External Coatings

The main objective of external coatings on pipelines is corrosion control. It can also provide mechanical protection to linepipe. Anti-corrosion coatings available for offshore pipelines are shown below:

• Asphalt
• Coal tar enamel (CTE)
• Fusion Bonded Epoxy (FBE)
• 3LPP
• 3LPE

For pipelines that require internal and external coating, normally internal coating is done before application of external coating unless the external coating requires pipeline heating that would damage internal coating.


Asphalt and Coal Tar Enamel
Asphalt, bitumen, and coal tar enamel (CTE) coating is 5-6 mm thick flood coating applied as molten material onto a rotating linepipe. This coating is reinforced with 1 or 2 layer of fiberglass to secure a better cohesion. Asphalt Enamel (AE) is a hot applied coating that has long been used in the offshore pipeline coating industry.

Pros:

• Inherently rough surface (good in preventing slippage between concrete and coating for concrete coated pipe)

Cons:

• Coherent is not strong and provide relatively poor adhesion to steel.
• Poor cathodic disbandment resistance compare to FBE and 3 layer polyolefin systems (3LPP and 3LPE).
• Prone to mechanical damage during construction and installation if applied on pipeline or riser without concrete coating.

Fusion Bonded Epoxy
FBE coating is a 0.4-0.6 mm thick, thin film coating with strong chemical bond with steel. It is a thermosetting resin applied in the form of dry fine powder onto the rotating linepipe being heated up to approximately 260 Celsius. At this temperature, the epoxy powder melts and flow all over the pipe surface. The pipe is then cooled down by water quenching. FBE coating can withstand temperature up to 95 Celsius. FBE top coated with plasticized formulation can be applied for HDD installation pipeline.

According to Ref. 3, FBE is not much used for marine pipelines as it is prone to impact damage and does not resist concrete coating by impingement.

Pros:

• Good adhesion to steel (suitable for usage on offshore pipelines and components, i.e valves and pumps.
• Flexible. (suitable for reeled pipelines application)
• Can be used with or without concrete coating.

Cons:

• FBE coating is slippery (For application on concrete coated pipe, it is recommended to provide anti-slip bands to reduce the risk of slippage between concrete and FBE coating)
• Susceptible to mechanical damage during handling and transportation
  

Multilayer Polyolefin System
3-layer Polyolefin Coating (3LPP, 3LPE) consists of:

• Base layer: FBE
• 2nd layer: polyethylene or polypropylene
• 3rd layer: polyethylene or polypropylene

It is called 3LPP if polypropylene is being used; 3LPE if polyethylene is used. Both Polypropylene and polyethylene are suitable for HDD installation pipeline coating as the natural self-lubricating characteristic of these materials minimizes the damage to the coating during installation process. 3LPP and 3LPE can withstand design temperature up to 130 Celsius.

Pros:

• Low moisture and oxygen permeation
• Strong adhesion properties of FBE
• Toughness characteristics of Polyolefin (high electrical resistance and long life expectancy)
• Resistant to handling and transportation damage
  

Cons:

• Polyolefin coating is slippery (For application on concrete coated pipe, t is recommended to provide anti-slip bands to reduce the risk of slippage between concrete and polyolefin coating)

References:

1. Palmer A. C., King R. A. (2004) Subsea Pipeline Engineering. Tulsa, Oklahoma: PennWell.
2. DNV-RP-F102, Pipeline Field Joint Coating and Field Repair of Linepipe Coating, 2003.
3. Braestrup M. W., Andersen J. B., Anderson L. W., Bryndum M. B., Christensen C. J., Niels Rishoj (2005) Design and Installation of Marine Pipelines. Oxford, UK: Blackwell Science.