That is factually incorrect. They don't double, the defense rating increases by 2500 points for every special port blown. If you blow dock, all of the others go up by 2500, and dock will go up too when it rebuilds. There is one uniform rating that effects all special (class 0 and 9) ports and it is incremented by 2500 every time.
The actual damage that the ports can do to you and can withstand from you is randomized, but it tends to increase with the defense rating. Still it is possible to blow dock, get a bump on the defense rating, but still have stardock not attack any higher the next time given the randomness.
Here's some data I collected a while back. Defense rating, number of waves it took to kill dock, and the blasts it shot back for.
Quote:
2500 1 wave No blasts
5000 2 waves Blasts: 15970
7500 3 waves Blasts: 28965, 4649
10000 3 waves Blasts: 42033, 22158
12500 4 waves Blasts: 75533, 43363, 21131
15000 5 waves Blasts: 125054, 43298, 30528, 11615
17500 6 waves Blasts: 181169, 54167, 53423, 26096, 12243
20000 7 waves Blasts: 244978, 91911, 54473, 58286, 90809, 32373
22500 7 waves Blasts: 155248, 141245, 68369, 57032, 30985, 39287
25000 8 waves Blasts: 163857, 240527, 177985, 156587, 105470, 45298, 20005
It would be nice if the current dock rating could be edited and transferred via TWA. Would save sysops a lot of hassle.