In the Rhine-Meuse delta many scour holes are present. These scour holes form a potential risk on the surrounding bridges, levees, weirs, dikes and tunnels. In general, scour holes can develop when there is a local change in hydrodynamic conditions or erosive capacity. Extensive research has been done in the past on scour holes close to hydraulic structures in a homogeneous subsoil, however, a large amount of scour holes are not related to hydraulic structures. Additionally, in deltas, the subsoil mostly consists of alternating layers of soil with different erosive capacities. This experimental study focused on a simplified situation of scour holes in heterogeneous subsoil with a three-dimensional character. The bed of the flume consisted of a concrete layer with a hole in the centre in which sand was exposed to the flow. Water depth, flow velocity and the size of the geometry were systematically varied to investigate their influence on scour hole development. Additionally, general observations were performed on the flow structures and scour development in and around the scour hole. It was observed that the scour development is different compared to the results of earlier investigations as a result of the non-erodible top layer. The process of undermining is a limiting factor for sediment transport out of the scour hole. The water depth showed a significantly smaller influence on the scour depth compared to existing empirical formulas. Additionally, the size of the geometry seems to be the most important varied parameter in this study. Relative to the size of the geometry, the scour depth remained approximately equal with different experimental conditions. Compared to two-dimensional experiments, the scour holes became deeper, which suggests importance of three-dimensional flow for scour hole development.

Current trends show that the offshore wind industry is evolving into deeper waters, with larger turbines and consequently larger foundations. In Europe 80% of the wind turbines are installed on monopile foundations and this is expected to be the dominant choice in the future. The shallow geology of most of the wind farms is characterized as sand. A known phenomenon around foundations in sandy soils is the occurrence of scour, due to a disturbance in the flow caused by the waves and currents. Scour affects the embedment depth and natural frequency of the turbine. In order to prevent these effects a commonly used technique is to install a rock blanket around the monopile, to act as scour protection. Monopiles are mostly driven into the soil with a hydraulic impact hammer. The blows of the hammer result in an elevated noise level. Within the German EEZ strict rules are set regarding the allowable noise level. Noise mitigation measures, such as a noise mitigation screen, are used to prevent exceedance of the sound exposure level, see figure 1. In an attempt to reduce the Levelised Costs of Energy (LCOE) of offshore wind, cost-saving optimizations are considered such as the installation of a single layer scour protection before foundation installation. The conventional two layer system requires two mobilization campaigns one for the filter and the second for the armour layer, after monopile installation. The single layer solution results in a reduced installation time and no risk of collision with the monopile. The NMS is lowered on the scour protection and the monopile is driven through the NMS. During the installation the NMS penetrates in the scour protection and deformations are observed, see the area close to the monopile in figure 2. Remedial works are very expensive and therefore one would like to know what the impact is on the performance of the scour protection during the lifetime of the wind farm. The project is based on a review of literature regarding relevant topics such as: soil mechanics, scour protection, noise mitigation and installation processes. Because of the problem’s novelty, the available research is limited. In order to acquire insight in to the problem there is a need to investigate which processes are involved and relevant. This is done by collecting all available field data from selected offshore wind farms installed with a NMS, such as CPT data, survey data and hammer logs. The analysis of the available data shows that the main factor influencing the NMS penetration is the combination of the use of a NMS together with the hammering procedure. Based on the available soil, hammer log, NMS and pile penetration data it is believed that the NMS imprint is a consequence of compaction of the underlying soils and scour protection. Compaction of underlaying soils is seen as having a beneficial effect on the scour protection system. Further it is investigated whether a large NMS penetration combined with a high relative mobility for the scour protection during storm conditions can lead to undesirable effects. Time series of environmental conditions were reconstructed from field measurements and numerical model hind casts. These time series serve as input to assess the relative rock mobility between installation and most recent bathymetry survey. Locations are verified for the imprint depth and relative mobility and a potential worst case is identified. The identified worst case is evaluated and validated against model tests of similar conditions. The outcome of the study is that limited rock movement has occurred from the edges of the imprint into it, but that the scour protection is still performing well after a 5yr design storm. This is also confirmed by model tests. A potential weak-link is identified in the slope of the imprint where the scour protection thickness is reduced. Practical mitigation and monitoring measures are proposed, but further research is needed to verify whether the risk of winnowing can appear within the slope of the deformed part of the imprint.

