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Centrifuge modeling of cyclic loading induced by ice sheet |
Author: Date:2008-05-12 |
Suction-based applications originated in Europe, was first used as a basis for anchoring the floating structure, the 1994 European Europipe16 /. 11E will be the first marine oil production platform built-in suction basis. As a result of this construction has a basis for fast and easy to transport and install, shallow depth of the investigation, you can re-use, etc., so gradually in the marine engineering has been widely used and developed.
Since the 1990s, China's oil sector on the basis of this had a strong interest in the preparation and development of oil and gas in the Bohai Sea area to apply. There are areas in the Bohai Sea in winter's frozen period of three months or so, the formation of sea ice exciting dynamic load on the stability of the platform poses a serious threat. Move into the Bohai Sea ice edge platform designed to control one of the load. To that end, commissioned by the Chinese Academy of Sciences Institute of Mechanics, combined with the geological conditions of the Bohai Sea region, Tsinghua University carried out loads of ice-suction centrifugal model test basis.
Centrifugal model of the dynamic load test simulation is very important. The first generation of dynamic loading equipment is pneumatic jacks, the gate valve used to control movement back and forth, the structure imposed on the vertical and horizontal cyclic loading, the load lower frequency, more or less at about 1 Hz. The second generation is the use of servo-hydraulic vibration can be imposed on the structure of 0.001 ~ 10 Hz cycle of the load; shaking table centrifuges used hydraulic servo-exciter fall into this category, the frequency can reach 350 Hz, but exciting time is very short, generally less than 2 S. Introduced the Institute of Mechanics, Chinese Academy of Sciences and Qinghua University for the joint development of the centrifuge and electromagnetic vibration in the next 100 g of centrifugal model test. It can be 100 g of centrifugal acceleration, the marine platform suction imposed on the basis of a long-lasting (20min), high frequency (100 Hz) levels of cyclic loading, peak load up to 98 N above.
1 Electromagnetic exciter
Restrictions on the suction-type action on the basis of the peak load of ice for 980 kN, a frequency of 1 Hz. According to the centrifugal model similar to the law, 100 g for the vibration under suction-type basis to impose an amplitude for 98 N, a frequency of 100 Hz the level of dynamic load.
Figure 1 for the electromagnetic vibration of the structure of the profile. Figure 2 for the exciter control system diagram. Exciter mainly by the permanent magnet and coil of thin-walled silver consisting of dynamic composition. Permanent magnet in a dynamic form of the strong magnetic field, when alternating current to pass, the dynamic result of the ever-changing electromagnetic force and the magnetic field in the movement back and forth through the dynamic excitation under the force of inertia imposed on the model. Table 1 shows the design parameters of the exciter.
The design of two key technologies: ? exciter in the field of high centrifugal acceleration, the dynamic weight of 100-fold increase. Dynamic place in the permanent magnet in the shape of the circle and move in the same space. In order to reduce the wear and tear magnetic field, the very small space. The increase in self-gravity, the dynamic role of the moment created by the deformation, so that the dynamic vibration occurred deviated from the track, the deformation of the dynamic and may never even stuck a magnet for friction. In order to overcome the difficulties in the dynamic set up at both ends of the thin circular spring, will move in the ring magnet to fly in space. Spring from thin-oriented to limit the role of the dynamic in the direction of the circumference of the deformation. At the same time, with a dynamic linear precision bearings connected, so as to ensure the dynamic vibration does not occur deviated from the track.
In order to ensure the model as a result of vibration of the box will not have a vibration and vibration, vibration, the smaller the better the quality. However, the equipment is usually loaded with the quality of the output is directly proportional. Consider the technical requirements of a good use of conductive silver moving into a coil around the circle. The dynamic quality to 500 g, for the quality of the exciter l4kg.
