Section 3 Internal blast

3.1.1 Internal blast is defined as that which occurs from detonation of a high explosive from a hostile weapon or detonation of a ship’s own ammunition inside the hull envelope. In an internal explosive loading situation the loading on a boundary can be characterised by a series of decaying reflected pressure waves (blast impulses) followed by the rapid formation of a slowly decaying static pressure (Quasi static pressure QSP) as shown in Figure 2.3.1 Typical blast pressure time history.

3.1.2 The magnitude of the initial blast impulse is related to the distance from structure under consideration to the explosion. The reflections are a function of the compartment geometry. The QSP is dependent on the compartment volume with the rate of decay related to the vent area.

3.2.1 The threat protection levels for a given vessel should be determined through a vulnerability analysis against customer specified threat weapons. In the absence of such a study the following levels may be used as a guide:

• Level I Watertight bulkheads at R_4N ≥ 1 and zone bulkheads at R_4N > 1
• Level II Watertight bulkheads at R_4N ≥ 1,5 and zone bulkheads at R_4N > 2
• Level III Watertight bulkheads at R_4N > 3 and zone bulkheads at R_4N > 3

R_4N is the normalised blast resistance 2,5 m high, 4 mm thick, mild steel, fillet welded bulkhead.

3.3.1 The Rules are aimed at limiting the spread of blast damage to compartments adjacent to that directly affected by the explosion. For an explosion where the ratio of charge size to compartment volume is small, it may be possible to limit the damage to the affected compartment.

3.3.2 Ships complying with the requirements of this section will be eligible for the IB1 notation. Where further analysis or testing is used to determine the blast resistance of the structure an IB2 notation may be assigned.

3.3.3 For the IB2 notation, the assumptions made for initial deformations are to be submitted. Where these differ from normal ship building practice, the details are to be recorded on the approved plan.

3.3.4 There are specific scenarios such as fuel air explosions within aircraft hangars where the internal blast wave characteristics will need to be specially considered on request.

3.4.1 For level III protection all plate bulkhead materials are to have a sulphur content less than 0,01 per cent. This may be achieved by the specification of through thickness properties in accordance with the requirements of Ch 3, 8 Plates with specified through thickness properties of Rules for Materials.

3.4.2 Consideration should be given to the use of austenitic electrodes for fillet welding of ferritic materials subject to high strain loading. In selecting the filler material, consideration is to be given to the material’s proof strength and elongation; the 0,2 per cent proof stress of the filler material as welded is to match the strength of the ferritic parent steel plate, and the elongation to failure is to be as great as possible. Care should be taken to ensure that coatings are maintained as far as practicable. Where such materials are in wet or immersed areas, special attention is to be given to corrosion protection and the selection of a material that is not prone to chemical or electro-chemical attack. Details of the weld procedure are to be submitted for approval.

3.5.1 Structural failure can be caused by either the impulsive loading or the dynamic loading imparted by the combined blast waves and QSP. Normally if the weapon is sufficiently large to cause failure by impulse it will also fail under a dynamic loading assessment based on a step function to the QSP level. For the purposes of general design the step function to the QSP level assessment can be used as the loading criteria to determine failure. Safety or mission critical areas should be specially considered.

3.5.2 The actual threat level used in the calculation and areas of the ship to be protected are to be specified by the Owner and will remain confidential to LR.

3.5.3 The QSP can be determined from the following:

where

3.6.1 The blast resistance for a given bulkhead material, thickness and joint style can be determined as a proportion of 2,5 m high, 4 mm thick, mild steel, fillet welded bulkhead using the following formula based on a combination of explosive tests and analytical techniques:

3.6.2 The primary mode of failure for bulkhead structures is through the edge connection. Alternatives to the basic fillet weld have been assessed and incorporated in the joint type factor presented in Table 2.3.2 Joint type factor, K _j .

3.6.3 Alternative joint types may be used but are to be categorised using a dynamic joint test and blast assessment. For novel designs a further large scale controlled blast test of the proposed arrangement is to be tested. LR can provide details of the test and analysis requirements on request.

3.7.1 Piping that passes through the bulkhead is to be fitted with expansion pieces either side of the bulkhead. In addition, piping and other penetrations are to be arranged at the edges of the bulkhead where the relative movement is less as shown in Figure 2.3.2 Blast bulkhead penetrations.

3.7.2 Bulkhead attachments are to be kept to a minimum and designed for good dynamic performance.

3.7.3 The strength of doors if fitted will be specially considered. Steps are to be taken to prevent their detachment from or pushing through the surrounding structure.

3.7.4 Consideration should be given to the use of flexible collars around deep girder penetrations through the blast bulkhead to allow relative movement but retain watertight or gas-tight integrity.

3.7.5 Under blast loading, large displacements of the bulkhead may occur. Any nearby structure or equipment is to be located so as to provide a minimum clearance of 350 mm from the bulkhead. This distance is generally appropriate for deck heights of between 2 m and 3 m. The minimum clearance for other deck heights will be specifically considered.