You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application Ser. No. 61/496,835, filed Jun. 14, 2011. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates in general to well drilling and in particular to a shear ram assembly for a blowout preventer (“BOP”) that has guide arms to move a well pipe away from a side wall of the bore of the shear ram assembly during shearing. 
     BACKGROUND OF THE DISCLOSURE 
     Offshore drilling rigs normally employ a riser to connect the subsea wellhead with the drilling rig. A blowout preventer is located at a lower end of the riser. Land rigs also use blowout preventers. A blowout preventer is a large assembly having many features for closing the riser in the event high pressure in the wellbore begins pushing the drilling mud upward. Those features include an annular blowout preventer that seals around pipe regardless of the diameter. The blowout preventer also has pipe rams that are sized to close and seal around pipe strings of certain diameters. The blowout preventer also has shear rams that will shear a drill pipe string or a production tubing string in the event of an emergency. 
     Pipe shear rams have two rams, each of which has a blade mounted to it. Pistons move the rams toward each other to shear pipe extending through the blowout preventer. One blade is located at a higher elevation than the other and slides over the lower one when the shear rams close. Often the pipe string will be off-centered relative to the axis of the bore extending through the shear ram assembly. The blades have converging side portions that lead to a central area for centering the pipe as the rams close. In some pipe ram assemblies, a remote possibility exists that the pipe string will remain at an outside wall of the bore during closure, which could lead to incomplete shearing of the pipe string. 
     SUMMARY 
     The pipe shear ram assembly has a housing with a bore extending vertically for the passage of a pipe string. The assembly has first and second ram blocks, each having a blade, the blades being positioned such that one slides over the other when the first and second ram blocks are moved forward to a closed position. Two guide arms protrude from the first ram block toward the second ram block. Each of the guide arms has a tip and a wedge surface on an inboard side extending rearward from the tip. The wedge surfaces are farther apart from each other than the blade and guide the pipe string toward an axis of the bore in the event the first and second ram blocks are moved to the closed position. 
     In one embodiment, a rearward edge of each of the wedge surfaces is located substantially the same distance forward of a face of the first ram block as an upper edge of an outboard end of the blade of the first ram block. Preferably each of the wedge surfaces is located in a vertical plane. A distance between rearward ends of the wedge surfaces is less than a diameter of the bore in one embodiment. Each of the wedge surfaces may be flat. 
     Each of the blades may have a length between outboard ends that is less than a diameter of the bore. A pair of recesses are located on the second ram block adjacent opposite ends of the blade of the second ram block for receiving the guide arms when the first and second ram blocks move to the closed position. Each of the recesses has an upper wall. The guide arms are mounted to the first ram block so as to avoid sliding contact with the upper walls as the ram blocks move to the closed position. The bottoms and outboard sides of the recesses may be open. In one embodiment, the blade of the first ram block slides over the blade of the second ram block when in the closed position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the ram blocks of a shear ram assembly in accordance with this disclosure. 
         FIG. 2  is a side view of the ram blocks of  FIG. 1 . 
         FIG. 3  is a top view of the ram blocks of  FIG. 1 . 
         FIG. 4  is a bottom perspective view of the ram blocks of  FIG. 1 . 
         FIG. 5  is a front view of the upper ram block of  FIG. 1 . 
         FIG. 6  is a top view of the blades and guide arms of the ram blocks of  FIG. 1 , shown in an open position. 
         FIG. 7  is a top view of the blades and guide arms of  FIG. 6 , shown in a partially closed position. 
         FIG. 8  is a perspective view of the ram blocks of  FIG. 1  installed within a subsea blowout preventer assembly. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , shear rams  11  are shown removed from the housing or bonnet in which they are located. Shear rams  11  are part of a ram BOP assembly that is part of a stack assembly. In the case of offshore drilling, the stack assembly is located at the lower end of a riser extending downward from a drilling vessel. The lower end of the BOP stack assembly secures to a subsea wellhead on the sea floor. The BOP stack assembly will normally also contain pipe rams, variable bore rams, and a quick disconnect mechanism for disconnecting from the riser in the event of an emergency. When actuated, shear rams  11  will close the through bore and also shear pipe in the well, such as drill pipe, tubing, or casing. 
