Patent Publication Number: US-8113275-B2

Title: Multipart frac head with replaceable components

Description:
RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 11/787,575 filed Apr. 17, 2007. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to hydrocarbon well stimulation equipment and, in particular, to a multipart frac head with components that can be replaced permits the frac head to be refurbished in the field. 
     BACKGROUND OF THE INVENTION 
     The exploitation of marginal gas wells has necessitated an increase in the volume of proppant pumped through a frac head and associated wellhead isolation equipment during certain well stimulation operations. More than 10,000,000 pounds (4,500,000 kg) of proppant (e.g., frac sand, sintered bauxite, or ceramic pellets) mixed with a fracturing fluid such as “slick water” may be pumped down a wellbore at rates of up to 300+ bbl/minute during a multi-stage well stimulation procedure. As understood by those skilled in the art, pumping millions of pounds of abrasive proppant through a frac head at those rates causes abrasion, commonly referred to as “wash”, even if the frac head is designed to be abrasion resistant. 
     Frac heads are normally constructed from a frac head body of alloy steel (e.g. 4140 steel) with a central passage that provides a conduit for directing high-pressure fracturing fluids into a frac mandrel. The frac mandrel provides pressure isolation for pressure-sensitive wellhead equipment and conducts the fracturing fluid into a casing or a tubing of a well. Side entries are drilled through the frac head body to communicate with the central bore, and inlet ports are welded into the side entries. The outer ends of the inlet ports provide connection points for “frac irons”, which are steel pipes that conduct the high-pressure fracturing fluids from frac pumps to the frac head. Frac heads are generally built with 2-5 inlet ports. Each inlet port must be carefully welded into the frac head body by a skilled welder after the parts are pre-heated to 400°-600° F. to prepare them for welding. The welder builds up layers of weld metal to secure each inlet port. The weld must secure the inlet ports against 10,000-15,000 psi of fluid pressure induced by the frac fluids and violent mechanical forces transferred from the frac irons, which frequently vibrate and oscillate with significant force in response to flow obstructions and/or unbalanced pump loads. After all of the welding is completed the frac head is post-heated to 1100-1150° F. for about an hour/inch of thickness of the thickest part, and controllably cooled to below 300° F. before the welded areas are ground to a finished surface. Alternatively, the grinding may be performed before the post-heating. After complete cooling, paint is applied. All of the skilled labor, time and materials required to build the frac head makes it expensive to construct and to own. 
     Furthermore, when a frac head becomes worn due to wash, it has to be transported to a specially equipped machine shop to be refurbished. This may require transporting the heavy frac head hundreds or thousands of miles for repair. To refurbish the frac head, the washed surfaces have to be machined down to a consistent internal diameter to prepare them for welding, an operation known as “over boring”. If an inlet port or a bottom flange/adapter is too worn, it may have to be completely cut out and replaced with a new component. After machining, the frac head is heated (400°-600° F.) to prepare it for welding before weld metal is built up on the machined surfaces to a required thickness to restore the frac head to original specifications. Once the welding is completed the frac head must be post-heated to 1100-1150° F. for about one hour/inch of thickness of the thickest part for stress relief, and controllably cooled to below 300° F. The frac head is then re-machined to provide a smooth bore to inhibit abrasion. If any defects are discovered after machining, the entire heating, welding and post-heating processes must be repeated. Not only is refurbishing a frac head a time-consuming and expensive operation, the welded repair is never as resistant to abrasion as the original parts. Furthermore, the repaired frac head must be returned to the field, which again entails transportation expense. 
     In order to reduce the cost of maintaining frac heads, abrasion-resistant frac heads were invented, as taught for example in Applicant&#39;s United States Patent application number 2006/0090891A1 published on May 4, 2006. Abrasion resistant frac heads significantly reduce frac head maintenance, but cannot eliminate it. Because abrasion-resistant steels are brittle they cannot be used to line a bottom end of the central passage through the frac head, which is subject to impact and compression forces. Consequently, even abrasion-resistant frac heads require occasional maintenance besides the replacement of abrasion-resistant liners. 
     There therefore exists a need for a frac head that can be refurbished in the field. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a multipart frac head with removable components that can be refurbished in the field. 
