HP-UX Series 700 machines will boot single user as follows
from the boot disk prompt.  This basically just does an initial
program load of the hpux kernel in runmode single.

b pri ipl
hpux -is /hp-ux

Hardware security (via motherboard jumper) can be enabled.
If so enabled, a password will be required to boot single user.  If
you don't know that password, un-jumper the motherboard switch.