Flume Tank Testing of Offshore Wind Turbine Dynamics with Foundation Scour and Scour Protection
Publication: Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 146, Issue 5
Abstract
Scour erosion processes can occur at seabed level around offshore wind turbine monopile foundations. These scour processes are often especially severe at sites where mobile sediments, such as sands, are present in the superficial seabed soils. Loss of local soil support to the monopile, caused by scour erosion, can lead to significant changes in the dynamic characteristics of the wind turbine support structure. This can result in accelerated fatigue damage, owing to the applied cyclic loads from the wind turbine generator, especially at the rotor frequency. Although scour erosion can be controlled by appropriate scour protection systems, there is a lack of knowledge to support the design and optimization of these protection measures, to ensure that the dynamic performance of the wind turbine support structure remains within acceptable limits. This paper describes an experimental campaign conducted on a 1:20 scale model of a driven monopile foundation and wind turbine support structure, founded in a prepared sand test bed in the Fast Flow Facility flume (HR Wallingford, UK). Scour processes were induced by applying cycles of flow. Experiments were conducted to investigate the influence that these scour processes, and selected concepts for preventative and remedial scour protection, have on the dynamic characteristics of the monopile–tower system. The paper describes the experimental procedures that were adopted, and provides an assessment of the results.
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Acknowledgment
This research project is supported through funding from E.On Climate and Renewables (recently renamed RWE Renewables) and HR Wallingford, and by grant EP/L016303/1 for Cranfield University, the University of Oxford, and Strathclyde University, Centre for Doctoral Training in Renewable Energy Marine Structures (http://www.rems-cdt.ac.uk/) from the UK Engineering and Physical Sciences Research Council (EPSRC). Byrne is supported by the Royal Academy of Engineering under the Research Chairs and Senior Research Fellowships scheme.
We gratefully acknowledge the work of team members at HR Wallingford, E.On, and Oxford University, who aided in the implementation and execution of the testing program. In addition, we gratefully acknowledge Fugro for triaxial testing performed as input to the soil characterization for the experiments.
Notation
The following symbols are used in this paper:
- B
- small strain shear modulus coefficient;
- b
- scour protection width;
- D
- pile diameter;
- DR
- sand relative density;
- dxx
- sand particle diameter coefficient (xxth percentile);
- E
- Young's modulus;
- EI
- flexural stiffness;
- e
- soil voids ratio;
- fn,m
- natural frequency for mode m;
- GS
- soil specific gravity;
- G0
- soil small strain shear modulus;
- HB
- depth of sand below pile tip;
- hw
- water depth;
- L
- pile embedment length;
- LX
- geometric length (of structure component X);
- MTop
- structure top mass;
- Ncyc
- number of flow cycles;
- NM
- number of top masses;
- n
- soil porosity;
- pref
- reference stress;
- p′
- mean effective stress;
- qc
- CPT bearing stress;
- r
- radial distance from pile centerline;
- SG
- global scour bathymetry reduction;
- SL
- local scour bathymetry reduction;
- ST
- total scour bathymetry reduction;
- Tf
- elapsed flow time;
- tsp
- scour protection thickness adjacent to the pile wall;
- tw
- pile wall thickness;
- depth averaged flow velocity;
- v
- displacement;
- x
- coordinate in the streamwise direction;
- y
- coordinate in the spanwise direction;
- z
- distance above pile toe;
- γ
- soil unit weight;
- ζm
- damping ratio for mode m;
- ν
- kinematic viscosity of water;
- ρ
- material density;
- ρw
- water density, assumed 1,000 kg · m−3;
- soil effective vertical stress;
- ϕm
- phase angle for mode m; and
- ωn,m
- circular natural frequency for mode m.
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Published In
Journal of Waterway, Port, Coastal, and Ocean Engineering
Volume 146 • Issue 5 • September 2020
Copyright
© 2020 American Society of Civil Engineers.
History
Received: Sep 24, 2019
Accepted: Feb 24, 2020
Published online: Jun 11, 2020
Published in print: Sep 1, 2020
Discussion open until: Nov 11, 2020
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