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Explosion and Fire Response Analysis for FPSO
C H A P T E R 49
Explosion and Fire Response Analysis for FPSO 49.1 Introduction Hydrocarbon explosions and fires have been identified as major potential hazards in offshore installations. Extreme explosions and heat will pose serious consequences for safety, assets, and the surrounding environment. A number of explosion and fire accidents in offshore installations have occurred in recent decades such as the Piper Alpha accident (the accident that occurred on board the offshore platform Piper Alpha in July 1988 killed 167 people and cost billions of dollars in property damage; Figure 49.1) and the Deepwater Horizon accident, which occurred on April 20, 2010, killed 11 workers, and resulted in the largest oil spill in the United States (Figure 49.2). Before the Piper Alpha accident, research into fire and explosion was only through experience and statistical analysis. Experts mainly focused on the lack of fire-fighting procedures and recommendations for its improvement. With the development of modern
Figure 49.1 The Piper Alpha accident.
Marine Structural Design. http://dx.doi.org/10.1016/B978-0-08-099997-5.00049-6 Copyright © 2016 Elsevier Ltd. All rights reserved.
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Figure 49.2 The Deepwater Horizon accident.
methods of quantitative analysis, more and more studies are done using modern computer technology. A lot of effort now has been put into the prediction and controlling of explosions and fires in offshore installations. Risk-based approaches, rather than traditional prescriptive approaches, have begun to be more extensively applied in offshore designs.
49.2 Accident Causation Analysis Analyzing the cause of accidents is an important part of risk management. Usually, the accident risk that exists in the system will be found and its characteristics will be discovered and identified by inspecting and analyzing the system. According to the characteristics of hazardous energy, the source of the first hazard is defined as hazardous substances, which could be released accidentally into the system; the source of secondary hazard is referred to as all kinds of unsafe factors that exhaust measures to keep energy stable, such as loading and unloading, storage of goods, and hot work. The first kind of hazard is energy, which can cause the accidents and determine the severity of accident consequences. The source of the secondary hazard is a necessary condition to decide whether the accident occurs and affects the feasibility of the accident. The cause of an offshore oil platform accident is shown in Figure 49.3. Based on this theory, the main cause of fire and explosion accidents is shown in Figure 49.4. As a matter of fact, most of the reasons for fire accidents is their relationship with human factors. The International Maritime Organization (IMO) found that almost 80% of marine accidents occurred because of human error. The results further illustrate human action is an important factor affecting the safety of the whole system.
Explosion and Fire Response Analysis for FPSO
Figure 49.3 The cause analysis of marine accidents.
Figure 49.4 The main cause analysis of fire and explosion.
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49.2.1 Formal Safety Assessment Formal safety assessment (FSA) is a kind of integration and systematic analysis method. Its purpose is to make all aspects of the specification, design, operation, and inspection effective and to improve safety at sea comprehensively and reasonably. This includes the protection of life and health, the marine environment, and property. A summary of this method is available as follow. Phase I, Identification of dangerous sources: Its purpose is to evaluate the system to identify all the possible dangers and find out the cause and consequences of the possible accidents, then make a list in order of the degree of dangers to further analyze major risks and put forward a corresponding control scheme. Phase II, Risk assessment and management: In this phase the procedures are used to evaluate the risk of all kinds of danger: find the distribution of risk and the overall level of risk, focus on the high risk area and the main factors affecting the level of risk, and sort the risk in an acceptable level. Phase III, Risk restraining project: The effective measures are put forward to reduce risk on the basis of hazard identification and risk assessment. Phase IV, Cost and benefit evaluation: Calculate the costs of each risk control measure and the benefit from reduced risk degree. Phase V, Suggestions and decision: Select the optimal risk control measure.
49.3 Phase I: Identification of Dangerous Sources There are many methods to identify the dangerous sources. For example, the expert investigation method (EIM), preliminary hazard analysis (PHA), fault tree analysis (FTA), event tree analysis (ETA), failure mode and effect analysis, and hazard and operability. 1. Expert investigation method The main task of the investigation is to find all kinds of potential dangers and measure the consequences. Many dangers are difficult to confirm with statistical methods and causal reasoning in a short period of time in marine structure engineering. For example, there are relatively few available statistical data for the fire and explosion accidents on the floating production storage and offloading (FPSO). It is obvious that EIM is prevalent to identify the danger sources. The two most commonly used methods are brainstorming and Delphi. a. Brainstorming Brainstorming can also be called playing imagination. It includes two stages: Stage 1: The organizer reviews the criterion of imagination and the purpose of the meeting, and then team members express their opinions fully and put forward a series of ideas.
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2.
3.
4.
5.
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Stage 2: The ideas are then analyzed comprehensively. It can neither be despised nor accepted blindly. This method can be used for selecting risk identification and risk controlling measures. b. Delphi The Delphi method is a typical method of risk identification task directed by the famous American consultancy the Rand Corporation. This method is widely used in the decision-making process. Preliminary hazard analysis PHA is a qualitative analysis method to evaluate the internal risk factors and the degree of risk in the system. It can be the previous step to the FTA. Fault tree analysis FTA is one of the most important analysis methods of risk and security systems engineering. FTA is a longitudinal analysis method, a graphic deductive method from the top to the basic events. It can be used for qualitative evaluation and calculating the degree of system failure probability. One top event should be designated. The top event is the accident that is not expected to happen. During this period, construction of the fault tree is core. Event tree analysis ETA is a kind of inductive method. Contrary to FTA, ETA starts with the basic event, the probability of unexpected events can be roughly calculated, and finally, the top event will be found. Usually ETA can be combined with FTA, and work together to complete the analysis. Failure mode and effects analysis Each element is investigated to detect potential various failure modes in the subsystems or components. Then precautionary measures will be put forward.
49.3.1 The Structure Function of Fault Tree 1. Basic conception Assuming FTA is made up of n different kinds of independent events, a binary random xi variable equal to the state of the ith bottom events ei is defined to be: 1 ei occur xi ¼ i ¼ 1; 2; ..n (49.1) 0 ei not occur Binary random variable f expresses the status of the top event T: 1 T occur f¼ 0 T not occur
(49.2)
912 Chapter 49 The state variables of the top event are completely determined by the bottom event state variable values, as the top event state is completely determined by the status of the basic events. Define f to be the function of X ¼ (x1, x2,., xn), and assume the equation is as follows: f ¼ fðxÞ
(49.3)
f(x) is called the structure function of the fault tree. 2. Correlation function a. Correlation of the bottom event: If xi satisfies Eqn (49.4): fð1i ; xÞ 6¼ fð0i ; xÞ
(49.4)
then it can be said that the bottom event ei has an effect on the structure function. b. The correlation function: If the structure function f(x) meets the following properties, then f(x) is called the correlation function. e Each variable value xi (i ¼ 1, 2, ., n) has an effect on the structure function. e f(x) is correlated in terms of xi (i ¼ 1, 2, ., n) and is not diminishing. In the logic operation, the structure function of the fault tree composed by logic gates always accords with (b). The structure function is called correlation function if it coincides with property (a). Coherent structure function has the following properties: e f(0) ¼ 0; e f(1) ¼ 1; e Given state vectors X and Y, if X Y, namely xi yi, then f(x) f(y); e Suppose f(x) is a structure function composed with n independent events, then the equation is set up as follow: n
n
i¼1
i¼1
X xi fðxÞ W xi
(49.5)
c. Indicates that the fault tree concerning the state of the system is between the same kind of units in the series system and parallel system composed of the same unit. Transform the equation as follows: fðxÞ ¼ xi fð1i ; xÞ þ ð1 xi Þfð0i ; xÞ
(49.6)
In terms of the fault tree composed of n independent events, its structure function can be expanded n times. So the basic form of the equation is as follows: fðxÞ ¼
n X Y xyi i ð1 xi Þ1yi fðyÞ Y i¼1
(49.7)
Explosion and Fire Response Analysis for FPSO where xi is 1 or 0, and
P
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Y indicates summation of the state vector valued Y.
