A seal assembly, method for sealing a seal assembly and a modular seal unit for a rotating control device for use in an offshore environment. The modular seal unit includes a first outer housing, a first seal housing lockable within the first outer housing, and a first sealing element disposed on a lower end of the first seal housing. The first sealing element includes a throughbore configured to receive a drill pipe, and a sealing surface configured to seal against the drill pipe. The modular seal unit also includes a first connector configured to couple the first seal housing to the first outer housing, and a second connector configured to couple the first seal housing to one selected from a second outer housing of a second modular seal unit or a running tool adapter.