The effect of installation advancement ratio on interaction between helices of screw piles for offshore renewable energy

Upscaled screw piles have been proposed as a novel silent foundation solution for both offshore jacket structures and anchoring in various renewable energy applications. Replacing conventional pitch-matched installation approaches by over-flighted installation approaches has been proposed for single-helix screw piles to reduce installation vertical crowd force and torque whilst enhancing in-service tensile capacity and stiffness. This study, using DEM modelling, extends the scope of investigation to consider multi-helix screw piles and investigates installation and tensile in-service response, addressing the effect of helix spacing and installation advancement ratio together. The findings suggest that a pile featuring smaller helix spacing may necessitate less vertical force and torque for installation than a single-helix pile if over-flighted installation approaches are used, although the first (bottom) helix needs to be ‘deeply’ embedded to avoid the reduction of tensile capacity induced by additional helices. This differs from piles installed by pitch-matched approaches where additional helices typically increase installation force and torque. DEM micromechanical analyses help unveil the mechanisms behind the macroscopic response of such screw piles.