Some Aspects of Very Large Offshore Structures 



'Jf 



(<*> n ) = S «J n ) 



Jfan 



ffi 



CJ 



(29) 



In figures 18, 19 and 2 the spectral densities of the sea- states applied 

 during the tests, are given together with the predicted and measured 

 spectral densities behind the cylinder. There is a good agreement. 



The tests with the moored tanker were performed in the spec- 

 tra 2 and 3, with significant wave heights of 3.3 6 m and 5. 05 m. The 

 most important test results are stated in Table II. 



The most remarkable outcome of the experiments is the 

 considerable reduction in the mooring line force, due to the presence 

 of the cylindrical structure. The reduction in the force is relatively 

 much higher than the reduction in the wave height. This can possibly 

 be explained by the fact, that the drift force plays an important role 

 in the behaviour of a moored ship, this drift force being proportional 

 to the square of the wave height. 



If, for instance, we have a wave with frequency k-) = 0. 8, it 

 follows from figure 17, that the wave height is decreased by 20 per 

 cent, at the position of the moored tanker, and consequently the drift 

 force is decreased by 36 per cent, compared with the drift force in 

 the undisturbed waves. 



In figure 21 the results of the present tests are compared 

 with results obtained from the statistics of tests performed at the 

 Netherlands Ship Model Basin with different single point mooring 

 systems. For this comparison the following dimensionless coefficients 

 were applied : 



1 CO 



- for the mooring line force 



2/3 <~ 

 pgV g 



1/3 



and 



for the wave frequency 



CO 



60 



V 



J PP/J 



in which : 



v = the displacement volume 



L = the length between perpendiculars. 



PP 



971 



