Far left:
Detail of a button-bit used for advancing the HDD bore through the marine sedimentary rock formation
Left:
HDD Drill rig tricone bit for advancing the pilot bore
To ensure that the wetlands were preserved, the team
utilized horizontal direct drilling (HDD) to bore beneath the zone. Instead of the traditional method of digging a trench and backfilling, half-mile long HDD bores connecting the two sites were drilled up to 40ft below ground. The method poses challenges. To drill a bore of such
length, drilling fluid had to be pumped into the bore under pressure to clear the cuttings. However, if the pressure levels were too high, the fluid would be forced to the surface, potentially harming surrounding flora and fauna. To mitigate this risk, the project team deployed an
intersect bore technique, drilling from both ends and meeting in the middle. This approach reduced the amount of pressure needed to remove the cuttings and ensured that the sensitive wetland environment was protected. By employing hydrofracture calculations provided
by Jacobs, The HDD Company maintained allowable annular pressures in the bore.
This involved adjusting pump pressures and refining
the drilling fluids recipe to ensure the pressures remained within a designated pressure envelope. Think of this envelope as a ‘speed limit’ for the bore, which is informed by real-time data from subsurface tooling. The HDD Company were able to adjust in real time based on the actual field conditions.
NAVIGATING THE OREGON COAST Oregon’s coastal geology created a second major challenge. The project team was tasked with positioning the conduit exit points beyond the surf zone, where they could be used to route the cables to the offshore test site. As noted, traditional trenching methods were ruled
out due to their potentially destructive impact on the marine environment and disturbance to the beach and this led the team to consider the HDD approach. However, Oregon’s coastal area presents notable challenges for HDD drilling, with several projects in the state being abandoned over the years due to specific
Left:
Construction of the underground vault in the parking lot of Driftwood Beach State Park used to transition the offshore conduits to onshore
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