According to SOLAS chapter - 2, part 1, regulation 29.16, every tanker of more than 10,000 GT shall comply with the following:

• The main steering capability due to a single failure in any part of one of the power actuating systems shall be regained in not more than 45 seconds.
• The main steering shall comprise at least two identical power actuating systems, each capable of meeting the requirements. Loss of fluid from one system shall be capable of being detected, and the defective system shall automatically get isolated so that the other system shall remain fully operational

Considering the above regulatory requirements, given below is a “Fail Safe steering gear” suitable for use on a tanker of more than 100,000 T DWT.

Shown in the diagram is a “Fail safe steering gear” having two independent power actuating systems that can

• Work simultaneously in normal operation, meeting the requirement OR
• Work independently and meet the requirement
• In the event of loss of fluid from any one system, it can be detected and isolated automatically so that the other system can remain fully operational.

Working:

• The system incorporates two sets of electric-driven pumps. Both main and auxiliary pumps are on the same shaft. The main pump shown in the diagram is a variable delivery pump
• The variable delivery pump takes suction from the tank and supplies hydraulic oil to the ram cylinders. The oil flow of the pump is determined by the pump actuating lever
• The movement of the pump actuating lever is controlled by the rudder angle order given by the bridge with the help of a bi-directional control valve
• A two-way shock relief valve is fitted between the two cylinders to release the pressure from one side of the cylinder to the other side in case of pressure increase in one of the cylinders due to heavy seas
• By-pass valves are also fitted between two cylinders, which are normally shut during operation. When one system is stopped, there is a pressure drop, as the auxiliary pump has also stopped this opens the by-pass valves, thus removing the hydraulic lock of the ram operation.
• Auto isolation valves in the system are there to isolate one system in case of any failure.

Sequence of events during hydraulic oil leak:

Case 1: Consider an oil leak from any pipe for cylinders 1 and 2 with the No. 1 pump running:

1. No. 1 tank level will come down to L1, and it will sound an alarm on the bridge and in ECR
2. When the tank level further drops to L2, i.e. low-low level, the no. 1 pump stops.
3. Stopping the No. 1 pump also stops the attached auxiliary pump. So the line pressure drops, due to which the normally closed by-pass valves ‘X’ and ‘Y’ open.
4. Simultaneously, the auto isolation valves ‘A’, ‘B’ and ‘C’ will be operated by an electric signal. A, B and C are normally open valves. The electric signal will close them. So, systems 1 and 2 will be completely separated. Thus, the defective system, I.e. system 1, is isolated.
5. Along with the operation of the auto isolation valves, the No. 2 pump will start automatically. The attached auxiliary pump will act on the by-pass valve ‘Y’ and close it. This enables cylinders 3 and 4 to be in normal operation.
6. It should also be noted that since system 1 is completely isolated, there is no oil pressure to operate the bypass valve. So the by-pass valves remain open, thereby removing the hydraulic lock for the ram movement in cylinders 1 and 2

Case 2: Consider an oil leakage from any pipe of cylinders 3 and 4 with the No. 1 pump running:

Points 1, 2 and 3 are the same as case 1

1. Simultaneously, the auto isolation valves ‘A’, ‘B’ and ‘C’ will be operated by an electric signal. This will shut the normally open valves A, B and C. Thus, systems 1 and 2 will be completely separated
2. Along with the operation of auto isolation valves, the No. 2 pump will start automatically. The attached auxiliary pump will act on the by-pass valve ‘Y’ and close. So, cylinders 3 and 4 will come into normal operation.
3. Now, since the leak is between the pipe of cylinders 3 and 4, the level of the no. 2 tank will drop to L1 and give an alarm.
4. The level will further drop to L2, but the pump will not stop and changeover to ensure that the leak is from the pipe of cylinders 3 and 4
5. When the no. 2 tank level drops to L3, the no. 2 pump stops and the no. 1 pump starts to operate the steering using cylinders 1 and 2
6. Starting the no. 1 pump will ensure that the by-pass valve ‘X’ is shut, and stopping the no. 2 pump will ensure that the by-pass valve ‘Y’ is open

This ensures the operation of the steering Gear with the defective system fully isolated.