In the Dutch coastal waters more and more wind turbines on monopile foundations are built. An important design parameter is the first natural frequency of the wind turbine. The first natural frequency is dependent on environmental conditions and different soil conditions such as scour. This research gives a better insight into the sensitivities of these influences on the first natural frequencies. The first objective was to determine the sensitivity of different influences on the first natural frequencies in a full scale test environment. The first natural frequencies of four wind turbines (two protected against scour and two not protected against scour) in the Eneco Luchterduinen wind farm are constantly monitored and the natural frequencies are identified. It was found that only the water level has an identifiable influence on the identified first natural frequency and that the design natural frequencies and identified natural frequencies differ significantly. After the analysis from the full scale test different influences on the first natural frequency in a numerical computer model are investigated. The computer model predictions confirm that the water level has an identifiable effect and only small effects of backfilling of the scour hole on the first natural frequency are found. The third objective was to compare the sensitivity of different influences on the first natural frequency obtained from the computer model and from the full scale test. The effects of the water level are comparable. And the effect of backfill of the scour hole was found in the model but not in the full scale test. Therefore no conclusion is made on the effect of backfill nor on the presence of backfill. It was found that in order to mitigate the differences in first natural frequency of the test turbines and the model the soil should be modelled twenty times stiffer. The main influence on the first natural frequency of the offshore wind turbines in the Eneco Luchterduinen wind farm is the soil stiffness. The only other identifiable influence was the water level. Also in the design of the wind turbine the soil stiffness is underestimated significantly, implying overconservative designs.

Over the recent years, the Dutch Continental Shelf has seen the erection of various wind farms in an effort to counter climate change by replacing fossil-based power plants. Since the majority of these sites use scour protections which have a substantial impact on the surroundings, the Dutch government increasingly stimulates and requires ecological enhancement of such structures in order to restore the natural habitat and maintain the marine ecosystem of the North Sea. Oyster enhancement has been one of the methods used for the ecological enhancement of offshore wind farm scour protections. However, studies report that a successful structure for the harbouring of an oyster population is yet to be developed. Existing structures were found to be unstable or unable to provide a suitable habitat for the oysters (Bouma, Belze, et al., 2013; Stokes et al., 2012). The study at hand aims to develop a successful design for oyster broodstock structures for nature enhancement in offshore wind farms. It was conducted for Van Oord as part of project Ecoscour. To establish design criteria for the oyster broodstock structures, oyster habitats and the Borssele V site were analysed and a brainstorm session with partners from project Ecoscour was set up. The final criteria concerned: ecology; structural integrity; commissioning & construction; and monitoring. The design criteria were used to develop several viable structure designs. In order to rapidly assess the strength and stability of potential designs, a Python-based open source tool was built named Econstruct. With this tool, geometries can easily be evaluated under various influences, accounting for the effect of marine fouling. This could be used for quick investigations of loads, stability and structural integrity. The Econstruct tool was validated and analysed for sensitivity for the input parameters, of which the input radius and current velocity had the highest influence on stability. Using the environmental input parameters that hold for the Borssele V wind farm, a terraced design was proposed. After consultation with the manufacturer of the structure, D & M engineers, this proposal was altered to account for manufacturing limitations. The definitive design was evaluated using the Econstruct tool and found to be stable. It was then manufactured, and ultimately placed in Borssele V in October 2020. Several recommendations are made for future research and the design of ecological enhancement structures for offshore wind farms. Most importantly, the Econstruct open source tool can be used in the future for the design of structures for other ecological enhancement projects such as coral reef restorations.

