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Durham Research Online Deposited in DRO: 10 June 2020 Version of attached le: Accepted Version Peer-review status of attached le: Peer-reviewed Citation for published item: Cerfontaine, B. and Knappett, J. and Brown, M. and Davidson, C. and Al-Baghdadi, T. and Sharif, Y. and Brennan, A. and Augarde, C. E. and Coombs, W. M. and Wang, L. and Blake, A. and Richards, D. and Ball, J. (2021) 'A Finite Element approach for determining the full load-displacement relationship of axially-loaded shallow screw anchors, incorporating installation eects.', Canadian geotechnical journal, 58 (4). pp. 565-582. 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Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk Title A Finite Element approach for determining the full load-displacement relaonship of axially-loaded shallow screw anchors, incorporang installaon effects Author list Benjamin Cerfontaine*, Jonathan A. Knappe, Michael J. Brown, Craig S. Davidson, Therar Al-Baghdadi, Yaseen U. Sharif, Andrew Brennan, Charles Augarde, William M. Coombs, Lei Wang, Anthony Blake, David J. Richards and Jon Ball *Corresponding author Author details Benjamin Cerfontaine, BSc, MSc, PhD MSCA Research Fellow, School of Science and Engineering, University of Dundee, Fulton Building, Dundee, DD1 4HN, UK ORCID: 0000-0002-4833-9412 Email: [email protected] Jonathan A. Knappe, MA MEng PhD GMICE Professor, School of Science and Engineering, University of Dundee, Fulton Building, Dundee, DD1 4HN, UK ORCID: 0000-0003-1936-881X Email: [email protected] Michael J. Brown, BEng PhD GMICE Reader, School of Science and Engineering, University of Dundee, Fulton Building, Dundee, DD1 4HN, UK ORCID: 0000-0001-6770-4836 Email: [email protected] Craig Davidson, BSc MSc Research Associate, School of Science and Engineering, University of Dundee, Fulton Building, Dundee, DD1 4HN, UK ORCID: 0000-0002-4843-5498 Email: [email protected] Therar Al-Baghdadi, BSc, MSc, PhD Geotechnical Engineer, Municipality of Karbala, Karbala, Iraq ORCID: 0000-0002-7368-4285 Email: [email protected] Yaseen U Sharif, BSc, MSc PhD student, School of Science and Engineering, University of Dundee, Fulton Building, Dundee, DD1 4HN, UK ORCID: 0000-0002-3620-7500 Email: [email protected] Andrew J. Brennan, MEng PhD GMICE Senior Lecturer, School of Science and Engineering, University of Dundee, Fulton Building, Dundee, DD1 4HN, UK ORCID: 0000-0002-8322-0126 Email: [email protected] Charles Augarde, BSc MSc DPhil CEng FICE Professor, Department of Engineering, Durham University, Durham, DH1 3LE, UK ORCID: 0000-0002-5576-7853 Email: [email protected] Will M. Coombs, MEng PhD Associate Professor, Department of Engineering, Durham University, Durham, DH1 3LE, UK ORCID: 0000-0003-2099-1676 Email: [email protected] Lei Wang, PhD Research Assistant, Department of Engineering, Durham University, Durham, DH1 3LE, UK Email: [email protected] Anthony Blake, BEng, PhD Research Fellow, Faculty of Engineering and the Environment, University of Southampton, SO17 1BJ, UK ORCID: 0000-0001-5718-7900 Email: [email protected] David J. Richards, BEng MSc PhD CEng MICE Professor, Faculty of Engineering and the Environment, University of Southampton, UK ORCID: 0000-0002-3819-7297 Email: [email protected] Jon Ball, EurGeol Bsc. (Hons) CGeol FGS Chief Geotehcnical Engineer, Roger Bullivant Ltd, Swadlincote, UK Email: [email protected] Main text word count: 7602 Number of tables: 6 Number of Figures: 16 1 A Finite Element approach for determining the full load-displacement relaonship of axially- loaded shallow screw anchors, incorporang installaon effects B. Cerfontaine*, Jonathan A. Knappe, Michael J. Brown, Craig S. Davidson, Therar Al-Baghdadi, Andrew J. Brennan, Charles Augarde, William M. Coombs, Lei Wang, Anthony Blake, David J. Richards and Jon Ball ABSTRACT Screw anchors have been recognised as an innovave soluon to support offshore jacket structures and floang systems, due to their low noise installaon and potenal enhanced upli capacity. Results published in the literature have shown that for both fixed and floang applicaons, the tension capacity is crical for design but may be poorly predicted by current empirical design approaches. These methods also do not capture the load-displacement behaviour, which is crical for quanfying performance under working loads. In this paper, a Finite Element methodology has been developed to predict the full tensile load-displacement response of shallow screw anchors installed