Preliminary geotechnical design of offshore wind turbine monopiles in Türkiye using FEM

Date

2024-9-05

Author

Taşbaşı, Atasev Doğukan

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In this study, suitable offshore wind energy sites in Türkiye are identified based on the literature and Türkiye’s future energy planning. These sites are Bandırma, Bozcaada, Gallipoli (Gelibolu), İzmir, Karabiga, Kıyıköy and Silifke. For each of these sites, reasonable wind turbines are selected and site-specific load conditions are determined according to the existing literature and databases. Finite Element Method (FEM), more specifically the PISA Method via PLAXIS MoDeTo, is used to determine the optimum preliminary monopile dimensions (diameter and length), for three generic uniform soil profiles at the seabed, namely, soft clay, medium dense sand and medium stiff clay, at each of the sites, assuming 50 m water depth. For 7 different offshore sites, and 3 different generic soil profiles at each site, a total of 21 design cases are analyzed. Ultimate limit state (ULS) and serviceability limit state (SLS) -having 4 different load combinations- are investigated. In FEM analyses, clays are modelled with NGI-ADP, and sands are modelled with Hardening Soil small strain constitutive models. Monopiles are modelled according to Timoshenko Beam Theory. Long-term serviceability in terms of accumulated displacement and rotation caused by cyclic loads are not considered in this study. For 6 of these sites Bandırma, Bozcaada, İzmir, Karabiga, Kıyıköy and Silifke IEA 22 MW wind turbine is selected and the monopile diameters of 10.5 m, 11.5 and 12.5 m are considered. For Gallipoli, LW 8 MW wind turbine is considered and monopiles having diameters of 7.5 m and 8.5 m are used. For a medium dense sand soil profile, 10.5 m-diameter monopile having embedded lengths in the range of 38 m to 53 m; for a medium stiff clay soil profile, 11.5 m-diameter monopile and embedded lengths in the range of 49 m to 72 m satisfies the design requirements in all 7 sites. For soft clay soil profile, for Bandırma, the only design that satisfies the design requirements is a monopile having 12.5 m diameter and 76 m embedded length; and for Bozcaada, a reasonable diameter and embedded length could not be obtained within the ranges of values used in this study. It can be concluded that, for Bozcaada, in a soft clay soil profile, a monopile with larger than 12.5 m diameter, or for 12.5 m diameter, larger than 90 m embedded length might be necessary, or foundation types other than monopile might be considered. It is concluded that, for monopile design, soft clay soil profile is the most challenging condition as compared to other soil profiles. This study demonstrates the applicability of the PISA method in engineering practice using PLAXIS MoDeTo and presents preliminary design results for offshore monopile foundations in soft-clay, medium-stiff clay and medium-dense sand soil profiles under monotonic loading. Because of the limitations of this study (not available long-term wind, wave measurements at specific sites, not available site-specific seabed soil data, not considering the long-term accumulation of cyclic deformation and rotation in serviceability limit state etc), monopile dimensions obtained in this study should not be used for final design.

Subject Keywords

Offshore wind energy, Monopile design, Renewable energy, Finite element method, PLAXIS

URI

https://hdl.handle.net/11511/111478

Collections

Graduate School of Natural and Applied Sciences, Thesis