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that can increase box bulging to the point
of separation, whereas premium thread
forms with reverse-angle load-flanks, such
as RQ™ and XQ™, limit box bulging.
Since heat check cracks are the result
of fatigue loading, they always start
from the surface and form perpendicular
to the direction of the expansion and
contraction, which is longitudinal or axial
on a drill rod and easily pass across turns
of thread. Fatigue failures that result
from excessive drilling loads or excessive
deviation always produce cracks that start
internally and form perpendicular to the
axis, or circumferentially, and typically
follow the thread. In other words, since
there are no drilling loads that act
circumferentially, the only way to form
longitudinal or axial cracks is through
rapid heating and cooling.
Also, consider that the fatigue strength
of any steel is less than only 50% of its
normal ‘yield strength’ and that hardened
steel is much more brittle than tempered
steel. When subjected to excessive fatigue
loading, the maximum expected life of
any steel is less than three million cycles
of alternating load. In terms of a spinning
wireline drill rod, this represents less than
a few days operation — at most. This
same material limitation is behind fatigue
failures in drill rod joints when subjected to
excessive deviation.
These limitations hold true for all
wireline drilling industry carbon and alloy
steel grades, all tube forming processes,
all parallel and variable wall tubing, and
all heat treatment configurations. This is
because the transformation temperature
is determined by carbon content and
does not worsen significantly without
abnormally high levels of carbon (for
example, the grade AISI/SAE 1541
has excessive carbon and a reduced
transformation temperature, which was
a common grade before heat treatment
became popular). Also, whether a drill
rod was initially heat treated or not, steel
will transform when frictionally heated
above its critical temperature, regardless of
previous material properties.
Further, these material limitations hold
true regardless of whether a drill rod is new
or used; that is, transformation properties
do not change with use. Operators who
suffer from heat check cracking often claim
to have older rods that did not crack, which
is just a coincidence. That is to say, whether
or not a heat check cracking incident occurs
depends on a difference in the loading —
possibly in the same hole — rather than
any difference in the drill rods.
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Avoiding heat check
cracking failures
In summary, heat check cracking is the
result of an application issue where there is
heat generated by lateral contact pressure
and insufficient lubrication, leading to
fatigue failure.
Lateral contact pressure, or ‘drag’, can
obviously be created with a misaligned
drill rig or when the drill rod string
passes through a deviation, but more
typically is associated with drill string
buckling, due to excessive drilling torque,
rotational speed, and thrust or ‘weight
on bit’. A reduction in any of these
operating parameters will reduce the
lateral contact pressure.
However, in some cases, drill rod
midbodies may have been permanently
twisted or ‘bent’ due to dynamic overload
or due to roller rod handlers, which adds
to lateral contact pressure. Alternatively,
select a free-cutting drill bit which will
allow for reduction in torque and thrust.
You can use a Boart Longyear shoulder
thickness wear gauge to inspect for
uneven wear around the box shoulder
circumference, which can provide an early
warning to adjust operating parameters or
improving lubrication.
It is helpful to apply and maintain
an appropriate lubricant or grease
coating on the outer surface of the drill
string, which can directly reduce the
friction factor and the heat generated.
During rod pulls, visually inspect how
the coating has worn away as a good
indicator of the degree of lateral contact.
‘Shiny one-sided’ wear patterns, either at
the box ends or with a ‘slow’ one-quarter
spiral turn over the rod length, indicate
excessive loading.
While drilling fluids contribute to heat
check hardening by quenching heated
surfaces, maintain a high fluid pressure
and consider polymer additives to improve
lubrication, and reduce frictional heating
and quench severity.
Finally, consider this recommendation
from the IADC/SPE: “When users are
confronted with the cause of failure, the
initial reaction is often disbelief … It is
difficult to believe that tool joints can be
heated above their critical temperature
while drilling … in the presence of
mud circulation. From time to time,
knowledge of the in-hole friction heating
and cyclic quenching phenomena and
characteristic longitudinal cracks need to
be reintroduced to the field. Users and
inspectors need to recognise the evidence
and cull out the affected joints.” b
Various wireline coring drill rod samples with
heat check hardening and cracking.
About the author
Chris Drenth has a Bachelor’s degree
in mechanical engineering and has
been with Boart Longyear since
1995. He has been involved with
the design of the company’s coring
products and tooling throughout his
tenure and has held the role of global
engineering director for performance
tooling since 2008. Chris has filmed
videos on performance tooling
(specifically on Genuine Q™
Wireline and Roller Latch) for Boart
Longyear’s YouTube channels.
JANUARY 2019 MINING MIRROR
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