Summary details
| Industry | Oil and Gas – Upstream
|
| Asset Type | 6.6kV Electric Generator driven by gas turbine engine |
| Issue | Deeper diagnosis to avoid un-necessary maintenance following high vibration reading |
| Equipment Type | Gas Turbine driven salient pole rotor 4 pole 34MVA 50Hz generator |
| Benefit | Justification / decision to proceed with major overhaul of generator including complete rotor rewind; PLUS decision NOT to proceed with similar overhaul on the other generator. Saving from avoidance of work AND avoidance of exposure to risk of operating for 12 months without backup |
Equipment Role and Criticality
These two generators are used as the main supply of power to a complete operating platform. They are both approaching 30 years old, and there were concerns as to their future length of life. Some strange phenomena had been observed, with high vibration under some situations. The platform can run with only one generator, but it can place limitations on some operations.
Situation leading to this investigation
Concerns had been raised about whether some form of major overhaul was required to the generator, and if so what would be appropriate. The manufacturer had been called in and tests had been done on the stator windings, and some minor corrective work carried out. Possible suggestions of appropriate work included complete replacement of the stator windings, which could possibly be done in situ, or purchase of a complete new rotor and in-situ replacement. Each option would have different implications for the cost of the work, the quality of repair possible, and the impact on ongoing operations on the platform.
Faraday Predictive were asked to carry out tests, with a particular area of interest being the condition of the rotor, including the possibility that there could be insulation faults in the rotor windings.
The tests done by Faraday Predictive showed there was indeed a signal indicating rotor winding problems – the signals suggested that the strength of magnetic field created by the four poles were not all the same. Given the rotor winding wiring puts the same current through the windings on each pole, the only way a difference could occur is if there were turn-to-turn faults in the rotor windings.
The decision was taken to remove the generator from the platform and send it to a specialist overhaul shop, to be completely stripped down and rebuilt, at a cost of around half a million pounds. This revealed that there was indeed a problem of turn to turn faults in the rotor, which could only be tested by this complete strip down, enabling readings of rotor resistance to be measured separately for each of the four poles. The rotor structure, with four salient poles, can clearly be seen in the photograph below:
There was also corrosion of the copper of the windings visibly breaking through the insulation as can be seen here.
Once the generator had been completely repaired and re-installed on the platform, Faraday Predictive were asked to take a further set of tests. These confirmed clearly that the fault seen before the strip down had gone away, simultaneously confirming that the diagnostic interpretation of these signals had been correct, and that the repair had been carried out satisfactorily.
In addition, tests were carried out on the second generator. Given it is of the same age as the first one, it was suspected that it might be suffering from similar problems, and serious consideration was being given to removing it for a similar major overhaul.
The Faraday Predictive P100 system showed that there were no indications of the same problem with this second generator, which meant that the operators could avoid the cost and disruption that would have been associated with the overhaul.
In addition to the cash saving of £500,000 from not doing the overhaul, leaving the second generator in operation on the platform represents a significant reduction in the risk that operating with no spare generator would represent. Given the downtime costs of the platform are approach $1m per day, this factor is even more significant than the direct maintenance cost saving.
What to do next
If you are interested in applying Faraday Predictive condition monitoring systems to your plant, give us a call on 0333 772 0748, or email us on info@faradaypredictive.com.
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