In this paper the phenomenon of the slowly varying drifting force on a moored object in a random sea will be explained and illustrated from the results of a number of model tests with a rectangular barge. These tests, conducted at the Netherlands Ship Model Basin, were an extension of an earlier executed program.
It will be shown that, using the results of measured or calculated drifting forces on an object moored in regular waves, a prediction can be made of the drifting forces induced by wave trains consisting of regular wave groups.
Also for an irregular wave train the drifting force on the barge could be computed as a function of time, which made it possible to calculate the surge motion of the barge The results of tests and calculations show a reasonable agreement.
In the last few years the problems concerning the mooring of objects in random seas have gained much attention as a result of the necessity to load and discharge big tankers in open sea or due to the fact that the sea bottom had to be explored and exploited from vessels operating from the water surface.
A floating object moored in waves will generally be subjected to forces causing horizontal and vertical motions and moments causing angular motions about horizontal and vertical axes. In this paper the horizontal surge motion of a rectangular barge moored by means of linear springs in head waves will be dealt with. The surge motion can be split-up in a mean excursion, a slowly varying motion and a higher frequency oscillation around the slowly varying position.
The period of the higher frequency oscillation is equal to that of the wave motion and since a considerable amount of literature is available concerning this part of the motion, it will not be treated in this paper. From the results of model tests in regular waves the mean drifting force on the barge could be determined as a function of the wave frequency. Using these data the long periodical surgemotion of the barge was calculated for different stiffnesses of the mooring system for the condition that the barge was moored in a wave train which consisted of regular wave groups. The results of those calculations are compared with model test results. From these and earlier executed tests it is clear that resonance phenomena may occur when the period with which the wave groups encounter the barge equals the natural period of the surge motion of the moored barge. Further it appears to be possible to calculate the drift force induced by regular wave groups when such a wave train is considered to consist of two regular waves with a small difference in frequency.