The ultimate goal of the Corrosion Engineer is to ensure that materials performance matches with the desired economics of the plant or field. Where a materials solution can often be viewed as an expensive capital outlay, corrosion mitigation measures (such as chemical treatment) represent an extremely effective and affordable operational solution. The effectiveness of these treatments, however, is often only proven retrospectively at the time of maintenance inspection or via corrosion coupon analysis. Where critical process upsets occur, the operator may be exposed to unforeseen risks arising from loss of structural integrity or as a result of catastrophic localized (pitting) corrosion.

Technology is available which enables monitoring of materials performance on-line, such that data are available either continuously or on a regular basis. Of these, multi-technique electrochemical corrosion monitoring affords the operator a simple yet effective means of continuously reviewing the corrosion status of materials on-line and in conjunction with process data. Localized corrosion is differentiated from uniform corrosion, enabling the potential for catastrophic corrosion to be rapidly assessed and mitigated. The net result of this is the opportunity to directly and positively impact upon the corrosion status and see the benefit of remedial action “as it happens”. Using an “open system” computer allows real time on-line and off-line data to be converted into information that can be used to control rather than just report on problems as they occur. In this way information can flow across a range of plant computer and information systems to bring together disparate data sources into a single location to maximize the benefits – a synergistic approach.

This paper draws on some thirty years of experience of oil- and gas-field and refinery chemical application and corrosion monitoring, to provide a comprehensive approach to troubleshooting critical process problems. Specifically, Oil and Gas Exploration & Production and Crude Refining processes are reviewed.

Keywords: Chemical; Corrosion; Process; Safety; Environmental; Total Cost of Operation; Oil; Gas; Exploration; Production; Crude; Refining; Data Acquisition

Corrosion is a fact of life, but that fact does not make it inevitable. The annual costs of corrosion widely quoted for the USA and Europe amount to 3% of GDP, yet research has shown us that 70% of this cost could be avoided.

Another means of improving profitability of an asset is to extend plant operation beyond design life, but this is often at odds with the initial design and operating criteria adopted during the past years. Changing the operational focus of a facility after operating against a fixed life changes the life cycle costings, and requires a change in the whole management philosophy of operation to one of minimum corrosion damage to extend life.

The desire to reduce corrosion and similar "operating and maintenance" costs is a prime strategic objective for industry, but the reality of the situation is that these costs will continue to exist until the right tools for the job become established at every plant.

There is, however, good news which is that corrosion can be regarded quite simply as a process, in the same manner as production, refining or manufacturing. The fact that it is a process means that corrosion can be managed. What is required is a means of establishing:

• the existence of corrosion
• the contributory causes of corrosion
• a measure of the significance of the corrosion as regards detrement to plant profitability
• a feedback mechanism to prove how effective corrosion control measures have been

The Cost Efficiency Conundrum

For an increasing number of companies, a prime strategic objective is the progressive reduction of operating and maintenance costs (Total Cost of Operation). The means of achieving this is often to "plan" for problems by forecasting plant breakdown, this being supported by labor-intensive plant inspection and implementation of risk assessment programs.

The traditional approach to dealing with corrosion has been to view it as inevitable, a "necessary evil" that is addressed by replacement of critical areas if plant when the projected lifetime is approached. Where catastrophic failure has been experienced in the past, excess stocks of machine parts and piping etc. are carried to tide over breakdown periods. If an unforeseen plant breakdown occurs, great expense arises in attempting to salvage production capacity. However, once production has been lost, the investment returns for the plant can only be regained through future improved efficiency or extended operational life.

Unscheduled maintenance activities such as corrosion-related failures have the effect of pushing back all of the scheduled maintenance in a classic Catch 22 manner. Lack of scheduled maintenance leads to more unscheduled activities, which leads to less of the scheduled procedures, etc..

The ideal scenario, then, is to maximize revenues and production by pro-active control rather than relying on a reactive approach.

The Illusion of Control

Corrosion affects many aspects of the operation of a facility; not only the availability of plant but also the throughput and overall ROI (return on investment) of an asset through the reduction of corrosion costs.

