The API RP 2GEO (2011) p-y method is an industry standard approach to predict the lateral pile behaviour of slender oil platform piles with length (L) over diameter (D) ratios > 10. However, offshore wind turbines are founded on large diameter monopiles with an L/D ratio < 8, which are rather rigid piles. There are concerns about the applicability of the standard p-y approach to piles that have low L/D ratios and it is expected that other soil reaction terms such as base shear, base moment and distributed moment also play a role in rigid pile behaviour (Davidson & Donovan, 1983). Due to the increasing demand for renewable energy, the offshore industry is taking a leap when it comes to the development of offshore wind parks. Because power generated by a wind turbine is directly related to rotor diameter and thus to monopile dimensions, it is expected that monopiles with even lower L/D ratios will become standard. With the prospect that 27 % of European energy should be generated by renewable energy sources by 2030 (EUCO 169/14, 2014), efficient design methods for offshore wind turbine foundations are desired. Therefore, in this thesis, the performance of the current design approach is analysed and an investigation is conducted into the development of a new design method for large diameter piles in sand that are tailored to the offshore wind industry. In 2013, the PISA (PIle Soil Analysis) project, a joint industry research project, has been established to develop a new design method for large diameter monopiles under lateral loading. As part of the project, pile load tests (PLTs) and cone penetration tests (CPTs) were performed in Dunkirk, in sand. This new (unpublished) design method includes the derivation of soil springs, that represent the additional soil reaction terms, by using complicated Finite Element Method (FEM) calculations which require input from advanced laboratory tests. Even though these calculations are accurate, they are also time-consuming. In the past two decades, several cone penetration test (CPT)-based p-y methods have been published. With these methods, p-y soil springs are derived directly from CPT data and used as input in a pile-response model, avoiding the necessity of advanced laboratory testing and FEM calculations. In this thesis, the performance of CPT-based p-y methods, compared to the current approach is evaluated by using the API RP 2GEO p-y method (2011) together with CPT-based p-y methods published by Novello (1999), Dyson & Randolph (2001), Li, Igoe & Gavin (2014) and Suryasentana & Lehane (2016) to simulate the pile behaviour of short (L/D=3), medium (L/D=5.25) and large (L/D=8) piles that were tested in Dunkirk during the PISA project. A pile-response model is built in MATLAB to perform these calculations quickly and accurately. It is found that the API p-y method generally overestimates the pile response compared to results of CPT-based p-y methods and PLT measurements, especially for the short pile. Therefore it can be concluded that the current API p-y approach is not sufficient for predicting short pile behaviour. The CPT-based p-y methods predict stiffer responses, yet not stiff enough to match PLT measurements of the short pile. This strengthens the expectations that for short pile behaviour, apart from the p-y term, also other soil reaction terms play a role. Still, the largest contribution to pile displacement y is determined by the lateral soil pressure p (Byrne, et al., 2015a). Therefore, the development of a new p-y method based on the longest pile (L/D=8) that was available from the PISA PLT data set is investigated first. This investigation involves processing PISA PLT data into pile displacement- and soil pressure profiles. During this process, it is found that the direct differentiation of raw PISA measurement data comes with problems and results into wiggly soil pressure profiles. Curve fitting the moment profile has been applied in an attempt to overcome this issue. In this thesis, all ‘simple’ MATLAB curve fitting techniques have been analysed for the pile with L/D=8. The curve fitting techniques that have been investigated can be separated into four different types: global polynomials, interpolation splines, smoothing splines and least-squares splines. The ‘simple’ MATLAB curve fitting techniques in combination with the PISA PLT data did not result in a trustworthy pressure profile. Hence it was not feasible to continue the research into a new CPT-based p-y method based on the ‘simple’ MATLAB curve fitting techniques and the PISA data that was available. However, a future work approach is presented that can be used as a guideline for further research into the development of a new CPT-based design method for large diameter monopiles. A step-by-step approach is given for the complete derivation of CPT-based p-y curves from PLT data, together with the first steps for deriving springs for the additional soil reaction terms. The future work approach elaborates on data gathering, PLT testing, PLT data processing, curve fitting, how to the link p-y curves to qc data, and what formulas can be used as a preliminary basis for the derivation of the base shear, base moment and distributed moment. Following the future work approach created in this thesis, an accurate and cost efficient design method for offshore wind turbines could be developed that positively contributes to the energy transition and a sustainable environment.

Offshore wind has achieved many milestones considering this relatively new and emerging industry. Its huge advancement is mostly driven by lowering the overall project cost and renewable energy targets which are set by the EU. One of the potential cost savers can be found in the foundation structure design in which the monopiles are the most used choice. The area from which the design of these structures can be improved is quite complex and it can be related to the field of Dynamic Soil-Structure Interaction (DSSI). Offshore Wind Turbine (OWT) has several damping sources but in the last few years soil damping phenomena has attracted a lot of interest within the offshore wind research community because it is believed that potential benefits can be achieved if the contribution of this damping source can be estimated with reliability. Offshore wind design codes do not provide a methodology for the estimation of this damping type and therefore there is a need for the research. For the purpose of soil damping estimation, a geotechnical Finite Element (FE) software – Plaxis 3D was utilized. Firstly, the chosen soil model was verified through the three-step verification process which involved a comparison between experimental field data and numerical data from Plaxis. Once successfully verified, the soil model is utilized further in the design of the simplified OWT structure which is exposed to the dynamic wave and wind loads based on the serviceability and ultimate limit conditions. What is of the particular interest leads to the free vibration phase and monopile positioning (position of structure at the start of free vibration) from which the necessary information regarding the decay of displacement amplitudes is obtained and further analysed with the logarithmic decrement method for damping estimation. Adding on soil damping, as part of research additional topics were also considered and they relate to soil behaviour and natural frequency. The final results are compared to the other research papers and they are found to be well within the proposed literature damping range which highly depends on a certain combination of monopile geometry, soil type and loading conditions. From the research it was concluded that soil damping value increases as the wave and wind loads increase. Also, it was proven that a damping potential in offshore wind exists and that eventually soil damping should be implemented in the future offshore wind design codes.

