Abstract:
A top drive system for wellbore operations, the top drive system, in at least certain aspects, including a motor, a main shaft driven by the motor, the main shaft having a top end and a bottom end, a speed reduction system interconnected with the main shaft, a washpipe shaft interconnected with the speed reduction system for driving the washpipe shaft at a speed of rotation less than a speed of rotation of the main shaft. This abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims, 37 C.P.R. 1.72(b).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention and the application for this patent claim priority under the U.S. Patent law §120 of U.S. Application Ser. No. 60/902,798 filed Feb. 22, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention is directed to wellbore drilling top drive systems; to washpipe apparatuses for such systems; and to sealing systems for rotating members. 
         [0004]    2. Description of Related Art 
         [0005]    Drilling fluids are transmitted to drilling swivels and top drive drilling systems through a high-pressure swivel apparatus known as a “washpipe” with a seal known as a “washpipe packing” assembly. This packing assembly consists of a tubular component which is held stationary, and through which the drilling fluid flows under high pressure. A rotating seal assembly of contact lip seals is mechanically fixed to and rotates with the main shaft of the top drive or swivel, and forms a dynamic seal against the outer surface of the tubular washpipe as the main shaft rotates while drilling. Due to the high pressures and surface speeds involved in this arrangement, service life of the washpipe packing is limited. Drilling fluid leaks are therefore common on many drilling rigs, causing contamination and damage to associated components and environmental disruption. 
         [0006]    The prior art discloses a variety of top drive systems; for example, and not by way of limitation, the following U.S. patents present exemplary top drive systems and sealing assemblies: U.S. Pat. Nos. 4,458,768; 4,807,890; 4,984,641; 5,433,279; 6,276,450; 4,813,493; 6,705,405; 4,800,968; 4,878,546; 4,872,577; 4,753,300; 6,007,105; 6,536,520; 6,679,333; 6,923,254. 
         [0007]    In top drives and rotary drilling the washpipe typically sealingly engages a set of circumferential seals contained within a seal housing. Often, the washpipe remains stationary while the seals and the seal housing rotate. Such seal assemblies have conventionally included a series of reinforced, elastomeric, chevron-type seals interspersed with a series of reinforcing back-up rings. In certain prior art systems, one seal is exposed to full hydraulic pressure on one side, and atmospheric pressure on the opposite side. Full differential pressure of the drilling mud acts on one seal until that seal fails and the next seal in the assembly then acts as the primary seal. Some prior art swivel designs have sought to compensate for potential run-out and offset problems by allowing the washpipe and the seal housing to articulate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention, in certain aspects, provides a shaft sealing assembly in which an auxiliary shaft rotated by a rotating main shaft is connected to and rotates with the shaft via a speed reducer, e.g., a gear drive apparatus, between the main shaft and the auxiliary shaft so that the auxiliary shaft rotates at a reduced speed compared to the speed of the main shaft, e.g. about one-half the main shaft speed, thus exposing seals dynamically sealing against the auxiliary shaft to a surface which is moving at the reduced speed, thus enhancing seal life. 
         [0009]    In one particular aspect the main shaft is the main drive shaft of a top drive used in wellbore operations, e.g. drilling operations, and the auxiliary shaft is in fluid communication with a washpipe through which drilling fluid or mud flows to and through the top drive system. 
         [0010]    The present invention discloses, in certain embodiments, a top drive system with a drive motor; a drive quill or main shaft; a gear system coupled to the drive motor; a top drive support system for supporting various items; and a washpipe shaft according to the present invention connected with the top drive main shaft via a speed reducer for rotation at a speed less than that of the main shaft to enhance the life of seal assemblies, e.g. with packings used to seal against the washpipe shaft. 
         [0011]    Accordingly, the present invention includes features and advantages which are believed to enable it to advance top drive shaft sealing technology. Characteristics and advantages of the present invention described above and additional features and benefits will be readily apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments and referring to the accompanying drawings. 
