Model Tests On Pile Group Effect And Cyclic Effect Of Jacket Foundation Of Offshore Tand Turbines

As one of the most important and technology-mature renewable energy sources, wind energy is significant for the world’s energy development strategy. The jacket foundation normally consists of four piles and a relatively rigid jacket, which results in a relatively large lateral stiffness and is little influenced by environmental loads. So far, the jacket foundation has shown considerable prospects in engineering applications with the increase of both the capacity of the single wind turbine and water depth. These structures are frequently subjected to lateral loads from wind, waves, and earthquakes, so the lateral bearing capacity of foundation for offshore is the most important issues for the engineering design of the jacket foundation in extreme cases.Regarding the normal pile group foundation, the soil-pile-cap interaction makes the bearing capacity and deformation of the jacket foundation differ from that of an isolated single pile.Besides, the lateral loads applied on the high superstructure transfer a very large moment load onto the top of the jacket foundation, which could cause significant changes of both axial force and shear force on the piles. Thus, the pile-soil interaction of the jacket foundation of offshore wind turbines is somewhat different from other types of pile group foundations. The static analysis approach for traditional pile group foundations is clearly limited when applied in the design of jacket foundation for offshore wind turbines under lateral loads. Besides, lateral cyclic loads from wind, waves, and earthquakes can lead to excessive deformations and changes in the bearing capacity of large-diameter piles. It can lead to accumulated deformation. These phenomena are extremely detrimental to the normal operation of deflection-sensitive equipment, such as wind turbines. Thus, the cumulative deformation of jacket foundation under lateral cyclic loads during their service lives is the critical design issue. However it seems that the lateral pile-soil interaction of tetrapod jacket foundation under cyclic lateral loads has not been understood very well at present and there is few approach to analyze the cumulative deformation resulting from cyclic loads. So it is of great significance to carry out researches on the pile group effect and cyclic loading effect of lateral loaded jacket foundation.Centrifuge model tests were carried out to study the response of jacket foundation subjected to lateral monotonic and cyclic loads in sand soil and soft clay soil to investigate the pile group effect and cyclic loading effect, based on which analysis models were also presented for engineering design. The main research works and results are as follows:(1) The model tests were carried out on the centrifuge ZJU400 in Zhejiang University. The prototype jacket foundation was the foundation of a 3 MW wind turbine in Guishan offshore wind farm in China. Centrifuge model tests were conducted on a jacket foundation in saturated sand soil and soft clay soil, including 2 static loading tests and 9cyclic loading tests.(2) Based on the centrifuge model tests carried out on a tetrapod jacket foundation in sand subjected to monotonic horizontal loads (Zhu et al, 2014), the deformation and internal force of the piles as well as the lateral pile-soil interaction were discussed. The result revealed that the pile group effect and the uplifting axial force of the pile shaft are the main reasons for the difference of horizontal subgrade reaction in the soil around the back-row pile(s) and front-row pile(s).Modified p-multipliers relating to the loading direction, the pile deformation and the axial force of the pile shaft are proposed for all piles of the tetrapod jacket foundation as well as the corresponding p-y curves for the analysis of the complicated pile-soil interaction of the jacket foundation.(3) Centrifuge model tests were also carried out on the same tetrapod jacket foundation subjected to monotonic horizontal loads in soft clay. The deformation and internal force of the piles as well as the lateral pile-soil interaction were investigated. It was found that the horizontal bearing capacity of jacket foundation in the soft clay is smaller while the effecting range of the lateral load is larger when compared with that of jacket foundation in the sand. The test results also showed that the back-row pile of the jacket foundation was likely to be pulled out, which may cause the failure of jacket foundation in the soft clay.(4) This paper reports field tests of two large-diameter offshore piles in soft clay under lateral loads in Guishan Offshore Wind Farms in Guangdong Province of China. The results showed that the initial stiffness and ultimate soil reaction per unit length of the present derived p-y curves of the driven piles were obviously larger than that of the American Petroleum Institute (API)p-y curves for the investigated soft clay soil. This could be due to the reconsolidation of soil around the pile after it is driven. Thus, a new analytical model considering re-consolidation effect was proposed to obtain the lateral bearing capacity of the isolated pile in soft clay.(5) Also, the group pile effect of the jacket foundation in soft clay was investigated, which indicated that the difference of p-y curves of the front-row pile and back-row pile of jacket foundation in soft clay soil is much smaller than that in sand soil. It means that the group pile effect would play dominant role in the lateral pile-soil interaction of jacket foundation in soft clay instead of axial force of the pile shaft. Finally, a corresponding analytical model considering the re-consolidation effect and pile group effect was presented.(6) A series of centrifuge tests on jacket foundation under one-way and two-way cyclic loads were performed in saturated sand soil to study the cyclic loading effect. The test results showed that the impact of loading frequency, loading direction and loading amplitude on the lateral and vertical response of pile foundations. And systematic researches had been conducted on the pile-soil interaction under cyclic loads. It was found that the accumulative displacement of the jacket foundation under one-way cyclic loads is linear with Log(N). A cyclic degradation factor t is defined and calculated based on the derived cyclic p-y curves to analyze the degradation of pile-soil interaction and the accumulative displacement resulting from cyclic loading,which is related to the cyclic soil reaction ratios. And the corresponding expression of t for the front-row pile and back-row pile was presented. The test result also indicated that the two-way cyclic loads may not lead to accumulative displacement, for the loading amplitude was relative small. Finally, the vertical response of jacket foundation can be empirically divided into two stages: the irregular settlement stage and the entirely settlement stage. It can be attributed to the densification of sand with cyclic loading.(7) The same centrifuge model tests in soft clay were conducted to study the response of jacket foundation in soft clay under cyclic loads along border direction, which revealed the same cyclic loading effect of the jacket foundation in soft clay under one-way cyclic loads. However,the cyclic loading effect can be more remarkable than that in sand. Besides, the reconsolidation after cyclic loading may lead to the entirely settlement of jacket foundation in the soft clay.In this paper, centrifuge model tests were carried out on a tetrapod jacket foundation of an offshore wind turbine subjected to horizontal mono tonic and cyclic loads to study the pile group effect and cyclic loading effect. The deformation and internal force of the piles as well as the lateral pile-soil interaction were investigated. And the correspongding analytical models are porposed to analyze the lateral behavior of jacket foundation for offshore wind turbines.