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Unique approach solves pipeline exposure, river erosion.

Williams recently took a different approach to solving a pipeline exposure problem caused by river erosion in the Guadalupe River, near Victoria, TX. Instead of relocating the pipe and letting the river meander naturally, a decision was made to stabilize the pipeline and re-establish the river bank and shoreline to prevent further erosion.

Williams Senior Project Engineer Mark Bordelon said the erosion and pipe exposure was due to ever increasing flood events since the historical flood of 1998. Over time, approximately 70 feet of riverbank was lost, resulting in an exposure of a 120-foot section of the 26-inch diameter Transco natural gas transmission line that had been installed under the Guadalupe River bed in 1950.

According to the Williams' official, the Mainline A pipeline is operated by Transco, an affiliate of Tulsa, OK-based Williams Companies Inc. The route of the line begins in Hidalgo County in South Texas and runs 2,000 miles to the Metro New York area. While the pipeline transports approximately 10 percent of all natural gas consumed in the U.S., the portion crossing Victoria County moves about 12 MMcf/d.

Bordelon indicated that while a significant portion of the transmission line was exposed, it had not been taken out of service nor did it have any punctures or other problems that were a cause for concern. Engineering stress analysis of the exposed pipe was performed to ensure the safe, continued pipeline operations. There was, however, an initial concern that debris in the river could have damaged the protective corrosion coating on the pipeline. Subsequent diver inspection of the pipeline determined that the pipeline corrosion coating was intact.

He also pointed out that several engineering alternatives to solve the erosion problem were considered: 1) horizontal directional drilled (HDD) replacement crossing; 2) installation of a pipe support to brace the unsupported span coupled with Palisades geowebbing to allow siltation around the pipe; and 3) rebuilding the bank and armoring the pipe with a combination of rip rap and concrete pre-cast mats.

The decision to stabilize the pipeline and shoreline with a combination of rock rip rap and concrete mats proved to be the most economical as compared with an HDD installation.

"Stabilization costs were found to be approximately half the cost of a directional drill and could be completed in a shorter period of time," Bordelon explained. "Keep in mine that a proposed HDD installation would have entailed drilling both the Guadalupe River and the adjacent Victoria Barge Canal due to the associated geometry with a drilled crossing. This meant that the exit location of the directional drill would have to be located on the other side of the canal, thus requiring a 3,000-foot drilled crossing along with an additional 1,200 feet of conventional pipe lay at each end to accommodate the respective tie-ins on both sides of the river." The river would continue to exhibit lateral bank migration after drilling the replacement crossing.

Bordelon also pointed out that access to the job site was poor. The exposed pipeline section on the Guadalupe River was located approximately 2 1/2 miles down a gravel road and another 3 1/2 miles down the grassy pipeline right-of-way.

To alleviate the access issue, Invista, which operates a nearby chemical plant, agreed to allow crews to work on their property and construct a temporary bridge across the Guadalupe to provide access to the eroded area on the river's west bank.

Contract award

Bordelon said the turnkey contract to provide erosion stabilization and protect the pipeline, valued at $1 million, was awarded to Submar Inc. of Houma, LA.

Prior to the contract award, Submar had completed the initial design phase of the project. After award of the contract, Submar finalized all design and construction details. This work, carried out at the company's Houma office, was under the direction of John Lewis, who also served as project manager during the construction phase.

According to Lewis, Submar's crews mobilized to the job site in late May. Also in the preliminary design phase, he had visited Medina Crushed Stone in Medina, TX, to evaluate and select the 12,000 tons of rock that would be required for the project.

Continuing, he noted that the first work on the project focused on construction of the 240-foot long, 30-foot wide temporary bridge over the Guadalupe River.

Lewis explained that the bridge was constructed using the Flexifloat construction system provided by Robishaw Engineering of Houston.

In describing the system, he said it used pre-fabricated modular steel barge and auxiliary attachments that can be connected into larger assemblies of various sizes and shapes.

"The barges are designed to accommodate varying water depths, which turned out to be a real advantage," he said. "At the start of the project, the water depth was running at about 16 feet. By the time the project was completed, the river level had fallen to 13 feet and some of the barges were sitting in mud."

Lewis noted that four spuds were used to secure the bridge in place. At each end, board road ramps were constructed to accommodate the slope of the river bank which reached heights of up to 28 feet.

Without the temporary bridge, Submar's crews would have had to construct a significant amount of board roads and the dump truck drivers charged with delivering the 12,000 tons of rock for the project would have had to contend with the tough logistics that the project location presented with one-way truck access. In addition, building a board road for the entire 3 1/2 mile site access was cost prohibitive.

Instead, the drivers were able to use an existing gravel access road to the floating bridge to access the site and deliver an estimated 550 dump truck loads of rock to the job site. Later, when construction was under way, Submar's crews were able to use front-end loaders to make final placement of the rock.

