Why are gas turbines used to power some of the loads (gas compressors) on an FPSO? Why not increase the existing gas turbine generators' capacity and the gas compressors could have been powered by electric motors and VSD.

Is there any limit in terms of MW for the total loading on FPSO beyond which turbine to be used as driver?
(or)
Is there a limit to the amount of individual load, say, above this much MW, that load shall be driven by a turbine?

 

Selk, So many questions.

First of all it's pretty likely that many people on this forum (myself included) aren't exactly sure what a FPSO is. I did a quick search on the World Wide Web, and one of the meanings of FPSO (in the oil & gas industry) is Floating Production Storage & Offloading, so it's like a big "barge" loaded with many pumps and processing equipment for crude (and maybe some natural gas.?.?.?) as well as storage for crude and processed fluids/gases.

Anyway, the most likely reason that gas turbines are widely used as prime movers for compressors and large pumps is that based on the application they are the most cost-effective devices. They can produce high torque with a small footprint (space), some can be started and loaded very quickly, and some can burn crude as well as some processed/refined product(s). I'm sure that electric motors or even reciprocating engines can do many of the same things, but for some reason--again, probably because they are cost-effective and economical for the application, moreso than other types of prime movers.

Also, many of these FPSOs used large amounts of heat for processing, and the gas turbine exhaust is a great source of heat. If electric motors were used to drive the compressors and pumps then the heat would have to come from another source, and, yes, the gas turbine-generator set(s) could be a source, but maybe not enough on its own to meet the requirements of the facility.

That's my best, scientific guess. If there was a more cost-effective method I'm sure it would be more common.

Also, if an electric motor develops an "internal problem" it most likely would have to be taken off the FPSO and sent to a shore-side facility for repair (though some companies are now winding generators in place without shipping them to a service shop for re-winding, so I don't see why it couldn't be done with motors, too). Gas turbines, and I'm speaking primarily of heavy duty gas turbines (not aero-derivative gas turbines), can be maintained and repaired in place (though it's probably not the most "comfortable" place to work as many of these kinds of facilities are cramped and space is limited).

If aero-derivative gas turbines were used, they can be more easily removed and replaced with a "spare" engine while the other is sent to a shore-side facility for maintenance and/or repair.

As a former poster to Control.com said several times, "Engineering is a series of compromises." And economics is a VERY large driver of engineering compromises.