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.

Vibration reduction of main hulls using semiactive absorbers

An active bow absorber for vibration reduction of hull girders is presented. In this investigation, the chain locker of ships is designed as the tuned mass of the absorber, which is driven by a hydraulic system. Since the chain locker is very heavy, the absorber will have a high efficiency for vibration reduction. Including the dynamics of the hydraulic system, the control law of the absorber is derived basing on the optimal control and optimum estimation theory. Finally, frequency and time response are considered for a bulk carrier to understand the feasibility of this design. From the simulations, we find the bow absorber is highly effective for vibration reduction of hull girder due to main engine excitation, especially for the lower frequency mode, such as v-2, v-3 and v-4 modes. The required hydraulic force and the displacement of the actuator for reducing the resonance exciting are reasonable. The results also show the effect of vibration suppression by the bow absorber is similar to that by the stern absorber even for the excitation in stern.

Design of bow absorber for vibration reduction of hull girders

64) Robust Control for the Vibration Attenuation of Superstructures

A robust control methodology applied for active vibration reduction on superstructures using the H∞ synthesis is presented Based on this technique, the control law may be calculated according to the required objective The designed control system will automatically meet the performance and robustness properties, which are given in the specifications Finally, the superstructure of a cargo ship subject to wind gust and foundation oscillation is investigated The results show that the active vibration control system designed by this scheme has an effective vibration attenuation for both excitations In addition, this controller will maintain high performance even when there is a large perturbation in the system parameter

Robust Control for the Vibration Attenuation of Superstructures

Structural problems caused by wave induced hull vibration were experienced by a series of six landing craft operated by the Royal Australian Navy. These craft, of maximum displacement of 520 tonnes, were designed primarily for the transport of cargo from ships off-shore to water terminals and undeveloped beaches, but they have also been operated on long sea voyages, often in rough seas. It was decided to fit a damped vibration absorber in each vessel to increase the structural damping and minimise the vibration response. The absorber used has a mass of 10 tonnes, is supported on air springs, and is installed in the bows of the ship. The paper describes the design and installation of the vibration absorber, and gives the results of evaluation tests carried out in calm seas and in rough seas. Several years of operational experience have now been gained and the absorber has proved to be very effective in reducing hull vibration in these craft. The paper is followed by a discussion.

P A Farrell

Papers

Reduction of Hull Vibration in a Landing Craft by Means of a Vibration Absorber