Page 57 - Marine Structural Design

34 Part I Structural Design Principles

The structural response to waves used in the global structural analysis of a ship is calculated
based on the ship's Response Amplitude Operations (RAOs) when exposed to regular
sinusoidal waves, for different wave headings and frequencies.
The structural integrity of the ship is assured by implementing a number of different
combinations of loads, wave periods, and heading angles. For each situation, a number of load
components are calculated, such as external wave pressure, acceleration of the liquid cargo
and ballast tanks, accelerations at several stations along the ship's length, wave-induced
bending and torsional moments together with the corresponding shear forces along the length
of the ship, and the ship's motion in roll and pitch modes.
The short-term response of the ship is obtained through the evaluation of the seaway spectrum,
which is assumed to be stationary in a period of a few hours. The long-term response and the
probability of exceedance of the load are evaluated from the short-term prediction.
The hull girder loads are calculated from a number of components. The most significant of
these components are the still-water moments and shear forces resulting from the weight of the
ship, cargo, and buoyancy. The second major component of hull girder loads is, the dynamic-
induced loads that include the vertical and horizontal bending moments, shear forces and
torsional moment. These dynamic loads result from wave motions encountered by the ship.
The classification rules are used to determine the still-water bending moments and shear forces,
as these are mainly dependent on the loading conditions of the vessel. A more detailed
analysis is required when determining the dynamic aspects of the hull girder loads. Such
analysis is based on the sea conditions that the vessel is bound to encounter over its lifetime.
Normally, a 20-year service life is chosen and appropriate wave data is selected. The result of
such an analysis determines the extreme values that are used to calculate a design value for the
hull girder loads.
When determining the hull girder loads, the vertical bending moments and shear forces are
calculated first. Then tables and other sources of data are used to calculate the ratio of vertical
to horizontal bending moment and shear force. These ratios are mainly dependent on the ship's
dimensions and loading conditions.
External Pressure
Determining the external pressure acting on a ship is a more complicated process than the
calculation of hull girder loads. This is because the external pressure is influenced by a larger
number of parameters such as hull form, wave motion characteristics, ship speed, heading
angles in the sea, etc. The methods and theories used to determine the external pressure on a
ship are usually based on a number of assumptions, such as having a wall-sided hull, small
motions of the vessel, and being in an inviscid fluid. Thus, one has to be careful when
predicting a value for the external pressure.
The external pressure on a vessel is determined by initially dividing the vessel into two parts.
The pressures distributed over 40% of the length of the vessel, centered around the amidships
are normally very similar from ship to ship. Thus, the calculation of the pressure in these
regions is relatively straightforward and is done by applying the results of a complete
seakeeping analysis of full form tankers and bulk carriers. Formulae are used for the pressure
applied over the rest of the ship, since the pressure vanes significantly from one ship to the
next depending primarily on the hull form.
In a simplified form, the total external pressure PE, on a ship may be expressed as (ABS,
2002):