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Abstract

To support the United Kingdom's goal of achieving net-zero greenhouse gas emissions by 2050, with a target of 50 GW of offshore wind energy production by 2030, additional offshore wind farms must be considered. This master dissertation focuses on analysing soil displacements for an offshore wind turbine (OWT) supported by a 35-meter-long, 6-meter-diameter hollow steel monopile foundation in the Southern North Sea near Immingham, UK. Employing FEA using PLAXIS 2D, the study conducted soil displacement analysis utilizing the Mohr-Coulomb model with parameters derived from borehole No. 81/52A data at the specified location. Hand calculations were employed to determine the cyclic current and wind loads affecting the OWT tower, subsequently transferred to the embedded monopile.
Considering the entire soil stratigraphy, variations in the density of the first 8-meter layer of clay soil are considered due to uncertainties in soil parameters, impacting soil lateral, vertical, and total displacements. Results reveal a clear correlation between soil density and displacement patterns, with maximal strains occurring at greater depths as soil strength decreases. Stiffer soils exhibit shallower displacements, aligning with expectations.
Optimization of monopile diameter and length is conducted to meet the maximum permissible settlement criteria of 25mm, as specified by Das (2005, p. 285). The results revealed that an optimal monopile length of 39m with a diameter of 4.5m met the specified requirements.
Subsequently, an integrated analysis was conducted for the optimal monopile using the Cam-Clay model with assumed soil parameters extracted from secondary data from Schofield and Wroth (1968, p. 109). This analysis revealed contrasting settlement outcomes, where the Mohr-Coulomb model showed a higher settlement value of 24mm, while the Cam-Clay model exhibited a lower settlement value of 16mm, suggesting a potential cost-effectiveness reducing construction expenses by requiring a shorter monopile length, for the given assumptions. These results suggest that the lower settlement value obtained with the Cam-Clay model may be attributed to its requirement for more sophisticated soil parameters. Additionally, based on the critical state theory describing its ability to reflect the soil's response during consolidation, the higher settlement results obtained with the Mohr-Coulomb model could imply its reliance on envelope failure criteria, neglecting the influence of consolidation.
The results contribute to advancements in geotechnical engineering knowledge by identifying significant trends in soil displacement patterns and comparing the performance of different geotechnical models. This enhances understanding of soil behaviour in marine environments, thereby informing the design of sustainable renewable energy infrastructure.

Course: Civil Engineering - MSc - C0618

Date Deposited: 2025-01-16

URI/permalink: https://library.port.ac.uk/dissert/dis14665.html