TAPS CORROSION-CONCLUSION INHIBITOR-INJECTION PROGRAM SHOWING EARLY PROMISE

Peter M. Ricca
Alyeska Pipeline Service Co.
Anchorage

The treatment system implemented last year at TransAlaska Pipeline's pump stations has employed chemical inhibitors in a large-scale, corrosion-mitigation program.

Ultrasonic inspection in 1988 revealed internal corrosion predominantly in low-velocity piping. There, emulsified water had precipitated out of the crude oil and supported microbial corrosion.

TAPS developed a system of manual batch treatment to treat the numerous dead legs in the pump-station facilities along with procedures for monitoring inhibitor effectiveness. The first article about the program (OGJ, Apr. 22, p. 73) focused on the corrosion sites and project design. This conclusion deals with treatment and monitoring procedures and presents some preliminary results.

PROCEDURES, SCHEDULES

As monitoring fittings were installed in the dead legs, corrosion coupons or electric-resistance (ER) probe data and fluid samples were gathered to yield a brief baseline on internal pipe environment, prior to inhibitor treatment.

Because of the "fast-track" nature of the project, background data collection was limited to 30-45 days. High rates of process-water accumulation and very high sulfate-reducing bacteria (SRB) concentrations were confirmed inside the pipe.

Both coupons and ER probes confirmed very low weight loss or general corrosion rates. The onset of pitting, however, occurred on the coupons in less than 30 days.

Inhibitor injection was begun typically 30-45 days after fittings were installed.

The injection treatment typically treated 9-12 pipe segments at each pump station. The injection sequence started with the pipe segment farthest upstream of the relief tank and then proceeded downstream so that inhibitor would not be flushed out of the previously dosed lines.

Tanks and sumps were typically treated monthly. In instances in which tank turnover rate was high, tanks were dosed on a biweekly basis. The frequency of injection established for all categories of dead legs is shown in Table 1.

Inhibitor treatment was set up on a regular schedule in the spring of 1990.

One concentrate-injection truck (Fig. 1) was stationed at Pump Station 3 on the north side of the Brooks Mountain range and served Pump Stations 1-6.

The second truck was stationed in Fairbanks and served Pump Stations 7-12. One injection crew would start at Pump Station 1 and work southward, picking up equipment and chemicals as it proceeded.

Including travel, the biweekly treatment cycle of the 12 pump stations could be accomplished in 10-12 days.

At the end of a shift, 2-4 days were available for pump equipment and vehicle maintenance. Bulk inhibitor injection was scheduled around the biweekly concentrate treatments.

The bulk trailer was hauled by commercial carrier (Fig. 2) between the batch plant in Anchorage and the remote pump stations. The bulk truck was scheduled to rendezvous with the injection crew, which operated the generator and pump on the bulk truck.

Once established, this schedule and routine worked quite well.

MONITORING TECHNIQUES

To indicate the efficacy of MIC-inhibitor treatment at pump stations, extensive monitoring of pipe corrosion was performed.

A variety of techniques was used to determine inhibitor effectiveness. These included the following:

Electric-resistance (ER) probes
Water-chemistry analysis
Corrosion coupons
Bacterial analysis
Inhibitor residual analysis
Ultrasonic inspection (UI).

After a 1 -year evaluation of the six techniques, a longterm monitoring program for all TAPS pump stations was developed.

Electric-resistance probes were inserted into 10 monitoring fittings in the bottom of the pipe. These devices measure resistance change by material loss on a probe inserted into the process fluid.

Tube loop and cylindrical probes made from AISI C1010 steel were installed at various dead-leg locations.

ER probe data, which were confirmed later by corrosion coupons, indicated that general corrosion rates (based on weight loss) were < 1 mil/year (< 0.0025 cm/year).

Instantaneous ER probe readings and water-chemistry information indicated, however, a wide variability in the corrosiveness of process water.

This variability was not unexpected because TAPS receives and transports crude from six different producing units.

Because of the low overall corrosion rates observed, ER probes were subsequently moved from pump station dead legs to the incoming meters at Pump Station 1. At these new locations they could be used to detect abnormal slugs of oil-process water received into TAPS.

Water chemistry was also used to evaluate corrosion. Process-water samples collected from the bottoms of TAPS pipe varied widely (5,000-75,000 mg/I.) in total dissolved solids.