Figure 49.5 is as an example of a fault tree that explains Eqn (49.5); there are five bottom events, Y ¼ (y1, y2, y3, y4, y5). All 32(25) possible states of the fault tree are presented in Table 49.1; once f(y) is calculated, f(x) is obtained. From Eqn (49.7), the following equation is found: 4ðxÞ ¼ ð1 x1 Þð1 x2 Þx3 x4 ð1 x5 Þ þ ð1 x1 Þð1 x2 Þx3 x4 x5 þ ð1 x1 Þx2 ð1 x3 Þx4 x5 þ // þ x1 x2 x3 x4 ð1 x5 Þ þ x1 x2 x3 x4 x5 (49.8)
Ga a Gc
Gb b
c
x4
Ge
x1
Gd d
e
x3
Gf
x3
x5
f x2
x5
Figure 49.5 A typical example of a fault tree. Table 49.1: Dimensions and geometric properties of the frames Symbol hw (mm) tw (mm) bf (mm) tf (mm)
Main Frame 700 13 300 24
Secondary Frame 300 10 300 15
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49.4 Phase II: Risk Assessment and Management Fires and explosions are continuous threats on offshore oil and gas installations. The dominant fire and explosion events are associated with hydrocarbon leaks from flanges, valves, equipment seals, nozzles, etc. Fires and explosions both result from combustion associated with hydrocarbon gas leaks. Figure 49.6 shows a sample calculation of the frequency of leaks using ETA. The methods mentioned above are often unable to identify frequency of gas leaks and ignition probability properly. More refined methods for their calculation are thus required on the basis of simulations. The EFEF JIP (explosion and fire engineering of FPSO units) has developed more refined methods to calculate the frequency of fires and explosions. Jeom Kee Paik is the leader of the ongoing 27th Joint Industry Project on the Explosion and Fire Engineering of FPSO Units (EFEF JIP). The aim of the EFEF JIP is to develop state-of-the-art technologies for the quantitative assessment and management of the risk of hydrocarbon explosions and fires in offshore installations. A framework for the quantitative assessment and management of the risks associated with fires and gas explosion requires the identification of both the frequency and consequences of these incidents.
Figure 49.6 Explosion event tree analysis for a leak event.
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49.4.1 Procedure for Fire Risk Assessment and Management Figure 49.7 presents the EFEF JIP procedure for the fire risk assessment and management of offshore installations.
Figure 49.7 EFEF JIP procedure for the fire risk assessment and management of offshore installations.
916 Chapter 49 Risk is defined as a product of frequency and consequence. Thus, the main purpose is to accurately calculate the frequency and consequences of specific events within the framework of risk assessment and management. To estimate the risk level of a structure, the identification of danger source and action effects of fire are vital. Each of the fire scenarios can be simulated by computational fluid dynamics (CFD). This establishes and characterizes the fire load profiles based on time and space in terms of temperature and heat amount. The CFD modeling techniques employed are significantly essential to the accuracy of these CFD simulations. There are eight random variables to formulate fire scenarios via sampling techniques: • • • • • • • •
Wind direction (X1) Wind speed (X2) Leak rate (X3) Leak duration (X4) Leak direction (X5) Leak position in the x direction (X6) Leak position in the y direction (X7) Leak position in the z direction (X8)
The design fire loads can be decided by the fire load profile with respect to time, temperature, and heat dose and converted to software using finite element analysis, that is, ANSYS, ABAQUS, to realize the non-linear structural response. The properties of the fire resistance of steel are the main factors affecting the structural integrity of fire. A noncontinuous segment plot based on the definition of Eurocode (Franssen and Real, 2010) is shown in Figure 49.8. When it is at 400 C the mechanical properties of steel significantly decrease. On the other hand, the heat from fire flows into steel, which is a good conductor compared to other materials. Thus, fire can lead to the collapse of steel structures. The frequency of fire is the likelihood of accidents, leaks, and ignitions. The frequency of leaks and ignitions can be obtained by ETA. Finally, the risk is calculated. If the calculated risk level is greater than the acceptable risk level, then the system must be redesigned, or such other control measures must be taken such as a fire wall, passive fire protection, or deluge/water spray. Acceptable risk level is normally defined in terms of the probability of damage exceeding the main safety functions or probability of accident escalation.
49.4.2 Procedure for Explosion Risk Assessment and Management In general, the response assessment methods include three main levels of analysis: screening check, strength level analysis (SLA), and ductility level analysis (DLA).
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Figure 49.8 Mechanical properties of steel with temperature as the plot of Eurocode’s definition.
The screening check checks for the safety and reliability of the whole structure. The SLA is used to estimate the failure of most topside components during early detailed engineering as a linear elastic analysis. However, the demand for the DLA from ship owners has increased recently rather than the preference for the SLA for most of the topside modules of FPSO/FLNG (floating liquefied natural gas) offshore facilities. Figure 49.9 presents the practical approach widely used in the offshore industry based on API RP 2FB. Kim et al. (2014) present the DLA methodology for the topside modules under blast loads in terms of the offshore industry calculation method. Explosions create pressure waves and the energy releases take place in a very short period creating a shock front in which a peak pressure occurs. Following this, the overpressure drops very rapidly and reaches a negative phase. The typical simplified pressureetime curve is shown in Figure 49.10. In Kim et al. study, a two-time step process was applied to simulate the dynamic response for blast load by using ABAQUS software. The first step is a static analysis for self-weight stabilization, and the next step is a dynamic analysis for actual structural response under the blast loading. The purpose of the DLA method is to verify the nonlinear dynamic structural behavior under the dynamic load (blast loading). A certain amount of resonance may occur in the vibrating response of the topside structures due to both the maximum positive and
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Figure 49.9 Flow chart of structural assessment against blast.
negative deflections and forces with accelerations of each structural component. It is therefore of importance to evaluate the appropriate vibration mode shape, depending on the specific relation between the natural frequency of the structure itself and triangular impulsive loads with the blast duration. Obviously, the natural frequency is of major importance to the dynamic analysis and it can be found by modal analysis. The strain and stress of all members can be obtained during the dynamic analysis. To ensure the structure is safe all of the members should not exceed the correct plastic strain. Once the DLA cannot satisfy the criteria of assessment, some blast mitigation should be carried out. There are two methods to minimize the damage for explosion accidents. One is to decrease the frequency of accidents in gas release, formation, and ignition of an explosive cloud. The other is to reduce the consequence by installing a safety system.
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Figure 49.10 Simplified pressureetime curve.
Corrugated and flat-plate type blast walls are generally used for the purposes of reducing the explosion consequences.
