Screw pile design optimisation under tension in sand

Many applications in offshore engineering, such as floating or jacket-founded wind turbines or wave energy converters, require a significant uplift capacity of their foundations to be kept in place. Straight-shafted or suction piles in sands have a limited uplift capacity as they resist by friction only. In contrast, screw piles or screw anchors are a promising solution which provides a similar capacity to plate anchors and does not generate disturbance for marine mammals (e.g. from pile driving operations). The optimisation of the screw pile design does not rely only on the geotechnical assessment of the uplift capacity based on soil strength, but also on operational (installation requirements) and structural (helix bending, core section stress, limiting steel plate thick-ness) constraints. This paper develops a methodology for the design optimisation of screw piles under pure ten-sion in sand, incorporating all of these constraints, based on simplified analytical or semi-analytical approaches. The results show that the uplift capacity provided by an optimised screw pile is able to meet the needs of the offshore industry, across a range of soil densities and different applications (jacket foundation pile or tension leg platform anchor), providing that adequate installation plant could be dev

Centrifuge dataset for screw pile installation and uplift

The dataset describes the installation and uplift loading of a screw piles in a sand bed prepared by dry pluviation. The full details of the procedure can be found in the related paper. Control of screw pile installation to optimise performance for offshore energy applications (2021). Cerfontaine, B.*, Brown, M.J., Knappett, J.A., Davidson, C., Sharif, Y.U., Huisman, M., Ottolini, M. , Ball, J. Géotechnique The different phases of each test are described in Figure 1. The data sampling rate is equal to 10ms. 1. The centrifuge is spun up to the final g-level (50g at mid-depth of the pile). 2. A period of rest (few minutes). 3. Installation of the pile at a given advancement ratio (AR), defined as the vertical displacement of the pile per rotation, normalised by the helix pitch (helix height, measured at mid-plate). The rotation rate is fixed at 3RPM and the vertical rate depends on the AR. 4. The torque motor is disengaged (torque is released) and another rest period is allowed. 5. The pile is uplifted of 10mm at a rate of 1mm/min. 6. (Optional) The pile is unscrewed up to the surface. 7. The centrifuge is spun down. The measured data are given at model scale and their units indicated between brackets. 1. The vertical position [mm] of the pile, whose tip is initially one or two mm from the sand surface. The position is measured from the top and position downwards. 2. The vertical force [kN] at the top of the pile, which is positive downwards (pile in compression). 3. The torque [Nm] measured at the top of the pile, which is positive clockwise. There is an offset for each measured data. The offset considered in the paper to zero the different measurements is the value measured at the end of the first rest period. A summary of all the tests main parameters is given in the summary Excel file

Optimisation of screw anchor lateral capacity in sand for offshore renewable energy applications

Screw piles or screw anchors are a promising solution to anchor floating offshore renewable energy devices, such as wind turbines or tidal turbines. The installation generates limited noise (driven piles are noisy) and can be undertaken in all soil conditions. Although they are mainly used for their large uplift capacities, screw anchors can also be designed to provide significant lateral resistance. The optimisation of screw anchor design does not rely only on the geotechnical assessment of the uplift capacity based on soil strength, but also on operational (installation requirements) and structural (helix bending, core section stress, limiting steel plate thickness) constraints. This paper develops a methodology for the design optimisation of screw anchors under lateral loading in dense sand, incorporating all of these constraints, based on simplified analytical or semi-analytical approaches. The results show that it is possible to optimise the anchor design, maximising the anchor lateral capacity, whilst minimising the anchor weight. The maximum embedment depth and then the anchor capacity is mainly limited by the maximum torque available during installation and the short-pile to long-pile failure mechanism transition respectively

Optimised design of screw anchors in tension in sand for renewable energy applications

The offshore deployment of floating offshore structures such as wind turbines or wave energy converters is expected to strongly increase during the next decade. 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 uplift capacity while their installation generates far less noise and vibrations than pile driving. This paper incorporates within a single procedure geotechnical and structural constraints related to the installation and operation of screw anchors 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 representative case study. Results show that single screw anchors are more efficient (e.g. shorter and lighter) than driven piles to sustain tension loading. A sufficient capacity could be developed to address the needs of wave energy converters. However, tension requirements for tension-leg platform wind turbines would probably require groups of anchors

Effects of installation advancement ratio on cyclic uplift response of a single-helix screw pile:Experimental and numerical investigation in sand

A screw pile consists of one or several helices connected on a central straight shaft or core. These kinds of piles are widely used onshore and typically installed by applying torque at the pile head with additional vertical compressive force (also referred to as crowd force) if required. Current standards and industrial guidelines recommend that installation follows the pitch-matched approach i.e. advancement ratio AR = 1.0 [1], which means that the pile vertical penetration for each rotation equals the helix pitch, because it is suggested that this leads to reduced installation disturbanc