Dehydration Unit Failure in CCS Hubs – Managing Risk through Detailed Analysis

Abstract Carbon Capture and Storage (CCS) hubs are considered a cost-effective means for industries to reduce carbon dioxide (CO2) emissions through access to shared infrastructure. However, impurities in the emitters’ CO2 streams alter the physical properties and behaviour of the fluid, introducing integrity risks. CO2 is highly corrosive in the presence of agitated bulk condensed water, with carbon steel pipework corrosion rates potentially ranging from 0.5 mm/year to greater than 10 mm/year. The primary control mechanism is to set a specification on impurity levels, including water. Regardless, individual emitters, albeit infrequently, will experience control failures that result in off-specification fluids entering the network. This manuscript considers a CCS hub dehydration plant failure and demonstrates how detailed simulations facilitate effective risk management. A dehydration upset will result in wet CO2 being fed into the dry (previously on-specification) carbon steel pipeline network. Accurately simulating the migration of the wet CO2 and the propensity for free water drop-out are critical to establishing an appropriate operator response. The first challenge is to determine the solubility of water in the CO2-rich fluid. A literature review of experimental data and equation of state performance for water solubility in CO2 is presented, with commentary on impurity impact. The second challenge is to model the water dew point line, determine the quantity of free water formed as a function of pipeline operating conditions, and understand the resultant extent of water migration. This work demonstrates the use of a thermohydraulic simulator to track the concentration of water through the system over time. Different simulation scenarios of off-specification events are assessed, and the study demonstrates that modelling the exposure to risk of free water driven corrosion in the CO2 pipeline is subject to uncertainty and highly sensitive to model setup. It is therefore important to understand the simulation models and simulators in use as different interpretations of simulation results may lead to different operator responses, making an informed modeller / engineer essential to determining the risk during off-specification events and hence advise the appropriate response. The results also show that the appropriate action depends on the circumstances, making predictive simulations critical. A design analysis, as presented here, can only investigate a limited number of scenarios, leading to the conclusion that subsequent operations would benefit from an online flow management tool (FMT) to provide situational awareness and track the water distribution through the entire network. An FMT will provide the operator with far more information than physical metering alone to inform risk-based control strategies.