Partnering with a coupling
expert can help meet or exceed the industry’s most rigorous tolerance and quality
DEL WILLIAMS, PTR Group consultant
In the production of oil and gas, oil country tubular goods (OCTG),
such as casing, tubing, piping and pipelines, are the foundation of well and
pipeline design. As a result, the industry mandates that OCTG products are manufactured
according to American Petroleum Institute (API) standards and graded, based on performance
and materials used.
Within this broader category, precision threaded
couplings play a critical role in securely,
reliably connecting multiple lengths of casing and piping of varying sizes. These
couplings typically have internal threads (box) that are machined to match the
external threads (pin) of the casing. Unfortunately, couplings are under tremendous
pressures, high loads and extreme torque requirements are pushing the limits of
first-generation connections beyond the API and International Organization for
Standardization (ISO) standards required of OCTG products.
OCTG challenge. The core issue is that oilfield
couplings fatigue and fracture, due to stress concentration, particularly if reused
several times. Extreme stress, torque and internal pressures can all contribute
to coupling failure. Exposure to acidic fluids can also cause accelerated corrosion
and degradation in the coupling material. The extreme forces and harsh environment
can also compromise the coupling’s threaded connection, leading to leakage, loosening
and eventual failure.
The result can be catastrophic. When
couplings crack, split, rupture, fall off or cause a pipe string to break, it can
lead to substantial financial loss while posing serious environmental, health
and safety risks. Although couplings that meet established API and ISO
standards have traditionally been able to withstand most of these stresses, current
industry practices have increased internal pressures and pushed conventional couplings
beyond the yield point.
As a result, the semi-premium and
premium couplings are now in demand, and specialty machine shops that produce OCTG
threaded couplings are stepping forward to resolve oilfield challenges by using
advanced machining techniques, unique design features and specialized coatings
to manufacture couplings that surpass API and ISO requirements, Fig. 1.
More precise, specialized couplings required.
The greater demands placed on couplings in oil and gas operations are encouraging
a market shift toward more precise and specialized couplings. This is the
belief of Philip Moroco
at PTR Group, a multi-company business enterprise that offers superior contract
manufacturing of components and subassemblies in metal and plastics.
semi-premium and premium couplings require very close tolerance machining, approximately
0.001 inches (in.) for thread leads and tapers, and 0.0005 in. on the thread profile,”
says Moroco. “While
most CNC machinists can hold very close tolerances, it can be challenging to make
precision couplings that meet the required specifications, given the types and grades
of material used,” Fig.
notes that the material commonly used for these couplings is hot rolled,
seamless ASTM 4,125 through 4,140, quenched and tempered to the requirements of
API P110. The medium-carbon chrome-molly steel is known for its
hardenability and toughness. Thus, it provides these connections superior
mechanical properties for deep wells.
According to Moroco, semi-premium couplings
are machined to tight tolerances, but are based off an established
API thread profile. This can include a buttress thread profile (a screw thread profile with an asymmetric shape)
that allows the thread to have low friction and withstand greater loads in a
single direction. Premium couplings are machined to the tightest tolerances and
may incorporate substantial design changes, often to the threads, Fig. 3.
the wedge-style thread, which is designed to create the largest possible contact
surface area at make-up to provide superior compression and torque strength,
compared to couplings with conventional threads. However, wedge-style threads
are much more difficult to manufacture than standard or semi-premium options. “When tightened, a wedge-style thread engages
to create a very strong metal-to-metal seal in the connection,” says Moroco, adding that the four-to-one
thread lead also speeds assembly of the coupling to the casing.
Manufacturing high-quality, cost-effective
couplings. American Cap
Co., the division of PTR Group that specializes in machining precision couplings,
can operate virtually 24/7 with three shifts and has produced as many as 40,000
units in a month for an OCTG client. Although the company can source the raw materials,
they prefer to receive the metal coupling stock from the customer. This provides
what Morocco describes as a “win-win,” because American Cap can hold the stock
in inventory and convert the parts in what he calls “quick-turn coupling manufacturing”
while the customer avoids paying the standard markups.
