Section
3 Forward of the forward cargo tank
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.
- 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 FE, 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.
- The deck plating thickness and supporting structure are to be
suitably reinforced in way of deck machinery and topside units.
3.3 Bottom structure
3.3.4
Bottom floors.
- 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.
- 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.
- 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.
- 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.
- 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.
3.4 Side structure
3.4.2
Side shell local support members.
- Longitudinal framing of the side shell is to be carried as far
forward as practicable.
- 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.(b) and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.(c).
- 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.
- 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.
- 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.
- Perforated flats are to be fitted to limit the effective span of
web frames to not greater than 10 m.
- 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:
- 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.
- 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 Table 3.2.6 Design load sets
for plating and local support members (see continuation).
- Configurations with multiple cross ties are to be
specially considered, in accordance with Pt 10, Ch 3, 3.4 Side structure 3.4.3.(d).(iv).
- 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.
- 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
3.5.3
Deck primary support structure.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- The scantlings of pillars are to comply with the requirements
in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.4.
- Where the loads from heavy equipment exceed the design load of
Pt 10, Ch 3, 3.11 Scantling requirements 3.11.4, the pillar scantlings are to be determined based on the
actual loading.
3.6 Tank bulkheads
3.6.1
General.
- 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.
- 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.
- The thickness of tank boundary plating is to comply with the
requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.(a).
- 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.(b) and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.(c).
- 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.
- 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.
- 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.
- Watertight boundaries are to be fitted in accordance with Pt 4, Ch 3, 5 Number and disposition of bulkheads.
- 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.
- The thickness of boundary plating is to comply with
the requirements in Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.(a).
- 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.(b) and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.2.(c).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Pillar bulkheads are to comply with the following
requirements:
- the distance between bulkhead stiffeners is not to
exceed 1500 mm;
- where corrugated, the depth of the corrugation is not
to be less than 100 mm.
3.11 Scantling requirements
3.11.2
Plating and local support members.
- For plating subjected to lateral pressure,
the net plating thickness, tnet
, is to comply with the requirements of Table 3.2.3 Thickness
requirements for plating, where Ca
is taken as given in Table 3.3.2 Permissible
bending stress coefficient for plating.
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
|
- For stiffeners subjected to lateral
pressure, the net section modulus, Znet
, is to comply with the requirements of Table 3.2.4 Section modulus
requirements for stiffeners, where Cs
is taken as given in Table 3.3.3 Permissible
bending stress coefficient for stiffeners.
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
|
- For stiffeners subjected to lateral
pressure, the net web thickness based on shear area requirements,
tw-net
, is to comply with the requirements of Table 3.2.5 Web thickness
requirements for stiffeners where Ct
is taken as given in Table 3.3.4 Permissible shear
stress coefficient for stiffeners.
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.
- 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.(d).
- 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.(d) for all applicable design load sets in Table 3.2.6 Design load sets
for plating and local support members (see continuation).
- 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.(e) for all applicable design load sets in Table 3.2.6 Design load sets
for plating and local support members (see continuation).
- 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.(b) and Pt 10, Ch 3, 3.11 Scantling requirements 3.11.3.(c) when subjected to the applicable design load sets.
3.11.5
Pillars.
- 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 Table 3.2.6 Design load sets
for plating and local support members (see continuation), 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. |
|