in sand for praccal use, incorporang the effects of a pitch-matched installaon. The methodology is based on a two-step process. An inial simulaon, based on wished-in-place conditions, enables the idenficaon of the failure mechanism as well as the shear strain distribuon at failure. A second simulaon refines the anchor capacity using soil-soil interface finite elements along the failure surface previously idenfied and also models installaon 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. KEYWORDS Screw anchor, Helical Pile, Sand, Finite element modelling, Design INTRODUCTION Screw anchors or piles are a foundaon technology that may provide significant upli capacity for offshore applicaons (Byrne and Houlsby 2015; Houlsby 2016) while avoiding pile driving nuisance for marine inhabitants (Bailey et al. 2010). Screw anchors consist of one or more steel helices (150-400mm diameter), aached to a core of smaller diameter and are used onshore to anchor relavely light structures (Perko 2009). These anchors are screwed into the soil by applying a torque and a crowd force to ensure penetraon with a minimum soil disturbance (Perko 2009). Such anchors, if appropriately scaled-up, may be suitable to provide the very large tension requirements of boom-fixed jacket structures (e.g. 20MN, (Byrne and Houlsby 2015)) or floang tension-leg plaorms (e.g. 10MN, (Bachynski and Moan 2014)) for offshore wind turbines. The upli capacity of shallow screw anchors was invesgated by Davidson et al. (2019) through centrifuge tesng in medium-dense and dense sand. The centrifuge upli capacies were compared with results published in the literature, as shown in Figure 1. This figure presents a non-dimensional bearing factor, Nγ, obtained by normalising the upli capacity with respect to the helix embedment depth H, the area of the helix and the buoyant unit weight γ’, (1) Centrifuge results are consistent with the other experimental results, as shown in Figure 1. Bearing factors obtained by Ilamparuthi et al. (2002) constute the upper bound of the results presented, especially at larger relave embedment. This is probably due to their relavely small scale, being tested at 1g, leading to a more pronounced effect of dilatancy on the soil response. Conversely, centrifuge tests provide a lower bound. Centrifuge results of Dickin (1988) were reported for comparison, but were related to square plate anchors, which have been shown to provide lower upli capacity (Giampa et al. 2018a). Byrne and Houlsby (2015) stated that mul-foong structures such as tripods or jacket structures will become necessary to deploy wind turbines in deeper water. In this case, the tensile capacity is the crical design case and screw anchors can provide sufficient capacity. However typical analycal approaches (e.g. Mitsch and Clemence 1985) may significantly overpredict the screw anchor capacity for these large scale applicaons. The recent semi-analycal method proposed by Giampa span style="font-family:Calibri; font-style:italic">et al. (2017) for shallow anchors which is based on peak fricon and dilatancy angles for shallow anchors, assumes that the failure mechanism can be described by a shallow wedge, whose inclinaon to the vercal direcon is equal to the dilatancy angle. This finding is similar to the work of White et al. (2008) for the upli of buried pipelines and has been theorecally jusfied for anchors by Vermeer and Sutjiadi (1985). However, the method is limited to single helix screw anchors and does not provide any load-displacement (sffness) informaon, which is very important for jacket structures and tension-leg plaorms, as the axial sffness controls the global rotaonal sffness of the wind turbines. For instance, the rotaon of boom-fixed wind turbines must typically be kept below 0.5° to ensure safe operaon (Achmus et al. 2009). Finite Element modelling enables the predicon of the enre tensile load-displacement relaonship, but few studies have previously tackled this problem for screw anchors in cohesionless soils due to the difficules in capturing the effects of installaon (a large displacement process) on capacity. Those approaches which have been proposed for modelling the problem rely on back-calculated parameters, characterising the soil properes around the anchor, to reproduce field or experimental tests (e.g. Papadopoulou et al. 2014; Mosquera et al. 2015; Perez et al. 2018) without which upli capacies are overesmated (e.g. Gavin et al. 2014) due to an incorrect modelling of the strength mobilised at failure.