Since 80% of Iran’s export commodities is from petroleum they have a very shallow economy.  This means that any threat to oil exports could drastically hurt their economy and this is why sanctions against Iran have been aimed at its oil industry.  China is the biggest importer of Iranian oil and it was thought that even though these sanction would hurt the Iranian economy China would be able to increase their import of Iranian oil to keep that industry a float.  However, with the repeal of the Chinese insurers from oil tankers this trade partnership could be threatened.  While this may not affect China very much it could be disastrous for Iran.  Also, there are still several insures out there that can cover Chinese tankers but those numbers are dwindling.