Section 4 Fragmentation and small arms protection

4.1.1 For the fragmentation protection notation FP there are three distinct types of threat that can be defined. Each threat has a different mechanism and will affect the structure in different ways leading to different survey requirements.

4.1.2 Level I is concerned with an internally detonating threat and the protection is arranged to prevent the fragments reaching compartments other than the detonation compartment.

4.1.3 Level II is concerned with an externally detonating threat and the protection is arranged to prevent the fragments from that threat getting in to the ship.

4.1.4 Level III is concerned with a large internally detonating threat and the protection is arranged to prevent the fragments penetrating different zones or areas of the ship outside the detonation zone or area.

4.1.5 Small arms protection SP affects the structure in a different manner still as it attempts to prevent the threat weapon getting inside the ship.

4.1.6 For the design assessment of the ship it is necessary to define the extent of the fragmentation protection. Therefore not all areas of the hull may be protected. Typically a list of critical compartments is generated and the fragmentation protection arranged around these in the manner described in Vol 1, Pt 4, Ch 2, 4.4 Information required of the Rules for Naval Ships. For level III the protection may be described as being certain bulkheads.

4.1.7 Where the structure is found to be outside the limits defined in this Section, the condition should be reported to the LR Naval liaison office.

4.2.1 The Rules define the fragmentation resistance in terms of thickness of steel. However, for the larger threats, special materials such as rolled homogenous armour (RHA) or composite materials may be used. Repairs or modifications to areas with fragmentation or small arms protection should be carried out using materials of the exact specification and arrangement as those on the approved drawings.

4.2.2 The hardness of the material, typically defined by the Brinnell hardness number, can be used as a measure of the relative fragmentation resistance but it is no guarantee of the fragmentation resistance. Any alternative materials used should be qualified by testing or analysis undertaken by specialists.

4.2.3 Fragmentation protection materials are not to be welded unless they are of LR grade steel in which case the normal precautions concerning welding and heat treatment are to be applied.

4.3.1 In an assessment of the capability of the ship structure against fragments or small arms, the resistance is based on the thickness of the material. In general overall diminution may only be tolerated down to the rolling tolerance and no allowance can be given below that. The graphs in the Rule calculations include the LR rolling allowance as defined in Ch 3, 1.5 Surface quality of materials of the Rules for Naval Ships. Furthermore no local diminution can be tolerated.

4.4.1 Alignment and tolerances should be in line with the normal requirements of Ch 3 Construction Procedures.

4.5.1 The fragmentation resistance is affected by the thickness or arrangement of material as defined in Ch 16, 4.2 Materials. Any modifications should ensure that the material replaced is identical to, or proven as being of equivalent fragmentation resistance to, the material on the approved plans. Where necessary the capability of an alternative arrangement can be proven by calculation or fragmentation trials.

4.6.1 As part of the approval process for a material, it may be necessary to witness fragmentation trials, where this is necessary, the LR office responsible for the classification of the ship or liaison with the Navy should be contacted.