Part G - Guidance

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 (P_gd )max is the associated liquid pressure determined using the maximum design accelerations.

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

• .3 P_eq should be the greater of P_eq1 and P_eq2 calculated as follows:

  • Peq1	=	Po + (Pgd )max (MPa),
    Peq2	=	Ph + (Pgd site)max (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 P_gd 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 (P_gd )max or (P_gd site)max should be considered. The above formula applies only to full tanks.

LR 4.28-01 For determining Z_β, see also Figure 4.3 and Fig. LR 4.3.

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:

• K

  =

  13GM/B, where K ≥ 1 and GM = metacentric height (m);

• A = ; and

LR 4.28-02 For the purpose of calculating the acceleration components in 4.28.2 the length of the ship for determination of scantlings shall be taken as the Rule length (L) and amidships is to be taken as the middle of the Rule length, L, measuring from the forward side of the stern. The breadth and the Rule length are to be taken as defined in Pt 3, Ch 1,6 of the Rules for Ships.

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.

where:

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.