Fryer D.,MTMC Ltd |
Gorrie J.,MTMC Ltd
Journal of Ocean Technology | Year: 2010
This paper is about lifting and lowering big loads using davits or cranes mounted on ships that are moving in a seaway. It applies particularly, but not exclusively, to situations in which the load is transferred between the ship and the water surface. Examples include launching and recovering lifeboats, survival craft and fast rescue craft. In these examples both the lifting ship and the lifted load may be subject to excitation by the waves. During lifting and lowering, the system behaves as an undamped compound pendulum. The motion includes two distinct, but coupled, modes, illustrated in Figure 1. The first mode is swinging of the suspended load about a fulcrum which is the sheave of the davit or crane. The natural frequency of the swinging mode is determined by the length of the suspension L (the distance between the sheave and the centre of gravity of the load). The natural frequency reduces as the suspension length increases. The second mode is rocking about a fulcrum, which is the hook that attaches the load to the lifting wire or fall. The natural frequency of the rocking mode is determined by the length of the suspension R (the distance between the hook and the centre of gravity of the load). If R is less than the rocking gyradius of the load, the rocking frequency increases as R increases. During lifting, the swinging frequency increases but the rocking frequency remains constant, because L reduces but R does not change. The coupling between the two modes varies throughout the lift. During lowering, the swinging frequency reduces but the rocking frequency remains constant because L increases but R does not change. Both modes are capable of being excited by either continuous or transient inputs. Continuous excitation may be provided by the continuous motion of the lifting ship. Transient excitation may be provided if the load is not stationary and directly under the davit when it is lifted from the surface of the water. If the load and the ship are both in a seaway, both forms of excitation are almost certain to be present. The amplitude of oscillations caused by transient excitation is determined by the initial error in the lift-off position of the load, and persists throughout the lift because the system is virtually undamped. The amplitude of oscillations caused by continuous excitation increases throughout the time the load is suspended. Hence the coupled swinging and rocking motion described in this paper becomes a problem when lifting high-mass, high-gyradius loads. The motion can be dangerous, especially if the load is a rescue craft or lifeboat, in which case it may contain people. However, it is possible to control or suppress the motion, and that is the subject of this paper. © Journal of Ocean Technology 2010.