Tubular strings used to construct and complete wellbores typically comprise tubular segments joined by threaded connections. The operations of assembling such tubular strings and installing them into a wellbore, and disassembling such strings and removing them from a wellbore, are commonly referred to as “tubular running” (i.e., running the tubular string either into or out of the wellbore). Where the tubular string involved is a string of casing (as opposed to, for example, a string of drill pipe or a string of production tubing), these operations may alternatively be referred to as “casing running”. The process of making a threaded connection between two segments of a tubular string is commonly referred to as “making up” the connection, and the process of disconnecting two segments is called “breaking out” the connection.
For conventional drilling rigs that use a rotary table to rotate the drill string, tubular-running operations typically require the use of apparatus such as slips, elevators, and spiders to carry the weight of the tubular string while a tubular segment is being either added to or removed from the string, as well as power tongs to apply torque for make-up and break-out of threaded connections. For drilling rigs that use a “top drive” to rotate the drill string instead of a rotary table, it is increasingly common to use tubular-running tools attached to the top drive quill to structurally and sealingly connect to the upper end of a tubular segment, enabling the top drive to be used for make-up and break-out operations, and to react vertical hoisting loads (from the weight of the string) and torsional loads induced during connection make-up and break-out, while also enabling continuity of drilling fluid circulation through the top drive and the tubular string. Top-drive-equipped drilling rigs can also be used for “casing drilling” (or “drilling with casing”), which are terms used to describe the increasingly common practice of drilling a wellbore with a drill bit connected to the lower end of a casing string, such that the wellbore will already be cased once it has been drilled to the desired depth. This method replaces the separate sequential operations of drilling the wellbore, extracting the drill string, and inserting a casing string, as in traditional wellbore construction methods, with the single operation of drilling the wellbore with a permanent casing string.
Regardless of the type of drilling rig being used, most tubular-running operations (including casing drilling operations) require some type of tubular-handling equipment carrying grip elements having grip surfaces for engaging and gripping either the outer or inner surface of a tubular workpiece, in conjunction with the application of a radially-oriented normal force, to develop sufficient traction to prevent both axial and circumferential sliding of the grip surface relative to the tubular surface under application of applied axial and torsional loads during hoisting and during connection make-up and break-out.
The resultant of these applied loads is thus carried as a tangential shear load across an interfacial engagement region between the grip surface and the surface of the tubular. To prevent sliding, the product of the normal force multiplied by the effective friction coefficient of the interfacial region (i.e., the traction limit) must always be greater than the combined applied loads acting as a shear force transmitted across the interfacial engagement region to the tubular surface in the region of contact. In the majority of industry wellbore construction and completion applications, the grip mechanisms rely on some form of mechanical feedback—so-called “self-activating” grips—where increased applied load correlatively increases the normal force acting across the interfacial region, in the manner of a mechanical advantage.
The self-activating grip mechanisms of such tubular-handling apparatus are typically configured to include the familiar wedge grip configuration—directly in the case of hoisting equipment such as slips, and in a radially-adapted form in the case of power tong grips. For tubular-handling applications, this wedge grip configuration is generally characterized by two or more opposing, movable grip elements or “wedges” acting together in a generally annular space between the handling equipment body on one side (i.e., the sliding surface) and the surface of a tubular workpiece on the other (i.e., the grip surface), and arranged so that activation force applied to the grip element tends to cause sliding of the wedges against the handling equipment on a selected cam, ramp, or wedge angle so as to move the grip elements radially toward the workpiece, and thus urge the grip surfaces of the grip elements into contact with the workpiece. The activation force, in the foregoing context, is to be understood as the vector sum of the tangential load carried by the grip surface and any additional force otherwise applied to the grip element and also acting in a tangential direction such as implemented in hydraulic or pneumatic “powered” slips.
The ratio of grip element movement in the direction of loading to radial movement (relative to the hoisting equipment body) is controlled by the selected wedge angle, and together with friction forces arising on the sliding surface defines the mechanical force advantage of the system—i.e., the radial force acting normal to the workpiece surface, divided by the net activation force. Self-activation occurs when at least a portion of the activation force is provided by applied load acting tangentially on the grip surface. In typical manual slips, the activation force is provided almost entirely by the applied axial load, where only the gravity load of the slip assembly adds to the axial hoisting load. As an additional constraint on such equipment relying on largely self-activating wedge-grip-type mechanisms, it is also often necessary for the system to be self-releasing upon unloading—meaning that the wedge angle slope cannot be less than the static friction coefficient acting upon unloading.
