A self-contained mobile repair unit for repairing wells includes the hydraulic and pneumatic tooling required to do a variety of jobs including the installation and removal of an inner pipe string, sucker rods and a pump. The repair unit, hydraulic tooling and pneumatic tooling share a common engine and a common process monitor. Access to data gathered by the monitor is restricted at the job site itself. Instead, the data is transmitted to a remote home base for the purpose of monitoring operations form a central location.

CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of application Ser. No. 09/058,477 filed on Apr. 10, 1998, now U.S. Pat. No. 6,079,409. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject invention generally pertains to equipment used for repairing wells that have already been drilled, and more specifically pertains to mobile repair units that frequently travel from one site to another. 
     2. Description of Related Art 
     After an oil rig drills a well and installs the well casing, the rig is dismantled and removed from the site. From that point on, a mobile repair unit is typically used to service the well. Servicing includes installing and removing inner tubing strings, sucker rods, and pumps. The variety of work requires a myriad of tools. When the tooling is not closely associated with the mobile repair unit, the right equipment may not be available when needed. 
     Moreover, the work is carried out by a company that typically owns and operates several mobile repair units. The units are often operating at the same time at various remote sites. Some sites may be separated by hundreds of miles. This makes it difficult to stay abreast of the status at each of the sites. 
     Typically, a supervisor will travel from site to site. However, this is inefficient and often critical steps of an operation get carried out unsupervised. At times, accidents occur in the absence of an unbiased witness. 
     SUMMARY OF THE INVENTION 
     To avoid the problems of today's mobile repair units, a first object of the invention is to closely associate hydraulic and pneumatic systems with a mobile repair unit by having them share a common power supply and monitoring system. 
     A second object of the invention is to provide a remotely accessible mobile repair unit with the necessary equipment to make it universally adaptable to do a variety of work such as removing and installing an inner tubing string, sucker rods, and pumps. 
     A third object is to provide a mobile repair unit that senses and transmits, to a remote home base, data that identifies the extent to which an inner tubing string was stretched prior to flooding the well bore with fluid. 
     A fourth object is to identify from a remote location key events, such as the time of transition of installing steel sucker rods to installing fiberglass ones. 
     A fifth object is to restrict local operator access to a system that monitors the operation of a mobile repair unit so an unbiased and unaltered record can be recorded and maintained of the complete system and activity of the mobile repair unit. 
     A sixth object is to convey to a remote location a record that helps explain events that led to an accident at the work site. When the information is conveyed to a remote site, it is not likely to be destroyed by the accident itself, such as a fire. 
     A seventh object is to remotely identify an imbalance of a mobile repair unit caused by wind or leaning inner tubing segments against its derrick. 
     An eighth object is to remotely distinguish between the raising and lowering of an inner tubing string to help establish the cause of an accident. An added benefit is to be able to place the proper predetermined tension on a packer or tubing anchor being set. 
     A ninth object is to enable one to remotely identify when a mobile repair unit is operating for the purpose of determining the amounts to be invoiced for the work performed. 
     A tenth object is to provide a method of alerting a home base of a hazardous level of hydrogen sulfide gas present at a remote work site. 
     These and other objects of the invention are provided by a self-contained mobile repair unit having a universal set of hydraulic and pneumatic tooling for servicing well equipment such as an inner pipe string, a sucker rod and a pump. The repair unit and tooling share a common engine. An extendible derrick supporting a hoist is pivotally coupled to the frame of the repair unit. A monitor senses the load on the derrick and conveys that information to a remote home base where the time of critical events is identified. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a mobile repair unit with its derrick extended. 
     FIG. 2 is a schematic view of a pneumatic slip in a locked position. 
     FIG. 3 is a schematic view of a pneumatic slip in an open position. 
     FIG. 4 is a schematic illustration of a set of hydraulic tongs. 
     FIG. 5 is a side view of a mobile repair unit with its derrick retracted. 
     FIG. 6 is an electrical schematic of a monitor circuit. 
     FIG. 7 is an end view of an imbalanced derrick. 
     FIG. 8 shows digital data associated with a time stamp. 
     FIG. 9 illustrates the raising and lowering of an inner tubing string. 
     FIG. 10 shows an inner tubing being lowered. 
     FIG. 11 shows an inner tubing stopped at a predetermined depth. 
     FIG. 12 shows an inner tubing being locked in a conventional manner to another casing. 
     FIG. 13 shows an inner tubing being stretched. 
     FIG. 14 shows pre-stretched inner tubing locked within an outer casing. 
     FIG. 15 shows a first steel sucker rod (with a pump) being lowered into an inner tubing string. 
     FIG. 16 shows a second steel sucker rod being lowered into an inner tubing string. 
     FIG. 17 shows a first fiberglass sucker rod being lowered into an inner tubing string. 
