Case Study: Optimizing Pipeline Hibernation with Corrosion Mitigation
Introduction
Once a pipeline has been hydrotested and dewatered, there can exist an extended period during which the pipeline will lay dormant before it is commissioned. During hydrotesting, and depending on the type and source of the test medium, various contaminants can be introduced into the pipeline system, such as water, chlorides, oxygen, carbon dioxide, hydrogen sulfide, and bacteria, which can contribute to corrosion of the internal pipe surface.
Corrosion Mechanisms
Internal corrosion of pipeline systems is essentially the loss of material on the inner pipe wall, which reduces the strength of the pipeline, and is one of the main contributing factors to pipeline ruptures and leaks. The primary corrosion mechanism typically seen in pipeline systems is related to pitting corrosion, which is due to a localized breakdown of scales on the pipe wall in the presence of an electrolyte (water and salt). When iron dissolves, it does so as a positively charged ion (anodic reaction). The electrons produced through the reaction move through the metal wall to another location where they are in turn consumed in a reaction that produces a more alkaline environment (cathodic reaction). The dissolved iron in the electrolyte typically reacts with water and oxygen to form a corrosion deposit (rust).
A secondary corrosion mechanism that is commonly seen in pipeline systems is related to Microbially Induced Corrosion (MIC), which is caused by the presence of bacteria whose by-products initiate the corrosion cycle. Sulfate Reducing Bacteria (SRB) and Acid Producing Bacteria (APB) can create Hydrogen Sulfide (H2S) and other low pH byproducts. As biofilms are formed, the low pH by-products can become trapped underneath the film creating acidic environments that can result in material loss.
Corrosion Mitigation
To ensure the reliability of a pipeline system, it is crucial to proactively protect it with a well designed and executed hibernation program. To reduce or eliminate the effects of corrosion, multiple aspects of the corrosion cycle must be controlled. The mitigation strategies for controlling internal corrosion of inactive pipeline assets include the following:
Pigging
Following a successful pipeline hydrotest, the system should be dewatered and cleaned using a series of foam pigs and cleaning pigs, respectively. This will ensure that the pipe is free of standing water, and that there are no solids or debris remaining on the pipe walls which could trap moisture.
Pipeline Drying and Dehydration
Displacing dry gas through the system will reduce the atmospheric moisture inside the system to a target dew point, which will aid in minimizing water condensation due to fluctuations in temperature.
Corrosion Inhibition
Although drying the system to a low dew point will provide a great degree of corrosion protection, there is usually always going to be trace amounts of moisture that will exist in crevices (flanges, valves, instrumentation, etc.), so applying a corrosion inhibitor can provide an extra layer of protection. Corrosion inhibitors can be applied to pipeline systems in a batch between two interface pigs and works to slow the rate of corrosion at anodic or cathodic sites by forming a protective film on the metal surface.
Pipeline Drying and Dehydration
Displacing dry gas through the system will reduce the atmospheric moisture inside the system to a target dew point, which will aid in minimizing water condensation due to fluctuations in temperature.
Biocide Treatment
Trace amounts of water can continue to exist in pipeline systems even after pigging and drying have been completed. To combat the effects of MIC, a biocide can be included with a corrosion inhibitor batch to eliminate any microbes that may remain in the pipeline system.
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Inert Blanketing
Removing the oxygen in the pipeline system and replacing it with an inert gas (chemically inactive), like nitrogen, will eliminate the effects of atmospheric corrosion.
Case Study
A newly constructed 26.4 km NPS 10 and 11.9 km NPS 12 pipeline system in central Alberta was going to be temporarily hibernated following hydrotest and before commissioning. The prime contractor for the project would be responsible for dewatering, cleaning, and drying each pipeline with a methanol wash. In designing a predictable inhibitor and biocide delivery program, STEP collaborated with a specialty pig manufacturer in the supply of fit-for-purpose pipeline batching pigs tailored for creating a positive seal between dissimilar fluids. The objective was to use a well sealing lead and tail pig that would resist migration of gas from the pipeline backpressure while also evenly coating the pipe wall with the inhibitor blend, respectively.
Although drying a pipeline system with methanol will provide some degree of corrosion protection, there will always be trace amounts of moisture that will exist in small crevices, so applying a corrosion inhibitor and biocide batch mixture will ensure that any fluid remaining in the pipeline is noncorrosive and free of microbes.
Challenge
For the chemical batching technique to be effective, a detailed application design is required to ensure that the internal surface of the pipe is clean, and the volume and contact time of the chemical batch with the pipe wall is sufficient. For larger diameter pipelines, it is also crucial that the batch slug remains intact throughout the entire application process to ensure the entire circumference of the line is coated with inhibitor. This is especially true for pipelines with major downhill elevation changes, as the hydrostatic pressure of the chemical batch may cause speed excursions of the lead pig resulting in poor circumferential coverage of the internal pipe surface.
Solution
STEP began by engaging in key stakeholder discussions with the client and it was decided to consult with a third-party upstream chemical group that provides corrosion control products globally for a variety of oilfield systems. The proposed chemical inhibition system combined an organic, film forming water soluble corrosion inhibitor, and a liquid biocide effective at controlling Sulfate-Reducing Bacteria (SRB) and Acid-Producing Bacteria (APB). Case Study A newly constructed 26.4 km NPS 10 and 11.9 km NPS 12 pipeline system in central Alberta was going to be temporarily hibernated following hydrotest and before commissioning. The prime contractor for the project would be responsible for dewatering, cleaning, and drying each pipeline with a methanol wash.
In designing a predictable inhibitor and biocide delivery program, STEP collaborated with a specialty pig manufacturer in the supply of fit-for-purpose pipeline batching pigs tailored for creating a positive seal between dissimilar fluids. The objective was to use a well sealing lead and tail pig that would resist migration of gas from the pipeline backpressure while also evenly coating the pipe wall with the inhibitor blend, respectively.
Figure 2 contains an example of the batch run simulation for the NPS 10 pipeline system. STEP conducted a thorough analysis of each pipeline system elevation profile and the recommended volume of each inhibitor batch to determine the anticipated hydrostatic and dynamic pressures that would be acting on the system during the batch run. In doing so, STEP was able to determine a recommended line pack pressure that would ensure a constant pig train velocity across the entire length of each pipeline with a particular focus on downhill segments where pig speed excursions are more likely to occur.
Following the evaluation of the elevation profile and inhibitor batch characteristics, STEP recommended using nitrogen gas as the displacement medium for the pipeline inhibitor blend. Doing so would replace the oxygen in each pipeline system with inert nitrogen gas to provide the best possible protection against corrosion for long term hibernation. In addition, nitrogen pumping equipment provides fully variable rate control which allows the operator to ensure a constant pig speed without having to rely solely on venting at the receiving end. Using the anticipated dynamic and hydrostatic pressure evaluation on each pipeline system, STEP was able to determine the required nitrogen displacement rate and the required nitrogen volume for the project.
Summary
STEP was able to perform a thorough analysis of each pipeline system to deliver a predictable and controlled pipeline inhibitor batch run that would ensure the proper application of the corrosion inhibitor on the internal circumference of the pipe. In performing a detailed analysis of each pipeline system, STEP was able to successfully execute the above inhibitor batching runs for the client in the predicted amount of time and was able to remain within 15% of the budget. In addition, the decision to displace the inhibitor batch with nitrogen gas would provide the client’s asset with the best possible corrosion protection for short term hibernation.
Website: stepenergyservices.com