Like the poster said about the prenominal ordering... if adjective are of the SAME priority, use commas. But let's say you were SPECIFICALLY on a search for abandoned turtle shells. Let's say you were on an important quest JUST for these abandoned turtle shells... and you didn't care about any occupied turtle shells. In fact, the whole point of your story would then revolve around.
A huge abandoned turtle shell...
A tiny abandoned turtle shell...
A stinky abandoned turtle shell...
Even though the "abandoned" part is an adjective, it is SO important that it becomes a part of the indivisible phrase "abandoned turtle shell" which becomes it's own special entity. In this case, even though "abandoned" becomes a VERY important quality of the turtle shells, it becomes a less important adjective (because, in a sense, it's just a regular part of our turtle shell now)... so the most important adjectives are the modifiers "huge" and "tiny" and "stinky" which now becomes more important because they differentiate our abandoned turtle shells.