Vessel adapted to perform subsea wellbore related operations involving a riser string between the subsea wellbore and the vessel, e.g. drilling and/or wellbore intervention, said vessel comprising a hull having a deck, said vessel comprising: - a riser storage hold present within said hull below said deck, which riser storage hold comprises storage racks adapted to store therein parallel stacks of multiple riser sections and/or pre-assembled riser stands in horizontal orientation, which riser storage hold has a floor, side walls, and a roof, - an elongated riser transfer opening between said deck and said roof, said riser transfer opening extending in a direction parallel to said storage racks, said riser transfer opening having a length and a width so as to allow for transfer of a single riser section or a single riser stand in horizontal orientation via said riser transfer opening out of and into the riser storage hold, - a riser transfer station arranged within said riser storage hold below said riser transfer opening and provided with a transfer elevator that is adapted to raise and lower a single riser section or a single riser stand in horizontal orientation thereof so as to pass the riser section or a riser stand through the riser transfer opening, - a first crane arranged within the riser storage hold, said first crane being adapted to lift and lower a single riser section or a single riser stand (85,95) at least allowing for removal of a single riser section or a single riser stand riser stand from a storage rack (80,90) and for placing a single riser section or a single riser stand into a storage rack respectively, and said first crane being adapted for transverse transportation of a single riser section or a single riser stand at least between said transfer station and a position above a storage rack, wherein said first crane is a first overhead travelling beam crane which comprises: - a travelling beam extending in a direction parallel to said storage racks and supported at each end thereof on a crane rail perpendicular to said storage racks, - a winch trolley provided with one or more winches and displaceable along said travelling beam, - multiple riser grippers adapted to engage on a single riser section or a single riser stand at spaced gripping locations thereof, wherein, possibly, an elongated gripper frame is suspended by one or more winch driven cables from said winch trolley, said gripper frame being provided with said riser grippers, wherein the riser storage hold is provided with first length storage racks adapted to store therein single first length riser sections and/or pre-assembled riser stands each having a length of at least 100 ft. (30.48 m), e.g. of 120 ft. (36.57 m) or 150 ft. (45.72 m), and wherein the riser storage hold is provided with second length storage racks adapted to store therein single second length riser sections each having a length of between 50 ft. (15.24 meters) and 90 ft. (27.43 meters), e.g. of 75 ft. (22.86 meters), and wherein the first crane is adapted to transfer a second length riser section between each of said second length storage racks and the transfer station, and to transfer a first length riser section or riser stand between each of said first length storage racks and the transfer station, characterised in that the transfer elevator comprises a first elevator unit and a second elevator unit that are spaced apart in direction parallel to said storage racks, wherein each of said first and second elevator units are adapted to selectively operate stand-alone or in unison, wherein, for operating stand-alone, each elevator unit is adapted to raise and lower a second length riser section, and wherein, for operating in unison, said first and second elevator units are further adapted to raise and lower a first length riser section or single riser stand. - a riser storage hold present within said hull below said deck, - an elongated riser transfer opening between said deck and said roof, said riser transfer opening extending in a direction parallel to said storage racks, said riser transfer opening having a length and a width so as to allow for transfer of a single riser section or a single riser stand in horizontal orientation via said riser transfer opening out of and into the riser storage hold, - a riser transfer station arranged within said riser storage hold below said riser transfer opening and provided with a transfer elevator that is adapted to raise and lower a single riser section or a single riser stand in horizontal orientation thereof so as to pass the riser section or a riser stand through the riser transfer opening, - a first crane arranged within the riser storage hold, said first crane being adapted to lift and lower a single riser section or a single riser stand (85,95) at least allowing for removal of a single riser section or a single riser stand riser stand from a storage rack (80,90) and for placing a single riser section or a single riser stand into a storage rack respectively, and said first crane being adapted for transverse transportation of a single riser section or a single riser stand at least between said transfer station and a position above a storage rack, - a travelling beam extending in a direction parallel to said storage racks and supported at each end thereof on a crane rail perpendicular to said storage racks, - a winch trolley provided with one or more winches and displaceable along said travelling beam, - multiple riser grippers adapted to engage on a single riser section or a single riser stand at spaced gripping locations thereof,