Quantification of Uncertainty in CFD Simulation of Accidental Gas Release for O & G Quantitative Risk Assessment

Quantitative Risk Assessment (QRA) of Oil & Gas installations implies modeling accidents’ evolution. Computational Fluid Dynamics (CFD) is one way to do this, and off-the-shelf tools are available, such as FLACS developed by Gexcon US and KFX developed by DNV-GL. A recent model based on ANSYS Fluent, named SBAM (Source Box Accident Model) was proposed by the SEADOG lab at Politecnico di Torino. In this work, we address one major concern related to the use of CFD tools for accident simulation, which is the relevant computational demand that limits the number of simulations that can be performed. This brings with it the challenge of quantifying the uncertainty of the results obtained, which requires performing a large number of simulations. Here we propose a procedure for the Uncertainty Quantification (UQ) of FLACX, KFX and SBAM, and show its performance considering an accidental high-pressure methane release scenario in a realistic offshore Oil & Gas (O & G) platform deck. The novelty of the work is that the UQ of the CFD models, which is performed relying on well-consolidated approaches such as the Grid Convergence Index (GCI) method and a generalization of Richardson’s extrapolation, is originally propagated to a set of risk measures that can be used to support the decision-making process to prevent/mitigate accidental scenarios.

Simulation of Multi-Gas Jet Flows by Use of Quasi Gas Dynamic Equation System

In the present paper, we use the quasi gas dynamic (QGD) model together with a finite volume method for the simulation of a gas jet inflowing region filled with another gas in the presence of gravity forces. A flow picture for such flow strongly depends on the gases density ratio. Numerical simulations are held for a region filled with air under atmospheric pressure. Three variants of inflowing gas are considered: methane (light gas), butane (heavy gas) and helium (extremely light gas). A difference between flow pictures for these test cases is demonstrated. Results obtained with the presence of wind in the air are also compared. Grid convergence is established by use of different spatial meshes. Here, the the QGD model demonstrated good efficiency in modeling multi-gas jet flows. The calculations were also used for the adjustment of the numerical method parameters.

Direct numerical simulation of hypersonic boundary layer transition over a lifting-body model HyTRV

Direct numerical simulation (DNS) of transition over a hypersonic lifting body model HyTRV developed by China Aerodynamics Research and Development Center is performed. The free-stream parameters are: the free-stream Mach number is 6, the unit Reynolds number is 10000/mm, the free-stream temperature is 79 K, the angle of attack is 0, and the wall temperature is 300 K. Weak random blowing-and-suction perturbations in the leading range are used to trigger the transition. A high order finite-difference code OpenCFD developed by the authors is used for the simulation, and grid convergence test shows that the transition locations are grid-convergence. DNS results show that transition occurs in central area of the lower surface and the concaved region of the upper surface, and the transition regions are also the streamline convergence regions. The transition mechanisms in different regions are investigated by using the spectrum and POD analysis.

CFD Gas Explosion Simulation on LNG-Fueled Ship

The International Maritime Organization (IMO) has issued the regulation of the Emission Control Area (ECA). This area was designated to control the air pollutant which emitted by conventional fuel from a ship. Thus, LNG fuel could be an alternative fuel to deal with a new regulation. However, the utilization of LNG fuel in the ship should be proven for its safety and reliability. The scope of this work was to capture the blast overpressure as an accidental load. The open-deck fuel gas supply system (FGSS) of the hypothetical Oil Tanker was used as a target for the explosion model. The parameters of the scenarios consisted of the gas cloud size and gas cloud center of gravity in X, Y, and Z directions. Each parameter contains a likelihood, and it should be converted as probability density functions (PDF). Thus, it can be utilized to generate 50 scenarios by Latin Hypercube sampling. This research used the ExSim CFD (Computational Fluid Dynamics) code to simulate a gas explosion. The grid convergence and software validation were also conducted to understand the software performance. Finally, the overpressure and gas explosion frequency from scenarios could be utilized to determine the exceedance frequency diagram. This diagram could be used as a prediction of the gas explosion on the LNG-fueled ship.

Free Surface Effect Assessment of the Flow Around the DARPA SUBOFF-5470 Submarine Using OpenFOAM

An important parameter to submarine navigation and control is determining the hull speed relative to the surrounding sea. The object of this paper is to find possible locations for speed sensors on a submarine. The probes should be fitted to the hull areas where the water flow is free of turbulence structures, vortices, or bubble formation to obtain a reliable measurement. In this work, a rational procedure is proposed to identify the probe installation site on the hull of the DARPA SUBOFF-5470 submarine, through numerical simulations using OpenFOAM. Three different depth conditions at three different navigation speeds were considered to assess the free-surface effect. First, verification and validation procedures were completed at deep water conditions (H/D = 5.4). The results of this analysis indicate a convergence ratio of 0.49 with an uncertainty of 0.04%SC. Later, a grid convergence analysis was completed at periscope depth conditions (H/D = 1.1), within the highly nonlinear Froude range. These results show an oscillatory convergence with an uncertainty of 0.78%. Finally, the hull region between 5 and 15% from the bow of the submarine length is recommended for installing the speed probe, considering the linearity of the flow, without high gradients and vortex structures.

