Without an effective and appropriate corrosion monitoring system in place, upstream operations could be susceptible to significant asset damage and product loss, which could lead to declining profitability and potentially catastrophic environmental and reputational damage.
However, new technologies and customized solutions exist to help these operations get ahead of corrosion issues and ensure the continued health of their field assets.
Growing threat of corrosion
Corrosion is a major threat to the integrity of upstream production assets, with flowlines, water injection systems, and import and export lines particularly vulnerable.
Corrosion often is linked to the presence of water and the use of carbon steel. Changing process conditions such as lower flow rates increase the probability of water holdup, and as assets age there tends to be a rise in water cuts. Since water is heavier than oil and gas, corrosion also often is found at the bottom of pipes, where access is a particular challenge for most monitoring technologies.
Fields where CO2 and H2S are common are also susceptible to corrosion. Sour production (high H2S) fields also provide challenges for corrosion monitoring with respect to the form of attacks, monitoring technologies and safety.
Corrosion can be devastating. It can lead to production losses, metal losses, and safety and environmental setbacks as well as having a highly negative impact on the economics of the field. The National Association of Corrosion Engineers, the professional body for the corrosion control industry, predicts the total annual cost of corrosion in the oil and gas production industry is $1.372 billion.
Importance of corrosion monitoring
While corrosion can be controlled in several ways (e.g., through material selection of corrosion-resistant materials or process optimization and the use of corrosion inhibitors like chemicals), a systematic and effective corrosion monitoring strategy is vital.
Effective corrosion monitoring enables corrosion inhibitors to be verified and optimized, extends the service life of equipment and allows users to plan maintenance and avoid unscheduled production stops. Corrosion monitoring also improves the safety of personnel and the platform and helps protects the environment.
Corrosion monitoring technologies can be separated into two categories: intrusive in-line corrosion monitoring and nonintrusive corrosion monitoring.
Intrusive (in-line) corrosion monitoring technologies include weight loss coupons that involve inserting preweighed samples of steel in a pipe, retrieving them after a period of time (e.g., three to six months), cleaning them and weighing the difference.
Electrical resistance probes are also an important in-line technology whereby corrosion is measured by the amount of resistance experienced by the probe and, based upon recorded changes, metal loss over time is calculated.
Finally, there are linear polarizaton-resistance probes that are based on electrochemistry and where the ratio between the applied voltage and current response gives the polarization resistance, which is inversely proportional to the corrosion rate.
Nonintrusive corrosion monitoring technologies are led by the electric field signature method (FSM), which is based on distributing an electric current through a monitored object (most commonly pipes or pipelines) and measuring the voltage drop between an array of sensing pins installed outside the object.
The first measurement is called the field signature, and later measurements are compared to the initial signature. Through changes in voltage drop due to changes in wall thickness, corrosion can be identified and quantified. Figure 1 shows field signature monitoring of a pipeline powered by solar panels.
FIGURE 1. Corrosion monitoring of a pipeline using the nonintrusive FSM and powered by solar panels is shown. (Source: Emerson Automation Solutions)
Finally, there are nonintrusive ultrasonic measurements. Installing ultrasonic sensors permanently on the pipe for regular measurements at the spot provides increased stability and sensitivity. Sensitivity for ultrasonic measurements in field is typically 10 to 20 micrometers for changes in wall thickness. It is also possible to determine surface conditions inside the pipe from the form of the wave signal.
Selecting the right applications
What technologies are most suitable for each application? In such cases, it is important to:
- Establish the purpose and objectives of the monitoring strategy;
- Determine the information needed; and
- Review the upstream applications and identify the best mix of monitoring technologies to generate the required information.
Proactive process control (e.g., chemical inhibitor tuning) would point to high-sensitivity and fast-response technologies. In-line probes provide the highest sensitivity and fastest information on changes in corrosion rates so immediate action can be taken.
Integrity management/permanent inspection would point to wall thickness sensors that can be widely distributed on a platform.
For localized corrosion where there is an intense attack at localized sites on the surface of a component while the rest of the surface is corroding at a much lower rate, the FSM and its ability to detect such corrosion would be most suitable.
Similarly, in areas where there is difficult access for monitoring locations, such as the bottom of underground pipelines, nonintrusive methods that allow installation on the pipeline would be most suitable.
Finally, in sour conditions where conductive iron sulfide (FeS) deposits reduce the value of probe measurements and where safety issues related to probe retrieval and installation are paramount, nonintrusive monitoring such as FSM or ultrasonic measurements provide an affordable and safer alternative to corrosion probes.
Benefits of combining technologies
The combination of corrosion technologies often provides the best and most reliable monitoring.
Benefits include the increased reliability of corrosion data via independent technologies, the combination of high-sensitivity/fast-response technologies for the immediate tracking of corrosion rate changes and direct wall thickness change monitoring for the confirmation and verification of asset conditions. Finally, there is the combination of total area coverage for localized corrosion alongside the direct assessment of pipe/vessel wall thickness.
In the case of sour production where conductive deposits of FeS may cause measurement problems, monitoring strategies can be adapted from using in-line probes and ultrasonic wall thickness sensors in combination, where H2S concentrations are moderate, to replacing probes with ultrasonic sensors where H2S concentrations are high. Weight loss coupons can also be used in parallel.
The acceptance of WirelessHART as an industry standard also has increased the availability and affordability of online communications, bringing with it proven reliability and the elimination of cabling costs.
WirelessHART is a wireless sensor networking technology that combines a range of applications through the same gateway and provides online communications to a variety of data management solutions and/or control systems.
Both Emerson’s in-line corrosion monitoring and nonintrusive ultrasonic measurement systems are available with WirelessHART communications and can be combined into a single wireless-based corrosion monitoring solution.
Specialized software tools also are available for collecting data from sensors and installed monitoring tools. Figure 2 provides details of such a data management system with a status window on the left for each sensor and color codes that state the condition at each location.
FIGURE 2. A data management system from Emerson has a status window on the left for each sensor and color codes that state the condition at each location. (Source: Emerson Automation Solutions)
Corrosion monitoring today should be based on each individual case, with the technology choice based on the information needed, how the information will be used and how the wide array of technologies available on the market can be adopted. The result will be qualified, accurate and relevant information in the ongoing fight against corrosion.
Have a story idea for Operator Solutions? This feature highlights technologies and techniques that are helping upstream operators overcome their challenges. Submit your story ideas to Group Managing Editor Jo Ann Davy at email@example.com.
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