Single pull macgregor type hatch cover.pdf - Cochin University of ...

Table 3.4 Access equipment strength calculations
Factor Weather deck Tween-deck
Maximum bending stress,
kgf/cm square
Maximum shear stress,
kgf/cm square
965 1200
700 700
Maximum deflection 0.0028 * span 0.0035*span
Source: Cargo Access Equipment for Merchant Ships.
The top plate of a typical hatch cover panel may be from 6-13 mm thick
depending on the spacing of the beams (generally, thickness=spacing/100). This plate
is stiffened by beams spanning the hatch cover, usually fabricated ‘tee’ beams having
a depth of about 4 percentage of the span and spaced 500-1000 mm apart. The panel is
completely by side and end plates which may be from 8.20 mm thick.
3.3.1.5 Deformation
As the ship become larger and hatchways take up a greater percentage of the deck
area, so the question of hatchway deformation becomes more important. Traditionally
ships have fairly small hatchways and so have derived a considerable amount of their
strength from their decks. As hatchways have increased in width, so the deck’
contribution to the longitudinal and the torsional strength of the hull girder has
declined, being limited to the strips of deck outboard of the coaming and between the
hatches. A ship with hatchways more than 70 percentage of the beam in width has
approximately half the torsional rigidity of a similar ship with hatchways which are
only 40 percentage of the ship’s beam. Compensation in the form of thickened plating
and/or box girders may be required.
Fig 3.3 indicates the extent of hatchway deformation that may be encountered
in a large bulk carrier loaded in alternate holds. Although the deformation is not
excessively large and is not permanent, they can be sufficient to allow sea water to
enter and parts of the hatch cover to fracture.
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