     Shear rams  11  include an upper ram assembly  13  having a face or forward end  15 . A semi-circular seal groove  16  is located on the upper side of upper ram block  13  for receiving a portion of an elastomeric seal. An upper shearing blade  17  mounts to forward end  15 . Upper blade  17  has a forward face  23  with an upper edge  19  and a lower edge  21 . Lower edge  21  extends forward farther from forward end  15  than upper edge  19  in this example, resulting in face  23  inclining relative to vertical. Face  23  is also generally concave or converging, resulting in the center of face  23  between its outboard ends  24  being recessed relative to the more forward portions of face  23  at outboard ends  24 . A variety of different shapes for upper blade  17  may be employed. 
     Pipe guide arms  25  are located on the outboard sides of upper ram block forward end  15 . Each arm  25  could be formed integrally with upper ram block  13 , or they could be otherwise attached, such as by welding or fasteners. Each arm  25  has a vertically oriented inboard side  27  extending forward from a base  37  of each arm  25 . Base  37  is designated to be where arm  25  joins forward end  15 . Also each arm  25  has a forward end or tip  29 . A wedge surface  31  extends from a junction with a forward end of inboard side  27  to tip  29 . Each inboard side  27  is parallel with a longitudinal axis  28  of shear rams  11  in this example. Wedge surface  31  may be a flat vertical surface, as shown, that is at an acute angle relative to longitudinal axis  28 . In this embodiment, wedge surface  31  is at an angle of about 30 degrees relative to longitudinal axis  28  and to inboard side  27 . Rather than flat, wedge surface  31  could be a curved surface. Each guide arm  25  has an upper side  33  that is flat and in a horizontal plane in this example. A chamfer or bevel  34  optionally may be at the intersection of upper side  33  with tip  29 . 
     As shown in  FIG. 2 , upper side  33  is spaced at a lower elevation on upper ram block forward end  15  than upper blade lower edge  21 . Upper side  33  is not located directly under upper blade  17  in this example because inboard side  27  of each arm  25  is approximately the same outboard distance as one of the upper blade outboard ends  24 , as shown in  FIG. 3 . Also,  FIGS. 2 and 3  illustrate that tip  29  extends forwardly more than upper blade  17  from forward end  15 . The junction of inboard side  27  with wedge surface  31  is approximately in vertical alignment with the junction of upper shear blade upper edge  19  and outboard end  24 . Wedge surface  31  joins inboard side  27  approximately the same distance from upper ram block forward end  15  as the distance from forward end  15  to the upper edge  19  of upper blade face  23  at outboard end  24 . 
     Each guide atm  25  has an outboard side  35  that extends from arm base  37  to tip  29 . Outboard side  35  may be at an acute angle relative to longitudinal axis  28 . In this embodiment, the acute angle is about 15 degrees relative to longitudinal axis. Tip  29  has a smaller height and width than base  37 . 
       FIG. 4  shows a lower side  39  of each arm  25 . Lower side  39  is shown as being flat and in a plane parallel with upper side  33  ( FIG. 3 ). A beveled surface  41  may join an outboard edge of lower side  39  with outboard side  35 . Beveled surface  41  is shown to be in a plane inclined relative to horizontal. 
     Referring still to  FIG. 4 , and also to  FIG. 5 , the lower side of upper ram block  13  has a sheared pipe end recess  43 . Recess  43  has a rear wall portion  45  that joins side wall portions  47 . Wall portions  45  and  47  are vertical walls positioned to receive the upper end of a well pipe after it has been sheared. Rearward wall portion  45  is illustrated as being partially cylindrical and blends with side wall portions  47 , which are straight. Other shapes for shear pipe end recess  43  are feasible.  FIG. 4  also illustrates a T-shaped connector slot  49  on the rearward end of upper ram block  13  for connecting to a piston rod. 