     The invention therefore provides a multipart frac head, comprising: a frac head body having a plurality of inlet ports threadedly secured in respective side entries in the frac head body, and a bottom leg socket in a bottom end of the frac head body; and a bottom leg removably secured in the bottom leg socket, the bottom leg comprising an inner end having an elongated pin thread that cooperates with the box thread in the bottom leg socket to secure the bottom leg in the bottom leg socket, the elongated pin thread extending beyond the bottom leg socket when the bottom leg is secured in the bottom leg socket and is engaged by a box thread of a lock nut that is tightened against the bottom end of the frac head body to lock the bottom leg in the bottom leg socket, and an outer end that includes an external shoulder with an upper side that supports a threaded union nut and an underside with a metal ring gasket groove. 
     The invention further provides a multipart frac head, comprising: a frac head body having a plurality of inlet ports threadedly secured in respective side entries machined in a sidewall of the frac head body, and a bottom leg socket in a bottom end of the frac head body that comprises a box thread and a seal bore located inwardly of the box thread; a bottom leg removably secured in the bottom leg socket, the bottom leg comprising an inner end received in the seal bore and an elongated pin thread that cooperates with the box thread to secure the bottom leg in the bottom leg socket; and a lock nut threadedly secured to an outer end of the elongated pin thread, the lock nut being tightened against a bottom end of the frac head body to lock the bottom leg in the bottom leg socket. 
     The invention yet further provides a multipart frac head, comprising: a frac head body having a plurality of threaded side entries retained in respective inlet ports, and a bottom leg socket that comprises a box thread and a seal bore located inwardly of the box thread; a bottom leg threadedly secured in the bottom leg socket, the bottom leg comprising an inner end received in the seal bore, the inner end cooperating with high-pressure seals in the seal bore to provide a high-pressure fluid seal around the bottom leg, and an elongated pin thread that cooperates with the box thread to secure the bottom leg in the bottom leg socket; a lock nut threadedly secured to an outer end of the elongated pin thread, the lock nut being tightened against a bottom end of the frac head body to lock the bottom leg in the bottom leg socket. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional diagram of one embodiment of a multipart frac head in accordance with the invention; 
         FIG. 2  is a schematic cross-sectional diagram of another embodiment of the multipart frac head in accordance with the invention; 
         FIG. 3  is a schematic cross-sectional diagram of yet another embodiment of the multipart frac head in accordance with the invention; 
         FIG. 4  is a schematic cross-sectional diagram of a further embodiment of the multipart frac head in accordance with the invention; 
         FIG. 5  is a schematic cross-sectional diagram of the multipart frac head similar to the frac head shown in  FIG. 1 , with a bottom leg that includes a funnel-shaped section to reduce an internal diameter of an outlet of the frac head to permit the multipart frac head to be used with wellhead isolation equipment with a through-bore of a size corresponding to the reduced internal diameter; 
         FIG. 6  is a schematic cross-sectional diagram of the multipart frac head similar to the frac head shown in  FIG. 1 , with a flanged adapter that includes a funnel-shaped section to reduce an internal diameter of an output of the frac head to permit the multipart frac head to be used with wellhead isolation equipment having a through-bore of a size corresponding to the reduced internal diameter; and 
         FIG. 7  is a schematic cross-sectional diagram of the multipart frac head similar to the frac head shown in  FIG. 5 , with a flanged adapter that includes the funnel-shaped section, and a bottom leg with a segmented wing nut. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention provides a multipart frac head with components that can be replaced to permit the multipart frac head to be refurbished in the field, so that costs associated with maintenance operations are reduced. In one embodiment the multipart frac head has a removable bottom leg. Since most abrasion in a frac head occurs in the bottom leg where converging streams of abrasive frac fluid are most turbulent, the removable bottom leg permits the multipart frac head to be refurbished in the field before it must be returned to a machine shop to be completely overhauled or recycled. In another embodiment the bottom leg and the inlet ports of the multipart frac head are all removable and can be replaced. This permits the multipart frac head to be built using only machined parts. No welding is required. The inlet ports as well as the bottom leg of the multipart frac head can be replaced in the field, reducing construction and maintenance costs and further reducing transportation costs associated with frac head maintenance. 
       FIG. 1  is a schematic cross-sectional diagram of one embodiment of a multipart frac head  100  in accordance with the invention. The multipart frac head  100  has a frac head body  102  and a plurality of inlet ports, two of which ( 104   a ,  104   b ) are shown. Frac heads are normally equipped with 2-5 inlet ports. In this embodiment the inlet ports  104   a ,  104   b  are welded to the frac head body  102  using methods well known in the art. Each inlet port  104   a ,  104   b  includes a respective central bore  106   a ,  106   b  in fluid communication with a mixing chamber  108  of the frac head body  102 . A top end  110   a ,  110   b  of each inlet port  104   a ,  104   b  terminates in a pin thread to which a frac iron adapter  112   a ,  112   b  is connected by a wing nut  114   a ,  114   b , also in a manner well known in the art. 
