Determination of characteristic accidental actions by Jørgen Amdahl Largely based on material developed by Torgeir Moan Oslo, November 2009 1 Outline General Risk Analysis Explosion actions Fire actions Ship collision Dropped Object Miscellaneous 2 1
I.4 Relevant Accidental Actions and their Measure of Magnitude 3 1Explosion loads (pressure, duration - impulse) scenarios explosion mechanics probabilistic issues characteristic loads for design 2 Fire loads (thermal action, duration, size) 3 Ship impact loads (impact energy, -geometry) 4 Dropped objects 5 Accidental ballast 6 Unintended pressure 7 Abnormal Environmental loads 8 Environmental loads on platform in abnormal floating position ALS relating to structural strength (NPD,1984) A P, F Estimate the damage due to accidental loads (A) at an annual probability of 10-4 - apply risk analysis to establish design accidental loads P, F Fault tree Critical event Event tree End events Accidental load 4 E Survival check of the damaged structure as a whole, considering P, F and environmental loads ( E ) at a probability of 10-2 Target annual probability of total loss: 10-5 for each type of hazard 2
Risk analysis methodology End events 5 Causual chain Methods -Fault tree -Influence diagrams. Accidental event -Prelim. hazard analysis -HAZOP -FMECA.. Reliability methodology: Consequence chain -Event tree -Evacuation models -Simulation - classical reliability (on/off components, system analysis) - structural reliability - human reliability (data for simple operations; effect of training) Consequences Basis for Risk Analysis 6 Estimate actions with annual probability of occurrence: 10-4 Empirical data - 23 000 years of experience required for 90% chance of one event Accumulated no. of platform years world wide: - fixed platforms: ~ 150 000 - mobile units: ~ 15 000 - FPSO: ~ 1 500 Theoretical analysis (Fault tree/event tree) The basis for risk analysis is the facts that : a) almost every major accidental event has originated from a small fault and gradually developed through a long sequence or several parallel sequences of increasingly more serious events, culminating in the final event, b) it is often reasonably well known how a system will respond to a certain event. By combining knowledge about system buildup with knowledge about failure rates for the elements of the system, it is possible to achieve an indication of the risks in the system (Vinnem, 1999; Moan, 2000b). 3
Risk Analysis Process The risk analysis process normally consists in the following steps: - define and describe the system - identify hazards - analyse possible causal event of hazards - determine the influence of the environmental conditions - determine the influence of active/passive safety systems (capacity; reliability, accident action integrity, maintenance system..) - estimate event probabilities/event magnitudes - estimate the risk 7 Risk Risk Analysis Planning Risk Risk Acceptance Criteria System Definition Hazard Identification Risk Risk Reducing Measures Frequency Analysis RISK RISK ESTIMATION Consequence Analysis Risk Risk Picture RISK RISK ANALYSIS Risk Risk Evaluation Tolerable Unacceptable 8 Acceptable 4
Application of Risk Analysis Risk estimate risk = probability times consequences compare estimated risk with acceptable risk Risk mitigation implementation of risk-reduction actions (to comply with acceptable risk or ALARP) e.g. carry out ALS design check. See Section III.2. estimate accidental actions for design Revise risk estimate 9 IV.3 Risk mitigation Probability reducing measures for hazardous situations escalation of hazardous situations Consequence reducing measures design of load bearing structure passive measures; fenders, fire protection active measures relating to safety and support systems measures relating to contingency equipment and organization 10 5
