Benefits of Detailed CFD Ventilation Analysis During Early Design Phases. Josué Quilliou Gexcon Consulting

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1 Benefits of Detailed CFD Ventilation Analysis During Early Design Phases Josué Quilliou Gexcon Consulting

2 12 95

3 NORSOK S-001: Natural ventilation in hazardous areas shall be as good as possible and shall as a minimum provide an average ventilation rate of 12 AC/h for 95 % of the time. Shall is used to indicate requirements strictly to be followed in order to conform to this NORSOK standard and from which no deviation is permitted, unless accepted by all involved parties. H-001 (Principles and requirements for the heating, ventilation and air-conditioning systems on offshore production installations): Natural ventilation in hazardous module areas is from a safety point of view considered sufficient when an average ventilation rate of 12 AC/h in areas is met for 95 % of the year. [ ] If required by project specific risk analysis or to optimise working environment conditions, adequate ventilation requirements in a hazardous area can be calculated for dilution of estimated fugitive emissions. [ ] The quantity of air can be calculated from data in API 4589 using the methodology given in API RP 505.

4 API API RP 505 is used for the calculation of the minimum air introduction rate to achieve «adequate ventilation», based on the evaluation of fugitive emissions (API 4589) (=hydrocarbon leakage rate anticipated under normal operations). Method based on Module Ventilation Rates Quantified, Oil and Gas Journal, W.E. Gale, December 23, 1985, p41 API RP 505: Sufficient dilution air must be added to the space in question to ensure that the concentration of flammable vapor or gas is maintained below 25 percent of the Lower Flammable Limit (LFL) for all but periods of process upset, abnormal equipment operation, rupture, or breakdown. Weaknesses: Data collected in the 90s on facilities located in the Gulf of Mexico Is it relevant for FPSOs and FLNGs?

5 API Example Safety factor Components for Offshore O&G Fugitive hydrocarbon emission rate: 1 kg/h Minimum volumetric air flow: 6.8 m 3 /min For a m 3 module, minimum ACH should be 0.05 (factor of 240 compare to NORSOK) Components for Onshore Gas Production facility Fugitive hydrocarbon emission rate: 4.2 kg/h Minimum volumetric air flow: 28.6 m 3 /min For a m 3 module, minimum ACH should be 0.21 (factor of 57 compare to NORSOK)

6 Outcomes NORSOK requirement of 12 AC/h, 95% of the time: Fairly conservative requirement. Criteria related to air quality requirement for personnel (OSHA, NFPA 45) Such specific criteria can be misleading for consultant. Necessity to change our approach and rather focus on how to reach a better ventilation AC/h not the most suitable parameter to quantify ventilation level (high dependency on volume of the area of interest) Down to one number not representative of the whole trend

7 Benefits of Detailed CFD Ventilation Analysis During Early Design Phases Why such particular interest: Natural ventilation act as a passive safety measure by helping diluting and dispersing released gases. Key is to not only assess but enhance ventilation level Quantify variations can be valuable and highlight weaknesses and give insights on possible solutions and optimisations, hence the value of performing such analysis during early design phases. Fast and inexpensive CFD is determinant as it allows to dive into the details in an illustrative way, giving you a better understanding of the flow field. 2 ways to evaluate ventilation simulations: CFD with 3D visualization of the air flow Tabulated AC/h values

8 Benefits of Detailed CFD Ventilation Analysis During Early Design Phases Time consuming Missing the big picture Misleading if large difference in the Wind statistics Realistic and accurate picture of the ventilation potential

9 Benefits of Detailed CFD Ventilation Analysis During Early Design Phases

10 Benefits of Detailed CFD Ventilation Analysis During Early Design Phases Numerical Comparison: Root Mean Square Deviation Trends give you most of the essential information and it can be sometimes helpful to go down to one value, as representative of the real conditions as it can be (cf. 95th percentile ). The closer to 0 the NRMSD is, the more likely the curves are. Remain important to relate this value to exceedance curves.

11 Thank you for your attention!