Ask the Experts: What different types of coatings are applied to pipelines?

External coatings may be applied for corrosion resistance, thermal insulation, or weight coating, for example, while internal coatings can provide corrosion resistance and flow improvement.

Fusion-bonded epoxy (FBE)
FBE is a corrosion resistant coating with a proven track record over the last 20-30 years. The required thin layer (250-500µm) gives excellent corrosion resistance from -40°C to 85°C. However, damage can result if the pipes are not handled correctly or if they are subjected to significant impacts. FBE is, however, flexible enough to allow field pipe bending and gives excellent cathodic disbondment protection.

3-layer polyethylene (3LPE)
The first layer is FBE to provide corrosion resistance. The second layer is an adhesive to bind the FBE to the topcoat. This adhesive is polyethylene-based and is 150-200µm thick. The topcoat is extruded polyethylene at a coating thickness of 2-4mm. The polyethylene gives excellent impact and abrasion resistance. This multi-layer system gives temperature resistance up to 80°C. The disadvantages compared to FBE are higher raw material costs and slower production rates.

3-layer polypropylene (3LPP)
This system is similar to 3LPE except that a polypropylene adhesive and topcoat are used. Temperature resistance is increased to 110°C.

Concrete
Concrete is used as a weight coating in order to anchor the pipe on the seabed. The concrete thickness is usually 50-100mm, which also gives a degree of impact resistance. The density of the concrete can vary, but is usually 3040kg/m³. Concrete is usually applied on top of a corrosion resistant coating. An impingement or wrap process can be used to apply concrete.

Multi-layer foam coats
These coatings usually consist of 3LPP, with additional layers of foamed polypropylene to give a total system thickness of 30-100mm. The voids in the foam reduce thermal conductivity to give these systems excellent insulation properties. These systems are used for deep-water applications and the compressive strength of the foam must be high enough to cope with the pressures at these depths. The oil is at elevated temperatures and must be kept hot to avoid solid material forming that can slow down flow rate and potentially block pipes.

Polyurethane
Polyurethane is used as an insulation coating. The insulation properties are enhanced by incorporation of polymer or glass microspheres. Glass microspheres are utilised when high compressive strength is required for deep-water applications. The polyurethane is formed by mixing a polyol and isocyanate together immediately prior to pouring into a mould surrounding the steel pipe. Once this system is cured by heat and time, the solid polyurethane is formed. This process is not continuous and is therefore slow. Due to the high cost of the moulds, this coating is not viable for short pipelines (<10km)

In order for any of these coatings to perform well, it is essential to ensure that the pipes are of high quality and are prepared correctly. Before coating, all pipes need to be visually inspected and repaired if necessary before they are washed and dried. Once dry, the pipes need grit blasting to the client’s smoothness specification (usually ISO 8501-1 SA2.5). A further pre-treatment (e.g. chromate wash) is necessary to ensure excellent adhesion of the first layer to the steel.

The correct application of the coatings is also critical. For example, if the pipe is too hot during application, the FBE will not bond well to itself or the steel surface. However, if the FBE is too hot, it fully cures before the adhesive layer is applied preventing good adhesion between these layers, which leads to poor peel tests.

Specialist internal coatings

The general trend within the main oil producing countries has been towards exploitation of oil and gas fields containing highly corrosive hydrocarbon products. In response to this shift, an advanced internal coating was developed. This coating, called TK 236, is a high temperature, epoxy-novalac coating system with an advanced molecular structure. When the coating is applied to carbon steel pipe in combination with its proprietary phenolic primer system, the result is a coating with extreme low gas permeability, excellent adhesion, pH range of 3-13, extremely high gloss and a very high level of corrosion protection at temperatures up to 200°C.

TK 236 can be offered as a total coating system. The connection of the line pipe, spools and fittings in the field is made by a special ‘Thru-Kote’ UB Sleeve weld connection. This gives the following particular benefits for the user:

• No special joint end preparation, such as spigot and socket, is required.
• Internally-coated pipes can be laid in the field without any special laying or installation system.

The UB Sleeves have a special ‘Heat Wrap’ already applied around them, which enables a full strength butt weld to be made, without causing any damage to mastic or lining.

Phil Collier is Coating Manager at Corus, and has a BSc (hons) in Material Science. Corus is an Anglo-Dutch company formed in 1999 by the merger of British Steel and Koninklijke Hoogovens. The company employs around 50,000 people worldwide, with the capacity to produce 21 million tonnes of steel a year. It offers a full project management service for pipelines worldwide, utilising a wide range of coatings from our joint venture operation and integrated supply partners. For all enquiries, please e-mail: corusenergy@corusgroup.com