     FIG. 18 shows a second fiberglass sucker rod being lowered into an inner tubing string. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a retractable, self-contained mobile repair unit  20  is shown to include a truck frame  22  supported on wheels  24 , an engine  26 , a hydraulic pump  28 , an air compressor  30 , a first transmission  32 , a second transmission  34 , a variable speed hoist  36 , a block  38 , an extendible derrick  40 , a first hydraulic cylinder  42 , a second hydraulic cylinder  44 , a first transducer  46 , a monitor  48 , and retractable feet  50 . 
     Engine  32  selectively couples to wheels  24  and hoist  36  by way of transmissions  34  and  32 , respectively. Engine  26  also drives hydraulic pump  28  via line  29  and air compressor  30  via line  31 . Compressor  30  powers a pneumatic slip  84  (FIGS.  2  and  3 ), and pump  28  powers a set of hydraulic tongs  52  (FIG.  4 ). Pump  28  also powers cylinders  42  and  44  which respectively extend and pivot derrick  40  to selectively place derrick  40  in a working position (FIG. 1) and in a lowered position (FIG.  5 ). In the working position, derrick  40  is pointed upward, but its longitudinal centerline  54  is angularly offset from vertical as indicated by angle  56 . The angular offset provides block  38  access to a well bore  58  without interference with derrick pivot point  60 . With angular offset  56 , the derrick framework does not interfere with the typically rapid installation and removal of numerous inner pipe segments known as a inner pipe string  62  and sucker rods  64  (FIG.  16 ). 
     Individual pipe segments of string  62  and sucker rods  64  are screwed to themselves using hydraulic tongs  66  which are schematically illustrated in FIG.  4 . The term “hydraulic tongs” used herein and below refer to any hydraulic tool that can screw together two pipes or sucker rods. An example would include those provided by B. J. Hughes company of Houston, Tex. In operation, pump  28  drives a hydraulic motor  68  forward and reverse by way of valve  70 . Conceptually, motor  68  drives pinions  72  which turn wrench element  74  relative to clamp  76 . Element  74  and clamp  76  engage flats  81  on mating couplings  78  of a sucker rod or inner pipe string of one conceived embodiment of the invention. However, it is well within the scope of the invention to have rotational jaws or grippers that clamp on to a round pipe (i.e., no flats) similar in concept to a conventional pipe wrench, but with hydraulic clamping. The rotational direction of motor  68  determines assembly or disassembly of couplings  78 . Transducer  80  is used to provide a 0-5 VDC signal  82  that in one embodiment of the invention indicates the applied torque to couplings  78 . 
     Referring to FIGS. 2 and 3, when installing inner pipe segments  62 , pneumatic slip  84  is used to hold string pipe string  62  while the next segment  62 ′ is screwed on using tongs  66 . Compressor  30  provides pressurized air through valve  86  to rapidly clamp and release slip  84  (FIGS. 2 and 3, respectively). A tank  88  helps maintain a constant air pressure. Pressure switch  90  provides monitor  48  with a signal that indirectly indicates that repair unit  20  is in operation. 
     Referring back to FIG. 1, weight applied to block  38  is sensed by way of a hydraulic pad  92  that supports the weight of derrick  40 . Hydraulic pad  92  is basically a piston within a cylinder (alternatively a diaphragm) such as those provided M. D. Totco company of Cedar Park, Tex. Hydraulic pressure in pad  92  increases with increasing weight on block  38 . In FIG. 6, first transducer  46  converts the hydraulic pressure to a 0-5 VDC signal  94  that is conveyed to monitor  48 . Monitor  48  converts signal  94  to a digital value, stores it in a memory  96 , associates it with a real time stamp, and eventually communicates the data to a remote home base  100  by way of a modem  98 . 
     In the embodiment of FIG. 7, two pads  92  associated with two transducers  46  and  102  are used. An integrator  104  separates pads  92  hydraulically. The rod side of pistons  106  and  108  each have a pressure exposed area that is half the full face area of piston  108 . Thus chamber  110  develops a pressure that is an average of the pressures in pads  92 . One type of integrator  104  is provided by M. D. Totco company of Cedar Park, Tex. In one embodiment of the invention, just one transducer  46  is used and it is connected to port  112 . In another embodiment of the invention, two transducers  46  and  102  are used, with transducer  102  on the right side of unit  20  coupled to port  114  and transducer  46  on the left side coupled to port  116 . Such an arrangement allows one to identify an imbalance between the two pads  92 . 
     Returning to FIG. 6, transducers  46  and  102  are shown coupled monitor  48 . Transducer  46  indicates the pressure on left pad  92  and transducer  102  indicates the pressure on the right pad  92 . A generator  118  driven by engine  26  provides an output voltage proportional to the engine speed. This output voltage is applied across a dual-resistor voltage divider to provide a 0-5 VDC signal at point  120  and then passes through an amplifier  122 . Generator  118  represents just one of many various tachometers that provide a feedback signal proportional to the engine speed. Another example of a tachometer would be to have engine  26  drive an alternator and measure its frequency. Transducer  80  provides a signal proportional to the pressure of hydraulic pump  28 , and thus proportional to the torque of tongs  66 . 
