Nonintrusive inspection assesses wall loss in gas-plant separator

Bill Browne
CATS International Ltd.
Merseyside, U.K.

• British Gas International's Hannibal, Tunisia, plant processes natural gas produced from the Miskar offshore field (Fig. 1). [12,433 bytes]
• Ultrasonic inspection of a vessel wall is conducted manually (Fig. 2). [7,332 bytes]
• An immediate display of corrosion-monitoring data is provided from the ultrasonic survey (Fig. 3). [7,660 bytes]

Ultrasonic surveying at British Gas Tunisia Ltd.'s (BGTL) Hannibal plant in Tunisia accurately characterized the extent surface erosion caused by flow impingement in a separation vessel.

The procedure also provided a base-line data set in preparation for installation of continuous condition monitoring.

Miskar field facilities

BG plc has been operating in Tunisia since 1988 and is currently the country's largest foreign investor, with offices in Sfax and Tunis.

Operating as British Gas Tunisia Ltd. (BGTL), it is one of the largest foreign holders of Tunisian oil and gas exploration and production acreage, with interests in three exploration permits totaling 5,640 sq km.

These include a 100% interest in the Miskar field, 125 km offshore in the Gulf of Gabes, which currently supplies 70-80% of Tunisia's total daily gas demand via a sales contract with Societé, Tunisienne de l'Electricit, et du Gaz (STEG), the Tunisian state electricity and power distribution company, which purchases up to 5.1 million normal cu m/day (MMcmd).

Discovered in 1975 by Elf Aquitaine after the successful test of the Miskar-1 well, this field was appraised with four additional wells and two-dimensional seismic survey.

Following technical and commercial evaluation, the original contractor allowed the exploration permit to expire and, in 1988, the Amilcar Permit, which covers 1,584 sq km., was obtained by Houston Oil & Minerals, Tunisia. This permit was subsequently transferred to BG's local subsidiary, BG Tunisia Inc., in 1989 and is now held jointly with the Tunisian state oil company Enterprise Tunisienne d'Activities Petroleum (ETAP).

Since 1996, natural gas from this field has moved ashore via 124-km pipeline, consisting of a 122-km subsea and 2-km buried onshore stretches, and processed at the Hannibal plant, 32 km south of the industrial city of Sfax.

The production platform contains topsides gas facilities capable of processing up to 9.6 MMcmd of gas which is then transported through the 124-km pipeline to the Hannibal plant.

Built by Bechtel and commissioned in 1997, the Hannibal installation removes nitrogen, carbon dioxide, and hydrogen sulfide from the gas as it comes ashore (Fig. 1 [12,433 bytes]). Part of the treatment process entails the separation of the carbon dioxide and hydrogen sulfide from an activated amine solution.

The two carbon-steel columns used for this process have been in service for 2 years and operate at slightly less than 100° C. They are approximately 3.35 m internal diameter x 30 m tall incorporating dished ends and inlet/outlet nozzles penetrating the cylindrical portion of the vessels.

The first major scheduled maintenance shutdown was performed at the Hannibal plant during summer 1997 when access was gained to one of the amine stripper columns and a visual survey of internal condition performed.

This revealed that above-average material loss had occurred where flow from the inlet nozzle had impinged upon the diametrically opposite internal vessel surface. An unplanned entry was made into the second column where the same erosion was discovered but to a lesser extent.

Physical measurements taken at the time were used to assess the extent of wall-thickness reduction, confirm the integrity of the vessels, and estimate their residual life expectancy if the degradation process were allowed to continue.

This assessment made clear that the safe working life of the units would be significantly reduced unless steps were taken to inhibit the effects of the observed erosion.

Diverter installed

Consequently, BGTL adopted an approach, which considered all safety, productivity, and commercial implications.

The aim was to quantify the extent of the problem and identify cost-effective and non-disruptive methods to provide condition and trend data that would lead to effective remedies.

Evaluation of various solutions led to installation of a distributor to divert flow from the vessel wall and to erode sacrificially to protect the vessel itself.

