Courtesy of T3 Energy Services

Blowout preventers contain valves with tapped holes for sealing off oil or gas.

Like the parts they thread, taps for oil and gas parts can be complex.

As the Deepwater Horizon drilling platform disaster and subsequent oil spill in the Gulf of Mexico have amply illustrated, extracting crude from the Earth is no easy task. Tapping parts for the oil and gas industry can also be complicated.

The components are typically big, measuring 2 ' to 3 ' in diameter or larger. Workpiece materials range from stainless and alloy steels to nickel-base superalloys.

Parts that require threaded holes include blowout preventers, Christmas tree stack-up assemblies, fracture valves, oil flanges, modules for mud pumps, valve assembly bonnets, crankshafts, riser flanges and sucker rod couplings.

Courtesy of OSG Tap & Die

OSG developed the Hy-Pro HXL taps for horizontal machining applications and the VXL taps for vertical tapping applications for the oil and gas industry.

Blind-Hole Applications

Spiral-flute taps are commonly applied in oil and gas parts, mostly for blind-holes in valves. "In the oil and gas industry, there are many wellhead parts bolted to other parts," said Al Zaitoon, sales and marketing manager for YG-1 Tool Co., Vernon Hills, Ill. "When they get into production, (drilling companies) must have various types of valves to control flow rates of the gas or oil. These valves have a lot of flanges, which are used to connect various components."

But spiral-flute taps are difficult to design because they must include mechanisms for evacuating chips. Removing chips is challenging because they can adhere to the tap, causing the tap to break or the user to recut chips.

"We found the problem in a lot of blind-hole applications was the taps were failing because very long chips were getting 'bird nested' around the shank," said Jeff Stephens, engineering supervisor, OSG Tap & Die Inc., Glendale Heights, Ill. "And when the tap was reversed during machining, sometimes the tap fractured due to the chips being wound up in the flutes."

Spiral-flute taps draw chips upward. Through-coolant taps are particularly effective in blind-hole applications because the coolant helps evacuate chips—but only so much.

OSG has developed two taps for blind-holes in oil and gas parts: the Hy-Pro HXL for horizontal machining applications and the Hy-Pro VXL for vertical machining applications.

"For the horizontal, we shortened the thread lengths and changed the core diameter to allow for more chip-pocket room toward the back, but still have enough strength in the front," Stephens said. "Rather than producing long chips, the HXL breaks the chips into small 6 and 9 shapes, almost like a drill."

He added that the web diameter and flute form of the VXL was changed to allow more chip-pocket room toward the back. Instead of winding around the shank, the chips push from the shank for effective evacuation. "You are still going to have a longer chip, but the VXL inhibits bird nesting."

Another tap available for oil and gas industry parts is OSG's Exotap VC-10. Rockwell Precision Inc., Houston, employs it for parts used by oil field service providers. The 11-employee shop uses the VC-10 for blind-hole applications in alloy steel workpieces. "It breaks up the chips and lasts a long time," said Daniel Cotrino, COO for Rockwell. "Also, the thread finish is really nice even if I just use coolant. I had to put oil on the tap (we used previously) to get a good finish, which is time consuming, but I can just use straight coolant on the VC-10."

Tension/Compression Holders

Instead of rigid tapholders, tension/ compression toolholders are recommended when applying spiral-flute taps for blind-holes to dramatically improve tap life and thread quality. "With rigid tapping, there is no room for error," Zaitoon said. "It is better to have tension and compression. Tension/compression toolholders have a bit of 'float' inside, so you don't bind and break the tool."

But, depending on the toolholder, tension and compression can be challenging when applied to spiral-flute taps. Spiral-flute taps tend to feed themselves faster in tension/compression toolholders. If there is not enough tension or too much, the first few threads become oversize. "There are no threads at the top of the hole," said OSG's Stephens. "It is easy to tell when overfeeding occurs because there are feed marks on the front side of the flank of the tap."

To avoid overfeeding, users can decrease or increase toolholder compression or manually reduce the feed rate by 5 to 10 percent from recommended levels. "It is usually trial and error," Stephens said.

Tap Design

Common workpiece materials for oil and gas parts include 4140 and 4340 alloy steels, stainless steels and nickel-base alloys, such as Inconel and Monel, which provide strong corrosion resistance for subsea applications.

As with any application-specific product, tool design is based on the workpiece material characteristics. "Tap features, such as substrate, rake angle, helix angle, relief, number of flutes and chamfer, are all influenced by the material to be machined," said Mark Hemmerling, director of marketing for Walter USA LLC, Waukesha, Wis., which makes the Paradur Inox line of taps for the oil and gas industry.

Typically, spiral-flute taps for stainless steels have a high spiral and a high cutting angle. This allows a freer cutting action because stainless steels have a tendency to rapidly workharden. Inconel also has a tendency to workharden and requires taps with extra relief and a strong cutting angle for rigidity. The cutting angle is typically neutral or slightly positive, which provides it with excellent chipping resistance and rigidity. Otherwise, the material can shrink around the tap during cutting. Once that happens, it can compress and break the tap, Stephens noted. (All materials shrink slightly after tapping or drilling and the amount of shrinkage varies from material to material. The problem is most pronounced in difficult-to-machine materials.)

Courtesy of Walter

Walter Prototyp's Paradur Inox taps with THL coating are for threading oil and gas parts.

Also, with higher pitch diameter and OD reliefs, there is less tap contact with the hole. When that occurs with the tension/compression toolholder, the tap itself has an even greater tendency to overfeed. "It is all interrelated," Stephens said.

