Dynamic Behaviour Of Platform Cranes

Abstract

ABSTRACT Mathematical models of varying complexity are developed to predict forces occurring in a typical offshore platform crane while lifting from a supply boat. Computer programs based upon these models are used to assess the usefulness of these simulations. It is shown that a simple two degrees of freedom model can be usefully employed at the feasibility design stage whereas a more sophisticated multi-degree of freedom model is required for detailed crane design. These programs are used to investigate the influence upon the induced loads of various operational parameters including the load lifted, the jib angle, the line speed and the instantaneous deck velocity. It is concluded that high dynamic loads will frequently arise in such cranes and that specifically written computer programs are necessary for effective derating. or design. A statistical approach to the assessment of supply boat motions is used to show that maximum deck velocities can greatly exceed those predicted by the simpler methods previously used. INTRODUCTION As oil and gas exploration moves into more exposed waters supply boats are required to operate in increasingly severe conditions. This means that the dynamic stresses induced in offshore platform cranes lifting from these boats are likely to increase for three different reasons. Firstly, there will be a tendency for supply boats to stand further off from the platform with the consequent increased lifting radii resulting in increased stresses. Secondly, higher stresses will be produced should higher line speeds be used to reduce danger to the supply boat, the supply boat personnel, and the load. Thirdly, larger supply boat motions mean that the effect of a misjudged lift will prove more severe. Since exploration has moved into the North Sea there have been numerous crane failures, and recognition of the dynamic aspect of offshore lifting is given in the derating rules used when lifting in conditions other than a flat calm. In spite of this, the accident statistics for offshore crane operations continue to make sad reading, indicating shortcomings in the derating criteria adopted. It is self-evident that to design or derate an offshore platform crane effectively a study of the crane's behaviour over a wide range of conditions must be made. Unfortunately, due to the complexity of offshore cranes, an explicit solution is possible only if the crane structure is reduced to a very simple model. Such models have been studied previously (1,2,3" but they tend to give unrealistic results and they fail to give any information about the forces in many of the primary crane components. For a more realistic prediction of response mathematical models of greater sophistication must be employed. Such models normally use step-by-step numerical techniques to integrate the equations of motion of the structure through the time domain starting from initial, known, conditions. Essentially the conditions at time "t" are used to predict the conditions a short interval of time "ilt" later.