The next question was how to monitor the effectiveness of the remedy and ensure that the rate of wall-thickness reduction had been reduced to acceptable levels.

One approach would have been to perform periodic internal visual surveys to monitor condition. The rate of material loss before introduction of the flow diverters, however, made frequent intervention necessary. This approach was deemed too expensive and disruptive.

Conventional ultrasonic wall-thickness measurement that used "key-point" digital ultrasonic inspection provided a possible answer, but the coverage and repeatability afforded by point measurement failed to meet requirements.

The conventional approach to wall-thickness assessment entails measuring the time lapse of ultrasonic signals, generated by a normal compression-wave transducer reflected off the opposing wall of the material under test.

Because ultrasonic travels at known constant velocity for a given material, it is possible to translate this time-lapse measurement into distance traveled, or thickness.

This is done by manually applying a normal (0°) compression wave transducer usually on a 4 or 6-in. grid and taking spot readings of thickness with a digital meter or analogue flaw detector to achieve the desired area coverage.

Limits of approach

However, this simplistic approach has several significant limitations:

• Corrosion or erosion is often random. There is no guarantee, therefore, that the selected measurement points on the grid represent the worst case. Nor are there sufficient points to achieve the required area coverage to determine the extent or characteristics of the problem.
• Conventional thickness-measurement techniques rely upon visualization (or "gated" electronic capture and transit-time measurement) of the signal reflected from the back wall of the material through which the ultrasonic beam has traveled. This ignores or eliminates any signal responses from within the body of the material itself. If the material exhibits structural degradation or has such defects as cracks or lack of weld fusion, in isolation or in combination with wall-thickness reduction, overall integrity is reduced.
• Manual ultrasonic methods are highly subjective in terms of application and interpretation, especially with respect to accurate reproduction of exact location (Fig. 2 [7,332 bytes]). This is an important consideration during monitoring of condition and propagation trends. Because eroded/corroded surfaces are irregular, they do not constitute perfect or normal reflectors, and the resulting signal is not necessarily returned to the receiver. This can result, at its worst, in complete loss of signal or, at best, in a reflected waveform which comprises low amplitude and/or repeat echoes from the various facets of the surface under test. This can, obviously, give erroneous results.
• Other non-destructive techniques, such as radiography and electro-magnetic methods, offer only a comparative basis of assessment. In addition, they are often logistically impractical under plant operating conditions.

Ultrasonic measurement

Having installed the distributors and restarted the plant, British Gas then contacted continuous corrosion-monitoring specialists Rabco Industrial Inspection Services, Poole, Dorset (U.K.), for recommendations on the most suitable method of ongoing assessment.

Recognizing that an accurate base-line survey was required before any meaningful corrosion monitoring could be initiated, Rabco contacted CATS International Ltd., Merseyside, U.K., which offers plant condition assessment with a range of computerized imaging techniques.

CATS determined that the most appropriate approach was to perform through-wall visualization with ultrasonic measurement. This would be carried out in conjunction with computerized processing capable of creating a realistic and proportionate color-graphic image of the internal surface.

This image could then be directly compared with the known characteristics of the eroded area and quantified during the shutdown.

In July 1997, a team surveyed the affected areas on both vessels and produced internal-surface contour maps which exactly corroborated the visually observed and physically measured profiles recorded when the vessels were accessed some months before.

The technique employed by CATS digitally records all position-related signal waveforms which are processed on-line to create a color-coded depth map of wall thickness. This is displayed immediately on a three elevational image showing X - Y, transverse profiles and a plan elevation which illustrates the extent of the damage.

Inspection is performed by manually scanning a normal compression wave ultrasonic transducer over an area. The position of the transducer, which is coupled to the inspection surface with high-temperature gel, is continuously logged electronically.

Position-related ultrasonic signal waveforms are continuously recorded, digitized, and stored so that as-recorded data can be retrieved and analyzed off-line. These waveforms are also processed on-line to build up a three-elevational, scale-graphical image of the through-wall condition of the material covered by the ultrasonic beam (Fig. 3 [7,660 bytes]).

