Late proposals by Brazil and a number of developing nations for optional extensions to this deadline for certain ships, needed to keep their economies on track, were also included in the legislation. Marpol tankers or the smaller, category 3 ships which have either double bottoms or sides running the entire length of their cargo tanks will be able to trade up to their 25th year if they are flagged in a state opting to allow such extensions. Similarly, other Marpol or category 3 tankers will be able to trade up to 2017 under extension flags. However, port states may decide not to allow such extension vessels into their ports or offshore installations. In either case, flag and port states will tell IMO and the acceptance or rejection will apply across the board.
Port states can refuse to accept ships that have been extended.
There is a provision for flag states to extend and for port states to accept. They have to take it to IMO, of course. It's not a case of a ship arriving in port and them stringing a wire across and saying 'you can't come in'. The EU states have already said that they will not allow extension vessels to call at their ports. There is, though, nothing to stop such ships passing through national waters.