The transition to a decarbonised energy system requires gathering, transport and distribution over short and long distances of CO2 and H2. For such systems, concerning offshore applications, the track record is very limited or null. The scope of this paper is to provide an overview of critical safety aspects and knowledge gaps associated with CO2 and H2 offshore pipelines. This will pave the way for a novel methodology to assess technological risk and will open the path for designing the roadmap to develop new tools for the evaluation of the hazards and their consequences. The starting ground of the novel methodology is the review of the state of art of safety aspects for CO2 and H2 offshore pipeline systems. The paper presents the status of international regulations, applicable tools and methodologies for safety analysis in the new transport scenarios and the available data on fluid release and its consequences (asphyxiation, flammable gas clouds etc). In addition, a specific approach to underwater dispersion modelling is proposed as well as the effort to collect experimental data for validation purpose. The review of the state of the art revealed that, particularly for the offshore system, safety issues are compounded by limited or no experience, lack of accident statistics on which to base risk assessment, limited availability of experimental data on underwater release and dispersion of the product into the atmosphere, toxicity and impact on health, safety and the environment. Last but not least, international regulations need to improve and reach a sufficient level of definition and coverage of topics has not yet been achieved for engineering to have a solid regulatory footprint. In order to ensure that subsea pipeline systems meet the safety and environmental requirements of companies, regulations and international standards, this paper proposes a novel methodology to develop a risk assessment process, from the initial phase during design to the operational life of offshore pipeline systems, exploiting and adapting Saipem knowledge of hydrocarbon risk analysis and consequence modelling tools available to date.

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.