49.5 Phase III: Risk Restraining Project In terms of fire prevention and control of offshore platforms, strict precautions should be formulated to avoid fire and explosion caused by hydrocarbon combustion. The specific measures are described below. 1. Improve the layout The platform’s location should avoid an earthquake-prone belt and lightning-vulnerable areas, and fuel storage areas should be set in a well-ventilated downwind position and kept at a maximum distance from sources of ignition. In the process of building a layout for the firing of containers and work over well completion and temporary facilities, special measures should be raised. A reasonable arrangement of a firewall will help prevent flames from spreading and provide a heat insulation barrier. Meanwhile, the firewall should avoid adverse effects of large amounts of hydrocarbon steam and combustible gas gathering. 2. Team management For most instances of accidents, human error is the main factor. Workers manage the main body and object management, education of staff, and safety training. The education process, stricter accordance with operating instructions, and not overloading operations should be strengthened in offshore productions. Strengthening the platform patrol inspection work will guarantee a better flow of work.
920 Chapter 49 3. Management of equipment Offshore equipment suffers from perennial water and wind erosion. Therefore, strengthening the maintenance of equipment is necessary. There is a special process that forbids hot maintenance. It is necessary to periodically blow down parts of the equipment to prevent fires and explosions caused by high pressure. Natural ventilation components shall be equipped with a spark and fire detector to prevent sparks. 4. Strengthen the inspection and maintenance of lightning protection and electric bonding facilities Offshore platforms are flammable and explosive areas. Oil and gas production operation should be stopped during a thunderstorm. In order to avoid fire accidents caused by static electricity, certain electrostatic protection devices are necessary. Electrostatic discharge facilities should be in good condition and connection parts fixed firmly. Operators should dress according to the rules and not wear chemical fiber clothes. A human body electrostatic touch release facility should be set up before workers take up their quarters. 5. Strengthen the inspection and maintenance of electrical facilities Electrical safety is a comprehensive technology, requiring both engineering and organization. It includes insulation protection, barrier protection, safety distance protection, grounding lightning protection, leakage protection, automatic control equipment, etc. Offshore equipment ages quickly and erodes due to long exposure, so it is important to strengthen the maintenance inspection daily so as to prevent electrical components aging and short circuiting. 6. Hot surface protection Structure surfaces with abnormally high temperatures should not come in contact with liquid hydrocarbon, oil, and flammable gases. 7. Security system and fire control facilities A platform security system should be put in place to detect abnormal occurrences and prevent accidental fires. A combustible gas detector should be installed to detect the concentration of combustible gases. When the concentration reaches the explosion limit, it should provide a warning and truncate the source of the fire. In order to prevent the spread of fire, a fixed or semifixed type of foam fire extinguisher is needed. Finally, increase the staff’s education of extinguishing equipment use and their ability to make an immediate response.
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49.6 Examples of Explosion Response of FPSO 49.6.1 Introduction FPSO has become the mainstream of the development of offshore oil gas production field because of the advantages of transferable and reusable. Research into the safety and reliability of FPSO is of great significance. The authors conducted a quantitative risk assessment of combustible gas under explosion hazards. We made a simulation of the leakage and explosion process of combustible gas in FPSO by FLACS CFD and made an analysis of the explosion load characteristics. Then we imported data of the simulation into the ANSYS/LS-DYNA computations to make an analysis of structural response of the offshore platform. The contribution of this study is demonstrated with an applied example using a hypothetical topside structure of an FPSO that is exposed to hydrocarbon explosions. We then presented a procedure for the non-linear structural response analysis of offshore installations with a focus on explosions.
49.6.2 Gas Dispersion CFD Simulations The FLACS code, which is a three-dimensional transient finite volume CFD program, is used to simulate gas dispersion and explosion events. The commercial version of the FLACS code provides the results at a limited number of monitoring points and/or panels. To demonstrate the applicability of the FLACS code in the simulations of gas dispersion, an example of an explosion analysis in an offshore module is considered. Figure 49.11 shows the layout and principal dimensions of this hypothetical topside module of the FPSO. The gas dispersion simulation is performed to characterize the gas cloud size, which is affected by various factors including leak rates, duration times, positions, and wind conditions. Gas Dispersion Scenario Gas dispersion is as follows: • • • • • • • •
Leak rate: 4 kg/s Leak location: (6,6.75,2) Start time: 0 s Leak duration: 40 s Temperature: 20 C Gas composition: methane 91%, ethane 7%, propane 2% Wind speed: 3 m/s Wind direction: þX
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Figure 49.11 Layout and principal dimensions of the hypothetical FPSO topside module, (a) side view, (b) ichnography.
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The Spatial Distribution of Gas Concentration
Figure 49.12 The spatial distribution of gas concentration at t ¼ 40 s, (a) 3d view, (b) y ¼ 6.5 m.
Actual Gas Cloud and Equivalent Gas Cloud The cloud shown in Figure 49.12 is the actual gas cloud. The equivalent gas cloud volume is defined when the equivalent ratio defined in Eqn (49.9) equals 1. ER ¼
ðmfuel =moxygen Þactual ðmfuel =moxygen Þstoichiometric
(49.9)
924 Chapter 49 where mfuel and moxygen are mass of gas and oxygen in actual or stoichiometric conditions, respectively. Effect of Leak Rates In the simulation process of flammable gas leakage, we set the speed as 1, 2, 3, and 4 kg/s, respectively. The volume of actual gas cloud and equivalent is showed in Figure 49.13.
49.6.3 Gas Explosion CFD Simulation A flammable gas explosion is influenced by many factors, such as leak rates, leak location, combustion source location, and so on. Gas Explosion Scenario The gas dispersion is as follows: • • • • • • • • • •
Leak rate: 4 kg/s Leak location: (6,6.75,2) Start time: 0 s Leak duration: 40 s Temperature: 20 C Gas composition: methane 91%, ethane 7%, propane 2% Wind speed: 3 m/s Wind direction: þX Combustion source location: (8,6.75,3) Ignition time: 40 s
Figure 49.13 The effect of leak rates.
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Figure 49.14 shows the allocation of 128 monitoring points to read overpressures (P) and combustion product mass fraction (Prod) on the positions of interest. Figure 49.15 shows the distribution of combustion product mass fraction (Prod) at 40 s and Figure 49.16 shows overpressures (P) at monitor point 28.
Figure 49.14 The allocation of 128 monitoring points, (a) monitoring points on mezzanine deck and process deck, (b) monitoring points on main column.
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Figure 49.15 The distribution of combustion product mass fraction (Prod) at 40 s, (a) three dimensional space, (b) y ¼ 6.5 m 3d profile, (c) y ¼ 6.5 m 2d profile.
49.6.4 Nonlinear Structural Response Analysis The structural response analysis is undertaken for a situation in which the topside structures are subjected to explosion loads. Structure Model Figure 49.17 shows the model of a target structure with SHELL163 elements. The structures are made of mild steel and the material property is presumed to be plastic
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Figure 49.16 The overpressure at monitor point 28.
Figure 49.17 LS-DYNA structure model.
kinematic. The ends of all eight columns are set to be fixed and all other boundaries are in a free condition. The Distribution of Structure Stress, Displacement, and Strain Each monitoring of the overpressure curve is loaded into the structure, and we get the following platform structure displacement and stress distribution. Figure 49.18, Figure 49.19, and Figure 49.20 show the von Mises stress distribution, displacement distribution, and strain distribution, respectively, at 0.12 s. The Distribution of Displacement on Main Columns We select the four elements on the column as shown in the Figure 49.21 and the overpressureetime curve is showed in Figure 49.22. The results show that the overpressures on the upper and bottom of the main column are higher compared with the overpressures on the middle.