         [0012]    Certain embodiments of this invention are not limited to any particular individual feature disclosed here, but include combinations of them distinguished from the prior art in their structures, functions, and/or results achieved. Features of the invention have been broadly described so that the detailed descriptions that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. There are, of course, additional aspects of the invention described below and which may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention. The claims of this invention are to be read to include any legally equivalent devices or methods which do not depart from the spirit and scope of the present invention. 
         [0013]    What follows are some of, but not all, the objects of this invention. In addition to the specific objects stated below for at least certain preferred embodiments of the invention, there are other objects and purposes which will be readily apparent to one of skill in this art who has the benefit of this invention&#39;s teachings and disclosures. It is, therefore, an object of at least certain preferred embodiments of the present invention to provide: 
         [0014]    New, useful, unique, efficient, non-obvious sealing for drive shafts, for top drive shafts, sealing arrangements for such shafts, and methods of their use. 
         [0015]    The present invention recognizes and addresses the problems and needs in this area and provides a solution to those problems and a satisfactory meeting of those needs in its various possible embodiments and equivalents thereof. To one of skill in this art who has the benefits of this invention&#39;s realizations, teachings, disclosures, and suggestions, various purposes and advantages will be appreciated from the following description of preferred embodiments, given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. The detail in these descriptions is not intended to thwart this patent&#39;s object to claim this invention no matter how others may later attempt to disguise it by variations in form or additions of further improvements. 
         [0016]    The Abstract that is part hereof is to enable the U.S. Patent and Trademark Office and the public generally, and scientists, engineers, researchers, and practitioners in the art who are not familiar with patent terms or legal terms of phraseology to determine quickly from a cursory inspection or review the nature and general area of the disclosure of this invention. The Abstract is neither intended to define the invention, which is done by the claims, nor is it intended to be limiting of the scope of the invention or of the claims in any way. 
         [0017]    It will be understood that the various embodiments of the present invention may include one, some, or all of the disclosed, described, and/or enumerated improvements and/or technical advantages and/or elements in claims to this invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0018]    A more particular description of embodiments of the invention briefly summarized above may be had by references to the embodiments which are shown in the drawings which form a part of this specification. These drawings illustrate certain preferred embodiments and are not to be used to improperly limit the scope of the invention which may have other equally effective or equivalent embodiments. 
           [0019]      FIG. 1  is a schematic view of a prior art top drive drilling system. 
           [0020]      FIG. 2  is a front view of a top drive system according to the present invention with washpipe apparatus according to the present invention. 
           [0021]      FIG. 3  is a cross-section view of one embodiment of part of the system of  FIG. 2 . 
           [0022]      FIG. 4  is a cross-section of one embodiment of part of the system of  FIG. 2 . 
           [0023]      FIG. 4A  is a schematic side cross-section view of the system according to the present invention. 
           [0024]      FIG. 5  is a cross-section view of part of the embodiment of  FIG. 4 . 
       
    
    
       [0025]    Presently preferred embodiments of the invention are shown in the above-identified figures and described in detail below. Various aspects and features of embodiments of the invention are described below and some are set out in the dependent claims. Any combination of aspects and/or features described below or shown in the dependent claims can be used except where such aspects and/or features are mutually exclusive. It should be understood that the appended drawings and description herein are of preferred embodiments and are not intended to limit the invention or the appended claims. On the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. In showing and describing the preferred embodiments, like or identical reference numerals are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
         [0026]    As used herein and throughout all the various portions (and headings) of this patent, the terms “invention”, “present invention” and variations thereof mean one or more embodiment, and are not intended to mean the claimed invention of any particular appended claim(s) or all of the appended claims. Accordingly, the subject or topic of each such reference is not automatically or necessarily part of, or required by, any particular claim(s) merely because of such reference. So long as they are not mutually exclusive or contradictory any aspect or feature or combination of aspects or features of any embodiment disclosed herein may be used in any other embodiment disclosed herein. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 1  shows a typical prior art drilling system with a derrick DK supporting a top drive TD which rotates drill pipe DP. The top drive is supported from a travelling block TB beneath a crown block CB. A drawworks, DS, on a rig floor RF raises and lowers the top drive. The top drive moves on a guide track GT. 
