Many are saying that the IP turbine will overspeed if disconnected from the compressor and I can see a transient overspeed in a very short interval after decoupling.
But once the IP compressor is no longer being driven, compression and resultant exhaust flow will be pretty much limited to what the HP can stuff into the cans and I would think that there would not be that much gas flow to drive the IP turbine, especially with the many stages of the IP compressor simply freewheeling.
So either:
- the IP overspeed happens near instantaneously on decoupling
- the HP gas flow is sufficient to overspeed the decoupled IP turbine
