As offshore wind energy expands, the environmental impact of monopile foundations on ocean stratification has become a growing concern. Thermal stratification occurs when layers of seawater with different temperatures form due to solar heating, creating a stable density gradient that limits vertical mixing. However, monopile-induced turbulence can disrupt this layering, a process known as destratification, which may have significant ecological consequences.

This study investigates a novel perforated monopile design as a potential solution to mitigate these effects. Using Computational Fluid Dynamics (CFD) simulations in OpenFOAM, the research compares the hydrodynamic behavior of regular and perforated monopiles, focusing on turbulence intensity, wake formation, and destratification rates.

The results indicate that perforated monopiles reduce turbulence intensity and modify wake dynamics compared to regular monopiles, by allowing partial water flow through the structure and reducing recirculation zones. However, their impact on destratification rates remains minor. While perforations alter hydrodynamic behavior, they do not significantly mitigate stratification breakdown.

These findings suggest that while perforated monopiles influence flow dynamics, their effectiveness in reducing destratification is not significant enough to definitely recommend these structural modifications. Future research should explore different perforation patterns and other alternatives develop more sustainable offshore wind turbine foundations.

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.

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.

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.