This thesis comprises an in-depth review of monopile foundations for offshore wind turbines under monotonic and cyclic loads. The review evaluates current design methodologies using advanced finite-element analyses and highlights inadequacies of conventional monotonic p-y curves, historically tailored for the oil and gas industry, for use in offshore wind turbine foundations. The initial chapter dives into the effects of cyclic loads on soil stiffness around monopile foundations, indicating divergence in the scientific community. In terms of soil damping for monopile foundations, the study underscores the complexities of damping and the general neglect of directly calculated soil damping ratio. By comparing the conventional API p-y curves, the PISA design method, and the new ISO/API p-y curves with three-dimensional finite element analyses, a discerning evaluation emerges, pinpointing the strengths and drawbacks of the current engineering methodologies used widely in the industry. Overall, the literature study concludes the pressing need for refined, evidence-based geotechnical strategies in monopile foundation design and assessment. Considering a critical gap in research regarding soil damping for monopile structures, this research, conducted at the SAGE laboratory of the University of Surrey, investigates the damping behaviour of stiff piles in sandy soil. Three monopile sizes with slenderness ratios between 3.75 and 7.5 were examined at forcing frequencies from 0.1 to 5 Hz for 10,000 to 50,000 cycles. Employing experimental methods, a novel approach for measuring soil damping is suggested, effectively isolating soil material damping from other damping sources in contrast to the prevalent research practice of back-calculating soil damping from the overall system damping. In processing the experimental data, validations with research databases identified discrepancies, particularly at the pile toe, emphasising the superiority of the model test data for industry design guidance. This research provides a detailed discussion on the effects of soil damping ratio due to variation in L/D ratio, ULS loading, number of cycles and frequency variation. The observations reveal that the soil damping around the stiff pile exhibited a non-linear fluctuating shape. This research introduces the “zonal method”, which intends to reduce the design complexities surrounding soil damping ratios by linearising its non-linear profile. Dynamic Simple Shear tests run simultaneously with experimental model tests, provided results for direct comparison, hinting at possible computation of soil damping ratio using element test apparatus only.

For future work, the zonal concept introduced in this thesis requires additional refinement to enhance its precision and applicability. This refinement can be achieved by integrating soil cyclic element tests, which would consider variations in soil vertical pressure and soil density across different zones. Subsequently, the findings from the soil element tests need to be systematically correlated with the results from the soil model tests, establishing a comprehensive and coherent model that accurately reflects the soil-pile interaction in various soil conditions and loading scenarios.