SCR Vs. VFD

Actually the terms are too general to be able to draw any conclusions like those that you requested. It is a little like comparing apples to oranges and making conclusions about the health benefits of one verses the other. (Assuming that was the only fruit to which you had access.)

The other half of the system which you failed to mention is the motor that is being driven by one of the two systems that you did mention. Since there are so many types of motors on the market, it would be a fallacy to consider that all Oil Rigs use the same exact motor. Even if they did, you would have to take that into consideration to approach an answer such as reliability and efficiency. So, in conclusion, the motor is part of the "system".

Lastly, this blog works best for specific questions. If you are trying to get a "feel" for a couple of types of systems, perhaps you would get better information from someone who repairs, rebuilds, or rewinds motors. Otherwise it usually boils down to which one is the least expensive to buy or run. Even that question depends on your outlook as to which cost (initial purchase or annual operating cost) is most important.

You are talking about two different types of motor speed control. When you say SCR, your talking D.C. motor drives. When you talk about VFD's your talking about A.C. motor drives in which the reason for both is speed control.

That being said, almost all electric propulsion and motor speed control have evolved into VFD's. More reliable, less expensive. Downside - Harmonics.