Section 1 Identification of critical areas

1.1.1 LR has applied direct calculation procedures in the structural appraisal and approval of new buildings and in various investigations on special service craft of both steel and aluminium alloy construction. Through these procedures and the wealth of information collected on the LR fleet database, a number of locations have been identified where good design, workmanship and alignment during construction are particularly important. These are usually locations where high stress variations can be experienced during the lifetime of the craft. These are referred to as critical locations and are highlighted in this Chapter.

1.1.2 This Chapter identifies the critical areas within various structural elements of the hull structure and transverse bulkheads.

1.1.3 In Ch 4, 2 Structural details the structural detail design improvements that can be applied to increase the fatigue life of the structural components are provided. These detail improvements are intended to give the designer guidance for meeting the design criteria for structural detail components.

1.1.4 The application of 2 and 3-dimensional finite element analyses techniques to the hull structure enables the global and local capabilities of the hull structure to withstand static and dynamic loadings to be assessed. Such analyses will enable those high stress locations and joints within the craft to be readily identified. Such locations will then, by their very nature, be at risk to fatigue damage unless appropriate measures are taken at the design stage and subsequently during construction.

1.1.5 Extensive ‘in service’ experience of the performance of existing craft structures, already provide an awareness of those critical locations which merit particular attention either due to stress or alignment difficulties.

1.2.1 Stress concentrations occur in both the primary and secondary structures of all craft and are identified during the design process by such means as finite element calculations. The designer will modify the detail to alleviate the stress concentration either by redesign or increase in scantlings. However, even after modification that area will still, in general, be exposed throughout the life of the craft to stresses higher than in surrounding areas.

1.2.2 At the design appraisal stage, a plan of the structure should, where appropriate, be prepared indicating by the Builder or designer these regions, and consideration can then be given, by the production team, into the appropriate methods of construction and the tolerances to be applied in order to remain within the assigned design parameters.

1.3.1 The very nature of steel/aluminium construction requires the assembly of a multitude of structural components into blocks within an assembly shop and then the erection of these blocks within a building dock or on a building berth. The welded interface between structural components in sub-assembly areas can be reasonably controlled; however, the welded connections between large prefabricated blocks in the building dock or on the building berth cannot be so easily controlled due to the sheer size of the blocks being handled.

1.3.2 The most critical type of joint is the welded cruciform joint where it is subjected to high magnitudes of tensile stress normal to the table member of the joint. The double bottom construction lends itself to the block construction. The interfaces between these blocks and those formed by the primary transverse structure may lie in areas of high stress. Critical cruciform joints are also found within the prefabricated blocks and also require close attention to alignment, but this is more easily achieved.

1.3.3 It can readily be seen that the combination of stress concentration and misalignment is to be avoided if the fatigue strength is to be satisfactory during the service life of the craft.

1.4.1 The bottom shell area of high speed craft is subjected to the highest cyclic loading throughout the life of such craft.

1.4.2 The fatigue fractures in bottom longitudinal end-connections of higher tensile steel and aluminium alloy has been well documented, and constructional details in way of these connections, designed to increase fatigue life, are now incorporated by many Builders as standard. It is, therefore, important that due consideration be given to this detail at the design stage to reduce the risk of fatigue cracking during service.

1.4.3 Detailed recommendations are given herein for the critical areas, see Ch 4, 2 Structural details.