My experience with gas pipelines is on the gathering end, 3, 4, 6 and 8 inch mostly. Slugs of water, or condensates are common on those, but the facilities are built to handle them as well and they are generally no big deal. I would have assumed the larger transmission lines would have some kind of equipment to deal with the infrequent slugs that must occur, but I suppose the pressures and velocities are higher too.