Investment into offshore wind farms has been growing to address the growing threat of climate change. The majority of offshore wind turbines (both current and planned) are founded on monopiles, large circular steel pipe piles ranging from 4.0 m – 7.5 m in diameter. Based on available borehole records, most planned wind turbines in the UK will be founded in overconsolidated clay deposits. Monopile design is done via usage of the well established p-y curves. However, there are issues with the usage of the p-y curves. Firstly, the curves may be unsuitable to model the monopile’s behaviour as it is expected to behave similarly to a rigid pile rather than flexibly. Secondly, the curves may not accurately estimate the initial pile-soil stiffness. Thirdly, the curves are not comprehensive enough to account for the accumulated strain and stiffness changes resulting from cyclic loading. Considering these issues, research was carried out to improve the current design of monopiles in clay by carrying out displacement controlled monotonic and load controlled cyclic load tests in a centrifuge. Results from monotonic tests suggest that the DNV (2014) design methodology to construct p-y curves in clay based on Matlock’s (1970) soft clay criterion significantly underestimate stiffness. Findings suggested that the experimental p-y curves could be characterised through modification of the criterion. Modification of the criterion produced estimates that matched the 3.83 m monopile experimental curves. Pile toe shear force was observed to contribute little to ultimate lateral resistance and stiffness. Despite the marginal contribution, an effort was made to characterise the pile toe shear force. Estimates of the modified criterion on the 7.62 m monopile did not match the observations, indicating that further research should be carried out to improve the modified criterion. The cyclic tests displayed two distinct regimes; the stiffening regime and the softening regime. Results suggests that cyclic loads of different characteristics influence the locked in stress conditions of the soil which in turn influence the excess pore pressure behaviour, hence dictating whether the stiffening or softening regime takes place. Suggestions were made regarding the conditions that dictated whether the stiffening or softening regime would take place. In the stiffening regime, the stiffening rate decreased with increasing strain while as the accumulated rotation rate increased with vertical load for the same cyclic load magnitude. The softening regime was determined to be extremely detrimental as the high rates of softening and accumulated rotations could cause failure of the system in the short-term. Recommendations were made to estimate the cyclic stiffness and accumulated rotations resulting from both stiffening and softening regime.

This research study developed basic dynamic models that can be used to accurately predict the response behavior of a near-shore wind turbine structure with monopile, suction caisson, or gravity-based foundation systems. The marine soil conditions were modeled using apparent fixity level, Randolph elastic continuum, and modified cone models. The offshore wind turbine structures were developed using a finite element formulation. A two-bladed 3.0 megawatt (MW) and a three-bladed 1.5 MW capacity wind turbine were studied using a variety of design load, and soil conditions scenarios. Aerodynamic thrust loads were estimated using the FAST Software developed by the U.S Department of Energy’s National Renewable Energy Laboratory (NREL). Hydrodynamic loads were estimated using Morison’s equation and the more recent Faltinsen Newman Vinje (FNV) theory. This research study addressed two of the important design constraints, specifically, the angle of the support structure at seafloor and the horizontal displacement at the hub elevation during dynamic loading. The simulation results show that the modified cone model is stiffer than the apparent fixity level and Randolph elastic continuum models. The effect of the blade pitch failure on the offshore wind turbine structure decreases with increasing water depth, but increases with increasing hub height of the offshore wind turbine structure.

A literature review of current design and analysis methods for offshore wind turbine (OWT) foundations is presented, focusing primarily on the monopile foundation. Laterally loaded monopile foundations are typically designed using the American Petroleum Institute (API) p-y method for offshore oil platforms, which presents several issues when extended to OWTs, mostly with respect to the large pile diameters required and the effect of cyclic loading from wind and waves. Although remedies have been proposed, none have been incorporated into current design standards. Foundations must be uniquely designed for each wind farm due to extreme dependence on site characteristics. The uncertainty in soil conditions as well as wind and wave loading is currently treated with a deterministic design procedure, though standards leave the door open for engineers to use a probability-based approach. This thesis uses probabilistic methods to examine the reliability of OWT pile foundations. A static two-dimensional analysis in MATLAB includes the nonlinearity of p-y soil spring stiffness, variation in soil properties, sensitivity to pile design parameters and loading conditions. Results are concluded with a natural frequency analysis.

The installed capacity of offshore wind turbines in Europe is increasing with the monopile being the most common type of foundation. During its lifetime an offshore wind turbine is exposed to high dynamic loads which eventually can result in the fatigue of the substructure. This thesis will show how the linear damage accumulation approach based on the Miner’s rule can be used to estimate the damage induced on the substructure of an offshore wind turbine using measurements from strain gauges. Furthermore, the most important environmental influences will be illustrated and the different stress concentration factors and the size effect introduced in the industry guideline DNVGL-RP-C203 will be analysed towards their effect on the calculated lifetime.

The high-cycle accumulation (HCA) model of Niemunis et al. is applied for the simulation of model tests on monopiles subjected to high-cyclic loading with up to five million loading cycles. A new finite element program for geotechnical applications written by the authors is used for this purpose. The simulations with the HCA model show a good agreement with the deformations measured in the model tests.

