Abstract:
A pre-load monitor is arranged for use between a between a washer face of a fastener and a bearing surface of a fixture arranged to receive a portion of a shank of the fastener. The pre-load monitor, which can be in the form of a washer, includes a piezoelectric generator which generates the electrical power to measure, monitor and report the pre-load. The resulting electrical power produced is proportional to the pre-load and can be stored to provide the energy required to function electrical, electronic and communication circuits included in the washer. Alternatively, the pre-load monitor can be a half- or split ring (or a C-ring) constructed similarly to that of the washer, or the ring can be a two-piece gasket construction that includes an array of the washers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to systems and methods used to determine whether a correct pre-load is, or has been, applied to threaded fasteners such as nuts, bolts, and machine screws. More specifically, the invention relates to these systems and methods as applied to pressure-containing equipment like that used in the oil and gas industry. 
         [0002]    For the correct and safe on-going operation of equipment, particularly pressure-containing equipment, it is important that the correct pre-load is applied to fasteners and that the load is consistent thereafter. 
         [0003]    Traditional techniques of determining fastener pre-load typically rely on measuring the torque applied to the fastener when the correct pre-load is achieved. (Or, turning a nut through a specified angle that stretches the bolt the desired amount). However, differences in tolerances, lubrication, and friction, might require different torques be applied to similar fasteners in a fastening application to achieve the same preload among all of the fasteners. This is a significant limitation of traditional techniques and can lead to highly inconsistent forces, for example, around the circumference of a bolted flange. 
         [0004]    Ultimately these differential forces can lead to failures, pressure escapes and fasteners coming apart. These situations can result in environmental damage and harm to personnel. 
         [0005]    Prior art systems and methods modify the fastener itself and generally make use of ultrasonic techniques. For example, U.S. Pat. No. 4,899,591 to Kibblewhite discloses an ultrasonic transducer made of a thin piezoelectric sensor consisting of a piezoelectric film sandwiched between two thin electrodes. The transducer is permanently mechanically and acoustically coupled to a head of a fastener and is used to determine the length, tensile load, stress, or other tensile load-dependent characteristics of the member by ultrasonic techniques. 
         [0006]    U.S. Pat. No. 7,412,898 B1 to Smith et al. makes use of a fastener having a bore that includes a grounding pin and a radio frequency identification (“RFID”) transponder or tag exposed to the bore (see also US 20060022056 A1 to Sakama et al. disclosing a bolt with an RFID tag and chip for storing information and an antenna for transmitting wirelessly the information stored on the IC chip). The transponder generates a response signal with a unique identifier in response to a radio frequency signal from a transmitter. The grounding pin contacts and grounds the transponder to block transmittal of the response signal when a tensile load applied to the fastener is less than a minimum tensile load value. 
         [0007]    U.S. Pat. No. 8,683,869 B2 to Herley et al. discloses a system and method of monitoring fastener pre-load using an ultrasonic transducer on the fastener and electronically transmitting the measured pre-load to a monitoring station for analysis (see also U.S. Pat. No. 7,698,949 B2 to Akdeniz et al. incorporating ultrasonic transducers with load bearing washers). The transducer, which is mounted to the head or threaded end of the fastener by an adhesive, is excited by a voltage pulse and the “time of flight” of the ultrasonic wave is used to indicate pre-load (relative to zero load time of flight). A reader or cap can be placed over the head of the fastener to engage the electrical contacts of the transducer and communicate via an electrical lead to the monitoring station. Alternatively, a radio frequency identification (“RFID”) tag can be mounted on the fastener and operably connected to the transducer. 
         [0008]    U.S. Pat. No. 6,378,384 B1 to Atkinson et al. discloses a force sensing device in the form of a washer that includes force sensitive resistors fabricated as thick film resistors which are printed and fired onto an electrically insulated substrate material. The washer is used to monitor pressure pulses of an underlying process or operation that a fastener is in communication with, not fastener pre-load. Others have experimented with ceramic washer designs that make use of piezoelectric filaments connected to a handheld device. 
