Over the past 5 years, a substantial research effort aimed at optimising the design of offshore wind turbines has led to significant reductions in the projected cost of developing offshore wind. Optimising the geotechnical design of these structures, through modern analysis techniques such as 3D Finite Element Modelling (FEM), has played a key role in helping to reduce costs. This paper presents a methodology for accurately modelling monopile behaviour using Cone Penetration Test (CPT) data to calibrate the non-linear stress dependent Hardening Soil (HS) model. The methodology is validated by comparing the modelled behaviour to field tests on a range of pile geometries. The paper also demonstrates how the soil-pile reaction response curves can be extracted from the FE model by isolating the stresses on each element of the pile. The contribution of each component to the overall lateral resistance is shown to vary with the pile geometry and is examined using the extracted soil reaction curves.

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This paper presents the results of field tests performed to investigate the field behaviour of winged-monopile foundations. The principle of the winged monopile is that steel plates are attached to a standard monopile (in the area near the ground or seabed surface) to increase the foundation stiffness and lateral resistance. The experimental tests described in this paper consisted of load tested driven instrumented prototype scale standard (reference) monopiles and piles with varying wing geometries at two sand sites. The overall load–displacement performance and mobilised bending moment profiles were examined to assess the potential benefits of adding wings to monopiles. Experimental p–y curves were developed for the piles to analyse how the presence of wings influenced the soil– structure interaction of the foundation system. The use of simplified p–y methods for predicting the winged-pile response was assessed. The experiments proved that the addition of wings greatly improved the lateral resistance and stiffness of the piles; however, the results suggest that conventional p–y curve methods are limited as they cannot account for the effect that the enhanced stresses mobilised by the wings have on the strength and stiffness response of the pile below the wing location.

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Lateral loading is often the governing design criteria for piles supporting offshorewind turbines and with the recent growth of this sector, the reliability of traditional design approaches is receiving renewed interest. To accurately abeb the behavior of a laterally loaded pile requires a detailed understanding of the soil reaction that ismobilized as the lateral deflection of the pile occurs. Currently, the p-y curvemethod iswidely adopted tomodel the response of laterally loaded piles. The limitations of existing p-y formulations arewidely known, and there is acceptance that load tests on large-diameter stiff monopiles are urgently required to formulate appropriate designmethods. However, interpretation of the data frominstrumentation placed on stiffmonopiles is not straightforward. This paper proposes an optimization technique to derive the soil reaction profile along the shaft of instrumented piles, fromwhich the correlated p-y curves can then be obtained. Themethod considers force equilibrium, pile deflection, and additional boundary conditions. A set of fourth-order polynomial equations are abumed to model the soil reaction profile under each load step during a monotonic load test. By minimizing the difference between themeasured and calculated bendingmoment and considering equilibriumof the shear forces acting on the pile, the soil reaction profile and the concentrated tip resistance can be obtained simultaneously. A stiff instrumented test pile installed in over-consolidated sand was load tested and the resultswere used to test the performance of the proposed method. The results are compared with othermethods used in literature and practice. Themethod provides a consistent framework to derive p-y curves from measured strain data. The results of the field test and derived p-y curves confirmed that existing designmethods do not accurately capture the lateral loading response of piles in dense sand.

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In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

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Monopiles have been by far the most common support structure used for offshore turbines, with approximately 75% of existing wind farms founded on these large diameter steel tubes EWEA(2014). However, despite the widespread prevalence of monopiles across the wind sector, the design tools commonly used by industry have typically evolved from those developed by the oil and gas sector, which apply to significantly different design conditions. This paper introduces some of the design approaches available in practice and identifies some of the limitations of current offshore codes. Finite Element Methods (FEM) are suggested as a means of more accurately considering offshore soil behaviour, although the importance of accurate calibration of these models against real data (lab and/or field data) is stressed. Novel means of determining the in-situ frequency response are also discussed and the potential implications for monopile design at different sites. Finally, some design aspects of XL monopiles are considered that suggest monopiles may be pushed into ever increasing water depths.@en