Abstract:
The most commonly used method
for
non-destructive
testing (NDT)
of welded tubulars in
underwater
locations is
magnetic particle
inspection (MPI). This method
is
effective
in terms of crack or
defect detection, but
requires much
diver
effort.
This work examines the
use of
Hall effect probes
for
crack
detection
and measurement
in
steel specimens and underwater pipelines and structures. A
simple theory of magnetic
leakage fields is developed,
and
how
such
fields
relate to crack characteristics.
The
finite
sizes of the Hall probes employed are taken into
account, and an analytic expression
for the field from
a
tapered crack
is developed. Practical magnetic signals
from
a cracked
Y-jointed tubular are taken, and shown to be
consistent with
MPI indications.
A double
probe system
is
proposed which enables crack
depth
measurement
to be
made
irrespective
of a
knowledge
of
the
crack width or
level
of magnetisation
in the
specimen.
Experiments
using a prototype
double
probe system show
encouraging results on artificial cracks
in
small
specimens, though there is
a troubling unknown
background
bias
effect
in the measured signals.
An instrument
using a time differentiated
probe signal
has been developed
which
is
capable of
detecting
a crack
in
a Y-joint at a scan
height
of up to 5mm
with a
level
of
magnetisation rather
less than that
used
by MPI. A method
of continuously monitoring a crack
in
a Y-joint is
also
described,
using multiple
differential
pairs of probes.
The method
is found to give
indications
consistent and
comparable with MPI.
