Page 401 - Pipelines and Risers

368 Chapter 19

been used to determine the acceptability of defects. ECA procedures typically rely on the
application of Crack Tip Opening Displacement (CTOD) test results to determine
maximum allowable defect sizes. The values of defect length are founded upon plastic
collapse calculations which are based on assumptions regarding the flow stress and the
yieldtensile strength ratio of girth and parent metal welds.

Pipeline welds are traditionally inspected using visual examination and radiography. Recently
there have been a number of advances in Non-Destructive Testing (NDT) equipment suitable
for pipeline weld inspection.

Radiography systems are available which produce a real-time image of the weld being
inspected. Normally, a radiograph of the weld is produced by exposing a suitable piece of
film. The film is then processed and developed prior to viewing for interpretation. The real-
time systems produce the image of the weld on a screen which can be viewed without the
need for film processing. The radiographic image is stored on digital laser disc as a permanent
archive and offers instant retrieval. The time to inspect each weld is reduced compared to
traditional methods.

As an alternative to radiography, high speed ultrasonic inspection is available. This method
has become a standard NDT method for inspecting GMAW (onshore) pipeline girth welds in
Canada. Currently available high speed ultrasonic equipment is capable of inspecting a 40-
inch diameter girth weld in 90 seconds. The inspection can be performed immediately on
completion of production welds. A limitation of this technique is that it is not reliable for wall
thickness below 10 mm. For project wall thickness above 10 mm ultrasonic inspection is a
viable option. The use of automated ultrasonic inspection for onshore and offshore pipeline
welding may reduce construction costs.

Non-standard pipeline diameters should be considered. Optimization of the pipe ID based
on modeling of the pipelines in detailed design may demonstrate that the linepipe cost can
be reduced by procuring pipe of the exact ID required as opposed to selecting the larger
standard size, for examples on the Britannia gas pipeline. Conversely, it may be of benefit
to modify the design flowrates to enable selection of a more economical size of pipe.

Elimination of mill hydrostatic test with appropriate increased NDE.

Wall Thickness and Construction
Given two similar design conditions, increasing the grade of linepipe in simplistic terms will
correspondingly decrease the wall thickness and therefore provide cost benefits. In addition to
this, a thinner wall thickness will also have various impacts on construction activities. A
thinner wall thickness will require less field welding and therefore, in theory, has the potential
to reduce constructiodlay time. At present there is insufficient data to make a direct like-for-
like comparison between, say, X70 and X65 for a given pipe diameter.