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Clasification Society Rules and Regulations - Rules and Regulations for the Classification of Offshore Units, January 2016 - Part 10 SHIP UNITS - Chapter 3 Scantling Requirements - Section 3 Forward of the forward cargo tank

Section 3 Forward of the forward cargo tank

Parent topic: Chapter 3 Scantling Requirements

3.1 Symbols

3.1.1 The symbols used in this Chapter are defined as follows:
L = Rule length, in metres
L2 = Rule length, L, but need not be taken greater than 300 m
B = moulded breadth, in metres
D = moulded depth, in metres
TSC = deep load draught, in metres
TLT = minimum design light load draught, in metres
E = modulus of elasticity, in N/mm2
σyd = specified minimum yield stress of the material, in N/mm2
τyd = N/mm2
s = stiffener spacing, in mm
p = design pressure for the design load set being considered, in kN/m2
g = acceleration due to gravity, 9,81 m/s2
k = higher strength steel factor, defined in Pt 10, Ch 1, 3.1 General 3.1.7.

3.2 General

3.2.1  Application.
  1. The requirements of this Section apply to structure forward of the forward end of the foremost cargo tank. Where the forward end of the foremost cargo tank is aft of 0,1L of the unit’s length, measured from the F.P., special consideration will be given to the applicability of these requirements and the requirements of Pt 10, Ch 3, 2 Cargo tank region.
3.2.2  General scantling requirements.
  1. The deck plating thickness and supporting structure are to be suitably reinforced in way of deck machinery and topside units.
3.2.3  Structural continuity.
  1. Scantlings of the shell envelope, upper deck and inner bottom are to be tapered towards the forward end. See also Pt 10, Ch 3, 1.6 Tapering and structural continuity of longitudinal hull girder elements.
  2. All shell frames and tank boundary stiffeners are to be continuous, or are to be bracketed at their ends.
3.2.4  Minimum thickness.
  1. In addition to the required scantlings given in this Section, the plating and stiffeners are to comply with the minimum thickness requirements for the cargo region given in Pt 10, Ch 3, 2.2 General 2.2.4 and Pt 10, Ch 3, 2.2 General 2.2.5, except as given in Pt 10, Ch 3, 3.2 General 3.2.4.

    Table 3.3.1 Minimum net thickness of structure forward of the forward cargo tank

    Scantling location Net thickness (mm)
    Pillar bulkheads 7,5
    Breasthooks 6,5
    Floors and bottom girders 5,5 + 0,02L2
    Web plating of primary support members 6,5 + 0,015L2

3.3 Bottom structure

3.3.1  Plate keel.
  1. A flat plate keel is to extend as far forward as practical and is to satisfy the scantling requirements given in Pt 10, Ch 3, 2.3 Hull envelope plating 2.3.1.
3.3.2  Bottom shell plating.
  1. The thickness of the bottom shell plating is to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
3.3.3  Bottom longitudinals.
  1. Bottom longitudinals are to be carried as far forward as practicable. Beyond this, suitably stiffened frames are to be fitted.
  2. The section modulus and thickness of the bottom longitudinals are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2 and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
3.3.4  Bottom floors.
  1. Bottom floors are to be fitted at each web frame location. The minimum depth of the floor at the centreline is not to be less than the depth of the floors within the cargo tank region.
3.3.5  Bottom girders.
  1. A supporting structure is to be provided at the centreline, either by extending the centreline girder to the stem or by providing a deep girder or centreline bulkhead.
  2. Where a centreline girder is fitted, the minimum depth and thickness is not to be less than that fitted in the cargo tank region, and the upper edge is to be stiffened. Where a centreline wash bulkhead is fitted, the lowest strake is to have thickness not less than required for a centreline girder.
3.3.6  Plate stems.
  1. Plate stems are to be supported by stringers and flats, and by intermediate breasthook diaphragms spaced not more than 1500 mm apart, measured along the stem. Where the stem radius is large, a centreline support structure is to be fitted.
  2. Between the minimum design light draught, TLT , at the stem and the deep load draught, TSC , the plate stem net thickness, tstem-net , is not to be less than:

    tstem-net = mm, but need not be taken as greater than 21 mm

    Above the deep load draught, the thickness of the stem plate may be tapered to the requirements for the shell plating at the upper deck.

    Below the minimum design light load draught, the thickness of the stem plate may be tapered to the requirements for the plate keel.

