A method for assembly of a riser string onboard a vessel for performing subsea wellbore related operations involving said riser string, the riser string extending between a subsea wellbore and the vessel in a wellbore related operation, wherein the vessel includes a hull, a riser storage storing therein, in horizontal orientation, multiple pre-assembled riser stands and/or multiple individual riser stand, a deck structure having an elongated transfer opening above said riser storage, a transfer elevator, wherein multiple pre-assembled riser stands, each being pre-assembled from multiple riser sections connected end-to-end and each having a length of at least 150 ft. (45.72 m), and/or multiple individual riser sections each having a length of at least 150 ft. (45.72 m), are stored, in said horizontal orientation, in the riser storage.