CFD Simulations in the Ocean Engineering Department Daniel Carvalho Marcelo Martinelli Paulo Roberto Pagot Raphael Coelho

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1 CFD Simulations in the Ocean Engineering Department Daniel Carvalho Marcelo Martinelli Paulo Roberto Pagot Raphael Coelho

Introduction The Ocean Engineering CFD Group works on multidisciplinary simulation tools for several problems related to offshore systems Recently

2 Introduction The Ocean Engineering CFD Group works on multidisciplinary simulation tools for several problems related to offshore systems Recently there were many different cases where Computational Fluid Dynamics was successfully employed Most applications were performed using either CFX or FLUENT

3 Case Studies Summary CASE I : Torpedo Anchor Hydrodynamics CASE II : Platform Gas Flare Cold Simulation CASE III: Thermal Plume Dispersion CASE IV: High Pressure CO 2 leaking

4 CASE I Torpedo Anchor Hydrodynamics Torpedo Anchor C

5 Problem Description Torpedo are gravity type anchors, launched by free fall Y Z X During its launching procedure, its orientation must be the most vertical aligned as possible Its stability may be evaluated through the calculation of the hydrodynamic moments generated by its movement through the sea water CFD simulations were employed to compared different torpedo models concerning their hydrodynamics loads

6 CFD Methodology ANSYS-CFX Simulations Steady-State Regime 20m/s Velocity (based on terminal velocity) Volumetric mesh (~ 1 million nodes) One inclination angle with respect to vertical for each simulation Execution time (round 2h for each angle) Inclination on the plane R45 R0 R45 Inclination Plane

7 C4 C7 C2 C9 C5 RC3 Parameterized Geometry C6 C8 RC11 C12 Allows easy and automatic setup of different geometric configurations Ref.: RT TEO 003/2012 Feasible for geometrically similar anchors ØC1 RC10

8 Workbench Automatic Simulation Setup Simulation and Analysis Geometry Meshing Parameters Table

9 Results Comparison of torpedo models in terms of vertical drag The restoration moment diagram can be used to evaluate the anchor susceptibility to unstable trajectory Streamlines

10 Conclusions Different torpedo types were compared in terms of vertical drag and restoration moments; The hydrodynamic coefficients obtained may be used by rigid body dynamic tools to simulate the whole system behavior; The parameterized model allows to include optimization tools, together with other analysis required (eg. geotechnical); The torpedo anchor C was designed and launched on Pre-salt oil fields.

11 CASE II Platform Gas Flare Cold Simulation

12 Problem Description Gas burners have difficulties to stably burn gaseous hydrocarbons with high CO 2 concentration; Open flame combustion reaction zone happens where the mixture is flammable and the flow velocity matches the flame propagation velocity; Poor gases (high CO 2 ) have small flame propagation velocity;

13 Solution Methodology Description Cold (without combustion) CFD simulations; to identify spatial regions of intersection between: flammable regions and regions where the flow velocities are smaller than the flame propagation velocity; If such region of intersection exist, the combustion may be stable;

14 Generated Mesh Size: 2,36 million nodes Only prismatic elements inside the pipe Cylindrical Far field (30m diameter and 50m high)

15 Ignitability Analysis Two criteria must be simultaneously achieved: Concentration between LFL and UFL + Velocity <10m/s

16 Lower Flamability Limit: 5% Reference Values Upper Flamability Limit *: 15% Turbulent Jet Flame Speed**: 10m/s *This limit actually varies with the CO 2 content and was locally adjusted, when necessary based on CHEN, C.C.; LIAW, H.J.; WANG, T.C.; LIN, C.Y., Carbon Dioxide Dilution Effect on Flammability Limits for Hydrocarbons, Journal of Hazardous Materials, 163, pp , ** According to GANT, S.E.; PURSELL, M.R.; LEA, C.J.; FLETCHER, J.; RATTINGAN, W.; THYER, A.M.; CONNOLY, S., Flammability of Hydrocarbon and Carbon Dioxide Mixtures, Process Safety and Environmental Protection 89, pp , 2011.

17 ConcCH4Molar Jet Axial Flammability Results Jet Axial Line CH4 QMAX CH4 QMIN Mix QMAX Mix QMIN LII LSI - CH4 LSI - MIX Z [m] Flammable length reduced when CO 2 is added to the gas ΔZ Mix ΔZ CH4

18 Volumetric Results Field Results FLASH Flow: Nm³/day Concentration (v/v): 44,4% CO 2 55,6% CH 4

19 Volumetric Results Field Results BURN Flow: Nm³/day Concentration (v/v): 0,0% CO 2 100% CH 4

Volumetric Results Field Results DON T BURN Flow: 100.

