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.

In response to the UNCCC held in Paris in 2015 the need to reduce the global warming, due to CO2 release in atmosphere, led to a new business for the capture and storage of CO2 in dedicated deep water reservoir. In this sense the transport of the CO2 at low temperature, necessary to condensate the gas, through offshore pipeline is a commercial and technical valid strategy. One of the issues related to the transport of a condensate gas is the thermal exchange between the transport system, in this case offshore pipelines, and the environment. The gas is usually carried by ships in a liquid phase at very low temperatures, for example −30 °C in case of CO2. The fluid is introduced into the pipeline at the same temperature to not further consume energy for warming up. The design of the offshore pipeline subject to these operating conditions, very cold fluid internally and a water temperature slightly over 0°C at external side, can be affected by the ice formation around the pipe. The ice thickness formation is primarily governed by the external convection coefficient. For the offshore pipelines located in deep waters where the sea currents are negligible, only the natural convection phenomena can occur on the external surface of the pipeline. Considering steady state scenario the heat transfer from the internal fluid to the external environmental is governed by the thermal resistance of each component of the system like fluid, steel, anticorrosion coating, thermal insulation if any and external convection due to the seawater. The low temperatures of both seawater and ice formation, approximately at −2°C, allow to be close to the maximum value of the seawater density: usually this occurs at a slightly colder temperatures depending on salinity and water depth (for the fresh water the maximum is at 4°C). The natural convection is driven by the buoyancy effect due to fluid density variation with temperature: the scenario described above lead to minimizes these effects and consequently the heat transfer due to the natural convection (increasing the thermal resistance). Most of the correlations in literature are related to different temperature ranges, far away from this particular situation: a numerical investigation using computational fluid dynamics technique has been performed. The analysis is executed by means of commercial CFD software FLUENT: the model is based on a two dimensional grid of a pipe submerged in water. In this paper: • The state-of-the-art about the natural convection coefficient estimate for submerged cylinders proposed by different authors through Nusselt number assessment; • A description of the proposed numerical approach is given highlighting the different approaches based on the boundary layer behavior; • A typical application is shown.

The transport of CO2 through offshore pipelines is one of the last business that the Operators are beginning to face, in line with the coming needs for climate change mitigations. The scenario for CO2 Capture, Transport and Storage anticipates capture and treatment at local plants, the transportation by ships in a liquid phase at low temperatures (close to −30 °C) to a terminal for the following offshore submarine transportation in a pipeline up to an injection well, for the final (and permanent) storage underground. In order to optimize the operating costs for CO2 transport via pipeline, and to reduce energy consumptions, no heating is applied from ship to pipeline inlet. In such case, the pipeline will reach approximately a temperature of −30 °C in the initial landfall section. The design of the offshore pipeline subject to this operating conditions, very cold fluid inside and a sea water temperature slightly over 0°C outside (North Sea), must face the possibility of ice formation around the pipe. For the Northern Lights project, this possibility has been analyzed and the HDD (Horizontal Directional Drilling) at landfall resulted the only section where the ice formation could jeopardize the pipeline integrity. Detailed assessment for both normal operating conditions and contingency cases has been performed. In the former case, a steady state thermal analysis with analytical method (thermal resistances) has been applied to calculate both the longitudinal, along the pipeline axis, and radial temperature profile: all the water inside the HDD freezes. Therefore, a water circulation system has been studied to prevent the ice formation. The pumping system required to ensure enough water flow has been dimensioned considering pressure losses inside the HDD. Power consumption in the order of 3 kW is expected. The breakdown of the pumps has been analyzed in order to determine the available time before the sea water freeze inside the HDD obstructing any circulation. A transient analysis has been carried out simulating the temperature after water circulation arrest. Both analytical and Finite Element Model have been used to calculate the transient process causing water freezing.