Small-diameter pile installation by the GBM Works Vibrojet®

Abstract

The offshore wind energy sector requires efficient and environmentally conscious installation of founda-tion piles. Impact-hammering generates significant noise pollution, requires mitigation measures, andcan cause pile fatigue, while vibro-hammers face limitations in achieving target depths in dense soils. This thesis investigates the technical feasibility of applying the GBM Works Vibrojet® – a new method combining vibro-hammering with water jetting near the end of the pile to fluidize internal soil and reduce friction for installing small-diameter piles (0.5-4 meters) used in jacket structures and floating windturbine foundations.

The study employs literature reviews, adapts existing soil resistance (SRD) and bearing capacity (API) models, and introduces a ”Vibrojet® potential reduction ratio” to quantify the resistance of the soil inside the pile, which can potentially be reduced to zero. It analyses various pile dimensions and soil conditions representative of the North Sea. Findings suggest the Vibrojet® can likely be scaled down for small-diameter piles, although submerged installation and the soil plugging effect, for piles with a diameter smaller than 1.5 meters, need further study. The potential reduction in the resistance of the soil in the pile is comparable to monopiles for unplugged small-diameter piles but significantly higher for plugged piles. Consequently, the impact of the Vibrojet® on the axial bearing capacity in comparison to impact-hammering is similar to that of monopiles (20-25% reduction) in the analysed non-uniform soil conditions for unplugged piles, but can be substantially higher for plugged piles, particularly shorter ones.

The study concludes that installation by the Vibrojet® has technical feasibility for small-diameter piles, offering significant soil resistance reduction potential, especially in plugged conditions. While the bearing capacity reduction for unplugged piles is manageable, the impact on plugged piles requires careful design consideration, particularly given the axial loading demands on jacket and mooring piles. Recommendations include using more advanced CPT-based models, conducting physical tests, and further investigating the precise effects of fluidization on soil properties and bearing capacity.