Section 3 Military distinction notations

3.1.1 By its very nature a naval ship will be required to face and resist a variety of threats and it will be necessary to incorporate particular features to address those threats.

3.1.2 Some of the features required are already incorporated in the notation 100A1 NS. However, where the operational requirement demands, additional or specific levels of performance, special features notations such as those listed in Vol 1, Pt 1, Ch 2, 4 Surveys – General may be assigned showing protection against the effects of a particular threat.

3.1.3 Unless specifically requested these notations will be assigned at an appropriate level which will remain confidential to the Owner. It is the responsibility of the Owner to specify the threat levels suitable for their requirements. The agreed threat levels will not appear in the Register Book or be published in any other form. Only the notation MD will be used to show that some military features have been incorporated and constructed in accordance with LR’s Rules and Regulations for the Classification of Naval Ships.

3.1.4 A distinction is made between:

• levels of threat, describing the magnitude of the missile, torpedo, mine or bomb; and
• method of analysis which may be performed at differing levels of complexity.

3.1.5 In an effort to establish links between the different military loads, default levels of threat have been assigned. A distinction is made between levels of above water and underwater threats, as certain ships may be at greater risk from one or the other depending upon their operational requirements. They are summarised in Table 1.3.1 Relationship between notations.

3.1.6 In addition to the hull class notations defined in Vol 1, Pt 1, Ch 2 Classification Regulations, ships complying with the requirements of this Chapter will be eligible to be assigned the additional class notations defined in Vol 1, Pt 1, Ch 2, 2.2 Definitions 2.2.2 and Vol 1, Pt 1, Ch 2, 2.2 Definitions 2.2.4 or descriptive notes as defined in Vol 1, Pt 1, Ch 2, 2.2 Definitions 2.2.8.

3.2.1 As described in Table 1.3.1 Relationship between notations the external blast notation is normally independent of the internal blast and fragmentation notations as the threats that produce a survivable blast effect usually have a reasonable stand off. Typically, significant blast loading will arise from externally detonating threats such as far field nuclear at large stand offs and fuel air explosions at moderate stand offs. For an externally detonating conventional weapon, the blast will normally be insignificant but there will usually be a fragmentation threat. The external blast notation may also be independent of the residual strength notation unless the plastic deformation from an external blast renders certain structure ineffective with respective to global strength. For example a superstructure which contributes to longitudinal strength.

3.2.2 Usually, both internal blast and fragmentation will result from an internally detonating threat and are therefore linked, for example, a missile threat as shown in Table 1.3.1 Relationship between notations. For a particular threat it is recommended that both fragmentation and internal blast assessments will be made to the same level of threat for the structure adjacent to the point of detonation. Consideration should be given to the precise nature of the blast loading and fragmentation pattern of the threat.

3.2.3 If transverse bulkheads are used to limit the longitudinal spread of damage then the decks and side shell will probably be damaged such that a residual strength assessment is required to ensure that the global strength is not compromised. This should be to the same threat level as the internal blast threat. Longitudinal blast resistant bulkheads, box girders or service tunnels could be used to maintain the longitudinal effective material of the hull girder.

3.2.4 A residual strength assessment of the above water structure can be carried out for any threat level under any threat, independently of the other above water threat notations. This is because the ship may still retain function even though it has not been specifically armoured against the internal blast or fragmentation arising from such a threat. The residual strength notation is normally required for sea skimming missile threats that may remove significant areas of above water structure.

3.3.1 Shock enhancement should be aimed at providing ruggedness and to verify at a low level, equipment and system operation is maintained and at a higher level, equipment is retained and the hull does not rupture. Notation is currently confined to structure and concentrates on local damage that can be addressed by close attention to quality of construction and by adopting good constructional detail. Shock effects give rise to equipment and system damage. Shock is a different mechanism from whipping, therefore a whipping assessment will not generally be required to the same level of a shock assessment, though it may be necessary to check that the shock threat assessed will not have a significant whipping load. Residual strength assessments may be appropriate for shock threats depending on the extent of local damage.

3.3.2 Whipping is caused by proximity detonation of a charge that excites the main hull girder at a low-order (two node) natural frequency which may cause significant structural damage at a relatively low charge weight. Shock effects therefore may be relatively low order and it will not always be necessary to undertake a shock analysis. In addition a whipping analysis may not be necessary for threats which detonate on contact or for steel ships under 70 m in length. Due to the nature of whipping effects (usually the plastic collapse at a section of the hull), a residual strength calculation is not normally appropriate for a whipping threat because the damage from the direct shock is usually limited.

3.3.3 Residual strength assessments of underwater threats are normally concerned with contact mines or torpedo impacts. These will remove a certain amount of hull structure the effect of which is to be assessed by the residual strength calculation. Shock or whipping threats will only require a residual strength notation where there are significant amounts of local deformation to the hull girder. Significant damage is defined as that which reduces the global strength below the design margins.

3.4.1 In addition to levels of capability determined by the threat level specified, there are also different methods of assessment. The method of assessment will depend on three aspects:

• The level of the threat. At higher levels of threat, the requirements of the rules may become uneconomical or impractical and a more in depth analysis is required.
• Applicability of the rule formulations. If the threat level is outside the range of applicability of the rule formulations further analysis will have to be undertaken.
• Acceptance criteria, dependent upon whether the threat is to be assessed against elastic or plastic collapse criteria.

3.4.2 Three methods of assessment are shown in Figure 1.3.1 Assessment methods 1. In general the same threat level can be specified in each case however, it is the responsibility of the Owner to specify the correct levels to meet their specific requirements.

1. The analysis of military loads can most simply be assessed using the elastic model created for rule analysis. This will result in an acceptable but conservative solution.

2. The next more complex method uses an elasto-plastic or ultimate strength model.

3. Finally more complicated processes such as 3D dynamic analysis can be used to determine the loading for the elasto-plastic model. Normally this will be carried out for local areas of interest.

3.4.3 Once an ultimate strength model has been created for the appropriate sections along the hull it may be utilised for a variety of military notation calculations, as shown in Figure 1.3.2 Assessment methods for higher threat levels.

3.4.4 The damage required for the residual strength calculation can be defined in a variety of ways for a variety of threats, collisions or groundings. Non-military damage is defined in Vol 1, Pt 6, Ch 4, 4 Residual Strength Assessment, RSA and military damage by the damage radii in Vol 1, Pt 4, Ch 2, 7 Residual strength or specifically from external blast and vulnerability calculations. The results from a vulnerability analysis can be used for input to a variety of military notations and, in general, formal vulnerability assessments will be required for higher threat levels, see Vol 1, Pt 4, Ch 1, 2.1 General 2.1.4

Figure 1.3.1 Assessment methods 1

Figure 1.3.2 Assessment methods for higher threat levels