Steering Gear
Steering Gear
Steering Gear
Steering Gear
THE SINGLE
Steering Gear
FAILURE CRITERIA (Safematic design of steering system)
Some oil tankers, gas tankers & chemical tankers are required to have a safematic design of steering system i.e. no single failure on a steering should make the steering of the ship inoperative and thus give the rudder a chance to become free.
Steering Gear If, in the 4 Rams hydraulic system described the leakage had developed at say point "X" (as happened in case of `MT Ameco Cardiz' that resulted in total loss of the tanker and its cargo), there would be no way to isolate this leak to have the steering capability of the system available to the ship after the leak. To take care of such leaks (i.e. leaks outside isolating valves of the rams), safematic designs have been developed.
Steering Gear Safematic Design: Method No.1 (Also called 100% redundancy method), refer figure All valves are automatically controlled and electrically operated valves. In this arrangement System I supplies to ram No. I & 2 . System 2 supplies to ram No. 3 & 4 . However, as it is not practical to have both systems work simultaneously, and therefore the only combination of operations available are:
Steering Gear
Combination No. 1:
Isolating valve I & 2 open, By pass valve 5 shut; system No. I In operation. Isolating valve 3 & 4 shut, By pass valve 6 open; system No. 2 stopped Top two rams are in operation, working on system No. I and are providing 100% torque. Bottom two rams & system No. 2 is shut.
Steering Gear Combination No.2: Isolating valve 3 & 4 open, By pass valve 6 shut, system No. 2 in operation. Isolating valve 1 & 2 shut, By pass valve 5 open, system No. stopped Bottom two rams are in operation, working on system No. 2 and are providing 100% torque. Top two rams & system No. l is shut.
Steering Gear Now the logic circuit of safematic design in this method will cause the condition of operation to change from Combination No. I to Combination No.2 or Combination No.2 to Combination No. 1 (depending on which Combination was in operation when the heavy loss of oil LLL (Low low level) alarm is actuated).
Steering Gear Example if Combination No.1 is in operation, (i.e. system No.1 & rams No. 1& No. 2 are in operation) & LL (Low level) alarm actuates. This alarm takes care of normal leakage & can be set right by filling in some oil in the tank during the watch.
Steering Gear However, if LLL (Low low level) alarm follows immediately, it indicates a heavy loss of oil and logic circuit will cause the condition of operation change from Combination No.1 to Combination No.2. It may be noted that as said earlier even though we have 4 rams, only 2 rams are in operations at any given time. Thus each set of 2 rams should provide 100% torque. Most ships will not employ this method to take care of “single failure criteria, as this method means a 100% duplication and a costly one.
Steering Gear
Steering Gear Combination No. 1: Isolating valve I,2,3 & 4 open, By pass valve 5 & 6 shut; system No. I or system No. 2 working. All 4 rams are in use and provide together 100% torque. Combination No.2: Valve ‘a’ & ‘b’ shut. Isolating valve I & 2 open, By pass valve 5 shut; system No. I working. Isolating valve 3 & 4 shut, By pass valve 6 open; system No. 2 shut. Top two rams are in operation, working on system No. I and are providing 50% torque. Bottom two rams & system No. 2 are isolated.
Steering Gear Combination No.3: Valve ‘a’ & ‘b’ shut. Isolating valve 3 & 4 open, By pass valve 6 shut, system No. 2 working. Isolating valve 1 & 2 shut, By pass valve 5 open, system No. stopped. Bottom two rams are in operation, working on system No. 2 and are providing 50% torque. Top two rams & system No. l are isolated.
Steering Gear Logic circuit of this safematic design shall on detecting a heavy leak actuate low low level (LLL-1) alarm and cause change over from condition No.1 to condition No.2 or condition No.3. And if the leak persists in the system, it will actuate (LLL-2) alarm. This will cause change of the condition further from condition No.2 to condition No.3 or condition No.3 to condition No.2 (depending on what was the existing condition.)
