[DRAFT] TheApplication of 3D CFD Simulation for Risk and Safety Assessment in Oil and Gas Industry Facilities Rev00d

The Role of 3D CFD Simulation In Structural Design and Explosion Risk Assessment Muhammad Zulkifli, Mochamad Safarudin GexCon Indonesia

I.

Introduction

In the oil and gas industry, the stake holders have been dealing with safety issues from time to time either in design engineering phase, modification or operation of the facility. These safety issues sometimes could lead to major accidents in the facility such as fire, explosion, and dispersion of toxic gas to inhabited area, etc. Such events could result in catastrophic consequences that lead to casualties, property damage, and pollution. Major accidents occurred in the past have given knowledge and insights for developing guidelines to improve safety. Design and engineering of the facility should fulfil its functional, health, and safety requirements defined by regularity standard where it’s applied (e.g. NORSOK 2010, API etc.). Including the risks of explosion and fire are addressed in the design engineering guidelines to make sure the risk assessed qualitatively and quantitatively. To achieve that there are number of methods have been used to estimate the physical effect of major accident event such as explosion. The explosion overpressure in the far field area is usually estimated using simplified method such as TNT equivalent, Multi Energy or Baker Strehlow, etc. However, for predicting in the explosion region and the near-field these methods are not sufficient. Over simplification may render to conservative results and overlook the chance to understand how risks can be made acceptable. In addition, these methods do not take into account the interaction of the flow and actual geometry during the explosion event. This flow phenomena can be modelled in 3D computational fluid dynamics (CFD) in which all governing equations of flow, energy, momentum, mass and thermal are solved numerically using power of computers. The interaction of the flow and actual geometry during the explosion could create turbulence that enhance the mixing of fuel and air thus result in increase of explosion overpressure. Accurate prediction of explosion overpressure in surrounding area is important in order to evaluate whether secondary explosion to nearby equipment would lead to escalation. Figure 0-12 illustrate the comparison between these methods and CFD (FLACS). In the past, the cost of this method is high as the price of high performance computer still expensive. With the advancement of computer technology in recent years, the cost of high performance computer has become more affordable and made the implementation of 3D CFD for safety study sensible.

Figure 0- 12 Comparison of overpressure vs. distance between simple methods and FLACS simulations [Error: Reference source not found3]

3D Computational Fluid Dynamics (CFD) can be used to simulate the dispersion, fire and explosion phenomena. Its physical effects such as overpressure loading, radiation heat load, and gas concentration level could be predicted as well. These are important information that can be used for consideration in safety assessment studies. Figure 0-34 to Figure 0-78 shows the visualization of 3D CFD simulation results using FLACS for various applications such as dispersion, fire and explosion.

Figure 0- 34 3D flow visualisation of flammable gas dispersion simulation results using FLACS

Figure 0- 56 3D flow visualisation of overpressure of explosion simulation results using FLACS

Figure 0- 78 3D flow visualisation of fire simulation results using FLACS

However, a validated software or code should be used in conducting safety study using CFD in order to ensure accuracy and reliability of the tool itself i.e. CFD software. It is should be proven valid against the experiment and test results. Despite the CFD modelling is not substitute of actual experiment but it provides cost and time effective alternative solution to predict/estimate the physical effects of the dispersion, fire and explosion event. Figure 0-910 shows the comparison of experiment result versus FLACS simulation result. It is important to be noted, that sometimes the user need to make assumption and simplification of the problem in modelling dispersion, fire or explosion using CFD software. Despite the advantage feature of the validated CFD software, modelling mistakes or errors can lead to unreliable, unrealistic, under predicted or over predicted results of the CFD simulations. Therefore, it is highly important to conduct the CFD simulation and analysis by the user who is fully understand and has experience in CFD modelling. This will ensure that simulations are done in as realistic way as possible. FLACS is one of the CFD software used for consequence modelling such as dispersion, fire and explosion. It is one of the validated CFD software that has been developed specifically for modelling dispersion, fire and explosion. FLACS has become an industry standard and is the required tool in several oil and gas companies for their explosion hazard assessments. It is developed by Gexcon AS, a company founded by Christian Michelsen Research (CMR) Norway and some major oil and gas companies in joint industrial project.

Figure 0-910 FLACS simulation results submitted prior to BFETS full-scale test 24, flame (top) and pressure contours (lower) at time of maximum pressure. Predicted pressure curve compared with test result (left versus right) [1].

II.

