Complex sentence: Cargo tanks on gas carriers are not inerted, but the hold spcool using a small heel of cargo while the vessel is in ballast while retaining the explosion protection provided by the inert gas.
Split into simpler sentences: Cargo tanks on gas carriers are not inerted, but the hold space around them is. This arrangement allows the tanks to be kept cool using a small heel of cargo while the vessel is in ballast while retaining the explosion protection provided by the inert gas.