     The frac head body  102  has a top end of  118  with a central passage  120  in fluid communication with the mixing chamber  108 . In this embodiment, the top end  118  terminates in a threaded union connector described in Applicant&#39;s U.S. Pat. No. 7,125,055 entitled Metal Ring Gasket for a Threaded Union, which issued on Oct. 24, 2006, the specification of which is incorporated herein by reference in its entirety. The threaded union connector includes a pin thread  122 , a metal ring gasket groove  124  that receives a metal ring gasket  125 , and a socket  126  that receives a pin end  127  of a complementary threaded union connector of equipment  128  connected to the multipart frac head  100 . The equipment  128  is typically a high-pressure valve, but may be any other well completion, re-completion or workover equipment. The pin thread  122  is engaged by a box thread of a wing nut  130  supported by an external shoulder  131  of the complementary threaded union connector of the equipment  128 . 
     A bottom of the mixing chamber  108  has a funnel-shaped section that tapers inwardly to a central passage  132  of a bottom leg  134  received in a bottom leg socket  135  in the frac head body  102 . The bottom leg  134  has a top end  136  with a smooth outer diameter that enters a seal bore  138  in the bottom leg socket  135 . Two O-ring grooves  140   a ,  140   b  accept O-rings  141   a ,  141   b  that provide a high-pressure fluid seal around the top end  136  of the bottom leg  134 . An elongated pin thread  142  on the bottom leg  134  engages a box thread  144  in the bottom leg socket  135 . In this embodiment, the tapered bottom end of the mixing chamber  108  is lined with a wear-resistant insert  146 . Due to its position at the bottom of the mixing chamber  108 , the wear-resistant insert  146  protects the frac head body  102  from most of the abrasive turbulence caused by the confluence of frac fluid streams pumped into the mixing chamber  108  through the inlet ports  104   a ,  104   b . The wear-resistant insert  146  is held in place by the top end  136  of the bottom leg  134 . A lock nut  150  engages the elongated pin thread  142 . After the bottom leg  134  is securely secured in the bottom leg socket  135 , the lock nut  150  is turned up tight against a bottom end of the frac head body  102  to lock the bottom leg  134  in place and ensure that it will not back out of the bottom leg socket  135 . 
     A bottom end of the bottom leg  134  terminates in a threaded union connector described in Applicant&#39;s above-referenced United States Patent. The bottom end includes an external shoulder  152  that supports a wing nut  154 . A metal ring gasket groove  156  accepts a metal ring gasket (not shown) for the threaded union, and two of O-ring grooves  158   a ,  158   b  accept O-rings  160   a ,  160   b  for providing primary fluid seals for the metal ring gasket. 
     As is well known to those skilled in the art, the bottom of the mixing chamber and the bottom leg of a frac head are normally the parts most likely to wash. Consequently, the multipart frac head  100  is easily maintained in the field by replacing the wear-resistant insert  146  and/or the bottom leg  134  with new or refurbished replacement parts. 
       FIG. 2  is a schematic cross-sectional view of another embodiment of the multipart frac head in accordance with the invention. The multipart frac head  200  is constructed and assembled without welding. The multipart frac head  200  includes a frac head body  202  with a central passage that having a mixing chamber  204 . A plurality of side entries, only two ( 206   a ,  206   b ) of which are shown, are machined into a cylindrical sidewall of the frac head body  202  at right angles with respect to the mixing chamber  204 . Each side entry includes a seal bore  208   a ,  208   b . Each seal bore has two O-ring grooves  210   a ,  210   b  that accept O-rings  212   a ,  212   b , which seal against a respective inner end  218   a ,  218   b  of the respective inlet ports  220   a ,  220   b . Box threads  214   a ,  214   b  machined in the respective side entries  206   a ,  206   b  cooperate with elongated pin threads  222   a ,  222   b  to retain and the respective inlet ports  220   a ,  220   b  in the respective side entries  206   a ,  206   b . Lock nuts  224   a ,  224   b  which respectively engage outer ends of the respective elongated pin threads  222   a ,  222   b , lock the inlet ports  220   a ,  220   b  in the side entries  206   a ,  206   b.    
