does a quick search

It sounds like the high-pressure transmission lines, in the US, need to be about three feet (1 meter) down. Dunno if Russia builds to the same standard.

I don't know how large of a warhead you need

a shaped charge might differ

and how accurate you'd need to be to penetrate one. Russia may not even have its transmission pipeline network buried, for all I know.

But because they span huge areas and can be struck at any point, I'd expect them to be pretty difficult to defend. Even if you ran defenses all along them, you necessarily are thinning your defenses. Do that, and said defenses can't handle a concentrated attack at any one point.

The problem of being the defender is that the attacker has the initiative and gets to decide where to concentrate for an attack.

For should the enemy strengthen his van, he will weaken his rear; should he strengthen his rear, he will weaken his van; should he strengthen his left, he will weaken his right; should he strengthen his right, he will weaken his left. If he sends reinforcements everywhere, he will everywhere be weak.

Sun Tzu, The Art of War

It doesn't make much sense to hit the pipelines if the refining infrastructure is broken. In that case they don't have anything to send through the pipelines anymore anyway.

Hitting pipelines is more of a short-term measure to cause pain. They're relatively easy to repair (much easier than advanced refining equipment), and there's often some redundancy so that gas can be shipped through a different part of the network if one pipeline is breached.

On the other hand, the pipelines span such a huge area that they're basically impossible to defend. If you're able to constantly hit pipelines all over the place, you're causing significant strain on enemy logistics. However, as mentioned by another comment, the pipelines are often buried, which makes them practically immune to light drones.

All in all, I think Ukraines strategy of targeting refineries directly is probably better in the long term, even though they're more well defended. It also has the advantage of forcing russia to try to defend the refineries. Even russia understands that defending the pipeline network is basically impossible, so they would rather just take the hits and repair the pipelines, so you wouldn't get the secondary benefit of straining their air defences.