Striving for integrity

With pipeline corrosion costing oil and gas industry billions of dollars in repairs and lost production, we speak to expert Richard Norsworthy about preventative measures.

What are the main challenges today when it comes to pipeline integrity as related to external corrosion?
Richard Norsworthy. Though pipeline companies are doing a better job with cathodically protecting their pipelines, there are still many who misunderstand the use of pipeline coating in conjunction with cathodic protection. Many companies are still using types of pipeline coatings that shield cathodic protection current when the coating loses adhesion which allows water to penetrate and corrosion to develop. The amount and rate of corrosion will be dependent on several variables, but some coatings have developed a reputation for shielding cathodic protection when disbondments occur. This continues to be one of the main reasons for external corrosion on cathodically protected pipelines today. There are some coating types that will allow cathodic protection to be effective when disbondments occur, allowing the cathodic protection current to effectively protect the pipe surface under the coating.

How has the understanding of the interaction of cathodic protection and coatings evolved over the years? 
RN. Many do not understand the relationship between cathodic protection and coatings. Nearly all coatings used for pipelines today are good dielectric barriers which allow cathodic protection to spread and protect many miles with minimal current requirements. The question is why the industry still has external corrosion on cathodically protected and coated pipelines. Cathodic protection does not usually cause coating failures with these coating systems. The reasons most coatings disbond or fail because of poor coating selection, surface preparation and application. Soil stress issues affect many types of coatings. Cathodic protection can damage some coatings, but most blistering and coating damage on cathodically protected pipelines are not caused from cathodic protection. The cathodic protection just accelerates the failure.

What would be some best practices that one could recommend to a buried pipeline operator?
RN. When possible select coating types that will not shield cathodic protection currents if coating adhesion fails. These are called non-shielding coating systems. The coating types that have a proven history of non-shielding are fusion bonded epoxies (FBE) and the Polyguard RD-6 coating systems. Though these coatings may not be totally non-shielding in all situations, they both have proven field history, with laboratory testing to back up these claims. FBE has more than 50 years of service life and the RD-6 now has over 22 years of field service. There have been many internal line inspection tool runs in the pipes coated with these two coatings, yet it is rare to ever find external corrosion under either of the coatings when disbondments occur. It is also rare for these coatings to disbond when properly applied. Therefore, water under these coating types has not been a major problem. In the United States, the Department of Transportation has issued CFR 192.112 (2008) mandating non-shielding coatings if operators want to increase their operating pressure. If a coating company claims to have a non-shielding coating type, ask for their proof. Do not just take their word for it, scrutinise it.

Could you give an example of where your services and expertise aided a client with guarding their pipelines against corrosion?
RN. As mentioned above, the Polyguard RD-6 coating system has a very successful history with no reports of measurable external corrosion under the rare, known disbondments. As more companies find disbonded and shielding problems with other coating systems such as shrink sleeves and solid film backed tape systems, the RD-6 system is becoming the field applied coating system of choice for rehabilitation and girth weld coatings on new pipelines. The RD-6 system requires less surface preparation, is easier to apply and can be immediately back filled versus two part epoxies. Our market share is growing world wide as more and more corrosion control engineers learn that there are choices when it comes to field applied coatings. When selecting pipeline coatings, the non-shielding properties of a coating system may be more important than most other properties normally considered.

Richard Norsworthy has published numerous papers on corrosion control, and authored the chapter on Coatings for Underground or Submersion Service in Uhlig's Corrosion Handbook. He is a NACE instructor for courses in basic corrosion, CP, and coatings used in conjunction with CP. In 1995, he started Lone Star Corrosion Services, which became part of Polyguard Products in 2007.