Clasification Society Rulefinder 2016 - Version 9.25
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 6 Evaluation of structure for sloshing and impact loads

Section 6 Evaluation of structure for sloshing and impact loads

Parent topic: Chapter 3 Scantling Requirements

6.1 Symbols

6.1.1 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
Cb = block coefficient
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 N/mm2

6.2 General

6.2.1  Application.
  1. The requirements of this Section cover the additional strengthening requirements for localised sloshing loads that may occur in tanks carrying liquid and local impact loads in the forward and aft structure. The impact loads to be applied in Pt 10, Ch 3, 6.4 Bottom slamming are described in Pt 10, Ch 2, 4 Sloshing and impact loads.

6.3 Sloshing in tanks

6.3.1  Scope and limitations.
  1. The requirements of the LR ShipRight Procedure for Ship Units specify the methodology in assessing the scantling requirements for boundary and internal structure of tanks subject to sloshing loads, due to the free movement of liquid in tanks.
  2. The structure of cargo tanks, slop tanks, ballast tanks and large deep tanks, e.g. fuel oil bunkering tanks and main fresh water tanks, is to be assessed for sloshing. Small tanks do not need to be assessed for sloshing pressures.
  3. All cargo and ballast tanks are to have scantlings suitable for unrestricted filling heights.
  4. The following structural members are to be assessed:
    1. plates and stiffeners forming boundaries of tanks;
    2. plates and stiffeners on wash bulkheads;
    3. web plates and web stiffeners of primary support members located in tanks;
    4. tripping brackets supporting primary support members in tanks.

6.4 Bottom slamming

6.4.1  Application.
  1. Where the minimum draughts forward, TFP-mt or TFP-full , as specified in Pt 10, Ch 2 Loads and Load Combinations, is less than 0,045L, the bottom forward is to be additionally strengthened to resist bottom slamming pressures.
  2. For self-propelling units with conventional single screw, ship-type aft sections, additional strengthening against aft slamming will not normally be required. For units with full deep aft sections, strengthening to resist bottom slamming should be applied over 0,3L aft, using the requirements of Pt 10, Ch 3, 6.4 Bottom slamming 6.4.3 and Pt 10, Ch 3, 6.4 Bottom slamming 6.4.4 and the applicable draughts aft. Units with raised or unusual sections aft that may be susceptible to slamming will be specially considered, using the requirements of Pt 4, Ch 2, 4.3 Strengthening for wave impact loads and Pt 4, Ch 2, 5.2 Strengthening for wave impact loadsof the Rules for Ships.
  3. The draughts for which the bottom has been strengthened are to be indicated on the shell expansion plan and loading guidance information, see Pt 10, Ch 3, 1.2 Loading guidance.
  4. The section modulus and web thickness of the local support members apply to the areas clear of the end brackets. The cross-sectional shear areas of primary support members are to be applied as required by Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7 and Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7.
  5. For harsh service, special consideration should be given to strengthening of bottom forward in relation to the actual forces determined from model tests and/or direct calculations.
6.4.2  Extent of strengthening.
  1. The strengthening is to extend forward of 0,3L from the F.P. over the flat of bottom and adjacent plating with attached stiffeners up to a height of 500 mm above the baseline, see Pt 10, Ch 3, 6.4 Bottom slamming 6.4.2.

    Figure 3.6.1 Extent of strengthening against bottom slamming

  2. Outside the region strengthened to resist bottom slamming, the scantlings are to be tapered to maintain continuity of longitudinal and/or transverse strength.
6.4.3  Design to resist bottom slamming loads.
  1. The design of end connections of stiffeners in the bottom slamming region is to ensure end fixity, either by making the stiffeners continuous through supports or by providing end brackets. Where it is not practical to comply with this requirement, the net plastic section modulus, Zpl-alt-net , for alternative end fixity arrangements is not to be less than:
    Zpl-alt-net = cm3

    where

    Zpl-net = net plastic section modulus, in cm3, as required by Pt 10, Ch 3, 6.4 Bottom slamming 6.4.5
    fbdg = bending moment factor
    =
    ns = 0 for both ends with low end fixity (simply supported)
    = 1 for one end equivalent to built in and one end simply supported.
  2. Scantlings and arrangements at primary support members, including bulkheads, are to comply with Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7.
6.4.4  Hull envelope plating.
  1. The net thickness of the hull envelope plating, tnet , is not to be less than:
    tnet = mm

