Effective Use of CFD in Upstream Engineering
Sumanta Sarkar, Dy General Manager (R & D)
Tushar Bhad, Asst Manager (R & D)
Dnyandeo Ingale, Sr Executive Engineer
L & T Hydrocarbon Engineering Limited

In offshore oil & gas industry Computational Fluid Dynamics (CFD) has been used extensively as an analysis tool towards meeting contractual obligations , risk mitigation, evaluation of new technology/equipment, safety studies etc. The R & D centre of L & T Hydrocarbon Engineering Limited (LTHE) has carried out a number of studies using CFD analysis as tool.

The current study deals with the CFD analyses performed for an offshore production unit during design/engineering phase to finalize the layout of critical hot gas stacks and flare boom. The base case result showed passage of hot gas plume over helideck leading to unsafe condition for helicopter operation. The Flare boom was subsequently relocated to mitigate the problem .

Oil & Gas industries face frequent design challenges associated to fluid flow problems coupled with heat transfer in exploration, production, and process equipment. Fluid flows and heat transfer are inherently complex and governed by complex equations. Computational Fluid Dynamics (CFD) has become an important tool in solving many of these problems. In recent decades with tremendous growth in computational technology both in terms of hardware and software, it is possible to solve complex governing equations depicting a real life system numerically and able to deliver results within acceptable time limits. Commercial CFD codes are versatile and are able to produce visual results and quantification of parameters related to flow, heat transfer, species etc. These capabilities have made CFD an indispensable tool whenever practical analysis and engineering design work involving fluids is required.

Offshore installations are normally comprised of a number of systems and processes that release hot gases at substantial rate. Typical hot gas sources are exhausts of PGC modules, GT or DG set exhausts and Flares. For combusting flares, luminous flames are also present along with hot flue gases which cause substantial increase in surrounding temperature through radiation which might require higher metallurgy for flare boom and nearby structures. These hot gases emanating from different sources may also create turbulence and other thermal effects (increasing the ambient air temperature) that may severely affect helicopter operations, unless adequate risk reducing measures are taken at the design stage.

LTHE's in-house capability of providing advanced engineering solutions using computational technologies such as CFD, FEA etc have been gainfully utilised in the Hydrocarbon project business. For issues related to fluid flow, CFD analyses using commercial code have been used in predicting complex flow fields involving multiphase, mixing, combustion, heat transfer, and radiation. Typical process equipment or systems modelled were multiphase flow simulation in pipeline, manifold and separation equipment, design analysis of waste heat recovery units, dispersion analysis for flammable and toxic gases, analysis of radiation and heat transfer from flares and hot gas stacks, heli-deck environment assessments, design analysis of gas dehydration and gas sweetening units etc. These in-house analyses were specifically helpful towards meeting contractual obligations, risk mitigation, evaluation of new technology/equipment, safety studies etc. In -house modelling capability also helped to evaluate performance of OEM designs before installation, thereby reducing/eliminating uncertainties during commissioning/PGTR.

The current study deals with the CFD analyses performed for an offshore production unit during design/engineering phase to finalize the layout of critical hot gas stacks and flare boom. During operation of the platform and flaring, the high temperature off gases from the gas turbine (GT) exhausts and flare get dissipated in the atmosphere forming flue gas plumes. Based on the ambient air direction and velocity, the shape, size and direction of the flue gas plume changes. In the event of spreading of hot flue gas around the helideck area, it would rise the ambient air temperature around the helideck and may affect the helicopter operation. In order to ensure safe helicopter operation, it is required to estimate the rise in temperature around the helideck. To estimate such temperature enhancement (if any), a comprehensive CFD study was conducted for the offshore production unit involving the platform along with the HP/LP flares, exhaust ducts and the helideck.

CFD as an Analysis Tool
The basic idea behind CFD analysis is to set up equations for the conservation of mass, momentum and energy for the volume in which the fluid flows. This is achieved by dividing the computational domain volume into a finite number of small control volumes (cells). Conservation equations for mass, energy, momentum, species etc are solved numerically over the small control volumes. The equations are solved by an iterative process to arrive at a solution. As the number of calculations can become very high depending on the number of control volumes and the number of equations solved, a high processing speed computer is generally required to perform the calculations.

Governing Differential Equations
A Governing differential equation expresses a certain conservation principle , and each equation employs a certain physical quantity as its dependant variable and implies that there must be a balance among the various factors that influence the variable. The dependent variable of these differential equations are usually specific properties, e.g., mass fraction, velocity (momentum / percent mass) and specific enthalpy.

In general, if F denotes the dependent variable, then the general differential equations is

These four terms in the general differential equation are the unsteady term, the convection term, the diffusion term and the source term. If F is the specific property and ? is the density ?F denotes the amount of corresponding extensive property contained in a unit volume. The quantity ?( ?F )/ ? t is the rate of change of relevant property per unit time, the unsteady term. The quantity ? uF is the convection flux that is the flux carried by the general ? u field. The third term G is the diffusion coefficient and is specific to a particular meaning of F. The last term S is called source term, that is it can be the rate of heat generation for a conduction convection problem or the rate of generation of the chemical species in the chemical species problem or S = (G- ?e) where G is the rate of generation of turbulence energy and e is the rate of dissipation and (G- ?e) is the net source term for a turbulent flow model.

In the current study, continuity equation, x, y, z momentum equations and energy equations have been solved. Also turbulence was modeled using the k-e model and two equations for k and e have been solved.

