OPERATIONS & MAINTENANCE
Tese motors, which can range from 460V up to 13,200V, are found in crude oil export pumps, circulating water (saltwater, utility, fresh) pumps, fire pumps, cement pumps, gas compressors and fans. Medium-voltage generators in the main power plant, as well as low- voltage standby generators that back up the main generators and power lifesaving equipment are also critical to the operation of the platforms. Unfortunately, this equipment is subjected to ice, moisture, changing temperatures and dense salt-fog. Te salt- fog is one of the worst problems because the salt, combined with the moisture, can cause severe damage to the electrical insulation inside the critical motors and generators.
“Te offshore environment is probably one of the harshest environments for electric motors and generators,” says Donna Lee Hodgson, senior electrical engineer for Shell International Exploration and Production. Hodgson currently designs deepwater facilities for projects in the Gulf of Mexico. “I have seen motors come into the shop that have salt encrusted on the interior of the stator windings,” says Hodgson. Sand is also prevalent on deepwater platforms given the constant sandblasting and re-coating of carbon steel structures. “Basically, the sand that you are blasting
GenGuard is a patented and tested protection system
at the carbon steel structures around the equipment tends to get into the motors and generators too,” explains Hodgson. Te sand breaks down the insulation coatings on the windings, which leads to
The MotorGuard tool detects insulation breakdown in motors and generators 18
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premature failures. Te environment is so corrosive that Hodgson typically specifies enclosed or weather-protected motors. In addition, most offshore oil operations engage in a time-based preventative maintenance (PM) programmes. As part of the programme, insulation resistance tests are typically scheduled on a semi-annual or annual basis. Typically, insulation resistance tests are also conducted at the start of annual overhauls or planned outages, to identify any motors that need repairs. According to Hodgson, critical motors on the deepwater platforms are tested to determine the equipment’s Polarisation Index (PI). Te PI is used to determine the fitness of a motor or generator and is derived by calculating the insulation resistance of the windings using a portable megohmmeter. Te test begins with a reading of insulation resistance recorded at one minute, then a second test reading is taken for 10 minutes. Te ratio of the two measurements provides the PI, which should be above 2.0. Still, despite these types of tests, motors and generators can become compromised within only a few days in the corrosive, dirty environment and
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