A method of unloading cargo at sea from a cargo vessel to a cargo barge, comprising the steps of providing a catamaran vessel having a pair of spaced apart catamaran hulls and a pair of spaced apart arch shaped frames, each frame attached to each hull at a frame lower end portion; positioning the cargo vessel in between the hulls and under the top portion of the frames; placing one or more cargo barges next to one or both of the hulls, wherein the hull is in between the cargo vessel and the cargo barge; and unloading cargo from the cargo vessel to the cargo barge wherein the cargo travels from the cargo vessel, over the catamaran hull to the cargo barge.