Section 4 Residual Strength Assessment, RSA

4.1.1 This Section gives the requirements and procedures to be adopted for the application of the residual strength assessment procedure.

4.1.2 The following definition gives the basic default mission statement for the residual strength capability. The ship is required to have a 95 per cent probability of surviving for 96 hours, after sustaining structural damage as a consequence of military action, collision or grounding, in wave conditions that have a probability of occurring for 80 per cent of the time.

4.1.3 Three assessment levels are available for the residual strength assessment notation as detailed in Vol 1, Pt 6, Ch 4, 1.2 Hull girder strength notations

4.1.4 For the RSA1 residual strength assessments the residual strength after damage is to be verified using the simplified assessment method given in Vol 1, Pt 6, Ch 4, 4.4 Bending strength – Simplified assessment method RSA1 and Vol 1, Pt 6, Ch 4, 4.5 Shear strength – Simplified assessment method RSA1. This uses elastic theory, based on the remaining section moduli and area after damage, and determination of the buckling strength to resist the global hull girder loads.

4.1.5 For the RSA2 residual strength assessments, the residual strength after damage is to be verified using the ultimate strength analysis method which determines the ultimate strength of the hull after damage using direct calculation methods. The assessment criteria are given in Vol 1, Pt 6, Ch 4, 4.6 Bending and shear strength – Ultimate strength analysis method RSA2

4.1.6 For the RSA3 residual strength assessments, the residual strength after damage is to be verified using a recognised finite element code suitable for this type of analysis. The failure will be determined by the criteria implicit in the finite element code chosen. Several assessment codes are available and the calculation should be performed by a competent and experienced body with relevant experience.

4.1.7 All critical sections to be assessed are to be considered for all damage scenarios, irrespective of whether the critical section is damaged or not.

4.1.8 In addition, the residual strength of the hull girder or main deck following the failure of any single critical structural element is also to be considered. See Vol 1, Pt 3, Ch 2, 2.3 Definitions and structural terms . A review of the structural arrangement is to be made to establish the locations of any critical structure and a separate assessment made as to the impact of their individual failure on the strength of any main deck or the hull girder. The method of review and assessment are to be agreed with LR.

4.1.9 For damage scenarios that involve flooding of the ship, the effects of the flood water on the still water shear forces and bending moments are to be considered in the residual strength assessment at all critical locations whether they are damaged or not. In the latter case the capability of the undamaged critical section will be based on the structural capability of the intact section.

4.2.1 The extent of damage to be considered is defined below. Unless otherwise specified by the Owner it will not be necessary to consider the consequences of combining damage extents from different weapon threats or damage scenarios.

4.2.2 The extent of damage due to military threats is defined as the minimum of the shock or blast damage that is likely to result from a specified weapon threat. The weapon threat may be specified by any of the following:

• the residual strength notation threat level, see Vol 1, Pt 4, Ch 2, 7 Residual strength
• the Owner;
• as a direct consequence of requirements of other notations.

4.2.3 Collision damage to the side shell. The standard damage extent is to be taken as:

Level A

• 5 m longitudinally between bulkheads
• from the waterline up to the main deck
• inboard for B/5 m.

Level B and C

• 5 m longitudinally anywhere including bulkheads
• from the bilge keel up to the main deck
• inboard for B/5 m.

4.2.4 Grounding or raking damage to the bottom structure. The standard grounding damage extent is to be taken as:

Level A

• length of 5 m anywhere forward of midships
• upwards for 1 m or to the underside of the inner bottom, whichever is less
• breadth of 2,5 m.

Level B and C

• length of 0,1L _R anywhere forward of midships
• upwards for 1 m or to the underside of the inner bottom, whichever is less
• breadth of 5 m

4.2.5 For the Levels A and B residual strength assessment, the residual strength is to be considered at a minimum of three critical sections for each anticipated damage extent. The critical sections are to be taken in the midship region and near each quarter length location. For the Level C residual strength assessment, the residual strength is to be considered at all critical sections along the length for each anticipated damage extent.

4.2.6 The damage requirements used for the residual strength assessment should be clearly identified in the Loading Manual or Stability Information Book.

4.3.1 The effective geometric properties of critical transverse sections in way of the damaged area are to be calculated in accordance with Vol 1, Pt 6, Ch 4, 2.1 General and Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus. In defining the longitudinal position of each critical section, the shadow areas associated with the damage and other openings specified in Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.15 are to be considered together with the proximity of other openings, see Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus 1.4.17. The effectiveness of the superstructure may also need to be re-evaluated.

