Page 58 - Marine Structural Design

Chapter 2 Wave Loads for Ship Design and Classification 35

(2.3 1)
where,
pg = Specific weight of sea water
hs = Hydrostatic pressure head in still water
ku = Load factor

hDE = Hydrodynamic pressure head induced by the wave
The pressure distribution may be predicted across a vessel in both a lengthwise and girthwise
direction. Most of the data required in order to carry out such calculations are obtained from
seakeeping analysis of ships.
Internal Tank Pressure
The internal pressure in a tank, which carries liquids onboard a ship, is made up of three parts:
Hydrostatic pressure that is equivalent to pgh,
Changes in pressure head that are due to the pitching and rolling motions of the ship,
Inertial force of the liquid column due to the accelerations caused by the motion of the ship.
The internal pressure in a tank is calculated by a series of formulae specific to the shape of the
tank being analyzed. A number of different tank shapes exist, such as J-shaped, rectangular,
and U-shaped. Other factors that affect the internal pressure are the amount of liquid carried in
the tank, and the location and number of air pipes in the tank.
For example, a simplified formula used to determine the internal pressure in a liquid-carrying
tank is as follows (ABS, 2002):
p, =E(V+’uh,) (2.32)
where,

77 = Local coordinate in vertical direction for tank boundaries measuring from the
top of the tanks

ku = Factor that takes into consideration the resultant acceleration of the liquid due
to the ship’s motion

h, = wave-induced internal pressure head, including inertia force and added
pressure head.

2.5 References
1. ABS (2002), “Rules for Building and Classing Steel Vessels”, American Bureau of
Shipping.
2. Bhattacharyya, R (1978), “Dynamics of Marine Vehicles”, John Wiley & Sons, Inc
3. Bishop, R.E.D. and Price, W.G. (1979): “Hydroelasticity of Ships”, Cambridge
University Press.