500 µm (100 mils) over a thin primer, less than 25 µm (1 mil) thick. As the pipe rotates through the coating station, a 500 µm (20 mils) inner layer of inert porous glass fiber mat is simultaneously wrapped over the coating. This glass wrap reinforces the enamel much as steel does in concrete.

Figure 24.10: Coal-Tar Enamel with Glass Layer To be effective, the glass should be positioned in the outer one-third of the enamel. It should not be exposed to the air nor be allowed to contact the bare surface of the pipe.

Any moisture that contacts the glass can wick moisture to the pipe surface under the coating. If this occurs, a corrosion cell could be established at that location, and under-film corrosion could occur.

A heavier, more dense layer of glass-fiber mat, saturated with a coal-tar cutback solution, is applied to the outer surface of the hot coal-tar-enamel coated pipe. This outer wrap provides additional strength to the coating and, once installed, offers resistance to soil stress.

A final outer wrap of Kraft paper or whitewash is applied to the coated and wrapped pipe. This wrap provides a light background to spot obvious mechanical damage and

provides heat reflection while the pipe is in outside storage.

As a typical thermoplastic coating, coal-tar enamel is affected by changes in ambient temperature. At higher temperatures, the enamel will soften and flow and eventually may disbond from the pipe, while at lower temperatures the material may become brittle, crack, and disbond.

• Coating process

In the plant coating process, in the order shown, the pipe is:

− Delivered to a holding rack, identified according to shipping papers, and inspected for mechanical damage

− Conveyed through an open gas flame ring which removes moisture, mill lacquer (a temporary protective coating), loose rust, etc., and heats the pipe to about 38°C (100°F)

Figure 24.11: Flame Cleaning before Blasting

Figure 24.12: Rusty Pipe Pre-Blast

− Conveyed to a centrifugal wheel blast station where the pipe is blast cleaned with steel shot or grit, or a combination of both, to achieve the specified surface cleanliness (usually NACE No. 3/SSPC-SP 6 or better) and the required surface profile

Figure 24.13: Post-Blast Pipe Primed

− Primed as it exits the wheel blast unit

− Coated by a flood coat of molten coal-tar enamel, with simultaneous wraps of a:

∗ inner glass wrap

∗ outer glass wrap

∗ final outer wrap of Kraft paper

− Exposed to a water quench to cool and re-solidify the enamel

− Conveyed to a cooling and storage rack where the coated and wrapped pipe is further cooled, and the coating ends are beveled to expose the enamel for later field-joint coating as the pipe is installed

− Inspected for holidays with a high-voltage DC holiday detector; the holidays are marked and repaired

− Inspected for any obvious mechanical damage and repairs are made

− Transported to storage or to the pipeline right-of-way for installation

Once the pipe leaves the entry-holding rack, it rotates as it is conveyed through the various stations—cleaning, coating, wrapping, and quenching—until it reaches the final holding and inspection racks. Both the speed of rotation and the speed of travel can be varied, which permits adjustment of the cleaning process as well as the coating process.

At the enamel coating station, the hot coal tar emits volatile coal-tar oils and some white smoke, which intensifies with excess heating. With continued excess heating, the coal tar will emit copious yellowish-white fumes, which is a sign of degradation of the enamel coating, until finally the product becomes carbonized, brittle, and unsuitable as a protective coating.

Coal-tar epoxies and cutbacks are self-priming. However, coal-tar enamel requires a primer in order to develop a reliable bond to the pipe. Here, the purpose of the primer is to insulate the pipe long enough to prevent chilling of the enamel and to allow the primer and enamel to fuse together to develop the bond.

Extruded Polyethylene

Polyethylene materials may be extruded into pipe coatings by one of the following methods:

• Cross-head extrusion

After the pipe has been blast cleaned, it passes through an annular extrusion head (die) where a hot mastic (120°C [250°F]) is applied. This mastic is a butyl rubber/asphalt blend extruded uniformly on the pipe to a DFT of about 250 µm (10 mils).

Figure 24.14: Applying Mastic Primer

The primed pipe enters a second annular extrusion head where a seamless sleeve of polyethylene is extruded around the pipe to a DFT of 1,000 µm (40 mils). The sleeve is larger in diameter than the pipe being coated.

Just beyond the extrusion head, the pipe enters a water spray that cools the polyethylene and causes it to shrink onto the primed surface in somewhat of a “compression”

fit.

Figure 24.15: Extruded Sleeve of Polyethylene, Water Quenching, and Holiday Testing

During the cleaning, coating, and extrusion process, the pipe travels longitudinally, but does not rotate.

Pipe up to 76 cm (30 in.) can be coated by this process.

Extruded polyethylene has some memory and can shrink from the ends. The polyethylene does not bond to the mastic primer.