       SUMMARY OF THE INVENTION 
       [0009]    A pre-load monitor made according to this invention is arranged for use between a washer face of a fastener and a bearing surface of a fixture arranged to receive a portion of a shank of the fastener. The pre-load monitor, which can be in the form of a washer, includes a piezoelectric generator which generates the electrical power to measure, monitor and report the pre-load. The resulting electrical power produced is proportional to the pre-load and can be stored to provide the energy required to function electrical, electronic and communication circuits included in the washer. Alternatively, the pre-load monitor can be a half- or split ring (or a C-ring) constructed similarly to that of the washer, or the ring can be a two-piece gasket construction that includes an array of the washers. 
         [0010]    Conversely, applying a small stimulating signal to the washer can used to measure a response of the piezoelectric generator and monitor the washer for changes in the pre-load condition. The washer can include load sensing hardware such as a strain gage based load cell that detects the deformation of the piezoelectric generator when stimulated. 
         [0011]    The pre-load monitor can also include a “smart” tool such as a socket wrench in wireless communication with the washer either directly or via a socket. The tool can also be in wireless communication with its environment to provide positional information and to access and download data relating to the task to be performed, such as details of the pre-load required for individual fasteners. 
         [0012]    A method of measuring fastener pre-load using the pre-load monitor includes the steps of placing a clamping force on the pre-load monitor, measuring the fastener pre-load using the electrical power generated, broadcasting the electrical power data generated, and calculating the fastener pre-load based upon the broadcast electrical power data. The broadcasting step can be powered by a portion of the electric power generated by the pre-load monitor, and this power also can be stored by the pre-load monitor. 
         [0013]    The clamping force can be applied by the smart tool, and the tool can also be used to monitor fastener pre-load over time and apply make-up pre-load. The fastener pre-load can also be adjusted to account for a temperature gradient between the assembly environment and the operating environment of the fixture. 
         [0014]    Monitoring could also be done by stimulating a portion or layer of the pre-load monitor lying adjacent to the flexible piezoelectric material, detecting a deformation of the flexible piezoelectric material in response to the stimulating step; and measuring an amount of the deformation. The amount of deformation can be determined by way of a load cell. The stimulating step can occur by way of a voltage source provided by a power supply internal to or external of the pre-load monitor. 
         [0015]    A system of monitoring fastener pre-load includes a pre-load monitor arranged for use between a washer face of a fastener and a bearing surface of a fixture arranged to receive a portion of a shank of the fastener. The pre-load monitor has an upper and lower metallic layer that provides the mechanical properties needed to appropriately distribute the clamping force, a flexible piezoelectric material arranged to generate electric power in response to the clamping force, and a radio frequency identifier (“RFID”) tag arranged to broadcast an amount of the electric power generated. In one preferred embodiment, the pre-load monitor is in the form of a circular washer. In another preferred embodiment, a gasket material is arranged about the washer. As with the method, the pre-load monitor can be used in combination with the smart tool. 
         [0016]    The pre-load monitor can also include a stimulation layer lying adjacent to the flexible piezoelectric material and in communication with a power supply and a detection layer lying adjacent to the flexible piezoelectric material and arranged to detect a deformation of the flexible piezoelectric material. The pre-load monitor can also include an energy storage layer such as a battery or capacitor (or both) in communication with the flexible piezoelectric material. 
