Just conjecture on my part but this is how I see it shaking out.
First of all, Papelbon won't be in the bullpen, he'll be a starter, and a surprisingly good one at that.
As for Hansen, I highly doubt he starts in AAA. He very well may but I just don't see it. Depending on what happens with Timlin or if they sign another set-up man, Hansen could be the set-up guy. The closer's role is Foulke's---he's reportedly excited about coming back next season healthy with an eye on becoming a solid closer again.
Now here's where it gets interesting. When they get to the trade deadline, if Hansen is looking like the closer-in-waiting everyone deems him to be, they could be looking at a Lidge-Dotel scenario where, if they can get a nice package in return, they trade off Foulke and promote Hansen to closer. But again, that would only be if they can get a very attractive deal for Foulke AND feel Hansen is ready.
I would say the safest bet is, depending on if Timlin comes back or they sign another set-up guy, to look for Hansen to be the 7th inning guy and cut his teeth there or possibly even the set-up to Foulke.