In the previous article, we discussed the importance of coating steel pipes and several coating types, including single- and dual-layer epoxy and two- and three-layer polyolefin systems. This article describes other coating methods for steel pipes and their welded joints.
Coal Tar or Asphalt Coatings
Coal tar enamel (CTE) and asphalt coatings have been used for more than 80 years. CTE is a polymer-based coating produced by plasticizing coal tar, with mineral fillers added to achieve the desired properties.
The CTE coating system consists of four main components: primer, coal tar enamel, and inner and outer fiberglass wraps. The fiberglass improves mechanical strength and protects the pipe against soil stresses and potential damage during transportation and installation.
To apply this coating, coal tar enamel heated to 240°C is poured onto a rotating, primed pipe. An enamel-impregnated inner fiberglass wrap is immediately applied over this layer, followed by a second fiberglass wrap impregnated with coal tar. Finally, a single layer of kraft paper is wrapped around the pipe to protect the coating against UV degradation.
Liquid Spray-Applied Coatings
These coatings are usually two-component epoxy or polyurethane systems. They can be readily spray-applied at the installation site, making them suitable for rehabilitating existing pipelines.
Concrete Weight Coatings (CWC)
With growing global demand for oil and gas, declining onshore resources, and increasing interest in offshore reserves, oil and gas extracted offshore are transported through pipelines to coastal refineries. Since seawater acts as an electrolyte and causes pipe corrosion, an external corrosion-protection coating is essential. A concrete coating is also required to maintain pipeline stability on the seabed against waves, ocean currents, and buoyancy, and to protect steel pipes with anticorrosion coatings against potential damage, including impacts.
Concrete coatings can be applied using several methods. In the first method, concrete is deposited onto a belt that wraps around the pipe and compresses the concrete against its surface. Steel reinforcing mesh is simultaneously embedded in the concrete. Another method involves spraying concrete onto the pipe at high velocity. Sometimes, the two methods are combined: the pipe is first coated with cement using the first method, followed by cement spraying to achieve the desired thickness.
Polyurethane foam is commonly used to cover the welded joint areas. The foam is poured into a plastic or steel mold surrounding the pipe joint, filling the uncoated space.
Pipe Joint Coatings
The ends of each pipe section coated using the methods described above are left uncoated to allow joining and welding. After welding, the weld and surrounding uncoated areas must be protected. Various methods are available for coating these areas. Joint coating is particularly important because a weld can be the weakest point in a pipe. Welding and joint coating are performed at the pipeline installation site, so the selected method must be compatible with site conditions.
Some methods for coating field joints are described below. Table 1 also shows the main pipe coatings and suitable coatings for their field joints.
Heat-Shrinkable Sleeves: Crosslinked polyolefin with a mastic or hot-melt adhesive.
Hot-Applied Coal Tar Enamel: A system comprising primer, coal tar, inner and outer fiberglass wraps, and a final kraft paper wrap.
Flame-Sprayed Polyolefins: A multilayer system consisting of a thermosetting resin layer, usually epoxy, followed by a thermoplastic topcoat thermally sprayed onto the resin layer.
Cold-Applied Polymeric Tapes: Polymer-based adhesive tape systems that may include a primer and one or two layers of polymeric tape.
Fusion-Bonded Epoxies: Thermosetting epoxy powders that cure upon heating.
