These guys engage in actual naval combat on the high seas. I don't think they're afraid of lawsuits. If I were a lawyer, I wouldn't go anywhere near either side of that battle anyway.
The vandalism by the local chapter this year has been hilariously misguided. They destroyed a GM crop experiment that was investigating the damage GM crops do, attacked a building they thought processed cage chickens but actually cut off the supply of free range eggs and destroyed fences that were protecting reintroduced native animals from predators in protest against culling kangaroos.
At the end of the day that's why I'm perfectly happy with Greenpeace/etc doing their battling from the web. They get the ego rub they desperately desire and no fingers are brought close to dangerous machinery.
In theory, yes, Shell could sue. But it'd generate more PR and more negative attention. Shell want it to blow over. Litagation will not make it blow over.
You can bet Shell's legal team is already going over this in detail, and their response is not going to involve a bunch of retweet whoring on twitter.