Implied Risk of a given Accidental Action, A(i) Probability of total failure: under relevant F&E actions n ( i) P ( i ) P[ FSYS A ] [ FSYS ( A ) j P A j 1 ( i) j ] i = -Explosion -Fire -Ship impact A j (i) - accidental load (or, corresponding damage) relating to component (j) Further interpretation of the calculation of the 10-4 action - later 11 V.1-2. Explosions & Fires Explosion is a process where combustion of premixed gas cloud is causing rapid increase of pressure Fires is a slower combustion process No No Ignition Ignition Release Release of of Gas Gas and/or and/or Liquid Liquid Immediate Immediate Ignition Ignition Formation Formation of of Combustible Combustible Fuel-Air Fuel-Air Cloud Cloud (Pre-mixed) (Pre-mixed) Fire Fire Ignition Ignition (delayed) (delayed) Gas Gas Explosion Explosion No No damage damage Damage Damage to to Personnel Personnel and and Material Material Fire Fire 12 Implication of simultanous occurence of explosion and fire: Explosion can occur first and damage the fire protection before the fire occurs Fire Fire and and BLEVE BLEVE 6
13 Explosions Simulation of explosion pressure Explosion on offshore platforms Gas explosion, resulted from release of hydrocarbon General definition An explosion is defined as an event leading to a rapid increase of pressure. The pressure increase may arise from many different causes: Mist or gas (including vapors) in air or in other oxidizers Loss of containment in high pressure vessels Combustion of dust Run-away reactions Nuclear reactions 14 Explosion phenomenon Explosion testing Initiating event leak of hydrocarbons due to maloperation, failure of vents Pressure load (drag force) depends upon: type of gas and gas ratio ignition (location, intensity, ) venting (location, area, ) relief, water deluge geometry and size of the room Pressure determined by numerical methods (AutoReGas, FLACS) semi-empirical methods experiments 7
Types of explosions confined, partly confined, unconfined 15 Types of explosions 16 8
Explosion overpressure key issues Maximum pressure versus area Local design Global design Spatial distribution Rise time Duration Impulse 17 Historical explosion frequency Data: 1973-1997, 25 year, 34 explosions on offshore platforms in the North Sea UK sector (16) Norwegian sector (17) Dutch sector (1) Frequencies: Reference: Vinnem, J. E. (1999) 2.49E-03 1.08E-04 18 per explosion area year 9
Explosion load analysis Calculation of explosion load (explosion mechanics) semi-empirical; CHAOS numerical; FLACS, PROEXP, AutoReGas,... Probabilistic analysis probability of occurrence of scenario conditional distribution of pressure for a given type of scenario 19 Simple, semi-empirical procedure 20 10
Analysis based on analytical approach Release of hydrocarbons 21 Hydrocarbon contents Gas Oil Non-process 2-phase Condensate Source of release Flanges Valve Instrument taping Nozzles Source: 1801 HC release incidents, 1992-2000, HSE OTO2000 112 Release modes Jet release High flow velocity Strong flow field recirculation zone Evaporating pool Low flow velocity Wind, buoyancy dispersion In an open area a dense gas cloud may intrude into confined spaces, pose serious problem. West Vanguard Rig (1985) 22 11
Explosive cloud formation Dispersion pattern Dominated by the jet for medium and large jet releases Otherwise by ventilation Gas concentration level Depends on ventilation Volume fraction of fuel 23 Combustion process Chemical reaction CH4 2 O2 3.76N2 CO2 2H2O 2 3.76N2 ENERGY Pressure from combustion How fast the flame propagates Deflagration (1-500 m/s), peak pressure several bar Detonation (1000-2000 m/s), peak pressure 15-20 bar How the pressure can expand (Confinement) Positive feedback Turbulence Burning rate Flow velocity 24 12
Overpressure loads Result from increases in pressure due to expanding combustion products Drag loads Result from the passing flow of air, gases and combustion products Secondary loading Missiles Relative displacement and vibration Explosion loading 25 Analysis of explosion pressures Normally, most information is available from CFD analysis, FLACS etc. 26 13
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29 Estimation of explosion frequency Release of hydrocarbon Dispersion into explosive cloud Ignition P (explosion) = P(ignition dispersion, release) P(dispersion release) P(release) 30 15