         [0028]      FIG. 2  shows a system S according to the present invention with a top drive  1  with a drive motor  2 ; a gear system  3  coupled to the top drive  1  with a bearing support  4  and support links  4   a ; a washpipe apparatus  10  with a washpipe shaft according to the present invention; an elevator load ring  5 ; a mud saver system  9 ; a lower internal blowout preventer  6 ; a saver sub  7 ; a gooseneck  24 ; and a pipe gripper  8  with supports  8 A connected to the elevator load ring  5 . 
         [0029]    The present inventors have recognized the problems associated with the rotation of a shaft with respect to packing or seals and have realized that effective reduction of the speed of shaft rotation can enhance seal life and reduce seal wear. 
         [0030]    By reducing the surface speed at which the seals effectively operate, the present invention extends the service life of washpipe packing. Instead of the packing rotating at main shaft speed against a stationary washpipe, the present invention provides a speed reducing mechanism (e.g. a gear drive apparatus) which rotates a washpipe shaft at, e.g., one-half the speed of the main shaft, thus the washpipe packing is not subjected to the full speed of the main shaft. In one embodiment of the present invention, a similar dynamic sealing assembly is, optionally, added at the upper, formerly stationary, end of the washpipe. By rotating the washpipe shaft with the main shaft, but at a lower speed than the main shaft, e.g. at one-half main shaft speed, the or each of the sealing assemblies are only subjected to one-half the original surface speed (thus one half the original contact with a rotating surface), increasing the overall service life of the packing and washpipe assembly. Rotation of the washpipe shaft is driven by the main shaft through a speed reducing mechanism. 
         [0031]    As shown in  FIG. 3  an apparatus  10  according to the present invention has a main washpipe body  12  disposed above and in fluid communication with a main shaft M of the top drive system S. A washpipe shaft  70  projects into an upper packing assembly  16  and rotates with the main shaft M (but is connected to the main shaft only via a gear system described below). Upper and lower packing assemblies  16  and  18  respectively, have seals which seal against the washpipe shaft  70 . 
         [0032]    Drilling mud flows through a channel  22  of the gooseneck  24  into an interior  70   a  of the washpipe shaft  70  and from there down a channel C through the main shaft M into the drill pipe. 
         [0033]    Packing  15  of the upper packing assembly  16  does not rotate with the washpipe shaft  70 . The outer surface of the washpipe shaft  70  rotates against the packing  15 . The gooseneck  24  is connected to gooseneck support  25  with bolts  26 . For assembly purposes a support  28  is connected to the upper packing assembly  16  with a bolt (or bolts)  32  connected to a housing  34 . A bolt (or bolts)  36  connect the lower packing assembly  18  adjacent the main shaft M. Following assembly, the support  28  and the bolts  32 ,  36  are deleted. 
         [0034]    A primary gear  40  is secured to and rotates with the main shaft within the housing  34 . Teeth  40   a  on the primary gear  40  mesh with teeth  42   a  on planetary gears  42  which are mounted on shafts  44  of a gear carrier  46 . Ball bearings  48  in a lower part  52  of the housing  34  and ball bearings  49  under an upper part  54  of the housing  34  isolate the housing  34  from the rotation of the main shaft M, rotation of the primary gear  40 , and rotation of the planetary gears  42 . A shield  62  shields items below it from falling drilling mud and inhibits the leakage or flow of mud from going into apparatuses below the shield  62 , e.g. a top drive gear box. 
         [0035]      FIGS. 4 and 5  show another embodiment of a system  100  according to the present invention useful as a washpipe apparatus in the system S of  FIG. 2 . An s-pipe extension  101  is connected to a gooseneck  102  (sometimes referred to as a washpipe apparatus). Drilling mud flows through a channel  101   a  in the s-pipe extension  101  into a channel  102   a  through the gooseneck  102  down into a channel  121  of a washpipe shaft  120  and then to a channel  141  through a top drive shaft  140  driven by a top drive motor  105  (e.g. a rotating shaft projecting from a top drive gear box  104  (shown in dotted line). A connector  103  with seals  103   a ,  103   b , connects the s-pipe extension  101  to the gooseneck  102 . 
