Dynamic behavior of monopile supported offshore wind turbine is challenging due to complex long term wind and wave loading. Design of offshore wind turbine (OWT) structure primarily requires estimation of the fundamental frequency which needs to be kept away from excitation frequencies of wind and wave loading to avoid dynamic amplification of response and early fatigue damage. Global warming changes the wind and wave pattern due to pressure changes over the earth. The effect of climate change is having significant effect on wind speed, significant wave height and wave period, which in turn changes the dynamic behavior of OWT system. Fluctuating wind speed and wave height due to the effect of climate change may result in increased response and early fatigue damage. This study focuses on dynamic behavior of the monopile supported offshore wind turbine structure due to the climate change variability corresponding to 50 years future wind speed values. Historic wind data is utilized to project the future wind speed. The system is modeled using a beam on nonlinear Winkler foundation model. Soil resistance is modeled using American Petroleum Institute based cyclic p-y and t-z curves. The dynamic response and change in fatigue life of OWT structure is examined due to the effect of climate change and design implications are also suggested.
Most of the existing bridges with a total number of 700,000 in Japan were built in the period of high economic growth 40 ~ 50 years ago. Many of them suffered serious deterioration or damages. Seismic reinforcement for such bridges is urgently needed as a national policy. However, reinforcement methods for existing bridge foundations are being underdeveloped, and there is no well-developed method available currently. In this paper, the diagnosis and inspection method, or the needs for seismic strengthening of existing pile foundation was reviewed first. As a new seismic reinforcement technology for existing pile foundation, the composite pile method (patented, and registered in the New Technology Information System of the Japanese Ministry of Land, Infrastructure and Transport (NETIS)) by deploying ground improvement around existing pile foundation constructed in soft ground and liquefiable ground, was proposed based on the review. Different from others which are mainly focusing on improvement of lateral bearing capacity by installing additional piles or footing, the primary purpose of this method is to improve subgrade reaction at pile head, and increase the seismic resistance of the pile foundation. The feasibility of the proposed method was confirmed based on the results of large scale shaking table tests for group piles under various ground improvement conditions with Level 1 and Level 2 ground motions, such as lateral displacement and sectional force generated on the piles during shaking.
Demands for the testing of large prototypes in geotechnical engineering are increasing, while restrictions are recognized, such as costs and limited scale of testing facility. To resolve such demands and restrictions, Iai et al. (2005) proposed a scaling law by combining the scaling law for centrifuge testing with the one for 1-g dynamic-model testing. They call it the “generalized scaling law” in dynamic centrifuge modelling. The objective of the present study is to investigate and point out issues on the applicability of the scaling law through the technique of “new” modelling of models for dynamic behavior of pile foundations in both dry and saturated sand ground. Through comparison in the prototype scale, the applicability of the scaling law to bending moments was confirmed on the condition that centrifugal acceleration was more than 4.9 g for dry condition, and 14.4 g for saturated condition of 1/100 scale model.
Soil liquefaction caused by the earthquake vibration is a well-known soil dynamic characteristic and the disasters is self-evident. In this paper, we present a modified finite element-finite difference coupled method, embedding an adaptive time stepping procedure, and conduct the numerical simulations of seismic liquefaction hazards. The adaptive time stepping procedure consists an error estimator and a time stepping strategy, which is specific to the explicit solving scheme. Through the conducted examples of two dimensional embankment, it is found that the seismic liquefaction of soil foundations do induce huge damage to the upper and surrounding structures; by the proposed modified finite element-finite difference method, the displacement and the pore water pressure behaviors are well obtained compared with the traditional method. The computational efficiency is obviously improved; the considerable calculation cost will be saved to the huge model simulations.
In evaluating the damage caused by earthquakes, attention has been paid exclusively to ground liquefaction of sandy grounds during earthquakes. However, studies of post-seismic damage, especially damage to clayey/silty grounds have also been reported. In this paper, the behavior of an actual alternately layered sand-silt ground and foundation-superstructure system during and after an earthquake is investigated. The calculations are carried out using a 2D/3D soil-water coupling analysis program named as DBLEAVES that can not only describe the static and dynamic behavior of natural complex grounds, but also can solve soil-structure interaction problems. Two cases with different foundations (long-pile type foundation and dense short-pile type foundation) are analyzed. A rotating hardening elastoplastic model named as Cyclic Mobility model (CM Model) is adopted in the analyses to properly describe the nonlinear behavior of soils during and after large earthquake motions. The results show that the long-pile type foundation is more suitable to resist uneven settlement while the short-pile type foundation has a better resistance to ground liquefaction. No matter what kind of case may be, not only the liquefaction but also the long-term settlement after the earthquake should be taken into consideration seriously.
The method of calculation of bearing capacity of the foundation base with long-term non-linear deformation of clay soils under the regime of cyclic prolonged static loading. In the calculation of the bearing capacity of the base is considered a solid phase of development zones limit equilibrium, which is achieved for the foundation bases at the end of the formation of a hard core, deforming the ground.
Offshore wind farms are being built in pursuit of sustainable and environment-friendly energy. In order to construct these offshore wind farms with a reliable degree of safety, the seabed foundation of these turbines should be able to withstand the forces applied to it. Hence, it is necessary to investigate the dynamic behavior of the seabed under cyclic conditions due to long-term wave, wind, and tidal loading. In this paper, the long-term cyclic behavior of soils supporting wind turbine foundations is presented. Numerical analysis is conducted in order to simulate the behavior of sand at the center of the foundations, in addition to laboratory dynamic testing. The results show that long-term behaviors of marine soil could be very different from short-term behaviors, and should be considered in the dynamic design of offshore wind turbines.