2 1g under the conditions of vibration test
1g under the performance test is a natural gravitational field testing under the input and output wave vibration of relevance, such as phase and amplitude attenuation ratio; efforts to carry out vibration analysis of the characteristics that come to the constant electric power; and DY 25, a piezoelectric force sensor for calibration. 1 g due to the conditions of soil do not reflect the actual level of stress, the excitation load is not imposed on the basis of the model, but rather imposed on a fixed heavy entirely on rigid. In force between the sensors and put a heavy rubber mat in order to avoid a collision.
Figure 3 for a typical input and output wave form and amplitude curves. Exciter input and output sine wave form similar, but with the frequency increased, the amplitude was attenuated trends, such as 3 (c) above, the amplitude attenuation 0.25 to a range of more than 3.0 dB. Amplitude decay than the general requirements in _3dB, or loss of power too much. Through rate, the vibration characteristics of electric power for the constant l2.45 N / A, one of the most exciter Oh l5 A input current, it corresponds to the maximum dynamic load of semi-peak for the 147 N, load frequency range from basic Restrictions on the amplitude attenuation and force structure and stiffness associated natural frequency.
3 centrifuges in the dynamic simulation of ice load
Tsinghua University, in the 50 g-t geotechnical centrifuge, was 100 g of centrifugal acceleration to the initial centrifuge model test to test the performance of the exciter.
3.1 Preparation of centrifuge model
Silty soil to adopt sand, and its ratio to 2.69 Ping, both size and a half to 0.14 mm, permeability coefficient for the 5x10-4cm / s. 21 cm thick (a prototype 21 m) silty sand layer dry density control in accordance with sub-5 level is hit. Preparation of soil samples from the bottom up water, and then saturated exhaust. Suction-type centrifuge model-based sensors and the layout shown in Figure 4. Suction-buried by soil on the basis of thin-walled hollow steel drums and barrels at the top of the welded steel pipe components. In the thin-walled steel pipe on a fixed slide, the exciter exciting side under a steel ball, you can slide up and down movement in order to adapt to the deformation of the foundation.
3.2 100g move under the ice load simulation
Model Foundation in 100 g of centrifugal acceleration to complete the consolidation process, the consolidation period of 7 months (prototype). After the consolidation, the exciter first imposed on the suction pile 19.6 N static load. In this quiet, and then load on the basis of the imposition of the frequency and amplitude were 98 N and 100 Hz sine load level, load the prototype frequency and amplitude of 1 Hz and 9.8 X 10kN. Barrels in the action-load over 25 h. Figure 5 to 100 g of centrifugal acceleration under the vibration of the input and output waveform (prototype). In the process of loading and loading wave amplitude will remain basically unchanged.
Because of a vibration-based foundation of a system than 1 g test complex, 100 g under the exciter output similar to sine wave, however, the wave measured with the load of ice better similarity.
Dynamic load imposed after the soil around the foundation for more than static pore water pressure rise, the foundation's largest ultra-quiet surface of the pore water pressure can be effective stress of 20%. Figure 6 shows the initial excitation from the beginning of a prototype time 1.67 hours, a barrel near the top soil in the pore water pressure rose to a peak and then decline in the beginning of the process of change. At the same time throughout the course of excitation, that is, 25 h, the top settlement barrels a 60 cm. Subsidence curves appearing in two significant changes in the instantaneous, it may reflect the foundation is not connected with the vibration caused by the smooth (Figure 6).
4 summary
Institute of Mechanics, Chinese Academy of Sciences and Qinghua University jointly developed a centrifuge of electromagnetic power-loading equipment. The exciter in 1 g conditions, the input and output waveform of the relevance of good vibration can contribute to the design requirements.
100 g in the dynamic vibration under no deviation from trajectory, dynamic and non-permanent magnet friction. Exciter capable of suction-type basis of the requirements imposed by the frequency of the provisions of the peak level of dynamic load. Output waveform than 1 g under the conditions of poor, but in line with the basic characteristics of the ice load. Tsinghua University, thanks to Professor Li Weixian technical guidance. |