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Figure 49.18 The von Mises stress distribution at 0.12 s.
Figure 49.19 The displacement stress distribution at 0.12 s.
The Displacements on the Midpoint of the Main Girder For the whole deformation of the upper structure, it is vital to examine the deformation of the main girder. The displacements on the midpoint of main girder can effectively reflect the deflection of the frame.
Explosion and Fire Response Analysis for FPSO
Figure 49.20 The strain distribution at 0.12 s.
Figure 49.21 The location of four elements.
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Figure 49.22 Overpressureetime curve.
The Deflection of the Frame The location of the four midpoints of the main girder is shown in Figure 49.23 and the displacementetime curve is shown in Figure 49.24. The displacement of the C point has the closest distance from the explosion sources reaching a maximum at t ¼ 0.39 s with a value equal to 0.41 mm. The displacement of the C point is about 1.7 times the displacement of the D point.
Figure 49.23 The location of the four midpoints of the main girder.
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Figure 49.24 The displacementetime curve.
49.7 Example of Fire Response of FPSO 49.7.1 Fire CFD Simulation Fire Scenario The object of the fire CFD simulation is to simulate the gas cloud dispersion, gas cloud temperature, and heat fluxes that are time and space dependent. The fire load is correlated to the elevated temperatures obtained from the fire CFD simulation. One of the commonly adopted tools for fire CFD simulations is the fire dynamic simulation (FDS), which is a fire dynamic simulator, a CFD model of fire-driven fluid flow. FDS solves numerically a form of the NaviereStokes equations appropriate for low-speed, thermally driven flow with an emphasis on smoke and heat transport from fires. The fire example we simulate here is a pool fire, which is caused by combustible liquid burning on the surface. The following fire scenario was selected in the fire CFD simulation: Density of heat flow ¼ 24,000 kW/m2 Leak area ¼ 1.5 1.5 m Leak direction ¼ þZ Leak position in the X direction ¼ 10.75 m Leak position in the Y direction ¼ 6.5 m FDS Structure Model Figure 49.25 shows the layout and principal dimensions of a hypothetical topside module of the FPSO. All decks are supported by strong I-girders. But here in FDS, the decks are
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Figure 49.25 Layout and principal dimensions of the hypothetical topside module of the FPSO.
simplified as plates with different thicknesses. The process deck and mezzanine deck are 700 mm thick and the upper deck is 300 mm thick. The columns are also simplified as square columns because no round surface can be built in FDS. Figure 49.26 and Tables 49.1 and 49.2 present the geometric topology of the deck beams and columns. Using FDS, monitoring points should be reasonably assigned. Each frame has a monitoring point in the middle. Part of the points can be seen in Figure 49.27.
Figure 49.26 Topology of decks (red lines (dark grey in print versions) indicate main frames and dotted lines indicate secondary frames), (a) process and mezzanine deck, (b) upper deck. Table 49.2: Dimensions and geometric properties of the columns Symbol rinner (mm) router (mm)
Column 160 178
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Figure 49.27 Monitoring points.
FDS Results Smokeview is a separate visualization program that is used to display the results of an FDS. Figure 49.28 displays the heat flux at time 3.6 and 600 s. Figure 49.29 displays the temperature distribution at cross-section Y ¼ 7.6 m at time 10 and 600 s. Figure 49.30 displays the temperature distribution at cross-section X ¼ 9.8 m at times 10 and 600 s. Figure 49.31 displays the temperature distribution at cross-section Z ¼ 5.7 m at times 10 and 600 s.
Figure 49.28 Heat flux at different simulating times.
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Figure 49.29 Temperature distribution at cross-section Y ¼ 7.6 m at different times.
Figure 49.30 Temperature distribution at cross-section X ¼ 9.8 m at different times.
Figure 49.31 Temperature distribution at cross-section Z ¼ 5.7 m at different times.
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49.7.2 ANASYS Analysis Temperature Simulation The result of the ANASYS temperature simulation using SHELL131 is shown in Figure 49.32. The maximum temperature is 590.55 C. The mezzanine deck is influenced mostly while the process deck remains almost the same when there is no fire. Structure Analysis Take Q235 steel as an example. The elasticity modulus and yield stress of Q235 change with temperature in Table 49.3.
Figure 49.32 Temperature simulation in ANASYS. Table 49.3: The elasticity modulus and yield stress of Q235 Elasticity Modulus
Temp ( C) 16 100 200 300 400 500 600
Yield Stress
Ton-module
T Temp/t [ 16 C T Temp (MPa) T Temp/t [ 16 C T Temp (MPa) T Temp (MPa) 1.000 1.000 0.959 0.900 0.831 0.621 0.171
206,000 206,000 197,554 185,400 171,186 127,926 35,226
1.000 1.000 0.823 0.629 0.498 0.402 0.204
235 235 193 148 117 94 48
707 707 1800 1831 994 478 158
936 Chapter 49 Add the temperature load to the structure and uniform pressure to one of the main frames. Figure 49.33 shows all loads and restrictions on structure. Results Structure deflection in the Z direction is shown in Figure 49.34. The biggest deflection is 358 mm.
Figure 49.33 All loads and restrictions on structure.
Figure 49.34 Structure deflection in the Z direction.
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Figure 49.35 von Mises stress distribution.
The von Mises stress distribution is shown in Figure 49.35. The biggest stress is 247 MPa. It takes place in the red area in Figure 49.35.
References Bishop, S.R., et al., 1993. Nonlinear dynamics of Flashover in compartment fires. Fire Saf. J. 23, 11e45. Bossche, A., 1991. Computer-aided fault tree synthesis 1. Reliability Eng. Syst. Saf. 33, 217e241. Cai, K.Y., 1996. System failure and fuzzy methodology: an introductory overview. Fuzzy Sets Syst. 83, 113e133. Cullen, L., 1990. The Public Inquiry into the Piper Alpha Disaster. Health and Safety Executive, HMSO, London. Czujko, J., 2001. Design of Offshore Facilities to Resist Gas Explosion Hazard: Engineering Handbook. CorrOcean, Oslo. Czujko, J., Paik, J.K., 2010. Explosion and Fire Engineering of FPSOs (Phase II): Definition of Gas Explosion Design Loads. Final Report No. EFEF JIP-04eR1. Research Institute of Ship and Offshore Structural Design Innovation, Pusan National University, Busan, Korea. Franssen, J.M., Real, P.V., 2010. Fire Design of Steel Structures. ECCS Eurocode Design Manuals. Ernst & Sohn. Germany, Berlin. Gee, P.H., 2000. The evolution of offshore legislation. Trans. ImarE 112 (1), 3e10. HSE, 2005. Accident Statistics for Floating Offshore Units on the UK Continental Shelf (1980e2003). HMSO RR 353. Health and Safety Executive, London. Kim, T., 2014. The Nonlinear Dynamic Structural Response Analysis of the FPSO Topside Module under Blast Loads, ICTWS 2014, Busan, Korea. Msc/Cir.829, MEPC/Circ 335 Interim Guidelines for the Application of Formal Safety Assessment (FSA) to the IMO Rule-making Process, November 17, 1997. Nolan, D.P., 1996. Handbook of Fire and Explosion Protection Engineering Principles for Oil, Gas, Chemical, and Related Facilities. Noyes Publications, Park Ridge, NJ.
