Section 3 Load cases

3.1.1 The standard load cases, given in Table 2.3.1 Load combinations, are to be considered. The purpose of these load cases is to ensure that the longitudinal, transverse and shear strength of the hull structure complies with the acceptance criteria given in Vol 4, Pt 1, Ch 2, 5 Acceptance criteria.

3.1.2 Loads are to be applied as described in this Section or alternatively, as derived according to Vol 4, Pt 1, Ch 3 Load Development.

3.2.1 The load cases, 1–8, as described in Table 2.3.1 Load combinations, are to be comprised of the proportion of global load components as indicated. In each load case, one of the load components is maximised. The load components may be combined by superposition.

3.2.2 The load cases in Table 2.3.1 Load combinations are applicable only if the Rule loads are being used. If the loads are derived based on a ‘first principle’ approach, then the load cases described in Vol 4, Pt 1, Ch 3 Load Development are to be used.

3.3.1 In general, all load cases specified in Table 2.3.1 Load combinations are to be investigated to identify areas that require local fine mesh analysis.

3.3.2 The fine mesh analysis can be carried out by means of separate local finite element models, in conjunction with the boundary conditions obtained from the global coarse mesh model. Alternatively, fine mesh zones may be incorporated into the global coarse mesh model.

3.3.3 Where appropriate, secondary loads such as pressure loads are to be applied in conjunction with the boundary displacements obtained from the global model.

3.4.1 The still water loads are to be applied to the global finite element model regardless of whether Rule loads are being applied or loads calculated by direct calculation.

3.4.2 Hog and sag still water load cases are to be analysed fulfilling the following criteria:

1. Ship upright and at or near to the maximum draught.

2. The still water bending moment cases are to approximate, as far as is possible, the assigned, or specified, permissible still water bending moment condition distributions. It may only be necessary to include one loading condition; if there is not a large variation in the deadweight distribution.

3.4.3 The following still water loads are to be included:

1. Self weight – as generated from the modelled hull structure, suitably factored to achieve the specified steel weight, including the position of the LCG. In this respect, it may be useful to divide the model longitudinally into a number of material zones, each of which can have a separate factored value for the steel density.

2. Machinery, outfit and other equipment – all major items to be applied as point loads or pressure loads at the correct locations. Minor or unknown items may be included in the steel weight.

3. Buoyancy loads – to be applied as pressure loads , ρ_gh, on wetted shell elements, where h is the distance of the element centroid below the still waterline.

4. Ballast and fuel oil – to be applied as pressure loads on tank boundaries, based on the actual liquid head and density.

5. Containers – vertical point loads to be applied at each corner of the stack base, whether above or below deck.

6. Cargo, payload, passengers or vehicles – as point loads, uniformly distributed loads or pressure loads at the correct locations.

3.5.1 The vertical wave bending moment load cases are to be applied by using a static wave balance program. Separate cases are required for the hog and sag conditions. Each case is comprised of a wave with the following properties:

1. A wavelength equal to LBP

2. A wave crest amidships for the hogging condition and a wave trough amidships for the sagging condition

3. A sinusoidal wave profile

4. A wave height sufficient to induce the Rule hogging or sagging design vertical wave bending moment (VWBM) amidships, as given in Vol 1, Pt 5, Ch 4, 2.4 Vertical wave bending moment.

3.5.2 The wave height required to induce the required bending moment will need to be derived by trial and error using a suitable longitudinal strength program. The ship is to be balanced on the wave and the resulting draught, trim and wave parameters are to be used for determination of external pressure distribution. This wave is to be determined for both the hog and sag condition.

3.5.3 The pressure values to be applied should not include the pressure component or buoyancy loads due to the still water condition.

3.6.1 The Rule horizontal bending moment distribution, as described in Figure 2.3.1 Horizontal wave moment distribution, is to be modelled by applying longitudinal force pairs at each transverse bulkhead position in the plane of the side shell. These loads are to be distributed longitudinally along the depth of the side hull structure or of the centre hull structure at the forward and aft ends where the side hull structure has terminated. When integrated along the ship length, the incremental moment couples are to generate the Rule horizontal wave bending moment distribution specified in Vol 1, Pt 5, Ch 4, 2.6 Horizontal bending moment.

3.6.2 The horizontal bending moment couples are to be calculated in the same manner as described in Vol 4, Pt 1, Ch 2, 3.8 Component H – Longitudinal torsional moment 3.8.3 for the longitudinal torsional moment.

3.7.1 The splitting moment occurs when the waves force the side hulls away from (sag) or towards (hog) the centre hull, resulting in stresses in the cross-deck structure.

3.7.2 The splitting moment may be achieved by applying simplified line loads, in the directions depicted in Figure 2.3.2 Splitting moment, hog for hog and Figure 2.3.3 Splitting moment, sag for sag, at the following locations:

1. A line load is to be applied vertically on the length of the keel of each of the side hulls.

2. A line load is also to be applied transversely, near the keel in way of supporting structure, along the length of the outboard side of both side hulls.

3.7.3 The magnitude of the distributed loads should be determined so that the resulting splitting moment distribution along the cross-deck is equal to that specified in Vol 1, Pt 5, Ch 4, 3.1 Splitting moment.

3.7.4 Alternatively, uniformly distributed loads may be applied to the side hulls in order to achieve the Rule splitting moment distribution. In the sag case, these loads would act on the inside of the side hulls and in the case of the hog would act on the outside of the side hulls. The loads would consist of a hydrostatic head, as well as a dynamic component sufficient to generate the Rule splitting moment value.

3.8.1 The Rule torsional moment distribution, as specified in Vol 1, Pt 5, Ch 4, 2.7 Longitudinal torsional moment, is to be applied as depicted in Figure 2.3.4 Longitudinal torsional moment distribution and is to be modelled by applying a torsional moment at each transverse bulkhead position. This results in a stepwise torsional distribution, as depicted in Figure 2.3.5 Application of Rule hydrodynamic torque distribution to FE model.

3.8.2 The moment is to be applied as a distributed force couple on all nodes in the 'vertical' portion of the bulkhead where it intercepts the side shell.

3.8.3 The torsional moment, T, required at each bulkhead position can be calculated from:

3.8.4 Other proposed methods of modelling the Rule hydrodynamic torque distribution will be specially considered.

3.9.1 The torsional moment about the transverse axis is to be applied as depicted in Figure 2.3.6 Transverse torsional moment. The magnitude of the distributed loads should be determined so that the resulting torsional moment is equal to that specified in Vol 1, Pt 5, Ch 4, 3.3 Transverse torsional moment.

3.10.1 A static roll angle of 30° is to be applied to the model. The draft and trim are to be adjusted in order to balance the hydrostatic loads and the displacement. Still water loads are to be included in this load case. All loads subject to gravity are to be resolved into their correct components given the heeled position of the ship.