Abstract

The development of high resolution metal loss techniques has opened up the possibility of real time monitoring in applications that had previously been notoriously difficult to investigate, such as subsea or multiphase systems. The main impact of these developments has been on improvements in the application, evaluation and optimisation of corrosion inhibitors.

The following paper presents a number of case histories that illustrate the design and operating cost benefits that can be achieved through application of high resolution real time corrosion monitoring.

Introduction

High resolution metal loss techniques have opened up the possibility of real time corrosion monitoring in multi-phase systems that had previously been difficult to investigate. Furthermore, designs have been developed for demanding high temperature and pressure, deepwater developments. The main impact has been on improvements in the application, evaluation and optimisation of corrosion inhibitors.

Substantial improvements have been obtained in sensitivity and changes in corrosion rates can be rapidly detected even for thicker, long life, sensor elements. In addition, the design flexibility of sensor elements means that they can be configured as probes or flow through devices introduced into processes as spool pieces. Spools are manufactured to include pairs of rings, the same diameter as the pipeline, providing 360 degree monitoring of the plant material of construction, the weld material and other materials of interest.

This has had a number of significant benefits for both corrosion mitigation and operating cost including; the testing and selection of products under true process conditions, optimisation of chemical dosage, increased availability of inhibitor and overall cost reduction through better management of corrosion.

Overview of technology

The high resolution metal loss techniques employed in the following case histories are based on the same principles of operation as Electrical Resistance (ER). However, improvements in probe design, signal measurement techniques and analysis provide significantly higher levels of resolution and hence, of speed of response over traditional techniques. Resolutions of the order of 5 nanometers are available even on 1mm thick elements. This provides true real monitoring of corrosion. Unlike electrochemical techniques, ER based technology is not restricted to continuous aqueous phases but provides a reliable output in a wide range of multiphase systems. In addition, the fast speed of response allows a range of corrosion inhibitor products and dosages to be tested live in a system within days as opposed to weeks or months.

Not only can corrosion inhibitor monitoring be more widely applied in field conditions, but using the same equipment, laboratory evaluations be conducted under conditions more representative of the field.

The technology has allowed the development of new application configurations such as high resolution subsea probes designed to last the full life span of the system without replacement. These probes have been configured for the most demanding deepwater high pressure, high temperature operational conditions and are qualified to pressures up to 1553 bar and temperature of 180 degrees C in 2500 meter water depth to keep pace with the latest subsea completion technology. (See figure 1).

An additional development is that the sensing element can be configured in different forms to suit applications. A non-intrusive system has successfully been employed that provides circumferential resolution of corrosion. The system is based on the same high resolution metal loss technology. However, because the sensor elements can be employed in the form of closed rings, transverse shaped sections of pipe can be used.

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