LR V.6 Double bottom construction
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Clasifications Register Rules and Regulations - Rules and Regulations for the Construction and Classification of Ships for the Carriage of Liquid Chemicals in Bulk, July 2022 - LR V Structural Arrangements and Scantlings - LR V.6 Double bottom construction

LR V.6 Double bottom construction

Parent topic: LR V Structural Arrangements and Scantlings

LR V.6.1 General

LR V.6.1.1 The requirements of this Section are to be considered additional to those of LR V.7 Cargo tank boundaries and Pt 4, Ch 9, 3.3 Loading conditions*.

LR V.6.1.2 The depth of the double bottom is to be not less than as required by Ch 1, 2.6 Location of cargo tanks and is also to be not less than as required by Pt 4, Ch 9, 3.3 Loading conditions*. For high relative density cargoes, see also LR V.6.5 High relative density cargoes of this Section.

LR V.6.1.3 Due regard is to be paid to the depth of double bottom necessary to accommodate suitably sized access openings, and maintain the required standard of strength.

LR V.6.1.4 Arrangements are to be provided to enable double bottom tanks situated below cargo tanks to be filled with water ballast to assist in gas-freeing these tanks.

LR V.6.1.5 Side girders are to be arranged in way of longitudinal bulkheads.

LR V.6.1.6 Transverse continuity of inner bottom structure is to be maintained in way of longitudinal bulkheads. Where a double bottom is fitted in centre tanks only, the inner bottom is to be suitably scarphed into the wing tank structure.

LR V.6.2 Symbols

LR V.6.2.1 The symbols used in this Section are defined as follows:

h = vertical distance from the mid-point of the span to the highest point of the tank, excluding the hatchway, in metres
S = span of longitudinals, in metres, measured between span points as shown in Pt 3, Ch 3, 3 Structural idealisation*, but not to be taken less than 1,5 m.

LR V.6.2.2 Other symbols are defined in LR V.1.2 General definitions and symbols.

LR V.6.3 Inner bottom longitudinals

LR V.6.3.1 The section modulus of inner bottom longitudinals in ships fitted with double bottoms under the cargo tanks is to be not less than the greater of the following:

  1. Pt 4, Ch 9, 6 Inner hull, inner bottom and longitudinal oiltight bulkheads*.

  2. Z = 0,0081ksS 2(ρ(h+ 0, 9) + 10 (p V-0,2)) cm3.

LR V.6.4 Struts

LR V.6.4.1 Where struts are fitted, their scantlings are to be not less than as required by Pt 4, Ch 1, 8.5 Floors*. In addition, the axial stress in the strut and the shear stress in lapped end connections are not to exceed 108 N/mm2 (11,0 kgf/mm2) and 83,5 N/mm2 (8,5 kgf/mm2) respectively.

LR V.6.5 High relative density cargoes

LR V.6.5.1 Where cargoes of relative density in excess of 1,025 are to be carried, the height of the double bottom may be required to be increased and, in addition, the floor scantlings and associated arrangements are to comply with the requirements of this sub-Section.

LR V.6.5.2 The shear stress in double bottom floors, using net area in way of openings, is not to exceed 83,5 N/mm2 (8,5 kgf/mm2) in association with the greater of the two following loading standards:

  1. Empty cargo tank with draught to deck edge.

  2. Full cargo tank taking account of cargo relative density with draught = or , whichever is the lesser.

LR V.6.5.3 Where floor stiffeners are lapped to bottom and inner bottom longitudinals, a shear stress of 83,5 N/mm2 (8,5 kgf/mm2) is not to be exceeded in the welded connection. Where the floor stiffeners are directly welded to bottom and inner bottom longitudinals, an axial stress of 108 N/mm2 (11,0 kgf/mm2) is not to be exceeded, except where approved deep penetration welding is used, in which case the axial stress is not to exceed 147 N/mm2 (15,0 kgf/mm2). For the purpose of this calculation, the following two loading standards are to be applied:

  1. Bottom longitudinal connections, draught to deck edge.

  2. Inner bottom longitudinal connections, full cargo tank, taking account of cargo relative density.

LR V.6.5.4 Where collars or lugs are fitted the proportion of loading taken by the floor stiffener will be considered on the basis of the design of the connection and the requirements of Pt 3, Ch 10, 5.2 Arrangements at intersections of continuous secondary and primary members*.

LR V.6.5.5 Openings are not to be cut in double bottom floor panels adjacent to longitudinal bulkheads. Within a distance of 25 per cent of the floor span measured from supports, the edges of floor openings are to be re-inforced. Where the ratio of floor depth to floor thickness exceeds 100, the buckling strength of floor panels is to be verified by direct calculation.

LR V.6.5.6 Double bottom construction involving the use of struts is not, in general, to be adopted where the cargo relative density exceeds 1,5. Where struts are proposed for cargoes having relative densities greater than 1,025, consideration will be required to be given to the strut and longitudinal scantlings, and the strut end connections may be required to be bracketed.

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