King County owns and operates a combined sewer in Magnolia, a neighborhood in Seattle. The Magnolia CSO Conveyance Pipeline project was designed to capture excess flows during storm events and transfer them to a 1.5 million gal retention tank. In order to capture these flows, a pipeline was needed to redirect them from a steep gorge running from the high bluffs of the Magnolia neighborhood to the low point at Elliot Bay and carry them to storage on the other side of the bluff. Walsh Construction was awarded the project and elected to use HDD technology to install the pipeline. Mears Group was the subcontracted driller for the project and Underground Solutions Inc. supplied and assembled the Fusible PVC pipe used for the project.
The bore was constructed by setting up pits at both ends of the alignment, with 60-in. conductor casings driven on both sides of the alignment by auger-boring methods to provide for hillside slope stability and to seal the bore for drilling slurry management.
The pilot bore alignment was created with three horizontal curves, two vertical curves and an average slope from the upstream side to the downstream side of 1.8 percent. A blow-out preventer (BOP) was used at the conductor casing on the lower elevation end of the bore to maintain drilling slurry in the borehole during all phases of the drilling process. Pilot bore and reaming operations were completed with two 140,000-lb drill rigs. Pipe installation was completed with a 440,000-lb rig on the lower elevation side.
Houston Ship Channel
Laney Directional Drilling completed an 11,653-ft installation under the Houston Ship Channel, using the intersect method on a 12-in. steel propane pipeline along the crowded ExxonMobile corridor. NuStar Energy LP contracted Laney as a prime contractor for the HDD operations alone, while WHC Energy Services was the prime contractor for the mainline work.
Although the HDD operation was a North American record for length using 12-in. pipe, the bigger challenge was overcoming a 28-degree side bend in the middle of the intersect. The project was completed in just 23 days by acquiring a rare waiver from ExxonMobil to be allowed to drill 24 hours per day along the right of way. Laney’s portion of the project involved installing a section of pipeline through the ExxonMobil corridor, under the Houston Ship Channel and into the port of Baytown. The only trouble was finding space underground for the pipe.
There were more than 15 pipelines in the area where Laney was working. Laney faced challenges in designing the project. The engineering team had to sort through the existing pipelines to come up with a design to allow the drilling crew enough footprint to set up equipment. Laney has performed a number of challenging projects in its past, including long lengths, curves and intersects, but none had the combination of all three that led to the project being such a challenge. Laney deployed two of its in-house 750,000-lb HDD rigs for the project, using both rigs during the pilot bore and one rig to pull the product pipe into place. The project involved drilling at a total depth of 160 ft and 90 ft below the Houston Ship Channel, through fat clay, lean clay and silty sand.