Volume-Based Assessment of Erosion Patterns around a Hydrodynamic Transparent Offshore Structure
6. Summary and Conclusions
Only a few studies exist which provide an approach to analyze complex erosion patterns around offshore foundations. Therefore, hydraulic model tests were carried out, investigating the spatial erosion process in the near-field and vicinity of the hydrodynamic transparent jacket-structure in combined wave and current conditions. The main conclusions can be summarized as follows:
• Different volume analyzing methods and dimensionless parameters are introduced which can be generally adapted for any other offshore structure or coastal structure to reveal physical processes in complex erosion patterns. Therefore, eroded sediment volumes are normalized in relation to a structural volumeVD,ias well as in relation to the considered erosion area,VA,i,VI,iand the structural diameterDV,i, given in Equations (3)–(6).
• A comparison between locally (circle area of 6 D around each pile) and globally (area of 1.25 times the structure footprint) attributed erosion volumes revealed that wave dominated hydrodynamic conditions withUcw≤0.57 led to scour patterns which were dominated by local erosion around the piles (68% locally, 32% globally, forUcw=0.57). Furthermore, it is shown that the share of globally eroded sediment volume is significantly increased in current dominated conditionsUcw
≥0.75 (33% locally, 67% globally, forUcw=1.0).
• The literature reports that hydrodynamic interactions between groups of circular piles are small if the distance between them is larger than six times the piles’ diameter [14–16]. In the past, this was partly interpreted as a border beyond which global scour around jacket-type foundations may not occur [10]. In contrast to this, insights from the present study illustrate that the area of the seafloor affected by a supposedly transparent hydraulic structure is considerably larger than expected and is estimated to be 2.1–2.7 times the structure’s footprint for the present study.
• A comparison reveals that findings stemming from the present study generally agree well with in-situ data from field studies [8–10]. Similar areal distributions of eroded sediment volume with a stable maximum of the erosion intensity at 1.25 A (1.25 times the structure’s footprint) as well as a global scour extent in a similar range to the present study (2.1–2.7 A) is found from a comparison of in-situ data (2.7–2.8 A).
• To improve the prediction of global scour around jacket-type offshore foundations, empirical expressions (Equations (7) and (8)) are proposed to account for the areal development and extent of global erosion volumes and scour depths in the near-field and vicinity of the foundation structure.
The analysis and derivation is explained stepwise and is based on insights of the introduced methods. Furthermore, the knowledge of the extent of erosion patterns in relation to the erosion intensity, as well as of the value of the eroded sediment at different points, is useful for the design of a scour protection system around such complex foundation structures. While the former might be used to determine the required spatial extent of a scour protection, the latter helps determining the volume, which has to be refilled by a subsequently installed scour protection.
• Results allow a prediction of areas which exceed a certain erosion rate as well as a quantification of spatially eroded sediment in the near-field and vicinity of the foundation structure. By this means a structure-induced environmental footprint as a measure of eroded sediments partially affecting marine habitat can be exposed. Once eroded sediment is entrained into the water column it deposes behind the structure with the effect of burying marine habitats and can be transported over long distances due to long lasting vortices and an increased turbulence and mixing [5–7].
As a consequence, not only areas in the vicinity but also in the far-field of the structure can be affected, with potential impacts [2–4] on the marine wildlife and the ocean seabed environment in general. These potential impacts to the marine environment might represent an important hurdle for the future of wind technology in general.
Author Contributions:For this research article the specifying of the individual contributions of the authors is as follows: idea and concept of the article, M.W.; methodology, M.W.; investigation and volume calculations, M.W.;
resources, T.S.; writing—original draft preparation, M.W.; writing—review and editing, M.W., A.S., T.S. and A.H.;
supervision, T.S. and A.H.; project administration, T.S. and A.H.; funding acquisition, T.S.
Funding:The present study is part of the research project “HyConCast—Hybrid substructure of high strength concrete and ductile iron castings for offshore wind turbines” (BMWI: 0325651A). The authors gratefully acknowledge the support of the German Federal Ministry for Economic Affairs and Energy within the funded project. The publication of this article was funded by the Open Access fund of Leibniz Universität Hannover.
Acknowledgments:The authors gratefully acknowledge the support of T. Kreklow, and F. Faltin for their support in conducting the laboratory experiments and pre-processing of the 3D scan data. The authors thank also the FZK and in particular M. Miranda-Lange for support related to the 3D scanner.
Conflicts of Interest:The authors declare no conflict of interest.
Notations A
Reference distance times the structure footprint length;
A given times the structure footprint distance, for this structure 0.55 m in both directions, 1 A=0.55 m/0.55 m, A=x or y distance/structure footprint distance in x or y direction a1 Structure footprint area; for the present studya1=0.55m×0.55m
ai Interrogation areaaiin dependence toi
B Additional term of equation (9);B=10−2(−5.2Ucw+6.9) C Additional term of equation (9);C=10−1(3.8Ucw+4.9) D Pile Diameter of the main struts of the jacket structureDorDpile Dsleeve Diameter of the pile sleeve of the jacket structure
Dleg Diameter of the legs of the jacket structure
DV,i Incremental erosion depth; representing an erosion depth of the related interrogation area times the pile diameter
d50 Grain size for which 50% of the material by weight is finer
f Frequency
g Gravitational acceleration Hs Significant wave height KC Keulegan-Carpenter number
n Number of piles
S(f) Velocity frequency spectrum
U Orbital velocity at the bed in direction of the waves Uc Undisturbed current velocity at 2.5 D from bed U Mean current velocity of the vertical profile Ucw Wave-current velocity ratioUcw=Uc/(Uc+Um) Um Undisturbed maximum orbital velocity at 2.5 D from bed Urms Root-mean-square (RMS) valueUof at the seabed
VA,i Cumulative erosion volume;VD,iin relation to each normalized areaai/a1 Varea Additional term in equation (8) to account for the areal development of volumes VD,i Dimensionless erosion volume;Verosionof an interrogation areaaiin relation to the
structural reference volume
VI,i Incremental erosion volume; the net gradient volume
VD,i−VD,i−1
in relation to each corresponding areaai/a1−ai−1/a1.
Verosion Eroded sediment volume in m3below a reference value based on the pre-scans
θ Shields parameter
θcr Critical value of the Shields parameter
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