Centrifuge modelling of screw piles for offshore wind energy foundations

Screw piles (helical piles) can provide a viable, cost-effective and low-noise installation alternative to increasing the size of existing foundation solutions (e.g. monopiles) to meet the demand for the advancement of offshore wind energy into deeper water. Significant upscaling of widely used onshore screw pile geometries will be required to meet the loading conditions of a jacket supported offshore wind turbine. This increase in size will lead to greater installation force and torque. This paper presents preliminary results from centrifuge tests investigating the requirements to install screw piles designed for an offshore wind energy application using specially developed equipment. Results indicate that the equipment is suitable to investigate these screw pile requirements and that significant force is required for such upscaled screw piles, with 19 MN vertical force and 7 MNm torque for the standard design. Optimisation of the screw pile geometry, reduced these forces by 29 and 11% for the vertical and rotational forces respectively

Development of an instrumented model pile

An instrumented model pile has been realized to study the displacement pile installation effects in sand in physical model tests. The system includes a model pile, instrumented with axial and horizontal contact stress sensors, and a corresponding calibration apparatus. The development of the instrumented model pile, including numerical analysis of the mechanical response during testing, and an optimization of the instrumentation to minimize thermal effects are described. The performance of this new model pile is demonstrated using calibration measurements and an example application in a physical model test at an elevated stress level in the geotechnical centrifuge

New method for full field measurement of pore water pressures

A cost effective method to measure pore water pressures in mixed granular media is described using 40 miniature MEMS pore pressure transducers. High accuracy in a single point is exchanged for lower accuracy full field measurements adjacent to the strongbox wall. The system is easily de-aired and calibrated due to the fact that the transducers are installed inside the strongbox wall. Additionally, the proof of concept test shows that the transducers are sufficiently accurate for problems with large pressure difference such as consolidation of clay while being subjected to elevated stress levels in the geotechnical centrifuge

A new test setup for studying sand behaviour inside an immersed tunnel joint gap

During inspections of several immersed tunnels in the Netherlands, damage of immersion joints has been observed. In some cases the Gina seal has moved inwards from its original location, and in other cases a permanent elongation of the entire tunnel structure has been measured. For both cases it has been hypothesised that a seasonal expansion and contraction of the tunnel elements allows sand to enter the joint gap between elements during winter, where it is compacted during summer, leading to an increasing amount of sand in the joint gap over the years. In order to study this mechanism and assess its impact, a 1:3 scale model joint gap has been designed and constructed. This setup can simulate expansion and contraction cycles of the joint and measure stresses in the joint gap and deformations of the Gina seal. First test results are presented here and show that compaction of the sand entering the joint gap indeed occurs and leads to the observed large inwardsdeformations of the Gina seals

Centrifuge modelling of screw piles for offshore wind energy foundations

Screw piles (helical piles) can provide a viable, cost-effective and low-noise installation alternative to increasing the size of existing foundation solutions (e.g. monopiles) to meet the demand for the advancement of offshore wind energy into deeper water. Significant upscaling of widely used onshore screw pile geometries will be required to meet the loading conditions of a jacket supported offshore wind turbine. This increase in size will lead to greater installation force and torque. This paper presents preliminary results from centrifuge tests investigating the requirements to install screw piles designed for an offshore wind energy application using specially developed equipment. Results indicate that the equipment is suitable to investigate these screw pile requirements and that significant force is required for such upscaled screw piles, with 19 MN vertical force and 7 MNm torque for the standard design. Optimisation of the screw pile geometry, reduced these forces by 29 and 11% for the vertical and rotational forces respectively