Scour protections around monopile foundations of offshore wind turbines usually rely on loose rock as the main material, and on traditional construction methodologies. But how would a concrete prefabricated element perform instead of rock? This question, together with the particular focus on Xstream as main element, is at the heart of this master thesis. The research goal is to determine and describe the hydrodynamic stability of the Xstream elements, focusing on their application near monopile foundations under wave-only loading. The test program is mainly based on the conditions at the southern North Sea. While two main authors (Izbash (1935) and Shields (1936)) have set the foundation of the principles of stability of rocks under flows, the proliferation of offshore wind energy (and of submarine cables) motivated the creation of the JIP-HaSPro (Handbook of Scour and Cable Protection Methods), greatly based on studies by Broekema et al. (2023), and used in a considerable measure for this research. The present study was conducted as a physical modelling test program in the laboratory of BAM in Gouda, in a flume with a wave generator. A scale of 1:15.71 was used. Due to logistical reasons, two distorted scales were applied: the first (and main) distorted scale, with which correctly scaled orbital velocities near the bed were generated, while smaller water depths and wave heights (fitting in the flume’s height) could be applied; the second distorted scale consisted in the reduction of the pile diameter, so that it fitted in the flume’s width (this made the results somewhat conservative compared to the original prototype pile diameter of 10m); half monopile was adopted, adjacent to one of the flume walls. A rectangular form was adopted for the model scour protection, still conscious that a circular shape concentric with the monopile is the most suitable. The layer thicknesses were based on packing densities related to area. No sediment was used, i.e. the block layer was laid on a fixed bed. A group of 6 test series were carried out, each featuring a different layer thickness and two placing methods with different packing density as a result. Each test series consisted of 8 wave conditions with the same peak period (Tp,m = 3.2 s) and significant wave heights varying from 0.08m to 0.26m. The tests were 40 minutes long, aiming for a number of 1000 waves. Three types of movement were observed during the tests: rocking, internal displacements (i.e. all displacements larger than about 1 unit diameter), and removal from the protection (external displacements). The data collected consisted of a. wave data, measured by the wave gauges and processed in WaveLab; and b. block movement data, based on observations aimed at the three types of movement. The first feature noticed during the tests was the formation of patterns of erosion and deposition within the construction area, in the form of a trench and a mound, next to the half monopile and right behind it, respectively. The amount of blocks rocking grew gradually with the wave conditions, while both the internal and external displacements showed a delayed growing tendency with scattered peaks...

Recent research identified a novel scour-enhancing flow mechanism related to 3-D flow behaviour. Under specific conditions, vertical flow separation at the upstream crest of a scour hole can be suppressed as a result of lateral nonuniformities in the main flow. This so-called vertically attached flow state is associated with increased scour potential. The objective of this study was to evaluate the effects of lateral nonuniformities on scour-driving flow behaviour in setups with realistic width-to-depth ratios. Laboratory experiments form the backbone of this study. A measurement technique was developed that enables efficient and reliable determination of near-bed flow directions. The technique is based on tufts: small threads fixed to the bed that follow the local flow direction. An algorithm was created to convert video footage of tuft movement to numerical data on flow directions. Next, flow behaviour was studied through a set of experiments conducted in a simple laboratory setup that simulates a common scour situation in the field. Lateral nonuniformities were generated in a mixing layer downstream a lateral expansion of the inflow channel. Two distinct horizontal flow states were observed across the tested configurations: horizontal divergence and horizontal contraction. Horizontal contracting flow was more likely to occur in geometries with a short bed protection, a large horizontal expansion and a narrow inflow channel. The transition point between both flow states was narrow. Only a slight change in geometry could entirely turn around the flow state. The horizontally diverging flow state is accompanied with constant vertical flow separation at the slope's crest. The corresponding return current entails a near-bed flow in upward direction. The horizontally contracting flow state is related to an alternating vertical flow pattern in which vertical flow separation and vertical flow attachment take turns. An alternating vertical flow regime was never identified before in this context. The alternating flow state is not present in the jet's centre. Instead, vertical separation takes place here. Additionally, a consistent lateral near-bed flow from the mixing layer to the centre of the jet is present. The frequencies in which vertical attachment and separation alternate match the frequencies of two-dimensional coherent turbulence structures (2DCS) in the mixing layer. This similarity indicates a relation between mixing layer instabilities and suppression of vertical flow separation, which is a valuable new insight into the process of understanding nonuniformity-induced flow attachment. The presence of the horizontally contracting and vertically alternating flow state likely increases scour dimensions. As a scour hole develops, unidirectional lateral near-bed flow velocities stay relatively high, maintaining scour potential. Scour potential is largest in and around the mixing layer. This most likely results in a strongly 3-D scour hole with maximum scour depths in the areas directly affected by the mixing layer. Scour hole development in previous experiments on scour in similar setups confirms this scouring behaviour. The premise that excessive scour is caused by an alternative flow state provides the opportunity to further improve and extend the abilities of empirical scour prediction.