bearfoil
I think the first place to start is to look at an item on the Trent 900 cross-section drawing and also the engine cutaway. Of particular interest, note that the roller bearing for the rear HP spool sits directly below the frame and plenum which I believe contains the stub pipe that would normally feed oil to this roller bearing and the one under the IPT rotor disc. Now this stub pipe, for one reason or another, had begun to fail by means of a fatigue crack initiation, even perhaps undetected on a previous engine cycle or flight that permitted oil to begin to seep out into the plenum cavity. But on this flight upon climb out, the crack progressed to the extent the oil flow into the cavity became great enough to cause abnormal operation of the HP spool rear roller bearing and the spool began vibrating. The roller bearing under the IP rotor disc seemed to be unaffected. However, enough oil had collected in the plenum chamber and given the temperature in this area, ignited. The disc, especially in the bore area, began to heat beyond its material property capability and in a ductile manner began to stretch. It got to the point where restraint (580 bolt holes) was overcome by stretch, and the power drive arm failed at the bolt holes (a disconnect). The N2% begins to fall. The ECU, sensing this calls for more fuel and the N3% begins to rise. N2% steadies momentarily, then drops like a rock. The ECU calls for more fuel flow and the N3% continues to rise. But then the HP compressor outlet pressure starts to drop and fuel flow is reduced by the ECU. With no air feeding the HP compressor, it stalls, BANG! The hot gasses built up in the combustor (580 psi @ 98% N3?) and compressor release both forward and aft (flames) but upon reaching the fan by-pass are blown back by the fan through the bypass and may not have been seen emerging out the front of the engine.
Very quickly then, the free IPT rotor (blades and disc) in an over-speeding mode because of the high N3% moves rearward into the Stage 1 LPT stator ring, contacting it close to the fractured power drive arm continuing to overspeed from the "false bearing effect and then bursts, BANG!
Some Key Points:
The stub pipe may not have completely fractured dumping a large quantity of oil into the plenum at once, more gradual but progressive over time. In fact it could of broken completely through as a result of the breakup in this area of the engine, we don't know the time line.
Over speed of the IP rotor was probably not detected because N2% is measured off the IP compressor.
The rotor overspeed was not slowed because the initial contact with the Stage 1 LPT nozzle was too close to engine centerline. For example, if you turn your bicycle upside down and turn the pedals, try stopping the wheel close to the hub verses stopping it at the rim.
The HP spool rear roller bearing upset was great enough to cause detectable vibration (skidding/uneven contact) but not great enough to cause seizure.
The fuel flow to the combustor was reduced when the P30 pressure collapsed, but may not have been cut off completely, just reduced.
The burning streaks on the fan bypass cowling is characteristic of a major failure in the turbine area, blades, nozzles and discs and the compressor stall that is created.
Essentially, almost everything happened before the flight deck crew had a chance to react other than to begin to shut the engine down after receiving warnings 2 seconds before the disc burst.
I would suspect the ECU was reprogrammed to call for fuel flow reduction at the first sign of N2% fall off and N3% rise which might have prevented the disc burst situation.
Those are my thoughts on this engine failure.