Corrosion measurement has traditionally utilized off-line Electrical Resistance (ER) probes, portable techniques or weight loss coupons, leading necessarily to a reactive regime. Measurements are often only made available on a monthly or quarterly basis, meaning that plant engineers are locked into a strategy where they count how much damage has already taken place. Even though steps may be taken to alleviate any corrosion problems, the off-line nature of the traditional measurement methods mean that feedback on the success of remedial measures is not instantaneous. Whilst accountability is provided, there is only the "illusion" of control.

The fact is that corrosion damage does not happen at the same rate or by the same corrosion mechanism all the time, but takes place in episodes that are related to specific types of operational situation. In order to have any effective level of corrosion control it is vital to monitor corrosion activity in real time along with the process conditions that can cause the damage. In this way the operator can see the corrosion as it is occurring and carry out adjustments to minimize the damage, reduce the need for maintenance and lower the risk of unexpected failure. With this level of control in place and the corrosion rate known, it is possible to extend the periods between inspection and further reduce operation and maintenance costs.

The newer generation of on-line, real-time corrosion management system gives the operator the ability to monitor the condition of static plant (pipes, vessels, manifolds, etc.) in much the same way as vibration monitoring on pumps and turbines. Utilizing a systems integration approach, the rate and mode of attack can be correlated on-line with process parameters via network connection to distributed control and asset management systems (DCS and AMS).

Multi-Technique Corrosion Monitoring

The latest in real-time corrosion management systems utilize a combination of modern electrochemical techniques to evaluate the corrosion behavior of a material. The combined approach increases the confidence in the data, whilst examining localized as well as general corrosion. This technology is proven for application in condensing and multi-phased systems, and helps to increase understanding of the corrosion mechanism whilst providing a rapid and continual assessment of corrosion rate. The techniques utilized are briefly described:

Electrochemical Noise (EN) – proven advanced technology

Electrochemical Noise (EN) technology has been in use for almost twenty years, yet still represents the most advanced technology for real-time corrosion appraisal. It is the most sensitive technique available for on-line measurement and is unique in that the ‘signatures’ from EN sensors can be used to derive not only corrosion rates, but also mechanistic information about corrosion type (general or localized).

EN provides data which can be correlated with the mode of corrosion because the signals recorded are characteristic of the corrosion process. Special recently patented algorithms are available which can further distinguish the localized corrosion mechanism (e.g. pitting, crevice attack, stress corrosion cracking, bio-corrosion, etc.).

Of particular interest is the fact that the EN technique has application in low conductivity environments (Rs > 100,000 Ohms), where other electrochemical techniques are unable to function.

Linear Polarization Resistance (LPR)

Linear Polarization Resistance (LPR) monitoring involves measurement of the polarization resistance of a corroding electrode at low values of over-potential. Corrosion rate values are calculated from LPR, but assume a steady-state condition such that all estimates relate to the likelihood of uniform or general corrosion. This fact renders LPR incapable of providing localized corrosion information.

It is customary to use a combined EN/LPR probe for aqueous environments to maximize the corrosion monitoring information gathered.

Critical Process Corrosion Management

Tailoring the “total approach to corrosion management” to the specific needs of Oil and Gas Production and Refining Critical Processes is readily achieved utilizing modern technology systems. In specifying the scope of the system, some simple guideline rules can be applied as follows:

