Extreme urban crossing in the heart of Miami

Last summer, Brownline USA made a major contribution to a challenging trenchless project in the middle of Miami, FL. Installing underground power lines in such a busy urban area required absolute control and accuracy during the pilot bore.

The goal of this horizontal directional drilling was to install a 69 kV transmission line underground along a 1,200 m / 3,900 ft stretch. For a city such as Miami, installing utilities underground is important: it helps prevent outages when overhead power lines are damaged by storms or hurricanes over the Atlantic Ocean. This drilling was executed by contractor Laney Directional Drilling.

Absolute accuracy needed

The drill site was active 24 hours a day to complete the pilot hole. Because of the urban environment, the workspace at the entry side and the tolerance at the punch-out were extremely limited. There were also many existing underground utilities in this area, so absolute accuracy was a necessity for installing this cable.

The Drillguide Gyro Steering Tool is perfect for projects such as this because there is no risk of the surrounding infrastructure influencing the positional measurements. For an additional verification, the GPS track add-on was used as well.

Difficult underground conditions

Because of the varying soil layers, the pilot bore was completed alternating between a jetting assembly and mud motor. 52 m / 172 ft of casing was installed at the entry side to prevent the sandy layers from collapsing inward. Brownline’s Pressure-While-Drilling service was used to keep an eye on the downhole pressures. With these readings, it was concluded that the annular pressure needed to be reduced in order to continue drilling safely.

The design of this crossing was then changed from a single drill to an intersect. The annular pressure was reduced, which helped to prevent inadvertent returns from spilling out into the streets. Brownline USA contributed to the completion of this intersect drilling by providing the Radar add-on. This tool detects the distance between the two drill heads, which allows for a smooth joining of the two boreholes.

With the tools and services from Brownline USA, the challenges of this pilot bore were dealt with, and the crossing was completed.

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