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

must be guarded against. As these steels have low corrosion resistances, special
coating, e.g. epoxies, are often used on the underside of hatch covers where sweating
can cause accelerated corrosion. Other materials such as higher tensile steel and
aluminium are acceptable to the Classification Societies.
3.4.1 Higher Tensile Steel (HTS)
If the stress level within a structure were the only factor governing its scantlings, the
use of higher tensile steel instead of mild steel would result in weight savings of up to
15 percent. However stress is not the only governing factor. Deflection, minimum
thickness, ease of construction, initial and maintenance costs must all be taken into
account. Higher tensile steel (HTS) can sometimes be used for the beams of hatch
covers, with their top plates made from mild steel. This reduces the weight of the
covers, without incurring the extra cost of using all HTS construction. Thus, HTS is
sometimes used on hatch covers where, by reducing the cover weight slightly, it may
be possible to install less powerful operating mechanism. Construction entirely in
HTS is sometimes used in pontoon covers for container ships, to keep their weight
below the maximum lifting capacity of container cranes.
3.4.2 Aluminium
Aluminium structures can be 55-60 percent lighter than equivalent mild steel
structures. Most of the problems associated with higher tensile steel are not present
with aluminium. Since it is not as strong as steel, thickness have to be increased,
thereby alleviating problems associated with minimum regulatory deflection, buckling
etc., unless the design is such that deflection is significant. Welding aluminium does
however require special skills and equipment. Corrosion is less of a problem if the
correct grade of material is used and if precaution are taken where steel and
aluminium meet, e.g. at coamings. The spacing of stiffeners can be increased in an
aluminium structures this has the added advantage that the number of stiffeners and
hence the weight and cost of the structure can be further reduced. The major
disadvantage of aluminium is that a suitable grade of material for access equipment
costs approximately eight times as much per tonne as mild steel, with the overall
effect of increasing the cost of the cover by upto three times.
Aluminium covers have been fitted to deep tanks because they are so much
lighter. Since deep tank covers are often made in one piece, aluminium covers are
36