         [0017]    Objectives of this invention include providing a pre-load monitor and method of its use that (1) can be used on a wide variety of fasteners and bolted connections, including but not limited to pressure-containing connections; (2) does not require any modification to the fastener and instead uses fixtures such as washers or rings that distribute fastener load to measure pre-load; (3) does not rely upon ultrasonic techniques and instead uses the electrical power generated by the fixture when under load to measure pre-load; (4) can apply different torques to similar fasteners being used in an application so that all of the fasteners are under a correct, consistent pre-load; (5) provides a correct pre-load in an assembly environment given a temperature gradient experienced by a fastener when in an in-use or operating environment; (6) continually monitors fastener load and can signal whether any changes are occurring from pre-load conditions; (7) makes a complete audit trail of the fixture and its corresponding fastener or fasteners; and (8) can be used in combination with a “smart” tool so that prior knowledge of the application by the operator is not required. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a top plan view of a preferred embodiment of a pre-load monitor or fixture in the form of a circular washer having mechanical properties in addition to electrical generation and RF communication properties. The washer is sized for use in combination with the fastener typically used in a particular application. 
           [0019]      FIG. 2  is a cross-section view of the washer of  FIG. 1 . A piezoelectric material generates electrical power that is used to power the fixture and its communication capabilities and to indicate pre-load. 
           [0020]      FIG. 3  is a front elevation view of the washer of  FIG. 1  under load and broadcasting an RF signal that could be read by an operator, a remote operated or autonomous underwater vehicle (“ROV” or “AUV”), by a neighboring washer, or by a tool in communication with the washer. 
           [0021]      FIG. 4  is a side elevation view of a “smart” tool useful for applying make-up preload and communicating with the fixture. The tool includes a sensor interface, RF communication hardware, and a processor and a memory device capable of receiving and storing measurement data provided by the washer of  FIG. 1  (see also  FIG. 14 ). 
           [0022]      FIG. 5  is a top plan view of an array of the washers arranged about a flanged connection, plate or closure. 
           [0023]      FIG. 6  is top plan view of another preferred embodiment of the pre-load monitor or fixture in the form of a half- or split ring. The ring can be constructed similarly to that of the washer of  FIG. 2 or 7  for shared sensing or can be a two-piece gasket having an array of washers constructed similarly to that of the washer of  FIG. 2 or 7 . 
           [0024]      FIG. 7  is an isometric view of another preferred embodiment of the pre-load monitor or fixture in the form of a washer. The washer includes a stimulation layer having a connection tab to a power source and a detection layer having load sensing hardware. 
           [0025]      FIG. 8  is a top plan view of the detection layer of  FIG. 7 . The load sensing hardware can be a strain gage based load cell. 
           [0026]      FIG. 9  is a front elevation view of the detection layer of  FIG. 7 . The thickness of the layer is determined based upon the application. 
           [0027]      FIG. 10  is a top plan view of the split rings of  FIG. 6  arranged about a flanged connection, plate or closure. 
           [0028]      FIG. 11  is a top plan view of a molded rubber, one-piece C-ring that includes the washer of  FIG. 2 or 7  local to each fastener hole of the ring. 
           [0029]      FIG. 12  is a front elevation view of the two-piece gasket embodiment of the split ring of  FIG. 6  under load and connected to an external power source. 
           [0030]      FIG. 13  is schematic illustrating the tool of  FIG. 4  when in use in an RF (or similar) communication environment. 
           [0031]      FIG. 14  is a schematic illustrating the tool of  FIG. 4  as it interacts with other elements of the RF communication environment. 
