Similar symptoms on my 2011 MBP.
The shutdown would occur during the boot process as the progress bar is loading, about 50-60% of the way the machine would just shutdown, everytime.
I tried resetting the PRAM and SMC controller as per Apple's instructions. This didn't help directly.
I ended up going into internet recovery mode, and attempted to use the Disk Utility to repair/diagnose the main HD. Disk Utility couldn't mount it or perform First Aid.
So I opened up the terminal and ended up doing the following:
- Unlocking my encrypted drive.
- Mounting the drive with
mount_hfs and specifically ignoring journalling. If this step succeeds, I'd spend some time backing up important files to an external drive or USB stick, as this may be your only chance to do so if you've got an incurable issue with your HD.
diskutil disableJournal on the drive after it mounted.
diskutil enableJournal on the drive.
diskutil unmount the drive.
Step 1 is explained here (archive).
Step 2-5 are explained in a bit more detail here (archive).
My system then booted up correctly and seems to be working okay. I suppose the issue was a corrupt journal file that the system kept on failing on each time it booted, and by disabling/re-enabling the journal I effective "cleaned" it up. I can also confirm that steps 3 & 4 cleared out some large
*journal* files in the root of the problematic drive.