Increasing Spatial and Directional Accuracy for reporting Dimensioning DRAG loads with FLACS(-Risk)

Transcription

1 Increasing Spatial and Directional Accuracy for reporting Dimensioning DRAG loads with FLACS(-Risk) FLUG Meeting, Bergen 2016 (HAZARDS 26, May 2016, Edinburgh, UK) Nicolas Salaün (Gexcon AS)

2 Content Introduction & Background Benchmark using a real industrial assessment (offshore, existing facility) Discussion, Conclusions

3 1. Introduction & Background Context, Scope & Definitions

4 Context Cost-efficiency, innovation Economic context in puts high focus on cost-effectiveness sometimes translated into (over?) simplification of complex technical CFD safety studies Not systematically possible to do simplifications or fast-track analysis without reducing accuracy and add conservatism Cost reduction comes from smarter and less excessive / conservative design cannot necessarily be achieved by more simplistic approach Try to increase safety value for money: innovate using the potential from the complex CFD tools in order to deliver (more) useful findings in an appropriate amount of time

5 Scope & pursued goals Present a method for more appropriate and cost-effective design against DRAG loads Frame: Offshore risk & emergency prepardness, compliance vs. Norsok standard Focus: DRAG (or dynamic pressure or explosion wind ) Common practices assume: Uniform dimensioning DRAG load definition PER AREA DRAG loading mostly affect small bore items (piping) and is used as a basis for design 1. Analyst: Enhance practicality of presented results for dimensionning accidental load (DAL) Suggest comprehensive and design-effective improvements in reporting. Improve graphical representation to increase trust, understanding, awareness & agility 2. General: Demonstrate benefits for safety and design through an analysis of an actual installation Provide relevant information matching the engineering workflow Remove conservatism resulting from simplification in design process Facilitate smart assessment : easily reduce cost while maintaining same safety level

6 Definitions DRAG loading and resulting efforts Simplified absolute DRAG: DRAG [Pa] = 1 Τ2 ρ [kg/m 3 ] V[m/s] 2 V (t) DRAG (t) is a dynamic loading Absolute V Absolute DRAG is spatially distributed DRAG ~ V Vector field. DRAG is directional: DRAG x (V x ), DRAG y (V y ), DRAG z (V z ) Absolute FORCE DRAG = 1 Τ2 ρ V 2 C D W A DLF = DRAG C D W A DLF C D : drag aerodynamic coefficient (-) W A : normal projected area facing the flow (m 2 ) DLF: Dynamic Load Factor (-) when using a static-equivalent approach Suggestions to improve reporting of DRAG are simply based on these definitions

7 2. Benchmark using a real industrial assessment Existing Approach, Increasing Spatial Accuracy, Directional Dependancy, Visualisation

8 Existing approach: single DAL output Common practice DAL from DRAG loads reported as a single value on an AREA basis (module / deck ) Small bore items uniformly designed against the same DAL for DRAG, regardless of their location and/or orientation

9 Existing approach: single DAL output Common practice: load definition per area A sufficiently large number of CFD simulations is performed, here incl. a large array of local Monitoring Points (MPs) reporting time-history parameter(s) as-built geometry. Probabilistic aggregation use corresponding explosion frequency and the max. value over space of the max. DRAG values over time for all simulations 0.69 barg (0.13 barg) for 10-4 pr. year return DRAG load in main deck (mezz. deck) Raw level of information: 3 x 3 x 2 m 3 mapping, 945 MPs over 2 decks

10 Existing approach: single DAL output Common practice Benefits: more practical for risk assessment / design process to get a single DAL value Piping strength requirement in a module derived from a single exceedance chart Frequency of failure can be identified from the same chart Disadvantages: Excessive design in areas which aren t expose to explosion loading No credit given to clever pipe routing & engineering judgement Large degree of simplification involved causing challenges & potential unnecessary costs

11 Existing approach: single DAL output In reality: Spatial variance of DRAG according to velocity field due to inhomogeneous geometry Report output per individual location Directional dependancy of DRAG Obstacles and boundary conditions affect flow direction and resulting loading Report directional dependancy

12 Improving reporting Increase spatial accuracy

13 Refining Spatial Accuracy of DAL for DRAG Uses the actual spatial mapping to report location-specific results (no more spatial «aggregation») Existing Approach Individual MP grid-based mapping

14 Refining Spatial Accuracy of DAL for DRAG Individual reporting using 405 DAL values spread over 3 elevations Colour coding shows min. to max. values Blocked «cells» are blacked-out El. 1.0 m El. 3.0 m El. 5.0 m