3.3.7  Floors and girders in spaces aft of the collision bulkhead.
  1. Floors and girders which are aft of the collision bulkhead and forward of the forward cargo tank are to comply with the requirements in Pt 10, Ch 3, 3.3 Bottom structure 3.3.4 and Pt 10, Ch 3, 3.3 Bottom structure 3.3.5 and are to comply with the shear area requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3.

3.4 Side structure

3.4.1  Side shell plating.
  1. The thickness of the side shell plating is to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2. Where applicable, the thickness of the side shell plating is to comply with the requirements in Pt 10, Ch 3, 2.3 Hull envelope plating 2.3.4.
  2. Where a forecastle is fitted, the side shell plating requirements are to be applied to the plating extending to the forecastle deck elevation.

3.4.2  Side shell local support members.

  1. Longitudinal framing of the side shell is to be carried as far forward as practicable.
  2. The section modulus and thickness of the hull envelope framing are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2 and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
  3. End connections of longitudinals at transverse bulkheads are to provide adequate fixity, lateral support, and, where not continuous, are to be provided with soft-nosed brackets. Brackets lapped onto the longitudinals are not to be used.
3.4.3  Side shell primary support structure.
  1. In general, the spacing of web frames, S, is to be taken as

    S = 2,6 + 0,005L2 m, but not to be taken greater than 3,5 m.

  2. In general, for the transverse framing forward of the collision bulkhead, stringers are to be spaced approximately 3,5 m apart. Stringers are to have an effective span not greater than 10 m, and are to be adequately supported by web frame structures. Aft of the collision bulkhead, where transverse framing is adopted, the spacing of stringers may be increased.
  3. Perforated flats are to be fitted to limit the effective span of web frames to not greater than 10 m.
  4. The scantlings of web frames supporting longitudinal frames, and stringers and/or web frames supporting transverse frames in the forward region are to be determined from Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3, with the following additional requirements:
    1. Where no cross ties are fitted:
      • the required section modulus of the web frame is to be maintained for 60 per cent of the effective span for bending, measured from the lower end. The value of the bending moment used for calculation of the required section modulus of the remainder of the web frame may be appropriately reduced, but not greater than 20 per cent;
      • the required shear area of the lower part of the web frame is to be maintained for 60 per cent of the shear span measured from the lower end.
    2. Where one cross tie is fitted:
      • the effective spans for bending and shear of a web frame or stringer are to be taken, ignoring the presence of the cross tie. The shear forces and bending moments may be reduced to 50 per cent of the values that are calculated, ignoring the presence of the cross tie. For a web frame, the required section modulus and shear area of the lower part of the web frame are to be maintained up to the cross tie, and the required section modulus and shear area of the upper part of the web frame are to be maintained for the section above the cross tie;
      • cross ties are to be designed using the design loads specified in Pt 10, Ch 3, 2.6 Bulkheads 2.6.7.
    3. Configurations with multiple cross ties are to be specially considered, in accordance with Pt 10, Ch 3, 3.4 Side structure 3.4.3.
    4. Where complex grillage structures are employed, the suitability of the scantlings of the primary support members is to be determined by more advanced calculation methods.
  5. The web depth of primary support members is not to be less than 14 per cent of the bending span and is to be at least 2,5 times as deep as the slots for stiffeners if the slots are not closed.

3.5 Deck structure

3.5.1  Deck plating.
  1. The thickness of the deck plating is to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2 with the applicable lateral pressure, green sea and deck loads.
3.5.2  Deck stiffeners.
  1. The section modulus and thickness of deck stiffeners are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2 and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2, with the applicable lateral pressure, green sea and deck loads.
3.5.3  Deck primary support structure.
  1. The section modulus and shear area of primary support members are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3.
  2. The web depth of primary support members is not to be less than 10 per cent and 7 per cent of the unsupported span in bending in tanks and in dry spaces, respectively, and is not to be less than 2,5 times the depth of the slots if the slots are not closed. In the case of a grillage structure, the unsupported span is the distance between connections to other primary support members.
  3. In way of concentrated loads from heavy equipment, the scantlings of the deck structure are to be determined based on the actual loading.
3.5.4  Pillars.
  1. Pillars are to be fitted in the same vertical line wherever possible and effective arrangements are to be made to distribute the load at the heads and heels of all pillars. Where pillars support eccentric loads, they are to be strengthened for the additional bending moment imposed upon them.
  2. Tubular and hollow square pillars are to be attached at their heads and heels by efficient brackets or doublers/ insert plates, where applicable, to transmit the load effectively. Pillars are to be attached at their heads and heels by continuous welding. At the heads and heels of pillars built of rolled sections, the load is to be distributed by brackets or other equivalent means.
  3. Pillars in tanks are to be of solid section. Where the hydrostatic pressure may result in tensile stresses in the pillar, the tensile stress in the pillar and its end connections is not to exceed 45 per cent of the specified minimum yield stress of the material.
  4. The scantlings of pillars are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.5.
  5. Where the loads from heavy equipment exceed the design load of Pt 10, Ch 3, 3.11 Scantling requirements 3.11.5, the pillar scantlings are to be determined based on the actual loading.

