Abstract

The systematic selection approach for corrosion monitoring systems and locations for our oil and gas fields is described. A major factor in the selection process is the high level of H2S in some of our fields. This limits the number of monitoring systems that can successfully be used and also adds to the safety concerns. New probes for corrosion monitoring have been tested by us and the results of the trials are discussed here.

Introduction

Corrosion monitoring is a process whereby the condition of plant equipment is constantly evaluated and their remaining life are predicted. It is only part of the overall condition monitoring and plant integrity management.

Corrosion monitoring, in one form or another, is widely practiced in the oil and gas industry. There are two main reasons for carrying out corrosion monitoring:

  1. To assess the corrosivity of the environment and estimate the remaining life of the pipeline or vessel being monitored.

  2. To evaluate the effectiveness of chemical inhibition programs.

There are many forms of corrosion monitoring. These can be classified into many different groups according to the features of the monitoring system. Examples include:

  1. Intrusive and Non-intrusive

  2. Real time data providers and Historical data providers

  3. Those measuring corrosion rate and those showing other related parameters.

Examples of (1) include corrosion coupons (intrusive) and Hydrogen Probes (non-intrusive). For (2) we can cite Linear Polarisation resistance (real-time) and corrosion coupons (Historical); while examples of (3) include Linear Polarisation Resistance (corrosion rate measurement) and Bioprobes (bacterial population measurement).

Two of the oldest forms of corrosion monitoring are visual inspection and chemical analysis. Some of the useful information obtained from the latter method include total dissolved solids, total iron count, etc.

Corrosion monitoring is only one part of the overall Corrosion Management strategy. It should also never be taken as a substitute for proper design and material selection. Corrosion prevention is always preferable to corrosion control. However, in some cases total corrosion prevention is ruled out due to commercial or logistical reasons. In these cases corrosion control measures, such as corrosion allowance, chemical inhibition, etc. are utilised instead. In such cases corrosion monitoring becomes an integral part of the corrosion control strategy.

Why Use Corrosion Monitoring?

To choose corrosion monitoring means deciding to be proactive rather than reactive. To prevent rather than to fix. There can sometimes be a strong argument against corrosion monitoring. The basis of this argument are as follows:

  1. By correct material selection and corrosion control, one ceases the need for corrosion monitoring.

  2. The accuracy of some of the corrosion monitoring instruments is questionable.

  3. Intrusive type corrosion monitoring can create an extra safety hazard, particularly in high pressure sour systems.

  4. It can be labour intensive.

  5. Adds extra cost to the project.

The counter argument in favour of corrosion monitoring is as follows:

  1. Total corrosion prevention by material selection is not always economically feasible.

  2. Operational & environmental criteria used at the design stage may change during the design life of the plant.

  3. Monitoring can provide an early warning against corrosion, thus saving time and expenditure on unnecessary shutdown, repair and replacement.

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