     A threaded union connector  230  is machined at a top of the frac head body  202 . The threaded union connector  230  includes a peripheral pin thread  232 ; a metal ring gasket groove  234 ; and, a socket  236  that receives a pin end of a complementary threaded union connector of well stimulation equipment or flow control equipment mounted to the frac head (not shown). A bottom leg socket  240  is machined into the bottom end of the frac head body  202  concentric with the mixing chamber  204 . The bottom leg socket  240  includes a seal bore  241  located inwardly of a box thread  242 . The seal bore includes two O-ring grooves  254   a ,  254   b  which respectively accept O-rings  256   a ,  256   b . A top end  252  of the bottom leg  250  is received in the seal bore  241  and cooperates with the O-rings  256   a ,  256   b  to provide a high-pressure fluid seal between the bottom leg  250  and the bottom leg socket  240 . An elongated pin thread  244  on the bottom leg  250  engages the box thread  242  to lock the bottom leg  250  in the bottom leg socket  240 . A lock nut  260  engages an outer end of the pin thread  244  and is tightened against a bottom of the frac head body  202  to prevent the bottom leg  250  from backing out of the bottom leg socket  240 . The bottom leg  250  terminates in a threaded union connector of the type described above with reference to  FIG. 1 . The threaded union connector includes a pin end  262  with two O-rings  264   a ,  264   b  received in O-ring grooves  266   a ,  266   b . A wing nut  268  is supported by an annular shoulder  270  on a lower periphery of the bottom leg  250 . 
     As will be understood by those skilled in the art, any one of the inlet ports  220   a ,  220   b  and the bottom leg  250  can be replaced in the field. Consequently, the multipart frac head  200  is less expensive to maintain because it can be refurbished in the field by field hands using machined replacement parts. It is also less expensive to build because its constructed using only machined parts, so no preheating or skilled labor for welding are required. 
       FIG. 3  is a schematic cross-sectional view of another embodiment of the multipart frac head in accordance with the invention. The multipart frac head  300  closely resembles the multipart frac head  200  described above with reference to  FIG. 2 , except that the multipart frac head  300  has welded-in inlet ports  304   a ,  304   b , which are well known in the art. A central bore of each inlet port  304   a ,  304   b  receives a respective wear sleeve  306   a ,  306   b , as described in Applicant&#39;s above-referenced published patent application. A mixing chamber  308  of the frac head body  302  is lined by a first wear sleeve  310  and a second wear sleeve  312 . The first wear sleeve  310  includes a plurality of side entries  314   a ,  314   b  with sockets  316   a ,  316   b  machined in an outer periphery of the wear sleeve  310  which respectively receive inner ends of the wear sleeves  306   a ,  306   b . A top end of the frac head body  302  is machined to include a frac iron adapter  320  having a central passage  332  lined by a wear sleeve  334 . 
     A bottom leg  340  of the frac head  300  is received in a bottom leg socket  342 , which includes a seal bore  344  that receives a top end  341  of the bottom leg  340 . O-ring grooves  346   a ,  346   b  receive O-rings  348   a ,  348   b  to provide a fluid tight seal around the top end  341  of the bottom leg  340 . A box thread  350  in the bottom leg socket  342  is engaged by an elongated pin thread  352  on the bottom leg  340  to secure the bottom leg  340  in the bottom leg socket  350 . A lock nut  360  also engages and outer end of the elongated the pin thread  352  to lock a bottom leg  340  in the bottom leg socket  342 , as described above. A lower end of the bottom leg  340  is provided with a threaded union connector, which includes a wing nut  362  rotateably supported by a peripheral shoulder  364 . A bottom of the peripheral shoulder  364  includes a metal seal ring groove  365 . A pin end  366  of the threaded union connector includes O-ring grooves  368   a ,  368   b , which accept O-rings  370   a ,  370   b.    
       FIG. 4  is a cross-sectional schematic diagram of yet another embodiment of the multipart frac head in accordance with the invention. A multipart frac head of  400  is identical to the multipart frac head  300  described above with reference to  FIG. 3 , with an exception that the bottom leg  340  includes a wear sleeve  402  received in a wear sleeve socket  404  to further improve an abrasion resistance of the bottom leg  340 . As understood by those skilled in the art, the multipart frac heads shown in  FIGS. 1-4  are connected to a wellhead or wellhead isolation equipment, a top end of which is shown schematically at  406 . 
