Understanding the scour processes around offshore structures in shallow or intermediate waters is critical for ensuring their lifetime stability. Currently, there are many different formulations for simplified geometries such as monopiles, however the number of semi-empirical formulations available for complex structure types is very limited. In this context, an experimental test campaign was carried out at the IHCantabria facilities with the aim of evaluating the evolution of scour and the mechanisms that govern it on a jacket-type structure that is often used for offshore substations in offshore wind farms. The stability of this structure is ensured by pile clusters consisting of several elements, such as mud mats, pile sleeves, stiffeners and piles. Therefore, the complexity of the elements that are in direct contact with the seabed is maximal. A series of experimental tests were carried out considering the action of the current over a sandy seabed and under live bed regime conditions (ʘ>ʘcr). The physical experiments enabled us to identify the global scour patterns around the foundation, as well as the local scour around the most exposed elements. In general, local scour developed only around the elements in contact with the seabed (i.e., pile clusters). Local scour at the most exposed pile clusters occurred in two successive phases: (1) phase 1-mud mat scour (scour around the mud mats, resulting from the blocking effect of all pile cluster elements), and (2) phase 2-local pile scour (local scour around each pile). A dimensionless scour depth function was derived from the experiments to predict the total scour at the pile cluster contours. For this purpose, the pile cluster geometry was simplified as a vertical truncated cylinder (equivalent diameter Deq and height). The dimensionless scour depth [a,ß] was estimated considering only the equivalent pile cluster diameter [Seq = a *Deq, where a = 0.329 - 0.389] and taking into account a correction height factor because the pile clusters do not cover the entire water depth [Seq = ß*Deq*Kh, ß = 1.1 - 1.3]. The values obtained were consistent throughout the whole test series and are valid for predicting the total scour around similar complex-type structures (pile clusters).

The authors would like to thank DRAGADOS OFFSHORE for their support and funding for conducting this research. This study is part of the ThinkInAzul programme and was supported by the Ministerio de Ciencia e Innovación with funding from the European Union NextGeneration EU (PRTR-C17. I1) and Comunidad de Cantabria. This work is also part of the R&D project “Advanced methodology for foundation of critical structures in offshore wind farms (CREMA)” funded by MCIN/AEI/10.13039/501100011033.