netic flux that travel through or along
the tube or pipe. A receiver or detector
monitors the test signal and compares it
to a reference signal, a baseline established from a defect-free length of tube
or pipe, containing machined notches or
artificial defects mandated by the various
testing specifications. A material defect
causes a change in the way the test energy flows through the tube or pipe; the
receiver interprets this change as a flaw
and alerts the operator, marks the defect
location, or both.
The EC method is suitable for products that have wall thicknesses less
than 0.200 inch. Many applications require detection of both inside-diameter
(ID) and outside-diameter (OD) surface
notches. This requirement mandates either MFL or UT.
EC. The basis for an EC test is electromagnetic induction. An EC system applies an alternating current (AC) stimulus, typically between 2,500 cycles per
second (also known as hertz [Hz]) up to
100 kHz, to a coil that surrounds the tube
or pipe under test. The AC stimulus creates an alternating magnetic field around
the coil, causing an electrical current to
flow through the tube or pipe. This circular current, the eddy current, opposes
the primary field in the coil.
For ferromagnetic tubes the EC test
coil also is energized with a strong direct
current (DC) magnetic field by means of
a large, separately wound electromagnet. The function of the electromagnet is
to saturate the material under test magnetically. This allows much deeper excitation and removes any permeability
variations that are interpreted as noise.
UT. The UT principle uses sound
waves at frequencies beyond the threshold of human hearing ( 20 kHz), commonly in the frequency range of 2. 25 to
25 MHz. Ultrasonic testing uses either a
piezoelectric transducer or an electromagnetic acoustic transducer. The former generates the sound wave outside of
the object under test and needs a couplant to transfer that energy to the object under test; the latter generates sound
waves within the object under test and
needs no couplant.
MFL. MFL relies on a DC power
source to create a magnetizing field that
generates an intense magnetic flux in the
test material. Internal or surface defects
cause some of the magnetic flux to leak
beyond the material’s surface, where it
can be detected by a flux-sensing probe.
The amplitude and frequency of the voltage generated by the flux sensor corresponds to the severity and location of the
MFL can be a very powerful and economic method to test large-diameter,
heavy-wall pipe and tube, satisfying
many American Petroleum Institute (API)
testing requirements. This method can be
quick and dirty in that it doesn’t need a
Where and How to Test
Testing setups come in two varieties:
online and offline.
Online. Online testing takes place as
the tube or pipe is manufactured on the
mill. It’s a continuous test, running without interruption. It has an advantage in
efficiency in that it doesn’t add any steps
to the manufacturing process. A disadvantage is that it’s a one-and-done arrangement. If the operator questions the
result and wants to verify that the system
has indeed signaled a flaw and not generated a false positive, he can’t back up
the process and test that portion of tube
again. While online testing is a powerful
tool, it can’t be the only tool for oil and
gas products. Because the API mandates
that NDT must be done after final processing, an offline test must be the final
Offline. Offline testing takes place
after the tube or pipe has been formed,
welded, cooled, straightened, annealed,
cut to standard lengths, and gone through
other subsequent processes steps, such
as hydrostatic pressure testing.
Offline tests are discrete tests, check-
ing one length at a time. They require a
secondary handling, which adds time
and additional material handling and
requires more floor space, but the ad-
vantages are versatility, repeatable test-
ing, and a higher inspection level. Many
arrangements are available, and the op-
erator can test an item more than once
if necessary to verify a flaw’s presence,
size, or location. Alarm indications can
be proven (false positives can be ruled
out), and in some cases surface flaws can
UT processes require a coupling medium
between the transducer and the material
under test. The medium usually is water
or a gel. Processes include contact, immersion, bubbler, and rotary.
Contact. An ultrasonic contact test is
almost always an offline setup in which
the transducer is coupled to the object
under test and the couplant is a high-viscosity fluid. Contact testing normally
is a manual operation.
Immersion. Immersion testing is an
offline setup in which the transducer and
the test object are immersed in water.
When the transducer is set so it is perpendicular to the item under test, the
sound beam creates longitudinal waves
in the tube or pipe and can be used to
detect laminations. The same transducer
can be used to measure the wall thickness. When the transducer is set at an
angle to the item under test, the sound
beam likely creates shear waves, which
can be used to detect defects on the OD,
on the ID, or below the surface.
Bubbler. A bubbler system uses a column of flowing water inside a curved
shoe and can be online or offline. The
sound beam is projected through the
water perpendicular to the test surface to
produce longitudinal waves. It also can
be set to an angle to the surface to produce shear waves.
An alternative is an irrigated shoe in
which the sound path is inside a plastic
wedge and only a very thin water-cou-pling layer is necessary.
Rotary. A rotary system is an apparatus in which the transducers revolve
around the tube or pipe as it moves past
them. Although most rotary setups are
offline, on some occasions a mill can accommodate a rotary system.
Transducers are strategically positioned inside the rotary unit to allow full
volumetric coverage of the product to be