Using Month and year only to get all data

[hspc]

The 3rd paramter to CONVERT is used only when you convert to a character type, so it won't work in your case.
Instead, you should use DATEDIFF function like this:

SELECT vessel, voyage
FROM dneirout 
WHERE DATEDIFF(month,'20080901',dneirout.CONTAINER_OUT_DATE)>=0

Check the DATEDIFF function specs in BOL for more details.