Corrosion monitoring

Maintaining pipeline integrity is crucial in the oil & gas industry, and when pipeline defects are detected, or pipeline failures occur, understanding the cause of these events is of utmost importance.

Internal and external corrosion is a key concern in maintaining a pipeline to provide safe and reliable service.

One cause of corrosion is microbiologically influenced corrosion (MIC), and as much as 40% of internal corrosion in the oil & gas industry may be caused by MIC.

Bacteria within pipelines attach to internal surfaces and grow as biofilms.

The growth/maintenance of biofilms requires water, and even the low concentration of water in crude oil pipelines, or condensed water in gas pipelines, can be sufficient to allow patches of water-wet biofilms to grow.

The preferred type of sample for microbiological testing is biofilms that can be obtained from weight-loss corrosion coupons that may be present in a pipeline, from sidestream devices that are specifically designed for the collection of biofilm samples, or from pigging solids and other materials obtained during the cleaning of pipelines.

Accurate detection and quantification of corrosion-associated microorganisms requires that samples should be obtained and analysed as quickly as possible when microbial growth tests are used.

If samples are not analysed promptly erroneous data may result. Also, the cleaning of pipelines, generally referred to as pigging a pipeline, produces a valuable source of samples for microbiological testing, but the heterogeneous nature of pig returns makes it challenging to collect representative data if not done properly.

Another issue associated with pipelines is that MIC is not always considered when beginning an investigation of a pipe segment that is removed from service.

It is not uncommon to have a pipe segment removed from service and sitting in a warehouse for days or even months before the possible involvement of MIC is considered.

Microbial growth tests are the most common way to test for corrosionassociated microorganisms in the oil and gas industry, but fresh samples are required.

Significant changes in the quantification of microorganisms can result from storage conditions if a sample is not analysed immediately.

Storage of microbiological samples on ice (4 degres C) yields the best results, but storage at 4 degrees C for only one day can result in underestimates of the true microbial population (results may be only 10% of the true concentration). Storage of samples at higher temperatures can result in values that are 100 times higher than the results obtained with fresh samples.

The solids obtained from a pipeline pigging operation are very heterogeneous.

The recommended protocol for the sampling of pig returns involves the collection of a minimum of two composited samples from the pigging solids and from three different locations on the body of the pigging tool.

These two composited samples can then be used to inoculate microbial growth tests, perform testing to quantify adenosine triphosphate (ATP), and to perform genetic testing.

If pipe segments have been removed from service and fresh microbiological samples were not taken at that time, then microbial growth tests cannot be performed.

However, genetic testing may still be able to detect DNA from corrosion-associated microorganisms because genetic testing can detect DNA from both living and from dead bacteria.

Dry pipe segments that have been out of service for weeks or months prior to sampling generally do not yield detectable DNA unless the sample is obtained from a deep pit protected by corrosion deposits.

Microbiological testing is an important component in pipeline monitoring and maintenance programmes.

However, care must be taken to analyse samples as quickly as possible when microbial growth tests are performed, and collecting multiple composited samples is recommended when heterogeneous material such as pig returns are being sampled.

Genetic tests can detect DNA even from dead bacteria and can be used to test for the presence of corrosion-associated microorganisms on samples where microbial growth tests cannot be used, such as old pipe segments.

It is important to obtain comprehensive data in determining the corrosion mechanism in any given sample.

Physical and chemical tests, as well as microbiological or genetic tests should all be used.

Microscopic examination of the morphology of corrosion pits, chemical analysis of corrosion products, metallurgical analyses, microbiological analyses, and knowledge of the operating conditions and fluid composition when the pipe was in service should all be considered when making a determination regarding the likely corrosion mechanism.

John J. Kilbane is Intertek’s program manager for petroleum microbiology.