Coating Integrity Assessment on Off-shore Platform Pipeline

ESR Technology recently performed a coating integrity investigation for a major North Sea oil and gas operator. The aim of the investigation was to determine the cause of paint degradation on a gas lift pipeline located on an off-shore platform.

Areas of corrosion and blistering were seen in various locations along the pipeline, some coincided with pipe supports whilst others were in remote regions.  A refurbishment program that included a full re-painting program had been performed only 12 months prior to the degradation being identified. Ultrasonic testing showed that whilst the required minimum wall thickness had not been breached, some areas were approaching the limit.

Metallurgical sections were prepared and examined using Optical Microscopy and Energy Dispersive X-ray Analysis (EDX) from various areas of degradation.

Paint system

Cross-sectional analysis of the paint system revealed 5 individual layers with a total thickness of approximately 600 µm. The first layer was a zinc rich primer, typical of paints used in a marine environment. The thickness of the primer varied considerably, often due to the surface roughness of the pipe; in some areas no primer was present. The remainder of the paint layers were typical of titanium rich paints with fillers. 

Surface preparation

In order to obtain a satisfactory performance for corrosion protection the first stage of the coating application should be surface preparation. No contaminants such as chlorides, sulphides or blasting media should be present when applying the first coat primer; metallographic analysis showed this was not the case.

The surface should be keyed to ensure good adhesion, but not possess excessively coarse features such as gouges, grinding marks, deep scores. Achieving a uniform coating over such features will be difficult and can increase the likelihood of coating breach, particularly if the surface comes into contact with other components such as pipe supports or bands. Where blasting media is used, it should be ensured that all media is removed from the pipe surface prior to coating to ensure good adhesion and a consistent interface.

Suitable organic solvents or degreasing agents should be used to remove any potential contaminants. The removal of any dust should also be performed prior to painting; dust may be generated by nearby activities on the platform during remedial works.

Where over-painting is required the existing paint should be well adhered to the pipe surface and should be thoroughly washed to remove all contaminants, light abrasion may then be required to achieve the required key. Compatibility of new paint layers over existing layers should be considered. If the existing paint has flakes or blisters these should be re-prepared back to the substrate surface for localised repairs prior to full painting. A British standard lists the requirements for over-painting steel.

Blistering

Paint films are semi-permeable membranes that are permeable to water, but impermeable to dissolved solids. The water-soluble material at the interface that causes the osmotic gradient is generally either an inorganic salt, the products of corrosion, or retained solvents in the coating.

Research has shown that osmotic blistering is related to the presence of contaminants such as chlorides, sulphates, and other inorganic solubles at the interface. Blistering can also occur due to the presence of corrosion products such as Fe(OH)2, where chlorides and sulphates are simultaneously present.

Extensive blistering was observed, much of which had cracked and corroded leading to wall loss and thick scabs of corrosion product on the pipe surface. The presence of chlorides, sulphates and other impurities at the interface that promote corrosion were identified by EDX analysis.

Conclusion

The cause of the corrosion and blistering was due to embedded blast media beneath the repainted layer, and, corrosion beneath the primer layer. This indicated that both the repainted layer and the original paint had both been applied to an inadequately prepared surface; a significant factor in the contribution to premature onset of corrosion. Corrosion at pipe supports is a common problem, particularly in the oil and gas industry, due to the entrapment and stagnation of corrosive electrolytes, resulting in crevice corrosion.

It was recommended that all pipe work is adequately cleaned and prepared prior to painting, or, re-painting. Various standards are available which list the requirements to achieve good adhesion and integrity of the paint layer. Regular inspections should be performed, paying particular attention to pipe support areas.

If you would like to discuss any coating, corrosion or oil and gas related issues please contact Steve Gill on 01925 843425 or email This email address is being protected from spambots. You need JavaScript enabled to view it..

ESR Technology are the leading experts in bearing failure analysis and assessment. If you have any bearing related issues feel free to get in touch to discuss – This email address is being protected from spambots. You need JavaScript enabled to view it. or call 01925 843428.