20 Volumetric Results Field Results DON T BURN Flow: Nm³/day Concentration (v/v): 44,4% CO 2 55,6% CH 4

21 Volumetric Results Field Results BURN Flow: Nm³/day Concentration (v/v): 0,0% CO 2 100% CH 4

22 Conclusions The simulations showed how the CO 2 addition leads to smaller flammable regions Combining the flammability limits with the flame speed a criterium of ignitability was drafted The comparison between simulation and field visualization results for other flows and CO 2 concentrations should be done in order to validate the developed criterium More studies concerning the turbulence and the addition of combustion models should be included in future developments

23 CASE III: Thermal Plume Dispersion

24 Problem Description Simulation of temperature and CO 2 concentration fields due to a chimney near the tank Objective: to allow the risk analysis of oil tank heater project Location: Lubnor (Fortaleza)

25 Input values Main parameters: Temperatures: 300 o C (ave.) and 400 o C (max.) Chimney internal diameter=0.23 m Tank height=12.1 m Tank diameter=13.1 m Environmental temp.=30 o C Wind intensity and direction

26 CFD Methodology ANSYS CFX Simulations ANSYS Workbench Steady-state simulation Preliminary temperature distribution study without tank ( nodes)

27 Mesh density in near field CFD Methodology

28 Results Low Wind (1m/s) from chimney to stairs top (temp. Iso-surfaces: 60 and 80oC)

29 Results Aver. Wind (6.5m/s) from chimney to stairs top (temp. Iso-surfaces: 60 and 80oC)

30 Results High Wind (14m/s) from chimney to stairs top (temp. Iso-surfaces: 60 and 80oC)

31 CO 2 distribution for aver. Wind (8m/s) from chimney to stairs top Some results

32 Conclusions Identification of: High carbon dioxide concentration regions High temperature regions These results gave appropriate inputs for the chimney installation risk analysis

33 CASE I CASE IV: High Pressure CO 2 leaking Pre-Salt fields lead to rich CO 2 (>80%) and High pressures production and processing systems (>550bar). CO 2 under supercritical conditions. High pressures systems lead to: (i) low temperatures under depressurization or leaks at equipments, valves and flanges (brittle fracture risk); CO 2 presence leads to uncertainty and risks: (i) solid formation-erosion/plugging; (ii) huge physical properties variations specially close to critical point; (iii) changes in sound wave-> Fracture propagation Equipments could be modeled by process simulators Valves and Flanges requires CFD

34 Problem Description P de acordo com a despressurização expansão adiabática (isentrópica) + Q P total instantaneamente Expansão adiabática isentálpica a partir da pressão antes da BDV

Flanges CASE I CASE IV: High Pressure CO 2 leaking Modeling Difficulties due to under-expanded jets: high pressures (from 550 bars to vacuum - real gas); phase change (gaseous, liquid, super-critical

35 Flanges CASE I CASE IV: High Pressure CO 2 leaking Modeling Difficulties due to under-expanded jets: high pressures (from 550 bars to vacuum - real gas); phase change (gaseous, liquid, super-critical and solid CO 2 -multiphase); hypersonic flow (high order schemes); low velocity regions (odd-even decoupling for Density Based Solvers); high turbulent flow with BL-Shock wave interactions with heat transfer. Performed for ideal gas: under-expanded jets of literature, tests of PB and DB solvers (problems with DB solver), discretization schemes, different turbulent models (4 models) and wall treatment (3 types), y+ from 400 to 0.5, P from 50 to 400bars. Experiments Difficulties: high number of possible scenarios (hole sizes, jet angle, flange type, ), dealing with flammable gases, huge amount of CO 2. Performed for pure CO 2 : tests for 50bars and acquisition of facilities for large tests ( bars). Tests in SINTEF will work at 500 bars and under water conditions.

36 Results

37 Results Flange ASME -huge properties variation; -shock wave patterns new in literature. 1- Shock waves marked by CO 2 solid (experiment); 2- Plain jet with recirculation zones (experiment); 3-Flange temperatures and flow path lines (CFD).

38 Coarser mesh!!! 2mm step y + =400 Results

39 Conclusions New wave structure captured for partially confined underexpanded jets. The turbulent models seems to work quite well. Similar results for most of them with slightly changes form y + form 400 to 0.5. Real gas EOS would produce more than one root for twophase flow region. How Fluent decide which one to be used? It just report the existence of two roots How to deal with Phase change? Estimation of a source of solids with Eulerian model? Bulk region dominated by shock waves and very small solids particles. Simulate as single phase but change heat transfer properties if it is in solid region of phase diagram? Solids will destroy BL enhancing heat transfer. It would be necessary to project solid-gas frontier at the wall to determine region close to the wall subjected to solid properties.

40 OBRIGADO Final

CFD SIMULATION AND EXPERIMENTAL VALIDATION OF FLUID FLOW IN LIQUID DISTRIBUTORS

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