August 2015, Vol. 70, No.8
Features
Laney Completes Record-setting Direct Pipe Installation
In 2013, due to the surge in domestic natural gas production and abundant U.S. supplies, Houston-based Cheniere Energy made a prudent business decision. Recognizing the economic opportunity that could be derived from exporting (versus importing) natural gas, Cheniere Energy embarked on a major upgrade to the company’s Port Arthur, TX, liquefied natural gas (LNG) refinery. Originally built as an import facility, Cheniere Energy made a significant investment to repurpose the facility for exporting liquefied natural gas.
Because of the process Cheniere Energy employs to liquefy natural gas for the export market, the refinery would require very large amounts of potable water – a dramatic increase in the volume of water used previously.
To accomplish this, Cheniere Energy contracted with the city of Port Arthur to provide potable water at two points of service in Texas. At the recommendation of the city, Port Arthur-based engineering firm Arceneaux Wilson & Cole LLC was hired by Cheniere Energy to design a substantial water delivery infrastructure upgrade program for the city. The Port Arthur/Cheniere Energy facility pipeline was among several projects as a component of a complex, multi-faceted plan devised by Arceneaux Wilson & Cole to build more than 25 miles of transmission mains into an elevated storage tank and pump station for the city. The collection of projects was designed to double the capacity of the Port Arthur Water Treatment Plant from 20 million gallons to 40 million gallons per day.
Spring, Texas-based Laney Directional Drilling was hired to install a 36-inch HDPE water pipeline beneath the environmentally sensitive Sabine Neches Waterway and a critical hurricane flood protection levee near Port Arthur to deliver the additional volume of water required to the refinery.
Specializing in trenchless technology, Laney is a leading global horizontal directional drilling (HDD) company and the only U.S. HDD company providing integrated engineering, design and construction services.
Best solution
After carefully evaluating several factors that would come into play during the boring process, Laney selected the Direct Pipe installation method.
“Direct Pipe is a relatively new trenchless technology wherein a microtunnel boring machine and jacking pipe is installed in a curved manner with a Direct Pipe Thruster machine providing the motive force,” says Jwala Raj Sharma, Ph.D., P.E., project engineer with Laney Directional Drilling. “Compared to horizontal directional drilling (HDD), relatively low pressures can be achieved during the Direct Pipe construction process. It would have been very difficult to guarantee the very strict bore pressure tolerance required by the Army Corp of Engineers for the Sabine-Neches Ship Channel and hurricane protection system crossing using HDD.”
GeoEngineers, an international environmental engineering firm based in Seattle, WA, provided geotechnical, trenchless engineering and construction observation services. To design and obtain agency permission for the Direct Pipe crossing, GeoEngineers used its established Direct Pipe design methodology and program to develop a new engineering pressure model and pressure monitoring methodology specific to levees.
Ron Arceneaux P.E., PLS, partner and corporate secretary of Arceneaux Wilson & Cole, explains the main factors that led to the recommendation to employ the innovative Direct Pipe installation approach in more depth.
“This was the first Direct Pipe crossing permitted by the U.S. Army Corps of Engineers (USACE) to cross a USACE-regulated levee,” Arceneaux says. “There was a significant factor of safety pressure tolerance established by the Corps for crossing the hurricane protection levee. HDD was initially considered but was discounted due to relatively low soil formation limit pressures that indicated a higher than acceptable risk of hydraulic facture beneath the hurricane protection levee. With USACE guidance, we developed a design method for evaluating hydraulic facture risk for Direct Pipe installations that proved effective and successful.”
The pressure tolerance, which was not obtainable using HDD, had to be kept low because hydraulic fracture of soil during construction can lead to degradation of the foundation soil beneath the levee. This in turn can cause ground surface settlement, bearing capacity failure, undesired seepage flow path creation and other undesirable or even disastrous consequences.
Other significant factors cited by Arceneaux include the overall length and geometry of the Direct Pipe bore path (vertical, horizontal and combined curves required to avoid existing infrastructure totaling 3,457 feet in length and the large bore diameter required to accommodate the 48-inch diameter steel casing), confined workspace for preparing the launch pit, pipe fabrication space limitations and numerous agency reviews that had to be coordinated and consolidated to allow construction to occur.
“The predominantly clay soils also factored into the decision,” Sharma says. “It would have been difficult to achieve a factor of safety greater than two using HDD when crossing that kind of soil formation. It was possible, however, to achieve that factor of safety with Direct Pipe installation technology because the approach allowed us to maintain low fluid pressures in the soil formations during installation.”
Process
The innovative Direct Pipe technology combines the benefits of microtunneling and HDD drilling techniques while allowing for simultaneous borehole excavation and pipe installation in one step. The method can be applied to nearly any ground condition, making it an option for any installation crossing.
A small launch pit is created from which a microtunneling machine can begin excavation. The machine uses a slurry circuit to pump excavated material out of the pipeline. The pipeline is laid out on a series of rollers and is pushed through the borehole by a pipe thruster in varying increments. Throughout the excavation process, the tunnel face is controlled safely by slurry-supported tunneling technology. The single pass Direct Pipe process uses a steerable tunnel boring machine-cutting head to tunnel and push the pipe into place simultaneously, filling the void as it progresses.
“The Direct Pipe method allowed us to create the same type of bore path for the pipe as HDD, but with greatly reduced pressure at the cutter head because the cutter actually pulls the casing pipe through the strata,” Arceneaux says. “All of the hydraulic lines that are used for delivering drill fluids, as well as communication lines for directing the cutter head, are all contained within the casing pipe.”
The microtunneling machine was outfitted with a 52-inch soft material cutter head while sensors mounted directly to the machine continuously monitored pressures along the 90-foot deep bore path. After launching the Herrenknecht 750-metric ton Pipe Thruster, it took 22 days for Laney Directional Drilling crews to complete the 3,505-foot bore. The installation – completed with no significant pressure exceedances or delays – is a Unites States Direct Pipe record for the longest continuous bore using this innovative technology.
“Really, the only thing that slowed us down somewhat was all the mid-welds we had to complete along the way,” says Sharma. “Due to the limited work space, we didn’t have enough room in the staging area to string the entire length of the 36-inch HDPE pipe so we had to complete mid-welds every 400 feet. Otherwise, we didn’t have any equipment failures or significant delays. Most importantly, using Direct Pipe we were able to maintain the strict pressure tolerances required by the Army Corps of Engineers for the project. It’s a marvelous technology that hasn’t been done a whole lot in the U.S., but it’s certainly catching on.
“We are very proud of the record-setting accomplishment, but more so, we take great satisfaction knowing we were able to exceed the expectations of the city of Port Arthur and Cheniere Energy. We were all gathered around the exit target when the cutter head emerged, just inches of the target X we’d painted on the surface and marking our success.”
For More Information:
Laney Directional Drilling
(281) 540-6615, www.laneydrilling
Arceneaux Wilson & Cole LLC
(409) 724-7888, www.awceng.com
GeoEngineers
(206) 728‑2674, www.geoengineers.com
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