A finite element approach for determining the full load–displacement relationship of axially loaded shallow screw anchors, incorporating installation effects

Abstract

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.

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

“…Both load components increase with increasing uplift displacement, but the relative load sharing between the helix and the core decreases and increases respectively. A similar trend was observed for two other simulations (Cerfontaine et al, 2020), but not presented here) that were also validated against centrifuge data (an anchor at lower H/Dh in dense sand and medium-dense sand respectively), although the magnitudes of load carried by the helix (and core) were different at failure (75% of the total capacity was sustained by the helix in medium-dense sand).…”

“…To ensure the force equilibrium of the soil wedge, the shear stress distribution along the failure mechanism (τsoil) must be balanced by the internal normal distribution acting on the helix (σ'N) and shear stress distribution along the core (τc). A 2-step approximate numerical procedure (using the Finite Element software PLAXIS (2017)) has been previously established and validated against centrifuge tests to simulate the loaddisplacement relationships of screw anchors (Cerfontaine et al, 2020) under monotonic uplift loading, accounting for the effects of installation on the stress field within the soil. The behaviour of a particular anchor (helix diameter 1.7m, core diameter 0.88m, relative embedment ratio 7.4) embedded in dense sand (Relative Density, Dr = 84%) and described in detail in (Cerfontaine et al, 2020;Davidson et al, 2019) is given below as an example.…”

“…A 2-step approximate numerical procedure (using the Finite Element software PLAXIS (2017)) has been previously established and validated against centrifuge tests to simulate the loaddisplacement relationships of screw anchors (Cerfontaine et al, 2020) under monotonic uplift loading, accounting for the effects of installation on the stress field within the soil. The behaviour of a particular anchor (helix diameter 1.7m, core diameter 0.88m, relative embedment ratio 7.4) embedded in dense sand (Relative Density, Dr = 84%) and described in detail in (Cerfontaine et al, 2020;Davidson et al, 2019) is given below as an example. The helix is idealised as a horizontal plate attached to the shaft, which has been proven experimentally (Hao et al, 2019) and numerically (Al-Baghdadi, 2018) to be a good approximation.…”

“…The mesh was composed of triangular 15-node elements (PLAXIS, 2017) whose size is refined close to the anchor. The details of the modelling can be found in Cerfontaine et al (2020). The comparison of the numerical (Num.)…”

Developing Screw Piles for Offshore Renewable Energy Application

“…This work also showed that the failure plane that the shallow mechanism followed was defined by the dilation angle of the sand as found in other applications such as the uplifting of pipelines. It also suggested how existing analytical techniques could be improved to include the findings from FE analysis to give better predictions of screw pile uplift capacity (Cerfontaine et al, 2020).…”