With the increasing number of offshore wind power installations in Taiwan and the development of coastal geological research, related research has also become more and more comprehensive. Recently, Orsted Energy and National Cheng Kung University (NCKU) have also partnered to complete the world's first offshore wind turbine tower suction bucket jacket (SBJ) monopile sandbox quake test to confirm the earthquake resistance and soil liquefaction resistance of the technology and its applicability to the seabed of the west coast of Taiwan.
The narrow mountains in Taiwan have high and concentrated rainfall, water retention is not easy, and a large amount of sandy soil enters the sea along rivers. Therefore, offshore wind fields are concentrated in the western half of Taiwan in Changhua and Yunlin and here the seabed soil is mostly loose to medium compact sandy soil with a high risk of soil liquefaction during an earthquake. In order to stabilize wind turbines, the underwater foundation of offshore wind power at this stage mostly adopts the pipe frame type multi-pile foundation.
Although a pipe frame multi-pile foundation can ensure the stability of the fan in loose soil, the driving, piling, and further driving during the construction process will be accompanied by noise and vibration, which has a great impact on marine ecology. This is also referred to as a "piled foundation." However, although a tube-frame suction caisson foundation has the advantages of not producing noise, greatly reducing seabed vibration, and shallow embedding depth, soil type and soil strength must be considered before installation.
▲Tube frame suction caisson foundation （Source：Orsted）
This is why Taiwan's offshore wind power installations have previously adopted piling-based foundations. NCKU and Orsted have also partnered to discover the feasibility of applying this technology in Taiwan. Kuo Yu-Shu, a professor of water conservancy and marine engineering at NCKU, pointed out that in the early days, Taiwan was not familiar with offshore wind power technology, so it would naturally adopt European experiences. However, the piles of the tube-frame suction caisson are usually installed in relatively sticky soil and there is no experience in applying them to wind farms with high liquefaction potential.
▲Simulated installation of the suction caisson underwater foundation （Source：TechNews）
Usually soil liquefaction does not occur in soil deeper than 30 meters but the team simulated conditions such as liquefied soils up to 80 meters, a one in 50-year earthquake with a probability of 10% (the largest earthquake occurring once in 475 years) and other conditions. The team also used up to four meter a large model that conformed to the scale of a real turbine and a 1:47 caisson foundation for testing and, through the difficult suction installation method, three caisson foundations were installed and tested simultaneously.
The research results were positive. The suction caisson underwater foundation can provide sufficient stability for wind turbines and has the ability to withstand earthquakes and soil liquefaction, improving Taiwan's academic research energy.
In this study, Orsted also provided "real data" that is difficult to obtain in the academic world including actual size, future soil conditions, etc., to supplement the data required for the study.
Kao Chuan-Sheng, the project director for Orsted in Taiwan, pointed out that Orsted is the first offshore wind power company to apply a pipe-frame suction caisson foundation. After four years of seabed geological surveys and a better grasp of the geological conditions off Changhua, Orsted also believes that Taiwan could indeed adopt the basic technology of suction caisson and then entrusted a third-party certification agency to conduct a proof-of-concept. Project certification was obtained in February 2021 in cooperation with the Department of Hydraulic and Ocean Engineering of NCKU to perform large-scale sandbox quake tests.