04-04-2012 01:52 PM - edited 04-04-2012 01:54 PM
More experiences are being reported on the performance of dual fuel (diesel/ natural gas) generators for land-based drilling. Consistently, the diesel fuel displaced does not exceed an average of about 55%.
Does anyone have insight into the drilling conditions or generator characteristics that are establishing this limit. Obviously with the rising cost of diesel and falling cost of natural gas, additional cost savings with a higher use of gas in the dual fuel generator would be welcomed.
Do very low pilot fuel levels (5% to 15% diesel) favor more steady state loads? In other words, are the dynamic loads encountered with drilling operations the main reason higher percentages of gas cannot be reliably used?
04-07-2012 08:39 PM
The base engine and fuel system design probably have the most to do with maximum achievable substitution rates, at least in my experience.
With older engines, such as D399 and 3412's with older fuel systems, we could acheive substitution rates down to about 10% diesel, and they would do pretty good. Of course on those engines we modified them to Woodward electroninc governors and did some careful work integrating the fumigation system control with the diesel fuel governing.
Unit injected engines posed the biggest issues for higher substitution rates, mainly because lower than 25% diesel fuel injection rates at higher loads caused a number of fuel injector failures. All appeard the result of not enough cooling in the tip area and most failures root cause was due to high carbon deposits. The next biggest challenge, at leat in my experience was intergrating a fumigation system with a CAT ECM governing. On older 3500 B series engines didn't seem to be much of an issue as long as we kept about 30-40% diesel fuel rate. However as ECM files changed, mainly due to emissions, harder to get a good substitution rate without a bunch of alarms and possibly shutdowns.
Load factor and load cycles also play a part, again at least in my experience. Transient response is affected, but hard to actually calculate because so many factors can come into play, such as base governor dynamic rate, turbo match, actual load dynamics and fuel gas supply and control dynamics. Drilling is a bear, very short periods of high loads, longer durations of medium loads and longer periods of low loads, tough on SI engine engines and usually require load banks or something else to help spin them up and keep them from having low load problems like plug fouling. Dual fuel engines can do better, and be more tolerant to the low load issues, have better transient response, but the fact is most customers who use them get them on, get them working ok and go to work. I don't know of anyone who has actually spent the time required to really dial in a system for the best overall perfomance.
Too bad because you could record a typical load profile, and use a more capable load bank like a Chrestchic and program in that load profile, then use it to do your system tuning and development, but like I said, I don't think anyone has gone to that step.
The dealer I worked at did a number of dual fuel conversions until the local air board killed it, wasn't an SI engine so it was still a nasty dirty diesel, and the work and effort we spent got shelved or trashed. Now that it is becoming a good idea again in some areas I'm not sure anyone has really stepped up and done some really well thought out R&D work. Have heard a bunch of claims, but in talking to some of the current vendors no seems to have good technical answers.
Hope that helps, Mike L.