Centrifuge and DEM modelling of helical piles for offshore jacket foundations in sand

Two of the main challenges for the design of offshore wind turbine foundations are the need for silent installation techniques and for large tensile capacity. Helical piles, composed of one or several helices connected to a steel shaft can meet both challenges. However, the force and torque installation requirements necessary for offshore applications are large and must be reduced. By using centrifuge and Discrete Element Method (DEM) modelling this work demonstrates that the geometry and installation of helical piles can be optimised to reduce those installation requirements, while maintaining sufficient capacity. An original pile design made up of two shafts of increasing diameter connected by a transition piece, was installed at different penetration rates in sand. Results show that pile overflighting (low penetration rate per pile rotation) reduces the necessary crowd force to an acceptable magnitude for practical applications. The DEM simulations were validated against the centrifuge tests and revealed the installation mechanism. It was shown that a transition piece, composed of an internal helix leads to lower installation requirements than a conical transition. DEM simulations were also used to show that the design of the internal helix can be modified to further reduce the necessary installation torque.