Bendway weirs

In describing other major aspects of the job, Lewis said the three bendway weirs constructed upstream of the pipeline posed special challenges.

Construction started by cutting a ramp from the bank sloped down to the river, which was about 28 feet below the high bank. The water level at the time was running around 13 feet.

Constructed purely of rock, the largest of the three weirs was approximately 35-feet long, 14-feet tall and 26-feet wide at the base. The two weirs constructed upstream and downstream were slightly smaller, measuring about 30 feet in overall length.

Lewis noted that because of the soft bottom conditions, the weirs were constructed about two feet below mean low water level. This works out to a height of about 11 feet, with about three feet of base below the mud line.

He said, "Alignment of the weirs is important to control the flow of the river. In this case, the deepest part of the channel was right up against the right descending bank. Our goal was to direct the water off the toe and right descending bank out to the end of the weir."

The Submar project manager pointed out that while construction of the three weirs required about 8,000 tons of rock, not all of this went into the river. Along with building the weirs, three keys were built both up into the bank and onto the bank to prevent flanking of the river behind the weir.

Lewis noted that if the weirs are not keyed into the bank properly, and the river rises to a level that it starts to erode the bank, it could go behind the weir and create an island in the river. In this case, some of the keys reached lengths of 30 feet.

Stone toe protection

Another significant structure completed on the project was the Longitudinal Peaked Stone Toe Protection (L.P.S.T.P.) system.

Lewis indicated that construction started by placing rock at the toe on the western bank of the river and building a two-foot foundation below the mud line and then building up the rock until it reached the water elevation.

Next, two layers of large 24- to 36-inch diameter rip rap was placed parallel to the bank. The rock was installed in such a way as to form two peaks that reached an elevation equal to the pipeline as it sloped down into the river. In between the rip rap, four-inch diameter rock was placed until the peaks reach near the pipeline. Next, pea gravel and sand was placed around the pipeline to provide support, gain compaction, and fill the voids between the rip rap.

Over the rock, construction crews used a crane to place Submar-manufactured Revetment Mats to span over the top of the pipe. Each mat section is 8-feet wide, 20-feet long and 4 1/2 inches thick weighing 6,200 pounds. Each mat section is interconnected with binding straps to make a final product a continuous mat structure.

Submar installed a total of 45 pre-cast concrete revetment mats. Lewis said the mats were placed from the top of the 28-foot high river bank down to where the pipeline had four feet of cover in the river.

To secure the mats in place, they were trenched into the rock both in the water and on the bank.

Rockshield

Lewis noted that Submar's three-man dive team was responsible for establishing the length of the pipeline exposure, which proved to be about 90 feet from the overbend of the pipe projecting into the water. There was an additional 30 feet of pipe exposed from the overbend to the bank.

The dive team's responsibility also included wrapping the pipe, both above and below the water line, with a spaghetti type PVC, or Tuff N Nuff Rockshield, to prevent any abrasive materials from contacting the protective pipeline coating.

Shortly after the Tuff N Nuff Rockshield was secured to the pipe, crews followed with rock placement until it was level with the top of pipe. At this point, Submar Revetment mats were placed directly over the pipeline and then an excavator was used to trench the mats into the rock to secure them in place.

In describing other major job activities, Lewis said rock was used to provide a work area out into the water. "We did not dam up the river, we only constructed a small rock island to allow the front-end loaders and excavators to have a relatively dry place to work and gain the reach needed to place the large amounts of rock required on the project."

Permitting

It took Williams 7 1/2 months to receive a U.S. Army Corps of Engineers permit to stabilize the exposed pipeline. Williams demonstrated their commitment to maintaining pipeline integrity by initiating the start of construction within three weeks after receipt of permits. In addition, Williams felt that this was a long-term solution to protect the pipeline by mitigating any future stream erosion at this location.

Final site restoration included re-planting native hardwood trees set back from the bank and planting willow posts into the rock on the embankment. Willow trees will establish into the embankment and create a natural setting. This method re-establishes the bank and provides protection for the pipeline by putting in enough mass to resist the river's natural erosional forces. It is an alternative available to the pipeline operating company and it includes time-proven technology that the Army Corps of Engineers has used on the Mississippi River for approximately 50 years to maintain the commercial navigation of the river channel.

Conclusion

In conclusion, Bordelon said, "The restoration and stabilization project resulted in a stable bank and protection for the pipeline. The bendway weirs created a zone of calm waters which is beneficial to aquatic habitat. The landowner benefited by eliminating any future land loss at this location. This was a win-win situation for both the pipeline operating company and the landowner."
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Article Details
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Author:Tubb, Rita
Publication:Underground Construction
Geographic Code:1USA
Date:Nov 1, 2005
Words:1980
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