Water-chemistry analysis showed samples to be typically unsaturated in CO2 with a pH of 6.0-7.0. An occasional sample indicated a pH as high as 8.5-9.0. Low calcium levels (100-200 mg/l.) and moderate alkalinities indicated a lack of scaling tendency and a low potential for under-deposit corrosion.

The fairly neutral pH and alkalinity confirmed suspected low overall or acidic corrosion rates observed on ER probes and coupons.

Analysis of brine samples collected from numerous access fittings at bottoms of pipe indicated the presence of bionutrients. Adequate levels of dissolved sulfates (15-1,000 mg/l.) and temperatures 90 F. (32 C.) were detected which could support mesophilic and thermophilic SRB. Some water samples showed both the electron acceptor (sulfates) and electron donor (acetates and propionates) components present that could provide nutrients to both sulfate-reducing and acid-forming bacteria.

PROGRAM'S BACKBONE

Strip corrosion coupons were installed in approximately 180 monitoring fittings located on the bottom of dead-leg piping. These coupons form the backbone of the long-term monitoring program.

A pair of these coupons, made from AISI C1010 steel, are inserted into each access fitting and extend into the flow stream approximately 1/2 in. (1.27 cm). The coupons are 3 x 3/4 x 1/2 in. (7.62 x 1.91 x 0.318 cm) in size and have a tumbled smooth finish to make surface pitting more visible.

Coupons are removed and analyzed quarterly. The method of Byars and Gallop 1 was used for analysis of coupon exposures.

Immediately after removal, surface swabs are obtained from the coupons and cultured to determine concentrations of sessile bacteria. The coupons are then preserved with a vapor inhibitor until they can be cleaned and processed.

After cleaning to remove surface deposits, the coupons are dried and analyzed for weight loss, net pitted area, and pit depth.

Because corrosion was primarily microbial in origin, bacterial analysis of water collected from bottom-access fittings was used extensively in the monitoring program. Levels or activity of anaerobic planktonic (floaters) bacteria were measured with one or more of four quantitative or semi-quantitative methods.

The serial dilution method was used most widely to detect concentrations of SRB and general anaerobic bacteria in water samples. A lactate/acetate medium, developed especially for growing North Slope SRB, was used to culture and enumerate the SRB. 2

A second type of medium containing thioglycollate was used to culture general anaerobic bacteria (GAB). Six to ten serial dilution vials were used, and SRB and GAB results were reported as 10n organisms per ml of sample.

GENE-PROBE TESTING

In some instances for which very high accuracy was required, the gene-probe test 3 was used to enumerate sulfate-reducing and general bacteria in water and surface solids samples.

A gene probe is a technique in which a small fragment of genetic material (DNA) unique to a specific type of bacteria is introduced into a solution of lysed cells and binds to a corresponding segment of RNA or DNA of the targeted bacteria.

The binding of the gene probe to the targeted RNA or DNA can be monitored by chemiluminescence and can be used to determine precise concentrations of targeted bacteria in the lysed cell samples. Currently there are three gene probes available for detecting oil field bacteria:

The all-bacteria (AB) probe for detecting all (aerobic and anaerobic Eu-bacteria) viable bacteria.
The SRB2 probe which detects mesophilic and thermophilic (desulfotomaculum and desulfovibro) SRB.
The SRB3 probe which detects mesophilic (desulfobacter) SRB.

For spot testing of microbial activity, two other methods were used: the API (RP-38) sand-medium test 4 and the Hydrogenase test. 5

To indicate the actual inhibitor present in the fluid at the far ends of the treated dead legs, samples were taken and analyzed for inhibitor residue.

Analysis of inhibitor dispersed in oil was used as a measure of inhibitor persistence because direct field methods for detecting inhibitor film in situ were unavailable. Inhibitor residue in the oil phase indicates that material is present and available to refresh the protective film on the pipe surface.

Inhibitor concentration in the water phase indicates the water partitioning effectiveness of the inhibitor. These analyses were also valuable in determining leak and diffusion rates of inhibitor out of treated dead legs.

TAPS relies heavily on repetitive area scanning of dead pipe with ultrasonic inspection (UI) methods. The 1-in. (2.54-cm) grid or "course-scans" UI is used primarily for discovery work to indicate pipe condition.