       
    
    
     ELEMENTS AND NUMBERING USED IN THE DRAWING FIGURES 
       [0032]      10  Pre-load monitor 
         [0033]      20  Washer (ring or perforated plate) 
         [0034]      21  Uppermost mechanical (metallic) layer 
         [0035]      23  Power generation or piezoelectric layer 
         [0036]      25  Electronics (including communications) layer 
         [0037]      27  Energy storage or capacitance/battery layer 
         [0038]      29  Antenna layer 
         [0039]      31  Lowermost mechanical (metallic) layer 
         [0040]      40  Washer 
         [0041]      41  Uppermost mechanical layer 
         [0042]      43  Detection (load sensing) layer 
         [0043]      44  Load cell or strain gage wiring 
         [0044]      45  Piezoelectric layer 
         [0045]      47  Stimulation layer 
         [0046]      49  Connection tab 
         [0047]      51  Lowermost mechanical layer 
         [0048]      60  Two-piece sensing gasket 
         [0049]      61  Half- or split ring 
         [0050]      63  Array of washers  20  or  40   
         [0051]      65  Wire 
         [0052]      67  Load sensing wire connection point 
         [0053]      69  Power supply 
         [0054]      71  One-piece c-ring 
         [0055]      73  Molded portion of ring 
         [0056]      80  “Smart” tool 
         [0057]      81  Socket wrench 
         [0058]      83  Handle 
         [0059]      85  Socket 
         [0060]      87  Display 
         [0061]      89  Sensor 
         [0062]      91  Processor 
         [0063]      93  Signal generator 
         [0064]      95  Communications hardware 
         [0065]      97  Computer readable media 
         [0066]      98  Reader 
         [0067]      99  Memory 
         [0068]      100  Protective device 
         [0069]      101  Headset or ear protection 
         [0070]      103  Eyewear 
         [0071]      105  Optical device 
         [0072]      107  Glove 
         [0073]      109  Signal generator 
         [0074]      111  Communications hardware 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0075]    In the preferred embodiments, the pre-load monitor is a washer, ring, or perforated plate, each used alone or in combination with a gasket material, and intended for use under a head of a fastener or a nut to spread the clamping force exerted by the tightened fastener. The pre-load monitor includes upper and lower metallic layers that provide mechanical strength and a piezoelectric material that is used to measure the pre-load using non-ultrasonic techniques. The piezoelectric material is preferably a flexible material (rather than, for example, a ceramic) and can be a printed flexible piezoelectric material. Additionally, the piezoelectric material can form a single layer of the pre-load monitor. 
         [0076]    In one preferred embodiment, the pre-load monitor is a circular washer  20  made of a composite material which includes a piezoelectric material (see  FIGS. 1 &amp; 2 ). The upper- and lowermost layers  21 ,  31  of the washer  20  are the mechanical (metallic) layers, providing the mechanical properties normally found in a washer. A power generation layer  23 , preferably in the form of a flexible piezoelectric material, resides adjacent to the uppermost layer  21 . As the washer  20  compresses in response to a fastener being tightened, piezoelectric power is generated proportional to the applied load. Once the generated power reaches a predetermined level, the proper pre-load is achieved. In this way, each fastener in an application can obtain the correct pre-load and pre-load is consistent among the fasteners (see e.g.  FIG. 5 ). 
         [0077]    For any given application, the relationship between generated power and pre-load can be determined through routine experimentation using fastener testing techniques known in the art. Temperature effects could also be accounted for. For example, transitioning between 37° or 38° C. on the back deck of a drillship to about 3° or 4° C. on the seabed causes contraction that could potentially increase the load on the fasteners. A temperature-corrected pre-load could be determined and applied in the assembly environment to account for this contraction in the operating environment. 
         [0078]    Alternatively, in another embodiment of the pre-load monitor  10  a piezoelectric layer  45  of a washer  40  is sandwiched between stimulation layer  47 —which is stimulated with an incoming (external) signal and causes the piezoelectric layer  45  to deform (see  FIG. 7 )—and a detection or reaction layer  43 . The detection layer  43  is sized to suit the application, load and hardware required for sensing and can include load sensing hardware such as a load cell or strain gauge wiring  44  (see  FIGS. 8 &amp; 9 ). Because the piezoelectric layer  45  is constrained by the fastener load, the resulting force can be used to determine changes in dimensions (e.g. gap) and pre-load. An upper- and a lowermost mechanical layer  41 ,  51  provide the mechanical properties normally found in a washer. 
         [0079]    Stimulation could be repeated at pre-determined scheduled intervals to monitor the pre-load condition. The incoming signal could be provided wirelessly but is, more preferably, provided by an external power source (and its associated amplifier, oscillator and demodulator) in wired communication with the washer  40  (see e.g.  FIG. 12 ). In this way, real-time monitoring of fastener load is achieved. 
