Centrifuge Modelling of Monopiles under Cyclic Lateral Loads

Document Type : Original Research

Authors
M. Shahidikhah 1
M. moradi 2
1 Student
2 associate professor
Abstract
​Wind turbines are considered as an important element of the renewable energy structure. Offshore wind turbines are tending to be more efficient than onshore because wind speed and direction are more consistent. Monopiles are widely used for offshore wind turbines at present. They are always subjected to significant cyclic lateral loads due to wind and wave excitation. Monopiles are hollow cylindrical steel piles with a circular cross-section and a length to diameter ratio of less than 10 (L/D < 10). Currently, the design of monopiles is based on experiments performed on slender piles. Since monopiles behave rigidly, finding their action seems to be very necessary for accurate analysis and design of these structures.

In order to better understand the performance of monopiles under static and cyclic lateral loads, a series of static and cyclic lateral load tests was conducted on a stainless steel monopile in the geotechnical centrifuge. The main goal of this study is the examination of accumulated lateral displacement of a monopile foundation for an offshore wind turbine with a large diameter subjected to wind and wave loads. In this article, the lateral responses of a large diameter monopile under one-way force-controlled cyclic lateral loads are described and accumulated permanent pile shaft lateral displacements caused by cyclic lateral loads are discussed. All tests were performed in beam centrifuge. Monopile was installed in Firoozkooh-161 sand in this study. The centrifuge tests were carried out at different cyclic load and magnitude ratios insights into the ongoing development of net stresses and bending moment.

In this research, 4 Tests were designed and implemented to centrifuge modeling the action of monopiles in sandy soils. Tests were carried at physical modeling laboratory of the school of civil engineering at the University of Tehran. The first experiment was initially conducted to estimate the ultimate capacity of these piles, and then the obtained results compared to similar research findings. Three other experiments were carried out to evaluate their behavior affected by cyclic lateral load and to determine cumulative displacements and deformation state. Consequently, the results were finally compared with findings of other researchers, regulations, and relationships available related to other used piles (with a diameter less than 2 m) in geotechnical projects. Results of the study indicate that the use of available regulations and instructions in estimating the lateral load-bearing capacity of these piles was conservative. However, this fact can lead to the achievement of unreliable and upper-hand results. Thus, the existing relationships and regulations need to be changed to provide accurate results related to these piles.

The major findings of this study are presented below:

-The estimation of the monopile lateral bearing capacity is impossible with existing formulas, and this requires numerical or physical modeling.

-The behavior of the monopile structure under lateral loading is rigid until the failure limit, so the failure mechanism of the monopiles will be similar to the short and rigid piles.

-Cumulative lateral displacement of the monopile head is ascending in the number of cycles, and its rate in all cyclic tests is reducing.

-The monopile has rotated around a point in the depth of 30 to 75 percent of the driving length.

-The maximum bending moment value in all cycles has occurred in the depth of about 20% of the driving length.

Keywords

Subjects

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Modares Civil Engineering journal
Volume 19, Issue 4
November and December 2019
Pages 109-120