Abstract

We test the feasibility of employing an exclusively planar control effort to suppress unsafe ship-mounted crane pendulations induced by sea motions. The new crane configuration, designed to apply the control effort, is modeled and the proposed control effort, employing Coulomb friction and viscous damping, is applied. The three-dimensional nonlinear dynamics of the crane is then investigated.

The new crane configuration, dubbed Maryland Rigging, transforms a crane from a single spherical pendulum to a double pendulum system. The upper pendulum, a pulley riding on a cable suspended from the boom, is constrained to move over an ellipsoid. The major axis of the ellipsoid is the boom and the foci are the two points at which the riding cable attaches to it. The lower pendulum, the payload suspended by a cable from the pulley, continues to act as a spherical pendulum. Due to the geometry of the ellipsoid, the natural frequencies of the crane in the plane of the boom (in-plane) are almost equal to the out-of-plane natural frequencies.

The model is used to examine the response of a Maryland rigged crane to direct, in-plane, harmonic forcing. The frequency of the excitation is set almost equal to the crane’s lowest natural frequency. It is found that under this excitation and due to the one-to-one internal resonance between the lowest in-plane and out-of-plane natural frequencies, significant out-of-plane motions are induced by applying a purely in-plane forcing. Thus an in-plane control mechanism is not adequate for safe operation of the crane. To guarantee safe operation of a ship-mounted crane, one must apply both in-plane and out-of-plane control efforts.

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