System Plan of CO2 Marine Transport and Release in Deep Waters for Moving-ship Type of CO2 Ocean Storage

Global climate change due to the increasing concentrations of CO₂ in the atmosphere has become a great concern internationally. Ocean storage of the captured CO₂ from fossil-fuel burning is a possible option for mitigating the increase of CO₂ concentration in the atmosphere. That is, CO₂ would be separated from exhaust gas and collected at the large sources, and injected into the subterranean reservoirs or into the deep ocean to be isolated from the atmosphere for a sufficiently long time period. The moving-ship type of CO₂ ocean storage is a concept whereby captured and liquefied CO₂ is delivered by ship to a site and injected into the ocean depths by means of a pipe suspended beneath a ship as it slowly moves through the water. The released CO₂ will form plenty of droplets, and then they will rise gently due to the buoyancy, while reducing in the size to disappear by dissolution into the ambient seawater. Such vertical journey of CO₂ droplets as well as the horizontal movement of release point could distribute CO₂ to the planned region all over and control the initial pH changes. In case of bad weather conditions, CO₂ marine transport and operation on the sea should be adjourned although CO₂ would be captured at the plant every day. It is, therefore, required that the system would have the buffer storage at the port and the extra shipping ability to recover the delay of schedule. Since the large scale of such spare capability might lead to the increase in cost, it is needed to investigate how to plan the system allowed for weather conditions reasonably. In the previous studies of cost estimation for CO₂ marine transport and ocean storage, the rate of operation in total system was assumed as a constant value, and the necessary capacity of storage at the port was assumed as some specified days´ amount of captured CO₂ at the - plant. In this study, the time series model of sea state through one year is generated for the hypothetical ocean storage site off Japan, based on the wind data observed with satellite remote sensing, and then the simulations of CO₂ marine transport and operation on the sea are carried out considering the operational limit of sea state day by day. In this approach, the continuing bad weather days or the frequent occurrences of rough sea condition during the specific season are counted. In order to pursue higher efficiency of the operation on the sea, three kinds of system are applied for the simulations as; 1) CO₂ carrier and release ship are used separately and moored in parallel for CO₂ shift at the storage site, 2) CO₂ carrier and release ship are used separately and moored tandem for CO₂ shift, 3) ship is used as both a CO₂ carrier and release ship. The third one would not need mooring and CO₂ shift operations on the sea which are sensitive to the sea state, but would need long- and every-time operation of pipe handling. Finally, the results are assessed as the capital and running cost per CO₂- tonne to be treated.