     A telephone accessible circuit  124 , referred to as a “POCKET LOGGER” by Pace Scientific, Inc. of Charlotte, N.C., includes four input channels  126 ,  128 ,  130  and  132 ; a memory  96  and a clock  134 . Circuit  124  periodically samples inputs  126 ,  128 ,  130  and  132  at a user selectable sampling rate; digitizes the readings; stores the digitized values; and stores the time of day that the inputs were sampled. It should be appreciated by those skilled in the art that with the appropriate circuit, any number of inputs can be sampled. Page Scientific provides circuits that employ multiplexing to provide twelve input channels. 
     An operator at a home base  100  remote from the work site at which repair unit  20  is operating accesses the data stored in circuit  124  by way of a PC-based modem  98  and a cellular phone  136 . Phone  136  reads the data stored in circuit  124  via lines  138  (RJ11 telephone industry standard)and transmits the data to modem  98  by way of antennas  140  and  142 . In one embodiment of the invention, phone  136  includes a CELLULAR CONNECTION™ provided by motorola Incorporated of Schaumburg, Ill. (a model S1936C for Series II cellular transceivers and a model S1688E for older cellular transceivers). 
     Some details worth noting about monitor  48  is that its access by way of a modem makes monitor  48  relatively inaccessible to the crew at the job site itself. Amplifiers  122 ,  144 ,  146  and  148  condition their input signals to provide corresponding inputs  126 ,  128 ,  130  and  132  having an appropriate power and amplitude range. Sufficient power is needed for RC circuits  150  which briefly (e.g., 2-10 seconds) sustain the amplitude of inputs  126 ,  128 ,  130  and  132  even after the outputs from transducers  46 ,  102  and  80  and the output of generator  118  drop off. This ensures the capturing of brief spikes without having to sample and store an excessive amount of data. A DC power supply  152  provides a clean and precise excitation voltage to transducers  46 ,  102  and  80 ; and also supplies circuit  124  with an appropriate voltage by way of voltage divider  154 . Pressure switch  90  enables power supply  152  by way of relay  156  whose contacts  158  close by coil  160  being energized by battery  162 . 
     FIG. 8 shows an example of the data extracted from circuit  124  and remotely displayed at PC  164 . The values plotted at a point in time indicated by numeral  166  represent repair unit  20  at rest with engine  26  idling as shown in FIG.  1 . Numeral  168  showing weight on block  38  and high engine speed indicates the raising of an inner pipe string  62  as represented by arrow  170  of FIG.  9 . Numeral  172  showing weight on block  38  and low engine speed indicates the lowering of inner pipe string  62  as represented by arrow  174  of FIG.  9 . Points  176 ,  178 ,  180 ,  182  and  184  correspond to the conditions illustrated in FIGS. 10,  11 ,  12 ,  13  and  14 , respectively. In FIG. 10, an inner pipe string  62  is being lowered into an outer casing  186 . In FIG. 11, tubing pipe is stopped at a redetermined depth. In FIG. 12 pipe string  62  is rotated in a conventional manner to lock its lower end  188  to outer casing  186  (note slight torque at point  190 ). In FIG. 13 an upper end  192  of string  62  is raised until the pressure parameter at right and left pads  92  reach the predetermined limit indicated by numeral  194 . In FIG. 14 wedge  196  locks upper end  192  to casing  186 , and block  38  is disconnected from pipe string  62 . Points  198 ,  200 ,  202  and  204  correspond to the conditions illustrated in FIGS. 15,  16 ,  17  and  18 , respectively, which depict the lowering of a string of sucker rods having a pump  77  at its lower end. Intermediate points  199 ,  201  and  203  indicate tongs  66  screwing onto the first steel sucker rod  64  a second steel sucker rods  206 , a fiberglass sucker rod  208 , and a second fiberglass sucker rod  210 , respectively. Note the difference in torque and the incremental weight difference at pads  92  when changing over from steel rods to fiberglass ones. Points  212  correspond to the windy conditions illustrated by arrow  214  of FIG.  7 . The absence of data points beyond 12:00 indicates that the windy conditions prevented the crew from continuing, or it was Friday afternoon. 
     Referring back to FIG. 4, it should be noted that transducer  80  represents any one of a variety of devices that produce an electrical signal in response to a change in a sensed condition. In one embodiment of the invention, transducer  80  is actually a hydrogen sulfide gas detector with signal  82  serving as a gas detection signal that varies with a varying concentration of hydrogen sulfide gas  250 . An example of a hydrogen sulfide gas detector is a CONTROLLER 8000 provided by Industrial Scientific Corporation of Oakdale, Pa. 
     Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those skilled in the art. Therefore, the scope of the invention is to be determined by reference to the claims which follow.