Section
8 Construction of tankers with cargo tanks independent from the
ship’s structure
8.1 General
8.1.1 This Section
covers the requirements for the hull construction of tankers with
independent tanks and semiindependent tanks. For relevant cross sections, see
Figure 4.1.2 Examples of possible hull configurations for Tankers of the Types G. C and N of
Chapter 4 whereby the cargo tanks have been designated as ‘Type
of Cargo Tank 1’. The thickness of the hull envelope plating
and the scantlings of structural members are to be equivalent to those
given in Chapter 1, except as stated otherwise in this Section.
8.1.2 Tanks may
be supported by bearers of substantial construction. The bearers and
supporting brackets are to be arranged in line with floors and girders
in the bottom structure. Consideration will be given to proposals
to fit flat bottom tanks directly on to the top of bottom structure,
taking into account structural interaction resulting from different
structural stiffnesses.
8.1.3 Chocks
and other securing arrangements are to be fitted to avoid unintentional
displacement of the tanks caused by accidents such as collision and/or
flooding of the holds. The hull structure in way is to be reinforced
accordingly. When the cargo in the tanks can be heated, chocking arrangements
are to take account of expansion of the tanks.
8.1.4 The space
between the tanks and the surrounding ship’s structure should
be adequate to enable complete inspection of ship and tank structure.
8.2 Midship section modulus calculation
8.2.1 For the
calculation of the midship section modulus, see
Pt 3, Ch 3, 3.4 Calculation of hull section modulus. Where the top of semi-independent
tanks is continuous within the midship region and efficiently connected
to the ship’s structure, the plating and longitudinal stiffeners
of the top of the tank, and tank side to 0,1D m under
deck, may be included in the calculation.
8.3 Topside structure
8.3.1 For ships
where the cargo tanks are independent and whereby the topside structure
is not efficiently connected to the cargo tanks, the scantlings of
the topside structure are to comply with the requirements of Pt 4, Ch 1, 4 Deck plating and continuous longitudinal hatch side coamings . For all other configurations the
scantlings are to comply with the requirements of Pt 4, Ch 6, 3 Hull envelope framing. Where transverse stiffening
is provided in the top of the tanks, their scantlings are to be in
accordance with Pt 4, Ch 6, 4 Hull envelope framing – Transversely framed ships. Where
longitudinal stiffening is provided in the top of the tanks, their
scantlings are to be in accordance with Pt 4, Ch 6, 5 Hull envelope framing – Longitudinally framed ships.
8.4 Hull envelope plating
8.5 Double skin arrangement
8.5.1 The plating
of the inner skin is to comply with the requirements for deep tank
or watertight bulkheads, as applicable, see
Pt 3, Ch 7 Bulkheads.
8.6 Single bottom structure
8.6.1 A transverse
or longitudinal framing system may be adopted, but ships with a length, L, of more than 70 m and transverse framing are to have additional
longitudinal stiffening fitted at the bottom. The scantlings of floors,
transverses and longitudinals are to comply with the requirements
of Table 6.8.1 Single bottom structure of ships
with independent cargo tanks. The remaining
structural items are to comply with Pt 4, Ch 1, 6 Single bottom structure.
Table 6.8.1 Single bottom structure of ships
with independent cargo tanks
|
|
Item
|
Parameter
|
Requirement
|
| (1)
|
Floors
|
Web depth at
centreline
|
d
w= 40B mm
|
| Web thickness
|
t = 0,01d
w + 2,5 mm
|
| Modulus
|
The greater of:
|
|
|
a) Z = 7Ts
B
2 cm3
|
|
|
b) 
|
| (2)
|
Transverses
|
Modulus
|
The greater of:
|
| a) Z = 7T
S
B
2 cm3
|
b) 
|
|
See Notes 1 and 2
|
| (3)
|
Longitudinals
|
Modulus
|
Z = (3,95 + 0,04L
1) D
1
sle
2 cm3
|
|
See Notes 1 and 2
|
| Symbols
|
|
L, B, D,
T, S, s, le, Z, ρ and
t
|
= |
as defined in Pt 4, Ch 6, 1.12 Symbols and definitions 1.12.1
|
|
b
|
= |
breadth of tank support on the floor, in metres |
|
d
|
= |
mean height of the cargo tank, in metres |
|
d
w
|
= |
web depth of floor at centreline, in mm |
|
D
1
|
= |
D, but need not be taken greater than T + 0,4 m |
|
L
1
|
= |
l, but to be taken not less than 65 m |
|
Note
1. Where effective support at the
centreline of the ship is provided, the required section modulus of
the floors and transverses will be considered on the basis of a
reduced effective span.
Note
2. The formulae are based on the
assumption that the load from the tanks is evenly distributed over all
floors and transverses in way.
|
8.6.2 Floors
in a transverse framing system are to be fitted at every frame. Transverses
in a longitudinal framing system are to be spaced not more than 2,5
m apart. If the depth of floors is to be locally decreased to suit
the shape of the tank which is supported by them, the strength of
floors and transverses in way is to be maintained. When the loads
exerted by the tanks are not evenly distributed, the strength of the
bottom structure is to be verified by direct calculation.
8.6.3 On single
hull ships, the section modulus of side frames in way of bottom transverses
is to be increased by 100 per cent.
8.6.4 A single
bottom structure is not allowed on Tankers of Type C.
8.7 Double bottom structure
8.7.1 A transverse
or longitudinal framing system may be adopted, but ships with a length, L, exceeding 70 m are generally to be longitudinally framed.
The requirements of Pt 4, Ch 1, 7 Double bottom structure are generally
to be complied with. When cargoes of a higher specific gravity than
1,0 tonnes/m3 are carried or when the load from the tanks
is not evenly distributed over the bottom structure, the strength
of the bottom structure is also to be verified by direct calculation.
|