Reduction of offshore platform response with a liquid vibration absorber and seismic response of elevated liquid storage tanks

Lee, Seng Cheok (1979) Reduction of offshore platform response with a liquid vibration absorber and seismic response of elevated liquid storage tanks. Masters thesis, Memorial University of Newfoundland.

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Abstract

The thesis is divided into two parts - I and II. -- I Reduction of Offshore Platform Response with a liquid Vibration Absorber – The LVA consists of a liquid-filled cylindrical container mounted near the deck level of the platform. During wave excitation, the liquid in the container swirls into an oscillating motion which interacts with the platform motion to produce a reduction in the platform response. The reduction in the platform response is largely due to energy dissipation through damping of the liquid, and to a smaller extent the inertia of the liquid. In this investigation, the effectiveness of the LVA in reducing the dynamic response of the offshore platform model is studied. The finite element programme for dynamic analysis of two-dimensional fixed offshore platforms, developed by Duvall (1), has been extended by Glacel (2) to include damping effects and a tuned mass damper. This study describes further modifications of the work with an additional option of replacing the actual spring mass model by an LVA. The relatively rigid container wall is discretized by finite beam elements. The work of Housner (3) is employed account for the liquid sloshing loads on the container wall, while the liquid damping is based on a semi-empirical formulation by Stephens et al (4). The structural response of the model is determined for a digitized wave height spectrum, with and without the LVA in operation. The platform is analysed to determine the LVA effectiveness, and the variation of the system response with the LVA parameters. The various parameters considered are, cylinder radius, liquid height, liquid mass, frequency, and damping. Methods of frequency tuning, and damping device mechanisms are discussed. – II Seismic Response of Elevated Liquid Storage Tanks – The structure under consideration consists of a liquid storage tank mounted on an axisymmetric pedestal. The tank is a thin elastic cylinder with an axisymmetric dome-top and a conical base which is relatively rigid. A finite element model is presented for the seismic analysis of the structure. The shell mass and stiffness matrices are generated by using the computer code SAMMSOR-II (56). The procedure of Shaaban and Nash (45) is used to generate the added liquid mass matrix which accounts for the hydrodynamic effect on the tank wall. A digitized acceleration of an earthquake is provided as the ground excitation input, and the displacement response of the whole system determined by mode superposition.

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