Aggressive moves counter SRB in flooded subsea pipeline (стр. 2 из 2)

The multicup pig arrived ashore Jan. 15, 2009, and two out of four bottles in the serial-dilution test yielded indications at least 100 microbes/ml. APS reductase methodology showed up to 10,000 microbes/ml. These collective results showed large microbial populations were still within the trunkline and there was a continuing potential for MIC occurrence.

Other laterals

Following Jan. 15 passage of the pig through the damaged lateral, attention shifted to returning other laterals, not damaged by Hurricane Ike, to production. Initial concerns again focused on preventing hydrate formation in either the laterals or the trunkline.

Pigs launched from laterals on Jan. 21 and 27, 2009, traveled to the trunkline and were then pushed to the onshore gas processing plant by a 30-in. multicup pig from EW-873 at the upstream end of the trunkline. Fluid samples were again collected at the onshore facility, and they provided positive indications for SRB, with 100 microbes/ml based on serial dilution studies and 10,000 microbes/ml using APS reductase studies.

THPS treatment

THPS biocide was injected into the upstream end of the trunkline and was first evident in sampling associated with the pig arriving Feb. 5. A sufficient volume of THPS was injected into the trunkline to provide 1,000 ppm of THPS residual at the end of the trunkline based on the estimated volume of seawater remaining in the trunkline at the time. The first measurement, however, yielded only 76 ppm of THPS at the onshore facility.

Biocide treatments and pig runs followed a rotating schedule treating all laterals on the pipeline system, including those unaffected by Hurricane Ike, and the trunkline. Through February 2009 THPS residuals increased to roughly 720-820 ppm. The delay in getting the biocide distributed through the pipeline is undoubtedly related to the frequency of the pigging and the vertical incline of the pipeline.

Serial dilution studies were negative, showing microbial populations of SRB were below detectable levels.

The biocide was effective as APS reductase readings mostly measured zero. Samples collected Feb. 22 and Feb. 27, however, had APS reductase readings as high as 10,000 microbes/ml. The APS reductase test cannot distinguish between live SRBs and those freshly killed by a biocide, explaining how there can be differences in results between the serial dilution and APS reductase techniques. Results from serial dilution studies were taken as the most definitive measure of microbial populations.

Once microbial populations were eradicated, a batch treatment of a tenacious oil-soluble corrosion was applied to the trunkline, quickly reestablishing a protective inhibitor film, helping ensure the trunkline's ongoing integrity.

18-in. lateral

The next major operational change was returning the 18-in. lateral to service. The multidisc pig used to dewater the 18-in. OD lateral arrived at the ST- 200 platform Mar. 10, 2009.

During initial dewatering elevated levels of

appeared at the receiving barge where liquid was removed from the lateral, showing microbial activity in the pipeline. The H₂S required the fluid removed from the lateral be treated with an H₂S scavenger before transfer ashore for disposal.

scavenger was therefore injected ahead of the first pig to traverse the lateral back to the trunk line to reduce the concentration of dissolved within any remaining seawater resident in the lateral that would be pushed into the trunkline and transported ashore to Larose.

H₂S levels

The concentration of

dissolved in the fluids arriving at the onshore facility was measured for each pig run after the 18-in. lateral was brought on line. Analysis of the concentration of dissolved in the fluids arriving at the plant following each pig run showed a gradual increase from 50 ppm to about 650 ppm from mid- March through mid-June 2009 and then a decline to roughly 20 ppm.

Throughout this period, each of the laterals received biocide treatments on a regular rotation, and

scavengers were applied. Corrosion inhibitors also continued to be injected into the gathering system at the producing platforms at the same rates used before Hurricane Ike.

As the pH slowly increased to 7.3 from 6.5, concentrations of iron and manganese in the water samples gathered at Larose continued to slowly drop, suggesting corrosion was abating. Serial dilution studies throughout this period also showed microbial populations of SRB as below detectable limits; the microbiological processes were now being kept in check.

Continuing trend

Through the rest of 2009 and early 2010, continued serial dilution and APS reductase studies show microbial populations have remained below detectable levels, even after biocide residuals approached zero. The pH of the water removed from the system continues to be in the range 7.1-7.2, and the concentration of dissolved

has dropped to about 10 ppm. Excellent corrosion inhibitor residuals likewise indicate the system is properly treated. A spike in the amount of sand-quartz and iron particles removed during pigging runs in late 2009-early 2010 showed the need for a continuous pigging program to displace these denser particles through the gradual incline of the pipeline. These favorable results show the benefits of having implemented an aggressive chemical treatment and pigging program to regain control of internal corrosion mechanisms and thereby maintain pipeline integrity.