Section
7 Guidance
7.1 Guidance Notes for Chapter 4
7.1.1
Guidance to detailed calculation of internal pressure for static design
purpose
- This Section provides guidance for the calculation of the
associated dynamic liquid pressure for the purpose of static design
calculations. This pressure may be used for determining the internal
pressure given in Pt 11, Ch 4, 3.3 Functional loads 3.3.2.(d).
P
gd is the associated maximum liquid pressure determined using
site-specific accelerations.
P
eq is to be calculated as follows:
P
eq= P
o + P
gd (MPa)
- The internal liquid pressures are those created by the resulting
acceleration of the centre of gravity of the cargo due to the motions of the
ship unit referred to in Pt 11, Ch 4, 3.4 Environmental loads 3.4.2. The value of internal liquid pressure Pgd
resulting from combined effects of gravity and dynamic accelerations
shall be calculated as follows:
where
αβ
|
= |
dimensionless acceleration (i.e. relative to the
acceleration of gravity), resulting from gravitational and
dynamic loads, in an arbitrary direction β, (see
Figure 4.7.3 Acceleration
ellipsoids)
Note for large
tanks an acceleration ellipsoid, taking account of
transverse vertical and longitudinal accelerations should be
used
|
Z
|
= |
largest liquid height (in metres) above the point
where the pressure is to be determined measured from the tank
shell in the β direction (see
Figure 4.7.2 Determination of
internal pressure heads) Tank domes considered to be part
of the accepted total tank volume shall be taken into account
when determining Z
β unless the total volume of tank domes
Vd
does not exceed the following value: |
V
d
|
= |
|
where
V
t
|
= |
tank volume without any domes |
ρ |
= |
maximum cargo density (kg/m3) at the
cargo design temperature |
The direction that gives the maximum value of P
gd shall be considered. Where acceleration components in
three directions need to be considered, the ellipsoid shown in Figure 4.7.3 Acceleration
ellipsoids shall be used instead of the ellipse in
Figure 4.7.3 Acceleration
ellipsoids. The above formula applies only to full
tanks.
Figure 4.7.1 Determination of
internal pressure heads
See also
Figure 4.7.2 Determination of
internal pressure heads.
Figure 4.7.2 Determination of
internal pressure heads
Accelerations in three dimensions are to be
considered for ship units with independent spherical Type B tanks for
which the ellipsoid as shown in Figure 4.7.3 Acceleration
ellipsoids is to be used. Where loading conditions
are proposed including one or more partially filled tanks, the internal
liquid pressure to be used will be specially considered. See also
Pt 11, Ch 4, 3.4 Environmental loads 3.4.4.
Figure 4.7.3 Acceleration
ellipsoids
- Equivalent calculation procedures may be applied.
7.1.2
Guidance formulae for acceleration components
- The following formulae are given as guidance for the
determination of the maximum value of internal liquid pressure head P
gd, (see
Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.1, internal pressure).
In the transverse
direction, as shown in Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.1, the following apply:
The range of angle β is:
0 to βmax, with βmax = arc tan
For the longitudinal direction, βmax and
aβ
are to be determined with ax
substituted for ay.
7.1.3
Stress categories
- For the purpose of stress evaluation, stress categories are
defined in this Section.
-
Normal stress is the component of stress normal to the plane of
reference.
-
Membrane stress is the component of normal stress that is uniformly
distributed and equal to the average value of the stress across the
thickness of the section under consideration.
-
Bending stress is the variable stress across the thickness of the
section under consideration, after the subtraction of the membrane stress.
-
Shear stress is the component of the stress acting in the plane of
reference.
-
Primary stress is a stress produced by the imposed loading, which is
necessary to balance the external forces and moments. The basic
characteristic of a primary stress is that it is not self-limiting. Primary
stresses that considerably exceed the yield strength will result in failure
or at least in gross deformations.
-
Primary general membrane stress is a primary membrane stress that is
so distributed in the structure that no redistribution of load occurs as a
result of yielding.
-
Primary local membrane stress arises where a membrane stress produced
by pressure or other mechanical loading and associated with a primary or a
discontinuity effect produces excessive distortion in the transfer of loads
for other portions of the structure. Such a stress is classified as a
primary local membrane stress, although it has some characteristics of a
secondary stress. A stress region may be considered as local if:
-
S
1 ≤ 0,5
and
-
S
2 ≥ 2,5
where:
S
1
|
= |
distance in the meridional direction over which the
equivalent stress exceeds 1,1f
|
S
2
|
= |
distance in the meridional direction to another
region where the limits for primary general membrane stress are
exceeded |
R
|
= |
mean radius of the vessel |
t
|
= |
wall thickness of the vessel at the location where
the primary general membrane stress limit is exceeded |
f
|
= |
allowable primary general membrane stress. |
-
Secondary stress is a normal stress or shear stress developed by
constraints of adjacent parts or by self-constraint of a structure. The
basic characteristic of a secondary stress is that it is self-limiting.
Local yielding and minor distortions can satisfy the conditions that cause
the stress to occur.
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