Transcript Ship Construction

SHIP CONSTRUCTION
Group C
Basujit Chakravarty
Harsh Thakkar
Vimal Raj Menon
Shantanu Mishra
Explain, with the help of diagrams the effect
of dynamic forces on the structure of the
ship.
DEGREES OF FREEDOM
Six ship motions are illustrated
in the diagram. When these
motions are large then very
large forces may be generated.
These forces are often of a
local nature, but are liable to
cause the structure to vibrate
and thus transmit stresses to
other parts of the structure.
DYNAMIC FORCES
Dynamic Forces result from
• The ship’s motion in the sea
• The action of the wind and waves
• The effects of operating machinery
Effects of Dynamic Forces
• Sagging
• Hogging
• Racking
• Torsion
• Pounding or Slamming
• Panting
SAGGING
In a heavy seaway, a ship may be
supported at the ends by the
crests of waves while the middle
remains unsupported. If the wave
trough is now considered at midships then the buoyancy in this
region will be reduced. With the
wave crest positioned at the ends
of the ship, the buoyancy here will
be increased. This loading
condition will result in a bending
moment which will make the ship
to sag.
HOGGING
If the wave crest is considered at
mid-ships then the buoyancy in
this region will be increased. With
the wave trough positioned at the
ends of the ship, the buoyancy
here will be reduced. This loading
condition will result in a
significantly increased bending
moment, which will cause the ship
to hog.
This will be an extreme condition
giving the maximum bending
moment that can occur in the
ship’s structure.
RACKING
When a ship is rolling, the
accelerations on the ship’s structure
are liable to cause distortion in the
transverse direction. The deck tends to
move laterally relative to the bottom
structure, and the shell on one side to
move vertically relative to the other
side.
This type of deformation is referred to as ‘Racking’. Transverse bulkheads
primarily resist such transverse deformation, the side frames contribution
being insignificant, provided the transverse bulkheads are at their usual
regular spacing.
Its greatest effect is felt when the ship is in the light or ballast condition.
The brackets and beam knees joining horizontal and vertical items of
structure are used to resist this distortion.
TORSION
A ship traversing a wave train at
an angle will be subject to a
twisting moment (torque) and the
structure in ‘torsion’
The greatest effect occurs with
decks having large openings.
In some ships, a heavy torsion
box girder including the upper
deck is provided at the topsides to
accommodate the torsional
stresses.
POUNDING OR
SLAMMING
Heavy pitching assisted by
heaving as the whole ship is lifted
in a seaway may subject the
forepart to severe impact from the
sea.
The greatest effect is experienced
in the lightship condition.
To compensate for this, the
bottom over 30% forward is
additionally strengthened in ships
exceeding 65m in length and in
which the minimum draught
forward is less than 0.045L in any
operating condition.
PANTING
This is a stress which occurs at
the ends of a ship due to
variations in water pressure on
the shell plating as the ship
pitches in a seaway. This tends to
create and in-and-out movement
of the shell plating, known as
panting.
The effect is accentuated at the
bow when making headway.
Additional stiffening is provided in
the form of panting beams and
stringers.