Section 2 External blast

2.1.1 For the external blast notation there are three distinct types of threat can be defined, that from a conventional weapon, a fuel air weapon or a nuclear weapon. The size and type of threat has no influence on the survey requirements.

2.1.2 In addition to the levels and type of threat, there are different methods of assessment. The more complex methods denoted by EB3 and EB4, use different acceptance criteria and this will lead to different survey requirements.

2.1.3 For the design assessment of the ship it is necessary to define the extent of the external blast protection. Not all areas of the hull will be protected. External blast effects are limited to the structure above the design waterline. A typical extent will normally encompass the main hull above the design waterline, the main superstructure blocks and the main mast.

2.1.4 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.

2.2.1 In an assessment of the capability of a ships’ structure against external blast, the plating is normally more than capable of sustaining the blast loading and the thickness requirement driven by normal sea loads. Similarly but to a lesser degree the primary and secondary stiffening will also be defined by the sea loads rather than the blast loads. The reason being that the structural assessments use plastic criteria and the sea loads elastic ones.

2.2.2 The allowances for overall corrosion and degradation of plating and secondary structure can be the same as for normal ships’ structure.

2.2.3 The end connections of primary and secondary stiffening are, however, critical. The external blast structural assessment is based on plastic hinges forming at these points and any deterioration of the structure particularly a reduction in section in way of bracket toes, could significantly lessen the overall capability. Critical locations are identified in Figure 16.2.1 Critical areas for ships with external blast notation.

Figure 16.2.1 Critical areas for ships with external blast notation

2.3.1 For the more complex assessment methods, denoted by EB3 and EB4, it is essential that the alignment and tolerances are in accordance with the design assumptions. In most cases these should be in line with the normal requirements of Ch 3 Construction Procedures. If tighter tolerances are required, then they should be indicated on the approved plan.

2.4.1 The blast pressure on the ship structure is affected by the geometry of the above water portion. Blast pressure waves, like any other waves, can reflect and be magnified. The addition of structure that forms internal angles, i.e. three planes at 120° or less, can lead to pressures well in excess of the design. Thus any modifications to the build, or additions to the ship that cause such angles may require a reassessment.

2.4.2 Likewise, reflections caused by new deckhouse structure or a reduction in the gap between deck houses could lead to pressures different from those used during the design assessment and may require a reassessment. The minimum air gap is defined in Vol 1, Pt 4, Ch 2, 3.3 Notation assessment levels and methodology 3.3.4 of the Rules for Naval Ships.

2.4.3 All of the assessment methods used use the natural frequency of the structure to determine the response. Thus any changes to the structure that affect the natural frequency may require a reassessment. Two areas can have an effect:

1. Changes to the mass of the structural element mainly by the attachment of equipment. The worst case being large masses at the centre of unsupported spans.

2. Changes to the effective length of the structure by attaching it to large items of equipment or adding extra structure.

2.4.4 Significant changes are defined as those which alter the natural frequency of the structure by more than 10%, see Vol 1, Pt 6, Ch 2, 5 Dynamic loading of the Rules for Naval Ships.

2.4.5 In an assessment of the capability of a ships structure against external blast, significant deflections of plate and stiffeners may occur. Changes to equipment or structural modifications that do not allow this deformation to occur may lead to rupture of the structure. Typically deflections are limited to l/30, where l is the length of the beam.