Table 1. Performance Requirements of 

 Motion-Compensating Lift System 



Item 



Operational Requirements 



Payload capacity 



40,000 lb wet weight 



Operating deptVi 



6,000 ft 



Lift rate 



1 ft/sec 



Dynamic line 

 tension 



< ± 1 0% of static load 



Load motion 



<±0.5 ft with respect to the bottom 



Lift line 



Wire ropes up to 1-1/2 in. in diameter 



Ship limits motion 

 (maximum) 





Displacement 



Velocity 



Acceleration 



±9 ft 

 ± 7 ft/sec 

 ±8 ft/sec^ 



ram tensioner to control the traveling block. The 

 other concept utilized slipping servo-controlled 

 clutches to control both the line-storage/traction unit 

 and the traveling block. 



The passive concept, also called a "boom- 

 bobber" system, was based on the use of a soft spring 

 to support the load through a pivoted boom. 

 Operating at ship input frequencies significantly 

 greater than boom-spring resonance would decouple 

 ship motion from the payload, to give the desired 

 tension and motion control. 



Evaluation 



In the evaluation of the technical desirability of 

 each concept, a number of points were con- 

 sidered: (1) the question of whether the conceptual 

 system could meet the performance objectives, 



(2) the estimated overall performance capability, 



(3) the potential reliability of the system, and (4) the 

 physical characteristics such as size, weight, and com- 

 ponent arrangement. 



The four dynamic-system concepts had much in 

 common, including a power unit of about 1,200 hp 

 to provide the motion-compensation capability, an 

 active, servo feedback control system and associated 

 controls, and significant lift-line travel over sheaves 

 while compensating for support-platform motions. 



The probability of successfully meeting the perfor- 

 mance objectives varied considerably among these 

 four concepts; those based on a constantly slipping, 

 servo-controlled clutch drive system were preferred 

 because of the extremely fast response offered by this 

 approach. The hydraulic drive systems utilized in the 

 other two active concepts are inherently slower in 

 responding to control signals and thus provide poorer 

 dynamic control of line tension and payload motion. 

 The fifth concept, a passive boom-bobber 

 system, had the following advantages: 



1. Simpler in design and more compact than the 

 active system 



2. Suspension of the payload and lift wire on a 

 soft fluid spring, resulting in a significantly longer 

 natural resonant period for the spring than for the 

 ship-motion periods and effectively decoupling the 

 payload motion from support-platform motion 



3. Almost no lift-line travel over sheaves while 

 compensating for support-platform motion, resulting 

 in considerable reduction of fatigue wear of the lift 

 line 



4. Holding of the payload at any depth without 

 requiring power from the system 



5. Maximum power required for payload lifting 

 operation estimated at 200 hp 



6. Relatively small size of hardware 



In addition to these factors, a Navy-wide evaluation 

 team judged that the probability of the concept's 

 being successfully built was high and the performance 

 objectives could be met with such a system. There- 

 fore, the passive design concept was selected for 

 development. 



ANALYTICAL MODELS 



During the design process both the contractor 

 and CEL developed analytical models to predict 

 system performance. The models are similar in that 

 they are linear and use lumped parameters. 



The first CEL model, shown schematically in 

 Figure 1, ignored parameters such as lift-line elastic- 

 ity, system friction,* and payload drag. These 

 parameters were recognized as important but, because 



Including breakaway friction, or "sticktion." 