         [0080]    For make-up preload, a “smart” tool  80  preferably in the form of a socket wrench  81  can be used (see  FIGS. 4, 13 &amp; 14 ). The wrench  81  includes a sensor interface  89 , RF communication hardware  95 , and a processor  91 , a memory device  99  capable of receiving and storing measurement data provided by the washer  20 ,  40 , and computer readable media  97 . Additionally, the wrench  81  could include a quick response (“QR”) reader  98  to read a QR code of a fastener and then, based on this identification, automatically retrieve the necessary pre-load information associated with the fastener and stored in a database. Retrieval could be done wirelessly or by way of a USB connection in the handle  83  of the wrench  81 . When being used in an RF environment, the location of the wrench  81 , along with that of the washer  20 ,  40 , can be determined and recorded. 
         [0081]    The socket wrench  81  includes communications hardware  95  to provide bi-directional communication with the wifi-enabled environment in which it operates, and the wrench  81  is able to download from engineering or production databases data such as but not limited to the correct preload required for any given fixture as determined by such factors as location (manual or automatic) or to a reference such as a QR code. The wrench  81  can write data back to the database (business management software) to confirm information such as location, date, time, preload applied, serial numbers used (as appropriate), and temperature at the time of fastening. The sensor or sensor interface  89  can be located in the wrench  81  or the socket  85  or in a protective device such as a glove  107  (see  FIG. 14 ), and additional sensors such as microelectromechanical systems (“MEMS”) sensors could be included in the socket  85  to detect such parameters as orientation and angle. Communication with the washer  20 ,  40  can be direct by way of the socket  85  or wireless, including but not limited to magnetic, inductive, or near field means. A display  87  can be included on the handle  83  to indicate whether the pre-load is below, approaching, at, or beyond the correct predetermined pre-load (see  FIG. 4 ). A similar type of display could also be included on the washer  20 ,  40 . 
         [0082]    The visual indication of status may also be made available to the user by use of appropriate eyewear  103 . A wireless communications link between the wrench and an appropriate optical device  105  worn in front of the user&#39;s eyes can be used to display the alert colors within the user&#39;s field of view in a manner similar to a “heads-up” display. This visual indication technique may be incorporated into protective eye equipment commonly required to be worn in industrial environments. 
         [0083]    The wrench  81  may optionally include a signal generator  93  such as one including an audio means of alerting the user to the status of the preload condition (e.g. a distinct tone for each of below, approaching, at, or over the correct preload value). 
         [0084]    The audio settings may optionally be configurable to ensure satisfactory operation for a range of users and operating environments. The nature of the operating environment (e.g. background equipment noise or other sources of interference) will dictate which frequencies will be most difficult to distinguish. The alert tones generated by the signal generator  93  may be adjusted to avoid such background frequencies. Similarly, the age and aural condition of the user will determine their ability to hear certain frequency ranges (e.g. younger people tend to be able to hear higher frequencies that older people cannot). Therefore, the alert tones of the wrench  81  may be adjusted to suit the aural capabilities of the user. 
         [0085]    Further, the audio alert may be broadcast by the wrench  81  to a protective device  100  such as a headset  101  worn by the operator. The preferred embodiment for this would be a wireless communications link such as commonly used between a mobile telephone and an ear-piece/headset. This may be used to provide a direct audio signal between the wrench  81  and the user. A protocol such as Bluetooth would be one option for this link. This approach will allow the incorporation of the audio alert into ear protection worn by the user either as an integral part of the protection device, or such that the earpiece can be worn underneath or inside the protection device without compromising the performance of the protection. 
         [0086]    The signal generator  93  of the wrench  81  may optionally include a vibration means of alerting the user to the status of the preload condition. Similar to the audio embodiment, a distinct vibration pattern can be provided for each of below, approaching, at, or beyond the correct preload value. 
