| Engineering Structures are subjected to a variety of environmental loads. They may cause the structures vibrate violently, sometimes may cause them damage or even collapse. Conventionally, structures resist such environmental actions by means of the inherent strength of the structures, i. e. the capacity to store or consume the energy due to the deformation of the structures during vibration. Unfortunately, so far it is still impossible to predict the maximum intensity and characteristics of such environmental loads accurately that may take place during their life periods. Under such uncertain loads, safety will not be insured, that may probably lead the structures to severe damage or even collapse. To cope with this problem, the "structure control" scheme has been regarded as an effective means. That means some control devices are installed on the structure, which will resist the environmental loads jointly with the structure to reduce the structural vibration caused by earthquakes, winds, ices or other environmental loads. In the past twenty years, although many progresses on the theory and methodology of structural control have been achieved, there are still many difficulties existing in their applications. The appearance and developments of the pseudo-excitation method (PEM) and the precise integration method (PIM) in the past decade has exhibited an attractive and brand new approach to tackle this field. In this thesis, the use of these two methods is attempted in order to improve the efficiency and precision of existing structural control methods, and also narrow the gap between the theory and its applications. The research covers LQG control, precise instantaneous optimal control and time delay control. Under the support of the project "863" hi-tech research and development program of china: "Innovative ice-induced-vibration resistant techniques for Jacket Platforms" (No. 2001AA602015), the ice-induced-vibration control techniques of offshore platforms under ice loads were studied extensively. A laboratory model control systems was built, and some applications of the vibration isolation techniques to offshore platforms were tested. The research results have now been successfully applied to JZ202-NW platform and good effect has been achieved.The main innovative research work in this thesis can be summarized as follows:1. Earthquake is in essence a non-stationary random process. For the LQG control problems associated with such non-stationary seismic excitations, the conventional methods are rather complicated and inefficient. In this paper, however, by using PEM, some innovative schemes are developed. In particular, the closed-form solutions of LQG control for structures subjected to non-stationary random seismic excitations has been derived, which is not only very accurate, but also quite efficient. By means of this method, some numerical computations are given to show the influence of the local site effect to adjacent buildings. By using the closed-form solution, some useful conclusions related to the system parameters are obtained, e. g., for given weighted matrices Q and R, the variations of the predominant frequency and the damping ratios of the site influence heavily on the effectiveness of vibration depressing. This method is also applicable to problems associated with other kinds of evolutionary random excitations.2. PIM is used to renovate a variety of optimal control algorithms.(1) A series of precise algorithms for the instantaneous optimal control, including the open-loop control, closed-loop control and closed-open-loop control, are proposed by using PIM, instead of the conventional numerical integrations. These proposed new algorithms possess very high precision even for quite large integration step sizes. A few numerical examples are given to show the advantages of the present method.(2)Time-delays in the feedback loop have severely restricted the development and application of structure control techniques and have resulted in unsatisfactory control behaviour. The PIM and the balanced model reduction method are used in the time delay control to design the controllers and the feedback controller with satisfactory vibration control performance by using discrete parameters obtained via the PIM. The control law contains not only the current state, but also a few previous states which reflect the influence of time-delays, that enables the proposed method valid for very big time-steps. The proposed method considerably reduces the order of the controller without causing essential difference in the control effect, which makes the method proposed in this thesis more feasible in practical applications.3. Many offshore platforms are built in a cold area, and so serious ice-induced vibrations may frequently occur. So far, however, there is short of systematic research work in this respect, and no effective method to cope with such vibrations has been found. This is a big challenge! The department of Engineering Mechanics in Dalian University of Technology takes on the project "863" hi-tech research and development program of china: "Innovative ice-induced-vibration resistant techniques for Jacket Platforms". As a backbone of this project, I work for it in connection with my doctoral thesis. I have applied successfully the vibration-isolating technology to the depression of offshore platform ice-induced-vibration, established a mechanical model for flexible structures with a tip-isolator; produced isolation control laboratory devices used for platform deck vibration-isolating experiments, which have been practically applied. Based on the analysis of dynamic ice-loads and the ice-induced vibration for fixed ice-breaking cones, a creative idea was suggested by me to install vibration-isolating cones on the jacket platform, and reasonable design parameters were found through repeated tests. Experimental results show that these measures are quite effective, and can hopefully be further developed in order to resolve such ice-induced-vibration problems thoroughly.
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