Estimation of release frequency Calculate initial leak probability according to size categories 0.1-0.5; 0.5-1; 1-2; 2-4; 4-8; 8-16; 16-32; 32-64; >64 (NORSOK) Derive a representative transient leak profiles Duration, transient source strength Calculate release frequency for a representative segment Location of leak (at least 3 points in one module) Direction of leak (6 jet directions, possible diffuse leak) Model correct medium: evaporates from liquid, oil mist Calculations for all process segments 31 Estimation of dispersion frequency Calculate ventilation Wind directions should be considered with frequency and speed distributions from wind rose of the area. Simple relationship between gas concentration and the ratio between leak rate and ventilation rate may be established. Calculate equivalent stoichiometric cloud Dispersion simulation CFD models, 10-15 simulations per module Location of ignition sources until ignition should be considered 32 16
Ignition mechanism Upper and lower flammability limits [LFL, UFL] Temperature dependent Minimum ignition energy Fuel type & concentration Sufficient ignition strength (0.1-0.3 mj) Ignition sources Rotating equipment Electrical equipment Hot work 33 Estimation of ignition frequency 34 Identify ignition sources Type and location of ignition sources based on layout Relative position in possible gas clouds Exposure of ignition source to the cloud Based on dispersion simulations Conditional probability of ignition, given the exposure Continuous ignition sources Ignition probability is proportional with the volume being exposed to flammable gas. Intermittent sources P(t) = P exposure [V(t)]* P intermittent (t) Gas detection and actions that may influence probability of ignition and formation of a cloud should be taken into account. For Each Dispersion Scenario 17
Event tree for the event leakage of hydrocarbons from a process module, with explosion presssure as final event. Determination of design accidental load Idealized example Events omitted from further consideration Leakage of hydrocarbons End event: Overpressures Probabilities 1.8 2.0 0.3 0.4 0.7 1.0 0.2 0.1 0.05 0.01 0.1 0.05 0.3 0.2 0.2 0.1 0.4 0.3 10 50 Pressure: 0.01 0.05 0.1 0.2 0.3 0.4 0.7 1.0 1.8 2.0 Prob.( 10 4 ) 50 10 0.3 0.4 0.3 0.2 0.2 0.1 0.1 0.05 ( 10 4 ) 35 pressure corresponding to a probability: ~ 0.95 10-4 0.3 barg Pressure exceedance diagram for idealized example 2.5 2 Pressure 1.5 1 0.5 0 0 0.5 1 1.5 2 Probability of exceedance [ 10-4 ] 36 18
Typical Explosion Loads for Design Explosion scenario Process area Export riser area Wellbay area Structural component Overpressure (barg) Duration Impulse (kpa s) Dick girder (30%) 0.3-0.5 0.1 <1.4-2.0 Process roof 0.2 0.3 1;3 0.5 Central blast wall 0.3-0.7 0.2-0.4 1.5-2.5 Upper deck 0.2 0.3 1.7 Note: Overpressures are controlled by ventilation 37 Probabilistic Simulations FLACS PROBLAST Dispersion Analysis Gas leak location and direction Gas leak rate Environmental conditions Explosion Analysis Ignition location Gas cloud location and size Monte Carlo Simulation Probabilistic scenario definition Overpressure definition OVERPRESSURE EXCEEDANCE DATA 1.00E-02 1.00E-03 1.00E-04 Sto ichio metric max Inho mo g eneo us max Risk analysis Sto ichio metric averag e STRENGTH EXCEEDANCE DATA 1.00E-02 1.00E-05 1.00E-06 0.0 0.5 1.0 1.5 2.0 Pressure [bar] Probability of exceedance 1.00E-03 1.00E-04 1.00E-05 Stoichiometric max Stoichiometric average Acceptance 1.00E-06 0 50 100 150 200 38 Deflection [mm] 19
Estimation of explosion consequences Direct fatalities and injuries Structural damage Escalation fatalities 39 Structural and equipment damage structural damage Excessive deformations damage to fire protection, safety systems damage to process equipment, thereby causing escalation of accident 40 20
Survival analysis of platform deck structure suffering extreme explosion damage (Amdahl, 2003) P max > 5bar 41 Explosion risk reduction Reduce explosion frequency Reduce explosion consequence 42 21
Immediate causes of release All types of release degradation of material properties (26%) Corrosion/erosion (19%) Incorrect installation (11%) Operator error (11%) Major releases Operator error and failure to follow procedures (50%) Source: 270 HC release incidents, 2000-2001, HSE OTO2001 055 43 Reduction of explosion frequency Prevent hydrocarbon release Quality of maintenance work Material property Operator error and procedure Prevent formation of explosive cloud Natural vs. mechanical ventilation Reduce volume of hazardous areas Prevent ignition Reduce hot work Maintenance of electrical equipment 44 22