A color scale is used to represent depth through the material so that the resulting plan image can be used "at a glance" to assess the extent of any damage and its severity in terms of residual thickness.

Results are available immediately so that operating decisions and remedial measures can be implemented without undue delay.

Immediate graphics

Because all raw digital data are stored according to position, it is possible to process results off-line to any suitable presentation format.

For instance, topographical images can be reproduced in the form of a three-dimensional isometric contour map or merged with plant engineering drawings indicating location and orientation of inspection "windows."

Alphanumeric text can be added to produce report commentary on the results obtained.

All software within the system is Windows-based and therefore compatible with proprietary hardware and image processing and presentation software.

Consequently, data manipulation and reporting can be tied into customers' existing data-management systems without the need for expensive, custom-written software.

Also, because data are converted into a digital format, they can be transmitted by modem to any location worldwide. This provides interested parties with the information for decisions immediately.

The coverage and resolution of this approach allow the nature and therefore the cause of the damage to be determined. For instance, where damage has been caused by turbulent or differential multi-phase flow, the characteristics of the surface will be evident on the resulting image.

Because the technique captures the full signal waveform (as opposed to only the reflected back wall response), the entire through-wall condition is visualized. This means that material structural damage caused by stress or chemically assisted processes can be effectively reported.

The technique can also be modified to utilize one or more angle beam transducers which can be used to capture either reflected or diffracted signals propagated by through-wall defects such as cracks or weld root erosion.

In isolation, included or embedded defects may be quite permissible. In combination with adjacent wall-thickness reduction, however, they can reduce overall integrity.

The ambient temperatures around the vessels posed a further problem. Although the stripper columns were operating at less than 100° C., the ambient desert temperature was so high that prolonged human intervention and use of conventional methodology were impracticable.

Because of the high rates of data acquisition, however, and real-time image processing possible with CATS technique, the inspection team was able to map the affected areas non-intrusively with the plant on-stream.

This is clearly more cost-effective than methods requiring shutdown and human intervention. External access to the area of inspection was provided by temporary scaffolding at the appropriate elevation and lagging was removed locally to expose the external surface to enable ultrasonic contact scanning to be performed.

Follow-up inspection

To monitor the rate of progression of the defect, a follow-up survey was commissioned by BGTL in November and December 1997.

Using the same procedures and techniques as before, a CATS team reinspected the same area on the original column and conducted a survey on the second column. This second survey confirmed that the extent of erosion on both units had been arrested by the remedial measures implemented by BGTL.

Selected areas on the associated carbon-steel pipework were also then examined to establish whether the flow-related problem had caused any accelerated material loss.

This assessment revealed no such effects, but ultrasonic imaging of the 12 to 6-in. eccentric reducers in the loop revealed measurable significant material loss, characteristic of the eddy effects caused by geometry-induced cavitation immediately downstream of a control valve.

Given the amount of high-resolution coverage and the quality of images generated by the deployed techniques, both of the surveys serve as an excellent baseline for future monitoring of the vessels and their associated pipework.

The resulting cost savings gained by adopting this non-intrusive inspection and simple remedial approach have not yet been fully assessed.

Compared with alternatives of premature retirement/replacement, however, or with frequent surveillance involving shutdown and physical intervention, the costs involved must be significantly less in terms of both capital expenditure and avoidance of downtime and consequential loss of production.

The overall economic benefits must therefore be measured in the £10,000s.

Now that the conditions and trends have been established, accurate assessment can be made regarding the future integrity and life expectancy of the unit.

Further cost reductions are also possible if a continuous condition-monitoring system, currently being considered, were to be installed.

The outcome of the exercise further vindicates the 'value-added' benefits of strategic, risk-based condition assessment using modern technology.

Given the improved awareness and confidence of material condition, BGTL should now be able to prescribe cost efficient and timely measures for future assessment.

These could include use of permanently installed co-polymer transducers capable of continuously logging wall thickness and providing real status feedback to a central location in exactly the same way that process parameters such as temperature, pressure, and flow are monitored routinely.

The Author