The European offshore wind industry has experienced significant growth in the past decade, mainly focusing on shallow areas in the North Sea to reduce the Levelised Cost of Electricity (LCoE) and compete with fossil fuels. However, as shallow areas become scarcer and the industry seeks greater independence from government subsidies, a shift towards deeper waters is anticipated, and already observed in Europe. In the northern part of the North Sea (60-120 meters deep), jacket foundations are currently favoured, despite drawbacks such as extensive engineering efforts, weld requirements, challenging series production, and high costs. This misalignment with the industry's LCoE reduction goal highlights the need for a technologically viable and economically attractive foundation concept for waters in the 60-120-meter range.

To combat this challenge, perforated monopiles are being developed. The perforated monopile consists of a monopile with perforations, either circular or elliptical, around the splash zone, with the goal of reducing the frontal area, and thus reducing the hydrodynamic loads on the structure. These concepts aim to combine the ease of manufacturing of a monopile, with the reduced area affected by hydrodynamic loads that are common for jacket structures. The research done so far on these perforated monopiles has only looked at the reduction in hydrodynamic loads, which have proven significant. These reductions in hydrodynamic loads should enable the perforated monopiles to be used in deeper waters compared to regular, non-perforated, monopiles. They could provide a tempting alternative for the more expensive jacket structures, but more research is necessary, especially in analyzing other loads that the perforated monopile may be subject to.

This thesis aims to look at one such different load that affects this perforated monopile, namely the installation loads induced by hammering. The first part of this thesis will look at stresses and fatigue damage during the installation of non-perforated monopiles. The second part will analyze the increased stresses, possible losses in hammer energy, and increased fatigue damage, all due to the presence of perforations. Finally, several alternatives, such as different geometries of perforations and different hammer loads will be analyzed with regard to their effect on fatigue damage.

The fatigue damage due to installation is found to increase significantly due to the presence of perforations, increasing from 5% for non-perforated monopiles, to up to 118% and 112% for the two most promising geometries analyzed, thus proving a show-stopper for installation via impact hammer, if no measures are taken.

Changing certain parameters, however, either the geometries of the perforations, or the characteristics of the hammer used, shows that installation is indeed possible. Using different geometries of perforations, that maintain a significant reduction in area, shows installation is possible, whilst limiting the fatigue damage to 53%. A reduction in hammer force by a factor of 2, also decreases the fatigue damage by 34% on average. The use of a so-called vibro-hammer also shows promising, resulting in a halving of the fatigue damage compared to the use of an impact hammer, but more research needs to be done to confirm this final finding.

To conclude, this research shows that installation of a perforated monopile is possible, although most, if not all of the reduction in fatigue damage due to hydrodynamic loading is cancelled out by the increase in fatigue damage due to installation. Geometries and installation methods may exist that improve the fatigue life of the structure, but this research was unable to find them. Future research may be able to find geometries and installation loads that do reduce overall fatigue damage.

Further research is also necessary before perforated monopiles can be taken into service, such as the confirmation of the energy losses in installation due to perforations. Also, several other load cases need to be analyzed, to ensure the perforated monopile survives its designed lifetime.

The potential for developing more efficient offshore wind support structures becomes increasingly critical as the offshore wind sector is faced with challenges such as rising commodity prices and shrinking profit margins. These challenges not only impact the industry’s capacity to meet growing global demand but also hinder efforts to address the imperative challenge of decarbonization. As the offshore wind industry expands, the roles of cost-effectiveness and ongoing research will be pivotal in driving the advancement of offshore wind energy on a global scale.

The support structure is responsible for supporting the turbine, transferring loads to the ground, and allowing access for inspection and maintenance purposes. The environmental loads acting on an offshore wind support structure (OWSS) result from a combination of waves, wind speed, turbulence intensity acting on the shape of the turbine components. These loads can be grouped in a scatter diagrams that depict the probability of occurrence of a given wind wave combined condition which is also known as a "sea state". Different sea states lead to significant vibrations and stresses on the foundation, which can cause fatigue and failure over time.

