It is obvious that at lesser depths and/or lesser loads the safety factors 

 would increase. Since it is highly conjectural how many lifts of 600 tons at 

 6,000 feet will be made, the only safe thing to do is give the worst case extra 

 consideration. Therefore, assuming that a large fraction of the lifts will put 

 the system to its maximum and, in addition, assuming that the highest 

 factor of safety is the most desirable, the 10-3/4-inch OD pipe weighing 

 81 pounds per foot will be given a more detailed investigation. 



Dynamic Loads 



The preceding calculations for the factor of safety were based on 

 conditions of static loading. For sea-going lift systems, the dynamic loads 

 are extremely important and static conditions do not form a realistic basis 

 for design. Because of this, the anticipated conditions of dynamic loading 

 should be given. 



There are two types of dynamic loading which are important in the 

 analysis of this system; (1 ) loads incurred during a sudden stop and (2) loads 

 imposed on the pipe due to ship motions. 



Stresses Due to a Sudden Stop. Computing the stresses in the pipe 

 due to a sudden stop requires that the force-time loading be either assumed 

 or determined. It is extremely difficult, if not impossible, to determine the 

 value of a load as a function of time. Because of this problem, it will be 

 assumed that the load is applied suddenly over a limited duration and is 

 constant (i.e., a rectangular pulse load). For a given time period, the force 

 can be determined from impulse-momentum considerations. By use of the 

 dynamic load factor, it is possible to compute the total load due to a sudden 

 stop. The results are plotted in Figure B-2. 



It can be seen in Figure B-2 that a safety factor of two is possible for 

 all sudden stops at any depth, if the load-pipe string combination is decelerated 

 frorni 2 to feet per second in 5 seconds. Even if the stopping time is of the 

 order of 1 second, the assumed safe maximum is not exceeded except for 

 depths over 5,000 feet. As a basis for comparison, the stresses due to an 

 immediate stop (computed using strain energy) are plotted in Figures 

 B-3 and B-4. It is apparent that in the latter situation, the pipe string could 

 be subjected to extremely high stresses; however, immediate stops are 

 impossible to achieve, and the curves for the stresses imposed on the pipe 

 over short time intervals are more realistic. 



It is apparent that stopping the pipe string could be a risky operation, 

 if the initial velocity were too high. The stopping operation is delicate and 

 careful control has to be exercised over the entire sequence. Nevertheless, if 



106 



