Page 93 - Pipelines and Risers

66 Chapter 4

where:

pi= internal pressure
pe= external pressure
D= nominal outside diameter of pipe
t= minimum wall thickness
SMYS (T) = specified minimum yield stress at temperature T
SMTS (T) = specified minimum tensile stress at temperature T
Temperature derating is to be accounted for at elevated temperatures (above 5OoC).
The following usage factors in Table 4.1 apply to the hoop stress equation:

Table 4.1 Usage Factors for Hoop Stress Criteria.
Material Usage Safety Class
Quality Factor Noma1 High
Class
Class B, C ')
Class A~) 0.83 0.77 0.67

Note:
1) In order to apply these higher usage factors, the following additional requirements with respect to linepipe
manufacturing must be fulfilled:
SMYS < (mean - 2*Standard Deviations) of yield stress;
SMTS < (mean - 3*Standard Deviations) of tensile stress;
=2* Standard Deviations of the wall-thickness.
2) After Table 2 in Section 6.4 of IS0 13623 (1997).

Stress Criteria
For internal-over pressure situations, the allowable equivalent stress and allowable
longitudinal stress are q*SMYS(T) and the usage factor q given in Table 4.2 is after Table 3
in Section 6.4 of IS013623 (1997).

Table 4.2 Equivalent stw design factor.
Load Combinations design factor q
Construction and environmental loads 1 .o
Functional and environmental loads 0.9

However, moment criteria, in Chapter 4.4 are considered to have better accuracy for strength
design.