The perforated monopile consists of a monopile with holes around the splash zone to reduce frontal area, which reduces hydrodynamic loads on the foundation. Assesing the potential of perforated monopiles in deep waters was based on a comparative study of the loads acting on the monopile by developing a model that considers the structural response of the system to different sea states. FEM studies are essential in identifying potential stress concentrations and their effects on the overall integrity and safety of the structure. Thus, analysis focused on assessing the performance of both a reference monopile and a perforated monopile structural models under both parked and power production conditions, including sea states with 50-year and 1-year return periods. The simulations encompassed 35 distinct sea states and computed maximum stresses at critical locations, including the mudline, perforations, and the splash zone.

The study found that the perforated monopile displayed an average reduction of 17% in the maximum stresses found at the mudline compared to the reference monopile. This reduction was attributed to improved flow dynamics facilitated by the perforations. However, the splash zone of the perforated monopile experienced increased stresses around the splash zone of up to a factor of four, attributed to higher overturning moments around the perforated area. Next, the analysis continued with a fatigue life assessment, highlighting sea states that could potentially challenge the structural integrity and longevity of the monopiles over their intended operational lifespan.

The study explored alternative solutions, such as varying thickness parameters at high-stress areas and using different materials at the splash zone. Additionally, the possibility of different perforation geometries was considered, although it could impact natural frequencies and cause resonance with loading frequencies.

To conclude, the research underscores the importance of a comprehensive assessment of monopile designs in offshore renewable energy structures with a discussion and recommendations for further research on the subject. It emphasizes the need for careful consideration of design choices based on specific environmental conditions at the installation site. The findings contribute valuable insights into optimizing monopile designs for long-term performance and structural reliability in varying sea state conditions during power production scenarios.

To meet global green energy targets, the bottom founded offshore wind industry is looking for ways to economically expand markets to deeper waters. A reduction of the hydrodynamic load is necessary to achieve this. One option is to perforate the monopile around the splash zone. Here the related work of Q. Star is continued through the implementation of a 2D Navier-Stokes based LES CFD model with VMS closure in Gridap.jl. The numerical simulations are gathered to create a dataset on which a machine learning surrogate model is trained. The analysis does not include secondary design aspects such as manufacturing, installation, noise mitigation, scour, corrosion, acidification, or marine growth. Neither does it require secondary fluid phenomena such as 3D simulations, FSI, VIV, and fatigue.
Analysis of 2D perforated cylinders, built up parametrically using the wall thickness, angle of attack, diameter, inflow velocity, number of perforations and porosity as input parameters shows that the first two are of negligible influence, and number three and four can, at least for their mean values, be modelled accurately using Morison equations. A non-scaled diameter does prove important in reducing the random nature of frequency-based effects. The number of perforations and the porosity provide complex interactions both from a design-force mean and variability perspective, as well as for the frequencies and vibrations they generate. A 2D LES-VMS model gives the perfect trade-off between cost and accuracy, predicting a possible drag reduction of more than 50% compared to a closed cylinder with large diameter.
Different machine learning surrogate models are analysed with the goal of massively speeding up the analysis in the future. This is achieved by a factor of 350 thousand using Random Forests, Gaussian Processes and Neural Networks. Although the Gaussian Processes preliminary show the best accuracy, below 6% error for millisecond predictions, further work on Neural Networks could give them the advantage in future analyses.

The yearly installed percentage of offshore wind jacket substructures is rising. The most common installation method for a jacket structure nowadays starts by driving the foundation piles in the seabed through a pre-piling template. The jacket is then lowered on top until the legs are resting on the foundation piles through friction-based stopper connections. In order to rigidly fix the connection, grout is pumped into the annulus between the pile and leg. During the curing period of grout, generally taken as 24 hours, environmental loads cause the jacket to oscillate in various directions. As a result, the jacket leg will move relative to the foundation pile. This movement, which is called Early Age Cycling (EAC), can cause crack formation in the cured grout therefore decreasing the shear capacity of the connection. The DNV GL has restricted this relative movement to a conservative 1 mm within the first 24 hours due to a research gap on the subject. The strict regulation forces companies to use expensive EAC mitigation concepts of which the real effects are a debated issue.

The objective of this research is to gain insight into the modelling approach and the magnitude of EAC movements and investigate how they can most efficiently be minimized. This is achieved by investigating three phases: 1) simulating a number of load cases on a global jacket model and extracting interface forces near the seabed, 2) using these interface forces to assess the EAC movement on a detailed pile-leg reference model with a full circular stopper 3) analysing this reference stopper by testing three modified configurations. These configurations are designed with respectively two, three and four brackets yielding the same contact area. A sensitivity study is then performed by increasing the contact area.

