Generally, bunkers have a flashpoint of greater than 60 degree Celsius. It's quite comprehensible that in no way the ambient temperature of the bunker tanks are gonna go above 60 degree Celsius. If the bunker tanks cannot reach 60 degree Celsius, it cannot produce cargo vapours that are rich in hydrocarbons; thus no explosion can occur. And thus, inert gas is not being used in bunker tanks. Hope this has been helpful !

Function of Inert Gas is to prevent Explosion in side the oil tanks by reducing oxygen level in side the tank.

We know from fire triagle to start a fire or an explosion we need

Fuel - Cargo Vapour

Source of fire- ( Heat, Static electricty, spark)

Oxygen - from atmosphere

Since Flash Point of Bunker Oil is Above 60 Deg  cel , formation of cargo vapour  sufficient enough to support combustion is not possible .

Then you may be think why IG is introduced inside the cargo tank. Your Doubt and question is Genuine. The reason is Static electricity Discharge.

During loading and tank washing lot of static electricty will be generated , this energy will be accumulated and at some point it will discharge by creating spark  in side the tank atmosphere. this spark is good enough to start fire in oil. With enough supply of oxgen uncontrollable fire will be formed and leads to explosion.

To avoid this situation we need to remove oxygen from the fire triangle by introducing Inert gas into the cargo tank.