Section 3 Internal blast

3.1.1 For the internal blast notation there are three different levels of threat that are defined based on the ship size. The size and type of threat has no influence on the survey requirements.

3.1.2 In addition to the levels and type of threat, there are different methods of assessment. The more complex methods denoted by IB2, may require a different structural arrangement from that given in the rules. For this case, structural arrangement and detail will be given on the approved plan.

3.1.3 For the design assessment of the ship it is necessary to define the extent of the internal blast protection. Internal blast protection is aimed at limiting the horizontal spread of blast damage by the installation on blast resistant bulkheads. Only certain areas may be provided with protection, and different areas may have different structural arrangements to improve the blast resistance. A typical extent on a frigate ship may involve fitting blast bulkheads at zone boundaries or around critical compartments.

3.1.4 Particular attention should be paid to areas of high stress under the dynamic load, these are at the edges of the bulkhead as shown in Figure 16.3.1 Critical areas for ships with internal blast notation.

Figure 16.3.1 Critical areas for ships with internal blast notation

3.1.5 It should be noted that Vol 1, Pt 4, Ch 2, 3.2 Threat level determination of the Rules for Naval Ships contains specific requirements for the materials and in some cases weld consumables of blast resistant bulkheads.

3.1.6 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.

3.2.1 In an assessment of the capability of the ship structure against internal blast, the plating and stiffening act together as a membrane to resist the blast. In general, overall diminution may be tolerated down to the rolling tolerance but no allowance can be given below that.

3.2.2 Minor local diminution may be tolerated in the centre of the bulkhead away from the critical areas as defined in Figure 16.3.1 Critical areas for ships with internal blast notation. However, the edge connections of the bulkhead are critical and any local deterioration of the structure particularly in way of the stiffener connections, could lead to fracture initiation and significantly lessen the overall capability.

3.3.1 Unless specified, the alignment and tolerances are to be in accordance with the normal requirements of Ch 3 Construction Procedures. If tighter tolerances are required, then they will be indicated on the approved plan.

3.3.2 Particular care should be taken to minimise distortion if non-conventional connections are used at the bulkhead edges.

3.3.3 For blast bulkheads with full penetration welds or non-conventional connections at the boundaries, the normal NDE requirements, as defined in Vol 1, Pt 6, Ch 3, 8 Double bottom structures of the Rules for Naval Ships, should be supplemented by random NDE checks to the satisfaction of the Surveyor.

3.4.1 The capability of the blast bulkhead is affected by: the material grade, plate thickness, boundary connection, relative size and spacing of stiffeners and the penetrations or equipment attached to it.

3.4.2 All repairs and modifications are to be carried out using the materials, weld consumables and design detail contained in the rules or on the approved plan.

3.4.3 Additional penetrations are to be kept to a minimum and should be fitted in the corners of the bulkhead, as defined in Vol 1, Pt 4, Ch 2, 3.5 Quasi static pressure of the Rules for Naval Ships, where the relative movement is less. Cables and pipes into these penetrations should have a means to accommodate movement of the bulkhead.

3.4.4 Any cut outs in the bulkhead are to be ground smooth and have well rounded corners to reduce the risk of fracture initiation. Particular care should be taken round large cut outs in way of deck girders. In general, large cut-outs should not be closed with welded rigid collars. Alternative arrangements may apply where water or gas tight integrity is to be maintained.

3.4.5 The attachment of heavy items of equipment to the bulkhead is to be avoided, particularly in the centre where displacements are greatest. All items of equipment attached to the bulkhead are to have adequate seatings designed for dynamic performance.

3.4.6 Under blast loading large displacements may be experienced and equipment is to be located at least 350 mm away from the outermost part of the bulkhead or equipment attached to it. The bulkhead should be allowed to deform and should not be restrained by additional structure or equipment stays.

3.4.7 Blast bulkheads may be fitted with unconventional doors designed to resist blast loading. Any modifications to the door are to be carried out strictly in accordance with the approved drawings.