Motion Characteristics of Crane Vessels in Lifting Operation

ABSTRACT This paper deals with motion characteristics of crane vessels in lifting operation. Emphasis is laid here especially on the effect of coupled motion between a vessel and hook load. Linear equations for calculating motions of 8 degrees of freedom are derived. Results of calculation of motion responses in regular waves are shown to be in good agreement with those of experiments carried out on a scale model of an actual crane barge. Calculation of motion responses of a few types of crane vessels in irregular waves and a trial estimation of workability thereof are presented to show the influence of the coupled motion effect on performance of crane vessels in actual sea area. INTRODUCTION For those who design or operate crane vessels, especially such vessels as used in offshore construction or maintenance, one of the greatest concerns should be performance of crane vessels under severe environmental conditions; particularly their motion characteristics in waves are decisively important from the viewpoint of their safety and workability. In predicting motion of crane vessels in lifting operation however, it has been a common practice to assume that motion of the hook load is unilaterally dependent on that of vessels. Actually however, the effect of coupled motion of a vessel and hook load is so great that motion response of the vessel is quite different from the one without the effect. Therefore, it is necessary to treat a dynamical system including vessel and load to predict behavior of a crane vessel in lifting operation. In this paper, a theoretical method to calculate such coupled motion is introduced together with the results of calculation and experiments on a scale model of an actual crane barge in regular waves. Furthermore, this paper also offers results of a study by calculation of the coupled motion effect on performance of a few types of crane vessels in irregular waves and some trial calculations of workability thereof EQUATIONS OF MOTION AND OTHER THEORETICAL ASPECTS We assume that the amplitudes of waves and oscillations are small enough to permit application of linear theory. According to the current practice, we set up a right-handed Cartesian coordinate system O--X1, X2, X3 with X1 and X2 axes in still water surface and X3 positive upward. We define six components x1-x6 of vessel motion as usual: translational motion Xi of G along the Xi axis (i = 1–3) and rotational motion xi+3 about an axis through G parallel to Xi (i= 1–3). Furthermore, we describe motion of the hook load by its swing angles °1 and °2 about X1 and X2 axes respectively. Thus, we have a dynamical system of 8 degrees of freedom. We derive the equations of motion from Lagrange's equations for which we calculate kinetic and potential energy of the vessel, surrounding water and hook load.