Machining in the Oil Patch
Small lots and challenging tolerances test companies working in this demanding field
By Bruce Morey
There are many ups and downs in the market for oil-production equipment. "It's a cyclical business we're in. The current cycle is not as much fun as the upside. However, it's something we're used to," remarks Clay Williams, CFO of National Oilwell Varco (NOV; Houston). To illustrate, the number of active rigs operating in North America dropped from 2256 to 1505 from December 2008 to December 2009, a 33% drop in demand, according to the December 7, 2009 Baker Hughes rig-count report.
Although new investment in rigs is down, this has not stopped NOV from investing in technology. Operating rigs need to drill faster, farther, and cheaper—using fewer personnel and truckloads. They need to spend more time drilling and less time in nonproductive setup and teardown onshore. The challenge in deep-water production is sending drills driven by pipes into the ground below 2 or 3 miles (3.2 to 4.8 km) of seawater—then drilling tens of thousands more feet horizontally. High temperatures and pressures at such depths mean more parts must be made of cobalt alloys such as Inconel, as well as high-strength, low-alloy steels like 4130. Components may have to deal with ambient temperatures and pressures of 700°F (371°C) and 40 kpsi (276 MPa) someday.
Drilling is not just drilling, either. Equipment used to drill for gas is different from that used to drill for oil. Land rig equipment differs from that used offshore. Product variety is the key to profitability. "In the rig solutions business alone, which is about 50% of what NOV delivers, we can produce more than a million separate manufactured goods made in 28 separate plants," says Greg Renfro, VP of global manufacturing improvement for NOV.
He describes NOV as a 'mass-customization' manufacturer rather than a mass-production company. Careful design around core architectures for each product—like top drives or iron roughnecks—is important. So is the design of the machining process. "In one of our more sophisticated plants, the average Economic Order Quantity [production lot size] is about 3.5. With our machines, setups, and programs combined with aids like part-probing, we can make one of something as economically as we can make 10."
This has an impact on the machine-tool supply base. It needs to deliver flexible machines, tools, and supporting equipment. "If I go out and invest $2M in a big, four-axis floor mill, as an example, I might need to make 1000 different parts. Of those, there may be 12 I may not be able to produce anywhere else. Economically, I need all 1000, because in all probability I couldn't justify purchasing the machine only to produce those 12."
Versatility and volatility are familiar to smaller manufacturers as well. Representative of such companies is Hole Specialists (Tomball, TX), a 40-employee machine shop that specializes in producing precise, straight, long holes, primarily for offshore drilling equipment. Larry Robinson, president, points to three key elements of his success: expertise, fixturing, and machines.
His biggest challenge in oil and gas equipment is finish and straightness. Pipes and casings used in oil and gas production often need to slide through one another—perfectly, every time—without jamming thousands of feet below the surface. The American Petroleum Institute (API; Washington, DC) developed drift-diameter standards for holes in pipes to ensure downhole equipment does not stick or jam. "There is often a part, inside a part, inside a part, with a tool working inside," explains Robinson.
To drill its holes, the company uses 34 machines, among them multiaxis CNC milling machines and nine honing machines from Sunnen (St. Louis). A massive investment in specialty honing heads, along with versatile fixturing, much of it designed by Robinson himself, completes the equipment part of the equation. "For example, we have quick-changeover V-blocks precision made where we can switch from a 4" [102-mm] OD to a 30" [762-mm] OD if we need to," he observes. After first gundrilling many holes, he hones all of them to meet surface-finish and drift requirements. Honing helps especially in meeting drift-straightness tolerances. Today they create holes up to 18" (457-mm) diam and as long as 50' (15-m) that meet these drift tolerances. Robinson predicts that future requirements will be even more challenging. "I think they are going to specify drift [in the near future] as less than 0.0005"[0.013 mm] per inch [25 mm] from deviation. In fact, two years from now, I think they are going to be down to 0.004 or 0.005"[0.10 or 0.13-mm] per foot [305 mm]. That means that if you drill a hole 20' [6.1 m], you have to be within 0.050" or 0.100" [12.7 or 2.5 mm] on the other end. It gets rather difficult."
Dennis Westhoff, applications engineer and business development manager at Sunnen, agrees that size tolerances for oil and gas are not as challenging as straightness specifications and surface finish in cylindrical bores. "Our technique for controlling taper variation and drift to meet API specifications is to make our abrasive length longer in the honing tool," explains Westhoff. "If the straightness requirements get really tight, we will go to double-banked or triple-banked tools with two or three times the abrasive length of our standard tools. That gives a bridging effect to straighten out any banana bend in the bore. We also do fractional-size bores—small diameters—honing bore lengths that are 5 and 6' [1.5 and 1.8-m] long. Controlling a honing bore for a 3/8" [9.5-mm] ID hole that's 5' long requires finesse and sensitivity."
Low-cost honing technology has also recently been developed for the production of orifice meter tubes used to measure gas flows. More than a million orifice meters are used in the US, according to Sunnen. To help automate a manual process, Sunnen introduced the HTA tube hone, aimed at surfacing applications, where stock removal is in the range of 0.030" (0.76 mm).
