Section
12 Steel hatch covers
12.1 General
12.1.1 These
requirements apply to hatch covers on exposed decks in Position 1, see
Pt 3, Ch 1, 6.6 Position 1 and Position 2 6.6.1,
and are in addition to the following requirements:
-
Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.14, Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.16, Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.17, Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.25, Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.27, and Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.29.
-
Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.5 for the vertical
weather pressure load case and cargo load, if carried on the hatch
covers.
Note When cargo is carried on the hatch covers, Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.8 to Pt 3, Ch 11, 4.2 Steel covers − Clamped and gasketed 4.2.10 are also to be complied
with. Cargo loads are to be in accordance with Pt 3, Ch 11, 2.3 Load model 2.3.4 and Pt 3, Ch 11, 2.3 Load model 2.3.5. Pt 4, Ch 8, 11.2 Direct calculations is to be considered for compliance with Pt 3, Ch 11, 2.4 Allowable stress and deflection 2.4.1, Pt 3, Ch 11, 2.4 Allowable stress and deflection 2.4.2, Pt 3, Ch 11, 2.5 Local net plate thickness 2.5.1, Pt 3, Ch 11, 2.8 Net scantling of secondary stiffeners 2.8.1, Pt 3, Ch 11, 2.8 Net scantling of secondary stiffeners 2.8.4, Pt 3, Ch 11, 2.8 Net scantling of secondary stiffeners 2.8.5, Pt 3, Ch 11, 2.9 Net scantling of primary supporting members 2.9.1 and Pt 3, Ch 11, 2.9 Net scantling of primary supporting members 2.9.2. The vertical weather design
load needs not to be combined with the cargo load.
-
For hatch covers
subject to wheel loading or helicopter landing, Pt 3, Ch 9, 3 Decks loaded by wheeled vehicles and Pt 3, Ch 9, 5 Helicopter landing areas are to be complied with.
12.1.2 The
net plate thickness, t
net, is the calculated
minimum thickness of the plating and stiffeners. The required thickness
is the net thickness plus a corrosion addition, t
c,
given in Table 7.12.1 Corrosion addition t
c
.
Table 7.12.1 Corrosion addition t
c
| Hatch cover type
|
t
c, in mm
|
| (a)
|
Single skin
|
2,0
|
| (b)
|
Pontoon (double skin)
|
|
|
|
(i)
|
for the top and bottom plating
|
2,0
|
|
|
(ii)
|
for the internal structures
|
1,5
|
12.1.3 Material
for the hatch covers is to be steel according to the requirements
for ship’s hull.
12.2 Stiffener arrangement
12.2.1 The
secondary stiffeners and primary supporting members of the hatch covers
are to be continuous over the breadth and length of the hatch covers,
as far as practical. When this is impractical, sniped end connections
are not to be used and appropriate arrangements are to be adopted
to ensure sufficient load carrying capacity.
12.2.2 The
spacing of primary supporting members parallel to the direction of
secondary stiffeners is not to exceed  of the span of primary supporting members.
12.3 Closing arrangements
12.3.1 Panel
hatch covers are to be secured by appropriate devices (bolts, wedges
or similar) suitably spaced alongside the coamings and between cover
elements.
12.3.2 Arrangement
and spacing are to be determined with due attention to the effectiveness
for weather-tightness, depending upon the type and the size of the
hatch cover, as well as on the stiffness of the cover edges between
the securing devices.
12.3.3 The
net sectional area of each securing device is not to be less than:
where
|
a
|
= |
spacing
in m of securing devices, not being taken less than 2 m |
|
f
|
= |
(σY/235)e
|
|
σ
Y
|
= |
specified minimum upper yield stress in N/mm2 of
the steel used for fabrication, not to be taken greater than 70 per
cent of the ultimate tensile strength
|
|
e
|
= |
0,75
for σY > 235
|
|
|
= |
1,0 for σY ≤
235
|
12.3.4 Rods
or bolts are to have a net diameter not less than 19 mm for hatchways
exceeding 5 m2 in area.
