It can be seen that the buoyant force can reduce the axial stress by 

 about 10% of the stress due to the weight of the pipe string and a 600-ton 

 load. This is of great advantage in reducing the total stresses and indicates 

 that the pipe string should not be flooded. 



Tapering 



The desirability of tapering the pipe string is obvious. Sections of the 

 pipe closer to the load can be lighter than those closer to the ship. A pipe 

 string designed to take advantage of this principle is illustrated in Figure B-10. 



There are probably other designs which would be more optimal than 

 the one illustrated in Figure B-1 0; nevertheless, it is difficult to conceive of a 

 design which would result in significant increases in the safety factors. For 

 the design illustrated, the safety factors are not appreciably different than if 

 the pipe weighed a uniform 81 lb/ft throughout its entire length. This is due, 

 primarily, to the fact that the load (600 tons) is very nearly half the allowable 

 load for the pipes. However, the pipe string presented in Figure B-10 will 

 hold a 357-ton load at 6,000 feet with a safety factor of three. 



There is no reason to make a major effort in trying to achieve optimal 

 weight economy in the pipe string. The large load capacities of present pipe 

 handling equipment makes this unnecessary, not to mention the fact that 

 drill pipe of any kind and size represents only a small fraction of the total 

 ship capacity. In addition, steels of high strength-to-weight ratios require that 

 strict attention be paid to minimum wall thicknesses, since handling can cause 

 impact damage resulting in cracks which lead to fatigue failure. 



Fatigue 



While important, yield strength alone cannot be used as a basis for 

 design. It is not sufficient to know that sporadic accelerations of the load 

 will not impart forces greater than the yield strength. The usable life of the 

 pipe must be determined from the cumulative damage resulting from cyclic 

 loading. 



In the design of aircraft, the useful life of a structural member is 

 calculated using the equation 





0.3 



122 