4.3.2 Due attention is to made to the effectiveness of structure which may have been plastically deformed, as a consequence of the damage, on the ultimate strength after damage.

4.3.3 For Level 1 and 2 residual strength assessments, damaged areas are to be considered at approximately the longitudinal positions L _R/4, L _R/2 and 3L _R/4. The geometric properties of critical transverse sections in way of these damaged areas are to be considered. Other longitudinal positions may also need to be considered depending on the internal arrangement or structural arrangement of the ship or the specified residual strength requirements.

4.3.4 For Level 3 residual strength assessment, damaged areas are to be considered at all positions along the length. Typically, it is expected that the local critical transverse section between adjacent main watertight bulkheads will be evaluated.

4.4.1 If the simplified analysis method is adopted for residual strength assessment, the longitudinal strength of the ship at each critical section is to satisfy the following criteria for the hogging and sagging conditions:

where

σ_DRS is the hull girder bending stress at strength deck

σ_BRS is the hull girder bending stress at keel

f _hts and σ_o are defined in Vol 1, Pt 6, Ch 4, 1.3 Symbols and definitions 1.3.1.

4.4.2 The design residual strength stress due to the Residual strength design vertical bending moment, σ_rs, for each structural member is given by:

where

M _RRS is defined in Vol 1, Pt 6, Ch 4, 4.4 Bending strength – Simplified assessment method RSA1 4.4.1

4.4.3 It is not necessary to satisfy the plate panel buckling requirements for compressive stresses provided that shear buckling of plate panels and all buckling modes of failure for longitudinal girders and stiffeners are satisfied. The design factors of safety are given in Vol 1, Pt 6, Ch 5 Structural Design Factors

4.4.4 Consequently, the following Sections on buckling control are to be complied with, based on compressive stresses derived in accordance with Vol 1, Pt 6, Ch 4, 4.4 Bending strength – Simplified assessment method RSA1 4.4.2

1. Secondary stiffening in direction of compression, Vol 1, Pt 6, Ch 2, 3.7 Secondary stiffening in direction of compression

2. Secondary stiffening perpendicular to direction of compression, Vol 1, Pt 6, Ch 2, 3.8 Secondary stiffening perpendicular to direction of compression

3. Buckling of primary members, Vol 1, Pt 6, Ch 2, 3.9 Buckling of primary members

4.5.1 If the simplified analysis method is adopted for residual strength assessment, the shear strength of the ship after damage at each damaged critical section is to satisfy the following criterion:

where

δ₀ is to be taken as the minimum value of δ_i,

and

A _z, and δ_i, are to be calculated in accordance with the method in Vol 1, Pt 6, Ch 4, 2.3 Shear strength for the damaged section, see also Vol 1, Pt 6, Ch 4, 4.1 Application 4.1.9.

f _hts and τ_o are defined in Vol 1, Pt 6, Ch 4, 1.3 Symbols and definitions 1.3.1.

4.5.2 The design extreme shear stress due to residual strength design shear force for each structural member, τ _rs , is given by

where

Q _RRS is given in Vol 1, Pt 6, Ch 4, 4.5 Shear strength – Simplified assessment method RSA1 4.5.1

A _z, and δ_i are given in Vol 1, Pt 6, Ch 4, 2.3 Shear strength 2.3.6

4.5.3 The following Sections on buckling control are to be complied with based on shear stresses derived in accordance with Vol 1, Pt 6, Ch 4, 4.5 Shear strength – Simplified assessment method RSA1 4.5.2:

1. Plating subject to pure in-plane shear, Vol 1, Pt 6, Ch 2, 4.3 Natural frequency of plate.

2. Shear buckling of stiffened panels, Vol 1, Pt 6, Ch 2, 3.6 Shear buckling of stiffened panels.
   

The design factors of safety are given in Vol 1, Pt 6, Ch 5 Structural Design Factors

4.6.1 The residual strength capability of the damaged hull girder may be assessed using a direct calculation ultimate strength analysis method. In this case the longitudinal strength of the ship at each critical section is to satisfy the following criteria for the hogging and sagging conditions:

where

M _RRS < f _URS M _URS

Q _RRS < f _URS Q _URS

4.6.2 The ultimate strength of each critical section is to be derived by direct calculation using elasto-plastic analysis methods.

4.6.3 If the methods used to derive the ultimate strength do not include allowance for shear loading, then the shear strength requirements of Vol 1, Pt 6, Ch 4, 4.5 Shear strength – Simplified assessment method RSA1 are to be applied.