Quantitative Risk Assessment using 3D CFD

Oil and Gas Processing plant consists of complex processing equipment, piping, vessels, facilities, structures and etc. These items are dealing with flammable materials have potential to become accidental release source of flammable gas or liquid to atmosphere. When unfortunate

event occur, the released flammable material could be accidentally ignited and result in fire or explosion. Moreover, this initial fire or explosion event could initiate secondary explosion and escalation as the impact for the first explosion impair other equipment, vessel or piping containing flammable material. Therefore, mitigation of such situation should be considered since the early phase of engineering design. Preventive design and maintenance could minimize the chance or consequence of the accident. However, a quantitative analysis should be performed in order to assess the reasonable and practicable mitigation measure or emergency action. Water deluge, blast walls, and venting are some options of mitigation measure that could reduce the explosion impact. To assess and design the mitigation measure, the potential explosion impact should be investigated in the corresponding area of concern this is where experiment and simulations play important role. The CFD simulation and Finite Element Analysis are becoming more popular to be used for predicting the explosion load to equipment, structures, or piping. Meanwhile, the FEA is used to simulate and analyse the structural response during the explosion. The output of strains, stresses and deformations are observed to see how structure responded to the explosion loading. Figure 0-1112 show the diagram of quantitative explosion risk assessment using CFD and Finite Element Analysis. It takes into account the frequency of an accidental explosion event would occur and the consequence is estimated using CFD and FEA simulations. Thus the explosion risk can be assessed quantitatively.

Figure 0- 1112 The procedure for quantitative explosion risk assessment and management

Taking into account the leak frequency, leak rates, leak directions, wind speeds, wind direction, ignition model, and the frequency of explosion is calculated to represents how likely the explosion event would occur. These data area calculated based on the historical database and also actual statistical weather data of corresponded location. Then the outcome of event frequency calculation is combined with the outcome of consequence modelling to form an explosion exceedance graph as representation of corresponding quantified explosion risk. This method is known as probabilistic explosion risk analysis.

The probabilistic explosion risk assessment normally performed in accordance with the guidelines given in NORSOK Z-013, Annex G [Error: Reference source not found4]. NORSOK is applied for gas and oil installations on the Norwegian continental shelf. However, the methodology is not exclusively used only for Norwegian oil and gas, but it has been used for other oil and gas facilities around the world as well. The 3D geometrical model is required to perform the CFD simulation using FLACS. The 3D model either can be generated using external CAD software in Microstation *.dgn file format then converted into FLACS model or build internal inside FLACS itself. The same geometry model is used in all CFD simulations steps ventilation, dispersion, and explosion simulations. Figure 0-1314 depicts example of onshore facility geometry modelled in FLACS.

Figure 0- 1314 Example of FLACS 3D geometry model of an onshore oil & gas processing facility

Since the flammable gas cloud formed from gas released is strongly influenced by its interaction with the wind flow pattern in the facility then the assessment of ventilation condition in the facility is required. The purpose of ventilation simulation is to assess the actual ventilation condition inside the facility and determine the representative wind condition use for dispersion simulation. The wind speeds and directions statistic are the main input for the ventilation analysis, Figure 01516 shows the example of wind direction frequency. Normally 8 to 16 wind directions are simulated (depends on available wind statistic data) and the prevailing speed is used for ventilation simulations. The air change per hour (ACH) and wind flow pattern will be evaluated for every direction after the ventilation simulations are done. The ACH represent how good the ventilation rate inside the facility, which according to NORSOK standard for offshore facility at least for 95% of time the ACH shall be above 12. Another important factor is the wind flow

pattern to be used to identify the stagnant area inside the facility. Figure 0-1718 and Figure 01920 show examples of ACH exceedance curve and wind flow pattern for an of shore facility respectively.

Figure 0-1516 Wind directions frequency distribution [3Error: Reference source not found]

Figure 0-1718 ACH exceedance curve of an offshore platform

Figure 0- 1920 Wind flow pattern and velocity visualisation in FLACS

After the ventilation analysis are carried out and the representative wind speeds & directions are determined, the dispersion simulation can be performed. The purpose is to establish representative gas clouds that might be generated for various wind and release condition. To complement the simulated results, the flammable gas cloud volumes also estimated using frozen cloud assumption which is a method developed to estimate gas cloud volume by interpolation of available simulated result. Figure 0-2122 shows example of maximum and average gas flammable cloud volumes for every leak rates.

Figure 0-2122 Maximum and average flammable gas cloud volumes for every leak rates

The dispersion simulations give information regarding the various flammable gas cloud volumes that likely generated for various wind and release condition. The volumes are then categorized into several classes to define the gas cloud sizes that will be investigated in the explosion simulations. Other parameters that also varied and investigated are ignition locations and gas cloud locations. Centre and edge ignition locations are used as parameter variation as well as gas cloud location within the facility. The receptor points for measuring the overpressure are also defined at this stage. The maximum and average explosion loads, drag, pressure impulse and drag impulse are the variables that normally recorded on every receptor points in explosion simulations. The explosion loads are then extracted and combined with the calculated explosion probability to form an exceedance curve for each receptor targets. This graph is the outcome of the probabilistic explosion risk analysis. As illustrated in Figure 0-2324, the graph shows explosion load exceedance curve for particular receptor target i.e fire wall. The graph shows the relation between overpressure received by a fire wall in an offshore platform with its corresponding explosion probability per year. In this particular graph, there are six exceedance curves that show various mitigation scenario of actual water deluge activation time after high gas alarm. This type of analysis called sensitivity study where one parameter is varied to observe and compare its effect to the result.

