5 General Conclusions
This publication was designed to be used by non-expert readers looking at the storage or use of CO2 in the offshore environment.
Sections 1 and 2 explain the general properties of CO2, and briefly describe its thermodynamic and chemical behaviour, including phase changes. It concludes that the designer should gain a more detailed understanding of these properties and make allowances as a result. In particular attention should be given to:
- The very low temperatures that can be reached as the result of the depressurisation of the CO2 when released rapidly from a vessel or pipeline.
- The impurities present in the CO2 will have an impact on the thermodynamic, physical and corrosive properties of the mixture.
- The most significant property affected is the raising of the mixture's bubble point and its impact on the potential for two-phase flow, the reduced solubility of water in the mixture and the formation of CO2 hydrates.
Section 3 notes that inhalation of the CO2 itself, leading to asphyxiation, is the primary human health hazard of which the designer should be aware. However, it should be understood that accident frequency prediction and associated calculations did not form a part of the work carried out to develop this publication. In the case of a real project, risk analyses should be carried out, and these should include attributable consequences (severity) as well. Example consequence indications are alluded to in Annex C. Generally, it is thought that CO2 releases due to containment failure have a greater likelihood of being small in size and high in pressure rather than large volume small pressure escapes.
Impacts are described in terms of SLOD and SLOT, but it is worth reiterating that these are land based calculations. This points towards a need for further work in this area to produce equivalent data for CO2 concentrations at sea. In addition, this section concludes that it is possible to structure the specification, that is the make-up of the impurities within the CO2, to allow that the health and safety impact of the CO2 itself should outweigh the health impacts of the individual impurities.
Section 3's discussion of the modelling exercise, and Annex C draws attention to the limitations of simplistic dispersion modelling. However, it should be remembered that the software used was not specifically designed with offshore situations in mind. More accurate CFD modelling should be used in real dispersion modelling exercises, so that the appropriate degree of confidence can be obtained. In addition, there are areas where solids are likely to form (sublimation) as a result of rapid cooling of the CO2 when pipes depressurise, and these cannot currently be modelled.
Small high pressure unplanned releases are likely to be neutrally buoyant in sea water. However, modelling shows that clouds are likely to change shape under differing conditions. In some cases, closing the ESDV associated with a platform was shown to make the dispersion slightly more hazardous locally while reducing the duration during a full bore release.
In the final section, specific hazards are identified alongside mitigation techniques. In conclusion, the approach demonstrates problems can be designed around and the risks handled in a safe and acceptable manner in accordance with the UK HSE's As Low As Reasonably Practicable (ALARP) principle. The platform designer should carefully develop process designs and layouts to avoid the potential for CO2 riser pipes and flow lines to impinge on steel jacket legs and structural members in the event of a rupture or leak. This should reduce the risk of steel embrittlement and failure resulting from extreme cooling during an escape.
The designer is also able to draw on many existing areas of knowledge with respect to transporting fluids and gases. These are referenced, but include the BSI, EIGA, CCSA and the HSE. The publication then concludes that there are other risks involved that should be considered in the changing of the design of an offshore platform to use CO2 inespecially with respect to introducing an acid gas within the produced fluids on to an existing or additional platform.