The learning objectives of this section are to make shipboard operational
personnel familiar with the external forces experienced on a ship held in
still water due to cargo loading the external forces experienced on a ship
he'll do to sea forces from wave actions a ship structure may be considered as
the material that provides the strength and stiffness to withstand all of the
forces but the ship may reasonably be expected to experience the hell girder
comprising fore-and-aft ship and cargo areas must have the strength to
withstand the full range of internal forces from sources as light ship weight
fuel cargo and ballast the hell girder must also withstand forces occurring
during launching and dry docking and other result from sea forces like
slamming racking and thermal effects as well as sloshing in partly filled tanks
static forces are set up by general cargo dry bulk cargo liquids and tanks
the sea static forces from general cargo are transferred to the adjacent
structure only in the vertical direction while liquids in a tank setup static
forces like pressure differences which act in all directions the effect of
static forces from dry bulk cargo will be in between forces from general cargos
and liquids grain for instance will affect both horizontally inclined and
vertical surfaces the static sea pressure acting on the submerged hell
will balance the static weights of the ship and cargo since the forces are not
evenly distributed along the hell forces have to be transferred and bending of
the hell girder will take place
when the hold is unloaded the surplus of buoyancy would put the double bottom in
a hog mode with a partly loaded hold the double bottom may be straight when the
hold is completely filled the double bottom is sagged because the cargo
weight is greater than the sea water pressure the side share will also deform
because of the increasing sea water pressure on the side shell and the
hopper acting like the so called fixed point
dynamic forces will primarily act in the same plane as the static forces if they
work in the same direction the dynamic forces will be added to the static
forces the dynamic forces are set up by the water particles surrounding the ship
the hull girder as well as a local elements is affected the largest dynamic
forces are experienced in the bow area
there mentions of a structure based on worst-case considerations for the hull
girder worst case with regard to the effect of forces and moments due to
regular waves is when a wave length between the crests is approximately
equal to the length of the ship the dimensioning of the hell girder
longitudinal strength is based on the maximum design loading condition and
wave conditions causing highest stresses in deck and bottom the worst case
considerations are when the wave crest our fore and aft the ship will be in
sagged condition if the wave crest is amidships the ship will be in extreme
Hoggett condition
additional stresses originate from pounding and panting conditions
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bearing the brunt of the wave encounters and having the largest combinations of
pitching and setting movements the for ship is naturally exposed to dynamic
forces
three types of dynamic forces are considered slamming in way of
flat-bottomed forward at light draft impact pressure forces in way of abrupt
or flared bow pressure forces caused by green water on deck particularly on the
forecastle deck the results may be in dint of bottom plating indent in plating
and forecastle deck and main deck in way of pillars inside forecastle
ships were the relative large depths may be subjected to significant transverse
forces these forces are rolling and pitching forces acting in transverse
direction and causing significant stresses in the transverse bulkheads
we're one hold is empty and the other is full opposing forces are acting at each
side of the transverse bulkhead causing shear stresses in the ship's side this
is particularly relevant for the open hatch type of bulk carriers and the car
carriers
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another type of a dynamic force is sloshing in tanks that are partly filled
significant additional forces may be set up in the tanks filled with liquid cargo
or ballast this phenomenon takes place when the natural frequency of the liquid
coincides with a pitching or rolling mode of the vessel and no means of
hampering the movement of the liquid are introduced like swash bulkheads in oil
tankers to prevent harmful forces there may be restrictions on the filling
height of tanks for instance filling between 20 to 90% of the tank volume may
be prohibited guidance on how to avoid excessive liquid pressure will usually
be issued to the ship as a part of the loading manual
dynamic forces are very much influenced by the skill and with a care a ship is
operated good seaman ship and competent handling in bad weather are very
important factors of reducing the damages on the ship