12.3.5 Between
cover and coaming and at cross-joints, a packing line force sufficient
to obtain weathertightness is to be maintained by the securing devices.
For packing line forces exceeding 5 N/mm, the cross section area is
to be increased in direct proportion. The packing line force is to
be specified.
12.3.6 The
cover edge stiffness is to be sufficient to maintain adequate sealing
pressure between securing devices. The moment of inertia,
I
,
of edge elements is not to be less than:
where
|
p
|
= |
packing
line pressure in N/mm, minimum 5 N/mm |
|
a |
= |
spacing in m of
securing devices. |
12.3.7 Securing
devices are to be of reliable construction and securely attached to
the hatchway coamings, decks or covers. Individual securing devices
on each cover are to have approximately the same stiffness characteristics.
12.3.8 Where
rod cleats are fitted, resilient washers or cushions are to be incorporated.
12.3.9 Where
hydraulic cleating is adopted, a positive means is to be provided
to ensure that it remains mechanically locked in the closed position
in the event of failure of the hydraulic system.
12.3.10 Hatch
covers are to be effectively secured, by means of stoppers, against
transverse and longitudinal forces (acting on the forward end) arising
from a pressure of 175 kN/m2.
12.3.11 The
equivalent stress:
- in stoppers and their supporting structures; and
- calculated in the throat of the stopper welds; is not to exceed
the allowable value of 0,8σY.
12.4 Load model
12.4.1 The
pressure, p, in kN/m2, acting on the hatch
covers is given by:
-
For ships of
length 100 m or greater, for hatchways located on the freeboard deck, p is to be the greater of 34,3 or the following:
Where a hatchway is located in position 1 and at
least one superstructure standard height higher than the freeboard
deck, the pressure p may be 34,3 kN/m2.
-
For ships less
than 100 m in length, for hatchways located at the freeboard deck, p is to be the greater of 0,195L + 14,9 or the
following:
Where two or more panels are connected by hinges,
each individual panel is to be considered separately.
- where
|
p
FP
|
= |
pressure at the forward perpendicular |
|
|
= |
49,1 + (L –
100) a
|
|
a
|
= |
0,0726
for type B freeboard ships |
|
|
= |
0,356 for ships
with reduced freeboard |
|
L
|
= |
Freeboard
length, in metres, as defined in Regulation 3 - Definitions of terms used in the Annexes of
Annex I to the 1966 Load Line Convention as modified by the Protocol
of 1988, to be taken not greater than 340 m
|
|
x
|
= |
distance,
in metres, of the mid length of the hatch cover under examination
from the forward end of L.
|
-
For weather deck
covers for holds which may be flooded and used as ballast tanks and
holds in OBO, ore or oil and similar types of ship, the pressure p,
in kN/m2, due to the internal load for a member and position
under consideration is to be taken as:
where
|
q
|
= |
roll
angle, in degrees, but need not exceed 25° and is not to be taken
as less than 22° |
|
Y
|
= |
transverse
distance, in metres, from the side coaming at the coaming top to the
member and position under consideration. Both sides of roll are to
be considered. |
In way of holds for oil cargo, a load equivalent
to the inert gas pressure is to be applied over the full breadth of
the cover and added to the load corresponding to the liquid pressure.
However, where the rolling angle has been determined by direct calculations,
the load may be derived accordingly.
12.5 Allowable stress
12.5.1 The
normal and shear stresses calculated for the net section hatch cover
structures are not to exceed the values given in Table 7.12.2 Permissible stresses.
Table 7.12.2 Permissible stresses
| Failure mode
|
Permissible stress,
in N/mm2
|
| Bending
|
σa
|
=
|
0,80σF
|
| Shear
|
τa
|
=
|
0,46σF
|
| Symbols
|
| σF
|
=
minimum upper yield stress, in N/mm2
|
12.5.3 The
stresses in hatch covers that are designed as a grillage of longitudinal
and transverse primary supporting members are to be determined by
a grillage or a FE analysis. When such an analysis is used the secondary
stiffeners are not to be included in the attached flange area of the
primary members.
