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Screw anchors have been recognised as an innovative solution to support offshore jacket structures and floating systems, due to their low-noise installation and potential enhanced uplift capacity. Results published in the literature have shown that for both fixed and floating applications, the tension capacity is critical for design, but may be poorly predicted by current empirical design approaches. These methods also do not capture the load–displacement behaviour, which is critical for quantifying performance under working loads. In this paper, a finite element methodology is developed to predict the full tensile load–displacement response of shallow screw anchors installed in sand for practical use, incorporating the effects of a pitch-matched installation. The methodology is based on a two-step process. An initial simulation, based on wished-in-place conditions, enables the identification of the failure mechanism as well as the shear strain distribution at failure. A second simulation refines the anchor capacity using soil–soil interface finite elements along the failure surface identified previously and also models installation through successive loading–unloading of the screw anchor at different embedment depths. The methodology is validated against previously published centrifuge test results. This simplified numerical approach has been derived to approximate the results in a single step. more...

Due to their low-noise installation and relatively large axial capacity, screw piles have been proposed as an alternative foundation solution in dense sand for offshore renewable energy applications in deeper water. For this to occur, significant upscaling of onshore dimensions is required. Furthermore, the effects of certain geometric features on installation requirements are still not well understood. In this work, using the three-dimensional discrete-element method, the effects of base geometry, shaft diameter and helix pitch were investigated by simulating the full installation process prior to conducting axial compression and tension tests. The results of the investigation showed it is possible to optimise the geometry of the screw pile to reduce installation requirements, in terms of both vertical installation force (up to 61%) and installation torque (up to 39%), without reducing the axial capacity of the pile significantly. more...

Screw piles can be used as foundations for offshore energy applications, thanks to their silent mode of installation and considerable uplift capacity, although a significant upscaling of onshore dimensions is necessary. However, the crowd (vertical) force necessary to install upscaled screw piles was previously shown to be far too great for practical installation. Although guidance recommends that the pile's vertical displacement must be one helix pitch (helix height) per each pile revolution, it is shown in this paper that a lower vertical displacement per revolution can significantly reduce the necessary crowd force during installation or even generate some pull-in. In addition, it was shown that the uplift stiffness and capacity of the pile were enhanced by this installation process, at a shallow (relative) depth in sand. This paper gathers 19 centrifuge tests, with varying screw pile geometries (shaft diameter, base shape), sand relative density and advancement rates. A predictive framework for the pull-in potential of a given pile geometry was proposed to assess its ability to be installed with a reduced crowd force. more...

Existing guidance on the installation of screw piles suggest that they should be installed in a pitch-matched manner to avoid disturbance to the soil that may have a detrimental effect on the in-service performance of the pile. Recent insights from centrifuge modelling have shown that installing screw piles in this way requires large vertical compressive (or crowd) forces, which is inconsistent with the common assumption that screw piles pull themselves into the ground requiring minimal vertical compressive force. In this paper, through the use of the discrete element method (DEM), the effects of advancement ratio, i.e., the ratio between the vertical displacement per rotation to the geometric pitch of the helix of the screw pile helix, on the installation resistance and in-service capacity of a screw pile is investigated. The findings are further used to assess the applicability of empirical torque capacity correlation factors for large diameter screw piles. The results of the investigation show that it is possible to reduce the required vertical compressive installation force by 96% by reducing the advancement ratio and that although over-flighting a screw pile can decrease the subsequent compressive capacity, it appears to increase the tensile capacity significantly. more...

A set of simple finite-element modelling procedures that can be used to estimate the load–settlement behaviour of continuous helical displacement (CHD) piles in sand is presented. The approach makes use of a stress- and strain-dependent non-linear soil model that can be parameterised using basic soil data that can be determined through routine site investigation. The procedures are validated against a database of physical model tests (reported in a companion paper), where they are shown to be suitable for estimating the load–settlement behaviour of CHD piles within the serviceability range. In this way they are complementary to the analytical method reported in the companion paper for estimating the ultimate capacity of a CHD pile. In this paper, the finite-element method and analytical model are applied to four historical load tests on CHD piles conducted at three different sand sites. The modelling is further validated and used to discuss potential savings in pile material and therefore cost due to additional confidence in performance determination at both ultimate and serviceability limit states. more...

The continuous helical displacement (CHD) pile is an auger displacement pile developed in the UK. It has performance characteristics of both displacement and non-displacement piles due to the way in which it is installed. Based on field experience, it has been shown that the load–settlement performance of CHD piles installed in sand exceeds the current design predictions based upon conservative effective pile diameter and design parameters associated with auger bored or continuous flight auger in situ piles. In an effort to gain a greater understanding of the performance of CHD piles in sand compared with more conventional piling techniques, a programme of physical model pile testing (reported in this paper) and associated finite-element modelling (reported in a companion paper) was undertaken. The model testing programme established that greater shaft resistance may be developed for the piles than had originally been considered. Based upon the results of the model testing, recommendations for more appropriate approaches to the selection of end bearing and shaft resistance factors are made to predict ultimate load capacity in sand. more...