Barrel

Barrels could actually only be used to store and transport 25 units of Crude oil as solid as an alternate to pipes when Fluid handling is researched.

Empty barrel

The Empty barrel could be crafted and while Crude oil barrels are emptied the Assembling machine will also return an Empty barrel.

Recipe:	File:Steel-bar.png 1 + 1 => File:Barrel-empty.png 1
Req. technology:	File:Fluid-handling.png Fluid handling

Process	Input	Machine	Output
File:Empty-crude-oil-barrel.png Empty crude oil barrel	1 1	File:Assembling-machine-2.png or File:Assembling-machine-3.png	File:Barrel-empty.png 1 + File:Crude-oil.png 25

Crude oil barrels couldn't be crafted, they get filled in the Fill crude oil barrel process while using Empty barrels.

Process	Input	Machine	Output
File:Fill-crude-oil-barrel.png Fill crude oil barrel	File:Barrel-empty.png 1 File:Crude-oil.png 25 1	File:Assembling-machine-2.png or File:Assembling-machine-3.png	1

Barrels could get very handy in situations where pipes are no or a bad option, for example oil production in far distance outposts. With Barrels Crude oil could be transported with Transport belts, with Logistic robots, by the Railway network or in the avatars inventory.

A simple example with Transport belts: