The progression of lifting and lowering operations into ever-increasing depths of water has exposed a weakness in the long-established methods of assessing the torsional stability of multi-fall hoists. Instances of "cabling" have been reported on two-fall hoists despite using "low-rotation" ropes in situations where they were expected to perform satisfactorily.
The author explains how assumptions that worked perfectly well when heights of lift were a few hundreds of feet can no longer be safely applied to offshore operations in thousands of feet water depth. This article presents a new and more rigorous solution that reveals a secondary influence of height of lift on the torsional stability of multi-fall hoists. The revised theory is used to illustrate the severity of the problem faced by offshore operators and to examine alternative methods of solution.
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