Discussion. Port of Tema.

Abstract

said that quantitative data on the movement of ships at berths was rarely obtained and there was none in the case of Tema, but by all accounts the berths were remarkably quiet. There had been no case of a ship having to put to sea because of ranging.208. The contribution of the Hydraulics Research Station to this happy state of affairs was rather a modest one. There were three major factors in determining the movement of moored ships at berths, only one of which had been studied at the Hydraulics Research Station:(a) the waves in the vicinity; (b) the attenuation of waves by the harbour; (c) the response of the ship and its mooring system to waves.At the time when the harbour was designed there were virtually no wave records.None of the few records that were obtained was harmonically analysed because there were no techniques for making use of such analyses. 209. Again; at the time the Hydraulics Research Station was making its investigation, in 1955, no information on the response of typically moored ships to waves was available. One had merely mariners' views that waves around 1 ft highsome said waves around 2 ft high-were just tolerable for a ship lying alongside. The only one of the three problems that was amenable to study was the attenuation of waves by the harbour, and this was examined minutely in an undistorted model built to a scale of 1 :l 20. 210. Fig. 9 showed a typical response curve, one of a large group which together revealed the performance of the harbour. The ordinate was the maximum waveheight found wherever it occurred along one berth expressed as a ratio of the steady wave-height at sea; the diagram showed how this ratio varied with the period of the waves. The abscissa covered a range of periods from 7+ to 45 seconds. The figure showed that in the 10-S period wave band responses were very satisfactory, around 1/20. At the long end of the scale, around 45 S, the response rose to one third and there was another whole set of data relating to waves with periods extending from 40 S to 5 min, which showed responses generally in excess of unity. 21 1. One wondered whether the harbour's freedom from the very long waves in the 40-S to 5-min period band was due to the absence of such waves on this particular coastline. The speaker was inclined to think that this was not the explanation. On the contrary the waves normally reaching Tema formed a regular swell from distant storms and were precisely of the type likely to set up surf-beats.212. It could be demonstrated that very long waves, or surf-beats, were not too troublesome provided they were not accompanied by shorter period swell. One found that rather stiff ropes were desirable, giving the ship a natural period shorter than that of the surf-beats. The speaker thought that the great success of the design in excluding swell had led to quiet berths in spite of the presence of surf-beats from time to time. Unfortunately this must remain a matter of conjecture because no wave records were available.