Section
8 Spoil space and well structure
8.1 Symbols and definitions
8.1.1 The
symbols used in this Section are defined as follows:
|
h
|
= |
load
head, in metres, measured vertically as follows: |
-
For plating,
the distance from a point one-third of the height of the plate above
its lower edge to the sill of the uppermost overflow weir.
-
For stiffeners
or girders, the distance from the middle of the effective length to
the sill of the uppermost overflow weir.
|
s
|
= |
spacing
of stiffeners, in mm |
|
t
|
= |
plate
thickness, in mm |
|
A
1
|
= |
cross-sectional area of flange or stiffener, in cm2,
including coaming plating.
|
Table 12.8.1 Effective specific gravity
| Effective specific gravity less
than or equal to 1,4
|
Effective specific gravity greater than 1,4
|
| ρ≤
1,4
|
ρ
> 1,4
|
| for vertical boundaries
|
for boundaries which have an
angle, a with the horizontal plane
|
| ρef = ρ
|
ρef = 1,4
|
ρef = 1,4 + (ρ –
1,4)(cos α)2
|
8.2 General
8.2.1 This
Section provides for:
-
horizontally
and vertically stiffened boundary bulkheads to hoppers, and holds
intended for dredged spoil, to ladder wells and to spud wells,
-
protection against
flooding in the event of the ladder well or adjacent bottom plating
being damaged by objects dredged up by bucket dredgers, and
-
continuity of
transverse strength in spoil spaces and wing tanks abreast of spoil
spaces.
8.2.2 As
an alternative to the requirements of this section regarding primary
structure, scantlings may be derived on the basis of direct calculation
methods, see
Pt 4, Ch 12, 18 Direct calculations.
8.2.3
Continuity
of strength. Arrangements are to be made to ensure continuity
of strength at the ends of longitudinal and well side bulkheads. In
general, the design should be such that the bulkheads are connected
to bottom and deck girders by means of large, suitably shaped brackets
arranged to give a good stress flow at their junctions with both the
girders and the bulkheads.
8.2.4
Ladder well cofferdams. Ladder wells cutter suction dredgers and bucket dredgers
are to be isolated from the remainder of the dredger's structure by local cofferdams at
least 600 mm wide, or are to be otherwise protected to prevent serious flooding due to
the well side plating being breached by the ladder structure should this be damaged in
service. Ladder wells of bucket dredgers are to be isolated by cofferdams, the extent
and widths of which are to be sufficient to contain any damage to the well side
bulkheads or bottom shell plating that could result from the impact of large objects
brought up in the dredge buckets. In way of the buckets the cofferdam may be extended
outboard in the form of a local watertight double bottom.
8.3 Spoil space and well boundaries
8.3.1 The
plating thickness of spoil space boundaries is to be not less than
the following:
|
t
|
= |
0,0046s
f
 + 3,0 mm, or |
|
t
|
= |
8,5
mm, whichever is the greater |
In the case of grab dredgers the minimum thickness
is to be 10 mm. These thickness requirements also apply to the plating
of watertight box keels and inner bottom plating. The value of ρef used in the calculations and the height(s) of the overflow
weir(s) are to be clearly shown on the midship section plan.
8.3.2 Attention
is drawn to the high rate of wear that can occur on spoil space boundaries,
and it is recommended that an additional corrosion allowance of 3,0
mm be added on areas subject to particularly onerous conditions. Where
such an allowance is added, the fact is to be marked on the relevant
plans.
8.3.3 The
thickness of plating forming the sides and ends of bucket ladder wells
is to be not less than:
In no case, however, is the side plating to have
a thickness less than 12 mm nor is the well end plating to have a
thickness less than 8,5 mm. Plating forming the boundaries of suction
pipe ladder wells is generally to be as required for shell plating.
Corrosion allowance on well end plating below bucket ladders may be
2,0 mm.
8.3.4 The
thickness of spoil space and ladder well bulkheads may be required
to be increased where high shear forces are present.
8.3.5 Bulkheads
forming the boundaries of spud wells are to be of increased strength.
Each case will be considered on its merits, but in general such bulkheads
should have a thickness of not less than 12 mm.