Reduction of consequences Prevent high turbulence and remove blockage Arrangement of equipment Provide deluge activation on gas detection Reduce overpressure Might be ignition source Install fire and blast barriers Improve structural strength Worse Better 45 Further interpretation of the calculation of the 10-4 action Characteristic value of each type (explosion, fire, ship, impact,..) of action should correspond to: an annual probability of 10-4 for the given structure, by gathering all types of explosion events in all locations Characteristic actions of a given type on different components of a given structure Establish an action exceedance diagram for each component Allocate a portion (p i ) of 10-4 probability to each area, and determine the Q c for each component corresponding to the probability, p i. Refined approach by considering the expression for the total loss probability 46 n ( i ) P ( i ) P [ FSYS A ] FSYS ( A ) j j 1 P [ A ( i ) j ] 23
More information in Engineering Handbook on Design of Offshore Facilities to Resist Gas Explosion Hazard Design of Offshore Facilities to Resist Gas Explosion Hazard Engineering Handbook Ed. Jerzy Czujko More information on: http://www.corrocean.com/explosionhandbook 47 V.2. Fire hazards Initiating event leakage of hydrocarbons (rate,...) caused by: maloperation of equipment (vents,..); failure of vents, pipes,... other combustibles Factors of influence type of burnable material (oil, gas,..others) gas mixture ignition passive fore protection; water deluge,... ventilation Resulting load thermal action intensity (heat flux), duration and size Note: A large number of scenarios 48 24
Characteristics of active safety systems Gas/fire detection ESD/safety valve Type Location Detection logic Availability Test interval Type Location Sectionalisation Closing time Availability Fire protection 49 Deluge/sprinkler system Location of valve/deluge head Sectionalisation Availability Test interval Fires: types and scenarios according to source: blowout riser/pipeline failure process equipment failure -affects location and criticality scenarios ventilation vs. fuel controlled fire in enclosure pool fire in the open jet fire and fire balls fire in running liquid 50 25
Calculation of fire action (heat flux in time and space) Empirical, simplified methods FIREX Numerical methods for calculating: Combustion, radiation and convection: KAMELEON FIRE Probabilistic analysis probability of occurrence of scenario conditional distribution of fire action for a given type of scenario 51 Passive Fire Protection Design against accidental fires - Challenge The total surface of the structural components of a platform (potential exposed to fire) is ~50.000 m 2 (For comparison: Area of a football ground is ~7.000 m 2 ) If all surfaces should be protected, the total weight of the PFP would be 200-500 tonne. (corresponding costs of magnitude 100 mill NOK) Question: Do we need to protect all surfaces? 52 26
Fire and Explosion Design Against Accidental Fires often based on standard fires 1000 C HC (NPD) ISO Standard heat fluxes e.g. 100-250 kw/m2 Fire duration e.g. 10 min. 2hrs Exposed areas envelope curves 1 Hour Time Standard Temperature Curves Standard heat fluxes 53 Example traditional fire scenario: Pool fire from process inventories on cellar deck in 55 minutes 150 150 150 250 150 150 Main deck only 100 Snorre B Fire Event 4b Fire exposure [kw/m 2] 0-55 minutes Cellar deck and underside main deck 54 27
Typical Fire Loads for Design Fire scenario Process fire Riser fire Structural components Trusses in process module Heat action [kw/m 2 ] Duration Reference 300 15 min Gas leak from 1 st stage compressor, ESD fails Columns/MSB 200 2 hours* Oil fire from leak from export riser Column 300 15 min Large gas flame from leak in gas export riser Blowout Well bay trusses 300 2 hours* Ignited gas blowout at wellhead or BOP. Members engulfed by fire Columns 200/250 2 hours* Fire on sea fed by oil blowout (24m dia, 19 m length) Jet fire -1500 Temperature: 1500-1700 C Open pool fire 100-200 Temperature: 500-1200 C 55 Fire and Explosion Real fires different from standard fires Fire intensity depends upon amount of combustibles, fuel characteristics, environmental conditions etc. Heat exposure depends upon location with respect to fire, view of fire... Transient temperature development in the material is a highly nonlinear process, notably radiation The structure often possesses considerable reserve strength, i.e. load redistribution upon first member failure Advanced analysis tools are often required 56 28
