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There are many different designs for shooting panels floating around the oil patch, some good, but some bad or even profoundly dangerous. Below are some random notes on shooting panels, not a comprehensive design guide. Use or reliance on the information and materials contained herein is entirely at your own risk.
API Guidelines
For decades there was no guidance document for oilfield explosives safety. There existed some safety information from manufacturers and from various explosives trade organizations. Finally, on March 1, 1994, the American Petroleum Institute (API) published "Recommended Practices for Oilfield Explosives Safety," API Recommended Practice 67 (RP 67).
Electric Wireline Unit Power - In RP 67, API recommends that there be a main power switch in the electric wireline unit to control all power to the shooting system.
Cable Safety System - In RP 67, API recommends that the cable safety circuit be inside the electric wireline unit, and when in safe mode the cable safety system must open all cable connectors from the electric wireline unit, and shunt said conductors to armor through a nominal 5,000 ohms (5 kW). The 5,000 ohms must consist of a minimum of two (2) similar resistors in parallel with the net resistance being 5,000 ohms (5 kW). The safe mode must be assured with a lock-out feature. Electrical continuity from the cable safety circuit to the cable conductor must be assured and the wiring must have mechanical protection.
Firing System - In RP 67, API recommends that there be at least three deliberate actions required to fire an explosive device. At least one of the actions should require two hands, and one action should involve a spring loaded switch.
Safety Schemes
The API guidelines are a bit vague as to the best arrangement for a safe shooting panel. Some schemes in use are quite elaborate requiring multiple keys and/or security codes to arm the system. Others tie shooting control to the intercom on the rig floor such that shooting cannot occur unless an affirmative action is taken by the individual in control of the intercom unit. On simple shooting panels, we like to see at least three separate switches in addition to the autotransformer control. There should be a main power switch, an arming switch, and a firing switch at the very least. It is not a bad idea for the power switch to be a key operated device. The firing switch should be a spring loaded button or switch with some sort of guard (flip up toggle switch arming guards are very effective with spring loaded toggle switches). A similar guard device for the arming switch is a good idea as well. Separate jacks for "SAFE", "CCL", and "SHOOT" make for a safer panel than selector switches (see discussion below).
Observations
In thinking about shooting panel safety, one must consider the consequences of multiple safety feature failures. Always remember it is exceedingly difficult to make an electronic device foolproof since fools are so ingenious. What follows is a bit of design philosophy commingled with a few thoughts on bad designs we have encountered.
Shunt Resistors - Some years ago, it was considered good safety practice to short the logging line leads prior to shooting (usually the center conductor or conductors to the cable armor), but it is now known this is a dangerous practice. A dead shorted logging line can act like a giant loop antenna (especially unshielded or unarmored lines sometimes used in shallow plugging and abandonment or stimulation work). A far safer arrangement is a shunt resistor. Many different values have been used over the years, but as mentioned above the current API recommendation is 5,000 ohms (5 kW) derived from two 10,000 ohms (10 kW) resistors configured in parallel for redundant protection. We have seen half watt (1/2 w) shunt resistors burned out in shooting panels, so we recommend larger shunt resistors. It seems prudent to use at least five watt (5 w) resistors, but ten watt (10 w) might be an even better idea.
Filter Capacitors - The ripple filtration capacitor across the bridge rectifier in a shooting panel should be kept to the lowest practical value possible. Capacitors store energy, and that is potentially dangerous in a shooting circuit. Almost certainly, there were more Gearhart-Owen (GO / GOI) / Gearhart Industries / Mineral Logging Systems (MLS) shooting panels made than any other brand, and they got away with a mere one microfarad (1.0 µf) capacitor across the bridge rectifier in most of their designs. Some manufacturers use tens or hundreds of microfarad capacitors, and some even used multiple capacitors; these practices should be regarded as potentially unsafe. A bleeder resistor should always be connected across the filter capacitor; typically in the range of 150 to 330 kilohm (150-330 kW). A few manufacturers omitted the bleeder resistor, and again this practice should be regarded as potentially unsafe.