According to Jeff Kneubehl of Wheatland
Tube, American Cap has produced many types of API couplings under this type of
arrangement for connection to horizontal, above-ground line pipe, merchant
coupling and rigid conduit coupling. Wheatland Tube is a division of Zekelman Industries,
the largest independent steel pipe and tube manufacturer in North America. “American
Cap does everything needed for couplings. Moreover, their specifications,
identification and inspection are significantly beyond what we require,” says Kneubehl.
Kneubehl says Wheatland Tube initially
utilized American Cap as a source for plastic thread protectors of various
sizes for the ends of tubing and piping. The company designed the thread
protectors, made the injection molding tool, and manufactured the parts on
Impressed with the operation,
Wheatland Tube asked PTR group to produce API couplings. The ability to produce
precision parts from different types of materials, including many types of metals,
alloys and plastics, is the result of an aggressive strategy of acquisition by PTR
Group, American Cap’s parent company.
The company has strategically acquired
companies that offer complementary parts, with manufacturing differentiated by material
and product type, to broaden the breadth of their overall parts portfolio. The growth
strategy, which is known in M&A circles as concentric acquisitions, allows
PTR Group to share resources and facilities across companies to lower costs for
Conforming to API standards. Moroco notes that American Cap can customize
couplings to meet the certification requirements of any OCTG client. The
company currently manufactures seamless coupling products, including API–5L and
API–5CT for the natural gas and oil industries. The API – 5L couplings meet the
standards for use for conveying gas, water and oil. The design covers seamless
and welded steel-lined pipes and includes plain-end, threaded end, and
belled-end pipe, and pipe with ends prepared for use with special couplings.
API–5CT seamless couplings provide
standards for casing and tubing. This standard specifies the delivery
conditions for steel pipes, coupling stock, coupling material and accessory
material. The OCTG client can specify numerous factors, such as the grade of material
(i.e.-API or proprietary), operating conditions it must withstand, and assembly
process used, Moroco continued.
Todd Diehl, PTR Group manager, said American
Cap upgraded their production line by adding manganese phosphate coating, an
inorganic compound used to protect the surface of steel products in various
industries. Among general phosphate coatings, manganese phosphate provides the
highest hardness, as well as superior corrosion and wear resistance, compared to
traditional coatings like zinc phosphate. “Traditionally zinc phosphate has
been the thread coating of choice, but manganese phosphate is much more durable
and has a better coefficient of friction. That makes it a superior coating for wedge
style couplings,” says Diehl.
Innovative tracking capabilities. American Cap also introduced individual
tracking of each coupling, using a serial number for traceability, instead of tracking
items by lot, which is the industry norm for API. “The individual part traceability
allows us to track the item from the point the material arrives, through machining
and inspection, to when it penetrates the ground in drilling operations,” Diehl
continued. “We maintain a high level of quality and accountability. So, if the coupling
comes back for whatever reason, we have access to all the information about the
production of that particular part.”
World-class manufacturing reduces risk
and minimizes NPT. As
the oil and gas industry faces increasing demands for coupling reliability and
performance, OCTG manufacturers can meet the increased performance standards by
partnering with a coupling supplier that has an in-depth understanding and proven
expertise in materials science and thread coatings, Fig. 4. PTR Group’s manufacturing
engineers and connection experts can play a pivotal role in ensuring quality and
reliability of their customer’s tubular goods. The approach fosters greater
collaboration in parts design, engineering and fabrication to resolve evolving component
challenges and requirements. WO
DEL WILLIAMS is a PTR Group consultant and technical writer, based in Torrance, Calif. He
writes about health, business, technology and educational issues. Mr. Williams
has an MA degree in English from California State University, Dominguez Hills.