Section 9 ‘Guidelines’ for the acceptance of non-duplicated rudder actuators for oil storage units of 10 000 tons gross and upwards but of less than 100 000 tons deadweight

9.1 Materials

9.1.1 Parts subject to internal hydraulic pressure or transmitting mechanical forces to the rudder stock are to be made of duly tested ductile materials complying with recognised Standards. Materials for pressure retaining components are to be in accordance with recognised pressure vessel Standards. These materials are not to have an elongation less than 12 per cent, nor a tensile strength in excess of 650 N/mm².

9.2 Design

9.2.1 Design pressure. The design pressure should be assumed to be at least equal to the greater of the following:

1,25 times the maximum working pressure to be expected under the operating conditions required in Pt 5, Ch 19, 2.1 General 2.1.2.(b)
The relief valve(s) setting.

9.2.2 Analysis. In order to analyse the design, the following are required:

The manufacturers of rudder actuators should submit detailed calculations showing the suitability of the design for the intended service.
A detailed stress analysis of pressure retaining parts of the actuator should be carried out to determine the stresses at the design pressure.
Where considered necessary because of the design complexity or manufacturing procedures, a fatigue analysis and fracture mechanics analysis may be required. In connection with these analyses, all foreseen dynamic loads should be taken into account. Experimental stress analysis may be required in addition to, or in lieu of, theoretical calculations depending upon the complexity of the design.

9.2.3 Dynamic loads for fatigue and fracture mechanics analysis. The assumption for dynamic loading for fatigue and fracture mechanics analysis where required by Pt 5, Ch 19, 3.1 General 3.1.5, Pt 5, Ch 19, 8.3 For oil storage units of 10 000 tons gross and upwards but of less than 100 000 tons deadweight and Pt 5, Ch 19, 9.2 Design 9.2.2 are to be submitted for appraisal. Both the case of high cycle and cumulative fatigue are to be considered.

9.2.4 Allowable stresses. For the purposes of determining the general scantlings of parts of rudder actuators subject to internal hydraulic pressure, the allowable stresses should not exceed:

≤ f

≤ 1,5f

where

the lesser of

equivalent primary bending stress

equivalent primary general membrane stress

specified minimum yield stress or 0,2 per cent proof stress of material at ambient temperature

specified minimum tensile strength of material at ambient temperature

equivalent primary local membrane stress A and B are as follows:

	Wrought steel	Cast steel	Nodular cast iron
A	4	4,6	5,8
B	2	2,3	3,5

9.2.5 Burst test. Pressure retaining parts not requiring fatigue analysis and fracture mechanics analysis may be accepted on the basis of a certified burst test and the detailed stress analysis required by Pt 5, Ch 19, 9.2 Design 9.2.2 need not be provided.
The minimum bursting pressure should be calculated as follows:

where

as from Table in Pt 5, Ch 19, 9.2 Design 9.2.4

design pressure, as defined in Pt 5, Ch 19, 9.2 Design 9.2.1

minimum bursting pressure

tensile strength, as defined in Pt 5, Ch 19, 9.2 Design 9.2.4

actual tensile strength.

9.3 Construction details

9.3.1 General. The construction should be such as to minimise local concentrations of stress.

9.3.2 Welds.

The welding details and welding procedures should be approved.
All welded joints within the pressure boundary of a rudder actuator or connection parts transmitting mechanical loads should be a full penetration type or of equivalent strength.

9.3.3 Oil seals. Oil seals forming part of the external pressure boundary are to comply with Pt 5, Ch 19, 3.2 Components 3.2.3 and Pt 5, Ch 19, 3.2 Components 3.2.4.

9.3.4 Isolating valves are to be fitted at the connection of pipes to the actuator, and should be directly mounted on the actuator.

9.3.5 Relief valves for protecting the rudder actuator against over-pressure as required in Pt 5, Ch 19, 3.3 Valve and relief valve arrangements 3.3.3 are to comply with the following:

The setting pressure is not to be less than 1,25 times the maximum working pressure expected under operating conditions required by Pt 5, Ch 19, 2.1 General 2.1.2.(b)
The minimum discharge capacity of the relief valve(s) is to be not less than 110 per cent of the total capacity of all pumps which provided power for the actuator. Under such conditions, the rise in pressure should not exceed 10 per cent of the setting pressure. In this regard, due consideration should be given to extreme foreseen ambient conditions in respect of oil viscosity.

9.4 Non-destructive testing

9.4.1 The rudder actuator should be subjected to suitable and complete non-destructive testing to detect both surface flaws and volumetric flaws. The procedure and acceptance criteria for non-destructive testing should be in accordance with requirements of recognised Standards. If found necessary, fracture mechanics analysis may be used for determining maximum allowable flaw size.

9.5 Testing

9.5.1 Tests, including hydrostatic tests, of all pressure parts at 1,5 times the design pressure should be carried out, subject to any limitations imposed by valves and other components. Where additional testing of systems or subsystems following final assembly is required, the test pressure may be subject to any limitations imposed by valves and other components.

9.5.2 When installed on board the unit, the rudder actuator should be subjected to a hydrostatic test at the pressure, defined in Pt 5, Ch 19, 9.5 Testing 9.5.1, as well as a running test.

9.6 Additional requirements for steering gear fitted to units with Ice Class notations

9.6.1 See Pt 3, Ch 6 Units for Transit and Operation in Ice