The readme says that you cannot invade a port where 6 or more points worth of ships are based - a PD counts as 3, CA = 2, and DD = 1, so the Russians have 6 at Petrograd, so you should not be able to invade it.
However my opponent did so on turn 1 of our game!! eeek

Testing hotseat I found that it seems to only count the ships that are _in_ port - ie not out patrolling - rather than the ones that are _based_ at the port.
Perhaps someone could test other ports for this to see if it is a game-wide problem - I wont't be able to do so for a few days.