Van Oovtmevssen 



behind the structure, at the position of the midship section of the 

 tanker; 



b- Measurement of the mooring line force and of the surge and heave 

 motions of the bow of the tanker with the tanker moored to the cylin- 

 drical tank in irregular seas; 



c- Measurement of the mooring line force and of the motions of the 

 bow of the tanker with the tanker moored to a fixed pile of small dia- 

 meter, in the same sea-states as tests b. These tests were perfor- 

 med in order to determine the influence of the wave diffraction on the 

 behaviour of the moored ship. 



The different test arrangements are shown in figure 16. 



For the measurement of the wave height a wave transducer 

 of the resistance type was used. The force in the bowhawser was 

 measured by means of a strain gauge transducer and the surge and 

 heave motions of the tanker by means of a pantograph. 



The measurements in irregular seas lasted 210 seconds or 

 35 minutes for the full scale, which is regarded to be long enough to 

 obtain reliable statistic data. 



Besides the measurements, the wave diffraction at the posi- 

 tion of the midship section of the tanker was also calculated with the 

 potential theory. In figure 17 the calculated ratio of wave amplitude 

 behind the cylinder to incident wave amplitude j? g/ f % is given to a 

 base of the wave frequency GJ , together with some experimental va- 

 lues. With the aid of this curve of S a / f a , the energy spectrum 

 behind the cylinder can be calculated for any incident wave spectrum. 



The spectral density S ^ of the incident waves is defined by : 



S f (WJ dc^ 4 3 J (27) 



in which : 



.£" = the amplitude of the n th component of f (t) with cir- 



cular frequency cJ • 



Consequently, the spectral density of the waves at the posi- 

 tion of the midship section of the tanker can be found from : 



S* (W )dCJ = i-S 2 



[f 



£? n 2 an # 



* 

 an 



"1 2 



an 



(CJ ) 



n 



(28) 



970 