The minimum wall thicknesses detected from these scans are used in structural calculations which determine repair or pressure-derating protocol. "Fine-scan," that is, 1/8-in. (0.318-cm) grid-spacing inspections, have been initiated to supplement the 1-in. "course-scan" UI and will be used to monitor the inhibitor mitigation effort on a microscale.

Selected areas of active MIC were identified and permanent reference marks located on the pipe so that repeatable 1/8-in. grid "fine-scan" wall measurements can be made on 3-6 month intervals.

Because of the difficulty of leaving below-ground excavations open for extended periods, below-ground UI measurements are typically made on 12-month intervals. An automated magnetic tracker with focused transducers is being used to perform the "fine-scan" UI.

Additional statistical sorting and comparison routines were developed to map and process the higher data density UI scans. The "fine-scan" maps provide the resolution to compare isolated pit-corrosion cells as small as 1/4 in. (0.635 cm) in diameter.

From previous work it was noted that the isolated pits were generally associated with the highest corrosion rates. It was theorized that the current densities are highest in small pits where the SRB concentrate at the center of the corrosion cell. As colonies grow in size, SRB migrate outward in a ring-like geometry.

As an MIC corrosion cell spreads in area, given the same level of nutrients and microbial activity, the current densities responsible for metal dissolution diminish.

MEASURING EFFECTIVENESS

Several measures of effectiveness are being used to evaluate the inhibitor mitigation effort.

The first is the ability of inhibitor injection to reduce pitting corrosion.

"Course-scan" UI measurements indicated significant decreases in pitting corrosion rates in 1990, after inhibition began.

Limited corrosion-coupon data and some fine-scan UI also showed reduction in pitting area and severity. Ultrasonic inspections and corrosion-coupon measurements will continue in the future to measure and confirm corrosion rates.

The second measure used is the ability of inhibitor to reduce microbial populations in the dead legs. Fluid samples showed a dramatic decrease in concentrations of planktonic anaerobic bacteria immediately after inhibitor treatment. The bacteria] count, however, returned to initial levels within 1 week after inhibitor treatment.

This repopulation is probably from refreshment of bacteria from new water diffusing in from the flowing process stream. These data strongly confirmed the incidental biocidal effect of the inhibitors selected (Fig. 3a).

In the true dead legs, where minimum diffusion of new bacteria occurs, SRB activity remained absent for at least 3 months after treatment (Fig. 3b).

The third measure of effectiveness used is schedule-completion performance.

Timely schedule performance ensures that the selected inhibitors will perform as designed. Difficulties in the logistics of scheduling three pump trucks, equipment maintenance, crew changeouts, and chemical deliveries over poor roads under arctic conditions to 12 remote locations are formidable.

Variations from scheduled activities and unscheduled out-of-sequence injections can add substantial labor and transportation costs.

The completed injections are tracked closely against the planned schedule. If treatments varied significantly from targeted frequency, changes to procedures or application dosages can be made. Equipment maintenance was scheduled between routine biweekly treatments to minimize pump and generator problems.

During the first half of 1990, the overall portion of completion of inhibitor treatments as scheduled averaged 61%. This low completion rate was due to interruptions from other pipeline construction projects.

During the second half of 1990, the portion of scheduled completion reached 85%.

In future years, scheduled inhibitor treatment completions of 95%+ should be attainable.

ONGOING PROGRAM

Approximately 14 months into the program, the inhibition program is showing very promising, cost-effective results. Full evaluation of this mitigation effort and the success of the program to extend the life of pipe will require at least 1-2 years of extensive monitoring to be completely reliable.

The performance results discussed here must be considered preliminary.

REFERENCES

Byars, H.G., and Gallop, B.R., "An Approach to the Reporting and Evaluation of Corrosion Coupon Exposure Results," Materials Performance, Vol. 27, No. 11, November 1975, pp. 9-16.
Injectech Inc., Lactate-acetate SRB Test, Ochelata, Okla.
Hogan, J., et al., "Use of RNA Probes in Detection of Sulfate Reducing Bacteria," Poster 16, GenProbe Inc., San Diego.
Ramsco Inc., API Sand Medium Test, Soldotna, Alas.
Caproco Ltd., Hydrogenase Test, Edmonton, Alta.