Section 7 Residual strength
Clasification Society 2024 - Version 9.40
Clasifications Register Rules and Regulations - Rules and Regulations for the Classification of Naval Ships, January 2023 - Volume 1 Ship Structures - Part 4 Military Design and Special Features - Chapter 2 Military Load Specification - Section 7 Residual strength

Section 7 Residual strength

7.1 General

7.1.1 This Section details the determination of the threat levels and methodology to be adopted in the attainment of an RSA1, RSA2 or RSA3 notation.

7.2 Threat level determination

7.2.1 The level to which a ship will be expected to structurally survive an attack scenario that results in weapon damage is to be specified by the Owner and will remain confidential to LR.

7.2.2 The threat level may be defined for a range of warheads detonating at given internal positions or UNDEX stand off distances with defined longitudinal locations. The probability of weapon hit locations can be determined from threat analyses which can be used to select the appropriate charge locations for the assessment.

7.2.3 Generally, fragmentation scenarios are not included in residual strength assessments since the damage is usually localised. However, any significant structural damage resulting from fragmentation (as determined in Vol 1, Pt 4, Ch 2, 4 Fragmentation protection) must be considered. Similarly, any damage from an external blast threat weapon must also bep included (Vol 1, Pt 4, Ch 2, 2 External blast). In addition, residual strength calculations must be used in conjunction with level 2 whipping analysis (Vol 1, Pt 4, Ch 2, 6 Whipping).

7.2.4 Where the STAB notation is assigned, the damage scenarios included in the residual strength assessment are to include grounding/raking and collision damages which are consistent with the compartment damage criteria specified by the subdivision and stability standard, see Vol 3, Pt 2, Ch 6, 1 General.

7.3 Notation assessment levels and methodology

7.3.1 Ships for which a residual strength assessment is carried out will be eligible for a RSA1, RSA2 or RSA3 notation as defined in Vol 1, Pt 4, Ch 2, 7.3 Notation assessment levels and methodology 7.3.6.

7.3.2 The assessment of the residual strength capability of a ship is to be performed as defined in Vol 1, Pt 6, Ch 4, 4 Residual Strength Assessment, RSA.

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.

7.4 Definition of damage

7.4.1 The damage radius is a measure of the extent of the damage caused by specific above water attack scenarios. This is shown diagrammatically in Figure 2.7.1 Determination of damaged structure, where the assumption of a detonation mid-compartment is shown and the extent of damage is indicated by the extent of the damage radii. Assumptions about position and extent of damage radii are dependent on warhead characteristics. In general, the radii is to be vertically positioned such that it removes the maximum amount of material or has the greatest effect on the sectional inertia.

7.4.2 The damage radii can be determined from:

r = f z W 1/3 m
where
f z = scaled distance dependent on ship type
W = equivalent weight of TNT, in kg.

7.4.3 Once a damage radius has been determined, the simplest method of accommodating damage is to remove all structure within and touching the damage radii from the residual strength calculation.

7.4.4 For underwater shock (UNDEX), hull plating failure can be derived from angle hull shock factor, as defined in Figure 2.7.2 Angle shock factor, exceeding appropriate hull lethality levels. The angled shock factor may be determined from:

SF =
where
W = equivalent charge weight of TNT, in kg
R = distance from charge, in metres
f (θ) = threat angle function, see Figure 2.7.2 Angle shock factor.

Figure 2.7.1 Determination of damaged structure

Figure 2.7.2 Angle shock factor

7.4.5 Special consideration will be given to the effects of damage to box type strengthening structure, armour plating, high yield strength materials, double skin hulls and blast strengthened bulkheads.

7.4.6 Generic damage can be generated using pseudo-random hit probability algorithms and repeated application of such algorithms can be used to represent successive weapon hits. Specific damage is difficult to quantify but can be estimated by using damage radii techniques for above water structure and by assuming critical shock factor levels for below water structure.


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