The towing experiment both in still water and in waves using two types of model of offshore structures such as barge and semi -submersible has been carried out in order to investigate the dynamic towline tension. Furthermore, the forced oscillation test with the cable model has also been carried out to compare with the theoretical calculation.
According to the statistics of the accidents concerning the marine structures, the number of the accidents during the transportation on the sea, namely towing, is the most. It reaches as high as 54.4% out of the total number of accidents (Maeda, 1984). It has been reported that the drift of the marine structures due to the breakage of the towline covers 13.6% out of this kind of towing accidents. The impulsive tension on the towline during the unstable motion can be an important factor leading to the accidents of the towline breakage in the towing of the marine structures. The towline tension in still water is discussed at first in this paper as a first step to investigate the towline tension during the unstable motion in the case of general single towing using two representative models of the marine structure such as barge and semisubmersible. There are quite a few researches (Bernitsas, 1986, Nonaka, 1989, Haraguchi,1988) on the maneuvering motions during towing. As for the extreme towline tension from seakeeping motions, Fylling et al. (1979a) presented the comparison between the experimental results and the theoretical calculation for moderate non-linearities, provided that the dynamic force does not exceed the static force. Milgram (1988) developed simplified models for the horizontal towline dynamics and showed the experimental and theoretical results of the dynamic towline tension. Shin(1991a) has also made the comparison between the numerical model predictions using clipping-off model and existing experimental results.