3.6 Tank bulkheads

3.6.1  General.
  1. Tanks may be required to have divisions or deep wash plates in order to minimise the dynamic stress on the structure.
3.6.2  Construction.
  1. In no case are the scantlings of tank boundary bulkheads to be less than the requirements for watertight bulkheads.
3.6.3  Scantlings of tank boundary bulkheads.
  1. The thickness of tank boundary plating is to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
  2. The section modulus and thickness of stiffeners are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2 and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
  3. The section modulus and shear area of primary support members are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3.
  4. Web plating of primary support members is to have a depth of not less than 14 per cent of the unsupported span in bending, and is not to be less than 2,5 times the depth of the slots if the slots are not closed.
  5. Scantlings of corrugated bulkheads are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.4.

3.7 Watertight boundaries

3.7.1  General.
  1. Watertight boundaries are to be fitted in accordance with Pt 4, Ch 3, 5 Number and disposition of bulkheads.
  2. The number of openings in watertight bulkheads is to be kept to a minimum, compatible with the design and operation of the ship unit. Where penetrations of watertight bulkheads and internal decks are necessary for access, piping, ventilation, electrical cables, etc. arrangements are to be made to maintain the watertight integrity.
3.7.2  Scantlings of watertight boundaries.
  1. The thickness of boundary plating is to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
  2. The section modulus and thickness of stiffeners are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2 and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.
  3. The section modulus and shear area of primary support members are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3.
  4. Web plating of primary support members is to have a depth of not less than 10 per cent of the unsupported span in bending, and is not to be less than 2,5 times the depth of the slots if the slots are not closed.
  5. Scantlings of corrugated bulkheads are to comply with the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.4.

3.8 Superstructure

3.8.1  Forecastle structure.
  1. Forecastle structures are to be supported by girders with deep beams and web frames, and, in general, arranged in complete transverse belts and supported by lines of pillars extending down into the structure below. Deep beams and girders are to be arranged, where practicable, to limit the spacing between deep beams, web frames, and/or girders to about 3,5 m. Pillars are to be provided as required by Pt 10, Ch 3, 3.5 Deck structure 3.5.4. Main structural intersections are to be carefully developed, with special attention given to pillar head and heel connections, and to the avoidance of stress concentrations.

3.9 Mooring systems

3.9.1  Supporting structure.
  1. Where the structure is subjected to concentrated mooring loads from mooring arms or yokes, external turrets or mooring hawsers, etc. the scantlings and arrangements are to be specially considered. Finite element analysis of attachments to the hull is to be carried out to ensure satisfactory stress distribution of the mooring loads into the hull structure. The permissible local stress levels are to comply with the LR ShipRight Procedure for Ship Units and Pt 4, Ch 5 Primary Hull Strength, as applicable.

3.10 Miscellaneous structures

3.10.1  Pillar bulkheads.
  1. Bulkheads that support girders, or pillars and longitudinal bulkheads which are fitted in lieu of girders are to be stiffened to provide supports no less effective than required for stanchions or pillars. The acting load and the required net cross-sectional area of the pillar section are to be determined using the requirements of Pt 10, Ch 3, 3.5 Deck structure 3.5.4. The net moment of inertia of the stiffener is to be calculated with a width of 40tnet , where tnet is the net thickness of plating, in mm.
  2. Pillar bulkheads are to comply with the following requirements:
    1. the distance between bulkhead stiffeners is not to exceed 1500 mm;
    2. where corrugated, the depth of the corrugation is not to be less than 100 mm.

