Page 409 - Pipelines and Risers
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316 Chupter 19
As the strength of linepipe increases, weld metals of increased strength and sufficient
toughness are required to ensure overmatching behavior of girth welds.
19.6 Material Property Requirements
19.6.1 General
The purpose of this section is to describe the material requirements, and compare the
requirements for longitudinal direction and circumferential direction. Typically, the material
properties requirement in hoop direction are related to pressure containment hoop stress
criterion and bucklinglcollapse under external pressure, while longitudinal properties are
directly specified for bucklinglcollapse under bending and tension, and weldability. See Bai et
al. (2000).
It is beneficial from the viewpoint of manufacturing to allow hoop yield strength higher than
longitudinal yield strength. In the following, requirements will be described regarding Crack
Tip Opening Displacement (CTOD), yield stress, ratio of SMYS and SMTS, fatigue
properties and wall-thickness tolerances.
19.6.2 Material Property Requirement in Circumferential Direction
Necessary CTOD value requirements for Heat Affect Zone (HAZ) and weld metal are to be
established that are relevant for the specific design conditions with regard to type and extent
of longitudinal weld defects likely to exist. Typically, the required CTOD value is established
through ECA (Engineering Criticality Assessment) using British Standard PD 6493.
The extent of longitudinal weld defects that likely to exist, is defined in the operators’
welding qualification specifications. Typical values are: depth 3 mm and width minimum of
25 mm and pipe wall-thickness.
The required CTOD value, as calculated based on codes, is rather stringent, due to large
scatters in the CTOD values from tests. Practical experience from field use of the line pipes
have, demonstrated that there has been very little structural failure due to lack of CTOD value
in hoop direction for line pipes. It is therefore suggested to closely evaluate the following:
CTOD testing methods, scatters and statistical evaluation of scatters;
Possibility to reduce the number of CTOD tests;
Safety factors used in ECA determination of CTOD requirements;
ECA design equations and analysis methods.
Similar observations may be made on the CTOD requirements for the longitudinal direction.
