Section 3 Design

3.1.1 Self-elevating systems are to be designed with redundancy such that a single failure of any component will not cause an uncontrolled descent of the unit or impair the safety of the unit. Each leg is to be provided with a load indication and an overload alarm at a manned control station.

3.1.2 Braking devices are to fail safe in the engaged position in the event of a failure or interruption of the power supply to the lifting machinery.

3.1.3 Unless otherwise agreed by LR, the system is to be designed such that the rack tooth is the weakest component in the self-elevating machinery with regard to static mechanical strength.

3.1.4 The jacking system, together with the fixation system if fitted, is to be capable of adequately lifting and supporting the hull, or leg installation under all operating, survival and tow conditions.

3.1.5 The requirement for emergency jacking of the hull with full or part pre-load to stabilise the unit in the event of sudden leg penetration is to be considered.

3.1.6 The self-elevating mechanism is to be designed to pre-load the foundation to the design conditions and be capable of supporting a load not less than the maximum load for which the leg has been designed.

3.1.7 Unless otherwise agreed, the minimum design operating temperature of the jacking gear machinery is to be in accordance with Pt 3, Ch 1, 4.4 Minimum design temperature

3.1.8 In selecting the prime movers for the self-elevating machinery, consideration is to be given to the effects of friction at the mesh of the pinion and rack, and between legs and guides, together with uneven load distribution.

3.1.9 The control station from which the elevating and lowering machinery is operated is to be provided with all necessary monitoring, alarms and controls including hull alignment, prime mover running load pin position, running indication, overload alarms and indication of availability of applicable power sources, as appropriate.

3.2.1 All enclosed transmission gearing is to be designed in accordance with a National Standard acceptable to LR.

3.2.4 Material endurance strength limits are to comply with the requirements of a National Standard acceptable to LR.

3.2.5 Consideration is to be given to the loads applied to the gears during wet/dry tow conditions, as the gear teeth may be subjected to full load reversal. The design will be given consideration based on the simulated load analysis for the main pinion/rack tooth mesh.

3.3.1 The design of the final (main) pinion and rack is subject to special consideration but the requirements of Pt 5, Ch 23, 3.3 Main pinion and rack 3.3.2 to Pt 5, Ch 23, 3.3 Main pinion and rack 3.3.7 are to be complied with.

3.3.2 The nominal contact ratio of the mesh is not to be less than 1,05, taking into consideration the cumulative effects of the design and assembly tolerance values and allowable wear during operation of the guides/rack tips.

3.3.3 The material hardness of the pinion is to be not less than that of the rack tooth material.

3.3.4 The pinion is to have a factor of safety on tooth root bending of not less than 1,5 for both static and dynamic loading conditions.

3.3.5 Hertzian tooth flank contact stress is generally not to be greater than three times the yield strength of the rack material, or not greater than 3,5 times the yield for pre-load jacking.

3.3.6 The ultimate strength (collapse load) of the main pinion tooth is not to be less than 1,1 times that of the rack tooth.

3.3.7 Consideration is to be given to the loads being applied to the main pinion mesh during wet/dry tow conditions where full load reversal may be expected.

3.4.1 Nominal shaft stresses for the plain section solid shafting are to be calculated as follows:

where

3.4.2 The maximum stresses due to bending and torsion are not to exceed the values shown in Figure 23.3.1 Allowable stress - Shafting. The assessment of the maximum stresses should take into account the system overload conditions. The allowable stress limits in Figure 23.3.1 Allowable stress - Shafting include an allowance for stress concentrations at keyways, fillets shrink assemblies or other areas of stress concentration, not exceeding 3,0. Where an effective stress concentration exceeds this value, the design will be specially considered.

3.4.4 Shaft materials having properties outside the range covered by Figure 23.3.1 Allowable stress - Shafting will be specially considered.

3.5.1 A minimum factor of safety on slippage of 2,0 is to be achieved based on the maximum load.

3.6.1 The capacity of the sleeve or anti-friction shaft bearings is to be such as to carry adequately the radial and thrust loads which would be induced under all operating conditions.

3.6.2 Hydrodynamic radial bearings are to be lined with babbitt or other material suitable for the intended application and duty. They are to be properly installed and secured in the housing against axial and rotational movement.

3.6.3 Selection of the particular design of sleeve bearing is to be based on an evaluation of the journal velocity, surface loading, hydrodynamic film thickness, and calculated bearing temperature under all operating conditions.

3.6.4 Selection of rolling element bearings is to be based upon the bearing manufacturer’s recommendations for the design loading and application.

3.7.1 Braking devices are to have a combined static friction torque capacity, considering the mechanical efficiency of the drive gear, such that no fewer than 1,3 times the maximum design load, to be supported during normal operation, may be held without brake slippage.

3.7.2 Means are to be provided such that, in the event of failure of one or more of the self-elevating machinery units, the defective unit(s) can be mechanically isolated such that the effectiveness of the remaining units in raising/lowering the hull is not impaired.