On the Slow Motions of Tankers Moored to Single Point Mooring Systems

It is known that in steady current and wind in combination with irregular waves a tanker will not take up a steady position in the horizontal plane. Both full scale observations and model tests have shown that the behaviour of a tanker moored to a single point mooring system is greatly determined by the slow motions of the tanker in the horizontal plane. Due to these slow motions (fish-tailing and galloping) of the tanker high loads can occur in the bow hawser. In order to be able to solve the general SPM mooring problem of the slowly varying drifting behaviour of the object moored by means of a bow hawser, it is apparent that first a number of fundamental aspects will have to be studied separately. In this paper the results of a study on the dynamic stability and the natural frequencies of the modes of motions of the tanker in the horizontal plane, including the effect of the system parameters in steady current and wind are presented. A procedure is developed to obtain practical values of added mass and damping to calculate the nature of the stability and the natural frequencies of the system. To have insight in the sensitivity of the various system parameters on the stability and the natural frequencies, results of calculations are provided. Trends following from the calculations are compared with results of model tests. The results of the tests and the calculations show a reasonable agreement.