Accordingly, and as is well known in the art (within practical limits allowing for lubrication on typical sliding surfaces such as slips in a slip bowl), a wedge angle of approximately 9 degrees is commonly considered to be a minimum (for example, a diameter taper of 4 inches per foot, or a 9.46-degree wedge angle, is an API industry standard for slip bowls). Relating this to the mechanical advantage of wedge grip mechanisms for common ranges of lubrication means that the mechanical advantage of these systems is in the order of 3:1 (near sticking to unload) and more typically 4:1 when normally lubricated. The inverse of this mechanical advantage translates to effective friction coefficients in the order of 0.25 to 0.33 for the traction limit of a grip interface as described above. Therefore, the traction limit actually present must exceed this for all levels of applied load to avoid slippage, and thus to avoid the risk of damage to the tubular workpiece or a dropped string.
For common wellbore operations using carbon steel tubulars, and particularly in environments where there is a risk of contamination of tubular surfaces by contaminants such as mill varnish, paint, mill scale, granular debris (e.g., sand and dirt), drilling fluids, corrosion inhibitors, etc., sufficiently high traction limits typically are not reliably achievable from pure friction between available grip surface materials in normal contact with carbon steel tubular surfaces. To overcome this challenge, grip surfaces are commonly provided on elements known as dies. Dies for gripping carbon steel tubulars are typically made from high-strength steel, with grip surfaces machined to carry what are variously described as teeth or wickers. The dies are adapted for rigid but removable structural mounting to movable grip elements, so that they can be replaced when the teeth or wickers become worn or damaged.
For reliable gripping effectiveness, it is typically necessary for the teeth or wickers on the dies to achieve some degree of physical penetration into the surface of the tubular workpiece, to induce a sufficient effective friction coefficient under load to provide a sliding resistance significantly greater than would be achievable from the more “pure” friction coefficient of the interfacial region. This particular type of gripping mechanism (i.e., combining friction resistance with mechanical interaction resulting from physical penetration into the workpiece) will be referred to herein as an “interference grip” mechanism.
To promote such penetration and minimize wear (and thus increase die service life), the teeth or wickers are usually surface hardened (such as by carburizing) to give them a hardness greater than that of the target workpiece. Consequently, the local friction coefficient achieved on the hard tooth surface against dry carbon steel tends to be low, and particularly so when acting through trapped debris or other contaminants that can have a lubricating (i.e., friction-reducing) effect. Therefore, the load flank angle of a die tooth penetrating the workpiece surface must be sufficiently steep to counter the tendency of the tooth to climb out of engagement under load.
Additionally, there are other known variables that can affect the traction of the interference grip mechanism, in the context of normal stress influencing the extent and effectiveness of initial tooth engagement with the workpiece (i.e., initial “bite”) and final tooth penetration depth resulting from the maximum normal stress present when gripping. Included among these variables are tooth shape (e.g., wedge shape or pyramid shape) and tooth distribution (e.g., tooth distribution pattern and density). Furthermore, the tooth height relative to the “valleys” or void spaces between die teeth should be sufficient to penetrate any surface contaminant layer that might be present, and ideally should be arranged to accommodate the associated contaminant debris in a manner that will allow the contaminants to self-clean (i.e., to fall out of the intra-tooth valleys), or at least so as to enable periodic debris removal to avoid the loss of adequate penetration and consequent loss of effective tractive resistance.
In general, then, and as is well known for almost all such gripping applications, die teeth with a coarser-textured grip surface will have comparatively greater gripping effectiveness than die teeth with a finer-textured grip surface, but will typically cause deeper workpiece marking or surface damage. On the other hand, a finer-textured grip surface may be more susceptible to intra-tooth plugging and loss of penetration depth due to wear, and therefore may not be able to reliably and durably provide the necessary tractive resistance to prevent slippage across the interface between the dies and the tubular workpiece. Therefore, the design of a grip surface for a given application will typically involve a balancing of these practical considerations.
Where wellbore applications require the use of tubulars made from a corrosion-resistant alloy (CRA), such as but not limited to a stainless steel, more stringent constraints are usually placed on the handling equipment than for comparable carbon steel tubulars, in order to preserve the corrosion-resisting properties of the CRA tubulars. Tubular-running equipment is commonly required to use so-called “non-marking” dies.
Where the CRA tubular material is stainless steel, it is commonly a further requirement that the tubular-handling equipment must avoid contact with any material that might result in the transfer of “free iron” onto the surface of the stainless steel tubulars (such as would typically result from contact between stainless steel and conventional carbon steel). This is because iron transfer has a well-known deleterious effect on the ability of stainless steel to form and maintain an uninterrupted passivating oxide layer, which is essential to stainless steel's ability to resist corrosion. For these reasons, conventional grip elements using hardened carbon steel typically must be avoided when handling CRA tubulars—in this regard, see industry standards ISO 13680 (“Petroleum and natural gas industries—Corrosion-resistant alloy seamless tubes for use as casing, tubing and coupling stock—Technical delivery conditions”) and API 5CRA (“Specification for Corrosion-resistant Alloy Seamless Tubes for Use as Casing, Tubing, and Coupling Stock”).
