March 2012, Vol. 239 No. 3

Features

NYSEARCHs Cased Pipe Corrosion Rate Estimation Program

Currently, gas operators with pipelines in High Consequence Areas (HCAs) are struggling with how to best respond to pipeline integrity inspection requirements where the carrier pipe passes through a casing. The annular space of a cased pipe represents a unique engineering environment.

Operators need information that leads to a scientifically valid method of estimating corrosion rates in such environments to facilitate the risk assessment specific to cased pipe sections. Without cutting-edge tools and technologies that give accurate cased pipe information, costly excavations that significantly disrupt the public’s infrastructure cannot be avoided. Essentially, a better understanding of the conditions and mechanisms that cause external corrosion on a cased carrier pipe, including atmospheric corrosion, is required.

NYSEARCH, a sub-organization of the Northeast Gas Association, together with DNV in Columbus, OH and several host utilities, have completed a valuable study of external corrosion rates on cased pipe. The study has produced a final report which, together with its results, is a new body of information which documents corrosion rates specific to cased pipe. This material, in the hands of the pipeline integrity engineer, provides the necessary support to avoid the use of default textbook guidelines.

The objective of the Cased Pipe Corrosion Rate Estimation program was to develop a realistic understanding of the environmental conditions that influence corrosion and corrosion rates within cased sections. The information acquired through this study aimed to assist with the prioritization of casings by allowing the pipeline owner to focus resources on those with the most risk for corrosion.

To attain this information, corrosion engineers have facilitated live field tests to collect data for this study. They determined the actual corrosion rates in field installations. Engineers now concur that the study’s results demonstrate that considerably less conservative pitting rates than those referenced in the ASME B 31.8S/NACE RP 0502 can be applied when limited corrosion rate indicators exist. During cased pipe audits, regulators have shown interest in the results of this project for considering the merits of these more applicable corrosion rates. The project also creates awareness about those few locations where the ECDA standard rates are non-conservative.

The project was divided into four parts: Bench Scale Lab Testing, Small Scale Lab Testing, NYSEARCH/NGA Test Bed Testing and Live Field Testing. Corrosion rates were determined using newly developed coupon chains and electric resistance probes. Bench Scale Lab Testing established a broad range of corrosion rates. These rates are based on a matrix of possible conditions specific to the annular space of a cased pipe. Lab test environments covered conditions such as dry and saturated air, flooded, filled and empty casings – all using fresh water, chloride water, and corrosion inhibited water.

Once the Bench Scale corrosion rates were determined, they were verified by comparison to data collected from conditions reproduced in the Small Scale Lab Tests and Test Bed Tests. Finally, Live Field Tests were performed at NYSEARCH project funder sites. The field tests confirmed the methods of data collection and validated the results of the Lab and Test Bed Tests.

The corrosion rates provided from live field tests, test bed, and small scale lab tests should be considered first when identifying comparable environments for a risk assessment model. If a comparable environment is not found from one of these sets of data, the corrosion rates determined from the Bench Scale Lab Tests can be used with the understanding that they may be less representative than those collected from full scale tests.

In general, a comparison of the corrosion rates for the Small Scale Lab Tests and Test Bed Tests were consistent with the results for the Bench Scale Lab Tests. The study has produced information that covers more than 80% of the scenarios expected for cased pipe. The remaining scenarios represent what corrosion engineers feel are less likely conditions that can be included in a follow on study if needed.

Photograph showing Small Scale Lab Testing.

Photograph showing the coupon chain installed within a Live Cased Pipe.