Page 241 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

230 CHAPTER 9 LARS and TMS
Typically, the static braking system is hydraulically powered upstream of the winch’s hydraulic motor. The brake is only released once sufficient hydraulic pressure is applied to the motor (thus transferring the weight of the load from the brake to the winch).
Winches are specified based upon several parameters including safety margins and winch configuration:
Ratings and capacity
• Safe working load (specified at either bare or full drum)—the maximum operating load in order to achieve manufacturers’ safety margins
• Maximum line pull (specified at either bare or full drum)—typically equal to 1.25 times Safe Working Load and is generally the nominal stalling point of the winch
• Line speed—specified in line travel length per unit time—feet per minute (fpm), meters per second (m/s), or meters per minute (mpm) and can be measured at bare drum, mid-drum, or full drum since the line speed changes as a function of distance (i.e., wraps) from the drum
• Drum capacity—specified in linear feet (or meters) based upon a stated cable diameter (e.g., 3300 ft (1000 m) of 1.97 in. (50 mm) diameter cable).
Considerable round trip times for the vehicle (predive to postdive) can be experienced, and expensive, depending upon the winch line speed. For instance, if a winch with a line speed of 100 fpm (30 mpm) is used for a 6600 ft (2000 m) operating depth, the round trip time in both direc- tions is 132 min (66 min in each direction). If you add to this a 15-min predive procedure along with 1 min to get through the splash zone (in both directions) and a 15-min postdive procedure, the total nonworking “trip time” of the dive from start to finish is 164 min! If the vehicle arrives at depth only to find that a lightbulb is blown, the round trip to change the bulb is nonproductive time. One oil company is notorious for specifying a nominal line speed in its contracting, comput- ing the theoretical trip time and then comparing it to the actual trip time—then putting the entire spread on nonrevenue downtime for any variance. Ouch!!! Beware of the fine print.
Drum dimensions
• Drum core diameter—outer diameter of the drum in feet or meters (D in Figure 9.10)
• Drum core width—width from flange to flange in feet or meters (B in Figure 9.10)
• Drum flange diameter—flange diameter in feet or meters (H in Figure 9.10).
Overall dimensions
• Length—total length of the entire winch system including all ancillary components (base, frame, guard, etc.)
• Width—total width including hydraulic motors, control station, and other winch components
• Height—total height of the winch plus all framing and components. The skid should be added
to this to gain a full dimension for the entire LARS plus winch system
• Weight (without cable)—total weight of the winch including all components
Regardless of how the vehicle is deployed, it will require a winch that either handles a heavy- duty tether or armored umbilical. Depending on the depth of operation, the winch can add a large footprint to your spread.