(i) The Continuity Equation
Physical principle: Mass can neither be created nor destroyed.

That is the net mass flow into the control volume must be equal to the rate of increase of mass inside the control volume.


This equation is applicable to all flows, compressible or in-compressible, viscous or in-viscid.

(ii)The Momentum Equation
Physical principle: The time rate of change of momentum of a body equals the net force (body & surface force) exerted on it. With u denoting the x- direction velocity, then momentum equation is given below:

Here, is the viscosity, P is the pressure, Bx is the x direction body force per unit volume, and Vx stands for the viscous terms that are in addition to those expressed by div ( grad u).

(iii)k e Model for Turbulence
The model employed in the present simulation is the standard k-e model proposed by Launder and Spalding. This employs two partial differential equations to estimate the velocity length scales of turbulence:

In the above two equations, P represents the production term given as


(iv) Energy Equation
The general equation for energy conservation used in Ansys Fluent as given below,

Where keff is the effective conductivity (k + kt) where kt is the turbulent thermal conductivity, defined according to the turbulence model being used, and jj is the diffusion flux of species j. The first three terms on the right-hand side of equation 7 represent energy transfer due to conduction, species diffusion, and viscous dissipation, respectively. Sh Includes the heat of chemical reaction, and any other volumetric heat sources you have defined.
In Equation-7

Where, sensible enthalpy defined for ideal gases and for incompressible flows as

In above equation 9 is the mass fraction of species j and

The value used for Tref in the sensible enthalpy calculation depends on the solver and models in use

Assessment of Environment around Helideck
For heli-deck environment assessments, it is necessary to estimate the air temperature around helideck for several scenarios with respect to various operating conditions and ambient conditions. Here various operating conditions means the different mass flow rates of hot gases from the different exhausts and the different mass flow rates of combustible gases at the flare. Various ambient conditions are related to the different ambient air directions, velocities and temperatures. As per CAP 437 guidelines, for safe helicopter operation, variation of air temperature around the helideck should not exceed by more than 2C. Hot air flow, combined with a sudden change in air temperature, may pose serious threat to helicopter operation such as possible momentary stalling of helicopter engines due to sudden air density changes through the turbine compressors, significant reduction in helicopter lift capacity etc. These risks can be controlled by proper design and by operational measures involving certain helicopter flight limitations. The risk varies with helicopter type, and the risk level increases with large temperature gradients in the flight path.

In the current analyses CFD simulations have been carried out for a 3D Hemispherical domain of 400 m diameter comprising of upper deck, production deck, main deck, Helideck, GTG/PGC modules with exhaust ducts and HP/LP flare with flare boom at their respective locations. HP/LP Flare tips and GTG/PGC Exhausts have been modelled at the exact location with respect to elevation of the support structures. Several cases have been studied considering different wind direction, wind velocities and ambient temperature.

Steady state single phase multi component (consisting of CH4, CO2, O2, N2 and H2O) dispersion analyses have been carried out using ANSYS Fluent. Burning of the gases emanating from the HP/LP flare tips has been modelled considering equivalent methane as a single combustible gas. To take care of the effects of convective and radiative heat transfer from flares to the structures, combustion and radiation have been modelled using Fluent based eddy dissipation reaction model and Discrete Ordinate (DO) radiation model respectively. Velocity inlet boundary condition has been used at the air inlet corresponding to specific ambient air velocities and ambient air temperatures.

Results were analysed in terms of pathline and iso surface plots for temperature. While pathline plots gave idea about the flow path of hot gas streams, iso-surface plots for temperature 2C above the ambient air temperature (iso-surface for 42C against ambient air temperature 40C) gave visual representation about the 3D region where air temperature exceeds the 2C limit as described above. Results (Figure-1D) shows the iso surface plot for temp (42 C) and Figure 1C, show that the hot gases after combustion from the flares pass over the helideck leading to unsafe condition for helicopter operation. Weather data reveals this condition prevails during most of the period in a year.

Based on the findings of the first analysis, modification in the layout was suggested to relocate the flare boom (attached to one of the legs opposite to the helideck direction). For modified layout as shown in Figure-2A & 2B, another set of analysis was performed keeping all the operating and boundary conditions same as the previous study.

Results show that the iso-surface of flue gas plume @ 42 C (2C rise in temperature above ambient temperature of 40C) does not come in the vicinity of the helideck and hence helicopter operation would not get affected by the hot gases emanating from the flare.

Summary
LTHE's in-house capability of providing advanced engineering solutions using computational technologies such as CFD, FEA etc. have been gainfully utilised in the Hydrocarbon project business. At the R & D of LTHE, CFD analyses are performed to predict complex flow fields involving multiphase, mixing, combustion, heat transfer, and radiation. For helideck environment assessments, a steady state analysis combining the effect of fluid flow, combustion and radiation has been carried out to assess the air temperature around the helideck for various wind velocities and wind directions. Based on the findings of the study decisions were taken regarding position of the helideck vis-a-vis the flare location.

  1. For the existing layout, hot gas plumes from flare passes over the helideck which may affect the helicopter operation.
  2. Based on the results of the first analysis with existing layout, flare boom location has been changed and the study has been repeated keeping all conditions same as the previous study.
  3. Modification study revealed that hot gas plumes from flare would not pass over the helideck during most of the time in a year and hence the modification has been implemented.
Reference
  1. CAP 437 "Offshore helicopter landing areas -Guidance on standards" Civil aviation authority 2010.
Acknowledgement
Authors are thankful to the LTHE management for allowing publication of this paper.