Machining bigger parts faster is another way to respond to volatility. That's what MAG (Fond du Lac, WI) delivers in its VDF 800 DUS Big Bore CNC lathe, a new release to North America. With a 14"(365-mm) spindle diam, 17,600-lb (7990-kg) part capacity and 4500 lbf (20-kN) Z-axis cutting force, it's advertised with turning length sizes of 3.3, 6.6, 9.8, and 13.1' (1, 2, 3 and 4 m) as standard, and up to 65.6' (20 m) optional. It also runs fast—to 900 rpm—with a spindle motor rated at up to 74 hp (55 kW.) A two-speed transmission provides the higher rpm for fine finishing cuts and single-set-up machining of large parts, enabling work on smaller-diameter parts, according to Juergen Reingen, global product leader, horizontal turning for MAG. "You can also put a milling head on it, you can put grinding on it, there is much flexibility on this machine," he states.
Another angle on the machine's flexibility is its teach mode, using hand wheels to develop cutting routines rather than relying only on CAD-derived CAM programs.which it can still import. Reingen points out that it may take as much time to develop a CAM program for many oilfield parts as it does for a skilled machinist to develop the program manually.
Along with its newest lathe, MAG also provides horizontal boring machines, universal machining centers, vertical turning machines, and HMCs, all of which stress single-setup machining for large parts. Contouring heads on boring mills, for example, provide a programmable axis capable of turning and complex machining with fewer tools, according to the company. Automation, like thermal growth compensation on the spindle, also increases ease-of-use.
MAG sees part sizes increasing, no doubt because of deep-water drilling. This drives processing on one machine. "Not only is it more time-consuming to move parts that weigh 20.60,000 lb [9.1.27,240 kg], but when parts are moved from machine to machine, precision can be affected, because setup errors can stack up." MAG also notes that surface finish is more critical than absolute tolerance.frequently as fine as 16 µin. (0.4 µm) rms. Finish quality depends heavily on a rigid machine, and MAG enhances rigidity with patented technologies like thermal growth compensation for boring-mill spindles.
Selling machine tools today is a process of developing an engineered solution, according to Dana Scott of Mazak (Florence, KY). He agrees that market conditions in oil and gas dictate small lot sizes and variety. Not all have adapted as well as others, however. "When we study a shop, the first visible tell-tale sign of a problem is pallets of partly finished parts waiting to get on a machine. A shop might spend a week getting two parts through. They might run four or five different operations across four or five different machines," he explains. Combining operations raises the complexity of that multitasking machine. (Read—the price of the machine goes up.) "That's why it's not the price tag on the machine that is important; it's the price of the process."
Higher machining speed, he points out, provides its own flexibility. Mazak's newest offerings concentrate on reducing noncut time, keeping the spindle cutting for as long as possible. "We are using super-high-gain servos that accelerate to full velocity in 0.1 sec. We try to halve the time to accelerate the spindle to target rpm." Customer part tolerances are getting tighter as well, especially for items like pumps, valves, and other complex parts that often need five-axis CNC machines. Some customers need machines capable of delivering 0.0004" (0.01-mm) tolerance.
Scott points to three Mazak machines introduced in 2009 as especially important to oil and gas customers. The Megaturn MTN 900 VTC is a new-generation CNC vertical turning center featuring a maximum workpiece size of 36.2 x 31.5" (919 x 800 mm). It removes material at a rate of 58.6 in.3/min (960 cm3/min), which Mazak says is 1.4x higher than comparable VTLs. The Slant Turn Nexus 550M is a CNC turning center designed for large-part turning applications. It features a 10.8" (274-mm) OD bore mounted on a 60-hp (44.8-kW), 1000-rpm turning spindle capable of generating up to 5163 lb-ft (7000 Nm) of torque.
The spindle is also used as a C axis, capable of indexing to 0.001° increments for slotting cuts and drilled holes. This spindle is a CNC-controlled axis for milling. The machine comes with a 10-hp (7.5-kW), 4000-rpm high-speed milling spindle for roughing, grooving, end-milling, and face milling operations. The STN550M's total travels are 18.25"(464 mm) in the X axis and 129.88" (330 mm) in the Z axis, with a 40.7" (1034-mm) swing capacity. An NC servodriven programmable tailstock drives the W axis. The multitasking Quick Turn Nexus 450-II MY CNC turning center features a headstock with a 15, 18, or 21" (381, 457, or 533-mm) chuck, mounted on a 50-hp (37-kW), 2000-rpm integrated-motor main turning spindle.
"The top challenges [in machining oil production equipment] are the toughness of the materials and the complexity of the parts," agrees Dan O'Connor of Lyndex-Nikken (Mundelein, IL), a supplier of rotary tables. "These push the capabilities of both equipment and human talent. I've known of processes where the table remains clamped for a week or more while features are machined." The company offers big-bore rotary tables for downhole instrumentation, including the CNCB-350 and the CNCB-450. The latter features an 8"(205-mm) diam through hole with minimum increments of 0.001° with 15 arc-sec accuracy. Primarily used in the manufacture of drilling bits, large top-mount tables, such as the CNC-401T or CNC-501T, typically provide a five-axis capability for a four-axis machining center.
"In oil and gas production," says O'Connor, "I wouldn't say that cost is irrelevant, but the small increase in price required to buy a top-quality product is more than offset by the intense production demands imposed by this environment. One part can cost as much as 10x the price of the rotary table.
This article was first published in the February 2010 edition of Manufacturing Engineering magazine.