         [0036]    An upper seal carrier  106  connected too the gooseneck  102  houses seal assemblies  107  (which include seal carriers and seals) which seal against an outer surface of the washpipe shaft  120 . The upper seal holder  106  maintains the top of the washpipe shaft  120  in place. A lower seal holder  108  connected to the shaft  140  houses seal assemblies  109  (which include seal carriers and a seal) which seal against an outer surface of the washpipe shaft  120 . The lower seal carrier  108  maintains the bottom of the washpipe shaft  120  in place. 
         [0037]    A speed reducer  160  interconnected between the shaft  140  and the washpipe shaft  120  reduces the speed of the washpipe shaft  120  as compared to the speed of the shaft  140 . Thus the seals  107  and  109  seal against a shaft (the washpipe shaft  120 ) rather than against the lower shaft (the top drive shaft  140 ) and the seals are, therefore, subjected to a shaft surface rotating at a speed less than (e.g., in certain aspects, at least 25% less than, and, in other aspects about 50% less than) the speed of the lower shaft. 
         [0038]    Ports  106   p  and  108   p  house bolts  106   b ,  108   b , respectively, which maintain the seal assemblies  107 , respectively, in place. Bolts  102   b  bolt the gooseneck to other structure. A torque arrester  112  is connected to the washpipe, e.g. to the upper seal holder  106  and to the speed reducer  160 . This torque arrester  112  prevents the ring housing  164  of the speed reducer  160  from rotating with other parts of the speed reducer  160 . The ring housing  164  provides a housing for bearings and gears of the speed reducer  160 . A splash shield  114  is connected to the shaft  140 . 
         [0039]    A sun gear  162  is connected to the lower seal holder  108  and rotates with the shaft  140 . Planetary gears  166  (e.g., but not limited to, four planetary gears) mounted on shafts  167  to a planetary gear carrier  168  mesh with and are driven by the sun gear  162 . Splines  169  on the planetary gear carrier  168  mesh with corresponding splines  128  on the washpipe shaft  120  to rotatively drive the washpipe shaft  120 . 
         [0040]    The ring housing  164  houses a bearing assembly  170  with bearings  172  between the ring housing  164  and the sun gear  162 . The ring housing  164  houses a bearing assembly  174  with bearings  176  between the ring gear and the planetary gear carrier  168 . The sun gear and the planetary gears are sized, designed and configured to achieve a desired gear reduction to reduce the speed of the shaft  120  as compared to the speed of the shaft  140 . For example, in one particular aspect, the gears are designed so that the shaft  140  rotates twice as fast as the shaft  120 . In certain prior systems seals (like the seals  109 ) were subjected to a shaft (like the shaft  140 ) rotating at its operational speed and seals (like the seals  107 ) were held stationary. The seals (like two seals  107 ; and the seals  109 ) in systems according to the present invention are now subjected to a shaft rotating at a much lower speed. 
         [0041]      FIG. 4A  shows schematically a system  200  according to the present invention in which a washpipe shaft  202  connected to a washpipe  204  is interconnected via a speed reduction device  206  to a rotatable top drive shaft  208 . Seals  211 ,  212  seal against the washpipe shaft  202 . Both seals are subjected to a surface of the washpipe shaft  202  against which they seal that is rotating at a speed less than the rotational speed of the top drive shaft  208 . The seals  211  are in a seal holder  218  connected to the washpipe  204 . Although the seals  212  are connected to a lower seal holder  214  that is connected to the top drive shaft  208 , the seals  212  only “see” a surface speed of the washpipe shaft  202  which is less than that of the top drive shaft  208  since the washpipe shaft  202  and the top drive shaft  208  are rotating in the same direction. The speed reduction device  206  may be any suitable speed reducer, including, but not limited to, a gear system. 