12.6 Effective cross-sectional area of panel flanges for primary supporting
members
12.6.1 The
effective flange area, A
f, in cm2,
of the attached plating, to be considered for the yielding and buckling
checks of primary supporting members, when calculated by means of
a beam or grillage model, is obtained as the sum of the effective
flange areas of each side of the girder web as appropriate:
where
|
nf |
= |
2 if attached
plate flange extends on both sides of girder web |
|
|
= |
1 if attached plate
flange extends on one side of girder web only |
|
t
|
= |
net
thickness of considered attached plate, in mm |
|
b
ef
|
= |
effective breadth of attached plate flange on each side of girder
web, in metres |
|
|
= |
b
p,
but not to be taken greater than 0,165⋉
|
|
b
p
|
= |
half distance between the considered primary supporting member
and the adjacent one, in metres |
|
l
|
= |
span
of primary supporting members, in metres. |
12.7 Local net plate thickness
12.7.1 The
local net plate thickness of the hatch cover top plating is to be
not less than:
or 1 per cent of the spacing of the stiffeners or
6 mm, whichever is greater
where
|
F
p
|
= |
factor for combined membrane and bending response |
| = |
1,50 in general |
| = |
1,90σ/σa, where σ/σa ≥ 0,8, for the attached plate flange of primary supporting
members
|
|
s
|
= |
stiffener
spacing, in metres |
|
p
|
= |
pressure,
in kN/m2, as defined in Pt 4, Ch 7, 12.4 Load model
|
|
σ
|
= |
as
defined in Pt 4, Ch 7, 12.9 Net scantlings of primary supporting members
|
|
σ
a
|
= |
as defined in Pt 4, Ch 7, 12.5 Allowable stress.
|
12.7.2 For
double skin hatch covers, when the lower plating is taken into account
as a strength member of the hatch cover, the local net plate thickness
of the hatch cover bottom plating is to be not less than:
|
t
|
= |
5,0
mm, whichever is the greater |
where
|
s
|
= |
stiffener
spacing, in metres. |
12.8 Net scantlings of secondary stiffeners
12.8.1 The
required minimum section modulus, Z, in cm3,
of secondary stiffeners of the hatch cover top plate, based on stiffener
net member thickness, is given by:
where
|
l |
= |
secondary
stiffener span, in metres, to be taken as the spacing, in metres,
of primary supporting members or the distance between a primary supporting
member and the edge support, as applicable. When brackets are fitted
at both ends of all secondary stiffener spans, the secondary stiffener
span may be reduced by an amount equal to  of the minimum bracket arm length, but not greater than
10 per cent of the gross span, for each bracket
|
|
s
|
= |
secondary
stiffener spacing, in metres |
|
p
|
= |
pressure,
in kN/m2, as defined in Pt 4, Ch 7, 12.4 Load model
|
|
σ
a
|
= |
as defined in Pt 4, Ch 7, 12.5 Allowable stress.
|
12.8.2 The
net section modulus of the secondary stiffeners is to be determined
based on an attached plate width assumed equal to the stiffener spacing.
12.9 Net scantlings of primary supporting members
12.9.1 The
section modulus and web thickness of primary supporting members, based
on member net thickness, are to be such that the normal stress σ
in both flanges and the shear stress τ, in the web, do not exceed
the allowable values σa and τa, respectively,
defined in Pt 4, Ch 7, 12.5 Allowable stress.
12.9.2 The
breadth of the primary supporting member flange is to be not less
than 40 per cent of their depth for laterally unsupported spans greater
than 3,0 m. Tripping brackets attached to the flange may be considered
as a lateral support for primary supporting members.
12.9.3 The
flange outstand is not to exceed 15 times the flange thickness.
