Method for separating oil well substances by means of a system, including a slug suppression and gas removal unit suitable to be arranged to a well, the slug suppression and gas removal unit designed to suppress slug flow and remove free gas, and an inclined separator system connected to fluid outlet of the slug suppression and gas removal unit by means of a horizontal large diameter pipeline, wherein the inclined separator system includes at least two inclined tubular oil and water separators (11a-d) formed by an elongated outer tube and in the elongated outer tube arranged an elongated inner tube, and where oil well substances are introduced into an end of the inner tube via a feed section passing through the outer tube and into the inner tube, and where the inner tube comprises multiple holes or slots arranged in a longitudinal direction, wherein the method comprises avoiding free gas carry over/under between the slug suppression removal unit and the inclined tubular oil and water separators (11a-d) by providing a liquid lock, characterized by providing and maintaining the liquid lock by arranging liquid level interaction between a slope part of the slug suppression removal unit and at top of the respective inclined tubular oil and water separator (11a-d) close to equal height, and establishing and maintaining a water-wetted entrance to the respective inclined tubular separator (11a-d) by using sloped soft bends (31a-d) at respective inlets (12a-d) of the respective inclined tubular oil and water separator (11a-d) having a curvature in both vertical and horizontal direction and exhibiting an angle corresponding to angle of the inner tube and arranging an interface between oil and water in the respective inclined tubular oil and water separator (11a-d) at approximately 50-70 % of overall length, referred to inlet end of the respective inclined tubular oil and water separator (11a-d), as well arranging the location of the first hole or slots, referred to inlet end of the inclined tubular oil and water separator (11a-d), at least at the water side, starting approximately 1/3 of the overall length of the inclined tubular oil and water separator (11a-d). providing and maintaining the liquid lock by arranging liquid level interaction between a slope part of the slug suppression removal unit and at top of the respective inclined tubular oil and water separator (11a-d) close to equal height, and establishing and maintaining a water-wetted entrance to the respective inclined tubular separator (11a-d) by using sloped soft bends (31a-d) at respective inlets (12a-d) of the respective inclined tubular oil and water separator (11a-d) having a curvature in both vertical and horizontal direction and exhibiting an angle corresponding to angle of the inner tube and arranging an interface between oil and water in the respective inclined tubular oil and water separator (11a-d) at approximately 50-70 % of overall length, referred to inlet end of the respective inclined tubular oil and water separator (11a-d), as well arranging the location of the first hole or slots, referred to inlet end of the inclined tubular oil and water separator (11a-d), at least at the water side, starting approximately 1/3 of the overall length of the inclined tubular oil and water separator (11a-d).