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operator claimed higher operating ranges of 4 to 5 knots (7 to 9km/hr) current 

 speed. Since the majority of vehicles are sensitive to fairly low current 

 speeds, their work schedule is often modified to operate during slack tide. 

 This is particularly true around structures when the current can have a 

 significant vertical component in addition to its horizontal component. 



Water current influence on cable drag is obvious; in several instances the 

 drag was sufficient to restrain the vehicle from reaching and maintaining 

 the operating depth required. One solution to this problem has been to work 

 the vehicle from a short tether attached to a heavy clump or from a submerged 

 launcher. In this instance the clump or launcher may be placed on the bottom 

 and the vehicle works within the radius of its tether. Additionally, some 

 operators work in a "live boat" fashion and tow the launcher or clump while 

 the vehicle keeps apace with the surface ship and the major portion of cable 

 drag is imparted to that section between surface ship and clump. A somewhat 

 unique approach is taken by the Harbor Branch Foundation. The vehicle, CORD, 

 grasps the clump with its manipulator as it is being towed. When an object 

 of interest is observed the vehicle releases its hold on the clump, the 

 clump is bottomed, and the vehicle works around the clump on a short tether. 



One problem encountered when the vehicle is working from a towed clump is 

 that the clump will undergo heave-induced vertical excursions almost in a 

 1:1 ratio with the motion of the surface vessel. The result is a periodic 

 shortening and lengthening of the vehicle's tether which could either pull 

 the vehicle off station or produce an excess of tether which might entangle. 

 To circumvent these possibilities one operator uses a heave-compensated 

 winch which permits the clump or launcher to remain at near-constant depth 

 as it is being towed. 



4.10 SEA STATE 



All commercial ROV operators can deploy their systems up to and including 

 Sea State 4. Many exceed this limit and claim to have operated in State 6 

 and as high as State 8. The later performances have taken place from a semi- 

 submerged vessel. Since these sea states are equal to - and often exceed - 

 the average sea states where most ROVs are operating, there are very few 

 incidents where surface conditions have forced aborting operations. Unlike 

 manned vehicles, deployment of ROVs does not jeopardize human life and, there- 

 fore, operations in sea states higher than their manned counterparts is 

 common. 



One restriction placed on ROV operators is directly brought about by increasing 

 sea state. Insurance underwriters will not assure the vehicle beyond, in one 

 instance Sea State 4. In this respect sea state has limited the application 

 of ROVs, but the number of occurrences and the extent of the problems are 

 difficult to quantify. 



One possible reason that underwriters are reluctant to provide coverage in 

 higher sea states is the difficulty encountered in safely bx.ingj.n_g a surfaced 

 ROV sufficiently close to a rolling/pitching ship for retrieval. One operator 

 inflicted damage to his vehicle when the support ship rolled and drew the 

 ROV under its hull where it collided with the vessel as it rolled in the 