         [0087]    In another preferred embodiment, a protective glove  107  generates the vibration within the structure of the glove  107  by way of a signal generator  109  rather than the socket wrench  81 . The glove  107  includes hardware  111  for wireless or inductive communication with the wrench  81  or socket  85 . 
         [0088]    The socket wrench  81  can be battery powered and charged either at a base station or inductively, and could be part of a ROV or AUV tool. A solenoid or similar mechanism can be used to cause the wrench  81  to stop driving when the correct pre-load is reached. A signal from the washer  20 ,  40  could be provided to a hydraulic pump unit to regulate and limit output pressure to a torque tool when the desired pre-load is reached. 
         [0089]    Returning once again to the washer  20  (see  FIGS. 1 &amp; 2 ), in addition to the power generation layer  23  the washer  20  could include electronics and communication hardware layers such as a radio-frequency identification (“RFID”) tag or chip layer  25 —which would provide a unique identifier to its respective washer  20  and allow identification of washer location and replacement—and a radial or circumferential RF antenna layer  29 . The RF signal broadcast by the washer  20  could be read by an operator, a remote operated or autonomous underwater vehicle (“ROV” or “AUV”), by a neighboring washer  20  (see  FIG. 3 ), or by the tool  80  in communication with the washer  20  (see  FIG. 4 ). To protect the electronics and other sensitive components of the washer  20 , the washer  20  could be covered or wrapped with a protective material (the same holds true for washer  40 ). 
         [0090]    Communication with the washer  20  could be powered by an incoming wireless signal, a wired connection to an external power source (see e.g.  FIG. 7  showing a tab  49  providing a connection point and  FIG. 12  showing a power source  69 ), or by way of a capacitance or battery layer  27  in communication with the piezoelectric layer  23  (see  FIG. 2 ). Each washer  20  could share a single wired connection with its neighbors (like that shown in  FIG. 10 ) and communicate with a neighboring washer  20 . 
         [0091]    Regardless of the power source, the washers  20 ,  40  can be in communication with, and a part of, a subsea electronic information system like that disclosed in U.S. Pat. No. 8,683,869 B2 to Herley et al. or US 2014/0064029 A1 to Jaffrey (“Jaffrey”), both incorporated by reference herein. Jaffrey&#39;s system includes a subsea-located sensor interface box that includes a processor and a memory device capable of receiving and storing sensor measurement data like that provided by the washer. Additionally, the sensor interface box can be in data and power communication with the washer. A subsea retrievable data capsule may be used in connection with the box and recovered from subsea (independently of the other elements of the information system) for forensic analysis of the recorded data. A similar electronic information system could be employed in topside applications and applied to the tool  80 . 
         [0092]    In another preferred embodiment, the pre-load monitor  10  is in the form of a half- or split ring  61  (see  FIGS. 6 &amp; 10 ). The split ring  61  can be constructed similarly to that of washers  20 ,  40  for shared sensing or a two-piece gasket arrangement that includes an array  63  of washers  20 ,  40 . The array  63  can be connected by a common wire  65 , with a connection point  67  that places the array  63  in communication with a power source  69  (see  FIG. 12 ). Alternatively, a C-ring  71  can be used (see  FIG. 11 ). The rings  61 ,  71  operate similarly to that of washers  20 ,  40 . 
         [0093]    The split ring  61  or C-ring  71  can be a molded rubber  73  that contains the washer embodiment  20 ,  40  local to each fastener hole of the ring  61 ,  71 . The power source  69  to the ring  61 ,  71 —as well as to the individual washer embodiments  20 ,  40  previously discussed—can take many forms, such as a battery pack, an aqua battery, ROV, AUV, hand-held device, piezoelectric generator (such as responds to the vibration of the operated equipment), or a wave motion operated power supply (see  FIG. 12 ). 
         [0094]    The following claims define the scope of the invention, including the full range of equivalents to which elements of the claims are entitled.