A green foundation for offshore wind energy - helical piles

Offshore wind energy has been explored as one of the sustainable energy forms for nearly 30 years, since the first offshore wind farm installed in Denmark in 1991. So far, monopile foundations are still the major foundation type in waters up to 30 m deep. Monopiles, or open steel pipe piles in general, are often installed by driving or vibration. These installation methods inevitably generate noise and vibration waves that can be harmful to marine lives. Helical piles have been used widely as the foundations of transmission towers and pipelines to resist pullout loadings. Helical anchors consist of one or more helical-shaped circular plates welded to a central steel shaft. During installation, they are screwed into the soil by applying torque to the shaft. Due to the torque installation, the installation noise and vibration can be greatly minimised. The current design guidelines of helical anchors are for small diameter shaft that is 0.5 m or less and the offshore monopiles can have their diameters up to 11 m.

This paper reports the initial study of model multi-plate helical piles installed in a geotechnical centrifuge. The helical piles are rotationally installed under an elevated gravity in normally consolidated kaolin clay. The contribution of the shaft resistance as well as the individual plate bearing capacities are measured with attached combined axial and torque strain gauges. The tensile capacity of the piles with three helical plates with prototype diameter of 2 m each are studied with respect to post installation consolidation hardening. It is shown that the tensile uplift resistance increases significantly with consolidation time and its uplift capacity is greater than that of an equivalent driven pile.