TWI Industrial Member Report Summary 901/2008
By W Xu and W He
Some of the pipeline installations today and in the future require plastic design for strain levels up to 2-3%. Design criteria for avoiding plastic instability and collapse while allowing locally large plastic strains are formulated in some of the pipeline design documents. This approach is generally referred to as strain-based design.
Likewise, an engineering critical assessment (ECA) of flaws for specified design strains is termed as strain-based ECA. The DNV design document for submarine pipelines currently recommends BS 7910 for ECA. Although the basic stress-based failure assessment diagram (FAD) approach in BS 7910 can be used for strain-based design and assessment, it is necessary to convert the applied strains into applied stresses. It is therefore desirable to formulate ECA procedures in terms of applied strain and particularly to use the general concept of a strain-based FAD for ECA.
The fracture toughness of pipeline steels and associated weld metals has continued to improve over the years, such that brittle fracture can now be avoided at most normal operating temperatures. In parallel, the confidence of the pipeline industry in the use of ECA to determine flaw tolerance and weld acceptability criteria has grown significantly. Experience indicates that the margin of safety in terms of acceptable weld quality may be large in some ECAs using currently available standards.
For offshore pipeline installations, some changes in the details of ECA have already been made. Notably, these changes include the increased plastic collapse limiting flow stress for displacement controlled loading, the use of low constraint fracture toughness specimens and the use of medium-scale tests. The present work is part of TWI research activities to further improve ECA procedures for onshore and offshore pipeline installations.
- Investigate effects of crack size, loading and material behaviour on the elastic-plastic crack driving force Jep by finite element (FE) analysis.
- Explore the concept of a strain-based FAD for ECA of flaws in girth welds under large strains.