The demand for offshore wind farm installation is increasing in recent years as the concern in using sustainable energy source is rising. One of the primary steps in constructing offshore wind farm is the pile installation which is a high-risk activity due to the expensive offshore installation vessels requirement. Any factor which can result in delaying the pile installation process will lead to financial losses. Therefore, a comprehensive driveability analysis using an efficient pile model is favourable. From the driveability model, the suitable pile equipment can be selected consider the underlying soil condition. The selected equipment must be capable of installing pile into the target depth without overstressing the pile within the design time.

An essential component in driveability model is to estimate the static resistance to driving (SRD). The SRD in analogues with axial static pile capacity approaches. These approaches use the Cone Penetration Test (CPT) data to determine the axial pile capacity. Factors such as the friction fatigue effect, stress equalisation and soil plugging are related and affect axial pile capacity. These factors are integrated into driveability analysis in this study to provide a more reliable result when using CPT-based approaches to calculate static axial capacity.

Pile installation data records from Blessington, Ireland are used as a part of the axial static load test programme. Pile load tests have been performed on open-ended steel piles with a diameter of 0.34m. The site condition consists of glacial deposit dense sand. From this database, the performance of driveability models by using the available and modified CPT-based approaches (e.g., the UWA-05, ICP-05, and Fugro-05) are assessed in this study. The modified model considers the friction fatigue effect, the pile ageing effect, the soil plugging condition, the pile tip mobilisation and the base residual stresses while the SRD is calculated by using the available CPT-based approaches.

The recent CPT-based axial static capacity methods are investigated to see if they can be used as a reliable method to determine static resistance to driving (SRD) profiles. The SRD profiles are comparable to the axial static capacity approach which account for the pile ageing effect, the soil tip displacement, and residual stresses during driving. The pile ageing effect that is incorporated in the model as installation resistance is set for the time equals to zero, unlike with the static capacity load test which is derived after a certain time after end installation. The total resistance that is recorded for open-ended small diameter piles is calculated to model the pile tip mobilisation which is associated with the base resistance. The residual base stresses are modelled for each hammer blow during driving. The wave equation analysis uses a combination of the SRD profiles and dynamic soil components, pile properties and installation equipment resulting in total resistance as the blow count prediction.

This study provides information on how to model driveability analysis from the recent CPT-based axial static capacity approaches. The models one modified to include related factors that affected pile installation process. The performance of predicted blow counts result from unmodified and modified CPT-based methods are appraised and compared to the recorded blow count as a model verification. The UWA-05 modified model can be considered as an appropriate model to estimate pile driveability from CPT-based axial static capacity approach.

Wind turbines are nowadays the main means for producing renewable energy in Northern and Western Europe, and therefore research is conducted to secure their efficiency. One of the critical aspects of the windfarms is the foundation type, as it can define the functionality and the cost of each turbine. Monopiles have prevailed as the most efficient foundation, since 75% of the existing wind farms are founded in this way. However, despite their dominance, there are still mechanisms that affect their stability during their lifetime. Scour formation, which is the erosion of the soil around a monopile, is a crucial one that is the topic of this project. More specifically, the effect of the scour formation (both depth and type) in the stiffness and the lateral capacity in sandy soils has been investigated, followed by analyses about the efficiency of the scour protection layers. The methodology that was followed included numerical simulations, which have been performed in the finite element code of PLAXIS 3D. More specifically, 26 simulations have been conducted, in which the vertical load, the scour depth and type and the scour protection length were the parameters that have been investigated. The conclusions drawn could be divided into four categories, the vertical load, the scour depth and type and the scour formation effect in the stiffness and the lateral capacity. It was shown that the increase in the vertical load had a positive influence in the lateral capacity of the soil-monopile system. However, it could be characterized negligible, as the lateral capacity increase was less than 5%. In the next set of results, the scour depth impact in the stiffness and lateral capacity of the soil was investigated. It was observed that for the same type of scouring, the increase of the depth of the scour hole significantly reduced the soil resistance. It is noted though that scour up to 1.0D could be managed except for the global scour case. On the other hand, for larger depths the situation was becoming more critical and in depth of about 2.0D, the loss of the capacity was too great leading to a no-return state, for all scouring types. Then, the type of scouring impact was investigated, as local scour (narrow and wide) or global can occur. It was shown that the narrow type of scour was the most favorable case, which could approach the no scour case for small scour depth, while the global scour case was the most critical, as even in small scour depths could lead to dramatic reduction of the capacity. The main point is that the narrow case and the wide one can be manageable in depths up to 1.5D under certain conditions, while the global type seems to become a no-return case for much smaller erosion depths. The last part included the scour protection effect which was similar to the vertical load effect, as it had a positive influence in the lateral capacity of the soil-monopile system, but in a range of 5 to 15%. Therefore, it could be ignored in the design phase, unless a thicker protection layer is used. The graphs of the evolution of soil pressures and pile deflection clearly indicated that the monopile presents a rigid response, as the pile was rotated in all cases from a certain rotation point. The rigid behavior of the pile states that the API method is outdated as it is based on slender piles and it cannot capture the rotation point and the distribution of soil pressures that occur due to the rigidity of the monopile. The final output of this current report is a design recommendation, depicted in two normalized charts that correlate load and stiffness at failure displacements with the scour depth and type.