12.10 Hatch cover plating
12.10.1 The
compressive stress, σ, in N/mm2, in the hatch cover
plate panels, induced by the bending of primary supporting members
parallel to the direction of secondary stiffeners, is not to exceed
0,8 times the critical buckling stress σC1, to be evaluated
as defined below:
| σC1
|
=
|
σE1
|
when σE1 ≤ σF/2
|
|
|
=
|
σF [1 –
σF/(4σE1)]
|
when σE1 >
σF/2
|
where
|
σF
|
= |
minimum
upper yield stress, in N/mm2, of the material
|
|
σE1
|
= |
|
|
E
|
= |
modulus
of elasticity, in N/mm2
|
| = |
2,06 x 105 for steel
|
|
t
|
= |
net
thickness, in mm, of plate panel |
|
s
|
= |
spacing
of secondary stiffeners, in metres |
12.10.2 The
mean compressive stress σ in each of the hatch cover plate panels,
induced by the bending of primary supporting members perpendicular
to the direction of secondary stiffeners, is not to exceed 0,8 times
the critical buckling stress σC2, to be evaluated as
defined below:
| σC2
|
=
|
σE2
|
when σE2 ≤ σF/2
|
|
|
=
|
σF [1 –
σF/(4σE2)]
|
when σE2 >
σF/2
|
where
|
σF
|
= |
minimum
upper yield stress, in N/mm2, of the material
|
|
σE2
|
= |
|
|
m
|
= |
|
|
E
|
= |
modulus
of elasticity, in N/mm2
|
| = |
2,06 x 105 for steel
|
|
t
|
= |
net
thickness of plate panel, in mm |
|
s
s
|
= |
length of the shorter side of the plate panel, in metres |
|
ls
|
= |
length
of the longer side of the plate panel, in metres |
|
Ψ |
= |
ratio between
smallest and largest compressive stress |
|
c
|
= |
1,3
when plating is stiffened by primary supporting members |
| = |
1,21 when plating is stiffened by secondary
stiffeners of angle or T type |
| = |
1,1 when plating is stiffened by secondary
stiffeners of bulb type |
| = |
1,05 when plating is stiffened by flat bar. |
12.11 Hatch cover secondary stiffeners
12.11.1 The
compressive stress σ, in N/mm2, in the top flange of
secondary stiffeners, induced by the bending of primary supporting
members parallel to the direction of secondary stiffeners, is not
to exceed 0,8 times the critical buckling stress σCS,
to be evaluated as defined below:
| σCS
|
=
|
σES
|
when σES ≤ σF/2
|
|
|
=
|
σF [1 –
σF/(4σES)]
|
when σES >
σF/2
|
where
|
σF
|
= |
minimum
upper yield stress, in N/mm2, of the material
|
|
σES
|
= |
ideal
elastic buckling stress, in N/mm2, of the secondary stiffener
|
| = |
minimum between σE3 and σE4
|
|
σE3
|
= |
0,001E  a/(A l2)
|
|
E
|
= |
modulus
of elasticity, in N/mm2
|
| = |
2,06 x 105 for steel
|
 a
|
= |
moment
of inertia of the secondary stiffener, including a top flange equal
to the spacing of secondary stiffeners, in cm4
|
|
A
|
= |
cross-sectional
area of the secondary stiffener, including a top flange equal to the
spacing of secondary stiffeners, in cm2
|
|
l |
= |
span of
the secondary stiffener, in metres |
|
σE4
|
= |
|
|
K
|
= |
|
|
m
|
= |
number
of half waves, given in Table 7.12.3 Number of half waves
|
 w
|
= |
sectorial
moment of inertia (warping constant) of the secondary stiffener about
its connection with the plating, in cm6
|
| = |
 for flat bar secondary stiffeners
|
| = |
 'Tee' secondary stiffeners
|
| = |
 [t
f(b
f
2 +
2b
f
h
w + 4h
w
2) + 3t
w
b
f
h
w] 10-6
|
| = |
for angles and bulb secondary stiffeners |
 p
|
= |
polar
moment of inertia of the secondary stiffener about its connection
with the plating, in cm4
