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Research Papers: Ocean Renewable Energy

Experimental Study on Installation of Composite Bucket Foundations for Offshore Wind Turbines in Silty Sand

[+] Author and Article Information
Puyang Zhang

Associate Professor
State Key Laboratory of Hydraulic
Engineering Simulation and Safety;
Key Laboratory of Coast Civil Structure Safety,
Ministry of Education,
School of Civil Engineering,
Tianjin University,
Tianjin 300072, China
e-mail: zpy@tju.edu.cn; zpy_td@163.com

Zhi Zhang, Yonggang Liu

School of Civil Engineering,
Tianjin University,
Tianjin 300072, China

Hongyan Ding

Professor
State Key Laboratory of Hydraulic
Engineering Simulation and Safety;
Key Laboratory of Coast Civil Structure Safety,
Ministry of Education,
School of Civil Engineering,
Tianjin University,
Tianjin 300072, China

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received April 25, 2015; final manuscript received August 12, 2016; published online September 16, 2016. Assoc. Editor: Lance Manuel.

J. Offshore Mech. Arct. Eng 138(6), 061901 (Sep 16, 2016) (11 pages) Paper No: OMAE-15-1034; doi: 10.1115/1.4034456 History: Received April 25, 2015; Revised August 12, 2016

The composite bucket foundation (CBF) is a cost-competitive foundation for offshore wind turbines, which can be adapted to the loading characteristics and development needs of offshore wind farms due to its special structural form. There are seven sections divided inside the CBF by steel bulkheads, which are arranged in a honeycomb structure. The six peripheral sections with the skirt have the same proportions while the middle orthohexagonal one is a little larger. With the seven-section structure, the CBF has reasonable motion characteristics and towing reliability during the wet-tow construction process. Moreover, the pressure inside the compartments can control the levelness of the CBF during suction installation. Several large-scale model tests on suction installation of CBF have been performed in order to explore the feasibility of the tilt adjusting technique in saturated silty sand off the coast of Jiangsu in China. The composite bucket foundation in the tests has an outer diameter of 3.5 m and a clear wall height of 0.9 m. During the suction-assisted penetration process, the pressures in all the compartments were controlled to level the foundation in a timely operation. A convenient method is to improve the CBF inclination by controlling the inside differential pressure among the compartments. It can be commonly carried out by applying suction/positive pressure with intermittent pumping among the seven compartments. Another adjusting technique for a big tilt with deeper penetration is operated with decreasing the penetration depth achieved by suction-assisted lowering the relatively high compartments and positive pressures raising the relatively low compartments. Test results show that the reciprocating adjustment process can be repeated until the CBF is completely penetrated into a designed depth.

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Figures

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Fig. 1

Pictures of the composite bucket foundation: (a) The prototype composite bucket foundation and (b) a 2.5 MW offshore wind turbine supported by CBF

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Fig. 2

The test model of composite bucket foundation

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Fig. 3

Layout of different sensors: (a) on the top lid of CBF, (b) along the skirt of compartment 5 (view 1), (c) along the skirt of compartment 2 (view 2), and (d) pictures of sensors inside the bucket

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Fig. 4

The layout of the main equipment: (a) scheme of the equipment arrangement and (b) picture of the equipment of the CBF

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Fig. 5

CPTs results: (a) cone resistance and (b) sleeve friction

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Fig. 6

The illustration of the foundation penetration process: (a) penetration depth versus time and (b) the foundation with the tilting angle

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Fig. 7

Comparison among measured air pressure values and empirical formulae predictions: (a) pressures in all compartments and (b) required pressures

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Fig. 8

Variations in the pore pressure of the compartment 2: (a) inside and (b) outside

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Fig. 9

Variations in the earth pressure of the compartment 2: (a) inside, (b) outside, and (c) top lid

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