Mooring and Positioning of Vehicles in a Seaway 



drift ^ n n^ 

 2(pgLA2j 



ft. '•-sec. 



Fig. 17. Representative power spectrum of lateral drift force. 



It is the drift forces and moments whose mean values tend to move 

 and/or rotate the ship off position, with their level of fluctuation 

 causing dynamic responses that produce ship motion. Thus some 

 means of control must be applied to "modulate" the forces developed 

 by the thruster system in order to minimize the ship's average 

 motion relative to its desired position. 



The ship will experience drift-like forces in the lateral and 

 longitudinal directions, as well as a yaw moment, and the philosophy 

 of applying control forces to the ship will be aimed at countering 

 these forces by orienting the ship in the proper direction so that the 

 resultant force is acting along the ship's longitudinal axis and there 

 will be no significant moment. It would then be possible to use the 

 main propulsive thrust of the ship, assuming controllable pitch 

 propellers, to counter this resultant force. Lateral forces that are 

 developed by particular thrusters , or other force systems, will be 

 used to overcome any tendency of the ship to rotate out of its pre- 

 ferred direction due to any resulting yaw moments. The force 

 magnitudes in regard to average values can be estimated from the 

 results of some of the cited references given in this paper, and 

 some idea of the time history variations and msiximum magnitudes 

 expected relative to the average values can also be inferred fronn the 

 work presented here. 



1077 