• Systems Integration and Interrogation capabilities allow facility operations to be proactively managed by correlating data in “real-time” to refine production operating practices and chemical treatment programs, and to optimize costs. All important process and operational parameters, chemical and corrosion monitoring data should be incorporated. Adopting multi-technique corrosion monitoring maximizes the breadth of information gathering to identify the most complete solution to a given problem. This approach enables unlimited access to information to authorized users from remote network locations. Alarm levels are pre-configured, to allow less skilled personnel to handle the day to day ‘as it happens’ off-specification process quality to be flagged and corrected.
• Use Electrochemical Noise (EN) for process optimization, general and localized corrosion and bio-corrosion measurements and control, in mixed phase and hydrocarbon systems such as:
• Corrosion inhibitor and biocide treatment optimization
• Downhole ESP pump environment health check
• Corrosion control of wells; gathering; export oil, condensate and gas pipelines and top of gas line condensation corrosion
• Demulsifier treatment adjustment to optimize field separation and desalting operations
• Optimization of gas compression and gas export from sweetening and dehydration operations
• Optimization of pipeline pigging operation
• Use combined EN/ LPR for process optimization, general and localized corrosion and bio-corrosion measurements and control in aqueous systems such as:
• Corrosion inhibitor and biocide treatment optimization
• Focus on water injection and produced water re-injection stagnant areas, e.g. filters, desanders, depurators and coalescers
• Oxygen scavenger treatment optimization for water injection systems, particularly where water injection and produced water re-injection ratios may vary considerably
• Quality control of power fluids corrosion and bio-corrosion products
• Corrosion control of gas sweetening and dehydration operation, e.g. corrosion inhibitor optimization and reboiler under-deposit corrosion
• Materials and metallurgy including weld metal evaluation
• Optimization of pipeline pigging operation
• Advantageous utilization of reduced quality, cheaper crude feed
• Control of corrosion inhibitor treatment in open and closed cooling water systems
• Optimization of heat exchange efficiency and reduction in surface scaling
• Use ER for general corrosion measurements to predict plant lifetimes and reinforce the electrochemical real time methods such as:
• Lifetime corrosion prediction in oil, gas, condensate, hydrocarbon gathering and export pipelines, also water injection and produced water re-injection systems
• Use real-time acoustic sand probes to measure and control erosion such as:
• Erosion control of wells and oil gathering pipelines, also produced water re-injection systems
• Use oxygen analyser and galvanic probe to monitor oxygen ingress such as:
• Optimization of injection water oxygen scavenger and antifoam treatments, also deaerator mechanical operation
• Use hydrogen probes to measure and control hydrogen induced cracking (HIC) and Sulphide Stress Corrosion (SSC) induced cracking such as:
• Sweetening units
• Sour oil and gas production
• Materials, metallurgy and weld metal evaluation
• Use chlorine analyser to monitor system chlorination such as:
• To optimize system chlorination to ensure that treatment is continuous (uninterrupted). This will in turn facilitate oxygen scavenger treatment optimization
• Use weight loss coupons, bioprobes, NDT methods and fluid sampling to verify real-time data.
• Use unit-specific probes (e.g. MENTOR CHx) to track skin temperatures and to monitor under-deposit corrosion to minimize system fouling from corrosion products, scale, wax, etc..
• Use condition monitoring to enable fault prediction, tracking and solution to optimize ESP operation with respect to downhole environments, and to give health checks on compressors, turbines and other strategic equipment such as export pumps.

Examples of typical Oil and Gas Production & Refining systems, illustrating the opportunities for improved corrosion management at strategic plant locations, are shown in Scenarios 1 to 6.

Scenario 1: Critical Process Management in Oil & Gas Production - Oil Separation & Export - Inhibitor Performance in an Oil Flowline:

This scenario describes how real-time corrosion management can be used to verify Inhibitor Performance in an Oil Flowline. The mimic display above shows typical Oil Separation and Export plant, with corrosion inhibitor injection points and installed EN/LPR and ER probes.

The trend graph below shows the EN corrosion rate with Inhibitor A versus the immediately reduced corrosion rate upon switching to the higher performance Inhibitor B. The rapid response of EN, coupled with its ability to detect localized corrosion, enables the operator to see quickly that the localized corrosion has been brought under control by Inhibitor B.

Scenario 2: Critical Process Management in Oil & Gas Production - Gas Separation & Export - Glycol Performance in a Gas Flowline

This scenario describes how real-time corrosion management can be used to verify quality of regenerated glycol (MEG) in a Gas Flowline. The mimic display shows typical Gas Separation and Export plant, with chemical injection points and installed EN/LPR and ER probes. In this scenario, a Gas Flowline is injected with a mixture of MEG (monoethylene glycol - to control hydrate formation) and corrosion inhibitor. The MEG is regenerated onshore to remove salt and water, and then reinjected into the pipeline system. The decision has been taken to run on residual corrosion inhibitor. The long-term EN corrosion rate trend shows a gradual increase in corrosion rate, and a change from uniform to localized behavior. The corrosion management system outputs an EN Probe Alarm showing "High corrosion", and provides intelligent "what next" suggestions of "Check inhibitor levels" and "Check MEG Salt Levels".

