In previous articles, I wrote about the various factors that drill pipe design engineers consider when specifying the drill pipe for a particular rig or particular job. Does the drill rod’s diameter provide for a good uphole velocity for the desired hole size? Do the connections have enough torque strength to withstand the rig’s capabilities? Can the tube’s tensile strength withstand expected hook loads?
As rig manufacturers come out with newer rigs with newer capabilities, the factors in drill pipe design can change. These rig improvements can change drill pipe requirements from the “old-school standards.” For example, consider drill pipe run on larger blasthole rigs. The larger-diameter hole sizes demand larger tricone bits, which require a hefty amount of bit weight to operate properly. Bit weight in blasthole operations is typically supplied by the rig’s pulldown capabilities.
Old-school standards mean 8⅝-inch OD for 10¾-inch bits, 10¾-inch diameter for 12¼-inch bits. The outside diameters followed standard pipe/casing sizes. Typical air packages for the rigs drilling these blastholes had about 1,150 cfm and 125 psi. The drill pipes typically had a 1-inch wall. In those days, large blasthole rigs carrying names like Bucyrus-Erie, Marion, Gardner Denver and P&H ruled the marketplace. As time went by, a new name surfaced to dominate today’s larger rig market: the Pit Viper series by Epiroc, in my opinion, seems to be everywhere.
In a previous column, I discussed bit weight. The recommended bit weight for a hard formation tricone can be 6,000 pounds per bit inch. If you put out a rig to drill up to a 16-inch hole size, then you should best be able to produce the 96,000 pounds of push needed for the hard formation bit to perform to its maximum.
How do we know if drill pipe will not buckle under these higher pulldown strengths? Buckling in drill pipe occurs when we apply too much push, and the drill string bows out and can “snake” downhole. The wall of the drilled hole limits the amount of bow. In typical water well or deeper applications, this is not a factor. We all know that, outside of starting a hole, you are not supposed to push (that is, run with the drill pipe in tension).
I was once asked to offer a quotation for drill pipe to run on a Pit Viper 351. The mine manager inquired about the buckling strength of the drill pipe we offered. I did not know, as I had never gotten the question or considered this aspect in drill pipe design. I investigated on the internet and found complicated formulas full of engineering jargon. So, I did the next best thing. I contacted someone who would know: my friend Richard Melvin, a drill pipe designer from Drilco.
I asked him about the buckling point of 9¼-inch OD-by-1-inch wall drill pipe with 120,000 pounds of pulldown. According to a chart he had, it would buckle at a length of 85 to 90 feet. Hopefully, the drill operator would only apply as much weight as recommended — in the 66,000-pound range — and our specified tubes would be OK.
We have all heard about the straw that broke the camel’s back. Buckling works the same: You are OK until you are not.
We have all heard about the straw that broke the camel’s back. Buckling works the same: You are OK until you are not. In the case of columns in a building, it could lead to a catastrophic event. When drill pipe bows out, it creates fatigue and can damage the hole walls.
Looking at the various formulas for calculating buckling strength, such as Euler’s Formula, a couple of points become evident. First, length is an important factor. Length appears in the denominator and it is the length squared. If you double the length, you reduce your buckling strength by a factor of four. In the scenario behind this buckling inquiry, this doubling happens when you add the second drill pipe. Ideally, we want a drill pipe whose buckling length is beyond the depth range of our drill rig at the maximum down pressure — or at least a safety factor beyond the recommended bit weight.
Euler’s Column Formula for Critical Loads
F = π2 E I / L2
Critical load (F) equals pi (π) squared times modulus of elasticity (E) times moment of inertia (I) divided by column length (L) squared.
Second, the strength of the tube is not a factor. The important factors involve the tube’s stiffness and shape — like a tube instead of an I-beam.
The air packages on these larger rigs can top 3,000 cfm. This extra air means you can go from drilling an 11-inch hole to a 12¼-inch hole without changing to a larger diameter of drill pipe. In the old-school approach, you would trade out your 9¼-inch drill pipe for 10¾-inch drill pipe. Not now. This means your 9¼-inch drill pipe needs to handle the higher down pressures of these larger bits. To compensate, rig makers are recommended drill pipe with a heavier wall, going from 1- to 1¼- or even 1½-inches.
This can drive drill pipe manufacturers and suppliers nuts. It can also drive up drill pipe costs to well over $10,000 per stick. Not many steel mills can produce seamless tubes with these heavier walls at the length required to make a 35-foot drill pipe. The effort to provide a better, more versatile rig for the mining customer sent waves through the drill pipe side. For me as a supplier, I found my manufacturers were just not that good at these larger-diameter and heavier-wall drill pipes. If I were to tap into this aspect of the drill pipe industry, I needed to find new manufacturers.