Centrifuge Testing of Large Screw Pile Geometries for Offshore Applications

Screw piles have been recognised as an innovative solution to support offshore jacket structures or mooring lines for floating platforms (e.g. wind turbines, wave energy converters). Their main advantage is a low noise/low vibration installation process and a potentially enhanced tensile capacity. The two main challenges for design are the prediction of installation requirements (torque and force) and the uplift capacity of the piles. This work investigates the effect of successive uplift phases at intermediate depths during the installation of a screw pile in dense sand, as a means of reducing the installation requirements. The modification of the uplift capacity at the final deepest depth and the installation requirements are considered in detail. Centrifuge tests have been undertaken at the University of Dundee, where an actuator has been developed to conduct installation and testing of model piles in-flight. Results show that the uplift capacity is reduced due to the previous uplift phases, but only by 10 to 20% of to a virgin installation to the final depth. The reduction in the installation requirements is of the same order of magnitude. A torque correlation factor was calculated for each test and was shown to be dependent on the relative embedment depth.