• Energy Research

Scour processes and corresponding scour protection measures have been extensively studied for vertical cylinders. Several formulations for estimating scour and determining the optimum size of rocks for scour protection are widely available in the literature. However, when studying other types of structures, the geometric variability of the structures, limits the application of available semiempirical formulations. Scour phenomena around complex structures have been investigated experimentally in reduced-scale wave-current basins. However, the number of tests focused on support typologies different from monopile structures is still limited, restricting the availability of semiempirical formulations focused on conceptual design of scour protection solutions or in the estimation of the free scour development. In this work, a better understanding of the scour processes around gravity-based foundations (GBFs) and the performance of different scour protection solutions is gained via experimental results obtained from a large-scale test programme (1:35 test scale), including combined wave and current tests. The experimental tests were divided into two main types: (1) free scour and (2) scour protection tests. The main conclusion was that the triggering of a horseshoe vortex is the main driver of scour processes together with the contraction of flow lines at the foundation contours. Since monopile semiempirical formulations are not suitable for scour prediction, a new semiempirical approach was obtained from the test results. Moreover, two distinct scour mitigation methods were analysed (rock protection and concrete mattresses). Based on the undisturbed mobility parameter, statistical and dynamic limits were defined considering the rock grading tested. Finally, for concrete mattresses, the design driver was identified, and the adaptability of these types of solutions to seabed changes was verified

Bottom-fixed offshore wind turbines are generally built on continental-shelf sections that are morphodynamically active due to their shallow depths and severe wave and current conditions. Such sites are commonly protected against scour to prevent the loss of structural stability. Scour protection can be designed using static or dynamic solutions. Designing dynamic protection requires experimental validation, especially for singular or unconventional structures. This article presents an experimental method for the laboratory analysis of scour protection for jacket foundations placed at morphodynamically active sites. The test campaign was conducted within the project East Anglia ONE (UK) as part of the asset owner studies and aimed to evaluate operation and maintenance (O&M) aspects, independent of the contractor’s original design assessments. The physical experiments explored morphodynamic changes on the sea bottom and their importance to scour protection, as well as the importance of the history of the wave loads to the deformation of the rock scour protection. This was explored by repeating different cumulative tests, including a succession of randomly ordered sea states (Return Period (RP) 1-10-20-50 years). The experimental results show that the deformation of the rock sour protection was the greatest when the most energetic sea states occurred at the beginning of the experimental test campaign. The maximum deformation was at 5D50 when the first test was also the most energetic, while it was at 3D50 when not included as the first test, yielding a 40% reduction in the scour protection deformation.

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.

Abstract Understanding the hydrodynamic performance of floating energy converters is a complex challenge. Hence, physical modelling is necessary to evaluate the performance of innovative designs and validate them. The present paper shows the experimental work performed to validate a new floating semisubmersible structure which combines wave energy converters (3 Oscillating Water Columns, OWC) and wind harvesting (5 MW wind turbine). To characterize the global response of the platform, as well as the OWCs’ performance, an innovative wave tank testing campaign was carried out at the Cantabria Coastal and Ocean Basin (CCOB). The multi-use platform was characterized under the incidence of regular wave tests (with and without wind), operational sea states and survival sea states (combining waves, currents and wind). During the tests wind was reproduced with a portable wind generator and the wind turbine was simulated as a drag disk. The OWC air turbines were experimentally conceptualized by different diameter openings on the upper part of each OWC. This paper describes the experimental testing campaign carried out at the CCOB and presents the most significant experimental results obtained, such as natural periods, movements, loads on the mooring system or accelerations, which are representative of the performance of the multi-use platform presented.

European oceans are subject to rapid development. New activities such as aquaculture and ocean energy have gained importance. This triggers interest in “multi-use platforms at sea” (MUPS), i.e., areas at sea in which different activities are combined. MUPS are complex features with regards to technology, governance, and financial, socioeconomic, and environmental aspects. To identify realistic and sustainable solutions and designs for MUPS, the MERMAID project applied a participatory design process (PDP) involving a range of stakeholders representing companies, authorities, researchers, and NGOs. This paper evaluates if and how the participatory design process contributed to the design of multi-use platforms. It is based on interviews with the managers of the case study sites and a questionnaire administered to all stakeholders participating in the PDP workshops. Analyzing the four case studies, we conclude that the participatory design process has had a valuable contribution to the development of the four different designs of MUPS, even though the preconditions for carrying out a participatory design process differed between sites. In all four cases, the process has been beneficial in generating new and shared knowledge. It brought new design issues to the table and increased knowledge and understanding among the different stakeholders.

The EU H2020 MaRINET2 project has a goal to improve the quality, robustness and accuracy of physical modelling and associated testing practices for the offshore renewable energy sector. To support this aim, a round robin scale physical modelling test programme was conducted to deploy a common wave energy converter at four wave basins operated by MaRINET2 partners. Test campaigns were conducted at each facility to a common specification and test matrix, providing the unique opportunity for intercomparison between facilities and working practices. A nonproprietary hinged raft, with a nominal scale of 1:25, was tested under a set of 12 irregular sea states. This allowed for an assessment of power output, hinge angles, mooring loads, and six-degree-of-freedom motions. The key outcome to be concluded from the results is that the facilities performed consistently, with the majority of variation linked to differences in sea state calibration. A variation of 5–10% in mean power was typical and was consistent with the variability observed in the measured significant wave heights. The tank depth (which varied from 2–5 m) showed remarkably little influence on the results, although it is noted that these tests used an aerial mooring system with the geometry unaffected by the tank depth. Similar good agreement was seen in the heave, surge, pitch and hinge angle responses. In order to maintain and improve the consistency across laboratories, we make recommendations on characterising and calibrating the tank environment and stress the importance of the device–facility physical interface (the aerial mooring in this case).