A large diameter subsea gravity separator for ultra deep water conditions of 2000 to more than 4000 m includes a vessel having a shell which defines a cylindrical tubular portion and closure portions at axially both ends of the tubular portion. The shell is a sandwich shell which is dimensioned to provide for a predetermined collapse resistance of at least MPa and includes an inner steel layer, an outer steel layer and a concrete layer completely filling the space between the inner steel layer and the outer steel layer.