Anti Corrosion Coating for Subsea Pipeline

Offshore steel pipelines are normally designed for a life ranging from 10 years to 40 years. To enable the pipeline to last for the design life, the pipeline needs to be protected from corrosion both internally and externally.

-  Internal Pipe Coatings

The primary reason of applying internal coatings is to reduce the friction and therefore enhance flow efficiency. Besides, the application of internal coatings can improve corrosion protection, pre-commissioning operations and pigging operations. Increased efficiency is achieved through lowering the internal surface roughness since the pipe friction factor decreases with a decrease in surface roughness. In actual pipeline operation the improved flow efficiency will be observed as a reduction in pressure drop across the pipeline.

The presence of free water in the system is one of the reasons to cause the corrosion of inner pipeline. An effective coating system will provide an effective barrier against corrosion attack. The required frequency of pigging is significantly reduced with a coated pipeline. The wear on pig discs is substantially reduced due to the smoother pipe's surface.

The choice of a coating is dictated by both environmental conditions and the service requirements of the line. The major generic types of coatings used for internal linings include epoxies, urethanes and phenolics. Epoxy based materials are commonly used internal coatings because of their broad range of desirable properties which include sufficient hardness, water resistance, flexibility, chemical resistance and excellent adhesion.

Internal epoxy coating flow chart 
Source: http://www.jrdistributors.com/images/download/internal-epoxy-flowchart_image.jpg

 
-  External Pipe Coatings

This first external pipe coating layer is used to protect the pipe against corrosion. A single layer coating is used when the installed pipeline is always in a static, laterally stable condition lying on soils such as clay or sand. Additional layers of coating are used for additional protection, for weight to help the pipeline remain laterally stable on the seabed, or for providing insulation. A multi-layer coating is generally used in cases where the external environment tends to easily wear out the external coating (e.g., pipeline lying on top of rocky soil, calcareous material, etc.). Depending on the external environment and on the location or use of the pipeline, a single-layer coating or a multi-layer coating is required.

The properties that are considered desirable for deepwater pipeline coatings are:

• Resistance to seawater absorption
• Resistance to chemicals in seawater
• Resistance to cathodic disbondment
• Adhesion to the pipe surface
• Flexibility
• Impact and abrasion resistance
• Resistance to weathering
• Compatibility with cathodic protection

1. Single-layer coating
A single-layer coating is used when the installed pipeline is always in a static, laterally stable condition lying on soils such as clay or sand. Additional layers of coating are used for additional protection, for weight to help the pipeline remain laterally stable on the seabed, or for providing insulation.

The most common choice for single-layer coating for deepwater pipelines is Fusion Bonded Epoxy (FBE). For deepwater pipelines where there is no other requirement on the external coating, FBE is most frequently used. Most deepwater oil and gas transmission lines use FBE as they are extremely cost effective. This coating can be used in conjunction with concrete weight coating. The other coatings that can be used with concrete coating are coal tar enamel and coal tar epoxy and they are used with lower product temperatures. Both of these coatings are bituminous coatings reinforced with fiberglass. However, most bituminous coatings are not desirable due to environmental laws and decreasing efficiency (sagging, cracking, permeation, and chemical deterioration). The FBE field joint coating is carried out using the same coating material as millapplied coating. Further advantages include:

• Easy to repair
• Easy for coating application
• High adhesion to steel
• Good for pipeline operating temperatures

In the US and UK, FBE coating is preferred for offshore pipelines.

Fusion Bonded Epoxy (FBE) coating

Source: http://www.haihaopiping.com/images/products/Fusion%20Bonded%20Epoxy%20Coating%20System.jpg

2. Multi-layer coating
A multi-layer coating is generally used in cases where the external environment tends to easily wear out the external coating (e.g., pipeline lying on
top of rocky soil, calcareous material, etc.).

- Dual-Layer FBE
Dual-layer FBE coatings are used when additional protection is required for the outer layer such as high temperature, abrasion resistance, etc. For deepwater trunklines the high temperature of the internal fluid dissipates rapidly reaching ambient within a few miles. Therefore, the need for such coatings is limited for SCRs at the touchdown area where abrasion is high and an additional coating with high abrasion resistance is used. The Duval system consists of an FBE base coat (20 mils) bonded to a polypropylene coating (20 mils). The polypropylene layer provides mechanical protection.

Dual-layer FBE

Source: http://www.nbfangfu.com/login/eWebEditor/UploadFile/2009722152935779.GIF

 - Three-Layer
Three-layer PP coating consists of an epoxy or FBE, a thermoplastic adhesive coating and a polypropylene top coat. The polyethylene (PE) and polypropylene (PP) coatings are extruded coatings. These coatings are used for additional protection against corrosion and are commonly used for dynamic systems like steel catenary risers and where the temperature of the internal fluid is high. These pipe coatings are frequently used in pipelines that are installed by the reeling method. The field joint coating for the three-layer systems is more difficult to apply and takes a longer time. However, in Europe, PE and PP coatings are preferred because of their high dielectric strength, water tightness,
thickness, and very low CP current requirement.

Three-layer polypropylene

Source: http://globalpipesfield.com/wp-content/uploads/2013/04/3LAYERS2a.jpg

- Concrete Weight Coating
Concrete weight coating is used when stability of the pipeline on the seabed is an issue. The two common densities of concrete that are used are 140 lbs/cu. ft and 190 lbs/cu. ft. Higher density is obtained by adding iron ore to the concrete mix. Recently, higher density iron ore has been used to obtain concrete density ranging from 275 to 300 lbs/cu. ft for the Ormen Lange pipeline in the North Sea.

Concrete weight coating

Source: http://www.offshoreenergytoday.com/wp-content/uploads/2012/01/Trinidad-ShawCor-Providing-Weight-Coatings-for-Technips-Offshore-Pipeline-Project.jpg

Source:
Offshore pipelines - Dr. Boyun Guo (2005)