Risk Acceptance and Risk Communication Stanford, March 26-27, 2007 Development of Accidental Collapse Limit State Criteria for Offshore Structures by Torgeir Moan Norwegian University of Science and Technology T.Moan.26.03.2007 1
Outline Introduction Accident experiences ULS: R C /γ R > γ D D C + γ L L C + γ E E C FLS: D=Σn i /N i allowable D Safety management at large in the offshore industry Critical event Accidental Collapse Limit State Concluding remarks T.Moan.26.03.2007 2
Introduction Oil and gas are dominant sources of energy; 20 % of the oil and gas is produced in the ocean environment - affects world economy Safety - oil and gas represents energy with large potential accident consequences - operating in a demanding environment - loss of reputation could also be an expensive consequence of accidents Regulatory requirements: - National Regulatory bodies; (MMS, HSE, PSA (NPD) - Industry : API, NORSOK, - Class societies/iacs -IMO/ISO/(CEN) T.Moan.26.03.2007 3
Introduction (continued) Safety with respect to - Fatalities - Environmental damage - Property damage associated with failuere modes such as: Capsizing/sinking Structural failure Mooring system failure Unavailability of Escapeways and Evacuation means (life boats.) T.Moan.26.03.2007 4
Accident Experiences Safety: absence of accidents or failures Fault tree Critical event - Fatalities - Environmental Event tree damage - Property damage Technical-physical point of view - Capsizing or total loss of structural integrity commonly develops in a sequence of events Human and organizational point of view - Codes - Design, fabrication and use of individual structures T.Moan.26.03.2007 5
Failure or accident due to natural hazards Technical-physical causes: Wave forces exceeded the structural resistance Human organizational factors: State of engineering practice (codes) Errors and omissions during the design (fabrication) phases Severe damage caused by hurricane Lilli in the Gulf of Mexico - relating to assessment of - wave conditions or load calculation - strength formulation - safety factors Should the platforms have been stengthened if improved state of the art knowledge became available later? T.Moan.26.03.2007 6
Accident experiences for offshore platforms Number of accidents per 1000 platform years T.Moan.26.03.2007 7
ULS: R C /γ R > γ D D C + γ L L C + γ E E C FLS: D=Σn i /N i allowable D P,F A Safety management - Leak - Dispersion - Ignition E P,F Design criteria - ULS, FLS (and ALS) QA/QC of the engineering (design) process QA/QC of the as-fabricated structure QA/QC during operation (inspection of structural resistance, monitoring of loads) Event control of accidental events Evacuation and Escape Direct design for damage tolerance (ALS) T.Moan.26.03.2007 8
Causes of structural failures and risk reduction measures Cause Less than adequate safety margin to cover normal inherent uncertainties. Gross error or omission during life cycle phase: - design (d) - fabrication (f) - operation (o) Unknown phenomena Risk Reduction Measure - Increase safety factors or margins in ULS, FLS; - Improve inspection of the structure (FLS) - Improve skills, competence, selfchecking (for life cycle phase: d, f, o) - QA/QC of engineering process (for d) - Direct ALS design with adequate damage condition (for f, o) - Inspection/repair of the structure (for f, o) - Research & Development Quantitative method Structural reliability analysis Quantitative risk analysis None T.Moan.26.03.2007 9
Safety management the ALS check Structures should be designed to ensure that small damages do not develop into disproportionately large consequences loss of overall structural integrity loss of buoyancy/stability Specified damage Alternative principles for design against accidental actions: - design the structure locally to resist the action - accept local damage and design the structure against progressive collapse (in the manner which is relevant in connection with abnormal strength) T.Moan.26.03.2007 10
Accidental Collapse Limit State relating to structural strength (NPD,1984, later NORSOK) A P, F Estimate the damage due to accidental event (damage, D or action, A) at an annual probability of 10-4 - apply risk analysis to establish design accidental loads P, F E Survival check of the damaged structure as a whole, considering P, F and environmental actions ( E ) at a probability of 10-2 Target annual probability of total loss: T.Moan.26.03.2007 11 10-5 for each type of hazard
Failure modes - Hazards (accidental actions) Instability: Hazards Structural damage on submerged parts, including explosion in column Unintended pressure or ballast/cargo distribution Structural failure: Hazards Fire/ Explosion Fire on sea Ship impact Dropped objects Mooring failure: Hazards - Hull, mooring, risers - Safety equipment (escape ways and evacuation means) T.Moan.26.03.2007 12
