Shutdown & Restart Operations Design for a Large-Scale Offshore CO2 Transport Pipeline: Thermal Expansion Phenomenon Assessment

Abstract Aramis is a major CCS project, currently launching the EPCI phase to be built in the North Sea offshore Netherlands. As one of the most ambitious CCS projects, with an injection capacity of 22 Mtpa, Aramis will play a crucial role in helping the EU achieve its decarbonization goals for industries with high fossil-fuel consumption. The centerpiece of the project is a nearly 200km long transport pipeline entirely buried, connecting emitters to various offshore storage sites. Nearby emitters will supply CO2-rich streams in gas phase directly from their capture facilities into a low-pressure onshore collection network. The gaseous stream will be pressurized in a compressor station and fed into the Aramis pipeline in warm conditions. More remote emitters will supply CO2-rich streams in liquid phase transported by ships (the expected temperature is down to -25 °C) or by a long-distance pipeline (ambient temperature). The mixture of streams from diverse types of emitters and / or sources will result in significant variations in composition and temperature in the Aramis pipeline. A possible large gap between temperatures of the transported CO2-rich stream and the ambience causes a thermal contraction or expansion phenomenon under stagnant conditions. For example, in a situation in which the system is shutdown while the fluid is colder than the ambient temperature, a significant pressure increase can be observed, possibly exceeding the pipeline’s normal operating or design pressures. This pressure increase must be anticipated and managed in both planned and unplanned situations. The equilibrium pressure (also called Settle-Out-Pressure, SOP) and the CO2 phase behavior in these circumstances differ significantly from those observed in hydrocarbon systems. This phenomenon requires special attention for process safety and operating philosophy definition. The lack of industrial benchmarks at Aramis’ scale creates an additional challenge for a safe design. This work describes the flow modeling approach applied to simulate the shutdown operations for this large-scale CO2 transport network. It quantitatively illustrates the relevance of the temperature gradient on the SOP values for different operational scenarios. The main impacts on the operation philosophy and safety engineering design are presented. Moreover, details concerning the simulation methodology, including the selected Equation of State (EoS) and applied safety margin on simulation results, are also presented in this work.