938 Chapter 49 Paik, J.K. A New Procedure for the Nonlinear Structural Response Analysis of Offshore Installations in Fires. Paik, J.K., Czujko, J., 2009. Explosion and Fire Engineering and Gas Explosion of FPSOs (Phase I): Hydrocarbon Releases on FPSOseReview of HSE’s Accident Database. Final Report No. EFEF JIP-02. Research Institute of Ship and Offshore Structural Design Innovation, Pusan National University, Busan, Korea. Paik, J.K., Czujko, J., 2010. Explosion and Fire Engineering of FPSOs (Phase II): Definition Offire and Gas Explosion Design Loads. Final Report No. EFEF-03eR2. Research Institute of Ship and Offshore Structural Design Innovation, Pusan National University, Busan, Korea. Riding, J.F., 1997. Formal safety assessment (FSA): putting risk into marine regulations. Trans. ImarE 109 (2), 185e192. Spouge, J., 1999. A Guide to Quantitative Risk Assessment for Offshore Installations. Aberdeen: The Centre for Marine and Petroleum Technology, UK. Thomas, P.H., Bullen, M.L., Quintiere, J.G., McCaffrey, B., 1980. Flashover and instabilities in fire behavior. Combusion Flame 38, 159e171. Vinnem, J.E., 2007. Offshore Risk Assessment: Principles, Modeling and Applications of QRA Studies, second ed. Springer, London. Wang, J., 2006. Maritime risk assessment and its current status. Qual. Reliability Eng. Int. 22, 3e19.
Explosion and Fire Response Analysis for FPSO 49.1 Introduction Hydrocarbon explosions and fires have been identified as major potential hazards in offshore installations. Extreme explosions and heat will pose serious consequences for safety, assets, and the surrounding environment. A number of explosion and fire accidents in offshore installations have occurred in recent decades such as the Piper Alpha accident (the accident that occurred on board the offshore platform Piper Alpha in July 1988 killed 167 people and cost billions of dollars in property damage; Figure 49.1) and the Deepwater Horizon accident, which occurred on April 20, 2010, killed 11 workers, and resulted in the largest oil spill in the United States (Figure 49.2). Before the Piper Alpha accident, research into fire and explosion was only through experience and statistical analysis. Experts mainly focused on the lack of fire-fighting procedures and recommendations for its improvement. With the development of modern
Figure 49.1 The Piper Alpha accident.
Marine Structural Design. http://dx.doi.org/10.1016/B978-0-08-099997-5.00049-6 Copyright © 2016 Elsevier Ltd. All rights reserved.
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908 Chapter 49
Figure 49.2 The Deepwater Horizon accident.
methods of quantitative analysis, more and more studies are done using modern computer technology. A lot of effort now has been put into the prediction and controlling of explosions and fires in offshore installations. Risk-based approaches, rather than traditional prescriptive approaches, have begun to be more extensively applied in offshore designs.
49.2 Accident Causation Analysis Analyzing the cause of accidents is an important part of risk management. Usually, the accident risk that exists in the system will be found and its characteristics will be discovered and identified by inspecting and analyzing the system. According to the characteristics of hazardous energy, the source of the first hazard is defined as hazardous substances, which could be released accidentally into the system; the source of secondary hazard is referred to as all kinds of unsafe factors that exhaust measures to keep energy stable, such as loading and unloading, storage of goods, and hot work. The first kind of hazard is energy, which can cause the accidents and determine the severity of accident consequences. The source of the secondary hazard is a necessary condition to decide whether the accident occurs and affects the feasibility of the accident. The cause of an offshore oil platform accident is shown in Figure 49.3. Based on this theory, the main cause of fire and explosion accidents is shown in Figure 49.4. As a matter of fact, most of the reasons for fire accidents is their relationship with human factors. The International Maritime Organization (IMO) found that almost 80% of marine accidents occurred because of human error. The results further illustrate human action is an important factor affecting the safety of the whole system.
Explosion and Fire Response Analysis for FPSO
Figure 49.3 The cause analysis of marine accidents.
Figure 49.4 The main cause analysis of fire and explosion.
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910 Chapter 49
49.2.1 Formal Safety Assessment Formal safety assessment (FSA) is a kind of integration and systematic analysis method. Its purpose is to make all aspects of the specification, design, operation, and inspection effective and to improve safety at sea comprehensively and reasonably. This includes the protection of life and health, the marine environment, and property. A summary of this method is available as follow. Phase I, Identification of dangerous sources: Its purpose is to evaluate the system to identify all the possible dangers and find out the cause and consequences of the possible accidents, then make a list in order of the degree of dangers to further analyze major risks and put forward a corresponding control scheme. Phase II, Risk assessment and management: In this phase the procedures are used to evaluate the risk of all kinds of danger: find the distribution of risk and the overall level of risk, focus on the high risk area and the main factors affecting the level of risk, and sort the risk in an acceptable level. Phase III, Risk restraining project: The effective measures are put forward to reduce risk on the basis of hazard identification and risk assessment. Phase IV, Cost and benefit evaluation: Calculate the costs of each risk control measure and the benefit from reduced risk degree. Phase V, Suggestions and decision: Select the optimal risk control measure.
49.3 Phase I: Identification of Dangerous Sources There are many methods to identify the dangerous sources. For example, the expert investigation method (EIM), preliminary hazard analysis (PHA), fault tree analysis (FTA), event tree analysis (ETA), failure mode and effect analysis, and hazard and operability. 1. Expert investigation method The main task of the investigation is to find all kinds of potential dangers and measure the consequences. Many dangers are difficult to confirm with statistical methods and causal reasoning in a short period of time in marine structure engineering. For example, there are relatively few available statistical data for the fire and explosion accidents on the floating production storage and offloading (FPSO). It is obvious that EIM is prevalent to identify the danger sources. The two most commonly used methods are brainstorming and Delphi. a. Brainstorming Brainstorming can also be called playing imagination. It includes two stages: Stage 1: The organizer reviews the criterion of imagination and the purpose of the meeting, and then team members express their opinions fully and put forward a series of ideas.
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2.
3.
4.
5.
911
Stage 2: The ideas are then analyzed comprehensively. It can neither be despised nor accepted blindly. This method can be used for selecting risk identification and risk controlling measures. b. Delphi The Delphi method is a typical method of risk identification task directed by the famous American consultancy the Rand Corporation. This method is widely used in the decision-making process. Preliminary hazard analysis PHA is a qualitative analysis method to evaluate the internal risk factors and the degree of risk in the system. It can be the previous step to the FTA. Fault tree analysis FTA is one of the most important analysis methods of risk and security systems engineering. FTA is a longitudinal analysis method, a graphic deductive method from the top to the basic events. It can be used for qualitative evaluation and calculating the degree of system failure probability. One top event should be designated. The top event is the accident that is not expected to happen. During this period, construction of the fault tree is core. Event tree analysis ETA is a kind of inductive method. Contrary to FTA, ETA starts with the basic event, the probability of unexpected events can be roughly calculated, and finally, the top event will be found. Usually ETA can be combined with FTA, and work together to complete the analysis. Failure mode and effects analysis Each element is investigated to detect potential various failure modes in the subsystems or components. Then precautionary measures will be put forward.