The inspector may be required to inspect the polyethylene pellets used in the extrusion process for any sign of moisture. The inspector also may be required to check:

− The temperature of the asphalt/rubber mastic

− The WFT of the primer

− The DFT of the extruded coating

− That the holiday detector is connected and in proper working order

− For any mechanical damage to the coating

• Side extrusion of polyethylene coatings

Figure 24.16: Coated Pipe Film Shrinkage/Memory Steel pipe may be made using a lap weld. This type of weld leaves a void that must be filled in with an adhesive layer of polyethylene-type refined coating.

Figure 24.17: Side Extrusion Head

After large-diameter pipe (to 2.44 m [96 in.]) is abrasive blast cleaned, it rotates as it travels forward under a side-extruder head. Polyethylene beads under high temperature and pressure are forced through the side extruder into a hot film about 1,000µm (40 mils) thick.

Figure 24.18: Trimming Cutback

This hot film is attached to the pipe at an angle to its longitudinal axis. As the pipe rotates and travels forward, the hot film is wrapped onto the asphalt/butyl mastic primed pipe surface. The finished coating resembles a spiral wrap of tape. The polyethylene film fuses together at the overlap due to the latent heat of the hot extruded plastic.

The coated pipe is tested for holidays with a high-voltage DC holiday detector, using a conductive rubber electrode.

Hard-Adhesive Extruded Polyethylene

In this process, a material such as epoxy is applied to the bare pipe and is then over coated with one or more layers of polyethylene applied by the side-extrusion process.

Fittings, bends, valves, etc., may be coated with polyethylene by the sintering process. Sintering is a method of applying polyethylene powder directly to a heated surface in much the same way as FBE, except at a lower temperature

Weight Coatings

The wall thickness of pipelines installed onshore may vary with the product transported. A high-pressure

a line transporting a petroleum product. For onshore installation, pipelines with a wall thickness as low as 6.2 mm (0.25 in.) may be used. For offshore installation, especially in deep waters, pipelines may require a greater wall thickness sometimes exceeding 25 mm (1.0 in.):

• For greater strength

• To provide negative buoyancy to prevent the pipe from floating, especially pipe diameters larger than 20 cm (8 in.).

Frequently, this heavy wall pipe is coated with a weight coating such as:

• Asphalt mastic, which is a combination of protective and weight coating, fortified with heavy aggregate

• Concrete reinforced with metal wire mesh

Asphalt Mastic

Figure 24.19: Asphalt Mastic Coating Operation Asphalt mastic is produced only in the United States under the trade name SomasticTM. Use of this material has declined in recent years, such that its future is in some doubt.

This is a combination protective coating and weight coating commonly used for offshore operations.

The asphalt mastic is made by adding mineral fibers (formerly asbestos fibers, now glass), inert fillers, and very dense, very heavy aggregate to molten asphalt enamel (260°C [500°F]). The resulting mixture, which contains about 15% asphalt and 85% inert materials, is applied at about 155°C (300°F).

The hot mixture is extruded over the pipe surface that has been primed with an asphalt primer. Minimum coating thickness is 1.25 cm (0.5 in.). The coating often is over coated with whitewash, then supported on its bare ends and stored out doors in a spider-like formation until the coating cools and sets.

The whitewash reflects some of the rays of the sun and thus provides heat reflectance for the coating. This treatment is common with many thermoplastic coatings that have a tendency to soften and slump if not protected during storage.

The asphalt-mastic-coated pipe may be fitted with bracelet anodes (usually zinc) bonded directly to the pipe.

In effect, the lay-contractor installs the “sacrificial”

cathodic protection system as the pipe is being installed.

Bracelet anodes can be installed in this manner on pipelines with any type coating.

Concrete Weight Coating

Concrete may be applied over asphalt-mastic-coated pipe to provide additional negative buoyancy or it may be applied over other pipeline coatings.

Figure 24.21: Application of Concrete Weight Coating with Steel Wire Screen

Concrete weight coating may be applied:

• By the gunite method

• As a compression coat (extruded)

Figure 24.22: Typical Somastic/Concrete Field Joint Thickness of the concrete coating may vary from 5 to 30 cm (2 to 12 in.) depending on the weight requirements.

The annular space at the field joint must be filled to the diameter of the coating plus any concrete weight coating added. Quick-setting cement, asphalt mastic, or high-density polyurethane foam may be used to fill this space.

Field Application of Pipeline Coatings

As shown in Table 24.1, several mainline coatings can be applied both in the field and in a stationary plant.

Polyethylene tape is the most commonly used material for mainline (i.e., pipe coating less field-joint coating) coating in field applications. These materials are widely used for coating rehabilitation work in the field. To a lesser extent, coal-tar epoxy, epoxy polymer, concrete epoxy, coal-tar urethane, and 100%-solids elastomeric polyurethanes also are applied in the field.

A number of the products mentioned may be used in the field to coat the bare, welded field joints of plant-coated pipe; these materials will be discussed later today.