    where

    αp = correction factor for the panel aspect ratio
    = but not to be taken as greater than 1,0
    lp = length of plate panel, to be taken as the spacing between primary support members or panel breakers, in metres
    Pslm = bottom slamming pressure as given in Pt 10, Ch 2, 4.2 Bottom slamming loads 4.2.2 and calculated at the load calculation point, in kN/m2
    Cd = plate capacity correction coefficient
    = 1,3
    Ca = permissible bending stress coefficient
    = 1,0 for acceptance criteria set AC3.
6.4.5  Hull envelope stiffeners.
  1. The net plastic section modulus, Zpl-net , of each individual stiffener, is not to be less than:
    Zpl-net = cm3

    where

    lbdg = effective bending span, in metres
    fbdg = bending moment factor
    =
    ns = 2,0 for continuous stiffeners or where stiffeners are bracketed at both ends, see Pt 10, Ch 3, 6.4 Bottom slamming 6.4.3 for alternative arrangements
    Cs = permissible bending stress coefficient
    = 0,9 for acceptance criteria set AC3.
  2. The net web thickness, tw-net , of each longitudinal is not to be less than:
    tw-net = mm

    where

    lshr = effective shear span, in metres
    dshr = effective web depth of stiffener, in mm
    Ct = permissible shear stress coefficient
    = 1,0 for acceptance criteria set AC3.
6.4.6  Definition of idealised bottom slamming load area for primary support members.
  1. The scantlings of items in Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7 are based on the application of the slamming pressure defined in Pt 10, Ch 2 Loads and Load Combinations to an idealised area of hull envelope plating, the slamming load area, Aslm , given by:
    Aslm = m2
6.4.7  Primary support members.
  1. The size and number of openings in web plating of the floors and girders are to be minimised, considering the required shear area as given in Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7.
  2. The net shear area, Ashr-net50 , of each primary support member web at any position along its span is not to be less than:
    Ashr-net50 = cm2

    where

    Qslm = the greatest shear force due to slamming for the position being considered, in kN, based on the application of a patch load, Fslm , to the most onerous location, as determined in accordance with Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7
    Ct = permissible shear stress coefficient
    = 0,9 for acceptance criteria set AC3.
  3. For simple arrangements of primary support members, where the grillage effect may be ignored, the shear force, Qslm , is given by:
    Qslm = fpt fdist Fslm kN

    where

    fpt = correction factor for the proportion of patch load acting on a single primary support member
    = 0,5 (fslm 3 – 2fslm 2 + 2)
    fslm = patch load modification factor
    = , but not to be greater than 1,0
    fdist = factor for the greatest shear force distribution along the span, see Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7
    Fslm = Pslm lslm bslm
    lslm = extent of slamming load area along the span
    = m, but not to be greater than lshr
    lshr = effective shear span, in metres
    bslm = breadth of impact area supported by primary support member
    = m, but not to be greater than S
    S = primary support member spacing, in metres.

    Figure 3.6.2 Distribution of fdist along the span of simple primary support members

  4. For complex arrangements of primary support members, the greatest shear force, Qslm , at any location along the span of each primary support member is to be derived by direct calculation in accordance with Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7.

    Table 3.6.1 Direct calculation methods for derivation of Qslm

    Type of analysis Beam theory Double bottom grillage
    Model extent Overall span of member between effective bending supports

    Longitudinal extent to be one cargo tank length

    Transverse extent to be between inner hopper knuckle and centreline

    Assumed end fixity of floors Fixed at ends Floors and girders to be fixed at boundaries of the model
    NOTE

    The envelope of greatest shear force along each primary support member is to be derived by applying the load patch to a number of locations along the span, see Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7.

  5. The net web thickness, tw-net , of primary support members adjacent to the shell is not to be less than:
    tw-net = mm

    where

    sw = plate breadth, in mm, taken as the spacing between the web stiffening.
6.4.8  Connection of longitudinals to primary support members.
  1. Longitudinals are, in general, to be continuous. Where this is not practicable, end brackets are to be provided.
  2. The scantlings in way of the end connections of each longitudinal are to comply with the requirements of Pt 10, Ch 3, 1.10 Intersections of continuous local support members and primary support members.

