Modern high-resolution corrosion monitors, including electrical resistance probes, are available. These instruments can be installed in key locations and connected to a control room computer. Responsible engineers can observe the real-time corrosion trends from the control room along with other operational parameters. By seeing the corrosion data in real time, personnel can take immediate mitigation action in situations with high corrosion rates or risks.
Corrosion Inhibitor Injection
Chemical injection is an effective solution to many corrosion problems; however, injection rates below the specified dosage result in inadequate corrosion control and injecting excessive dosages is wasteful and not cost effective. Monitoring chemical treatments with data loggers obtains real-time data that can help streamline the chemical treatment process. Real-time data are helpful for maintaining the effectiveness and efficiency of the pump skids.
Corrosion inhibitor levels in injection tanks are important. Level control can be computerized and automated. In cases of low inhibitor levels, an alarm can alert the corrosion team so that the chemical supply can be promptly adjusted. This is more efficient than requiring personnel to conduct site visits, note the inhibitor level readings, and replenish the corrosion inhibitor if necessary.
Real-time chemical injection information from data loggers also can help determine the performance of the chemical treatment. Automating this process provides more accurate injection data than manual control during site visits because the inhibitor performance and chemical levels in the tanks are checked only at the time of the visit. Chemical injection quantities are then calculated based on the difference between the levels measured during the site visits. Automation is particularly useful for remote sites. By implementing automation, it is possible to continuously monitor the corrosion inhibitor injection and performance, make necessary adjustments when needed, and thereby minimize the corrosion of assets.
Monitoring Inhibitors in Pipelines
Identifying the corrosivity of fluid flowing in a pipeline or system is essential when adjusting the formulation of the inhibitors. Off-the-shelf inhibitor formulations may not always work. It is better to have tailor-made inhibitors that suit the specific conditions exactly.
The corrosivity of free water in oil lines can be tested by following the side stream technique (Figure 1). This can be carried out from a sampling point at the 6 o’clock position in the circumference of a wet crude oil pipeline. Tubing is connected from this sampling point to a small side stream separator that will yield oil-free water for testing. The water outlet of the separator is connected to a device with linear polarization resistance (LPR) probes affixed to it. By monitoring the LPR meter or using electrochemical impedance spectroscopy techniques, corrosivity can be checked.
This side stream equipment also can be used to inject and test inhibitor formulations. Preparing an inhibitor in a laboratory with collected samples may involve numerous, complex factors. There can be problems associated with dissolved oxygen (DO) ingress and liberation of dissolved gases from samples collected in containers and transported to a laboratory. Similarly, establishing system pressure, temperature, and other conditions in a corrosion testing laboratory is often very difficult.
Using side stream equipment for online sampling can simplify this work because the fluid exposed to the test electrodes is exactly the same as the pipeline fluid. Also, the system pressure, temperature, and other conditions remain consistent. Water from the side stream equipment also can be used for testing dissolved gases such as carbon dioxide (CO2), hydrogen sulfide (H2S), and DO. Additionally, chemical constituents in the water, such as dissolved iron and total iron, can be checked with the help of appropriate standardized field test kits.
Normally, corrosion coupons are available for placement in the 6 o’clock position within pipelines. Coupon holders are also available with sample collection systems. Fluid can flow through the coupon holder to the outlet that is connected to the side stream equipment. Considerable care, however, must be exercised in unscrewing the coupon holder to allow fluid flow while avoiding holder displacement under high pressure. It is also important to confirm that the fluid flow is not blocked by particles that could impede the collection of side stream samples. This method gives real-time data of fluid corrosivity and provides data that are also representative of actual field conditions. In side stream equipment, two probes can be connected in a series to check their accuracy.
Conclusions
A dynamic approach to corrosion monitoring is essential. Acquiring and analyzing real-time data greatly facilitates corrosion prevention. Traditional methods concentrate on detecting and evaluating corrosion after it has occurred. Corrosion monitoring techniques should concentrate on collecting real-time data to identify corrosion risks in time to prevent significant losses and failures.
Bibliography
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Murthy, T.L.N. and G. Kannayya Naidu. “A Sour Gas Problem in Sweet Crude Oil Storage Tanks.” MP 54, 2 (2015).
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Murthy, T.L.N. “Corrosion Monitoring and Inhibitors for Production Tubing in Gas Wells.” MP 54, 10 (2015).
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“Remote Monitoring System Tracks Cathodic Protection and Sensor Data.” MP 55, 1 (2016): p. 18.