Section 1 General requirements

1.1.1 This section is aimed primarily at assessing structure such that it can resist the military loads imposed upon it, however it is essential in naval ship design to consider the effects an item of equipment or structure can have on a variety of parameters. For example a winch support and mount may adequately resist the forces imposed upon it during normal operation and absorb shock loads but have an unacceptably high noise or radar signature.

1.1.2 Chapter 1 gives guidance on some of the additional issues that the designer must consider in the design of a naval ship. Whilst it does not always give the definitive answer on these topics, it will help identify the impact of structural design on the subject. An example is radar signature reduction. The guidance gives the geometric properties to avoid but it will not give detail on radar absorbent coatings.

1.1.3 Information is classified in two types. Firstly, design guidance for which further approval has to be sought once a suitable standard is specified, and secondly, design requirements which have to be met as part of the 100A1 notation or a specific notation such as LA(N).

1.2.1 Plans are to be submitted showing the manner in which the requirements have been met and the location of the structure within the vessel for those features that have either a special notation or are required as part of the notation 100A1 or ship type.

1.2.2 Details on the loadings applied to individual items, and by these items to the support structure are to be included. In some cases stiffness requirements will also need to be included, e.g. mast mounted equipment.

1.2.3 Plans, and where requested, calculations are to be submitted for the following features and loadings as appropriate:

• Replenishment at sea arrangements.
• Aircraft and vehicle tie down arrangements.
• Movable decks, ramps and lifts.
• Masts and support arrangements.
• Towed array or towed body arrangement.
• Weapon recoil and thrust loadings.
• Blast and efflux loadings and pressure curves.
• Arcs of fire and blast or efflux impact zones for weapons.

1.2.4 Arrangements for the following features are to be included with the hull structural plans listed in Vol 1, Pt 6, Ch 2, 2 Structural design. In addition calculations are to be supplied where requested:

• Vehicle decks.
• Helicopter decks.
• Berthing.
• Docking loads.
• Beach landing or grounding.
• Holding down arrangements.

1.2.5 Plans and supporting calculations should be submitted for the following notations:

• External blast (EB1, EB2, EB3, EB4).
• Internal blast (IB1, IB2).
• Fragmentation (FP1, FP2).
• Small arms protection (SP).
• Underwater explosion (Shock) (SH).
• Whipping (WH1, WH2, WH3).
• Residual strength (RSA1, RSA2, RSA3).

1.3.1 A naval vessel will generally require some form of signature control and the operational requirement will determine the extent to which this is necessary. Signature control can be achieved using a variety of methods both active and passive. This section deals with the passive methods that structure can influence.

1.3.2 With good structural design the signature of the vessel can be controlled to a certain degree with little cost. The methods listed in Table 1.1.1 Ship signatures can help achieve this.

1.3.3 It is beyond the scope of the Rules to provide further detail on signatures, however on request, Clasifications Register (hereinafter referred to as 'LR') is able to provide information on suitable organisations who are able to give specialist advice as necessary.

1.3.4 Special features notations for signature control will not normally be assigned. Some of the above features will form part of the Naval Ship notation ✠ 100A1 NS and are detailed in Vol 1, Pt 4, Ch 1, 3 Military distinction notations.

1.4.1 In addition to the requirements of Vol 1, Pt 6, Ch 2 Design Tools, ships having the following military distinction notations are to comply with the requirements of this section for the designated areas unless specified otherwise. The requirements apply to plates, stiffeners, fillet welds, butt welds and welded attachments:

• EB1, EB2, EB3, EB4 Above water portion of the hull, superstructure and upper decks assessed against external blast requirements.
• IB1, IB2 Blast bulkheads.
• SH Hull envelope plating.
• WH1, WH2, WH3 Shear strake, stringer plate (including margin angle), bilge strake, keel plate, garboard strake and hull inserts.
• RSA1, RSA2, RSA3 Shear strake, stringer plate (including margin angle), bilge strake, keel plate, garboard strake and hull inserts.

1.4.2 Crack arresting strakes of minimum Grade E are to be fitted in the following locations, from 0,2L _R to 0,8L _R, according to the notation assigned:

• SH Shear strake, stringer plate (including margin angle), bilge strake, keel plate, garboard strake and hull inserts in these areas.
• WH1, WH2, WH3 Shear strake, stringer plate (including margin angle), bilge strake, keel plate, garboard strake and hull inserts in these areas.
• RSA1, RSA2, RSA3 Shear strake, stringer plate (including margin angle), bilge strake, keel plate, garboard strake and hull inserts in these areas.

Where the hull envelope is made entirely from Grade D steel, crack arresting strakes of minimum Grade E need not be fitted in the specified locations.

1.4.3 Generally for joints between steels of different strength levels the welding consumable may be of a type suitable for the lesser strength.

1.4.4 For joints between steels of different toughness levels, the welding consumable is to be of a type suitable for the higher grade being connected.

1.4.5 The consumable used is to comply with the requirements of Table 1.1.2 Welding consumable grade. Other grades of steel will be specially considered, but in general, the toughness in the upward vertical direction is not to be significantly less than that of the parent plate, measured in the direction of rolling.

1.4.6 Where armour plating consisting of steels with a specified tensile strength of 1000 MPa or above is integrated into the ship’s structure by welding, consideration should be given to the susceptibility of these materials to hydrogen cracking. The use of normal strength or higher strength steel of toughness grade D or higher in terms of Charpy V-notch fracture toughness should be considered as an intermediate transition material where the adjacent material is not already of this grade.

1.4.7 The proposed welding procedures are to be submitted for review, and are to be chosen to minimise the risk of hydrogen cracking. The following is recommended:

1. The use of welding consumables and electrodes with low hydrogen content (less than 5 ml/100 g of deposited weld metal).

2. The weld preparation and welding apparatus, consumables and electrodes are to be clean, dry and free from other sources of hydrogen such as lubricants and grease.

3. Controlled preheat, interpass temperatures, cooling rates and post-heat treatment chosen in accordance with manufacturers' guidance and recognised welding standards.

4. Weld sequence chosen to minimise the formation of residual stresses.

5. The use of welding consumables of a higher strength than necessary should be avoided.

6. Non-destructive examination should not be carried out before a period of 48 hours has elapsed from the time welding is completed.

1.4.8 The use of mechanical fasteners to secure armour plating should be considered as a means of removing the risk of hydrogen embrittlement as a result of the welding process. Where higher strength steel fasteners with Vickers hardness above 320 HV are used, consideration should also be given to the susceptibility of these materials to hydrogen cracking.