Asymmetrical Heat Distribution Pattern in Miniature Heat Sinks Due to Conjugate Heat Transfer

Three-dimensional numerical simulations are performed to investigate the conjugate heat transfer of water within microchannel heat sinks. Validation process is performed through comparison between obtained numerical results and experimental data. The global deviation grid convergence index (GCI) is used to conduct solution verification and calculate observed order of accuracy. Conducted numerical analyses include hydraulic diameter range of 206–330 µm, aspect ratio of 1–4 and Reynolds numbers of 300 to 850. Heat is observed to distribute non-uniformly among microchannel side and bottom walls due to conjugate heat transfer. Results show that over 93% of heat is transferred to water through microchannel side walls at the aspect ratio of 4. It is observed that the heat distribution is more non-uniform destruction while microchannel aspect ratio gets larger.

The Influence of Mesh Resolution on 3D RANS Flow Simulations in Turbomachinery Flow Parts

The question of the difference mesh refinement degree influence on the results of calculation of the three-dimensional viscous gas flows in the flow parts of turbomachines using the RANS flow models and second order numerical methods is considered. Calculations of flows for a number of turbine and compressor grids on successively refining grids have been performed. We used H-type grids with approximate orthogonalization of cells in the boundary layer. The calculations were carried out using a CFD solver F with the use of an implicit ENO scheme of the second order, a local time step, and a simplified multigrid algorithm. When calculating the flow on fine grids, the following were used: convergence acceleration tools implemented in the solver; truncation of the computational domain with subsequent distribution of the results based on the symmetry property; the computational domain splitting into parts and computations parallelizing. Comparison of the obtained results is carried out, both in terms of qualitative resolution of the complex structure of three-dimensional flows, and in terms of quantitative assessment of losses. Grid convergence was estimated in two ways. In the first, the characteristic two-dimensional distributions of parameters obtained on different grids were visually compared. The purpose of such comparisons was to evaluate the sufficient degree of solution of both the general structure of the flow in grids and its features, namely, shock waves, contact discontinuities, separation zones, wakes, etc. The second estimation method is based on the grid convergence index (GCI). The GCI calculated from the three-dimensional density field was considered in this paper. It is concluded that for scientific research requiring high accuracy of calculations and detailing of the structure of a three-dimensional flow, very fine difference meshes with the number of cells from 106 to 108 in one blade-to-blade channel are needed, while for engineering calculations, under certain conditions, it is sufficient to use meshes with the number of cells less than 1 million in one blade-to-blade channel.

Dynamic effects of fault modeling on stair-step and corner-point grids

Fault modeling has become an integral element of reservoir simulation for structurally complex reservoirs. Modeling of faults in general has major implications for the simulation grid. In turn, the grid quality control is very important in order to attain accurate simulation results. We investigate the dynamic effects of using stair-step grid (SSG) and corner-point grid (CPG) approaches for fault modeling from the perspective of dynamic reservoir performance forecasting. We have performed a number of grid convergence and grid-type sensitivity studies for a variety of simple, yet intuitive faulted flow simulation problems with gradually increasing complexity. We have also explored the added value of the multipoint flux approximation (MPFA) method over the conventional two-point flux approximation (TPFA) to increase the accuracy of reservoir simulation results obtained on CPGs. Effects of fault seal modeling on grid-resolution convergence and grid-type sensitivity have also been briefly examined. For simple geometries, both SSG and CPG can be used for fault modeling with similar accuracy in conjunction with the pillar-grid approach. This is evidenced by the fact that simulation results from SSG and CPG converge to identical solutions. SSG and CPG yield different results for more complex geometries. Simulation results approach to a converged solution for relatively fine SSGs. However, a SSG only provides an approximation to the fault geometry and reservoir volumes when the grid is coarse. On the other hand, non-orthogonality errors are increasingly evident in relatively more complex faulted models on CPGs and such errors cannot be addressed by grid refinement. It has been observed that MPFA partially addresses the discretization errors on non-orthogonal grids but only from the total flux accuracy perspective. However, transport related errors are still evident. Grid convergence behaviors and grid effects are quite similar with or without fault seal modeling (i.e., dedicated fault-zone modeling by use of scaled-up seal factors) for simple geometries. However, in more complex test cases, we have observed that it is more difficult to achieve converged results in conjunction with fault seal modeling due to increased heterogeneity of the underlying problem.

The Performance of a Gradient-Based Method to Estimate the Discretization Error in Computational Fluid Dynamics

Although the grid convergence index is a widely used for the estimation of discretization error in computational fluid dynamics, it still has some problems. These problems are mainly rooted in the usage of the order of a convergence variable within the model which is a fundamental variable that the model is built upon. To improve the model, a new perspective must be taken. By analyzing the behavior of the gradient within simulation data, a gradient-based model was created. The performance of this model is tested on its accuracy, precision, and how it will affect a computational time of a simulation. The testing is conducted on a dataset of 36 simulated variables, simulated using the method of manufactured solutions, with an average of 26.5 meshes/case. The result shows the new gradient based method is more accurate and more precise then the grid convergence index(GCI). This allows for the usage of a coarser mesh for its analysis, thus it has the potential to reduce the overall computational by at least by 25% and also makes the discretization error analysis more available for general usage.