A design for ship stabilization by activated antiroll tanks

An activated antiroll tank system design for ship roll reduction was investigated. Considering the dynamics of the ship motion, tank flow, and variable-pitch impeller, a control law is derived based on optimal control and estimation theory. Finally, a series of numerical computations for a 1174 tonne ship without an antiroll tank system, with a passive tank system, and with the proposed activated antiroll tank system is performed. The results show that the proposed activated tank system has superior antiroll properties for free rolling, regular, and irregular sea loads to the passive tank system. In addition, this design has a very low sensitivity to variations in ship and tank dynamics. It shows that the proposed system is reliable and valid for practical use.     

Vibration reduction of main hulls using semiactive absorbers

A semiactive-type absorber for vibration reduction of main hull girders was investigated. The semiactive absorber system includes a moving mass, support springs, dynamic dampers, and a control system. Only a small electrical power supply is needed for control of the damper valve and the operation of the control system. In this paper, the dynamics of the ship's hull and the constraints of the semiactive absorber are described first. Then, a suboptimal operation law is derived based on the properties of the absorber and the theory of optimal vibration reduction. The numerical simulation results show that the semiactive absorber is more efficient in hull vibration reduction than the passive absorber during critical periodical excitation from the propeller. The vibration caused by multifrequency excitation can also be suppressed by the semiactive absorber. In terms of effectiveness, the semiactive absorber is almost as effective as the active absorber. In particular, the performance of the semiactive absorber is excellent in the reduction of high-frequency fluctuations.List of symbols C _h ⁽ⁱ⁾ damping matrices of the segmenti - C _sb structural damping coefficient of bending - C _ss structural damping coefficient of shear - C _v hydrodynamic damping coefficient - EI flexural rigidity - f _a force generated by the absorber - f _ad damper force of the semiactive absorber - f _ext total excitation force - F _ext ⁽ⁱ⁾ generalized load vector in segmenti - teÎ the identity matrix - J performance index - J _r rotatory moment of inertia - k _a stiffness coefficient of the absorber - K _h ⁽ⁱ⁾ stiffnes matrices of the segmenti - K _s A _s G _s shear rigidity - k _v hydrodynamic spring coefficient - l _k length of the segmentk - m _a mass of the absorber - M _ext total exciting moment - M _h ⁽ⁱ⁾ mass matrices of the segmenti - m _v mass moment of inertia - w _h deflection of the center line of the hull - W _h ⁽ⁱ⁾ vertical translation and shear slope of nodes in segmenti - ¯ w _d displacement of the absorber mass relative to the hull - ¯ w _a absolute displacement of the absorber mass - ¯ w (a, t) absolute upward displacement of the hull atx=a - slope deflection due to bending - slope deflection due to shear - Dirac delta function - _k ⁽ⁱ⁾ Kronecker delta function - _k distribution function - shape function vector