Passive Fire Protection Jet Fire Real fire - example Rupture of a gas pipe, gas ignites. Pressure drops from 300 bar to 0 within 15min. Pressure [bar] 100 Temp [C] 1000 Thin wall (Sec. Steel) Thick walll (Main Steel) Pressure 30 Time [min] 30 Time [min] Typical temperatures of unprotected steel Question: Does this accidental fire cause structural collapse? 57 Analysis of floating production platform subjected to accidental fires - Scenarios Case 1 Case 2 Case 3 Case 5 Case 6 Case 6b Case 7 Case 8 Case 9 Case 10 Description Leakage from north side of HP knockout drum at lower deck. HC gas release, 3 kg/s for 20 min. Pool fire from oil release of 12 kg/s constant for 60 min. Wind: 7 m/s from south Pool fire from oil leakage at south side of HP knockout drum at lower deck. Constant release rate of 6 kg/s for 60 min. Wind: 2 m/s from south Ignited HC gas release from 2 nd stage HP separator at main deck, 24 kg/s for 20 min. Pool fire from oil release of 12 kg/s reduced to 3 kg/s after 60 min. Wind: 2 m/s south Ignited HC gas release, 24 kg/s constant for 20 minutes from 27VG001 on main deck. Release directed downward. Wind: 2 m/s from south. Ignited HC gas release directed downwards. Release rate is 24 kg/s and constant for 20 minutes at upper deck. Wind: 2 m/s from south As for Case 6 but the wind is 7 m/s from north HC gas release directed eastward at the riser balcony from production riser,release, 30 kg/s, constant for 5 min. thenlinearly reducing to 0 at 20 min. Wind: 2 m/s from east. HC gas release northwards at the riser balcony from the gas export riser. Initial release, 5 kg/s, constant for 5 min. before linearly reduced to 0 after 120 min. Wind: 2 m/s from east. Leakage from methanol tank at cellar deck. Constant release rate of 10 kg/s for 60 minutes. Pool located west of tank: Wind 2 m/s from south. Oil leakage, pool at elevation 60 beneath the flare tower.. Constant release rate, 16 kg/s, for one minute, linearly reduced to 0 at 18 min. Wind 2 m/s from south 58 29
Oil fires in process module and at sea with/w.o wind Kameleon FireEx - /NTNU/SINTEF/ComputIT/ A CFD based, 3D and transient gas dispersion and fire simulator 59 Response to fire on riser balcony - east wall unprotected 60 Case 7 HC gas release at the riser balcony from production riser. The initial release, 30 g/s, is constant for 5 min. then linearly reducing to 0 at 20 min. The release is directed eastwards Wind: 2 m/s from east. Due to high momentum of the release, good mixing and hence high temperatures are obtained over a large area. Fire is at its highest after 5 min. 30
Response to fire on riser balcony - east wall unprotected Temperatures after 8 and 16 minutes After 16 minutes the maximum temperature is 960 C, and the temperature is more uniformly distributed over both the height and width. Hence, the increase of the functional loads is performed after 16 minutes. 61 Fire on riser balcony - east wall protected Unprotected With PFP With PFP applied the maximum temperature is only 150 C; the fire lasts only for 20 minutes and the wall is allowed the emit heat on the unprotected inner side. With PFP on the outer side, the wall is more vulnerable to inside fires 62 31
Floating production platform Case 1:Leakage from north side of HP knockout drum at lower deck. HC gas release, 3 kg/s for 20 min. Pool fire from oil release of 12 kg/s constant for 60 min. Wind: 7 m/s from south CFD-simulation Temperature field Mechanical util. Trusswork needs protection 63 Passive fire protection - principles The degree of protection is selected on the background of the experience obtained for the completely unprotected structure with respect to temperatures and degradation of resistance. The protection is relatively poor and represents typical minimum thickness of epoxy product. The thermal insulation effect is expressed in terms of the generic Effective Heat Transfer Coefficient, set to 5 [W/m 2 K] Protection: Truss-work members are protected on all faces Middle fire wall T400 protected on the east side PFP applied on the east side of T600; adverse effect on the temperatures when heated on the west side from fires in the process area, positive effect for fires on the riser balcony on east side 64 32