Michels Trenchless is using an emerging, specialised trenchless construction technology to build the critical shore approaches essential to connecting offshore transmission lines to onshore power grids.
As Australia accelerates development of offshore wind projects as part of the country’s strategy to decarbonise its energy sector, Michels is bringing to Australia the engineering and construction experience it developed on some of the largest offshore wind projects in the United States.
Although horizontal directional drilling (HDD) is often the go-to method for shore approaches, Michels has found that the Direct Pipe® installation method can be very effective in certain situations. Particularly well-suited for areas with challenging ground conditions, such as mixed soils, high groundwater levels, or unstable geology, Direct Pipe combines a steerable microtunnel boring machine and a powerful thruster to install a pipeline on an accurate alignment and grade. With closed-face excavation and a continuously supported borehole, Direct Pipe is effective for long, large-diameter pipeline installations with minimal environmental impact.
A recently completed project in the United States demonstrated how trenchless construction methods can be combined to construct landfalls and trenchless crossings; in this case, electrical conduits for a 176-turbine (2.6-gigawatt) wind farm located 24 nautical miles off the coast of Virginia.
During design of the Coastal Virginia Offshore Wind projects in Virginia, Michels recommended using Direct Pipe for the nine landfalls, all of which were mined from the beach out into the floor of the Atlantic Ocean. Although HDD would typically be used for the application, Direct Pipe allowed the client to move forward with the project with minimal environmental risk without impacting the capacity of the project. The result was the largest number of Direct Pipe installations completed at one time anywhere in the world. Additionally, 10 HDDs were performed to cross sensitive areas on land to install electrical conduits to house the new power cables.
The nine Direct Pipe landfall installations placed 42-inch-diameter steel casing from the beach to the ocean floor. Then, 36-inch HDPE conduit was pulled inside each steel conduit. Approximately 17,000 feet of HDPE pipe was placed inside the steel casing between the nine installations. Next, the steel casing was extracted, leaving the HDPE conduit in place.
Although it ended successfully, the project was not without challenges. The landfall installation included excavation of the entry pit to set up the Direct Pipe equipment. When Michels started the excavation for construction of the entry pit, it was discovered that the water table was approximately seven feet higher than previously anticipated. This led to a change in the dewatering method. Additionally, the test on the groundwater revealed the presence of per- and polyfluoroalkyl substances (PFAS). The project team resolved the issue by building an on-site water treatment plant to treat the groundwater before disposal. While this led to additional project cost and impacts on the schedule, the project team was able to make up time by completing the landfall installations in shorter durations than scheduled.
For more information, visit michelstrenchless.com.au
This article was featured in the August edition of Trenchless Australasia.
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