     Referring again to  FIG. 1 , a lower ram block  51  is illustrated in horizontal alignment with upper ram block  13 . Lower ram block  51  has a forward end  53  that is parallel to forward end  15  of upper ram block  13 . A top seal groove  55  in the upper side of lower ram block  51  receives an elastomeric seal and aligns with seal groove  16  to form a continuous seal when ram blocks  13 ,  51  are in abutment with each other. The seal is not necessarily circular. Lower ram block  51  has a sheared pipe end recess  57  for receiving the lower end of well pipe after shearing. Sheared pipe end recess  57  has a curved rear wall portion  59  that blends with two straight side wall portions  61 . Other shapes are feasible. 
     A lower blade  63  attaches to forward end  53  of lower ram block  51 . Lower blade  63  is at a lower elevation than upper blade  17 , as illustrated in  FIG. 2 . Lower blade  63  slides under upper blade  17  when shearing. An upper edge  65  of lower blade  63  is at a slightly lower elevation than lower edge  21  of upper blade  17 . Lower blade  63  has a lower edge  67  that is closer to lower block forward end  53  than the upper edge  65 . A face  69  extends between lower edge  67  and upper edge  65  and is thus inclined relative to vertical. As illustrated in  FIG. 2 , in this example, the inclination of lower blade face  69  is the same as the inclination of upper blade face  23 . Lower blade face  69  also recesses or converges to a central area that is closer to lower block forward end  53  than the outboard ends  70  of lower blade  63 , as shown in  FIGS. 1 and 3 . The length of lower blade  63  from one outboard end  70  to the other is the same as the length of upper blade  17  from one outboard end  24  to the other. 
     Referring to  FIG. 4 , which shows the bottom of lower block  51 , a recess  71  is located on lower shear block  51  along each outboard side outward and rearward from lower blade outboard ends  70 . Each recess  71  comprises a space or clearance provided along an outboard side to receive one of the arms  25  when ram blocks  13 ,  51  are closed. Each recess  71  is defined by a downward-facing upper side wall  72  and an inboard side wall  74 , side walls  72  and  74  being flat and perpendicular in this example. Each recess  71  is aligned with one of the arms  25  to receive the arm when in the closed or sheared position. Each recess  71  has a greater longitudinal length than the length of each arm  25 . Also, upper side wall  72  has a greater width than each arm  25 , and inboard side wall  74  has a greater height than the height of each arm  25  from its lower side  39  to its upper side  33  ( FIG. 2 ). Recess  71  is not a closed cavity as it has no outboard side wall or bottom side wall. There is no interference between upper side wall  72  and upper side  33  ( FIG. 2 ) of arm  25 . There is no interference between inboard side  27  of arm  25  and inboard side wall  74 . Arm upper side  33  could optionally slidingly engage recess upper side wall  74 , but preferably no significant vertical forces would be created on lower ram block  51  as a result. 
     A vertical center point of each arm  25  is approximately the same as a vertical center of lower blade  63 . When moving to the closed or sheared position, lower blade  63  will slide between the two arms  25  as the arms enter recesses  71 . The outboard ends  70  of lower blade  63  will be closely spaced from the inboard sides  27  of arms  25  as the arms enter recesses  71 . A T-shaped connector slot  76  for connection to a piston rod is located on the rearward end of lower block  51 . 
     Referring to  FIGS. 6 and 7 , a housing or bonnet  73  for shear rams  11  is illustrated schematically by dotted lines. Shear ram blocks  13 ,  51  (only a portion shown) are mounted in bonnet  73  and move toward and away from each other along axis  28  by pistons not shown). A vertical opening or bore  75  extends through bonnet  73 . The lengths of blades  17 ,  63  are somewhat less than the diameter of bore  75 . The distance between arm tips  29 , measured from the inside surface of each arm tip  29 , is approximately the diameter of bore  75 . Stated another way, the distance between wedge surfaces  31 , measured at the forward ends of wedge surfaces  31 , is approximately the diameter of bore  75 . The distance between the rearward ends of wedge surfaces  31 , is approximately the same as the length of blade  17  and less than the diameter of bore  75 . Bore  75  has the same diameter as the riser pipe (not shown) attached to bonnet  73  for receiving well pipe  77  extending from the vessel into the wellbore. 
