Destroy the Oil rig (iPad version)

Pirates deployed a giant oil-rig to your country’s water. To make the matter worse, they equiped hundreds of battleships to snatch all natural resources.
They are very vicious and cruel that they plunder and threaten every vessels passing this area.
Your mission is to destroy all the pirates’ battleships and rigs.
Attention: only when there are not any protecting battleships can the rigs be completely destroyed. Therefore, you need to destroy battleships firstly.
Rule: You get 1 point for each ship destroyed. After destroying the rig, you’ll come to the next challenge.
Wish you always a winner!