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Intersection Beneath The Mississippi: 7,700-Foot Bore Generates Many Challenges For Ranger
Long, deep directional drilling installations are never routine – each project is different with its own set of challenges.
Louisiana contractor Digco Utility Construction L.P, dba Ranger Field Services (a Quanta company) recently made a complex, 7,700-foot crossing beneath the Mississippi River. The project was part of a 26-mile pipeline to transport brine from a salt dome in Bayou Choctaw to a new chlor-alkali plant on the opposite side of the river at Geismar, approximately 60 miles upriver from New Orleans.
The project owner is Boardwalk Louisiana Midstream. The chlor-alkali plant is owned by Westlake Chemical and is projected to produce 250,000 electrochemical units (ECU's) annually to generate chlorine.
The 12-inch pipe was installed 150-feet under the river by horizontal directional drilling (HDD). Several factors set this river crossing apart from more typical long-pipe installations beneath water.
Two HDD machines made pilot holes from opposite sides of the river which intersected under the river. The bore from the entry side was 4,071 feet and the exit side bore was 3,629 feet.
“Two drill rigs were used because we were required by the Corps of Engineers to stay below a predetermined annulus pressure while boring beneath the two levees,” said Boyd Simon, P.E., Ranger division manager. “Pressure was constantly monitored by a downhole pressure tool to ensure we did not go above the required allowable pressure and to alert us if any inadvertent returns were experienced inside the Corps ‘Monitor Zone’ of the levees. If that occurred, grout curtains would have had to be installed. This would have added a significant cost to the project.”
Two Vermeer D330x5000 drill rigs were used and set up on opposite sides of the river. These machines are powered by 426-horsepower engines and develop 330,000 pounds of pullback and rotary torque to 50,000 foot-pounds.
“We used an Adtech mud motor on the entry side rig because of harder ground conditions and due to the fact we did not want to push on the drill string too hard,” Simon said. “The exit rig had softer ground conditions and this rig did not need to drill the entire bore length.”
Set-up space on each side of the river was 150 by 200 feet. On the entry side, the rig was positioned about 1,700 feet from the water’s edge. The exit drill was staged about 1,500 feet from the river.
Surface conditions consisted of soft gumbo with cemented sand and traces of gravel below the surface. GeoEngineers conducted soil sampling and design analysis for the project.