3.11 Scantling requirements

3.11.1  General.
  1. The design load sets are to be applied to the structural requirements for the local support and primary support members, as given in Pt 10, Ch 3, 2.6 Bulkheads 2.6.7. The static and dynamic load components are to be combined in accordance with Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7 and the procedure given in Pt 10, Ch 2 Loads and Load Combinations.
3.11.2  Plating and local support members.
  1. For plating subjected to lateral pressure, the net plating thickness, tnet , is to comply with the requirements of Pt 10, Ch 3, 2.3 Hull envelope plating 2.3.2, where Ca is taken as given in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.

    Table 3.3.2 Permissible bending stress coefficient for plating

    Acceptance criteria set Structural member Ca
    AC1 All plating 0,80
    AC2 Hull envelope plating 0,95
    Internal boundary plating 1,00
    AC3 All plating 1,0
  2. For stiffeners subjected to lateral pressure, the net section modulus, Znet , is to comply with the requirements of Pt 10, Ch 3, 2.4 Hull envelope framing 2.4.2, where Cs is taken as given in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.

    Table 3.3.3 Permissible bending stress coefficient for stiffeners

    Acceptance criteria set Structural member Cs
    AC1 All stiffeners 0,75
    AC2 All stiffeners 0,90
    AC3 All stiffeners 1,0
  3. For stiffeners subjected to lateral pressure, the net web thickness based on shear area requirements, tw-net , is to comply with the requirements of Pt 10, Ch 3, 2.4 Hull envelope framing 2.4.2 where Ct is taken as given in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.

    Table 3.3.4 Permissible shear stress coefficient for stiffeners

    Acceptance criteria set Structural member Ct
    AC1 All stiffeners 0,75
    AC2 All stiffeners 0,90
    AC3 All stiffeners 1,0
3.11.3  Primary support members.
  1. For primary support members intersecting with or in way of curved hull sections, the effectiveness of end brackets is to include allowance for the curvature of the hull. For side transverse frames, the requirements may be reduced due to the presence of cross ties, see Pt 10, Ch 3, 3.4 Side structure 3.4.3.
  2. For primary support members subjected to lateral pressure, the net section modulus, Znet50 , is to comply with Pt 10, Ch 3, 7.3 Scantling requirements 7.3.3 for all applicable design load sets in Pt 10, Ch 3, 2.6 Bulkheads 2.6.7.
  3. For primary support members subjected to lateral pressure, the effective net shear area, Ashr-net50 , is to comply with Pt 10, Ch 3, 7.3 Scantling requirements 7.3.3 for all applicable design load sets in Pt 10, Ch 3, 2.6 Bulkheads 2.6.7.
  4. Primary support members are generally to be analysed with the specific methods described for the particular structure type. More advanced calculation methods may be necessary to ensure that nominal stress levels for all primary support members are less than the permissible stresses and stress coefficients given in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3 and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3 when subjected to the applicable design load sets.
3.11.4  Corrugated bulkheads.
  1. Special consideration will be given to the approval of corrugated bulkheads, where fitted.

    NOTE

    Scantling requirements of corrugated bulkheads in the cargo tank region may be used as a basis, see Pt 10, Ch 3, 2.6 Bulkheads 2.6.6 and Pt 10, Ch 3, 2.6 Bulkheads 2.6.7.

3.11.5  Pillars.
  1. The maximum load on a pillar, Wpill , is to be taken as the greatest value calculated for all applicable design load sets, as given in Pt 10, Ch 3, 2.6 Bulkheads 2.6.7, and is to be less than or equal to the permissible pillar load as given by the following equation, where Wpill-perm is based on the net properties of the pillar:

    Wpill ≤ Wpill-perm

    where

    Wpill = applied axial load on pillar
    = P ba-sup la-sup + Wpill–upr kN
    Wpill-perm = permissible load on a pillar
    = 0,1Apill-net50 ηpill σcrb kN
    ba-sup = mean breadth of area supported, in metres
    la-sup = mean length of area supported, in metres
    Wpill–upr = axial load from pillar or pillars above, in kN
    Apill-net50 = net cross-sectional area of the pillar, in cm2
    ηpill = utilisation factor for the design load set being considered:
    = 0,5 for acceptance criteria set AC1
    = 0,6 for acceptance criteria set AC2
    = 0,6 for acceptance criteria set AC3
    σcrb = critical buckling stress in compression of pillar based on the net sectional properties, in N/mm2.
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