Figure 0-2324 Example of exceedance curves for a fire wall in an offshore platform with different water deluge activation time

The significance of the explosion exceedance curve is to quantify the explosion risk for any target of interest in the onshore or offshore platform. The information obtained then proceeded

for risk evaluation, designing explosion mitigation measure such as barriers or response analysis.

structural

For design consideration, explosion overpressures at 10-4 and 10-5 per year often used as the minimum value for design explosion overpressures. These probability values are often considered as the limit of significant probability. It is not practicable to design the facility to withstand the highest conceivable explosions load that could occur. In this way the design is balance between the probability of explosion overpressures and the provision of sufficient barriers to withstand the explosion loads. Structural response due to explosion is an important aspect to be considered in risk assessment and engineering design. It will help evaluate the physical effect occurred to the structure and the equipment during the explosion that might get overlooked. The dynamics loads during the explosion transferred to the structure that might result in excessive deformation, vibration, torsional moment or even structural failure. By using CFD simulation, the dynamic loads from explosion can be represented in more realistic way than other simplified method mentioned earlier. Figure 0-2930 shows the dynamics pressure loads on the floor deck generated from explosion simulation using FLACS of a FPSO module shown in Figure 0-2526 and Figure 02728.

Z Floor, plated deck

Y X

Figure 0-2526 Geometry of a FPSO module

Figure 0- 2728 Visualization of explosion simulation results showing explosion flame and its max overpressure exerted to the structure of FPSO module

Figure 0-2930 Explosion overpressure load on the module floor deck

III. Structural Response to Explosion Load Major catastrophes from gas explosion can result in large dynamic loads, greater than the original design loads for the piping, equipment and structure. Due to the catastrophic effects of the explosion loads, efforts have been made during past decades to develop methods of structural analysis and design to resist the explosion/blast loads. The analysis and design of structures subjected to blast loads require the understanding of blast phenomena and dynamic response of the various components of the structures. The structural response of the building structures can be determined by solving equation of motions involving mass and stiffness characteristics of the structural elements. In the early design stage, a single degree of freedom structural response analysis is adequate to obtain structural deflections and stresses caused by blast loads as depicted in Figure 3.16. However, in the detail design stage, more comprehensive approach involving multi degree of freedom response analysis should be performed. Finite Element Method can be used as numerical analysis tool to solve the equation of motions of multi degree of freedom response. In addition, material non linearity of the structural components and structural connections can be modeled using this method. Figure 3.16 illustrates stress contour as a result of commercial Finite Element simulation software of blast load subjected to a stiffened wall structure.

Figure 0-31 Single Degree of Freedom Response of a single story structure

Figure 0-32 Stress contour of a stiffened wall structure subjected to blast load

IV. Summary QRA provide practicable solution in designing safety barriers or facility. The barriers or facility can then be designed with balance between the probability of explosion differing magnitudes and maintaining provision of adequate resistance to withstand the explosion. 3D CFD simulations as a consequence modelling tool for Quantitative Risk Assessment (QRA) can provide more details of the physical effect that could occur due to dispersion, fire and explosion events. Coupled with structural response analysis, the analysis give better understanding regarding the physical phenomena of explosion, fire or dispersion which may overlooked if only simplified method applied to predict the consequence. The insight from understanding this would be beneficial in risk evaluation and decision making process for risk reduction measures. One major factor that simplified method in predicting explosion overpressure lacking is modelling actual geometry in the analysis. The interaction of the flow and geometry during the explosion can be modelled using CFD thus the turbulence enhancement effect in the explosion

overpressure is included inferring that the result predicted using CFD is more realistic. A realistic loads is important in structural response analysis especially the load is change with time. Over simplified dynamics load might hinder the actual physical effect that potentially occurred when the actual loads are applied.

References 1. J.R. Bakke and O.R. Hansen, GexCon AS, Norway. (2003). Probabilistic analysis of gas explosion loads. FABIG Newsletter, Issue No. 34. 2. Limited, U. O. (2003). Fire and explosion guidance. Part 1: Avoidance and mitigation of explosions - ISSUE 1. UK Offshore Operators Association. 3. P. Hoorelbeke, TOTAL Petrochemicals HSE; J.R. Bakke, J. Renoult, R.W. Brewerton, GexCon AS; C. Izatt, Ove Arup & Partners. (2006). Vapor cloud explosion analysis of onshore petrochemical facilities. 7th Professional Development Conference & Exhibition. Kingdom of Bahrain: American Society of Safety Engineers – Middle East Chapter (161). 4. The Norwegian Oil Industry Association (OLF) and The Federation of Norwegian Industry. (2010, October). NORSOK STANDARD - Z-013, Risk and emergency preparedness assessment. Edition 3.