         [0042]    The present invention, therefore, provides in at least certain embodiments, a top drive system for wellbore operations, the top drive system having: a main shaft with a main shaft channel therethrough; drive apparatus for rotating the main shaft at a main shaft speed; a speed reducer apparatus connected to the main shaft; a washpipe shaft connected to the speed reducer apparatus so that the washpipe shaft is rotatable by the speed reducer apparatus at a washpipe shaft speed less than the main shaft speed; the washpipe shaft having a top end and a bottom end, and a washpipe shaft channel therethrough; a washpipe having a washpipe channel therethrough, the washpipe shaft channel in fluid communication with the washpipe channel; the washpipe shaft channel in fluid communication with the main shaft channel; a first seal holder connected to the washpipe and holding first seal assemblies for sealing against the top end of the washpipe shaft; and a second seal holder connected to the main shaft and holding second seal assemblies for sealing against the second end of the washpipe. Such a system may have one or some, in any possible combination, of the following: the speed reducer apparatus reducing the washpipe shaft speed to at least 25% less than the main shaft speed; the speed reducer apparatus reducing the washpipe shaft speed to about 50% of the main shaft speed; the speed reducer apparatus is a gear system including first gear apparatus connected to the main shaft and second gear apparatus connected to the washpipe shaft, the first gear apparatus for driving the second gear apparatus; the first gear apparatus includes a first gear, the second gear apparatus includes a plurality of second gears, and the first gear drives the plurality of second gears to rotate the washpipe shaft; the speed reducer apparatus having a ring housing, and a torque arrester connected to the ring housing and to the washpipe; wherein the torque arrester is connected to the first seal holder which is connected to the washpipe; and/or wherein the drive apparatus includes a top drive motor and a top drive gear system driven by the top drive motor. 
         [0043]    The present invention, therefore, provides in at least certain embodiments, a top drive system for wellbore operations, the top drive system having: a main shaft with a main shaft channel therethrough; drive apparatus for rotating the main shaft at a main shaft speed; a speed reducer apparatus connected to the main shaft; a washpipe shaft connected to the speed reducer apparatus so that the washpipe shaft is rotatable by the speed reducer apparatus at a washpipe shaft speed less than the main shaft speed; the washpipe shaft having a top end and a bottom end, and a washpipe shaft channel therethrough; a washpipe having a washpipe channel therethrough, the washpipe shaft channel in fluid communication with the washpipe channel; the washpipe shaft channel in fluid communication with the main shaft channel; a first seal holder connected to the washpipe and holding first seal assemblies for sealing against the top end of the washpipe shaft; a second seal holder connected to the main shaft and holding second seal assemblies for sealing against the second end of the washpipe; the speed reducer apparatus reduces the washpipe shaft speed to about 50% of the main shaft speed; the speed reducer apparatus is a gear system including first gear apparatus connected to the main shaft and second gear apparatus connected to the washpipe shaft; the first gear apparatus for driving the second gear apparatus; and the drive apparatus includes a top drive motor and a top drive gear system driven by the top drive motor. 
         [0044]    The present invention, therefore, provides in at least certain embodiments, a method for dynamically sealing a drive shaft of a top drive system, the top drive system suitable for wellbore operations, the method including: rotating a washpipe shaft of a top drive system; sealing against a top end and a bottom end of the washpipe shaft, the washpipe shaft being part of the top drive system, the top drive system having a main shaft with a main shaft channel therethrough, drive apparatus for rotating the main shaft at a main shaft speed, a speed reducer apparatus connected to the main shaft, a washpipe shaft connected to the speed reducer apparatus so that the washpipe shaft is rotatable by the speed reducer apparatus at a washpipe shaft speed less than the main shaft speed, the washpipe shaft having a top end and a bottom end, and a washpipe shaft channel therethrough, a washpipe having a washpipe channel therethrough, the washpipe shaft channel in fluid communication with the washpipe channel, the washpipe shaft channel in fluid communication with the main shaft channel, a first seal holder connected to the washpipe and holding first seal assemblies for dynamically sealing against the top end of the washpipe shaft, a second seal holder connected to the main shaft and holding second seal assemblies for dynamically sealing against the second end of the washpipe, and rotating the washpipe shaft at a speed less than the speed of the main shaft; wherein the washpipe shaft speed is at least 25% less than the main shaft speed; wherein the washpipe shaft speed is about 50% of the main shaft speed; wherein the speed reducer apparatus is a gear system including first gear apparatus connected to the main shaft and second gear apparatus connected to the washpipe shaft, and the first gear apparatus for driving the second gear apparatus, the method further including: driving the second gear apparatus with the first gear apparatus; wherein the first gear apparatus includes a first gear, the second gear apparatus includes a plurality of second gears, and the first gear is for driving the plurality of second gears to rotate the washpipe shaft, the method further including: driving the plurality of second gears with the first gear to rotate the washpipe apparatus; wherein the speed reducer apparatus has a ring housing, and a torque arrester connected to the ring housing and to the washpipe; wherein the drive apparatus includes a top drive motor and a top drive gear system driven by the top drive motor, the method further including driving the top drive gear system with the top drive motor, and driving the main shaft with the top drive gear system. 
