Section 5 Shock

5.1.1 For the shock notation there are three levels of threat that are defined. SH1 is at a lower level and based on simple analysis of the structure. Levels SH2 and SH3 are based on a more in depth analysis and at a higher level. Whilst shock is, to a certain extent, dependent on the scantlings of the structure, it is mainly dependent on the details such as connections, particularly those that are stress concentrations and possible sites for crack initiation. These critical locations are defined for the different shock levels in Vol 1, Pt 4, Ch 2, 5 Underwater explosion (shock) of the Rules for Naval Ships.

5.1.2 Shock is primarily a problem for equipment and systems but it is important to ensure that the hull is not significantly damaged at the shock level the equipment is designed to survive. It is also be important to ensure that the hull has a survival capability above that of the equipment. The shock notation is aimed at maintaining that hull structural capability.

5.1.3 Shock has local effects, but the whole of the underwater portion of the ship has the potential to be affected by a shock impulse. Therefore all areas need to be considered in the assessment of shock capability. Once the shock is transmitted from the water into the hull, the magnitude of the shock pulse decreases and the time of impulse increases with distance from the wetted surface see Figure 16.5.1 Shock pulse throughout the ship structure. This means that the further away from the wetted surface of the hull an item of structure is, the less likely it is to be damaged by shock.

5.1.4 Exceptions to this are pillars or minor bulkheads that are directly connected to the hull and these can transmit the shock pulse into the centre of the hull structure.

5.1.5 In determining the nature of a defect or modification it is also important to consider it’s criticality, i.e. what it might affect. Defects which when ruptured lead to a compromise of the watertight or gas tight integrity or the release of liquids can be considered more critical.

5.1.6 Defects will also be more critical on smaller ships with less internal structure which are less tolerant of damage.

5.1.7 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.

5.2.1 In an assessment of the capability of the ship structure against shock, the overall scantlings of the ship are not critical, and allowances for overall corrosion and degradation of plating and secondary structure can be the same as for normal ship structure.

5.2.2 Some of the details and connections of primary and secondary stiffening are critical. Deterioration of the structure in these areas could lessen the shock resistance and lead to fracture initiation. Critical locations are identified in Ch 16, 5.1 General.

Figure 16.5.1 Shock pulse throughout the ship structure

5.3.1 For the more complex assessment method, denoted by SH2 and SH3, 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.

5.3.2 Particular attention should be paid to the weld quality and workmanship in critical locations as defined in Ch 16, 5.1 General. 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.

5.4.1 The shock loading on structure is dependent on the natural frequency of the structure. Thus any changes to the structure that affect the natural frequency may also affect the capability of the structure to resist shock. In addition, the changes could also affect the response of the machinery and negate any shock protection. Two areas can have an effect:

1. Changes to the masses of equipment or the addition of new equipment. The worst case being large concentrated 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.

5.4.2 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.

5.4.3 In an assessment of the capability of the ship structure against shock to level I, only modest deflections of the plating may occur and a clearance need not be considered.

5.4.4 In an assessment of the capability of the ship structure against shock to SH2 and SH3, 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. Equipment is to be located at least 350 mm away from the inner most part of the wetted hull structure or equipment attached to it.

5.4.5 Modifications to the critical locations, as defined in Ch 16, 5.1 General, should be carried out in accordance with the approved drawings.