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 behavior 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 behavior 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 will be presented. A procedure will be shown 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 will be shown. Trends following from the calculations are compared with results of model tests. The results of the tests and the calculations show a reasonable agreement.
An SPM system occupied by a tanker will generally be designed for a maximum operational condition with regard to the sea state. The sea state, at which the down-time starts (disconnecting bow hawser and floating hoses) will be determined for an important part by the force level in the bow hawser.
The behavior of a tanker moored to an SPM system is greatly determined by the slow motions of the tanker in the horizontal plane. Due to this slow motion (fish-tailing and galloping) behavior of the tanker a slowly varying force occur in the bow hawser. Superimposed on the slowly varying force in the hawser are the forces caused by buoy and tanker motions with wave frequency. The combination of these forces can lead to high peak loads in the bow hawser force.
The phenomena occurring while the tanker is moored, are complicated and will be influenced by a lot of parameters, for instance: the size of the tanker and loading condition, the elasticity properties of the buoy and bow hawser, the length of the hawser, the forces on the vessel exerted by wind, current, waves, astern propulsion and the underkeel clearance.
Some model tests have been carried out to illustrate the slow motion behavior in the horizontal plane of a tanker in ballasted and fully loaded condition moored to an SPM system in steady wind and current and in irregular waves.