15 Refining Spatial Accuracy of DAL for DRAG Results and comparison vs. «conventional» approach No simulation rerun, only processing existing results differently Quantified results: o 1:20 spread across the module o 3 specific regions return high DRAG loads. Covers only ~ 2.5% of module. o Max. value from conventional approach (~ 0.69 barg) is spotted. o Average value over the module is in the order of 20 % of the max. value o Several DAL values below 0.06 barg (green) Regions showing high DRAG loads can be correlated with surrounding geometry: trust Useful for decision-making when fiting new pipes / checking Safety Critical Elements (SCEs)

16 Improving reporting Investigate directional dependancy

17 Directional Dependancy of DAL for DRAG Supporting structures, anchors, restrainments all have «directional» requirements Typical North-Sea tunnel-shaped module Directional flows: V ~ V y, Highly directional DRAG ~ DRAG y Efforts ~ flow direction versus equipment orientation Export DRAG x, DRAG y, DRAG z from the CFD simulations Take advantage of this specificity to optimize pipe routing and/or directional supporting requirements

18 Directional Dependancy of DAL for DRAG Results for increased spatial accuracy AND spatial+directional dependancies (el. 1m) Directional dependency of 10-4 /year DAL for DRAG: x, y, z components, el. 1m

19 Directional Dependancy of DAL for DRAG Results and comparison vs. «conventional» approach No simulation rerun, as long as proper relevant output (DRAG x,y,z ) is specified at the start Quantified results (case specific!): Does not necessarily lower requirements but brings added value on prevailing directions Shows that systematic use of the max. absolute DAL in all directions is not relevant Consistent geometry related design (!) Useful for decision-making (routing, support type definition, SCEs check )

20 Improving reporting Improve graphical representations of results

21 Improving graphical reporting Corresponding 3D (spatial) risk contours represented into the CFD FLACS geometry 3D risk-contours: 10-4 /year spatial dependency of DAL for absolute DRAG (Pa) Results post-treated automatically in FLACS(-risk) Mapping definition: grid cell size

22 Improving graphical reporting Corresponding 2D (spatial) risk contours represented into FLACS geometry 2D risk-contours at 3 different elevations: 10-4 /year spatial dependency of DAL for absolute DRAG (Pa) Results post-treated automatically in FLACS(-risk) Mapping definition: grid cell size h=1 m h=3 m h=5 m

23 Improving graphical reporting Benefits New type of processing of the 3D result files in FLACS-Risk beta version, no rerun Better definition (grid cell level), better automation and less manual monitoring (no MPs) Increase understanding and awarness: «visualisation for decision», facilitate smart design Highlight high / low risk regions Cost reduction potentials Key assessment tool for decision-making (routing, support type definition ) to transfer knowledge to risk-owner

24 Discussion & Conclusion(s)

25 Summary Lessons learn from the benchmark Conventional area-based method (single value of DAL for DRAG) includes a large degree of simplification. Can be seen practical for design purposes - if can be used. Does not imply cost-effectiveness, only simplification. Improving reporting of DAL for DRAG using spatial distribution and directional dependency: Is easily accessible, no simulation rerun Shows location-specific risk exposure & echoes with clever qualitative pipe routing Is useful & solution-oriented for mechanical engineers (shows prevalent loading directions ) Drastically reduces the systematic over-conservatism

26 Discussion Mandatory prerogative Methods vary among consultancy companies both for load definition and response type Need to align adequate safety output vs. relevant response input under risk-owner authority: select accurate way(s) of reporting a given load output (time, space, absolute, directional ) in coherence with subsequent type(s) of mechanical assessment in an holistic coherent approach Main goals for cost-efficiency shall be to (oppose to systematic simplification): Provide useful & relevant results Visualise the obvious and gain trust out of that Balance accuracy and consistency to draw robust conclusions lasting in time

27 Conclusions Use (CFD) tools / suggest innovative add-ons to show relevant results more interactively Rewards for adequate level of complexity: Important margins can be reduced toward a safe equivalent design at lower costs Easier to meet requirements and comply with safety objectives Highlight specific problematic region(s) - if any. Solution oriented. Expected to significantly save costs on the mechanical design side Operational report useful for qualitative judgment, awareness and agile decision making Acknowledgement: L. Rogstadkjernet (Gexcon), P.E. Nilsen (Statoil), D. Siccama (Gexcon)

28 Thank you for your attention! Questions? Nicolas Salaün