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...