Cyclic torsional stresses would not usually be encountered in a 

 heavy-lift system employing pipe string. The pipe would not be rotated as 

 would the drill pipe on a deep-sea rig. While this is advantageous in designing 

 the system, it still has been found in drilling practice that torsion is not a 

 major cause of drill string failure. 



SHIP MOTIONS 



Floating vessels oscillate in two modes: forced and free. The 

 amplitude of forced oscillations is dependent upon the forces between the 

 waves and the vessel and on the relationship between the natural frequency 

 and wave frequency. The amplitude of the free oscillations is a function of 

 the way in which the energy is dissipated as the vessel moves. Figure B-1 1 

 illustrates the relationship between the natural frequency of the ship and the 

 frequency of the waves. The curve is the same as any resonant curve for a 

 simple oscillating system with damping. A ship usually operates in the area 

 around resonance, so there is a good chance that heave can exceed wave 

 height. The response of a vessel is not as simple as Figure B-1 1 illustrates, 

 since the ship motions will be modified if the wavelength is equal to the 

 length of the ship or considerably smaller. 



Two ships whose characteristics have been determined by laboratory 

 tests are discussed in following paragraphs. These ships are the T-2 tanker 

 and the C1-M-A Vl . They represent two important classes of ships that will 

 give a realistic range of response characteristics for an operation of the type 

 under consideration. The FORDS platform is investigated in some detail also. 



> 



Vessel Natural Period/Wave Period 

 Figure B-1 1 . Typical vessel response curve. 



126 