       FIG. 5  is a schematic cross-sectional diagram of a multipart frac head  500 , which is similar to the multipart frac had  100  described above with reference to  FIG. 1 . The multipart frac head  500  has a bottom leg  502  that is funnel-shaped to reduce an internal diameter (ID) of the frac head outlet  506 . This permits the multipart frac head to be used with wellhead isolation equipment with a through-bore of an ID the size of the frac head outlet  506 . A central passage  504  at a top end of the bottom leg  502  forms a bottom of a mixing chamber  501 . The central passage  504  tapers to the frac head outlet  506 , which has an ID of, for example, 2¾″, 3½″, or 4½″. By stocking bottom legs  502  with outlets  506  having different IDs, the bottom leg  502  can be changed as required to match an ID of the wellhead or wellhead isolation equipment to which the frac head  500  is mounted. The threaded union connector on the bottom end of the bottom leg  502  may be connected to a complementary threaded union connector on the top end of a flanged adapter  510  with a bottom flange  512  for mounting the frac head  500  to flanged wellhead or wellhead isolation equipment. 
       FIG. 6  is a schematic cross-sectional diagram of a multipart frac head  600 , which is similar to the multipart frac had  100  described above with reference to  FIG. 1 . The multipart frac head  600  has a bottom leg  602  that has a central passage  604  of a same diameter as a mixing chamber  601  of the frac head  600 . A flanged adapter  606  connected to a bottom end of the bottom leg  602  has a through bore  608  that is funnel-shaped to reduce an ID of a flanged adapter outlet  609 . This permits the multipart frac head to be used with wellhead isolation equipment with a through-bore of an ID corresponding to the ID of the flanged adapter outlet  609 . The through bore  608  at the top end of the flanged adapter  606  forms a bottom of an elongated mixing chamber  601 ,  604 . The through bore  608  tapers to the outlet  609  of a smaller ID, for example 2¾″, 3½″, or 4½″. By stocking flanged adapters  602  with outlets  609  having different IDs, the flanged adapters  602  can be changed as required to match an ID of the wellhead or wellhead isolation equipment to which the frac head  600  is mounted. The flanged adapter  606  has a bottom flange  610  for mounting the frac head  500  to flanged wellhead or wellhead isolation equipment. 
       FIG. 7  is a schematic cross-sectional diagram of a multipart frac head  700 , which is similar to the multipart frac head  500  described above with reference to  FIG. 5 . The multipart frac head  700  has a bottom leg  702  with a central passage  704  that is funnel-shaped to reduce an internal diameter (ID) of a frac head outlet  706 . This permits the multipart frac head  700  to be used with wellhead isolation equipment having a through-bore with an ID corresponding to the ID of the frac head outlet  706 . The central passage  704  at a top end of the bottom leg  702  forms a bottom of a mixing chamber  701  of the frac head  700 . The central passage  704  tapers to the frac head outlet  706 , which has a smaller ID, for example 2¾″, 3½″, or 4½″, as described above. By stocking bottom legs  702  with outlets  706  having different IDs, the bottom leg  702  can be changed as required to match an ID of the wellhead or wellhead isolation equipment to which the frac head  700  is mounted. The threaded union connector on the bottom end of the bottom leg  702  may be connected to a complementary threaded union connector on the top end of a flanged adapter  710  with a bottom flange  712  for mounting the frac head  700  to flanged wellhead or wellhead isolation equipment. As understood by those skilled in the art, the multipart frac heads shown in  FIGS. 5-7  are flanged to permit a bolted connection to a flanged wellhead or flanged wellhead isolation equipment, a top end of which is shown schematically at  720 . 
     In this embodiment, the bottom leg  702  is equipped with a segmented wing nut  714 , as described in Applicants published patent application 2006/0090891A1 referenced above. Each of the bottom legs for the frac heads  100 - 600  described above have the same outer diameter from the top end to the external shoulder that supports the wing nut for the threaded union connector. Consequently, a wing nut machined from a single piece of steel can be used for each of those bottom legs. Because of the shape of the bottom leg  702 , the segmented wing nut  714  is used instead, and a high-pressure elastomeric seal  716  well known in the art provides a fluid seal between the adapter flange  710  and the bottom leg  702 . 
     While various embodiments of the frac heads in accordance with the invention have been described, it should be understood that those embodiments described above are exemplary only. 
     The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.