Due to the presence of a monopile in a turbulent flow swirling vortices, known as horseshoe vortices, can form upstream of the structure near the cylinder-bed junction. Those vortices then travel downstream of the cylinder as they are being dragged along with the current. The horseshoe vortices can scoop up bed material surrounding the cylinder, weakening the foundation of the structure. This phenomenon is known as scour. A common protection method to reduce the effects of scour is the dumping of stones in the vicinity of the cylinder-bed junction. The aim is to alter the flow dynamics near the bed in such a way that the underlying sediment is mostly unaffected. However, the protection material itself is also subject to hydrodynamic forcing and can therefore be entrained as well. This would be described as damage to the protection layer as its effectiveness reduces due to re-exposure of the underlying sediment. It is for this reason that protection layers must be carefully designed depending on their use. The mechanism of the horseshoe vortex system and its damaging aspects are not completely understood yet, making engineering of specific protection layers challenging. To better understand the flow behaviour upstream of a cylinder-bed junction, where the formation of the horseshoe vortices takes place, several tests have been performed at the Eastern Scheldt Flume in the Hydro Hall of Deltares. Here different cylinder and bed protection configurations were subjected to turbulent current and wave conditions where the upstream flow fields were measured with a PIV measurement system. Since the obtained information of the tests is limited to a specific area, there was an interest in modelling the test cases by means CFD, while using the test results as validation. In this thesis several of the test cases have been modelled with the Improved Delayed Detached Eddy Simulation (IDDES) hybrid turbulence model, based on the Spalart-Allmaras formulation, which include the following: a) Open channel with a smooth bed and no monopile b) Open channel with a rough bed and no monopile c) Open channel with a smooth bed and a smooth monopile d) Open channel with a rough bed and a smooth monopile The modelling strategy used starts off with a basic simulation case (a) which is compared to the test results for validation of the simulated flow characteristics. This model is then elaborated on by inclusion of either a rough protection model (b) or a smooth cylinder (c). Both models are again validated with the test results so that the ability of the IDDES model to model the fluid-structure interaction is investigated. Finally, a simulation that includes both a rough bed and a smooth cylinder (d) is performed. For simulation models b, c and d an assessment is made for potential initial particle entrainment by means of hydrodynamic forcing indicators. Additionally, the effects of bed model surface roughness on the entrainment of bed material are briefly considered.

In this research the hydrodynamic stability of a ballast filled mattress under waves is investigated. There is checked if this can be applied as scour protection around a suction bucket jacket: an offshore wind turbine foundation that uses a vacuum to suck itself in position. A ballast filled mattress is a mattress made from polyester with so called spacer threads that take up tensile forces when the mattress is trying to deform. The combination of stiffness and combined weight make the ballast filled mattress a possible new scour protection.

For the offshore wind industry it is essential to further reduce costs to be competitive to traditional energy resources. One of the options to achieve so is optimizing support structure design. Using XL-monopiles allows for larger turbines, deeper water access and consequently can provide for a more cost effective support structure. Within this market, Energieonderzoek Centrum Nederland (ECN) operates with its in-house developed software package to evaluate turbine responses. To further improve the model, a better insight is needed in breaking wave impacts on structures. This thesis is focused on developing an enhanced method to evaluate breaking waves on monopile structures to identify the effect of slamming waves on XL-monopiles. Based on wave tank measurements, a method to identify slamming wave impacts is developed and tested. Later, those wave tank measurements are reproduced and the identification method verified. Wave tank experiments were carried out at the Atlantic Basin at Deltares within the joint industry research project WiFi. For the experiments, two monopile scale models were placed in the wave tank, both equipped with multiple load and pressure sensors. During the tests the monopiles were exposed to series of wave trains, the irregular wave trains with a total of approximately 5000 waves, created a large sample size needed for the experiment. The wave measurements from the tank are analysed numerically to identify breaking waves. In this thesis a slamming wave event definition is proposed as follows: • Front crest steepness 푆 should reach breaking limit • The slamming impact of the wave should be more than 4 times the standard deviation of the force time series The numerical computations are carried out using the potential flow solver OceanWave3D to generate a sea state with comparable characteristics to the wave tank measurements. At first, the generated sea state appeared to lack the necessary wave height. By adjusting the input in the OceanWave3D program, a sea state was found that matches the measurements. Wave energy dissipation is checked throughout the measurements wave tank and compared to dissipation in the numerical model. It was shown that both the wave tank and numerical model show resembling dissipation. Hydrodynamic loading on the monopile foundation is assessed using the kinematics obtained from the OceanWave3D model. Several methods were combined to come to the total wave loading. First the Morison approach was used to account for the non-slamming part for the wave load. Using OceanWave3D, the force coefficients are calculated applying DNV guidelines. The second part, the slamming part of the wave load, is obtained in multiple phases. First, based on the above mentioned slamming wave criteria, wave steepness and impact forces, the individual waves are evaluated and scored as potential slamming. Secondly, using conservation of momentum and kinematics derived from OceanWave3D, the slam load is calculated. This slam load is determined evaluating the impact velocity of all waves individually and added to the Morison load if identified as slamming. Finally, when the calculated impact loads from the numerical model are compared to the recorded loads in the wave tank measurements, large similarity can be noticed. After comparison of the developed slam load representation to the DNV method of slam load estimation, the result is a less conservative slam load approximation. This is due to the evaluation of slamming impact velocities per wave, opposed to the assumption of a single impact velocity for the whole sea state. The results of this thesis can further implemented in turbine response evaluation tools by ECN resulting in a more optimized calculation model. It will enable the design of more (cost) efficient XL-monopile structures.