For obvious reasons, it is highly advantageous to adapt the grip surfaces of existing handling equipment to handle and run CRA tubulars. Therefore, equipment suppliers have sought to do so generally within the constraints of the existing wedge grip mechanisms used to handle carbon steel as described above.
Among such known adaptations are so-called grit-faced dies, which are commonly referred to as non-marking dies. Grit-faced dies find use in many CRA tubular handling applications, particularly where some amount of surface contamination must be accommodated. The grip surface is provided by size-controlled tungsten carbide or similar hard grit particles brazed onto a substrate, with the grit particles being randomly distributed over the substrate surface and arranged to protrude from the bonding layer of brazed material in much the same manner as the teeth or wickers of conventional machined dies. Both the grit particles and brazing materials are selected to avoid iron contamination.
The protruding grit particles thus function in the same general fashion as penetrating teeth or wickers on conventional dies, and result in indentations in the gripped tubular workpiece. Accordingly, grit-faced dies rely on an interference grip mechanism. It will be apparent that the size and shape of protruding grit particles, the density and distribution of the particles, and the brazing layer thickness all affect the actual marking geometry of a grit-faced die in addition to the general characteristics and dimensions of a grip mechanism. By careful selection of these properties and attention to manufacturing processes, acceptably small amounts of marking can apparently be achieved for many CRA applications.
However, detailed measurements of marking geometry have shown that marking in excess of expected limits is not uncommon, particularly due to random variations in the grit particle height, shape, and distribution, and that the severity of resultant local plastic deformation affecting corrosion resistance can be highly variable and more severe than deformations caused by conventional machined wedge-tooth dies. Furthermore, in field environments it is difficult to prevent these dies from filling with debris and therefore losing their effectiveness to the extent that operational practice calls for either frequent cleaning or complete replacement.
In CRA tubular-handling applications where even the low level of marking provided by so-called non-marking grit-faced dies is deemed unacceptable, more purely friction-based non-penetrating dies may be used provided that care is taken to eliminate surface contaminants. For these applications, the grip surface may incorporate a smooth-faced elastomer, a semi-metallic material, or a soft non-ferrous metal material such as aluminum.
However, in addition to requiring the tubular surfaces to be clean and dry, these grip configurations frequently require specialized handling equipment capable of applying additional activation load to the grip elements and reacted by the handling equipment body (for example, powered slips in spiders), to supplement the self-activation provided by the directly-applied load. This takes away the ability to use standard equipment, and the higher resulting radial load may thus be further limited by tubular collapse resistance or elastomer extrusion resistance, thus necessitating the use of yet further specialized tongs, slips, or elevators.
FIG. 1 schematically depicts the mechanics involved in the interaction between the grip surface of a slip or wedge grip and a tubular workpiece. FIG. 1 is a free body diagram of a self-activated grip element (slip segment) 10 acting as a wedge between a workpiece (e.g., casing) and a tapered sliding surface (slip bowl). Although not shown in FIG. 1, the tapered sliding surface of the slip bowl would be slidingly engaged by the sloped surface 12 on the left side of the slip segment 10 in FIG. 1, and the outer surface of the workpiece would be engaged by the toothed face 14 on the right side of slip segment 10.
Accordingly, slip segment 10 is “pinched” between the casing and the sliding surface of the slip bowl during the application of increasing axial load. Normal force vector N acting on sloped surface 12 of slip segment 10 induces shear load vector μN acting along sloped surface 12. Radial force vector Fr, acting normal to toothed surface 14 in reaction to normal force vector N, urges toothed surface 14 of slip segment 10 into the workpiece, and must be high enough to induce a sliding resistance force across the interface between the workpiece surface and toothed surface 14 greater than the applied axial hoist load Fa in order to prevent sliding of slip segment 10 relative to the casing, both axially and circumferentially.
This simple system is governed by the following relationship between axial and radial forces during loading:
                                          F            r                                F            a                          =                                            1              -                              μ                ⁢                                                                  ⁢                tan                ⁢                                                                  ⁢                α                                                                    tan                ⁢                                                                  ⁢                α                            +              μ                                =                      G            ra                                              Eqn        .                                  ⁢        1            wherein:
Fa=axial load
Fr=radial load
Gra=Radial to axial force Gain (ratio) during load increase
μ=friction coefficient on cones
α=radial cone angle
Solving this equation to show the radial force as a function of the applied axial or hoisting load yields:Fr=GraFa  Eqn. 2