Protective pipeline coating is a critical component for pipelines connecting gas fields to end-users. External coatings provide corrosion protection to maintain the integrity of the high-pressure carbon steel pipeline, working hand-in-hand with a cathodic protection (CP) system, while internal coatings reduce friction within the pipeline.

When selecting a suitable coating it is important to consider the coating’s mechanical protection and damage resistance from pipe handling, trench backfill, soil conditions and suitability of the field joint coatings. In operation, the soil chemistry, pipeline operating temperatures and compatibility with CP are also critical factors.

Fusion bonded epoxy

The vast majority of pipeline coatings available for the gas transmission market are now based on fusion bonded epoxy (FBE). As with all long-term coating systems, FBE requires excellent steel surface preparation to ensure proper adhesion.

The pipe is blasted to a near white finish. The surface profile’s height and peak density are critical as they provide an ‘anchor’ for the FBE to adhere to. In order to ensure a pristine surface for coating, the pipe is generally subjected to a phosphoric acid treatment to reduce salt and other surface contaminates prior to the coating application. The phosphoric acid is then removed using a high pressure spray of low conductivity water, from a reverse osmosis system.

ERW steel line pipe is a high-quality product, generally requiring minimal removal of slivers/mill scale or other contaminates.

Another benefit of the FBE type coatings is the high level of steel blasting preparation, which removes mill scale, and also in combination with a high-quality protective coating, historically has been demonstrated to significantly reduce the potential for stress corrosion cracking (SCC) of the steel.

FBE is a powder coating. The FBE is applied as a powder, which has been electrostatically charged to form an attraction to the earthed steel pipe. The pipe is preheated using an induction coil to approximately 245˚C to ensure that the FBE powder has adequate time to melt, flow and crosslink.

Powders are formulated to have different ‘gel times’, which refers to the time it takes to transform from a liquid to a gelled state. The addition of a second layer of FBE or of an adhesive for a three layer polyolefin (3LPO) system has to occur prior to the gelling stage to ensure proper bonding. Temperature monitoring, spray booth design, gun placement and effectiveness, and the ability to vary the duration between application and the controlled cooling are all key factors requiring experience and training to consistently ensure the highest quality product.

There are three common external pipeline coating systems available for selection: single layer FBE, dual layer FBE and three layer polyolefins.

Single layer FBE

Single layer FBE is generally between 400 and 750 microns in thickness, consisting of a single layered application of the thermosetting FBE. As the FBE melts, it wets out on the heated, conditioned steel pipe surface and forms an excellent adhesion to the roughened steel substrate.

FBE is 100 per cent solid with no solvents. Epoxies have inherently strong chemical resistance with very few chemicals impacting the coating, although some chemicals may influence the rate of cathodic disbondment at a defect site. FBE is described as ‘friendly’ to CP. While the coating is an electrical insulator, FBE provides more opportunity than high insulating coatings, enabling CP to prevent corrosion at disbonded defect sites. FBE also has low oxygen permeability.

Dual layer FBE

Dual layer FBE consists of a layer of the standard anti-corrosion FBE, which provides a high level of adhesion and anti-corrosion protection to steel, combined with a second layer of FBE specifically designed to provide additional properties such as abrasion resistance (ARO). In this instance, the top layer has been modified with additional fillers to provide better gouge, abrasion, and impact resistance.

The standard thickness for gas transmission lines is typically between 500 and 800 microns and 1,100 microns nominal thickness for high duty, such as horizontal bores or directional drilling.

Other top layer options include rough coat, providing a profile-key for concrete over-coatings, and high temperature products.

The epoxy resin in both products is chemically similar and accordingly there is seamless integration as the topcoat is applied before the primary layer has gelled.

Another key benefit of single or dual layer FBE products is the wide variety of available joint coatings, including FBE, two-part epoxy, polyurethane or heat shrink sleeves.

Three layer polyolefins

3LPO coatings systems consists of an FBE anti-corrosion layer bonded to an outer polyolefin (polyethylene or polypropylene) top layer using a polyolefin adhesive tie layer, which chemically bonds with both the FBE (grafted anhydride link) and combines with the polyolefin topcoat. For onshore use, the standard top layer is a high-density polyethylene (HDPE), while polypropylene is used offshore or for higher operating temperature conditions. The FBE layer restricts oxygen passage and the polyethylene restricts water passage. HDPE provides the best combination of properties from the polyethylene range, decreasing permeability to both water and oxygen and providing higher toughness (compared to lower density products).

The standard thickness for a three-layer polyethylene (3LPE) system is 1,275 microns, consisting of 150 microns of FBE, 125 microns of polyolefin adhesive and 1,000 microns of HDPE. For more severe soil conditions, the HDPE layer may be increased to 1,500 microns or to 3,000 microns for more for heavy duty requirements.

Coatings to suit different environments

Standard coating types are produced around the world to match the typical temperature and operating conditions experienced by domestic onshore pipelines. Some of these products include:

• Standard FBE is designed for operating temperatures generally ranging from -30˚C up to 80˚C for wet conditions or ≈105˚C for dry operating conditions
• 3LPE is restricted by the properties of the polyethylene and as such its operating temperatures ranges from ≈-20˚C up to 80–85˚C
• Three layer polypropylene (3LPP) permits operating temperatures up to ≈ 105˚C using standard FBE powder.

Other products that have been approved by Bredero Shaw, are:

• High-temperature single layer and dual layer FBE for operating temperatures up to 140˚C
• FBE for 3LPP with operating temperatures in excess of 110˚C.