the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Mar 2025
Scour variability across offshore wind farms (OWFs): Understanding site-specific scour drivers as a step towards assessing potential impacts on the marine environment
Abstract. The development of offshore wind farms (OWFs) is critical to meeting renewable energy targets, but predicting scour around offshore wind energy structures (OWES) and the associated potential impacts on marine ecosystems remains a challenge. Using high-resolution bathymetry data, this study analyses field-measured scour depths at 460 monopiles at nine British OWFs. The analysis reveals a large spatial variability of relative scour depths (π/π·) between OWF sites, but also within individual wind farms. Principal component analysis (PCA) is used to identify significant drivers of this variability. When the entire data set is considered, results indicate that median grain size (π·50), relative water depths (β/π·), and the significant wave height (π»π ,99) are the most important influencing factors for the variability of scour depths. Other parameters investigated, such as Froude number (πΉπ), pile Reynolds number (π π), flow intensity (ππ,99/πππ), and current velocity (ππ,99), were found to have a less clear influence. Further sediment-specific analysis shows that relative water depth (β/π·) is a particularly relevant driver of scour at sites with fine (63 to 200 ππ) and medium sands (200 to 630 ππ), with larger scour depths occurring in shallower water depths. Findings from this study provide new insights into scour behavior across a range of spatial and environmental scales and lay a foundation for the transferability of scour prediction frameworks to new OWF sites. In the future, findings and datasets from this study are suggested to be used to estimate scour-induced sediment transport and thereby to provide a step towards the assessment of potential impacts of OWF expansion scenarios in the marine environment. By addressing the broader implications for regional sediment dynamics, this research contributes to the sustainable development of offshore wind energy.
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RC1: 'Comment on wes-2025-41', Anonymous Referee #1, 20 Apr 2025
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The paper investigates the variability of scour depths around monopile foundations in offshore wind farms (OWFs) and identifies the key drivers influencing these scour processes. The study utilizes high-resolution bathymetry data from 460 monopiles across nine British OWFs to analyze spatial and environmental factors affecting scour. It should be acknowledged that the measurement data from the present study are of great engineering value to predict the scour before the planning of OWF. The methodology of interpreting the data is reasonable, and the general conclusion from the interpretation is plausible. The primary problem is that the overall manuscript lacks physical analysis and correlation of the scour depth with the hydrodynamic quantities. This is understandable due to the lack of critical Shield parameters, which are directly linked to the scour process. Furthermore, in shallow water regions, it seems that the wave characteristics associated with the KC number play an important role in determining the scour depth. As pointed out by the authors, this parameter is not analyzed due to the limited measurement. Despite this lack, the physical interpretation of this manuscript can still be enhanced by estimating the values of the Shields and KC based on some assumptions. For example, combined with Re and the roughness associated with the grain size, the shear stress can be estimated based on a log law assumption. Combined with the dispersion relationship, knowing the water depth, the wave-induced velocity can be estimated so that the KC value can be estimated. Some other suggestions to improve the manuscript are listed as follows:
- On Page 6, it is stated that the Froude number influences the scour depth. However, Fr is more related to the free-surface waves. How can it be related to the scour depth at the seabed? The analysis of the mechanism, which is stated to be related to the pressure gradients at the pile, is not clear. How can the free-surface waves affect the pressure gradients around the pile? How are the pressure gradients associated with the scour process? The authors should describe these problems in more detail, at least by providing some reference studies.
- As an important component of the analysis, the process of performing the PCA should be elaborated in more detail. For example, how to arrange the current measurement data into matrices and how to compute the correlation angle should be introduced.
- On Page 17, the discussion on the influence of the water depth on the scour depth is not sufficient, especially regarding the unexpected decrease in the scour depth with the increasing water depth. The authors stated that it is expected that a large water depth led to a large boundary layer thickness. However, the boundary layer thickness is not necessarily related to the water depth and more associated with Re and seabed roughness. Furthermore, the discussion on the pressure fields is rather superficial. Why does the increasing water depth result in more uniform pressure fields? Why does thinner boundary layer lead to a large shear stress?
- It is difficult to understand the influence of Fr on the scour depth. Especially, as shown in Figure 6 (e), it seems that there are only two exceptional points, and the correlation is not strong.
- On Page 30 from Line 592 to 597, it is stated that the wave dynamics play an important role in determining the scour depth. The authors should provide a more detailed discussion on this subject by estimating the KC number based on the wave height and water depth.
Citation: https://doi.org/10.5194/wes-2025-41-RC1
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