Increasing energy demand and carbon emissions have driven the development of alternative energy solutions. One promising technology is wind energy. Wind energy technology developments has advanced substantially since the 1980s. Offshore wind turbines have become a major research focus, due to the promising offshore wind resource. However, challenges in offshore wind energy have arisen due to the additional wave loading and strong wind loads. Structural control systems have been implemented and researched in order to decrease dynamic response of these systems. The previous studies were successful at decreasing fatigue loads in the tower and support structure of offshore wind turbines. Giving these results, it is still unknown if the reduced loading enabled by structural control systems can allow for reduced material costs in the major structural components. This research examines on an offshore wind turbine's tower-monopile structure by adding several configurations of passive tuned mass dampers, while simultaneously reducing the thickness of the structure in order to reduce costs. A range of candidate tower-monopile systems are created, and simulated in FAST-SC with and without passive tuned mass dampers. Fatigue and ultimate loads are calculated and analyzed. A variety of design criteria are considered including fatigue and ultimate loads, as well as local and global buckling. The results demonstrate that the tower-monopile thickness may be reduced up to 6.2% and still satisfy all design criteria.

The study presented in this thesis investigates the passive exchange of enclosed water with seawater in an offshore wind turbine foundation. This thesis was undertaken in collaboration with Vattenfall R&D, Älvkarleby, Sweden. The water exchange is studied by utilizing Computational Fluid Dynamics (CFD) simulations. A standard monopile foundation, which is installed in Horns Rev 3 wind farm, is considered for the study. The considered geometry consists of two replenishment holes.  The study aims to develop a methodology to utilize CFD simulations to quantify the exchange rate of water. CFD enables studying the effects of different wave parameters and sea states on the economic exchange rate. However, the secondary aim to develop the methodology for the CFD simulations is also to utilize the available computational resources efficiently.  The CFD methodology incorporates the learning from experiments and utilizes a semi-circular domain to enclose the control volume. The results from a mesh sensitivity study establish that a coarser mesh in the domain and a finer mesh within the monopile, coupled with Implicit LES is appropriate to study the overall effect of wave motion on the exchange rate. Also, the additional term scalar transport, incorporated to study the change in concentration within the monopile, provided an appropriate and computationally efficient tool to visualize the variation in water concentration.

Scour is a process of removing soils around foundations by currents and waves. For the pile-supported marine structures such as the monopile-supported offshore wind turbines (OWTs) and the pile-supported bridges, scour can decrease the pile capacities and alter the dynamic responses of the structures. At present, there is not a widely accepted method to estimate pile axial or lateral capacity under scoured conditions. For example, different recommendations are used among the existing design standards for estimation of the vertical effective stress and the resulting capacities for single piles under different scour conditions. None of the existing standards or design practice has even considered the scour effects on the behavior of pile groups. Furthermore, the investigation into the responses of piles under multiple hazards of scour and earthquakes is rarely reported. To address the foregoing limitations, this study first introduces an analytical solution to determining the vertical effective stress of soils around single isolated piles under scoured conditions and uses it to examine the limitations of the existing standards in estimation of pile tensile capacity (Chapter 1). The effect of soil-pile interface friction is highlighted. Next, the study proposes new approaches to investigating the combined effects of scour and earthquakes on the lateral responses of the monopile-supported OWTs in sand (Chapter 2) and soft clay (Chapter 3). Lastly, simple and practical methods are developed based on the p-y curve framework for analyzing the lateral responses of pile groups in sand (Chapter 4) and soft clay (Chapter 5) subjected to static lateral loading. The proposed methods in this study were encoded into a series of open-source computer scripts for engineering practice. They were verified with the 3D continuum finite element (FE) analyses. Using the proposed methods, standard methods, and 3D FE method, parametric analyses were conducted to investigate the scour effects on the lateral behavior of the monopile-supported OWTs under crustal earthquakes and that of the pile groups under static loading. The factors considered in the parametric study included effects of scour-hole dimensions, soil stress history, soil density, soil-pile interface behavior, soil liquefaction potential, pile group configurations, etc. Through the parametric analyses, the standard methods were critically assessed by comparing the results to those calculated by the proposed methods and 3D FE methods, and some design-related issues were also discussed. / Graduate

The study is developed from five in-depth interviews with individuals from different walks of life who have interacted significantly with the North Sea. The study discusses change in the North Sea specifically in the development of fixed turbine wind farms and their physical and aesthetic effects. Observations speakers make as to changes in the North Sea and as to its beauty are contextualised and discussed using NASA satellite images, photographs and review of available academic literature, UK policy documents and law. This context includes a study of the industrialised North Sea with reference to the sediment sea plumes behind monopile turbines. The United Kingdom was selected for particular study of its wind farm development permissions process, including evaluations of seascape and the requirement of independence for expert evidence. Decline of trawler access to the North Sea is referenced to wind farm growth, and to adverse changes in public opinion leading to closure of the UK Dogger Bank to trawlers. Finality of wind farm development decisions is considered against the prospect of overturn by the courts. This aspect covers the application and development of principles relating to appeal by way of judicial review in the UK jurisdictions of Scotland, England and Wales, and Northern Ireland. The study identifies, and explains the English aesthetic evaluation of wind farms. It concludes that sea plumes are the result of a legal choice to allow permit applications to succeed without testing by reference to detailed in-sea turbine dimensions. In the permissions process (a) sea plumes are not evaluated by the seascape criteria applicable to coastal or off-coastal wind farms (b) deep offshore wind farms are instead evaluated by possible changes to character of the sea. The study further concludes that (i) the open horizon of the North Sea has been lost in significant part (ii) the combined aesthetic of transience, decay, and nostalgia underlies the aesthetic of the North Sea Maunsell forts (contrasted to Sealand), and also underlies attitudes to decommissioning wind farms, and (iii) concepts of sea beauty may be based on appearance or health, being regulated by different legal regimes in each eventuality (respectively the European Landscape Convention, or the OSPAR/ biodiversity/ habitat initiatives)