49.3.1 The Structure Function of Fault Tree 1. Basic conception Assuming FTA is made up of n different kinds of independent events, a binary random xi variable equal to the state of the ith bottom events ei is defined to be: 1 ei occur xi ¼ i ¼ 1; 2; ..n (49.1) 0 ei not occur Binary random variable f expresses the status of the top event T: 1 T occur f¼ 0 T not occur
(49.2)
912 Chapter 49 The state variables of the top event are completely determined by the bottom event state variable values, as the top event state is completely determined by the status of the basic events. Define f to be the function of X ¼ (x1, x2,., xn), and assume the equation is as follows: f ¼ fðxÞ
(49.3)
f(x) is called the structure function of the fault tree. 2. Correlation function a. Correlation of the bottom event: If xi satisfies Eqn (49.4): fð1i ; xÞ 6¼ fð0i ; xÞ
(49.4)
then it can be said that the bottom event ei has an effect on the structure function. b. The correlation function: If the structure function f(x) meets the following properties, then f(x) is called the correlation function. e Each variable value xi (i ¼ 1, 2, ., n) has an effect on the structure function. e f(x) is correlated in terms of xi (i ¼ 1, 2, ., n) and is not diminishing. In the logic operation, the structure function of the fault tree composed by logic gates always accords with (b). The structure function is called correlation function if it coincides with property (a). Coherent structure function has the following properties: e f(0) ¼ 0; e f(1) ¼ 1; e Given state vectors X and Y, if X Y, namely xi yi, then f(x) f(y); e Suppose f(x) is a structure function composed with n independent events, then the equation is set up as follow: n
n
i¼1
i¼1
X xi fðxÞ W xi
(49.5)
c. Indicates that the fault tree concerning the state of the system is between the same kind of units in the series system and parallel system composed of the same unit. Transform the equation as follows: fðxÞ ¼ xi fð1i ; xÞ þ ð1 xi Þfð0i ; xÞ
(49.6)
In terms of the fault tree composed of n independent events, its structure function can be expanded n times. So the basic form of the equation is as follows: fðxÞ ¼
n X Y xyi i ð1 xi Þ1yi fðyÞ Y i¼1
(49.7)
Explosion and Fire Response Analysis for FPSO where xi is 1 or 0, and
P
913
Y indicates summation of the state vector valued Y.
Figure 49.5 is as an example of a fault tree that explains Eqn (49.5); there are five bottom events, Y ¼ (y1, y2, y3, y4, y5). All 32(25) possible states of the fault tree are presented in Table 49.1; once f(y) is calculated, f(x) is obtained. From Eqn (49.7), the following equation is found: 4ðxÞ ¼ ð1 x1 Þð1 x2 Þx3 x4 ð1 x5 Þ þ ð1 x1 Þð1 x2 Þx3 x4 x5 þ ð1 x1 Þx2 ð1 x3 Þx4 x5 þ // þ x1 x2 x3 x4 ð1 x5 Þ þ x1 x2 x3 x4 x5 (49.8)
Ga a Gc
Gb b
c
x4
Ge
x1
Gd d
e
x3
Gf
x3
x5
f x2
x5
Figure 49.5 A typical example of a fault tree. Table 49.1: Dimensions and geometric properties of the frames Symbol hw (mm) tw (mm) bf (mm) tf (mm)
Main Frame 700 13 300 24
Secondary Frame 300 10 300 15
914 Chapter 49
49.4 Phase II: Risk Assessment and Management Fires and explosions are continuous threats on offshore oil and gas installations. The dominant fire and explosion events are associated with hydrocarbon leaks from flanges, valves, equipment seals, nozzles, etc. Fires and explosions both result from combustion associated with hydrocarbon gas leaks. Figure 49.6 shows a sample calculation of the frequency of leaks using ETA. The methods mentioned above are often unable to identify frequency of gas leaks and ignition probability properly. More refined methods for their calculation are thus required on the basis of simulations. The EFEF JIP (explosion and fire engineering of FPSO units) has developed more refined methods to calculate the frequency of fires and explosions. Jeom Kee Paik is the leader of the ongoing 27th Joint Industry Project on the Explosion and Fire Engineering of FPSO Units (EFEF JIP). The aim of the EFEF JIP is to develop state-of-the-art technologies for the quantitative assessment and management of the risk of hydrocarbon explosions and fires in offshore installations. A framework for the quantitative assessment and management of the risks associated with fires and gas explosion requires the identification of both the frequency and consequences of these incidents.
Figure 49.6 Explosion event tree analysis for a leak event.
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49.4.1 Procedure for Fire Risk Assessment and Management Figure 49.7 presents the EFEF JIP procedure for the fire risk assessment and management of offshore installations.
Figure 49.7 EFEF JIP procedure for the fire risk assessment and management of offshore installations.
916 Chapter 49 Risk is defined as a product of frequency and consequence. Thus, the main purpose is to accurately calculate the frequency and consequences of specific events within the framework of risk assessment and management. To estimate the risk level of a structure, the identification of danger source and action effects of fire are vital. Each of the fire scenarios can be simulated by computational fluid dynamics (CFD). This establishes and characterizes the fire load profiles based on time and space in terms of temperature and heat amount. The CFD modeling techniques employed are significantly essential to the accuracy of these CFD simulations. There are eight random variables to formulate fire scenarios via sampling techniques: • • • • • • • •
Wind direction (X1) Wind speed (X2) Leak rate (X3) Leak duration (X4) Leak direction (X5) Leak position in the x direction (X6) Leak position in the y direction (X7) Leak position in the z direction (X8)
The design fire loads can be decided by the fire load profile with respect to time, temperature, and heat dose and converted to software using finite element analysis, that is, ANSYS, ABAQUS, to realize the non-linear structural response. The properties of the fire resistance of steel are the main factors affecting the structural integrity of fire. A noncontinuous segment plot based on the definition of Eurocode (Franssen and Real, 2010) is shown in Figure 49.8. When it is at 400 C the mechanical properties of steel significantly decrease. On the other hand, the heat from fire flows into steel, which is a good conductor compared to other materials. Thus, fire can lead to the collapse of steel structures. The frequency of fire is the likelihood of accidents, leaks, and ignitions. The frequency of leaks and ignitions can be obtained by ETA. Finally, the risk is calculated. If the calculated risk level is greater than the acceptable risk level, then the system must be redesigned, or such other control measures must be taken such as a fire wall, passive fire protection, or deluge/water spray. Acceptable risk level is normally defined in terms of the probability of damage exceeding the main safety functions or probability of accident escalation.
49.4.2 Procedure for Explosion Risk Assessment and Management In general, the response assessment methods include three main levels of analysis: screening check, strength level analysis (SLA), and ductility level analysis (DLA).
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Figure 49.8 Mechanical properties of steel with temperature as the plot of Eurocode’s definition.
