Two HDD outfalls for offshore wind energy near Cape Cod

In the winter of 2021-2022, Brownline USA surveyed two pilot bores for HDD outfalls in Barnstable, Massachusetts. On the Cape Cod peninsula, cables were brought to shore for the first commercial-scale offshore wind farm in the United States.

The Vineyard Wind 1 project involves 62 wind turbines to be constructed in the Atlantic Ocean, south of the islands of Martha’s Vineyard and Nantucket. When completed, the wind farm will generate green energy for over 400,000 homes.

In order to connect the offshore turbines with the mainland power grid, contractor Melcar Group carried out two HDD outfalls on Cape Cod. The contractor brought in Brownline USA to navigate the planned bores using the Drillguide Gyro Steering Tool.

Difficult formation

Before drilling, 30 m / 100 ft of casing was installed to protect the beach at the entry point, a popular summer vacation spot. The soil conditions that were encountered here were a challenging mix of silt, sand, and gravel, with also boulders larger than 40 in / 1 m in diameter.

In December 2021, drilling started for the first HDD. While this bore was in progress, a second rig was installed and work was started on drilling the second HDD, roughly 20 m / 65 ft next to the first. Both bores were around 425 m / 1395 ft long and reached a depth of 35 ft / 11 m below the sea floor.

Brownline’s Pressure-While-Drilling (PWD) service enabled a continuous monitoring of downhole pressures during drilling. This helped prevent any inadvertent mud returns during the project, despite the challenging mixed soils.

The HDDs punched out within tolerances on the sea floor, 4.5 m / 15 ft below water level. With the accuracy of our Drillguide GST, the pilot bores were completed safely and efficiently.

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