It is possible to make this work
I would suggest it being like this:
Name:Deep Freeze
Cost:10 turns
Rarity:Rare
Effect:
Target fleet gains +1 danger per hour
If the targeted fleet is in motion, then its travel time is reduced by 25%.When it reaches port, then it can no longer travel.
As long as target fleet remains in port, its danger drop from attacks is halved.
only the player that was last attacked by this fleet can cast this card on the chosen fleet.
The player that this card targets can only be attacked by the caster of this card.