DNVGL - DNV-RP-F108
FRACTURE CONTROL FOR PIPELINE INSTALLATIONS METHODS INTRODUCING CYCLIC PLASTIC STRAIN
|Publication Date:||1 January 2006|
Scope and application
This Recommended Practice considers plastic straining during the installation phase.
The plastic straining shall be limited to typical reeling situations (around 3% nominal strain)
In addition to installation, the commissioning and operation phases must be considered in order to assure safe operation during the whole life of the pipeline.
Although some advice is given in the Recommended Practice, more specific requirements are given in e.g. DNV-OS-F101 .
The Recommended Practice describes:
- tests for characterisation of the materials fracture resistance
- Engineering Critical Assessment (ECA) procedures for determination of acceptable flaw sizes in girth welds
- a test program for validation of the assessment procedure.
The Recommended Practice assumes that the weld strength (combined effect of tensile properties and geometry of both the weld and HAZ) over-matches or even-matches the parent pipe.
If the strength of the weld under-matches that of the parent pipe, the advice and recommendations of this Recommended Practice may not be sufficient and specialist advice is recommended.
Guidance note 1:
This Recommended Practice is mainly based on experience from tests and finite element analyses as well as practical installation experience from modern linepipe steels of type API 5L X52 to X65 welded by modern, well proven, welding methods giving ductile weldments. Pipe dimensions have typically been 6 to 16 inch OD and wall thickness of 15 to 25 mm. The methodology is also considered to be applicable for X70, 13Cr Martensitic Steels and 22Cr / 25Cr Duplex Stainless Steels provided ductile weldments are documented.
Additional work may be necessary if there is a significant difference between the materials and welding methods employed in the pipeline and those mentioned above (e.g. significantly higher strength, significantly lower fracture resistance or significantly different welding methods), or, the predictions of crack extension by tearing differ significantly from what is observed in the Segment tests (see Sec.4).
In cases where extensive experience exists and can be documented both with the linepipe material and welding procedure it may be possible to reduce the amount of testing and analyses recommended in this Recommended Practice.
For pipe dimensions significantly smaller than mentioned above, e.g. umbilical tubes, other testing and evaluation methods should be considered.
In all these instances expert advice is recommended in order to optimize testing and analyses.
It is recognised that testing and ECA methods are still evolving and, consequently variations to this Recommended Practice may be acceptable provided these are supported by appropriate test and analyses results.
Guidance note 2:
Some steels may be susceptible to hydrogen embrittlement both from welding and from cathodic protection. This must be considered when specifying both welding and testing conditions.
In cases where the steel may be susceptible to hydrogen embrittlement and hydrogen could be introduced during welding it should be noted that after completion of welding, the hydrogen will diffuse out of the weld over time. If the time between completion of pipe welds and the plastic straining during pipe installation, is short compared to the interval between completion of the test welds and the testing, then the fracture resistance estimate may be unrepresentative of the real structural welds. This problem can be reduced by either reducing the interval between welding and testing, or by chilling the test weld after welding and maintaining the chill until start of testing; this will reduce diffusion of hydrogen out of the test weld.
Where hydrogen may be introduced during service, e.g. by cathodic protection or sour service operation, it may be necessary to pre-charge the specimen with hydrogen prior to the fracture resistance testing for the assessment of the operation phase.