|
| = |
 for flat bar secondary stiffeners
|
| = |
|
| = |
for flanged secondary stiffeners |
 t
|
= |
St.Venant's
moment of inertia of the secondary stiffener without top flange, in
cm4
|
| = |
 for flat secondary stiffeners
|
| = |
|
| = |
for flanged secondary stiffeners |
|
h
w, t
w
|
= |
height and net thickness of the secondary
stiffener, respectively, in mm |
|
b
f, t
f
|
= |
width and net thickness of the secondary stiffener
bottom flange, respectively, in mm |
|
s
|
= |
spacing
of secondary stiffeners, in metres |
|
C
|
= |
spring
stiffness exerted by the hatch cover top plating |
| = |
|
|
k
p
|
= |
1 – ηp to be taken not less than zero;
for flanged secondary stiffeners, k
p need
not be taken less than 0,1
|
|
ηp
|
= |
|
|
σ |
= |
as defined
in Pt 4, Ch 7, 12.9 Net scantlings of primary supporting members
|
|
σE1
|
= |
as
defined in Pt 4, Ch 7, 12.10 Hatch cover plating
|
|
t
p
|
= |
net thickness of the hatch cover plate panel, in mm. |
Table 7.12.3 Number of half waves
|
K
|
m
|
| 0 < K < 4
|
1
|
| 4 < K < 36
|
2
|
| 36 < K < 144
|
3
|
| (m - 1)2
m
2 < K ≤ m2 (m + 1)2
|
m
|
12.11.2 For
flat bar secondary stiffeners and buckling stiffeners, the ratio h/t
W is to be not greater than 15k
0,5
where
|
h, t
W
|
= |
height and net thickness of the stiffener,
respectively |
|
k
|
= |
235/σF
|
|
σF
|
= |
minimum
upper yield stress, in N/mm2, of the material.
|
12.12 Web panels of hatch cover primary supporting members
12.12.1 This
check is to be carried out for the web panels of primary supporting
members formed by web stiffeners or by the crossing with other primary
supporting members, the face plate (or the bottom cover plate) or
the attached top cover plate.
12.12.2 The
shear stress τ in the hatch cover primary supporting members web
panels is not to exceed 0,8 times the critical buckling stress τC, to be evaluated as defined below:
| τC
|
=
|
τE
|
when τE ≤ τF/2
|
|
|
=
|
τF [1 – τF/(4τE)]
|
when τE > τF/2
|
where
|
σF
|
= |
minimum
upper yield stress of the material, in N/mm2
|
|
τF
|
= |
|
|
τE
|
= |
|
|
E
|
= |
modulus
of elasticity, in N/mm2
|
| = |
2,06 x 105 for steel
|
|
t
pr,n
|
= |
net thickness of primary supporting member, in mm |
|
k
t
|
= |
5,35 + 4,0/(a/d)2
|
|
a
|
= |
greater
dimension of web panel of primary supporting member, in metres |
|
d
|
= |
smaller
dimension of web panel of primary supporting member, in metres. |
12.12.3 For
primary supporting members parallel to the direction of secondary
stiffeners, the actual dimensions of the panels are to be considered.
12.12.4 For
primary supporting members perpendicular to the direction of secondary
stiffeners or for hatch covers built without secondary stiffeners,
a presumed square panel of dimension d is to be taken
for the determination of the stress τC. In such a case,
the average shear stress τ between the values calculated at the
ends of this panel is to be considered.
12.13 Deflection limit and connections between
hatch cover panel
12.13.1 Load
bearing connections between the hatch cover panels are to be fitted
with the purpose of restricting the relative vertical displacements.
12.13.2 The vertical deflection of primary supporting members is to be not more
than 0,0056l, where l is the greatest span of primary supporting members.
|