Tables I and II show that frequencies as small as 1.5 rad/sec (the 

 smallest resonant frequencies of the cable) are more likely to be found at 

 smaller wind speeds, but the amplitudes are much smaller at smaller wind 

 speeds. Obviously, as the wind speed decreases, the chances for the success 

 of the operation increase. The largest wind speed that is safe will depend on 

 the stability of the vessel and the manner of attaching the cable to it. 



Two obvious ways of attaching the cable to the vessel are from the 

 center of gravity., and from the side through a boom. In the first., heaving 

 motions of the vessel in beam seas will be critical., while in the second, roll- 

 ing motions in beam seas will be the most critical. 



In Figure 4, the heave and roll response amplitude operators for 

 Cuss I* in unidirectional sinusoidal (regular) deep ocean beam waves are 

 plotted versus the frequency m . Scales for the period (equal to 2 tt/ou) and the 

 wavelength (equal to 2v!g/m^) are provided. For waves approaching the vessel 

 from other directions, heaving and rolling are less. These curves were ob- 

 tained from "The Motions of a Moored Construction -Type Barge in Irregular 

 Waves and Their Influence on Construction Operation," Contract NBy-32206, 

 U.S. Naval Civil Engineering Laboratory. 



In the case of attaching the cable to the center of gravity of Cuss I, 

 some meaningful conclusions as to the safe sea state can be drawn from Tables 

 I and II and Figure 4. Assuming that we can represent an irregular sea by a 

 regular one with amplitude equal to the average amplitude of the irregular sea 

 and frequency equal to the most frequent frequency of the irregular sea., then, 

 when the wind velocity is 20 knots, the input to the cable is UD = 1.4 rad/sec 

 and 1 UqI = (0.42) (2.5) = 1.05 ft. This input can be tolerated by the cable. 

 According to this representation of an irregular sea., a wind speed of 30 or 

 even 40 knots can be tolerated by this cable. Since the input amplitudes that 

 the cable can tolerate are small for high frequencies., perhaps a fairer repre- 

 sentation of an irregular sea by a regular one is by the average of the 10% of 

 highest amplitudes and the most frequent frequency or the highest frequency of 



*Cuss I, originally a nonpropelled freight barge, is 260 feet long and has a beam 

 of about 50 feet and a draft of about 11 feet at 3, 000 tons displacement. It has 

 been converted to a sea drilling vessel, and in March 1961, it was used suc- 

 cessfully by Project Mohole to drill in almost 12,000 feet of water at a site 40 

 miles east of Guadalupe Island, Mexico. Cuss I is used for our preliminary 

 computations, because it is the only vessel, of the type that could be used in 

 the present operation, for which we can obtain the amplitude response curves 

 from existing literature . 



27 



artbur KlLittliSnc. 



S-7001-0307 