The screening check checks for the safety and reliability of the whole structure. The SLA is used to estimate the failure of most topside components during early detailed engineering as a linear elastic analysis. However, the demand for the DLA from ship owners has increased recently rather than the preference for the SLA for most of the topside modules of FPSO/FLNG (floating liquefied natural gas) offshore facilities. Figure 49.9 presents the practical approach widely used in the offshore industry based on API RP 2FB. Kim et al. (2014) present the DLA methodology for the topside modules under blast loads in terms of the offshore industry calculation method. Explosions create pressure waves and the energy releases take place in a very short period creating a shock front in which a peak pressure occurs. Following this, the overpressure drops very rapidly and reaches a negative phase. The typical simplified pressureetime curve is shown in Figure 49.10. In Kim et al. study, a two-time step process was applied to simulate the dynamic response for blast load by using ABAQUS software. The first step is a static analysis for self-weight stabilization, and the next step is a dynamic analysis for actual structural response under the blast loading. The purpose of the DLA method is to verify the nonlinear dynamic structural behavior under the dynamic load (blast loading). A certain amount of resonance may occur in the vibrating response of the topside structures due to both the maximum positive and
918 Chapter 49
Figure 49.9 Flow chart of structural assessment against blast.
negative deflections and forces with accelerations of each structural component. It is therefore of importance to evaluate the appropriate vibration mode shape, depending on the specific relation between the natural frequency of the structure itself and triangular impulsive loads with the blast duration. Obviously, the natural frequency is of major importance to the dynamic analysis and it can be found by modal analysis. The strain and stress of all members can be obtained during the dynamic analysis. To ensure the structure is safe all of the members should not exceed the correct plastic strain. Once the DLA cannot satisfy the criteria of assessment, some blast mitigation should be carried out. There are two methods to minimize the damage for explosion accidents. One is to decrease the frequency of accidents in gas release, formation, and ignition of an explosive cloud. The other is to reduce the consequence by installing a safety system.
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Figure 49.10 Simplified pressureetime curve.
Corrugated and flat-plate type blast walls are generally used for the purposes of reducing the explosion consequences.
49.5 Phase III: Risk Restraining Project In terms of fire prevention and control of offshore platforms, strict precautions should be formulated to avoid fire and explosion caused by hydrocarbon combustion. The specific measures are described below. 1. Improve the layout The platform’s location should avoid an earthquake-prone belt and lightning-vulnerable areas, and fuel storage areas should be set in a well-ventilated downwind position and kept at a maximum distance from sources of ignition. In the process of building a layout for the firing of containers and work over well completion and temporary facilities, special measures should be raised. A reasonable arrangement of a firewall will help prevent flames from spreading and provide a heat insulation barrier. Meanwhile, the firewall should avoid adverse effects of large amounts of hydrocarbon steam and combustible gas gathering. 2. Team management For most instances of accidents, human error is the main factor. Workers manage the main body and object management, education of staff, and safety training. The education process, stricter accordance with operating instructions, and not overloading operations should be strengthened in offshore productions. Strengthening the platform patrol inspection work will guarantee a better flow of work.
920 Chapter 49 3. Management of equipment Offshore equipment suffers from perennial water and wind erosion. Therefore, strengthening the maintenance of equipment is necessary. There is a special process that forbids hot maintenance. It is necessary to periodically blow down parts of the equipment to prevent fires and explosions caused by high pressure. Natural ventilation components shall be equipped with a spark and fire detector to prevent sparks. 4. Strengthen the inspection and maintenance of lightning protection and electric bonding facilities Offshore platforms are flammable and explosive areas. Oil and gas production operation should be stopped during a thunderstorm. In order to avoid fire accidents caused by static electricity, certain electrostatic protection devices are necessary. Electrostatic discharge facilities should be in good condition and connection parts fixed firmly. Operators should dress according to the rules and not wear chemical fiber clothes. A human body electrostatic touch release facility should be set up before workers take up their quarters. 5. Strengthen the inspection and maintenance of electrical facilities Electrical safety is a comprehensive technology, requiring both engineering and organization. It includes insulation protection, barrier protection, safety distance protection, grounding lightning protection, leakage protection, automatic control equipment, etc. Offshore equipment ages quickly and erodes due to long exposure, so it is important to strengthen the maintenance inspection daily so as to prevent electrical components aging and short circuiting. 6. Hot surface protection Structure surfaces with abnormally high temperatures should not come in contact with liquid hydrocarbon, oil, and flammable gases. 7. Security system and fire control facilities A platform security system should be put in place to detect abnormal occurrences and prevent accidental fires. A combustible gas detector should be installed to detect the concentration of combustible gases. When the concentration reaches the explosion limit, it should provide a warning and truncate the source of the fire. In order to prevent the spread of fire, a fixed or semifixed type of foam fire extinguisher is needed. Finally, increase the staff’s education of extinguishing equipment use and their ability to make an immediate response.
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49.6 Examples of Explosion Response of FPSO 49.6.1 Introduction FPSO has become the mainstream of the development of offshore oil gas production field because of the advantages of transferable and reusable. Research into the safety and reliability of FPSO is of great significance. The authors conducted a quantitative risk assessment of combustible gas under explosion hazards. We made a simulation of the leakage and explosion process of combustible gas in FPSO by FLACS CFD and made an analysis of the explosion load characteristics. Then we imported data of the simulation into the ANSYS/LS-DYNA computations to make an analysis of structural response of the offshore platform. The contribution of this study is demonstrated with an applied example using a hypothetical topside structure of an FPSO that is exposed to hydrocarbon explosions. We then presented a procedure for the non-linear structural response analysis of offshore installations with a focus on explosions.
49.6.2 Gas Dispersion CFD Simulations The FLACS code, which is a three-dimensional transient finite volume CFD program, is used to simulate gas dispersion and explosion events. The commercial version of the FLACS code provides the results at a limited number of monitoring points and/or panels. To demonstrate the applicability of the FLACS code in the simulations of gas dispersion, an example of an explosion analysis in an offshore module is considered. Figure 49.11 shows the layout and principal dimensions of this hypothetical topside module of the FPSO. The gas dispersion simulation is performed to characterize the gas cloud size, which is affected by various factors including leak rates, duration times, positions, and wind conditions. Gas Dispersion Scenario Gas dispersion is as follows: • • • • • • • •
Leak rate: 4 kg/s Leak location: (6,6.75,2) Start time: 0 s Leak duration: 40 s Temperature: 20 C Gas composition: methane 91%, ethane 7%, propane 2% Wind speed: 3 m/s Wind direction: þX
922 Chapter 49
Figure 49.11 Layout and principal dimensions of the hypothetical FPSO topside module, (a) side view, (b) ichnography.
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The Spatial Distribution of Gas Concentration
Figure 49.12 The spatial distribution of gas concentration at t ¼ 40 s, (a) 3d view, (b) y ¼ 6.5 m.
Actual Gas Cloud and Equivalent Gas Cloud The cloud shown in Figure 49.12 is the actual gas cloud. The equivalent gas cloud volume is defined when the equivalent ratio defined in Eqn (49.9) equals 1. ER ¼
ðmfuel =moxygen Þactual ðmfuel =moxygen Þstoichiometric
(49.9)
924 Chapter 49 where mfuel and moxygen are mass of gas and oxygen in actual or stoichiometric conditions, respectively. Effect of Leak Rates In the simulation process of flammable gas leakage, we set the speed as 1, 2, 3, and 4 kg/s, respectively. The volume of actual gas cloud and equivalent is showed in Figure 49.13.
49.6.3 Gas Explosion CFD Simulation A flammable gas explosion is influenced by many factors, such as leak rates, leak location, combustion source location, and so on. Gas Explosion Scenario The gas dispersion is as follows: • • • • • • • • • •
Leak rate: 4 kg/s Leak location: (6,6.75,2) Start time: 0 s Leak duration: 40 s Temperature: 20 C Gas composition: methane 91%, ethane 7%, propane 2% Wind speed: 3 m/s Wind direction: þX Combustion source location: (8,6.75,3) Ignition time: 40 s
Figure 49.13 The effect of leak rates.