8.3.6 Where
non-watertight bulkheads are fitted in the side buoyancy tanks, the
thickness of the plating is to be not less than:
-
-
whichever is the greater. Where the bulkhead is in the form
of a wash bulkhead, the openings should be so arranged that, in general,
the distance from lightening holes to any slots cut to accommodate
side shell or bulkhead longitudinals is at least equal to 1,5 times
the depth of the slot. The edges of large openings are to be stiffened.
8.3.7 The
section modulus of framing on spoil space boundaries is to be not
less than:
where
|
c
|
= |
as defined in Pt 4, Ch 12, 8.3 Spoil space and well boundaries 8.3.7 for longitudinal framing |
|
|
= |
1,0 for transverse
framing |
|
γ |
= |
1,4 for rolled
or built sections |
|
|
= |
1,6 for flat bars. |
|
ρ
ef
|
= |
effective specific gravity, see
Pt 4, Ch 12, 8.1 Symbols and definitions 8.1.1
|
|
k
|
= |
higher
tensile steel factor, see
Pt 3, Ch 2, 1 Materials of construction
|
The section modulus of longitudinals below  is to be taken not less than the value obtained at  .
Table 12.8.2 Definition of c for longitudinal framing
| Symbols
|
Location
|
c, see Note 2
|
| F
B as defined inPt 3, Ch 4, 5.1 Symbols
|
 and above
|
1,0
|
|
|
|
0,85
|
|
|
0,2D above base (see Note 1)
|
|
|
|
Base line (see Note 1)
|
|
Note
1. For ships with hogging still water
bending moments in loaded conditions and for split hull vessels,
c = 1,0.
Note
2. Intermediate values are to be
calculated by linear interpolation.
|
8.3.9 For
non-watertight bulkheads, the modulus of the stiffeners may be 50
per cent of that required for intact bulkheads. The stiffeners are
to be bracketed at top and bottom.
8.3.10 Structure
supporting spud well plating and bulkheads below, and in way of, `A'
frames and dredging machinery supports, is to be of substantial construction,
account being taken of the dynamic loads likely to occur with the
dredging machinery in operation.
8.3.11 Horizontal
girders supporting stiffeners on spoil space and ladder well boundaries
are, in general, to have scantlings as required by Pt 4, Ch 1, 9.2 Watertight and deep tank bulkheads for deep tanks, with ρ
and h as defined in Pt 4, Ch 12, 1.5 Symbols 1.5.1 and Pt 4, Ch 12, 8.1 Symbols and definitions 8.1.1 respectively and with span, l
e for horizontal girders supporting vertical stiffeners
on longitudinal bulkheads, measured between bulkhead bracket and bulkhead
bracket, i.e. ignoring any struts which may be fitted between spoil
space girder and shell stringer. Alternatively, the section modulus
of these horizontal girders may be reduced by 40 per cent from the
formula value if struts are fitted on alternate frames between the
spoil space girder and a shell stringer. These struts should generally
be horizontal and are to have a sectional area as required for pillars
by Pt 4, Ch 1, 4.4 Deck supporting structure with ρ as defined
in Pt 4, Ch 1, 1.5 Symbols and definitions 1.5.1 and h measured
from the inboard end of the strut to the height defined in Pt 4, Ch 12, 8.1 Symbols and definitions 8.1.1. Web frames and girders are
to have scantlings as required by Pt 4, Ch 1 General Cargo Ships, with ρ and h as defined in Pt 4, Ch 12, 1.5 Symbols 1.5.1 and Pt 4, Ch 12, 8.1 Symbols and definitions 8.1.1 respectively.
8.4 Cross members
8.4.1 Cross-members
are to be fitted within the hopper space in line with the bottom and
side shell transverses and with the bulkheads in the side buoyancy
spaces. Where the spacing between the cross-members exceeds 4 m, the
scantlings of all primary members contributing to the continuity of
the transverse strength in the spoil space are to be verified by direct
calculations, see also
Pt 3, Ch 1, 2.2 Submission of direct calculations. Where a box keel is fitted on the centreline,
webs are to be fitted within the box keel to ensure proper continuity
of strength across the ship in way of the hopper cross-member. The
webs required within centreline watertight box keels may have a thickness
3,5 mm less than that required for the hopper cross-members with which
they are associated, but their minimum thickness is to be not less
than 6,5 mm.