Polyethylene Tapes

There are two types of polyethylene tapes:

• Laminated

• Co-extruded

The laminated tapes are manufactured by applying a compatible adhesive to one side of a pre-fabricated film of plastic (polyethylene). The film usually is of medium density and molecular weight designed to provide optimum strength, flexibility, and ductility for application under tension.

The tape can vary in total thickness from 375 to 1,250 µm (15 to 50 mils). The typical adhesive portion can vary in thickness from 125 to 500 µm (5 to 20 mils).

The adhesive is prepared by introducing certain raw materials into a calender that consists of two rollers of different diameters placed close together, and rotating at different speeds. The raw materials fed into the calender are chewed and blended together at reasonably high pressure, with heat being generated in the process. The hot adhesive is removed and fed through another series of rollers that carry the plastic film backing. Here, the hot adhesive and film backing are laminated together, slit to size and put on rolls.

Co-extruded tape is manufactured in an entirely different manner from the laminated tapes. The co-extrusion apparatus consists of three separate feeders, each of which carries a different raw material connected to a single extruder or die.

Resin

Figure 24.24: Co-Extruded Process

From three separate feeders, melted or heated materials are funneled through an extrusion die, and the materials

exit into a homogeneous tape rather than a laminated tape.

With laminated tape, the adhesive can be separated from the backing. One side of the co-extruded tape exhibits a smooth and dense film, and the opposite side resembles an adhesive surface. It is virtually impossible to pull the co-extruded tape apart into separate, identifiable layers.

Both materials are applied cold—no heat is needed—

under controlled tension over specially primed surfaces.

Tape Application

• Stationary Plant

Pipe coated in a stationary coating plant follows the same coating sequence as that used for the application of polyethylene by the side-extrusion process:

− Pre-inspection

− Pre-cleaning

− Blast cleaning and priming

− Tape application

− Inspection and loading out

• Field Application These tapes are used as a:

− Mainline coating

− Rehabilitation coating

− Field-joint coating

Figure 24.25: Field Tape Application

For mainline construction, the pipe is welded into a continuous length, and bent to conform to the contour of the ditch. Frequently, two side-boom tractors, which travel about 15 m (50 ft) apart along the open ditch, are used in the application of the tape.

The front side boom carries a cleaning and priming machine that fits around the pipe and is adjusted for its diameter, while the rear side boom carries the tape-wrapping machine that also fits around the pipe.

The two side booms, working in tandem, lift the pipe, and as they travel forward, the pipe is cleaned, primed, and wrapped with the tape.

Alternatively, a combination cleaning/priming/wrapping machine may be used. These specially built units are equipped to clean and prime the pipe with a fast-drying primer, then simultaneously wrap the tape over the primed surface.

In rehab work, a combination machine is used and the application procedure is similar. Because the pipe is still in service and still in its original ditch, there are some exceptions in the process:

− Only short sections can be cleaned and wrapped

− The machine is on the pipe in the ditch

− The machine must frequently be taken on and off the pipe to bypass the necessary pipe supports

Figure 24.26: Rehab Coating with Plastic Tape

Some Problems with Pipeline Tape Coatings Each generic pipe coating has its own unique set of problems, and those often associated with plastic tapes include:

• Cathodic disbondment due to the CP system

• Spiral corrosion due to poor seal at the tape overlap

• Soil stress, which may displace portions of the tape

• Attack by bacteria, generally on the adhesive

• Undercut corrosion at the pipe/coating interface

Figure 24.27: Fish Mouth, Poor Overlap Seal

Liquid Coatings

Liquid coatings on pipelines may be solvent-based, coatings such as coal-tar cutbacks, vinyls; or chemically cured, such as inorganic zinc, coal-tar epoxies, high-solids epoxies, vinyl esters, 100%-high-solids epoxy coal tars, coal-tar polyurethanes, and 100%-solids elastomeric polyurethanes.

Either type of coating must be applied over properly cleaned (minimum near-white blast), dry surfaces, according to the manufacturer's specific recommendation.

Application equipment can consist of a simple conventional air spray rig with coal-tar cutbacks and vinyls, high-ratio (30:1 or 45:1) airless spray units with coal-tar epoxies, epoxies, etc., or finally, with more sophisticated plural-component spray units for coal-tar polyurethane and 100%-solids elastomeric polyurethanes.

Coating selection, number of coats, recoat times, total thickness, etc., all vary with the generic material, service conditions, and customer demand.

Coal-Tar Epoxies

Epoxies and coal-tar epoxies have been used for years both in plant and field applications.

The coal-tar epoxies have the advantages of high-bond strength, better chemical resistance, and higher temperature resistance than the solvent cutback coatings.

Combined with its resistance to moisture, coal-tar epoxy coatings give excellent service for many years and are still used extensively.

In document Chapter 24 Pipeline Coatings (Page 22-43)