Tapping for oil: Drilling Performance

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.