In this study, preliminary results from scaled model tests (1:45) in a 3D-wave-current-basin are presented. A 4-legged jacket model has been subjected to waves, current and combined boundary conditions. The influence of the jacket geometry on resulting flow patterns is analyzed and compared to flow patterns around pile groups. In the consideration of current induced flow patterns along the jacket structure it becomes evident that a flow around a jacket structure cannot easily be approximated by direct transfer of knowledge from in-line pile groups. For blockage factors Af/At < 0.3 an increase in wave energy inside the structure towards a maximum of twice the incident wave energy has been documented.
Two key aspects of the expansion of offshore wind energy into greater water depths are the increase of substructure geometries and the preferred use of jacket-type structures. Jacket structures are considered hydrodynamic transparent structures. An offshore structure is regarded as hydrodynamic transparent if the progressing wave train is influenced or altered in physical presence of the structure only to a marginal degree. As base widths, pile diameters and degrees of structural blockage increase in size, the question arises as to which extent a large jacket foundation can still be classified as hydrodynamic transparent.
In-depth understanding of the current and wave induced flow field around jacket structures has yet to be achieved. The interaction of nonlinear influences on a geometrically complex structure and the resulting instationary flow processes form a highly multidimensional system. A discussion is required if a comparison to flow patterns around less complex offshore structures like pile groups can serve as an initial step to approach this system. The influence of the jackets larger main pile spacing, steel nodes, braces and cross-sectional or secondary piles has to be assessed.
Existing literature on jacket type offshore structures regarding laboratory tests and data collection is only rare. Main objectives of existing hydrodynamic studies are more complex structures (Murray et al., 1995; Gu et al., 1996) or focus on determining slamming forces from breaking waves (Aune et al., 2011; Navaratman et al., 2012; Jose et al., 2017; Arntsen et al., 2013; Loukogeorgaki et al., 2016; Chatjigeorgiou et al., 2018). Beyond these objectives, some hydrodynamic processes have been discussed in the context of scour research (Chen et al., 2014; Welzel et al., 2018; 2019). In contrast to the concept of hydrodynamic transparency, Welzel et al. (2018; 2019) found that wave-driven erosion due to flow contraction induce global scouring underneath and around a jacket structure.
