For an offshore floating operation, you get to the location, drill a 30-inch diameter hole, and run surface casing, so all of the tools are big and bulky at that stage. Then you put your BOP on and you start drilling smaller, consecutive-sized holes. You go from 30 inches at 200 feet, and you can drill down gradually with a smaller hole size, down to seven or eight and a half, then to a five-inch hole, anywhere up to 35,000 feet. You case every section of that hole off so that you start with 30 inches, then you go to 20, 13, 12, nine and five-eighths, seven inches, all the way to the bottom, and then five inches as a liner goes all the way there. You cement those casing strings all the way along the sides and then perforate holes in them to get into the formation, and that's how your oil and gas come in and flow up.
Water weighs 8.3 pounds a gallon. Formation pressures can be up to 16 pounds per gallon coming the other way. While you're drilling that hole and setting those pipes to create a pressure integrity conduit, you use a mixture—barite and other chemicals—so that you take water at 8.3 pounds a gallon and make it 18 pounds a gallon, and you pump that into the well as you drill the hole. Then that 16-pound-per-gallon force that oil and gas want to come in with, you're holding that down with the mud at 18 pounds a gallon. That's barrier number one.
Barrier number two, as you're drilling, is the BOP. As long as you keep that 18-pound-per-gallon mud in the well, it's going nowhere. It stays right where it is. To make the oil flow at the end, you put in all these special valves; then you reduce the weight of the fluid column under a controlled condition so that you let the formation fluid come in slowly, and then you send it for the production train. That's the gist of it.
Every time you run a casing string, you're holding it back with that mud weight, but when you get it done, you cement the outside and then you have a big plug in that thing until you drill through it again. Every time you drill through that cement plug, you have a different set of rams in that BOP that close on the different sizes of pipe that you used to make that hole.
The BOP has four sets of rams that are set for different sizes, but it also has an annular, which can close on any size, from 18 and three-quarters down to three and a half inches. They're another redundancy, but they're typically there for lower pressure holdbacks, whereas the big rams and the shears are the stuff that keep you from the 10,000- or 15,000-pound pressure.
So those are your barriers: the mud weight, the cement, the casing, and ultimately the BOPs.
The third barrier concept...I assume you're talking about the relief well.