Then, of course, there are the main defendants: BP, Transocean (which owned the Deepwater Horizon rig) and Halliburton (the US company that cemented the Macondo oil well). The defendants add another layer of complexity because they’ve all sued each other, seeking either to deny any of the blame for the spill or, in the case of BP, share it. That’s another judgment for Judge Barbier to make. It’s what happens when a mass of lawsuits are coralled together in a process known in the US as multi-district litigation.