The ultra sound or Xray that BP did has found at least 2 rams partially closed. Theory on another board/blog and mentioned here is that there was no lockdown ring on the 9-7/8" hanger / seal assembly. They state that 9000 ? psi arrived under the seal assembly it collapsed the casing at surface allowing one or 2 joints to come loose at a connection under the tension of the whole casing string and slingshot these into the BOP. Sounds plausible to me. No shear rams could cut through 2 strings of pipe , and it would be difficult to plug with a junk shot also. It seems the recent BP powerpoint also leaves the door open to no lockdown ring being installed, they have 2 pages there. One showing a wellhead schematic with a lockdown ring and one without.