6.5 Bow impact

6.5.1  Application.
  1. The side structure in the area forward of 0,1L from the FP is to be strengthened against bow impact pressures.
  2. The section modulus and web thickness of the local support members apply to the areas clear of the end brackets. The section modulus of the primary support member is to apply along the bending span clear of end brackets and cross-sectional areas of the primary support member are to be applied at the ends/supports and may be gradually reduced along the span and clear of the ends/supports following the distribution of fdist indicated in Pt 10, Ch 3, 6.4 Bottom slamming 6.4.7.
6.5.2  Extent of strengthening.
  1. The strengthening is to extend forward of 0,1L from the FP and vertically above the minimum design light load draught, TLT , see Pt 10, Ch 3, 6.5 Bow impact 6.5.2.

    Figure 3.6.3 Extent of strengthening against bow impact

  2. Outside the strengthening region, as given in Pt 10, Ch 3, 6.5 Bow impact 6.5.2, the scantlings are to be tapered to maintain continuity of longitudinal and/or transverse strength.
6.5.3  Design to resist bow impact loads.
  1. In the bow impact region, longitudinal framing is to be carried as far forward as practicable.
  2. The design of end connections of stiffeners in the bow impact region are to ensure end fixity, either by making the stiffeners continuous through supports or by providing end brackets. Where it is not practical to comply with this requirement, the net plastic section modulus, Zpl-alt-net , for alternative end fixity arrangements is not to be less than:
    Zpl-alt-net = cm3

    where

    Zpl-net = effective net plastic section modulus, required by Pt 10, Ch 3, 6.5 Bow impact 6.5.5, in cm3
    fbdg = bending moment factor
    =
    ns = 0 for both ends with low end fixity (simply supported)
    = 1,0 for one end equivalent to built-in and one end simply supported.
  3. Scantlings and arrangements at primary support members, including decks and bulkheads, are to comply with Pt 10, Ch 3, 6.5 Bow impact 6.5.7. In areas of greatest bow impact load, the adoption of web stiffeners arranged perpendicular to the hull envelope plating and the provision of double sided lug connections are, in general, to be applied.
  4. The main stiffening direction of decks and bulkheads supporting shell framing is to be arranged parallel to the span direction of the supported shell frames, to protect against buckling.
6.5.4  Side shell plating.
  1. The net thickness of the side shell plating, tnet , is not to be less than:
    tnet =

    where

    αp = correction factor for the panel aspect ratio
    = but is not to be taken as greater than 1,0
    lp = length of plate panel, to be taken as the spacing between the primary support members, or panel breakers, in metres
    Pim = bow impact pressure as given in Pt 10, Ch 2, 4.3 Bow impact loads 4.3.2 and calculated at the load calculation point, in kN/m2
    Ca = permissible bending stress coefficient
    = 1,0 for acceptance criteria set AC3.
6.5.5  Side shell stiffeners.
  1. The effective net plastic section modulus, Zpl-net , of each stiffener, in association with the effective plating to which it is attached, is not to be less than:
    Zpl-net = cm3

    where

    llbdg = effective bending span, in metres
    fbdg = bending moment factor
    =
    ns = 2,0 for continuous stiffeners or where stiffeners are bracketed at both ends, see Pt 10, Ch 3, 6.4 Bottom slamming 6.4.3 for alternative arrangements
    Cs = permissible bending stress coefficient
    = 0,9 for acceptance criteria set AC3.
  2. The net web thickness, tw-net , of each stiffener is not to be less than:
    tw-net = mm

    where

    lshr = effective shear span, in metres
    dshr = effective web depth of stiffener, in mm
    = permissible shear stress coefficient
    = 1,0 for acceptance criteria set AC3.
  3. The minimum net thickness of breasthooks/ diaphragm plates, tw-net , is not to be less than:
    tw-net = mm

