Currently applied offshore wind foundation construction schemes include monopile foundations, multi-pile foundations, and gravity-based foundations, among which monopile foundations, due to advantages such as convenient construction and relatively low cost, are the main type of offshore wind foundation at present. Compared with other types, wind power monopile foundations are convenient to construct, but because the verticality control of the wind power structure on a monopile foundation is relatively more difficult, a transition piece is added in the structural design: a sleeve is added to the upper part of the steel pipe pile foundation, the sleeve is connected to the foundation with high-strength grouting material, and the upper part is connected to the tower. The advantage of this is that the verticality of the steel pipe pile foundation does not require special devices for control, and the verticality deviation caused by vertical pile driving can be adjusted through the transition piece to achieve strict control of the overall verticality.
Figure 1 Monopile foundation (the section in the figure is the transition piece)
Because foreign enterprises monopolize certain materials and technologies, domestic wind power manufacturers face high costs and a situation of being constrained by others. Therefore, some manufacturers actively develop alternative materials and technological breakthroughs, while others bypass certain missing technologies and materials, increasing the risks during the construction and service period of wind power structures. Due to the strong support of national policies for the wind power industry, new attempts and new technologies continue to emerge, but immature technologies should not be promoted on a large scale in new wind farms. Instead, the promotion of new technologies should be advanced step by step with comparisons. If, tested by time, a new technology is indeed superior, it will not be too late to promote it in the wind power field; during that period, it can be compared with the original mature foreign technologies and processes to determine its practicality and feasibility. After all, investment in the wind power field is huge, and once damage or even scrapping occurs due to immature technology, the loss is immeasurable.
At present, there are two structural schemes for monopile foundations in China: one follows the mature foreign transition-piece monopile foundation scheme, and the other is a monopile foundation scheme that eliminates the transition piece and uses direct rigid flange connection. The latter scheme was proposed as an alternative under the background of foreign monopoly on high-strength grouting material for transition pieces. After analyzing the reasons for proposing the transition-piece monopile foundation scheme, it is not difficult to find that the structural scheme of eliminating the transition piece and connecting the monopile foundation with flanges has defects that are difficult to overcome. Although it breaks the foreign technological monopoly, it cannot avoid the inevitable risks of its existence.
Setting aside other advantages and disadvantages, let us analyze the pros and cons of the two schemes only from the direct reason for proposing the transition piece.
The advantage of the transition piece scheme is that it does not involve verticality adjustment of the pile foundation; instead, verticality is adjusted through the transition piece, so that the overall verticality meets the design requirements. Its limitation is that it places extremely high requirements on the structure of the transition piece, especially the grouting material filling the gap between the sleeve and the steel pipe pile. Its advantages include no disturbance to the seabed foundation soil, easy and precise control of the overall structural verticality, and the buffering of the transition piece making stress concentration less likely under vibration loads.
Figure 2 Bending moment of a monopile foundation in seabed soil layers
Figure 3 Horizontal displacement of a monopile foundation in seabed soil layers
For wind turbine monopile foundations in the marine environment, the cyclic loads on the structure have three influencing factors: nonlinearity of pile-soil interaction, degradation of ultimate soil resistance, and pile-soil separation effect. The direct flange connection scheme eliminates the transition piece. During pile driving, a centralizing device is needed to continuously adjust verticality. Not only is the verticality accuracy difficult to control, but it also causes disturbance to the seabed foundation soil, resulting in a loss of constraint force of the foundation soil on the pile foundation driven into it, making the horizontal bearing capacity smaller than that of the transition piece scheme from the initial stage. In wind farms in the East China Sea and South China Sea, which are frequently hit by typhoons, wind power structures are more likely to tilt or even overturn. If the centralizing device is used to control verticality to meet design requirements with high accuracy, the loss of horizontal bearing capacity reduces the overall structure's ability to cope with strong actions such as typhoons. Figure 2 shows the bending moment of a monopile foundation in seabed soil layers under a certain load, and Figure 3 shows the corresponding horizontal displacement of the monopile foundation under the same load. It can be seen that the bearing capacity is greatest at a soil depth close to the upper one-third. Under the continuous action of the centralizing device, the pile-soil bearing capacity gradually decreases, so the ultimate load that can be borne also gradually decreases, thereby losing the ability to resist strong loads. However, if the overall structural verticality does not meet the design requirements, since the height of the wind power structure above the seabed is more than twice the part driven into the seabed foundation, the eccentric load on the structure is very large when verticality requirements are not met. Coupled with the loss of constraint force caused by foundation soil disturbance, once a strong wind load acts, the risk of overturning is particularly high.
Therefore, under the current conditions of engineering science and technology, the verticality control methods and mechanical theories for offshore wind farm monopile foundations should refer to the mature standard designs and engineering cases of European countries. A good structural design method should be carefully considered, and its long-term service life should also withstand the test of time.
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