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Figure 49.14 shows the allocation of 128 monitoring points to read overpressures (P) and combustion product mass fraction (Prod) on the positions of interest. Figure 49.15 shows the distribution of combustion product mass fraction (Prod) at 40 s and Figure 49.16 shows overpressures (P) at monitor point 28.
Figure 49.14 The allocation of 128 monitoring points, (a) monitoring points on mezzanine deck and process deck, (b) monitoring points on main column.
926 Chapter 49
Figure 49.15 The distribution of combustion product mass fraction (Prod) at 40 s, (a) three dimensional space, (b) y ¼ 6.5 m 3d profile, (c) y ¼ 6.5 m 2d profile.
49.6.4 Nonlinear Structural Response Analysis The structural response analysis is undertaken for a situation in which the topside structures are subjected to explosion loads. Structure Model Figure 49.17 shows the model of a target structure with SHELL163 elements. The structures are made of mild steel and the material property is presumed to be plastic
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Figure 49.16 The overpressure at monitor point 28.
Figure 49.17 LS-DYNA structure model.
kinematic. The ends of all eight columns are set to be fixed and all other boundaries are in a free condition. The Distribution of Structure Stress, Displacement, and Strain Each monitoring of the overpressure curve is loaded into the structure, and we get the following platform structure displacement and stress distribution. Figure 49.18, Figure 49.19, and Figure 49.20 show the von Mises stress distribution, displacement distribution, and strain distribution, respectively, at 0.12 s. The Distribution of Displacement on Main Columns We select the four elements on the column as shown in the Figure 49.21 and the overpressureetime curve is showed in Figure 49.22. The results show that the overpressures on the upper and bottom of the main column are higher compared with the overpressures on the middle.
928 Chapter 49
Figure 49.18 The von Mises stress distribution at 0.12 s.
Figure 49.19 The displacement stress distribution at 0.12 s.
The Displacements on the Midpoint of the Main Girder For the whole deformation of the upper structure, it is vital to examine the deformation of the main girder. The displacements on the midpoint of main girder can effectively reflect the deflection of the frame.
Explosion and Fire Response Analysis for FPSO
Figure 49.20 The strain distribution at 0.12 s.
Figure 49.21 The location of four elements.
929
930 Chapter 49
Figure 49.22 Overpressureetime curve.
The Deflection of the Frame The location of the four midpoints of the main girder is shown in Figure 49.23 and the displacementetime curve is shown in Figure 49.24. The displacement of the C point has the closest distance from the explosion sources reaching a maximum at t ¼ 0.39 s with a value equal to 0.41 mm. The displacement of the C point is about 1.7 times the displacement of the D point.
Figure 49.23 The location of the four midpoints of the main girder.
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Figure 49.24 The displacementetime curve.
49.7 Example of Fire Response of FPSO 49.7.1 Fire CFD Simulation Fire Scenario The object of the fire CFD simulation is to simulate the gas cloud dispersion, gas cloud temperature, and heat fluxes that are time and space dependent. The fire load is correlated to the elevated temperatures obtained from the fire CFD simulation. One of the commonly adopted tools for fire CFD simulations is the fire dynamic simulation (FDS), which is a fire dynamic simulator, a CFD model of fire-driven fluid flow. FDS solves numerically a form of the NaviereStokes equations appropriate for low-speed, thermally driven flow with an emphasis on smoke and heat transport from fires. The fire example we simulate here is a pool fire, which is caused by combustible liquid burning on the surface. The following fire scenario was selected in the fire CFD simulation: Density of heat flow ¼ 24,000 kW/m2 Leak area ¼ 1.5 1.5 m Leak direction ¼ þZ Leak position in the X direction ¼ 10.75 m Leak position in the Y direction ¼ 6.5 m FDS Structure Model Figure 49.25 shows the layout and principal dimensions of a hypothetical topside module of the FPSO. All decks are supported by strong I-girders. But here in FDS, the decks are
932 Chapter 49
Figure 49.25 Layout and principal dimensions of the hypothetical topside module of the FPSO.
simplified as plates with different thicknesses. The process deck and mezzanine deck are 700 mm thick and the upper deck is 300 mm thick. The columns are also simplified as square columns because no round surface can be built in FDS. Figure 49.26 and Tables 49.1 and 49.2 present the geometric topology of the deck beams and columns. Using FDS, monitoring points should be reasonably assigned. Each frame has a monitoring point in the middle. Part of the points can be seen in Figure 49.27.
Figure 49.26 Topology of decks (red lines (dark grey in print versions) indicate main frames and dotted lines indicate secondary frames), (a) process and mezzanine deck, (b) upper deck. Table 49.2: Dimensions and geometric properties of the columns Symbol rinner (mm) router (mm)
Column 160 178
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Figure 49.27 Monitoring points.
FDS Results Smokeview is a separate visualization program that is used to display the results of an FDS. Figure 49.28 displays the heat flux at time 3.6 and 600 s. Figure 49.29 displays the temperature distribution at cross-section Y ¼ 7.6 m at time 10 and 600 s. Figure 49.30 displays the temperature distribution at cross-section X ¼ 9.8 m at times 10 and 600 s. Figure 49.31 displays the temperature distribution at cross-section Z ¼ 5.7 m at times 10 and 600 s.
Figure 49.28 Heat flux at different simulating times.
934 Chapter 49
Figure 49.29 Temperature distribution at cross-section Y ¼ 7.6 m at different times.
Figure 49.30 Temperature distribution at cross-section X ¼ 9.8 m at different times.
Figure 49.31 Temperature distribution at cross-section Z ¼ 5.7 m at different times.
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49.7.2 ANASYS Analysis Temperature Simulation The result of the ANASYS temperature simulation using SHELL131 is shown in Figure 49.32. The maximum temperature is 590.55 C. The mezzanine deck is influenced mostly while the process deck remains almost the same when there is no fire. Structure Analysis Take Q235 steel as an example. The elasticity modulus and yield stress of Q235 change with temperature in Table 49.3.
Figure 49.32 Temperature simulation in ANASYS. Table 49.3: The elasticity modulus and yield stress of Q235 Elasticity Modulus
Temp ( C) 16 100 200 300 400 500 600
Yield Stress
Ton-module
T Temp/t [ 16 C T Temp (MPa) T Temp/t [ 16 C T Temp (MPa) T Temp (MPa) 1.000 1.000 0.959 0.900 0.831 0.621 0.171
206,000 206,000 197,554 185,400 171,186 127,926 35,226
1.000 1.000 0.823 0.629 0.498 0.402 0.204
235 235 193 148 117 94 48
707 707 1800 1831 994 478 158
936 Chapter 49 Add the temperature load to the structure and uniform pressure to one of the main frames. Figure 49.33 shows all loads and restrictions on structure. Results Structure deflection in the Z direction is shown in Figure 49.34. The biggest deflection is 358 mm.
Figure 49.33 All loads and restrictions on structure.
Figure 49.34 Structure deflection in the Z direction.
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937
Figure 49.35 von Mises stress distribution.
The von Mises stress distribution is shown in Figure 49.35. The biggest stress is 247 MPa. It takes place in the red area in Figure 49.35.
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