PRCI PR-261-9711

L51906e

NOVA Research & Technology Corporation

Need: Pipeline integrity is a fundamental requirement for the commercially successful operation of a pipeline transmission system. Many threats to integrity exist including third party damage, geological activity, corrosion, cracking and other problems. External corrosion has been recognized for many years as one of the main deterioration mechanisms that may reduce the structural integrity of buried transmission pipelines. While external corrosion is a significant threat to pipeline integrity the majority of these problems do not result in ruptures of the pipeline but can produce leaks. Inspection of major lines using in line inspection (ILI) tools allows external corrosion sites to be detected and repaired in a timely fashion; however, not all pipelines are amenable to ILI. Other tools are needed to assess the potential threat of external corrosion to pipelines unsuited to ILI. The ultimate goal of this program is to support development of site specific risk assessment models that will help to prioritize pipeline maintenance activities with respect to the efficient discovery and remediation of significant external corrosion damage on pipelines where ILI cannot be used.

Benefit: A combined lab and field program has been performed to study the external corrosion behaviour of buried linepipe. The overall aim of the project was to attempt to correlate pipe damage with measurable soil properties, in particular the redox potential as measured by buried soil probes. The lab component of the project involved the measurement of weight-loss corrosion rates in three types of soil, under saturated and as-received moisture conditions, and under permanently aerobic, permanently anaerobic or cyclic anaerobic/aerobic redox conditions. The soil conditions and coupon corrosion potentials were monitored during the course of the tests. In the field component of the study, pipe-depth soil properties were monitored using permanent NOVAProbes to measure the redox potential, soil resistivity, pH and temperature. Daily readings were taken at some locations with the aid of a custom-designed datalogger. Corrosion information was obtained from buried weight-loss or electrochemical coupons, from pipe excavations or from data from repeat in-line inspection runs.

Result: Overall the results obtained here agree with previous field observations under similar conditions. One implication of these results is that changes resulting in cyclic aerobic-anaerobic conditions at pipe depth may be more of a threat to pipeline integrity than constant aerobic or anaerobic conditions. The rapid attack seen for some coupons in the cyclic lab tests also suggested that ingress of oxygen to an anaerobic corrosion site during excavation on a pipeline system could accelerate corrosion, at least briefly. The upper limit of aerobic corrosion rate for an anaerobic to aerobic transition is likely of the order of 0.1 mm/year.

Product Details

Published:

09/05/2001

Number of Pages:

143

File Size:

1 file , 3.3 MB