Section 6 Global loads

6.1.1 For the whipping WH, residual strength RSA and Extreme strength ESA notations, there are distinct levels and types of threat that can be defined. The size and type of threat has no influence on the survey requirements.

6.1.2 All three notations have a different method for determining the load as they all deal with different events, however, once that load is defined, the structural assessment method and acceptance criteria are common to all three.

6.1.3 There are different methods of assessment which are identified by the last figure of the notation. The more complex methods denoted by WH2, WH3, RSA2, RSA3, ESA2 and ESA3, use different acceptance criteria and this will lead to different survey requirements.

6.1.4 For all three notations, the effect concerns the global capability of the ship girder and maintaining capability is similar to maintaining the longitudinal strength of the ship for normal sea loads.

6.1.5 Thus, changes to the effective sectional inertia in the midship region where the loads are highest, are most critical.

6.1.6 For each of the notations it is important to establish the effective and non-effective structure. For the simple methods of assessment, denoted by WH1, RSA1, and ESA1 the definition of such structure is given in Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus of the Rules for Naval Ships. For the more complex methods of assessment, denoted by WH2, WH3, RSA2, RSA3, ESA2 and ESA3 the definition of such structure is given in Vol 1, Pt 6, Ch 4, 1.4 Calculation of hull section modulus of the Rules for Naval Ships. An example of the typical amount of structure at a section and that which is effective is given in Figure 16.6.1 Effective structure for a typical ship section. The structure deleted can be considered non-effective as it does not contribute to global longitudinal strength.

Figure 16.6.1 Effective structure for a typical ship section

6.1.7 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.

6.2.1 For the simple methods of assessment, denoted by WH1, RSA1, and ESA1 the allowances for corrosion and degradation of plating and secondary structure can be the same as for a normal ship structure.

6.2.2 For the more complex methods of assessment, denoted by WH2, WH3, RSA2, RSA3, ESA2 and ESA3, to maintain the capability of a ship structure, the local allowances for corrosion and degradation of structure can be the same as for a normal ship structure, see Ch 10 Corrosion. However, there should be a negligible reduction (less than 0,5%) in the top side and bottom areas of the section of effective material.

6.3.1 For the more complex assessment method, denoted by WH2, WH3, RSA2, RSA3, ESA2 and ESA3, 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.

6.4.1 For all three threat types and methods of assessment, the capability of the structure is dependent on the amount of material in the section. Any removal, of structure for hatches, doorways, trunking, shipping routes anywhere along the length could reduce the amount of effective material and hence the capability of the ship. The most critical sections are those within 0,3L _R to 0,7L _R or where the section inertia is at a minimum in way of large openings for lifts and uptakes or downtakes. The assessment of capability is normally made at these critical sections.

6.4.2 It is also important to note that openings have a 30° shadow area associated with them as defined in Vol 1, Pt 6, Ch 4, 1.5 General of the Rules for Naval Ships, and this should be taken into account when calculating the amount of effective material at the section under consideration.

6.4.3 The buckling capability of longitudinal structural members is important for the strength of the section. Changes that may affect the buckling capability are:

• Misalignment of primary structural members.
• Increases in the effective length of members by the removal of minor structure.
• Distortion of members outside the allowable tolerances during fabrication or due to repairs.
• Severing longitudinal structure by modifications without adequate reinforcement.

6.4.4 For the more complex methods of assessment, denoted by WH2, WH3, RSA2, RSA3, ESA2 and ESA3, the capability of a ship structure is also sensitive to the local geometry, in particular hard corners. These are formed when primary structure such as decks and bulkheads intersect. Longitudinal minor bulkheads, which for longitudinal strength purposes are considered ineffective, will, under the more complex assessment methods contribute to the strength of the section if they span between main watertight bulkheads.