Stopping the Oil Spill?

I work in the pipefitting/welding industry. I live in La. where this oil spill drama is unfolding. I've never worked offshore, but I have lots of experience in the petro/chemical plants. The pipe that is leaking doesn't appear to be very large in diameter. I can't understand why BP doesn't fabricate a large diameter pipe (36 inches or larger) and completely drop it over the top of this leaking pipe, from the ocean floor, to the water surface. The weight of the pipe will cause it to sink in the mud/sand, stopping oil from leaking from the bottom. The pressure from the leaking oil well will push up the pipe, to the surface where it can be recovered. It wouldn't take very long to fabricate this, if the flux core or submerged arc welding processes were used. Is this an oversimplified solution that wouldn't work in the real world?