The following post will help you with some derating factors you can use:
Cable Sheath and Armour Loss
A few years ago I came across a book [Underground power cables, by S.Y.King and N.A. Halfter]; which if I remember correctly may help you with calculating the current.
You can also Google for 'calculating sheath currents'. I just tired and very quickly came across some papers with various methods of calculating sheath currents.