The largest EAC movement within all models can be measured at the tip of the jacket leg. The location of this movement on the circumference of the leg varies based on the loading condition and stopper model. In general simulations on the reference model showed EAC movements below 1 mm due to a uniform stress distribution from the stopper to the foundation pile. For larger wave loads, sliding occurred resulting in large EAC movements. The initial modified configurations show significantly larger EAC movement when compared to the reference stopper. In general, the EAC movement decrease when the number of brackets increases. This is the result of a more even stress distribution around the circumference of the pile. A phenomenon visible for the two bracket stopper is rotation around the axis of the wave direction resulting in large EAC movement. This effect, denoted as moment induced rotations, should be limited by all means. A sensitivity analysis on friction showed that sliding could most efficiently be solved by increasing the friction coefficient. This is highly recommended since it greatly improves the performance of the stopper connection for larger waves.

This research could be further extended by performing a large sensitivity study to normalize the current results. This would be needed to verify whether the current conclusions can also be adopted for general use.

This report investigates the potential reduction of steel weight for offshore wind turbine supporting jacket structures, if conventional welded joints are replaced by innovative wrapped FRP joints. This new type of connection is under development by Dr. Marko Pavlovic at the Delft University of Technology, and shows outstanding fatigue performance compared to welded counterparts. As jacket structures suffer highly cyclic load, member thickness of current jackets is governed by the fatigue performance of welds. Due to the superior fatigue performance of wrapped FRP joints, substantial weight benefit is expected to be made. The study examines a jacket supported 5 MW wind turbine located in 50-meter water depth in the North Sea. The structure and model are based on the UpWind project. The model includes soil-structure interaction by non-linear depth-dependent springs along with foundation piles. Fatigue limit state (FLS) and ultimate limit state (ULS) are simulated by respectively five and three scenarios. The scenarios consider different combinations of wind (speed and direction), waves (height, period and direction) and current (speed and direction). Six 10-minute simulations are performed for each scenario with different wind turbulence and wave irregularity seeds. Wind and waves are applied in a single simulation, and normal force N and bending moments Mip and Mop time series are recorded at a selection of elements. The time series are post-processed in a self-written MATLAB procedure. For FLS, detailed fatigue analyses of welded joints are performed by evaluating crown and saddle hotspot stress, according to DNVGL-RP-C203. For every time step, the hot spot stress is calculated by applying geometry and load-dependent stress concentration factors (SCFs). Rainflow counting is applied, and the resulting stress range is projected on the details’ S-N curve to evaluate the damage. Linear Palmgren-Miner is applied to accumulate damage. A similar procedure, including stress concentration at thickness transition, is applied to calculate fatigue of elements. For ULS, welded joints are checked for chord face and punching shear failure. Members are checked for tension yielding, local buckling and global buckling. ULS calculations are performed for all time steps and according to Eurocode manuals. The unity check of both FLS and ULS is calculated for each individual member. Next, the member thickness is manually optimised to obtain the most optimal use of material. This optimisation is performed for three different cases with both mild S355 steel and high strength S690 steel. The welded steel structure, case 1, acts as a reference. The unwelded structure, case 2, is the lightest structure if joint fatigue does govern design. Case 3 gives the wrapped FRP structure and includes fatigue results obtained from small scale lab tests. Additionally, due to limited production length of steel tubular elements, it includes circumferential welds in the legs. The potential jacket weight reduction if wrapped FRP joints are applied is large, and the governing unity check shifts from fatigue to global buckling. For mild steel, the reduction of steel weight is more than 50%. The additional reduction of mass for high strength steel is low and not economical. The eigenfrequency of the wrapped FRP structure is viable, as it is outside operating frequencies. The results for the wrapped FRP structure are based on two major assumptions. Firstly, satisfactory joint performance can be obtained, and secondly, this can be accomplished by increasing wrapping thickness only. These assumptions should be verified by future experiments to support the weight reduction statement. In conclusion, the potential benefit of wrapped FRP joints to offshore wind turbine supporting jacket structures is large, and future experiments will show if, or to what extent, the full potential can be exploited.