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Ultrasonic Testing of SAW Pipe |
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Imperfections
Imperfections occurring in SAW pipe seams can broadly be classified into two
categories i.e.
(a) ‘Longitudinal’ imperfections which are generally parallel to the weld
seam e.g. lack of side wall fusion, toe cracks, centre line cracks, lack of
penetration etc, and
(b) ‘Transverse’ imperfections e.g. transverse cracking and volumetric type
imperfections, vertical gas holes, porosity etc.
Note - Chevron cracking does not fall into any of these two categories and
is a special problem requiring a special approach to detection, and is not
included in the proposals for auto equipment. Many of the above imperfections are readily detectable by radiographic testing techniques
especially those listed under ‘transverse’ imperfections and those
‘longitudinal’ imperfections which are not close to the parent metal/weld
metal interface.
These imperfections which, however, are close to the parent metal/weld
interface can be difficult to detect using radiography and ultrasonic is the
only reliable technique to detect e.g. toe cracks lack of side wall fusion
etc., which are generally accepted as being ‘critical imperfections’ in
terms weld seam integrity.
API 5L
API 5L requires mandatory ultrasonic inspection of the weld seam using the
following reference standard (targets) :
(a) Internal/External notches of a depth equal to 5% of the nominal pipe
thickness or a
1.6mm () thro’ hole – trigger/alarm at 100% of signal height, or
(b) Internal/External notches of a depth equal to 10% of the nominal pipe
thickness or a
3.2mm (⅛") thro’ hole-trigger/alarm at 33% of signal height.
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World-wide practice in using the variety
of allowable reference standards varies considerably, e.g.
(i) In Japan, either the thro’ hole is used (normal method) or
notches, located at the weld seam centre, are used.
(ii) In the USA, holes and/or notches are used, sometimes on the
weld centre and sometimes at the weld cap edges, dependent on the particular
pipe manufacturer more importantly on the type of ultrasonic test equipment
in use.
(iii) In Europe, the situation is somewhat different, though not
universally so. Basic European practice is to use notches on the weld edges
(in the parent material) on both sides of the weld seam for the detection of
‘longitudinal’ imperfections, and a single thro’ hole in the weld centre for
the detection of ‘transverse’ imperfections.
In SAW pipe mills in the UK for example, 5% deep notches are used,
internal and external on both sides of the weld, i.e. 4 notches (2 int/2
ext) for ‘longitudinal’ detection and the 1.6mm hole drilled radically thro’
the weld at weld centre is used for ‘transverse’ detection. It should
however be appreciated that many of the oil/gas majors impose supplementary
NDT requirements to API 5L on pipe manufacturers, where apart from the
normal reference standards (targets) used for establishing the test
sensitivity, other targets have to be introduced into a test piece to
establish the equipment’s capability of detection of such targets (and
checked on a regular basis during normal pipe production).
Moving Pipe vs Moving Probe Head
This question of whether to move the pipe past a fixed inspection point
or moving the transducer assembly along the weld seam with a the pipe
stationary is a difficult question to answer.
Most pipe manufacturers prefer to move the pipe as the u/s test facility
is then within the ‘flow’ of pipe thro’ the mill, and is good practice in
terms of flow logistics. When this approach is adopted it is normal practice
to load each pipe in turn onto a bogie or chain conveyor and, with the weld
at TDC, translate the pipe past the inspection point. Translation of the
pipe along a roller conveyor is to be avoided, due to the inability of the
probe head/tracking system to follow instantaneous movement of weld position
imparted to the pipe as the pipe-end engages each roller in turn. The
primary advantage of moving the pipe, apart from flow logistics, is that the
electronics is installed at a fixed location, usually in a controlled
environment within a cabin from where the equipment operator executes the
test.
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