8.4.2 The
upper edge of the hopper lower cross-members should, in general, be
a height of not less than  above the keel in ships with the number 100 in their character
of classification. The lower edge should be as low as practicable
after allowing for the proper design of hopper doors, suction passages,
etc. Lower cross-members may be fabricated from flat plate suitably
stiffened or may take the form of a hollow box, generally of triangular
cross-section.
8.4.3 The
scantlings of box-type cross-members should be determined from the
requirements for hopper bulkheads where applicable. When flat plate
lower cross-members are fitted, the thickness of the web is to be
not less than:
|
t
|
= |
(0,7B + 3) mm or 8,5 mm
|
whichever is the greater.
8.4.4 The
cross-sectional area of the cross-member web after deducting access
openings, lightening holes, etc. is to be not less than:
where
|
h
w
|
= |
height, in metres, of the uppermost hopper overflow weir above
the keel |
|
S
M
|
= |
spacing of the cross-member webs, in metres. |
8.4.5 The
upper edge of the cross-member is to be stiffened by means of a tube
having an outside diameter not less than:
where
|
l
s
|
= |
span, in metres, of the upper edge of the cross-member (to the
centreline box girder if fitted), and a thickness equal to the minimum
required cross-tie web thickness, or by an equivalent flange or structure.
The lower edge of the cross-member is also to be suitably stiffened. |
8.4.6 The
cross-member web is to be fitted with stiffeners, spaced not more
than 80t mm apart having a modulus of not less than:
8.4.7 The
transverse strength of primary structural members, such as upper and
lower cross members and wing tank bulkheads, forming transverse ring
systems are to be verified by direct calculations, e.g. finite element
calculations on the basis of loads arising from hydrostatic, wave,
spoil pressure and loadings on closing appliances of bottom openings.
The stresses are in general not to exceed the following values:
|
|
Bending + axial stress (σ
b)
|
130/k N/mm2
|
|
|
Shear stress (T)
|
70/k N/mm2
|
|
|
Combined stress
|
180/k N/mm2
|
8.5 Pillars within hoppers
8.5.1 Pillars
are generally to comply with the requirements of Pt 4, Ch 1, 4.4 Deck supporting structure, account being taken of the
maximum forces that can be applied by rams or other gear fitted for
the purpose of activating hopper doors or valves.
8.6 Continuous coamings
8.6.1 Continuous
coamings are to have a plate thickness of not less than 8,5 mm. A
minimum thickness of 10 mm is recommended for coamings on grab dredgers.
Where the depth of the coaming exceeds 80t, the plating
is to be stiffened by one or more horizontal members so spaced that
the width of the upper panel of plating does not exceed 65t and
the width(s) of the lower panel(s) do(es) not exceed 80t.
8.6.2 Where
the coaming is stiffened with flat bar members, the members are to
have a breadth not less than 0,04S
s and a
thickness not less than 0,05 times their breadth, or 8,5 mm, whichever
is the greater. They are to have a minimum inertia of:
where
|
|
= |
A
1 and I include the coaming
plating for mid-panel above to mid-panel below the stiffener, and
|
|
S
s
|
= |
spacing of the brackets required by this sub-Section, in metres. |
|
|
= |
Where stiffeners other than flat bars are used, they are to
have at least the same minimum thickness and inertia as required for
flat bars. |
8.6.3 The
upper edge of the coaming is to be stiffened by a fabricated flange,
box girder or equivalent structure having a width not less than 0,05S
s and an inertia not less than:
where
|
|
= |
A
1 and I include the coaming
plating down to mid-panel below
|
|
|
= |
The thickness and/or attachments of the stiffening member are
to be such as to minimise any likelihood of local instability under
compression loading. |
8.6.4 The
coamings are to be supported by substantial brackets spaced generally
not more than 3,0 m apart where the coamings have a height of more
than 600 mm, nor more than 2,5 m where the coamings have a height
of more than 1,0 m but on longitudinally framed ships the brackets
are to be arranged in way of each deck transverse. Additional brackets
may be required in way of the ends of hopper upper cross-ties, especially
those which themselves support hopper door operating rams or similar
equipment.
8.6.5 The
ends of continuous coamings are to be well scarfed into the ship's
structure at the ends of spoil spaces. Unless longitudinal deckhouse
bulkheads are fitted in this area, the coamings are to be extended
beyond the end of the spoil space opening for a distance of at least
one frame space, or 1,5 times the coaming height, whichever is the
greater.
|