Evaluation of dynamic behaviors of tripod suction pile based on closed form solution

Abstract

Offshore wind power refers to a facility that generates electric power using wind energy by installing turbines in the sea. Unlike oil and gas production structures where a vertical load is dominant, offshore wind turbines have a very large horizontal load, which typically corresponds to 100% to 150% of the vertical load, acting at a higher location relative to the top of the pile, thus generating a large rotational moment at the lower foundation. In addition, since it has a design life of more than 20 years, a long-term fatigue analysis is also an important design consideration because continuous cyclic loads caused by the wind and waves work even after the installation. In particular, there is a risk of destruction by resonance when the natural frequency of the structure coincides with the frequency of this external force because it has a certain period. Therefore, in order to design an offshore wind turbine safely, it is necessary to satisfy both the natural frequency requirements of the turbine system as well as the allowable displacements of the lower foundation. Until now, the foundation of offshore wind turbines that have been mainly used in Europe are mono-piles which are typically constructed by driving large steel pipe and a jacket foundation in the form of square trusses. However, in the domestic seas where fishery by small boats and offshore aquaculture are main activities , unlike Europe, constant complaints have been raised due to the noise and vibration caused by the hammering method and the secondary damage to the aquafarms due to the occurrence of floating particles. This study analyzes the installation mechanism and technical characteristics of the suction pile, a new concept of eco-friendly offshore wind power foundation that can solve the aforementioned problems, and based on this, a simple analysis method is presented which can significantly shorten the time required for the integrated load analysis. First, an analysis technique based on the influence factors that can grasp the dynamic characteristics of the entire system is derived. It is possible to calculate the horizontal and rotational displacements at the main points of the member, when external forces (horizontal force and moment) are applied to the tripod suction pile foundation system. The proposed analysis method can not only evaluate the characteristics of a suction pile with a large diameter and small aspect ratio(L/D) and the vertical force generated by the tripod structure, but also can analyze based on the basic physical properties of the soil, piles and turbines. Because the results can be obtained in a closed form, the dynamic behaviors of the turbine system can be expressed simply, accurately and intuitively with this simple method. Secondly, a numerical analysis modeling technique that can accurately simulate the joint stiffness of the tripod suction pile system is presented, and the accuracy of the analysis technique is verified through a scaled-down model and natural frequency tests. Thirdly, through the centrifugal model test, the preliminary safety evaluation for the full-scale test is carried out by duplicating the soil conditions in the field. Through the comparison of the centrifuge test results with the field test results, the effect of the soil-foundation-structure-interaction has been validated. Lastly, a full-scale turbine was installed on-site to evaluate the safety and applicability of the tripod suction pile, and a natural frequency test was conducted at each installation stage to observe changes in the dynamic characteristics of the system. The field test results have been used as a criterion for evaluating the accuracy and effectiveness of each analysis method through comparison with the results of simple analysis, numerical analysis, and centrifugal model test. As a result of the comparative analysis, the effectiveness of the proposed simple analysis method was confirmed by predicting the natural frequency (99% of the measured value) and the allowable rotation angle (96% of the integrated load analysis value) with very high accuracy. Accordingly, it is expected that it can be used as a design tool for rapid decision making in the early stages of front end engineering design (FEED) work or project feasibility analysis.