Impact of Environmental Conditions on Safe Depressurisation of CO2 Pipelines: A Discussion on Design and Feasibility

Designing pipelines for CO2 transport comes with unique challenges when compared to conventional oil and gas transportation systems. One of which is the proximity of the CO2-rich fluid phase boundary to typical operating conditions. There are also significant risks specific to non-routine, planned operations which cross this phase boundary - such as depressurisation. This paper discusses how changes in environmental conditions can impact the safe depressurisation of CO2 pipelines. During depressurisation of a CO2 pipeline, cold temperatures are a risk due to the high Joule Thomson (JT) coefficient of CO2-rich gas. When the contents of the pipeline transition from dense to gas phase, heat will also be absorbed from the system's surroundings to supply the latent heat of vaporisation. The combination of these factors means that the surrounding ambient conditions can greatly impact the requirements for safe depressurisation. To investigate this impact, the depressurisation of three representative CO2 pipelines have been investigated using thermohydraulic modelling software, considering varying ambient conditions from Wood's project experience. The results show that factors such as ambient temperatures, wind velocities/seabed current, and the thermal conductivity of the surrounding soil have a first order impact on the minimum temperatures expected during depressurisation. The properties of the soil, such as dryness and composition - rarely the focus of detailed environmental analysis - are noted to have a particularly high impact on the minimum temperatures expected. Depending on the minimum wall design temperatures and pipeline length, this can result in significant minimum durations required to safely depressurise CO2 pipelines. It should be noted that a reasonable and economical approach for depressurisation is to assume a constant heat flux. Such an assumption provides an order of magnitude estimate as a screening procedure to determine if a more detailed survey is needed. However, in reality, the depressurisation event would cause the temperature of the soil to drop, which impacts the heat transfer from soil to pipeline. This will be discussed on a high level, with reference made to the finite element method adopted by some industry leading software packages. The case studies shown provide an understanding of how forecast conditions during these operations can determine system design margins and increase operational risks in very different ways depending on the installed pipeline environment. The outcome is an increased awareness on the importance of early project phase CO2 transport insights for transport assurance and asset integrity, and an appreciation of current best practice for CO2 pipeline modelling.