Recently, there has been an increase in the demand for natural gas as a source of clean energy, which has increased the demand for LNG carriers. However, LNG carriers require a capital investment to obtain equipment for the regasification process, which prevents fires and explosions. Thus, on account of NIMBY, a CNG carrier is suggested that eliminates the need for regasification equipment. Meanwhile, carbon dioxide emissions are more and more regulated by international conventions such as the Kyoto Protocol. Because of this, $CO_2$ carriers have also received international attention as a methodology to transport and store $CO_2$ cargoes. Several vessels or tanks to transport and store $CO_2$ gas have been studied in various countries. This paper proposes a conceptual design for a 20ft container shaped tank to effectively transport small cargoes of $CO_2$ and CNG. The proposed pressure vessel or tank will be carried by a conventional containership or special cargo ship. The influences of the design parameters for proposed pressure vessel or tank. Including the materials, scantlings, and shape of the pressure vessel, are studied theoretically and computationally.