Hi Hitoshi,
I am a bit reluctant to call them stringers, as those are generally laid out on the inside of the webframes and fastened to them. This does not appaer to be the case here. Also they would have to be continuous from fore to aft to fulfil the primary task of a stringer, i.e. enhance fore/aft rigidity and prevent flexing. In old merchant/fishing type vessel construction I have seen a sizeable doubler welded and riveted the entire length ovf the vessel to the top hull plates P&S just below the main deck line. This acts just like a deep I-beam.
Another application of the word 'doubler' would be a plate welded atop another one to cover a hole (like a tire patch).
In the days of wooden sailing vsls these would be large long boards inside near the top of the frames and are called 'clamps'. On top of the clamp another long board would be installed at right angles carefully following the curvature of the hull, called the 'shelf'. On top of that the deck beams would be laid out.
Rgds, RB