Answer MEP Question 58

Q. With reference to steering gear hydraulic systems: A. Explain the factors that could contribute to failure of connecting flange leading to total loss of oil from the system. B. Describe an arrangement designed to ensure that the problem would not cause steering failure.

Steering gear piping and other components should be capable of withstanding the pulsating pressures which can arise due to dynamic loads resulting from external forces on the rudder, and rapid demands during maneuvering as well as the stresses due to vibration. 

The piping should he adequately supported in order to minimize the effects of vibration and flexure under pressure, and any excessive vibration should be investigated and appropriate supports provided. 

The piping should be examined cracks. Particularly carefully around attachments to flanges. The flexible pipes of pinned actuator steering gears should be examined for damaged and replaced in  cases of doubt regarding their suitably. Furthermore, as the weight of piping may impose significant loads on the connecting flanges, the securing arrangements should be periodically examined and any broken or corroded studs should be replaced with new studs to the original specification. At the same time, the tightness of the bedplate holding down bolts and other attachments, nuts and keys should be checked and the necessary remedial Measures taken. A thorough examination should also be carried out whenever it is suspected that the installation may have been affected by rudder contact. 

Recorded observations of the hydraulic pressure, in comparative circumstances (oil temperature and depth of loading), area useful indication of the condition of the gear, and higher than normal pressures could indicate jamming within the gear, misalignment of the rudder or, in the case of a gradual increase in pressure, excessive wear of the rudder bearings. The system should be periodically inspected for leaks, particularly at seals and joints. Exposed working surfaces (e.g. rams) should be kept clean. The Amoco Cadiz disaster focused attention on the fact that failure of the common hydraulic pipe system of a four-ram steering gear with duplicated power units, could result in rapid discharge of oil from the circuit and loss of steering capability. Four ram or double vane type gears with duplicated hydraulic circuits, as well as duplication of pumps, were developed. This arrangement cannot, however, be operated with both pumps running and the duplicated hydraulic circuits isolated from each other. The systems have to be connected in common for operation with both pumps. Either pump can provide hydraulic power for the combined circuit or for an isolated half, with the bypass open on the other part. 

The Hastie — Brown split system, shown here in split operation, is arranged to give two ram operations automatically in the event of loss of fluid from one system. 

Figure illustrates a four-ram gear which compiles with the International Convention for the safety of life at Sea(SOLAS) regulation (1974, amended 1981) relating to tankers, chemical tankers or gas carriers of 10000 grt and upwards (see especially Chapter 11-1 Regulation 29, paragraph 16). 

Two main power and servo power units draw from a two-compartment tank fitted with oil level switches arranged at three levels. Level 1 gives an initial alarm following loss of oil from either system. In normal operation one or both power units provide hydraulic power to all four rams. Continued loss of oil initiates or both of the level 2 switches. These energize their respective solenoid operated servo valves, causing the combined isolating and bypass valves to operate, splitting the system such that each power unit. Supplies two rams only. At the same time if one power unit is stopped it is automatically started. Further loss of oil and the system on which it is occurring will operate one of the level 3 switches. This will close down the power units on the faulty side. Steering then continues, uninterrupted but at half the designed maximum torque on the sound system. The defective system is out of action and isolated.