With the emerging necessity of carbon capture and storage (CCS), many companies are evaluating the possibilities of CCS implementation in their assets. Technical evaluation for converting existing fields to CCS projects includes various topics such as carbon dioxide (CO2) transportation and its economics among other topics. Selecting a method for CO2 transportation becomes important when the target site is distant from the CO2 source, particularly if located offshore. The Intergovernmental Panel on Climate Change (IPCC) special report on CCS has identified that a liquefied CO2 (LCO2) carrier would be the lowest-cost option for distances more than 1700 km. An LCO2 carrier can also be the best option when transporting CO2 abroad to benefit from the international carbon tax, which has been collecting global interest. Along with this increased interest in LCO2 carriers, shipbuilding and engineering companies are developing their ships.
When an LCO2 carrier is used for offshore CCS, the ship would be located right above the target site to minimize the length of pipelines. As this distance between the LCO2 carrier and the target reservoir is shorter than other transportation options, the traditional modeling approach uses a standalone model of the LCO2 carrier. This approach excludes pipeline models when estimating required operating conditions of the carrier assuming a fixed outlet boundary condition. However, this boundary condition may differ from the actual value. Furthermore, in real systems, operating conditions (i.e., pressure and temperature) are not constant over time. Ignoring the dynamic interaction with downstream pipelines may lead to subsequent differences in simulation results. The actual thermo-hydraulics behavior of LCO2 carrier cannot be reproduced when standalone models are introduced.
In this study, a standalone LCO2 carrier model and an integrated dynamic CCS model connecting the LCO2 carrier, injection equipment, riser, pipeline, and wellbore were developed. The standalone LCO2 carrier model predicts the behavior of a whole ship from two LCO2 tanks to the carrier's outlet, which would be connected to the riser of the CO2 injection system. The integrated model calculates the whole CO2 injection system from two LCO2 tanks to the target reservoir by linking the standalone LCO2 carrier model and a flow model starting from the riser to the injection wellbore. The simulation results showed that the required CO2 pump discharge pressure of the integrated model was 5 bar higher than the standalone model to meet the target flow rate. As the required discharge pressure increased, the average speed and power consumption of the CO2 pump increased by 2.5% and 7%, respectively. In this comparison study we demonstrated that the integrated model could accurately represent the overall system behavior. No risk of solid CO2 formation was identified during unloading of two LCO2 tanks.
By using the developed integrated model, three different case studies were conducted to analyze the effect of rigorous heat transfer in LCO2 tanks, simultaneous tank unloading, and initial startup operation on the thermal-hydraulic performance of the system, respectively. The first case demonstrated that modeling the tanks with high-thickness thermal insulation is close to an adiabatic condition. The required discharge pressure of the CO2 pump was the same, and the final pressure and temperature of the tank holdup increased by 1 bar and 2掳C, respectively. The second case showed that changing the operation from sequential to simultaneous unloading of the two LCO2 tanks removed the disturbances observed during the transition of tanks in the sequential case. This removes potential instabilities in the pump controller and avoids any impact on the injection system performance. The unloading time was only 20 seconds shorter, and the required pump discharge pressure was the same. The third case demonstrated that the integrated model could analyze the initial startup operation, which displaces nitrogen (N2) and methane (CH4) in the pipelines and wellbore with CO2, which standalone models cannot predict. It took 500 seconds to fully displace N2 and CH4 in the system with CO2. Furthermore, the required valve opening time (19 seconds after injection commences) to prevent backflow from the reservoir could be determined.
In conclusion, dynamically integrated modeling can help identify interactions that are not apparent in the traditional standalone modeling approach. The integrated model can evaluate system behavior and possible operational risks that cannot be observed in standalone models. Simulation results in this work demonstrated that the dynamically integrated CCS model captures more realistic behavior of the whole CO2 injection system to help optimize the design and operation of a CCS project. Developing a plan to address these interactions through the integrated dynamic simulation can result in a more stable operation.
