Page 97 - Pipelines and Risers

yD to take account for dynamic amplifications during a snap-through dynamic buckling
(Nystmm et a1 1997).

4.5 Fracture

45.1 PD6493 Assessment
Fracture of the welds due to a tensile strain is normally evaluated in accordance with PD 6493
(1991).

This assessment method uses a curve (Failure Assessment Diagram) which combines the two
potential failure modes: brittle fracture and plastic collapse.

Maximum weld flaws, described in Statoil R-SF-260, Pipeline Welding Specification, are to
be used as the basic input for the calculations. The flaw has been assumed as maximum
allowable defect due to lack of fusion between passes. The defects and material are assumed
as below:

Type : Surface flaw due to lack of fusion
Depth (a) :3mm
Length Qc) : 50mm
CTOD : 0.20 mm (at operating temperature)
Material : As for Parent material

Surface flaw is chosen as the worst case scenario from acceptable flaws specified in the weld
specifications. The partial safety factors recommended by PD 6493, are as below:

For levels 2 and 3, no additional safety factors are required where worst case estimates are
taken for stress level, flaw size and toughness, and all partial coefficients should be taken
as unity. (Appendix A.l of PD6493).