Dynamic characterisation and modelling of a slew drive system of offshore cranes

Abstract

Current dynamical models of the slewing motion of offshore cranes cannot predict the occurring vibrations during the design phase that arise during slewing. The relatively small pedestal crane on the Sleipnir experienced severe vibrations during the slewing motion in both the crane house and the boom, which were in the order of 1 and 5 Hz. As a result, the crane operator almost shook out of his chair. This research aims to create a dynamical model of a crane from controller to suspended load to predict the occurring vibrations during slewing during the design phase. The type of crane studied is an electrically driven pedestal mounted offshore crane with a lattice boom structure. A system analysis of the crane regarding the dynamics results in a list of dynamic effects. These are compared to obtain which dynamic effects are significant and which are negligible to include in the dynamical model. The equations of motion are derived to create the dynamical model, which consists of the controller, the slew drive system, the boom and the suspended load. The controller includes a PI-controller, a ramp function and play compensation. The ramp function creates a s-curve as speed input for the PI-controller. Play compensation brings the gears under tension to press out the play, which prevents backlash. A comparison between a model with and without backlash shows that the model with backlash results in vibrations consisting of two frequencies, which are in the same order of magnitude as the vibrations observed in the pedestal crane on the Sleipnir. While the model without backlash is not able to predict these vibrations. In addition, the frequencies of vibration correspond to the natural frequencies of the first two lateral bending modes of the boom. The results show that backlash in the system excites the natural frequencies of the boom. This means that the cause of the vibrations is the combination of backlash and the flexibility of the boom. The dynamical model calculates the same vibrations as were found in the pedestal crane on the Sleipnir. Thus, the proposed dynamical model can predict the occurring vibrations during slewing during the design phase. Experimental data should be obtained in the future to validate the model.