Switching Concerns - One of the absolutely nuttiest things we have encountered is the placing of mains / generator AC power on the same switch as the shooting / logging line. A couple of manufacturers did this to take the shooting line to ground (in older panels) or to the shunting resistor (in newer panels) whenever the power is off. However, this "safety feature" puts 120 volt AC within a fraction of an inch of the shooting line conductor. The problem is exacerbated in miniature switches with their even closer spacing than standard switches. We know of no accidents attributed to this design flaw, but any such panel should be regarded as potentially unsafe. Any panel with AC mains / generator voltage and the shooting / logging line on the same switch should immediately be taken out of service and modified by a competent service facility. We like to see double pole switches used for power switches and arming switches in shooting panels so both the "hot" and "neutral" sides of the AC supply get switched open (on the output side of the isolation transformer there is no real neutral until one side is taken to ground in the case of panels with an "AC" shooting option). Switching both sides is inherently safer especially in cases where the neutral and hot sides of the AC power supply might become accidentally reversed.
CCL and Shoot Selection - The safest arrangement is to have separate jacks for SAFE, CCL, SHOOT, and FREEPOINT (in portable freepoint panels) as opposed to a single jack and a selector switch. Using a switch introduces unnecessary additional risk of a malfunction resulting in crossed circuits. A number of safety compromises have been devised involving elaborate relay schemes and fancy switches, but using separate jacks is simpler and safer. Bell used a rotary selector switch on their portable freepoint panel which required crossing two safe positions before reaching the shoot position from the log or CCL positions; not a bad idea, but still more complicated and inherently more dangerous than the separate jack approach.
CCL Power Supplies - Shooting panels are often used in conjunction with shooting CCL surface amplifiers. For safety reasons, shooting CCL surface amplifiers installed in trucks are usually powered with the truck battery 12 volt DC power supply, but portable units are often configured to run off mains AC power. It is customary safety practice to not start the truck generator (Onan) until the shot is several feet into the well. Having the shooting CCL surface amplifier running off the truck 12 volt supply simplifies this practice and makes it possible to delay starting the generator for as long as is deemed prudent. When a shooting CCL surface amplifier is powered by mains / generator AC power, the same caution discussed above in "Switching Problems" applies. In the case of portable shooting panels with CCL capability, or any unit powered by mains AC power, said unit should be reviewed for safety by a competent service facility.
Isolation Transformers - Most oil patch shooting panels utilize an autotransformer (a/k/a Variac or Powerstat) to adjust the voltage when shooting. Autotransformers are peculiar critters in that they have only a single winding shared by both the primary and secondary. As a result, autotransformers do not isolate the secondary from the primary AC power supply. All shooting panels with an autotransformer should also have an isolation transformer in the circuit. The isolation transformer can be located either before or after the autotransformer, but we prefer after as extra protection for the expensive autotransformer.
Miscellaneous Considerations - To the extent practicable, the cable safety circuitry should be physically removed from close proximity to mains AC power (or any sources of power in the panel). It is recommended that only the best quality switches be used in shooting panels; avoid the lower quality imported switches for this application. Full size switches are probably safer than miniature switches, but miniature switches may be unavoidable due to space constraints. Teflon insulated wire is recommended for shooting panel wiring because of Teflon insulation's tough mechanical properties and high temperature characteristics (the insulation is slower to fail if the wire overheats in a shorting scenario). Shooting panels should be assembled with the utmost care in a neat and workman like manner with careful inspection and verification of wiring. Extensive testing should be done after assembly to verify all circuits work properly.
Slick Testing Trick
Shooting panels can be dramatically tested with one kilohm (1.0 kW) quarter watt (1/4 w) carbon film resistors. Said resistors simulate the characteristics of the bridge wire in a typical detonator (blasting cap). Connect the one kW quarter watt carbon film resistor to the appropriate shooting panel jack with a short jumper cable, and follow the prescribed procedure to "shoot" the resistor. There will be a flash of fire and a bit of black smoke so do not lay the resistor anyplace cosmetic damage or fire might result.
Here is a schematic of a Simple AC Shooting Box.
If you have any doubt about the safety of your shooting panel, take it out of service immediately, and have it checked by a competent service facility.
See also Shooting CCL Surface Amplifiers and Shooting Continuity Meters.
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