Assuming a straight line interaction relationship 



+ = 1.0 



"ult 



= 0.625 



Ogy = 0.625 a^n = 0.625(150,000) 

 a_„ = 94,000 psi 



This value of Ogy will have to be reduced by the appropriate safety 

 factor. In the most severe case, 600 tons at 6,000 feet, a^^ = 75,000 psi. 

 The factor of safety is therefore 04,000/75,000 = 1 .25; it increases, of course, 

 as the depth and/or load decrease. In any case, it appears that the pipe is 

 relatively safe from fatigue failure due to tensile loading only. As stated 

 earlier, Aa is approximately 10% of the static stress in the severest sea states, 

 so the above approximations are well on the safe side. 



Additional Dynamic Forces 



Angular ship motions about the horizontal plane create transverse and 

 torsional vibrations in the pipe, at the ship. The roll of the ship is the principal 

 cause of transverse excitation. This particular motion is important in deter- 

 mining the number of bending cycles imparted at the uppermost portions of 

 the pipe string. However, only the first few sections of pipe will be affected 

 by the slight amount of roll and the bending stresses will be small. For the 

 purposes of this study they can be ignored. 



125 



