7.3.3 There
are three methods of assessment that may be used to determine the
damaged residual strength of the hull girder.
- Simple 2D cross-section elastic model.
- 2D ultimate strength model.
- Advanced 3D Finite Element Methods.
7.3.4 In the
case of a mine warfare ship or NS3 ships, a 2D elastic analysis will
normally be sufficient. For most other naval ships, a 2D ultimate
strength analysis would normally be required to determine the damaged
residual strength at a particular frame location along the hull girder.
7.3.5 An advanced
3D analysis incorporating initial deformations and residual stresses
will be required when:
- More detailed information is required throughout one or more compartments
along the length of the ship which have been shown by the more simplified
2D ultimate strength analysis to be inadequate. This may be necessary,
for example, where there are large structural discontinuities in hull
girder strength.
- The ship design cannot be reduced to a 2D beam or ultimate strength
description, for example, when it has an unusual structural configuration.
7.3.6 A RSA1 analysis method uses a 2D elastic cross-section representation
and a failure level criterion based on the calculated bending moment
being greater than both the design hogging and sagging bending moments
at the sections considered to be most critical.
7.3.7 A RSA2 method of analysis uses a 2D ultimate strength beam representation
and a failure level criterion based on the section ultimate bending
moments being satisfactory compared to the design bending moments
in both hogging and sagging. This will require assessment using ultimate
strength calculations at no less than three damaged positions along
the length of the hull.
7.3.8 A RSA3 method of analysis uses a 3D definition of a section of the
hull girder and relies on geometric and material failure criteria
implicit in the chosen finite element code. It could also include
coupled Euler-Lagrange formulations to specifically account for internal
and external blast effects, UNDEX shock and whipping.
7.3.9 In each
case, it is to be demonstrated that the hull girder remains below
the defined design hogging and sagging design bending moment failure
limits for all prescribed threat scenarios.
7.3.10 For
certain ship types, such as minesweepers, it will be necessary to
carry out several levels of analysis. An elastic analysis should be
carried out for threat levels which are expected to be survived on
a regular basis and geometric and material nonlinear analysis at higher
threat levels for which the ship is expected to survive.