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Aggressive moves counter SRB in flooded subsea pipeline

Aggressive applications of biocide, use of hydrogen sulfide scavengers, and multiple pig runs can quickly eradicate even major infestations of sulfate- reducing bacteria in a repaired subsea natural gas pipeline following seawater incursion.

Sample analysis made it possible to understand conditions within the pipeline and measure the treatment's effectiveness, allowing Williams Midstream to regain control of internal corrosion mechanisms and quickly eradicate microbial populations, minimizing their threat to pipeline integrity.

The inefficiencies of multicup pigs in removing seawater from pipelines became apparent, particularly when there are major differences in elevation. Injecting biocide at the upstream end of the trunkline required six pig runs before expected concentrations were observed near the downstream end of the pipeline.

Theory holds that as the cup pigs pass over each girth weld, they flex and leave a small quantity of water, which runs down and pools at a low point, meaning numerous pig runs may be necessary to displace chemical products through pipelines, particularly if there are large elevation changes.

Background

Hurricane Ike made landfall at Galveston, Tex., at 2:10 a.m. on Sept. 13, 2008. As the hurricane crossed the Gulf of Mexico it ripped one of the major laterals from the central trunkline in an offshore pipeline system. The breach allowed about 75,000 bbl of seawater to enter the trunkline and about 40,000 bbl of seawater to enter the lateral.

The trunkline was repaired about 100 days after the breach, and dewatering operations commenced. Dewatering detected 90-ppm H2S ahead of the pig. During dewatering of the lateral 2 months later, dissolved S spiked to 10,000 ppm. Since normal gas production contained at most traces ofS , the observed concentrations of hydrogen sulfide were attributed to microbiological activities occurring after seawater entered the pipelines.

This article provides an overview of the pipeline repairs, initiatives to dewater the pipelines, biocide and hydrogen sulfide scavenger treatments, and the pigging program, all of which were implemented to reestablish control over sulfate-reducing bacteria, and the S they generated.

Post-Ike repairs

Fig. 1 depicts the affected offshore gas pipeline system in the Gulf of Mexico. A pressure drop on the blocked pipeline during Hurricane Ike indicated a leak. Investigation determined an 18- in. OD lateral had broken free from the 30-in. OD trunkline and the connecting end of the lateral now lay about 572 ft to the west of the previous connection point.

A elbow had previously connected the lateral to the top of the trunkline. Williams decided to replace the section of the trunkline at the location of the breach with a specially designed horizontal spool piece, which provided a T connection to the lateral at 3:00 o'clock. New pipe was laid from the breached end of the lateral to the spool piece.

Fig. 2 shows the breached section of the trunkline, after it was removed from the Gulf of Mexico. Removal occurred about 45 days after passage of Hurricane Ike. Careful examination of the section sought indications of internal and external corrosion or of other mechanical damage that would affect pipeline integrity. The visual appearance of the interior surface of the

pipeline was like new, fresh metal, with no indications of pitting or general corrosion.

Divers walked the pipeline to assess damage. Replacement spools and connectors-grippers were designed, built, and pressure tested, enabling the trunkline and lateral to be repaired and returned to production.

Fig. 3 depicts design of the new spool pieces for the trunkline and the connection to the lateral.

Initial dewatering Knowledge of the length and ID of pipelines, subsea topography, and gas pressure—measured at the offshore platforms and the onshore plant receiving the natural gas—allowed estimation of the volume of seawater that had entered the trunkline and lateral. Williams's first inclination was to pig the trunkline from deepwater to shore to remove the seawater after repairs. The volume of seawater in the trunkline, however, far exceeded the capacity of the onshore slug catchers, which were designed to accommodate condensates and limited volumes of produced water.

Williams instead decided to pig from shore to deepwater. Routine processing of onshore production at the gas plant made an onshore source of gas available.

Fig. 4 shows the slug catchers at the onshore plant in Louisiana. The estimated 75,000 bbl of seawater in the trunkline exceeded the slug catcher's 10,000-bbl capacity, even with a fleet of water trucks to assist in the draining and disposal of the seawater.

The first step in dewatering the pipeline was launching a low-density foam pig from the onshore pig receiver-launcher and pushing seawater to die offshore platform, where it would be routed through temporary piping to a large barge stationed next to the off- shore platform to receive, process, and store the water and any condensate.

In early January 2009 the operator dropped line pressure to minimize the risk of hydrate formation and launched a soft foam pig from the onshore facilities. This pig proceeded slowly to the offshore platform, propelled by onshore gas. Soft foam offered a balance between flexibility and maintaining a good seal for the first step of dewatering.

The pig moved at about 3-4 mph, or about half the normal pigging speed. Because numerous pigs had previously passed through the trunkline without showing adverse wear from the numerous girth welds, the soft foam pig was expected to pass without being ripped apart.

The trunkline originates at deep- water offshore platform

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