Offshore Operational Availability of Onboard Direct Injection of CO2 into Sub-seabed Geological Formations

Abstract

Offshore operational availability was investigated for the proposed handling system consisting of the carrier vessel equipped with dynamic positioning capability, on-board manual operation of picking-up the sea-surface float connected to the upstream end of the flexible riser pipe laid-dawn on sea floor.An offshore site was selected to collect the oceanographic off the southern part of Japan facing to the Pacific Ocean. The response amplitude operator of the proposed CO2 carrier vessel was calculated based on 3-D Singularity Distribution Method. Based on the result of the interview survey with captains of research vessels which showed that for offshore operations a vessel's pitch motion and service deck wetness are dominant factors which define a vessel's operational availability, the RAO of the pitch and relative wave elevation was evaluated.The hull motions and the relative wave elevation in unidirectional irregular waves and in multidirectional irregular waves were calculated by a conventional method based on the seakeeping theory using the RAOs, the wave spectrum and the directional wave spectrum.Under the assumption that the operational availability of the proposed CO2 carrier vessel is constrained by the pitch motion and the service deck wetness, the allowable upper limit values of the significant wave height were calculated by a conventional method based on the sea keeping theorem using the 1/10 maximum expected response value of pitch motion and the relative wave elevation evaluated in the above-mentioned calculation. The threshold value of operation availability was set at the pitch of 3.0 degrees and the relative wave elevation at the freeboard to the service deck, based on the result of the interview survey with captains of research vessels.Finally, the overall operational availability was estimated, on the assumption that the threshold value of operation availability was set at the pitch of 3.0 degrees and the relative wave elevation at the freeboard to the service deck.We concluded that the operational availability of the proposed CO2 carrier vessel reaches a target value of nearly 90% for both full year and four seasons in the case that freeboard of service deck for offshore buoy picking-up operations set greater than 2.3 m as a service platform.