Helical piles as a novel foundation system for offshore piled facilities

Driven piles are used to support the majority of offshore oil and gas structures with jacket structures supported by axially loaded piles still remaining the most common form of fixed offshore platform. Piles for conventional platforms are normally driven open-ended steel piles. This paper presents general considerations regarding the use of helical (or screw) piles as a novel foundation system for offshore structures. This type of pile has many advantages over driven piles, these include; ease of installation, the ability to remove using a reverse torque technique and cost savings due to much smaller installation vessel requirements. Other uses of helical piles are as support system for anchoring systems used for floating structures (e.g. tension leg platforms, SPARS, semi-submersible production platforms). To investigate their potential a joint research project was performed between BAUER Maschinen GmbH and University College Dublin (UCD). This project to date has involved small-scale laboratory tests, full-scale field tests and finite element modelling. This paper reports the results from finite element (FE) analyses that investigated the effects of the pile geometry, soil stiffness and soil friction angle on the compression and tension capacity of a helical pile in sand. In total, 48 models were created using Plaxis 3D. Axial tension and compression load tests were performed for piles with an embedment depth of 6m. In addition to the FE analyses, a series of laboratory model tests were performed in a large sample of sand at UCD. The primary purpose of these tests was to study the effect of installation method on the capacity of a helical pile. Wished-In-Place (WIP) piles (formed by placing the pile in the testing chamber prior to sample preparation), with helical helix-to-shaft ratios of 1.2 and 2.5 were load tested and their capacity was compared to the piles installed using the torque method. The results of the lab tests suggest that torque installation results in compaction of the sand around the pile. As a result the uplift capacity of the pile of the torque installed pile was much higher than the WIP pile. The wall thickness depending on the maximum applied torque and the permissible torsion stress of the steel is also briefly discussed.