Marc Kelderman wrote a helpful article on this topic:

http://orasoa.blogspot.com/2007/03/delete-bpel-instances.html

And one of the users there also notes:

Note there is a patch to 10.1.3.3 to purge old data (6372741). Its a fairly dumb one that deletes everything older than a given date. Trying to do something more elaborate we have noticed that if you use the script in this article as a basis then rows in xml_document table are not removed.

We will add more to this starting point soon.