Scenario 3: Critical Process Management in Oil & Gas Production - Seawater Injection System - Oxygen Scavenger Performance

This scenario describes how real-time corrosion management can be used to verify the performance of an oxygen scavenger in an offshore Seawater Injection System. The mimic display shows typical Seawater Injection plant, with chemical injection points and installed EN/LPR and ER probes. The corrosion management system is able to determine a correlation between the trends of the EN corrosion rate and the concentration of oxygen measured in the system. The mimic screen displays an alarm banner indicating "Check Oxygen Scavenger", which is also highlighted in the "Suggested Next Action" screen.

Scenario 4: Critical Process Management in Refining - Amine Unit - Amine Recirculation Pump Performance

This scenario describes how real-time corrosion management can be used to verify equipment performance in an Amine Unit. The mimic display shows Amine Sweetening plant, with flow and temperature measurement points and installed EN/LPR probes. In this scenario, the amine recirculation pump beaks down causing the amine recirculation rate to fall. The corrosion rate increases due to H2S overload, and the mimic screen displays an Alarm Banner which indicates "Loss of Flow". The corrosion management system is able to determine a relationship between the change in amine recirculation rate and the EN corrosion rate. In addition, intelligent Suggested Next Actions indicate "Check Amine Pump Alarm".

Scenario 5: Critical Process Management in Refining - Cooling Tower System - Chlorinator Performance

This scenario describes how real-time corrosion management can be used to verify Chlorinator performance in a Cooling Tower System. The mimic display shows Cooling Tower plant, with chlorine injection and measurement points and installed an EN/LPR probe. In this scenario, the Chlorinator failure leads to increased microbial corrosion activity which is observed on the EN trend graph as an increase in both rate and localization of the corrosion. The mimic screen displays an Alarm Banner which highlights "Low Chlorine Levels", and the corrosion management system is able to determine the correlation between the loss of chlorine and the change in corrosion characteristics. The Suggested Next Action screen indicates "Check Chlorinator Operation".

Scenario 6: Critical Process Management in Refining - Cooling Tower System - Heat Exchanger Performance

This scenario describes how real-time corrosion management can be used to verify Heat Exchanger performance in a Cooling Tower System. The mimic display shows Heat Exchange plant, with heat exchange efficiency measurement and EN/LPR technology installed within a MENTOR CHx system. In this scenario, scale inhibitor treatment has failed to adequately control scale deposition, leading to a loss of heat exchange efficiency and the occurrence of localized corrosion. The corrosion management system is able to determine a relationship between the EN corrosion rate trend and the change in heat exchange efficiency. The mimic screen displays an Alarm Banner showing "Heat Exchange Efficiency Loss", whilst the Suggested Next Action screen indicates "Check Scale Inhibitor Injection".

Key to Figures:

• = Sensor/Measurement Location
• EN = Electrochemical Noise
• LPR = Linear Polarization Resistance
• ER = Electrical Resistance
• CHx = Special probe to measure corrosion under heat transfer conditions
• CI = Corrosion Inhibitor

Effective corrosion management can only be achieved by understanding the corrosion and controlling it along with the process conditions that cause damage. This level of control can significantly reduce the need for maintenance, lower the risk of unexpected failure, extend the periods between inspection, and further reduce operation and maintenance costs. This is exactly where modern real-time corrosion management for critical process control is able to afford greater value opportunities over any other means of corrosion measurement.

References  Top

1. Teevens, P.J., "Electrochemical Noise - A Potent Weapon in the Battle Against Sour Gas Plant Corrosion: Over Three Years Operating and Turnaround Inspection Experiences in Two Canadian Plants", Australasian Corrosion Conference, Perth, Australia, December 1998.
2. Teevens, P.J., "Pressure Equipment Life Extension Through Advanced Corrosion Surveillance Methods", Paper 3, Session 8, Maintec, Birmingham, United Kingdom, March 1996.
3. Eden, D.A., "Electrochemical Noise - The First Two Octaves", Paper No. 386, NACE Corrosion 98, San Diego, California, March 1998.
4. Hladky, K., US Patent # 4,575,678
5. Eden, D.A., John, D.G., Dawson, J.L., US Patent # 5,139,627
6. Eden, D.A., Carr, R.N., Dawson, J.L., US Patent # 5,425,867
7. Eden, D.A., "ASTM Electrochemical Noise Workshop", Atlanta, 5 May