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 which 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.

Deep foundations maybe used in a range of soil types where significant foundation resistance is required but their installation is often associated with disturbance due to noise and vibration. Greater restrictions on use in urban and offshore environments is now commonplace. Screw piles and rotary jacked straight shafted piles are two potential methods of silent piling that could be used as alternative foundation solution, but the effects of certain geometric and installation properties such as installation pitch i.e. the ratio between vertical displacement and rotation, on the required installation torque and force in sand are not well understood. In this paper the effects of installation pitch and base geometry on the installation requirements of a straight shafted pile are simulated in 3D using the discrete element method (DEM). The installation requirements of straight shafted piles into sand have been validated against centrifuge testing, in three different relative densities. The DEM shows reductions in installation force can be achieved by increasing the installation pitch or including a conical tip. An existing cone penetration test (CPT) based prediction method for installation requirements has been improved to include the effects of installation pitch and base geometry for rotary installed piles in sand.

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 has been developed to predict the full tensile load-displacement response of 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 previously identified 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. A simplified numerical approach has been derived to approximate the results in a single step.

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Control of screw pile installation to optimise performance for offshore energy applications. Géotechnique.

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, a significant upscaling of onshore dimensions is required. Furthermore, the effects of certain geometric features on installation requirements are still not well understood. In this paper, the effects of base geometry, shaft diameter and helix pitch were investigated using the three-dimensional discrete-element method 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.

The offshore deployment of floating offshore structures such as wind turbines or wave energy converters is expected to strongly increase during the next decade, to face the appetite for green energy sources. The growing size of these structures' dimensions, inducing very large mooring forces, makes the anchoring solution adopted a critical issue for the commercial success of floating marine energy farms. The upscaling of the screw anchor technology from onshore to the offshore environment has been recently proposed as an efficient way of providing a large tension capacity while their installation generates far less noise and vibrations than impact pile driving. Most of recent studies on screw anchors have focused on separated geotechnical problems such as their uplift capacity or installation requirements. This paper incorporates within a single procedure geotechnical and structural constraints to calculate the optimal anchor geometry able to maximise the uplift capacity available. Performance envelopes for screw anchors have been derived in a parametric study, covering a broad range of soil conditions as well as in a case study, representative of offshore conditions. Results show that single screw anchors are more efficient (e.g. shorter and lighter) than driven piles to sustain tension loading. The results presented in this study support the applicability of screw anchors to be used as part of the mooring system for wave energy converters. However, tension requirements for tension-leg platform wind turbines would probably require the use of group of anchors.

Screw piles are well suited foundations for offshore jacket structures, as they can be installed without significant underwater noise and have a large axial capacity. However, installation requirements for such large piles must be reduced to enable their installation in the field. This work combines geometry and installation optimisation to lower force and torque installation requirements. An original pile geometry, composed of a large diameter upper section connected to a smaller diameter lower section by a transition piece, was tested in a geotechnical beam centrifuge. The advancement ratio (AR), describing the relative vertical movement per pile rotation, was varied below the threshold usually recommended. Results show that a low AR reduces the pile penetration resistance and even generates some pull-in, while the torque remains almost unaffected. The torque is mainly associated with the upper section of the pile, which has a greater diameter to resist lateral loading in service. The pile capacity in tension generally increases as AR is reduced and reaches a maximum for AR = 0.5, while the compressive capacity reduces. It was shown that a simplified method can be used to estimate pile capacity, providing that some AR dependent reduction factors can be calculated or assumed.