Is it viable to not running Norns?

Hi,

I´m getting back tot his army since a while.

I want to know your experiences with or without norns.

I want to play sub assault, but i dont know how "must do have" are the norns in this kind of list?

Also how many burrowers do we need? With only 2 ravenners and one trygon is enought? Also 2 normal Ravenners or 1 normals and 1 buffed ravenners?

Any thoughts?