Since the introduction of the first offshore wind farm in 1991, the demand for offshore renewable wind energy has experienced exponential growth all around the world. To supply this demand, the power rating and corresponding dimensions of offshore wind turbines have grown significantly. Due to the ever-shrinking availability of easily accessible shallow water sites and the abundance of high quality wind resources in deeper water, the industry is stimulated to come up with innovative, yet cost effective, solutions to tap into these deep water sites. Historically, the use of monopile foundations has been an important facilitator of cost reduction due to its relative ease of manufacturability, transportability and installability. Monopile foundations have, however, thus far only been used in relatively shallow water depths. With jacket-type and floating support structures remaining relatively costly, the question arises if the monopile could yet be scaled up further to be used in water depths beyond the current 40-60mfor future, 10+ MW wind turbines. The goal of this research is to investigate the technical feasibility of monopile foundations in the water depth ’gap’ of 60 to 120 meters, which is currently claimed by jackets, for large wind turbines and determine critical design parameters for up-scaling monopiles to these depths. With an eye on future developments a 15 MW reference turbine is adopted and to make the research widely applicable the Hywind Scotland site in the Northern North Sea is selected, a well-documented reference site with a very severe wind and wave climate. To define monopile designs, a parametric (static) monopile geometry optimization tool is developed in Excel, which transfers the environmental data to forcing components. The monopile geometry is optimized for first natural frequency and ULS resistance (yield and global buckling) by varying the outer diameter and wall thickness along the structure. It was found that the ULS check is governed by the inertial wave forcing during survival case (50 year return period storm conditions). As the acting wave frequency is way lower than the system natural frequency, the monopile can be adequately assessed using a static approach. To assess the effect of critical variables, the tool is used to define monopile geometries for water depths ranging between 60 and 120 m, target first natural frequencies of 0.15, 0.17 and 0.20 Hz and a range of soil types, while complying with known manufacturing limits. The results show that all designs are within manufacturing limits and resistance against ULS decreases for lower target frequencies, making 0.15 Hz monopiles unfeasible. In contrast to the ultimate limit failures, the fatigue damage is largely incurred from normal rather than ultimate wave states, which embrace the first natural frequency of the system, thus warranting a full dynamic analysis. In order to assess the fatigue resistance of 0.17 and 0.20 Hz monopiles, which pass the ULS check, an analytical full dynamic model is developed in Maple based on the fundamental equations of motion, including (added) mass, aerodynamic damping and soil/structural stiffness. The model is verified and validated against numerical modelling using ANSYS Finite Element Analysis software and found to be in very good agreement. The Maple model is then used to test the fatigue performance. It is found that the fatigue damage accumulated over the 25-year design lifetime is governed by the bending stress cycles induced by inertial wave forces. The model is used to generate a transfer function between this wave forcing and the resulting bending stress over the entire range of present wave frequencies. The transfer functions are used to transfer the wave scatter diagram to accumulated fatigue damage against a B2, C1 and D graded S-N curve. The analysis results show increasing fatigue damage for decreasing first natural frequency due to the increasing slenderness and more prominent interference with present wave frequencies, which causes the 0.15 and 0.17 Hz monopiles to fail on fatigue damage. Given the very demanding site conditions assumed in this research, more slender monopiles may be feasible in deep waters with a milder wave climate. Over all, the results show no fundamental technical limitations for a monopile supported 15 MW wind turbine in water depths of up to 120 meters, provided that the stiffness of the structure is sufficiently high. Since steel usage increases for increasing stiffness, technically feasible monopiles will need to be relatively heavy, thus costly. Therefore, a range of strategies to reduce steel weight have been quantitatively assessed. It is found that the amount of steel can be reduced up to 35% by adopting higher-grade steel types and improved weld quality. Based on the fundamental limiting factors for monopiles found, also a novel hybrid floating-fixed bottom concept is proposed aiming at steel weight reduction. Although the concept can be considered promising, it does add complexity to the system and unfortunately does not (yet) result in steel reduction.

The offshore wind industry in Europe has experienced significant growth in the past decade, with wind farm development mostly focusing on the shallow area's in the North Sea. Naturally, the market is driven to reduce the Levelised Cost of Electricity (LCoE) to become more competitive with fossil fuels and less dependent on government subsidies. A transition in wind farm development towards deeper waters is expected and already observed in the market, driven by decreasing availability of shallow area's and higher wind resource at far offshore locations. The majority of the northern part of the North Sea is between 60 - 120 meter deep, currently the jacket foundation is deemed as the foundation of choice for this water depth range. Despite several technological advantages of the jacket, the main downsides are the large engineering effort and welds required to produce such a foundation resulting in difficult series production and high costs. This does not align with the industry's ambition to lower the LCoE. As such, the need for a technologically viable and economically attractive foundation concept for waters between 60 - 120 meter deep arises. The goal of this research can be divided in to two parts. The first part is to determine the potential of conventional monopiles in this water range and identify the main limiting factors. To do so, a monopile is dimensioned at a selected reference location for three turbines representing the current, near future and future outlook of the market. The designed monopiles are tested for manufacturabiliy, Ultimate Limit State (ULS) and Fatigue Limit State (FLS) to identify the technical showstoppers. Next, in the second part, a novel monopile design is introduced and analysed to work around the identified limits. To dimension the monopiles for the three reference turbines a parametric dimensioning script is developed. The monopile geometry is dimensioned to have a selected first natural frequency of 0.20 Hz, based on the relevant frequency diagrams. Next, these geometries are tested against mudline ULS failure for the power production and parked condition load cases. Hereafter, an FLS check for the B1, C1 and D S-N curves is conducted based on the obtained scatter tables for site conditions. It was found that D-curve fatigue damage for non grinded butt welds is the main limiting factor for all dimensioned monopiles in deep water. However, industry experts believe that all welds can be grinded in the production process, eradicating the need to assess the D-curve. When assuming this statement to be true the newly obtained limits become ULS failure during parked conditions for the 15 MW reference turbine and manufacturability constraints for the 20 MW reference turbine. The Haliade X showed no limits within the specified water depth range when neglecting the D curve fatigue damage. A perforated monopile concept with reduced available area for wave loading is introduced. A Computational Fluid Dynamics (CFD) model based on the 2003 Menter Shear Stress Transport turbulence model is constructed for a perforated monopile to gain insights into how waves propagate through the structure and the forces associated with this. The CFD model is verified against experimental wave flume data before being used for further analysis showing a root mean square error of 0.0192 between model results and experiments. The CFD model is used to assess three geometries with different perforations and levels of porosity. No increased drag around the first natural frequency caused by the perforations, hinting to favourable dynamics, was found in any of the test cases. As such, the dynamic response of the three perforated monopiles was found to be unchanged when compared to a reference pile without perforations. Despite this, a significant reduction of lifetime fatigue damage was observed caused by the reduced forces acting on the structure resulting from the smaller frontal surface. Next, the mudline stresses are recalculated and a structural finite element model to assess the stress concentrations around the perforations is set up to verify the maximum allowable stress level threshold is not exceeded. A geometry was found which shows a 35.5% reduction of lifetime fatigue damage whilst stresses remain below the maximum threshold, hereby showing the potential of the perforated monopile. Implementing this perforation allows the use of monopiles up to 87 meter deep, limited by D curve fatigue. Reference piles without perforations were found to be infeasible for all assessed water depths, also limited by D curve fatigue. It is shown that the perforation concept can either be implemented to push the monopile foundation to deeper waters, or can be used to realise steel reduction at current water depths.