Oilfield Pressure Vessel
Failure
A
high pressure
stripper bowl adapter, containing a test plug failed
during pneumatic testing, resulting in a fatality. A stripper bowl is part of
a snubbing unit used in high pressure oilfield applications.
Metallurgical failure analysis focused on the failed
threads of the adapter ring being used at the time of the
stripper bowl failure. Finite element analysis was also
conducted to determine loads at the time of failure.
Metallurgical failure analysis resulted in determining
that deficient manufacture had resulted from improper
machining of a heat treated forging. Finite element analysis
(FEA) revealed that
the stripper bowl design was defective.
Photograph A: Overall view of high pressure stripper bowl test plug ejected from
the stripper bowl adapter ring during pressure testing.
Metallurgical failure analysis resulted in several
opinions and conclusions in this matter:
- According to the engineering
drawings, the manufacturer knew that several choices
of materials for construction were possible for the
subject stripper bowl adapter ring, hereafter
referred to as the "subject adapter." The
engineering drawing listed AISI 4130/4140/4142 as
possible materials of fabrication.
- The manufacturer allowed the
machine shop to select the alloy (AISI 4130,
AISI4140, or AISI 4142) to be used to fabricate the
subject adapter. The machine shop chose and
specified forged and heat treated AISI 4130 for the
subject adapter. The choice of AISI 4130, a low
hardenability steel, by the machine shop, as the
material of fabrication for the subject adapter ring
was inappropriate.
- The machine shop that
fabricated the subject adapter ordered a forged,
heat treated ring of AISI 4130 from the steel
forging provider. The machine shop should have known
that AISI 4130 is a shallow hardening, low
hardenability steel.
- The machine shop machined the
outer diameter of the subject forging from 12.63 ti
11.75 inches in order to fabricate the subject
adapter.
- The machine shop should have
know that when the outer surface of a forged and
heat treated section of AISI 4130 is removed, the
underlying inner core (or interior) metal will be of
lesser hardness as well as tensile and yield
strengths than the metal near the forging surface.
- AISI 4140 was chosen as the
material of fabrication for the stripper bowl test
plug. Hardness test result recorded on the hardness
map of a section from the test plug show that indeed
AISI 4140 has a much more consistent hardness
through the full test plug thickness. The
manufacturer and the machine shop should have been
well aware of the better and uniform strength
properties available in heat treated AISI 4140 and
AISI 4142.
- During laboratory testing of
the failed subject adapter, tensile specimens were
machined from the wall of the subject adapter.
- Tensile test results were
obtained from the test specimens fabricated from the
wall of the subject adapter. Tensile tests resulted
in reported yield strength values of 75,300 and
71,00 psi. However, the tensile specimens were cut
from an area of the subject adapter after the
machine shop had machined away the harder, stronger
outer rim of the forging, in order to fabricate the
subject adapter, leaving a softer core of AISI 4130.
- The tensile yield strengths
of 75,300 and 71,000 psi, obtained during tensile
testing, reflect that testing at this location is
not indicative of the original 1/4T (midradius)
hardness or strength. The testing laboratory took
their tensile test specimens from the only source
available, i.e., from the subject adapter wall which
had been fabricated from the center (core section)
of the AISI 4130 steel forging.
- The tensile test results
obtained were also probably low because of defects
in the sample preparation and tensile testing
procedure. The tensile test results also reflect the
fact that the tensile test samples were taken from
the center or core region of the forging, after
machining by the machine shop. Tensile test
specimens from these core or interior areas would be
expected to have a lower hardness, tensile and yield
strengths relative to the forging 1/4T (midradius)
values.
- Machining of the forged ring
was done by the machine shop in order to fabricate
the subject adapter. Reduced tensile and yield
strength properties would be expected for properly
heat treated AISI 4130 AFTER machining down the
outer ring diameter and increasing the forged ring
inner diameter. Low yield strength results are not
an indication that the steel forging company
supplied improperly or insufficiently heat treated
AISI 4130.
- Visual observation of the
deformed threaded lifting eye holes in the upper rim
of the stripper bowl adapter ring confirmed that
under the influence of the 15,000 psi test pressure,
the stripper bowl adapter experienced generalized
yielding. Generalized yielding, of the subject
adapter, dictates a design defect and is not the
result of -10% yield strength steel.
- Failure of the subject
adapter would have occurred regardless of whether
the adapter had a yield strength of 70,000, 75,000,
or 80,000 psi.
- It was appropriate for the
steel forging supplier to rely on the test results
of their outside testing laboratory in certifying
that the subject forging conformed to the required
mechanical properties.
- In filling the machine shop
order for forged and heat treated AISI 4130, the
steel forging company followed proper and prescribed
procedures. Nothing that the steel forging company
did contributed to the subject adapter failure.
- As a general principle, most
engineers and testing technicians believe that
"water is non-compressible" (at normal operating
pressures). However, the fact that water is slightly
compressible at high pressures, such as those being
used during the hydrostatic test of the subject
stripper bowl assembly at the manufacturer test
site, makes it incumbent that engineers and testing
technicians be made aware of such facts (and
inherent dangers thereof) before conducting
hydrostatic testing at elevated pressures.
- It is also quite possible
that the lack of an air release port on the test
plug resulted in some air entrapment in the subject
stripper bowl assembly prior to the initiation of
hydrostatic testing.
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 Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical consultant and accident investigator. He has testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- r.c.jerner@metallurgist.com |
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