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Wind Topic Analysis

Suction bucket jacket: Introduction

April 27, 2022 Wind InfoLink

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Suction Bucket Jacket debut in Taiwan

Ørsted’s Greater Changhua 2a phase-2 and Greater Changhua 4 offshore wind farm has passed the review for environmental impact comparative analysis report on March 2, 2022. Items changed include larger wind turbines and the installation of suction bucket jacket (SBJ). The project will deploy SBJs in certain areas to install foundation without piling, reducing underwater noise and its impact on marine mammals.

What is SBJ? 

A SBJ is a steel pipe with an open bottom and a closed top. The weight and differential pressure of a caisson insert the pipe to the seabed. A caisson sinks due to its own weight. After reaching the seabed, water pumping equipment activates, creating negative room pressure in an enclosed space between the caisson and seabed. At this point, the seabed creates friction on the inner and outer side of the skirt and the penetration resistance of the caisson. When the weight and negative room pressure of the caisson exceed resistance from the seabed, the structure will be installed successfully. SBJs can be built into monopod and multipod foundations, in replace of tripod and jacket foundations.

The installation of SBJs requires precise engineering analysis and calculation. Since soil condition affects the assessment of resistance greatly, preliminary assessment shall take the seepage force into account. For instance, soils with higher permeability and bigger grains cause sea water to flow into caissons, reducing resistance against sinking caissons, which consequently descend too fast, resulting in the buckling of steel pipe. In this case, thickness of the steel boards should be adjusted. Considering soil condition, SBJs work better with clay soil and low-strength soil.

No piling on installation, easier to disassemble on retirement

Compared to root-pile structures, SBJs have the following advantages:

  • Requiring no piling equipment during construction.  

  • Assembly can be done onshore, reducing working hours on the sea.

  • Relatively marine life-friendly, for it generates practically no noise.

  • As a result of the Archimedes' Principle, foundations can be separated from the seabed and completely removed by increasing the pressure within them.

SBJs are easier to install and dismantle, with lower construction costs and the opportunity to realize sustainable environment development.   

Followings are comparisons of types of underwater foundations:

各類型水下基礎比較圖

Conclusion

Presently, few wind farms have employed SBJs. Lack of design, installation experiences and construction costs yet to reach economics of scale all restrict its development. Given rising awareness of a sustainable environment, SBJs have great advantage in its eco-friendliness and is with high hope to become the dominant type of underwater foundation.

Sources

Achmus, M., Akdag, C. T., and Thieken, K. (2013). "Load-bearing behavior of suction bucket foundations in sand." Applied Ocean Research, 43, 157-165.
Cheng, W.-C., & Lin, L.-C. (2019, July). Legs of Offshore Wind Turbines: Brief Introduction on Underwater Foundation. Energy Information Platform. Retrieved May 3, 2022, from Energy Information Platform 
Kuo, Y.-S., Lin, C.-S., Yang, C.-Y., Chen, W.-T., & Hsu, C.-W. (2019). Effect of Installation Discharge on the Pullout Capacity of the Suction Bucket Foundation. Journal of Taiwan Energy, 6(3), 257–268. 
Bureau of Energy, Ministry of Economic Affairs

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