     Well pipe  77  may be of various diameters depending on the purpose and type of the pipe. Well pipe  77  could coincide with bore axis  79  or be offset to one side as illustrated. A side of well pipe  77  could be touching a side portion of bore  75 , particularly if buckling of well pipe  77  has occurred in the wellbore. 
       FIG. 6  shows shear rain blocks  13 ,  51  fully open. While fully open, no portion of blades  17 ,  63  or arms  25  will protrude into bore  75 .  FIG. 7  shows shear ram blocks  13 ,  51  in the process of being closed to shear pipe  77 . As blades  17 ,  63  advance into bore  75 , the guide arms  25  contact well pipe  77  and help pipe  77  to move towards bore axis  79 . The wedge surfaces  31  makes sure that any part of well pipe  77  placed anywhere in bore  75  can be brought within range of the shear blades to be completely sheared. The full diameter of well pipe  77  will be contacted by blades  17  and  63 . When blade  17  is located midway across bore  75 , there will be no large gap between outboard ends  24  of blade  17  and the sides of bore  75  as the gaps will be filled by arms  25 . 
     The shear ram assembly  11  as described will shear a pipe string  77  when closed, even if the pipe string  77  is initially located against an outside wall of the housing bore  75 . The pipe string  77  will not be pushed outward to a position resulting in an incomplete shearing. The guide arms  25  provide two functions. The first is to push or direct the pipe string  77  toward the center of the bore  75 . The second is to remove or support the side load on the cutting blade  77  when the pipe string  77  is off center. 
       FIG. 8  shows blind shear rams  11  installed within a typical subsea blowout preventer assembly. The blowout preventer assembly has a blowout preventer (“BOP”) stack  81  that includes a frame with a wellhead connector  85  at the lower end for connecting to a subsea wellhead assembly (not shown). Blind shear rams  11  are normally located above pipe rams, which in this example includes pipe rams  91 ,  93 , and  95 . Each pipe ram  91 ,  93  and  95  has rams with semi-cylindrical recesses on the mating faces for closing around a different size range of pipe. When closed, blind shear rams  11  will seal off the bore and if pipe is present, will shear the pipe. 
     A lower marine riser package (LMRP)  94  connects to the upper end of BOP stack  81 . An annular BOP  95  may be located at the lower end of LMRP  94 . Annular BOP  95  will close around any size of pipe and seal the annulus between the pipe and the side wall of the bore. Annular BOP  95  will also seal fully even if a pipe is not present. A flex joint  97  is located at the upper end of LMRP  94  to allow flexing of a lower end of a riser string  99  connected to flex joint  97 . Choke and kill lines  101  extend from below annular blowout preventer  95  to alongside riser  99  for pumping fluid into the well. In the event of an emergency, LMRP  94  and riser  99  can be detached from BOP stack  81 . Redundant control pods  103  mount to LMRP  94  and contain hydraulic and electrical circuitry for controlling movement of the various rams  11 ,  81 ,  91  and  93  annular BOP  95  and other equipment. Control pods  103  are retrievable from LMRP  94  and are connected to an umbilical (not shown) leading to the drilling vessel at the surface. 
     While the shear ram assembly has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the disclosure.

Summary:
A pipe shear ram assembly has a housing having a bore extending vertically therethrough for the passage of a pipe string. Ram blocks have blades positioned such that one slides over the other when the first and second ram blocks are moved toward each other to shear a well pipe. A pair of guide arms extend from one of the rams. The guide arms have tips protruding a greater distance than the blade. Each of the guide arms has an inboard wedge surface beginning at the tip and extending along a line that intersects the longitudinal axis for guiding the pipe string toward a bore axis of the bore in the event the first and second ram blocks are moved to the closed position.