    where

    s = spacing of stiffeners on the web, in mm. Where no stiffeners are fitted, s is to be taken as the depth of the web.
6.5.6  Definition of idealised bow impact load area for primary support members.
  1. The scantlings of items in Pt 10, Ch 3, 6.5 Bow impact 6.5.7 are based on the application of the bow impact pressure to an idealised area of hull envelope plating, where the bow impact load area, Aslm , is given by:
    Aslm = m2
6.5.7  Primary support members.
  1. Primary support members in the bow impact region are to be configured to ensure effective continuity of strength and the avoidance of hard spots.
  2. To limit the deflections under extreme bow impact loads and ensure boundary constraint for plate panels, the spacing, S, measured along the shell girth of web frames supporting longitudinal framing or stringers supporting transverse framing is not to be greater than:
    S = 3 + 0,008L2 m.
  3. End brackets of primary support members are to be suitably stiffened along their edge. Consideration is to be given to the design of bracket toes to minimise abrupt changes of cross-section.
  4. Tripping brackets are to be fitted where the primary support member flange is knuckled or curved. The torsional buckling mode of primary support members is to be controlled by flange supports or tripping brackets. The un - supported length of the flange of the primary support member, i.e. the distance between tripping brackets, sbkt , is not to be greater than:
    sbkt = m, but need not be less than sbkt-min

    where

    bf = breadth of flange, in mm
    C = slenderness coefficient:
    = 0,022 for symmetrical flanges
    = 0,033 for one-sided flanges
    Af-net50 = net cross-sectional area of flange, in cm2
    Aw-net50 = net cross-sectional area of the web plate, in cm2
    sbkt-min = 4,0 m.
  5. The net section modulus of each primary support member, Znet50 , is not to be less than:
    Znet50 = cm3

    where

    fbdg-pt = correction factor for the bending moment at the ends and considering the patch load
    = 3fslm 3 – 8fslm 2 + 6fslm
    fslm = patch load modification factor
    =
    lslm = extent of bow impact load area along the span
    = m, but not to be taken as greater than lbdg
    Aslm = bow impact load area, in m2, as defined in Pt 10, Ch 3, 6.4 Bottom slamming 6.4.6
    lbdg = effective bending span, in metres
    bslm = breadth of impact load area supported by the primary support member, to be taken as the spacing between primary support members, but not to be taken as greater than lslm , in metres
    fbdg = bending moment factor
    = 12 for primary support members with end fixed continuous face-plates, stiffeners or where stiffeners are bracketed at both ends
    Cs = permissible bending stress coefficient
    = 0,8 for acceptance criteria set AC3.
  6. The net shear area of the web, Ashr-net50 , of each primary support member at the support/toe of end brackets is not to be less than:
    Ashr-net50 = cm2

    where

    fpt = patch load modification factor
    =
    lslm = extent of bow impact load area along the span
    = m,

    but not to be taken as greater than lshr

    lshr = effective shear span, in metres
    bslm = breadth of impact load area supported by the primary support member, to be taken as the spacing between primary support members, but not greater than lslm , in metres
    Ct = permissible shear stress coefficient
    = 0,75 for acceptance criteria set AC3.
  7. The net web thickness of each primary support member, tw-net , including decks/bulkheads in way of the side shell, is not to be less than:
    tw-net = mm

    where

    bslm = breadth of impact load area supported by the primary support member, to be taken as the spacing between primary support members, but not greater than lslm , in metres
    ϕw = angle, in degrees, between the primary support member web and the shell plate
    σcrb = critical buckling stress in compression of the web of the primary support member or deck/bulkhead panel in way of the applied load, in N/mm2.
6.5.8  Connection of stiffeners to primary support members.
  1. Stiffeners are, in general, to be continuous. Where this is not practicable, end brackets are to be provided.
  2. The scantlings of the end connection of each stiffener are to comply with the requirements of Pt 10, Ch 3, 1.10 Intersections of continuous local support members and primary support members.
Copyright 2016 Clasification Society, International Maritime Organization, International Labour Organization or Maritime and Coastguard Agency. All rights reserved. Clasification Society, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as 'Clasification Society'. Clasification Society Register assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Clasification Society entity for the provision of this information or advice and in that case any responsibility or liability is exclusively on the terms and conditions set out in that contract.