As more and more producers turn from induction motors to more efficient permanent magnet motors (PMMs), they’re saving thousands of dollars in operating costs. But safety procedures are different for PMMs than for induction motors. Is your PMM provider offering you the best safety program?
Every electric motor can also be a generator if its shaft is turned by external forces, such as downhole pressure on an ESP. For induction motors, the current generated is insignificant, requiring no additional safety procedures.
With the rise of PMMs such as those used in Extract’s extraordinarily efficient and ESG-friendly SpeedFreq ESPs, things are different. If not secured, PMMs can generate enough power to cause serious injury—anything over50 volts is considered potentially lethal.
Take, for example, a PMM rated at 120 volts at 1,000 RPM.If something causes that motor to spin at 2,000 RPM, it could generate 240volts and current depending on the motor.
How It Can Happen
So, what causes a pump to rotate on its own? It’s really only possible when the pump is off, which happens when the pump is being installed or pulled.
If the well kicks during either situation, the back pressure in the kick can cause the pump to turn in an unplanned manner. While this is unlikely, it only has to happen once.
A loss of well control would be more likely to cause this, although even then, the pressure will take the path of least resistance, which would likely be through the casing. But the potential is still there, so we must take appropriate preventive measures.
Some ESP companies installing both induction motors andPMMs use the same crews for both. This switching back and forth has the potential to allow crews to forget to implement those extra safety procedures when they’re installing PMMs. At Extract, we take the extra precaution of specializing our teams. That means the PMM group installs only PMMs, so they can do the same safety procedures every time they set foot on the well site.
Here’s the challenge: PMMs are also used a lot at industrial sites, where the motor is easy to reach and to lock the shaft whenever necessary. ESPs, on the other hand, are located around 10,000 feet downhole and are a little bit harder to access. We must get creative to make them safe.
How to Prevent It
To prevent a blowout or kick from creating a dangerous current, it requires some combination of three safety procedures to eliminate unwanted shaft turning, to protect site workers.
We’ll discuss each procedure individually, including pros and cons of each, to show how we put the safety of everyone—our team and every person and piece of equipment on the site—at the top of our priority list.
We’ll start with shunting. This involves tying all three phases of the cable together, creating an electrical or magnetic brake. It forces the poles of the magnets to “stick” together increasing the force need to rotate the motor.
Shunting does two things: First, it limits the ability of the shaft to turn in an unplanned case. Second, because current can’t flow without also having the presence of voltage, this connecting of the three phases prevents any possibility of voltage flow.
Within shunting procedures there are two options—grounded and ungrounded. We recommend the ungrounded option because if the shunt must be removed—which happens sometimes while making final connections—we can meter the wiring to determine if there is current flowing, telling us that something indeed is rotating downhole, and that we must solve that before continuing. If the shunt is grounded that’s where the current will go, and there will be no way to detect its presence—and that unknown can be lethal.
Usually if there is current present there’s something very recognizable happening with the well itself—gas is escaping or the well is out of control, so the source of the rotation would be easy to locate.
Shunting is one of three ways to mitigate unplanned motor rotation and is always applied as we require at least two of the three mitigation processes to be in place.
We always apply shunts before entering the designated hazardous area. The application process itself can create a spark that could ignite any escaping hydrocarbons, so we’re going to be safe during that process as well.
In the next post we’ll discuss a second option, which involves plugging the tubing to keep pressure from flowing through the ESP during installation.
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