The rake angle for spiral-flute taps also depends on the workpiece material. Materials that produce long chips normally require a tap with a high rake angle. For example, taps for steel typically have a normal cutting edge rake. Stringy materials, such as stainless steels and aluminum, require taps with a higher rake angle.

Taps for threading materials for the oil and gas industry are typically made out of cobalt or P/M HSS and are coated. YG-1's Hardslick coating, for example, raises the surface hardness of the tap. "The harder the coating, the more wear resistance you get. The coating also decreases the coefficient of friction," Zaitoon said. "This allows for lower torque requirements, the chips don't stick to the tap and it leaves a good finish on the thread."

Many spiral-flute taps being used to make oil and gas parts are 8 pitch (tpi) and 1 " to 2½ " in diameter. "Anything with 8 tpi is an oil field-type of tap," said Zaitoon. "It is a strong, coarse thread. You need a big, strong thread to bolt things together."

Stephens added that 8 tpi is the ideal thread pitch in the 1 "- to 2½ "-dia. range that still allows for assembly and disassembly. Too many threads make it difficult to assemble or disassemble the part; too few reduces thread integrity.

Standard length for a 1 "-dia. tap is a little over 6 ", but specials are available that are 10 " or 12 " long or longer. For these bigger taps, more flutes are better—typically five or six. Although having fewer flutes enhances chip evacuation, "there is more rigidity with a larger number of flutes," Stephens said, explaining why OSG uses six. "This allows us to spread the wear across one more flute and thin the resulting chip. We were able to add the extra flute and not compromise chip-pocket space, due to our special flute form."

In spiral-flute tapping, it is important to find the proper surface footage because chip formation changes with speed. "Feed rate is governed by the spindle speed and the pitch," Hemmerling said. For example, at 20 sfm, a Walter Protoyp 1-8 Paradur Inox tap with a THL coating going 2 " deep in 316 austenitic stainless steel runs at 75 rpm, feeds at 9.4 ipm, requires 202 ft.-lbs. of torque and uses an estimated 2.4 hp.

Terry Sebastian, CNC programmer and general foreman for Numerical Precision Inc., Crosby, Texas, uses the Paradur Inox in blind-holes for subsea and offshore equipment. "I can run them at twice the normal feed and speed and get three to four times the tool life compared to other taps I've used," he said.

Applying Roll Form Taps

Roll form taps are also commonly applied to machine oil and gas parts. These taps displace material rather than cut it.

Jarvis Cutting Tools Inc., Rochester, N.H., produces deep-hole taps for use in oil field parts. Its 10-tpi Jarflo HSS, PVD-coated form taps are for threading through-holes in sucker rod couplings. Typically, the thread length of the coupling is 4 " and the workpiece material is 4130 steel. Tap diameters range from 5⁄8 " to 15⁄8 ".

"Sucker rod couplings are used in large oil field pumps," said Mark Ford, Jarvis' marketing manager. "Attached to the end of the pump arm is a sucker rod, which can go several miles down into the earth. You have to keep building it to go deeper and deeper, so the couplings that hold these rods together need to be very strong and long in terms of thread engagement." The rod diameters vary but can be as large as 2½ " in diameter.

The couplings are 10 tpi, the American Petroleum Institute standard. "Forming 10-pitch threads in and of itself is difficult, and these threads also need to be strong and durable," Ford said. "And 4130 steel has a propensity for workhardening. So you are using a form tap to quickly thread this very deep hole where it becomes increasingly difficult to get coolant down into the hole."

Courtesy of T3 Energy Services

In the oil and gas industry, many wellhead parts are bolted to other parts.

Most manufacturers form thread sucker rod couplings since the resulting threads are stronger than cut threads. Rather than cutting or breaking apart the individual grains in the work material, which happens when cutting, the grains are left whole and moved into position.

However, the correct operating parameters are critical when form threading. "There are some schools of thought that say you can run it too fast and it overforms into the minor diameter," Ford said. "Certainly, you can run it too slow and not get the material to move fast enough so that it underforms in the minor diameter. Usually, the rule of thumb is that you run it one and a half to two times faster than you would for cutting."

Unlike drilling, where an operator adjusts the chip load simply by changing the feed and speed, when tapping, the operator's only option is to make it go slower or faster. "Depending on the size of the tap, the speed changes quite a bit, but generally speaking for 10-pitch threads in 4130 steel, your sfm will be somewhere in the 30 to 40 range," he added.

Cheboygan (Mich.) Tap & Tool Co. is another supplier of form taps for the oil and gas industry. The company recently designed a tap for a 4100 steel coupler made by a Canadian oil company.

"It was unusual in that it was a two-start, 10-pitch tap," said Mike Kelly, engineering manager for Cheboygan. "As the threads go around in a circle, they have a helix angle that takes them from the beginning to the back. With a two-start, they are 180° apart so the lead is twice as long as a normal thread. Rather than 0.1 ipr, it travels 0.2 ipr. This allows the coupler to be assembled to another part faster."

Tapping parts for the oil and gas industry requires careful attention to detail. As recent events have proven, part failure can be catastrophic, so every effort must be made to create a perfect part thread. Whether that is done with spiral-point or roll-form taps, there's no cutting corners when threading oil and gas parts. CTE

About the Author: Susan Woods is a contributing editor to Cutting Tool Engineering. Contact her by e-mail at [email protected].

Contributors

Cheboygan Tap & Tool Co.
(800) 633-3133
www.cheyboygantap.com

Jarvis Cutting Tools Inc.
(800) 258-7162
www.jarviscuttingtools.com

OSG Tap & Die Inc.
(800) 837-2223
www.osgtool.com

Walter USA LLC
(800) 945-5554
www.walter-tools.com

YG-1 Tool Co.
(800) 765-8665
www.yg1usa.com