100 ft. MOVEMENT (ELASTIC) 



1 DEVIATION AT 0. 5 ft/sec 



MORE OR LESS UNRESTRAINED RADIUS 

 OF PLATFORM MOVEMENT 426 ft. 



Fig. 2. Platform moored to submerged buoy at 

 150 feet in 6,000-foot water depth 

 with upper mooring scope of 3 ; 1- 



WATER VELOCITY 



TIME, HOURS 



Fig. k. 



Distortion of surface tidal current by- 

 elasticity and slack in the moorage. 



Fig. 3- Richardson toroidal buoy on taut 

 elastic propylene rope. 



reversing current of semi-diurnal period has been 

 considered. To suit the shallow water case the 

 peak velocity has been taken as 1.0 fps while for 

 deep water the more reasonable value of 0.5 fps 

 is taken. 



Fig. k shows the kind of distortion to be 

 observed for the two simple moorings. In the 

 shallow water case the analysis is simple. Shortly 

 after slack water the elastic extension of the 

 mooring has been relaxed and the platform is free 

 to move with the current for a distance equal to 

 twice the slack in the mooring. In the simplified 

 case there is no relative motion between platform 

 and water during this period and the current meter 

 registers zero. Rather abruptly the mooring line 

 begins to tauten and the apparent velocity rises 

 sharply. The rise is sharp because the elastic 

 extensibility of the moorage is small compared to 

 the total excursion of the water during one half- 

 cycle (252 feet versus 13,770 feet). The measured 

 velocity never does reach the peak velocity 

 because some elastic extension of the moorage is 

 continuing at this point. When peak velocity is 

 past, the moorage begins to relax and moves the 

 platform against the current to maintain the 

 apparent velocity higher than it should be until 

 after the true velocity has reached zero. The 

 cycle then repeats in the opposite direction. 



In the deep water moorage the total water trans- 

 port in one half -cycle is 6,885 feet and the 

 elasticity amounts to 2,750 feet in addition to 

 the 1,200 feet of unrestrained motion. The result 

 is to attenuate the apparent peaks quite severely. 

 The 1,200 feet of slack contributes its period of 

 apparently zero velocity as before . The degree 

 of attenuation and phase lag depend greatly on 

 the stiffness of the mooring so the diagram in 

 this case must be regarded as illustrative rather 

 than in any degree quantitative. 



i*a 



