Lead Session Chair:
Stephan Barth, Managing Director, ForWind - Center for Wind Energy Research, Germany
Ionut-Emanuel Stroescu (1) F P Peter Frigaard (2) Morten Fejerskov (3)
(1) Universal Foundation A/S and Aalborg University, Aalborg, Denmark (2) Aalborg University, Aalborg, Denmark (3) Universal Foundation A/S, Aalborg, Denmark
Poster Award Winner
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Presenter's biographyBiographies are supplied directly by presenters at OFFSHORE 2015 and are published here unedited
Emanuel is a M.Sc. engineer in the fields of structural and coastal engineering. He studied civil engineering at Technical University of Iasi “Gheorghe Asachi” and completed his master in structural engineering at Aalborg University. He is currently employed at Universal Foundation A/S as project engineer and enrolled as industrial Ph.D fellow at Aalborg University. His research is related to scour and hydrodynamics.
Scour design for self-protecting bucket foundations used for offshore wind turbines
In the past several years significant amount of resources and research have been put in the suction bucket technology. The perspective for broad use of this foundation solution is feasible due to reduced costs, environmental friendly technology and versatility for larger MW wind turbines compared to monopiles. One of the reduction cost features is the ability of the bucket foundation to self-protect against scour within certain environmental ranges. Until now, all offshore metmasts installed on Universal Foundations (Bucket Foundations) by Universal Foundation A/S have no additional scour protection besides own geometry and have good response from real experiences.
The assessment of the scour mechanism around bucket foundations is fulfilled by the means of experimental analysis in the laboratory from Aalborg University. The erosion mechanism is studied for various assumptions with respect to geometry parameters. The experiment is carried for irregular waves which are imposed over a tidal current. The spectral inputs for irregular waves are kept constant while, current intensity and direction are changed. The analysis results are compared with reference monopile and with the survey results from real installations for bucket foundations and protected monopiles.
Main body of abstract
Scour is a dynamic process in sea state and so, is dependent on direction and magnitude of water flow. The alternation of the tide will vary the time of seabed shear stress with direction and magnitude. Consequently, the scour depth may increase or decrease. Hence, the scour is generated by tides and the backfill is ensured by waves. For large diameters the waves will not generate scour, however waves may reshape seabed with wave dunes.
The result from experiments shows that beside environmental actions, firstly, the ratio of the skirt diameter to shaft diameter and the misalignment of lid with seabed are influencing the general scour amount. Secondly, the load transfer webs from shaft to skirt are acting as spoilers on pipelines. Therefore, a local scour mechanism is developed, which will vary in depth, function of web orientation to flow. Scour is studied as effect analysis and shows a good correlation with the edge scour studies for protected monopiles from literature. The driven parameters for scour in protected monopiles studies are diameter of the protection layer and the berm height. These two are comparable with the bucket’s parameters diameter ratio and misalignment with seabed.
The experiment shows that bucket foundation have significant less scour than unprotected monopiles for which design codes are intended. The contribution of the study stays in the identification of scour depending parameters for bucket foundations. Moreover, experimental curves have been obtained for scour development and compared with prototype surveys.
The presentation focuses on the status of real experiences compared to experimental results to provide a better understanding of scour phenomena and design recommendations for bucket foundations.