         [0045]    The present invention, therefore, provides in at least certain embodiments, a top drive shaft system including: a main shaft with a main shaft channel therethrough; drive apparatus for rotating the main shaft at a main shaft speed; a speed reducer apparatus connected to the main shaft; a washpipe shaft connected to the speed reducer so that the washpipe shaft is rotatable by the speed reducer apparatus at a washpipe shaft speed less than the main shaft speed; the washpipe shaft having a top end and a bottom end, and a washpipe shaft channel therethrough; a washpipe having a washpipe channel therethrough, the washpipe shaft channel in fluid communication with the washpipe channel; and the washpipe shaft channel in fluid communication with the main shaft channel. Such a system may have one or some, in any possible combination, of the following: a first seal holder connected to the washpipe and holding first seal assemblies for sealing against the top end of the washpipe shaft, and a second seal holder connected to the main shaft and holding second seal assemblies for sealing against the second end of the washpipe; wherein the speed reducer apparatus reduces the washpipe shaft speed to at least 25% less than the main shaft speed; and/or wherein the speed reducer apparatus reduces the washpipe shaft speed to about 50% of the main shaft speed; and/or the speed reducer apparatus having a ring housing, and a torque arrester connected to the ring housing and to the washpipe. 
         [0046]    The present invention, therefore, provides in at least certain embodiments, a drive shaft system including: a main shaft with a main shaft channel therethrough; drive apparatus for rotating the main shaft at a main shaft speed; a speed reducer apparatus connected to the main shaft; a secondary shaft connected to the speed reducer apparatus so that the secondary shaft is rotatable by the speed reducer apparatus at a secondary shaft speed less than the main shaft speed; the secondary shaft having a top end and a bottom end, and a secondary shaft channel therethrough; a pipe having a pipe channel therethrough, the secondary shaft channel in fluid communication with the pipe channel; the secondary shaft channel in fluid communication with the main shaft channel; first sealing apparatus for sealingly contacting the secondary shaft for sealing a main-shaft secondary-shaft interface; and second sealing apparatus for sealingly contacting a secondary-shaft-pipe interface. 
         [0047]    In conclusion, therefore, it is seen that the present invention and the embodiments disclosed herein and those covered by the appended claims are well adapted to carry out the objectives and obtain the ends set forth. Certain changes can be made in the subject matter without departing from the spirit and the scope of this invention. It is realized that changes are possible within the scope of this invention and it is further intended that each element or step recited in any of the following claims is to be understood as referring to the step literally and/or to all equivalent elements or steps. The following claims are intended to cover the invention as broadly as legally possible in whatever form it may be utilized. The invention claimed herein is new and novel in accordance with 35 U.S.C. §102 and satisfies the conditions for patentability in §102. The invention claimed herein is not obvious in accordance with 35 U.S.C. §103 and satisfies the conditions for patentability in §103. This specification and the claims that follow are in accordance with all of the requirements of 35 U.S.C. §112. The inventors may rely on the Doctrine of Equivalents to determine and assess the scope of their invention and of the claims that follow as they may pertain to apparatus not materially departing from, but outside of, the literal scope of the invention as set forth in the following claims. All patents and applications identified herein are incorporated fully herein for all purposes. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are including, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.