What are you proposing to do here ... utility power generation, or emergency backup, or something else? It makes a difference, and you haven't told us.
It will be a wide grey area, not a fine sharp line, between the two.
A 1 MW = 1000 kW = 1300 hp piston engine is pretty big as piston engines go, it's pretty small as turbines go. An order of magnitude more is an impractical piston engine. An order of magnitude smaller gas turbine is impractical to get good efficiency out of.
If you mean an electric drive to something like a pump or compressor then up to about 5-7 MW is reasonable then after that the power of a turbine comes into play.
Define the question a bit more please.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
I'm looking for a cross over point in terms of MW between the use of Gas turbines over electric drive. From whatever I have read, it seems more of comparison of parameters such as reliability MTBF, Efficiency, CAPEX/OPEX, Emissions calculation, Foot print & weight etc. which may decide to go for turbine or electrical drive
You also need to think about where the power is coming from to drive your electric motors and how you manage to start them.
ON a FPSO there are many competing factors which don't apply to an onshore plant.
Also if you're planning on using the gas from the production facilities as fuel, many engines only like really nice sales quality gas, not the crappy stuff you get off the separators. Turbines on the other hand can be made relatively easily to use it.
Equally you won't find too many turbines below about 3 to 5 MW.
At that size you also don't get many closed cycle units so their efficiency sucks.
On an FPSO you don't normally have massive individual driver loads so usually you either get a power feed from shore or include a couple of frame 5's to generate your power and just run it all as some sort of decent voltage unless for some reason you have a huge single driven load. IME and IMHO.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
In the old world steam turbines were made quite small for things like boiler feed pumps, or electrical generation on steam engines.
But here it appears you are talking about gas turbines, and not steam, or water turbines.
If that had been stated, it would have helped.
When I was involved in offshore power (GEC Gas Turbines) There often a 'fight' to determine the power driver.
Offshore plants typically have no power feed from the shore grid, due to distance and expense. Also space and fault level / voltage of generation must be considered. E.g. 66kv transmission might make good sense until you try and find space for the transformers and switchgear!
So large mechanical rotating plant (pumps, compressors) are often gas turbine or reciprocating engine driven to reduce electrical load. Leaving power generation for smaller plant.
Then the operating and maintenance guys (and gals) get involved. They want commonality of drivers on mechanical and electrical plant for operational and maintenance ease.
Then the platform design guys tell them that they have made a nice decision, but there is no room on the platform for all these small sets.
One consideration is speed. In the MW+ power range, assume motors are limited to 3600 rpm. If you need higher speeds you'll need to use turbines or motors with expensive gearboxes.
