A Basic Study on the Transient Behaviors of Moored Structures on the Sea

Mooring systems consist of floating bodies on the sea surface, mooring chain and/or wire in the sea, and mooring chain and/or wire and anchors on the sea bed. In order to analyze the transient behaviors of these systems subjected by large external load due to wind, waves, and tide, it is necessary to know the dynamical and statical characteristics of chain and anchors on the sea bed. As quasi-statical tests of anchors have performed by Ura and Yamamoto(13), the holding power and the roll stability of anchors can be discussed on the basis of the formulas proposed. In this paper, it is shown that experimental results of added mass of anchors due to soil is considerably larger than their own mass, and they provide a good fit to the theoretical formula proposed. The longitudinal and transverse holding friction of chain is also tested and the results are given in simple formulas.On the basis of these results, computational simulation and experiments are performed in order to analyze the transient behaviors of floating body subjected by a large gust and these results show good agreements. It is also shown that a mooring line to the lee side is effective to prevent the main anchor from being dragged, even if its holding power is not so large as that of main mooring line.