The present invention relates to a downhole production casing string for insertion in a borehole in a reservoir, the downhole production casing string having a first end nearest a top of the borehole and a second end furthest away from the top, the downhole production casing string extending along a longitudinal axis and comprising at least one opening which during production allows hydrocarbon-containing fluid from the reservoir into the downhole production casing string, a plurality of casing parts having end sections and a base section between the end sections, the base section having an outer diameter, and at least one annular projecting element having an outer face and at least one helical groove arranged in or on the outer face and having an overall outer diameter which is larger than the outer diameter of the base section. Furthermore, the invention relates to a downhole production casing string system for completing a well downhole and to a method of implementing a production casing string according to the invention.