Section 2 Structural modelling

2.1.1 A 3-D plate element model of the ship is to be used. This model should extend over the full length, breadth and depth of the ship. In some cases, a half breadth model may be used with the appropriate symmetrical boundary conditions. However, to simplify the loading and boundary conditions, it is recommended that a full breadth model be used.

2.1.2 The scantlings to be modelled are to be in accordance with those required by the Rules, as well as those required by the Complementary Rules. If the Complementary Rules are the Rules and Regulations for the Classification of Naval Ships (hereinafter referred to as the Rules for Naval Ships) or the Rules and Regulations for the Classification of Special Service Craft (hereinafter referred to as the Rules for Special Service Craft), then these scantlings are ‘net’ scantlings. If the Complementary Rules are the Rules and Regulations for the Classification of Ships (hereinafter referred to as the Rules for Ships), the required scantlings are ‘gross’ scantlings. Any additional thicknesses added due to the Enhanced Scantlings notation are not to be modelled.

2.1.3 The model should represent, with reasonable accuracy, the actual geometric shape of the hull. All effective longitudinal material is to be included. Similarly, all transverse primary structure, i.e. web frames, watertight and fire divisional bulkheads are to be represented in the model. The superstructure should also be included in the model.

2.1.4 The FE model is to be represented using a right handed Cartesian co-ordinate system, depicted in Figure 2.2.1 FE Co-ordinate system, where:

1. x is measured in the longitudinal direction, positive forward from the aft perpendicular

2. y is measured in the transverse direction, positive to port from the centreline

3. z is measured in the vertical direction, positive upward from the baseline

Figure 2.2.1 FE Co-ordinate system

2.1.5 The size and type of plate elements selected are to provide a satisfactory representation of the deflection and stress distribution within the ship's structure. In general, the plate element mesh is to follow the primary stiffening. Typically, the following guidelines are applicable:

1. Longitudinally, one element between web frames or intermediate floors, if applicable;

2. Vertically, two elements between decks, stringers, or every second or third stiffener, whichever is the smaller distance;

3. Transversely, at least two elements spanning the cross-deck structure and elsewhere, sufficient elements to maintain a satisfactory panel aspect ratio.

2.1.6 Plate elements are to have an aspect ratio less than 3, particularly in areas of interest in the model.

2.1.7 All primary structure, such as deck plating, bottom and side shell plating, longitudinal and transverse bulkhead plating, transverse floors, superstructure blocks, deckhouse blocks and internal structural walls are to be represented by plate elements. Primary girders, deep beams, web frames, etc. are to be represented by at least three elements through the depth of the member in areas of interest.

2.1.8 In areas where local fine mesh analysis is potentially required, as specified in Vol 4, Pt 1, Ch 2, 2.1 Global model 2.1.9, the structure in way of this area is to be adequately represented.

2.1.9 Secondary stiffening members may be modelled using line elements having axial and bending stiffness (bars). These elements may be grouped as necessary at the plate boundaries.

2.1.10 Pillars are to be represented by line elements having axial and bending stiffness.

2.1.11 Shell openings, deck openings, door openings and window openings of a significant size are to be represented in the model such that the deformation pattern under hull torsion, shear and bending loads is adequately represented in way of critical areas.

2.1.12 The model is to accurately reflect shell and superstructure side recesses, sweep brackets and superstructure breaks. The basic mesh, as described in Vol 4, Pt 1, Ch 2, 2.1 Global model 2.1.5 may need to be further refined in order to include these features.

2.2.1 In general, detailed stress analysis is to be carried out in the following locations, see also Vol 4, Pt 1, Ch 2, 1.1 Application 1.1.4:

1. The connections of the side hull to the ends of the cross-deck structure.

2. The connections of the centre hull to the ends of the cross-deck structure.

3. The transverse bulkhead where the highest shear stresses has been identified from the global load cases.

4. Areas with discontinuities in structure such as in way of openings or at the termination points of major structure.

5. After reviewing the results from the global load cases, areas in way of high stress gradients and areas exceeding the stress criteria specified in Table 2.2.1 Stress acceptance criteria permissible stresses.

2.2.2 Evaluation of detailed stresses requires the use of refined finite element mesh density in way of areas of high stress.

2.2.3 The extent of the local finite element model is to be such that the calculated stresses at the areas of interest are not significantly affected by the imposed boundary conditions and application of loads. The boundary of the fine mesh model is to coincide with main supporting structural members.

2.2.4 The mesh size in the fine mesh regions is not to be greater than 100 mm x 100 mm. A finer mesh size, such as 50 mm x 50 mm, may be required dependent on vessel size and the detail being considered. The extent of the fine mesh region is to be in general not less than 10 elements in all directions from the area under investigation. A smooth transition of mesh density is to be maintained.

2.2.5 The structural geometry, particularly in areas of concern is to be accurately represented.