The purpose of this paper is to discuss the measurement of resistance of a ship or an offshore structure in transit or under tow. Since a small replicate of the prototype is used, several simulation difficulties are experienced in the small scale testing. A few of the difficulties of the resistance or drag test and the remedial measures used in the setup and in the scaling up of the model data are discussed here.
The standard measurement technique for the resistance of a ship in a towing tank employs a towing staff attached to a dynamometer used for the measurement of the towing resistance. The towing staff is attached to the towing carriage in such a way that the system allows the ship to heave and pitch about the attachment point on the ship, but restrains its motion in the transverse direction. The aft end of the ship model is sometimes provided with a guide system for the side restraint. Offshore structures, however, are often towed in a restraint position, such as in the simulation of current drag.
A two staff arrangement for the towing tests is seldom used and is considered unconventional by many of the traditional basins. This paper addresses the difference of towing a· floating offshore model with a single staff (including a guide) versus a two staff arrangement. A ship model was consecutively tested at the simulated transit and loitering speeds in the Offshore Model Basin (0MB) at Escondido, CA in both a two staff and the traditional one staff arrangement. Both the inline and transverse loads were measured during these tests. In addition, the heave and pitch motions of the ship model were also measured. It is found that the results from the two series of tests were identical.
Another area of uncertainty is the effect of Reynolds number. The distortion of Reynolds number in the model necessitates certain corrective measures in the model tests. Use of the turbulence stimulator and the quantitative difference in the results with and without this stimulator for an offshore structure model is shown. The simulation of cu t load on an offshore structure is often done by towing e model. The difference between the current load and the equivalent towing load on a particular floating structure model is shown. It is shown that the towing loa were generally smaller than the corresponding current loads.
