amplitudes are less than 6 feet at a maximum frequency of 1.40 radians per second. 

 As the cable length diminishes to zero it would appear that the design dynamic stress 

 will be exceeded by an increasingly large amount. From Figures 33 and 34 it is seen 

 that for both examples at shorter lengths of cable, a greater amplitude of oscillation 

 is relatively less safe than a small amplitude, as expected. Further calculations are 

 required to determine the form of the curves as the cable length approaches zero. 



On 13 April 1965, a Submersible Test Unit (STU) loaded with racks of 

 specimens was lowered to a depth of 2,500 feet by the Deep Ocean Engineering 

 Division of NCEL. The record of cable tensions during the lowering operation is 

 shown on Figure 35. The weight of the load in water was approximately 5,500 pounds, 

 the structure being in the form of an open truss, its base consisting of two flat plates 

 with a total cross-sectional area of approximately 150 square feet. The load was 

 lowered on a 1.3-inch-diameter polypropylene cable. From a depth of 450 feet the 

 descent was carried out at a steady rate of 132 feet per minute. At depths shallower 

 than 450 feet the lowering operation was intermittent. The wave excitation was 

 estimated to be in the form of a 6- to 10-foot swell with periods of 10 to 12 seconds. 

 A brief discussion of various properties of the record in the light of the calculations 

 given above is pertinent. The parameters of the cable and load were approximately 

 those used for the first design example, although values of the coefficients of drag 

 and virtual mass do not necessarily agree. 



The record shows an expected decrease of dynamic stress with increasing depth. 

 The immediate reduction in the mean load at point B of approximately 2,000 pounds 

 corresponds to a drag force on the structure with a coefficient of drag equal to 2.76 

 at a vertical velocity of 132 feet per minute. Since the load consisted of racks of 

 specimens orientated parallel to the flow, this drag coefficient is not necessarily that 

 corresponding to form drag alone; however, the contribution of tangential drag on 

 the cable can be shown to be negligible. The steady reduction of mean load from 

 450 to 2,500 feet is due to the gradual removal of the weight of a 2, 500-foot-long 

 steel cable suspended from the base of the STU to the ocean floor. 



From the design example given previously the expected maximum dynamic 

 stresses in the lowering cable were calculated at cable lengths of 83, 830, and 

 1,660 feet assuming that |Uq| equals 6.0 feet. Table VII summarizes these compu- 

 tations which were carried out for each cable length by determining the period of 

 the cyclic stress from the record. Values of l-^axl w®''® then found from the curves 

 given in the results, for the appropriate ^ and y. values. Hence, the maximum 

 dynamic stresses were determined as shown in Table VII and superposed on the stress 

 record as given in Figure 35. 



In view of the uncertainty with respect to values of C[), C^^, and |Uq| for the 

 operation represented by Figure 35, the comparison of the calculated dynamic stresses 

 with those determined experimentally is considered to be fair. It is noted that at 

 830 and 1,660 feet the stresses recorded exceed those calculated by a factor of 2. 

 As the load is lowered, only one variable changes, that being the cable length. 



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