Section 5 Design loads and combinations

5.1.1 The self-weight including weight of all steelwork, rails, welding, paint systems and platform decking are to be taken into account in the calculations. Where timber decking is used, due consideration of the moisture content of the timber is to be taken into account.

5.2.1 The design is to be based on the maximum distributed load per metre applied as a keel block loading along the centreline of the platform, or along the inner set of transfer rails and/or trestle feet, where docking directly onto the platform does not occur. See also 2.7.2.

5.2.2 The loading imposed on the platform from the cradle or bogie wheels is to be applied during transfer operations as follows:

• over the entire docking length of the platform to the shore end of the platform for end transfer operations;
• over the entire length of side transfer rails for side transfer operations.

5.2.3 The block or cradle arrangement is, in general, to be such as to ensure that the pressure on the hull of a docked ship is not greater than that for which its structure is suitable. In general, this pressure will be the range between 200 to 230 t/m². Particular circumstances may, however, result in a greater or lesser pressure being appropriate.

5.2.4 The effects of the forces required to overcome friction in the transfer system are to be allowed for in the horizontal strength of the platform. The friction force is to be taken as not less than 1,5 per cent of the cradle wheel loads when roller bearings are fitted to the wheels, and 4 per cent when plain or bushed bearings are fitted.

5.3.1 The access and decked-in areas of the platform are also to be designed for pedestrian and maintenance purposes to:

1. a superimposed load of 5 kN/m², uniformly distributed; and

2. a point load of 10 kN at any one point.

Higher values may be required to meet operational equipment criteria.

5.3.2 Where vehicular access is required, the decking and deck support structure are to be designed in accordance with Ch 6, 2 Loading and design criteria, as appropriate for the intended vehicles. These loadings will not normally influence the lifting capacity specified in Ch 5, 2 Lifting capacity.

5.4.1 Each shiplift and transfer system and its supporting arrangements is to be capable of withstanding:

1. The loading from the wind on both the ship and platform from the specified maximum wind speed in which the shiplift will continue to operate.

2. The loading from the wind on the platform from an extreme out-of-service wind speed based on a 1:50 year return period.

5.4.2 The design wind speeds are to be based on local climatology data. Where the wind speeds are not defined by reliable local meteorological records, the following values may be used:

1. 20 m/s for the normal in-service condition.

2. 63 m/s for the out-of-service condition.

5.5.1 Shiplifts located in areas of high earthquake risk are to be designed to be capable of withstanding the accelerations resulting from an Operating Basis Earthquake (OBE). This is the acceleration for which the installation is expected to remain operational. An event of this magnitude/intensity can be reasonably expected to be experienced at the site during the operating life of the installation.

5.5.2 Consideration of the maximum credible seismic event at the site may be required if catastrophic failure of the installation results in significant loss of life or unacceptable environmental damage.

5.6.1 Shiplift platforms and transfer systems are to be considered for the design loadings resulting from the following load cases:

1. Case 1: Operational: docking and transfer with no wind:

   The shiplift and transfer system are to be considered with respect to its self-weight plus the applied vertical load from the docked ship and transfer system, together with the horizontal loads resulting from the traction/friction loads during transfer operations.

2. Case 2: Operational: docking and transfer with wind:

   The shiplift and transfer system are to be considered with respect to its self-weight plus the applied vertical load from the docked ship and transfer system, together with the horizontal loads resulting from the in-operation wind speed (actual data to be provided or 20 m/s will be used) applied to both the ship and the platform, and also to traction/friction loads during transfer operations.

3. Case 3: Survival: ship on transfer system on land during extreme wind conditions:

   The transfer system is to be considered with respect to its self-weight plus the applied vertical load from the docked ship, together with the horizontal load resulting from the extreme wind condition (actual data to be provided or 63 m/s will be used) applied to both the ship and the platform. Where appropriate, consideration may also need to be given to the OBE (Operating Basis Earthquake) seismic event, either:

   1. separately; or

   2. together with the extreme wind condition.

5.6.2 In way of platform bilge blocks, the platform structure is to be designed for the maximum loads resulting from load case 2. This load is to be not less than 20 per cent of the maximum distributed load per metre.

5.7.1 The allowable stress, σ_a, is to be taken as the failure stress of the component concerned, multiplied by a stress factor, F, which depends on the load case considered. The allowable stress is given by the general expression:

where

5.7.2 The stress factors, F, for steels in which σ_y/σ_u ≤ 0,85, are given in Table 5.5.1 Stress factor F

where

5.7.3 For steel with σ_y/σ_u > 0,85, the allowable stress is to be derived from the following expression:

where σ_a and τ_a are defined in Ch 5, 5.7 Allowable stresses 5.7.1.

5.7.4 Steels with σ_y/σ_u > 0,94 are not generally acceptable and shall be specially considered.

5.7.5 The failure stresses for the elastic modes of failure are given in Table 5.5.2 Failure stress.

5.7.6 For components subjected to combined stresses, the following allowable stress criteria are to be used:

1. σ_xx ≤ σ_a

2. σ_yy ≤ σ_a

3. τ_o ≤ τ_a

4. 

where

5.7.7 The allowable stresses may be reduced in areas where openings or details in the structure may lead to the creation of stress concentrations.

5.7.8 For members subject to compression, the allowable axial stress for compression members is to be taken as the critical compressive stress σ_cr, determined in accordance with Ch 4, 2.18 Allowable stress – Compression, torsional and bending members 2.18.2 and multiplied by the allowable stress factor F, as defined in Table 5.5.1 Stress factor F .

5.7.9 The allowable stress for plate buckling failure is to be as the critical buckling stress σ_cb, σ_bb or τ_b as appropriate and determined in accordance with Ch 4, 2.21 Allowable stress – Plate buckling failure, and multiplied by the stress factor F, as defined in Table 5.5.1 Stress factor F .

5.7.10 The allowable stress for joints and connections are to be in accordance with Ch 4, 2.23 Allowable stress – Joints and connections and multiplied by the stress factor F, as defined in Table 5.5.1 Stress factor F .

5.7.11 The allowable stresses in sheaves, shackles and other loose items are to comply with the requirements of Ch 8 Fittings, Loose Gear and Ropes. Alternatively, they are to comply with a recognised National Standard.

5.7.12 Items of structure which are subjected to wind forces only, irrespective of load combination, may be determined on the basis of a stress factor of F = 0,85.