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.
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 P_gd resulting from combined effects of gravity and dynamic accelerations shall be calculated as follows:

where

α_β

dimensionless acceleration (i.e. relative to the acceleration of gravity), resulting from gravitational and dynamic loads, in an arbitrary direction β, (see Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.1)

Note for large tanks an acceleration ellipsoid, taking account of transverse vertical and longitudinal accelerations should be used

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 Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.1) 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 V_d does not exceed the following value:

V _d

where

V _t

tank volume without any domes

filling limit according to Pt 11, Ch 15 Filling Limits for Cargo Tanks

maximum cargo density (kg/m³) at the 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 Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.1 shall be used instead of the ellipse in Pt 11, Ch 4, 6.2 Type B independent tanks 6.2.1. The above formula applies only to full tanks.

Figure 4.7.1 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 Pt 11, Ch 4, 7.1 Guidance Notes for Chapter 4 7.1.1 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 β_max, with β_max = arc tan

For the longitudinal direction, β_max and a_β are to be determined with a_x substituted for a_y.

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

mean radius of the vessel

wall thickness of the vessel at the location where the primary general membrane stress limit is exceeded

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.