There presently exist numerous devices that may be used to grip tubular members while torque is being applied to the tubular member. Such devices include by way of illustration “power tongs,” “backups,” and “chrome tools” and various other devices for gripping tubular members. Examples of power tongs are disclosed in U.S. Pat. Nos. 4,649,777 and 5,291,808 to David Buck. Typically power tongs will have a set of jaws which are the actual components of the power tongs which grip the tubular member. One example of these jaws is set forth in U.S. Pat. No. 4,576,067 to David Buck. The jaws disclosed in U.S. Pat. No. 4,576,067 include a die member which is the sub-component of the jaw that actually contacts the tubular member. In U.S. Pat. No. 4,576,067, the face of the die that contacts the tubular member has ridges or teeth cut therein. Typically, the teeth are sized such that 5 to 8 teeth per linear inch are formed across the gripping surface of the die. When the jaws close upon the tubular member, these teeth firmly bite into the tubular member and prevent slippage between the tubular member and jaws when large torque loads are applied to the power tongs or the tubular member.
Another class of devices to which the invention pertains grips the tubular in order to hold the tubular against vertical movement. Typically, the tubular is part of a tubing, casing or drill string formed from a long series of tubulars and the drill string is suspended above and/or in the well bore. This class of devices includes conventional slips, elevators and safety clamps. Slips and safety clamps utilize the weight of the tubular and/or drill string, and, in some cases, an external preload, to force the gripping surfaces into contact with the tubular being gripped. By way of example, the gripping member of the slip will have a gripping surface or gripping die on one face and an inclined plane on an opposite face. A slip bowl or similar device having a second and supplementary inclined surface will be positioned around the tubular with sufficient space between the tubular and slip bowl for the gripping member to be partially inserted between the slip bowl and tubular. As described in more detail below, the movement of the gripping member's inclined surface along the slip bowl's inclined surface causes the gripping surface to move toward and engage the tubular. The die or gripping surface of prior art slips is similar to the above described power tong jaw dies in that the gripping surface generally comprises a series of steel teeth which bite into the tubular to grip it.
While the above described methods for gripping pipe has been successful in many applications, there are certain disadvantages. One disadvantage is that after gripping tubular members, the teeth from the die will leave deep indentations or gouges in the surface of the tubular member. These “bite marks” left by the teeth may effect the structural integrity of the tubular member by causing a weak point in the metal which may render the tubular member unsuitable for further use or may lead to premature failure of the tubular at a future date.
A second disadvantage is encountered when using the dies with corrosion resistant alloy (CRA) tubular members. Exotic Stainless Steel with large percentages of Chromium, Nickle, etc., are typical CRA materials used in the oil and gas drilling industry. Oil and gas production frequently occurs in high temperature, corrosive environments. Because the above described die teeth are normally constructed of standard carbon steel, the bite mark made by the die teeth tend to introduce iron onto the surface of the CRA tubular. In such environments, the iron in the bite mark can act as a catalyst, causing a premature, rapid corrosion failure in the CRA tubular.
A further problem is encounter in that many CRA materials such as stainless steel are work hardened materials. This means that the malleability of the material decreases after the material is mechanically stressed. In the case of stainless steel tubulars, the bite marks or indentations caused by the prior art die teeth produce localized “cold working.” The points at which the teeth marks have been made are then less malleable than the other sections of the tubular and therefore may create inherent weak points in the tubular's structural integrity. Additionally, prior art steel teeth are formed in a uniform pattern. A series of uniformly sharp teeth bite marks may manifest themselves as a major stress riser with an adverse impact significantly more detrimental than a few individual random marks of similar depth. Thus, an uniform pattern of indentations or bite marks will create more damaging internal stresses in the tubular than a non-uniform pattern of bite marks.
As an alternative to using dies with teeth on CRA tubulars, the industry has employed dies which have smooth aluminum surfaces engaging the tubular. However, because these smooth faced aluminum dies rely purely on a frictional grip of the tubular, these dies must employ significantly greater clamping forces than dies with steel teeth. This greater clamping force in turn increases the risk that the clamping forces themselves will cause damage to the tubular. Furthermore, even with high clamping forces, the aluminum surfaces often do not have a sufficiently high coefficient of friction to prevent slippage between the dies and the tubular at high torque loads or high vertical loads.
To overcome the problem of slippage between the aluminum surfaced dies and a CRA tubular, the industry has developed a method of using a silicon carbide coated fabric or screen in combination with the aluminum surfaced dies. This method consists of placing the silicon carbide screen between the tubular and the dies before the dies close upon the tubular. The dies are then closed on the tubular with the silicon carbide screen positioned in between. The silicon carbide screen thereby allows a substantially higher coefficient of friction to be developed between the dies and the tubular. However, this method also has serious disadvantages. First, the silicon carbide screen must be re-position between the tubular and die surface each time the dies grip and then release a tubular. Thus for example, when a drilling crew is making up or breaking down a long string of drill pipe, several pieces (typically 5 to 6) of the silicon carbide screen must be placed in position for each successive section of pipe being made up or broken out. This repeated operation can be extremely inefficient and costly in terms of lost time. Secondly, this process requires a member of the drilling crew to repeatedly place his hands in a position where they could possible be crushed or amputated. Thirdly, while providing greater resistance to torque than a smooth surfaced aluminum die, there may nevertheless be situations where such high torque forces are being applied to the tubular that the silicon carbide screen method does not prevent slippage between the die and the tubular.