Quantitative Risk Assessment of CO2 & H2 Offshore Pipelines-Advancing On Modelling Subsea Accidental Releases

The assessment of risks for human health and environment is a crucial step of the design of subsea pipeline systems. Well-recognized standards, such as DNV-RP-F107, recommend carrying out periodic risk assessment throughout the whole life cycle of a subsea pipeline system. The purpose of this paper is to present the upgrade of a lean and proprietary tool to assess the consequences of CO2 and H2 releases from subsea pipelines whenever a Quantitative Risk Assessment (QRA) is required. To quantify the risk for people and the environment involved in an accidental loss of containment of CO2 or H2 sealines, the physical effects of subsea releases need to be evaluated. The mathematical model described in this paper is based on state-of-the-art integral models developed for subsea releases. It models subsea plumes or subsea gas blowout considering the effects of sea current, sea salinity, sea temperature as well as the effects of impurities in the released stream. The model was validated through a comparison with a detailed Computational Fluid Dynamic (CFD) simulation and case studies available in literature. At present, the assessment of subsea CO2 and H2 releases, for QRA purposes, is performed either by very simplified and not validated approaches, which can lead to an overestimation of the consequences, or by complex CFD tools which require specific skills, high computational costs, and long duration of analysis often not in compliance with tight project schedules. The results of this paper show a sufficient level of accuracy of the in-house integral model with respect to other well-recognized integral models in the estimation of underwater plume behaviour, bubble zone extension at the sea surface, void fraction, and mean plume speed. Therefore, it can provide a suitable set of input data for simulation of atmospheric dispersion of CO2 and H2. The comparison of the results, carried out by means of a CFD tool on a set of case studies, shows a good agreement of the main predictive parameters. The model described is a suitable tool for consequences assessment in QRA studies for CO2 and H2 offshore pipeline projects concurring at the Net Zero objective, contributing to understand release impacts on safety and environment.