Beatrice DirectPipe® Trenchless Crossing Landfall
Eadon Consulting carried out the design work for a DirectPipe® crossing under a SSSI beach in northeast Scotland. Stockton Drilling carried out the crossing, which consisted of two parallel cable ducts to carry export cables from the Beatrice offshore wind farm in the Morray Firth, 50km to the north.
Key facts
Location
Moray Firth
Client
Beatrice Offshore Wind Farm
In Partnership with
Stockton Drilling
DirectPipe® is a trenchless crossing technology developed by Herrenknecht for installing pipes or ducts under obstacles. The pipe is fabricated in advance, and laid out on rollers in line with the crossing. A tunnel-boring machine (TBM) is connected to the front end of the pipe, and driven into the ground by a pipe thruster mounted at the surface near the entry point. The TBM can be steered in the ground to install the pipe in a shallow curve, similar to horizontal directional drilling (HDD). Unlike HDD, no reaming of the drilled hole is required, and no pullback stage, making it particularly well-suited to landfall applications where extensive marine temporary works can sometimes be needed.
Images courtesy of Stockton Drilling
Eadon Consulting carried out the tunnel and overbend profile designs, specified the pipe thickness to avoid local buckling, and estimated installation forces. We also carried out other design work relating to the project, including design of a closure at the sea-ward end to permit subsequent cable installation, and design of the anchorage of the pipe thruster to its foundation.
A particular challenge on the job was the design of the overbend profile, i.e. the shape of the pipe as it transitions from lying horizontally on the ground to a downward angle into the tunnel. Too tight a curve and the rollers or pipe can be overloaded, whereas a shallower curve leads to much more extensive excavations or support steelwork. This is much more problematic with larger pipe diameters because they are much stiffer. Sophisticated calculation techniques were used to quickly analyse the roller loads and pipe stresses at every stage of construction, allowing us to optimise the geometry. This allowed readily available rollers to be used, avoiding delay to the programme.
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