Part G Guidance
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - Resolutions - Maritime Safety Committee - Resolution MSC.370(93) – Amendments to The International Code for The Construction and Equipment of Ships Carrying Liquefied Gases In Bulk (IGC Code) – (Adopted on 22 May 2014) - Annex - Amendments to The International Code for The Construction and Equipment of Ships Carrying Liquefied Gases In Bulk (IGC Code) - Chapter 4 – Cargo Containment - Part G Guidance

Part G Guidance

Parent topic: Chapter 4 – Cargo Containment

4.28 Guidance notes for chapter 4

  4.28.1 Guidance to detailed calculation of internal pressure for static design purpose

  4.28.1.1 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 referred to in 4.13.2.4, where:

  • .1 (Pgd )max is the associated liquid pressure determined using the maximum design accelerations.

  • .2 (Pgd site)max is the associated liquid pressure determined using site specific accelerations.

  • .3 Peq should be the greater of Peq1 and Peq2 calculated as follows:

    • Peq1 = Po + (Pgd )max (MPa),
    • Peq2 = Ph + (Pgd )site (MPa),

  4.28.1.2 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 referred to in 4.14.1. The value of internal liquid pressure Pgd resulting from combined effects of gravity and dynamic accelerations should 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 4.1).

For large tanks, an acceleration ellipsoid taking account of transverse vertical and longitudinal accelerations, should be used.
Zβ = largest liquid height (m) above the point where the pressure is to be determined measured from the tank shell in the β direction (see figure 4.2).

Tank domes considered to be part of the accepted total tank volume shall be taken into account when determining Zβ , unless the total volume of tank domes Vd does not exceed the following value:

  • with:

    • Vt = tank volume without any domes; and
    • FL = filling limit according to chapter 15.
ρ = maximum cargo density (kg/m3) at the design temperature.

 The direction that gives the maximum value (Pgd )max or (Pgd site)max should be considered. The above formula applies only to full tanks.

  4.28.1.3 Equivalent calculation procedures may be applied.

  4.28.2 Guidance formulae for acceleration components

  4.28.2.1 The following formulae are given as guidance for the components of acceleration due to ship's motions corresponding to a probability level of 10-8 in the North Atlantic and apply to ships with a length exceeding 50 m and at or near their service speed:

  • - vertical acceleration, as defined in 4.14.1:

  • - transverse acceleration, as defined in 4.14.1:

  • - longitudinal acceleration, as defined in 4.14.1:

 where:

a0 =
L0 = length of the ship for determination of scantlings as defined in recognized standards (m);
CB = block coefficient;
B = greatest moulded breadth of the ship (m);
x = longitudinal distance (m) from amidships to the centre of gravity of the tank with contents; x is positive forward of amidships, negative aft of amidships;
y = transverse distance (m) from centreline to the centre of gravity of the tank with contents;
z = vertical distance (m) from the ship's actual waterline to the centre of gravity of tank with contents; z is positive above and negative below the waterline;
K = 1 in general. For particular loading conditions and hull forms, determination of K according to the following formula may be necessary:
  • K = 13GM/B, where K ≥ 1 and GM = metacentric height (m);
A = : and
V = service speed (knots);
ax, ay, az = maximum dimensionless accelerations (i.e. relative to the acceleration of gravity) in the respective directions. They are considered as acting separately for calculation purposes, and az does not include the component due to the static weight, ay includes the component due to the static weight in the transverse direction due to rolling and ax includes the component due to the static weight in the longitudinal direction due to pitching. The accelerations derived from the above formulae are applicable only to ships at or near their service speed, not while at anchor or otherwise near stationary in exposed locations.

  4.28.3 Stress categories

  4.28.3.1 For the purpose of stress evaluation, stress categories are defined in this section as follows.

  4.28.3.2 Normal stress is the component of stress normal to the plane of reference.

  4.28.3.3 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.

  4.28.3.4 Bending stress is the variable stress across the thickness of the section under consideration, after the subtraction of the membrane stress.

  4.28.3.5 Shear stress is the component of the stress acting in the plane of reference.

  4.28.3.6 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.

  4.28.3.7 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.

  4.28.3.8 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:

  and

  ,

 where:

S1 = distance in the meridional direction over which the equivalent stress exceeds 1.1f;
S2 = 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; and
f = allowable primary general membrane stress.

  4.28.3.9 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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