Estimating the Accidental Event Damage or accidental load with annual probability of occurrence: P = 10-4 Need homogeneous empirical data of the order 2/p = 20 000 years to estimate events by empirical approach Accumulated platform years world wide: - fixed platforms: ~ 180 000 - mobile units: ~ 20 000 - FPSO: ~ 2 000 Theory based on: - accidental events originate from a small fault and develop in a sequence of increasingly more serious events, culminating in the final event, - it is often reasonably well known how a system will respond to a certain event. T.Moan.26.03.2007 13 Account of all measures to reduce the probability and consequences of the hazards
Risk indicators for large scale accidents - monitoring of incidents (near-misses) Blow-out related incidents - uncotrolled hydrocarbon leaks - lack of well control Structure related incidents - structural damage, leak, collisions, loss of mooring line..) - ships on collision path, etc. Nonfunctioning barriers against large scale accidents - e.g lack of detection, deluge T.Moan.26.03.2007 14
Relevant Accidental Actions (Hazards) 1 Explosion actions (pressure, duration - impulse) scenarios explosion mechanics probabilistic issues characteristic loads for design 2 Fire loads (thermal action, duration, size) 3 Ship impact actions (impact energy, -geometry) 4 Dropped objects 5 Accidental ballast 6 Unintended pressure 7 Abnormal Environmental actions 8 Environmental actions on platform in abnormal position T.Moan.26.03.2007 15
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 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 T.Moan.26.03.2007 16
Probabilistic Simulations For example: explosion overpressure (given type, I of acidental action) (i) A jk FLACS PRO BLAST 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 Histogram of overpressure at each location (j) based upon different scenarios (k) 1.00E-03 1.00E-04 1.00E-05 Stoichiometric max Inhomogeneous max R isk analysis St o ichio met ric averag e 1.00E-06 0.0 0.5 1.0 1.5 2.0 Pressure [bar] T.Moan.26.03.2007 17
Abnormal strength (damage) - generic values for specific types of structures based on consideration of the vulnerability of the structural components. Examples: 1) slender braces in mobile drilling platforms (semi-submersibles) due to their vulnerability to ship impacts and fatigue. 2) Catenary mooring line 3) Tether in tension-leg platforms Abnormal degradation due to fatigue or corrosion T.Moan.26.03.2007 18
FE Analysis of damage and survival of the damaged platform Validation To be based on full scale experiments, laboratory tests For the use in a design project, the accuracy of predictions is to be identified. Laboratory experiments with corrugated panels PRESSURE [N/mm2] 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Experiment Analysis 0 0 20 40 60 80 100 DISPLACEMENT [mm] Courtesy: J. Czujko, 2001 Application of Methodology Topside structure on a jacket platform T.Moan.26.03.2007 19
Framework for Risk-based Design against Accidental actions i ) P ( i) = P[ FSYS D ] P[ D A ] FSYS j, k P[ A ( ( i ) jk jk ] For each type of accidental action probability of damaged system failure under relevant F&E actions probability of damage, D given A jk (i) (decreased by designing against large A j (i) ) probability of accidental action at location (j) and intensity (k) (i) P A jk is determined by risk analysis while the other probabilities are determined by structural reliability analysis. PFSYS [ D] Is determined by due consideration of relevant action and their correlation with the haazard causing the damage T.Moan.26.03.2007 20
Characteristic value of the accidental action at each relevant location -The characteristic value for each type of accidental action refers to a probability of exceedance of 10-4. for the platform as a whole. - In view of additive character of the P FSYS (i) [ ] (i) (i) P (i) = P FSYS D P D A P A FSYS jk jk jk - the exceedance probability level relevant for each location (k) is taken to be a portion of 10-4 for each location. Procedure for determining the characteristic accidental action (overpressure) on different components (j) of a given platform, can be determined as: - Establish exceedance diagram for the load on each component - Allocate a portion (pi) of 10-4 probability to each area, and determine the Qc for each component corresponding to the probability, pi - Determine the characteristic load for each component from the relevant load exceedance diagram and reference probability. T.Moan.26.03.2007 21
From Prescriptive to Goal-based to Prescriptive Approach - A goal-based approach is needed/practiced for new design concepts - (new function, layout, ) deepwater production platforms (spar, TLP, semi, FPSO) - Future challenges relating e.g. to arctic oil and gas operations require a Goal-based Approach - Design accidental actions tend to be more prescriptive when: experiences are accumulated after accidents T.Moan.26.03.2007 22
Typical Explosion Loads for Design Explosion scenario Process area Export riser area Wellbay area Significant experiences about Explosion Actions for design for commonly occuring cases Structural component Overpressure (barg) Duration Impulse (kpa s) Deck 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: goal-setting approach tends to become prescriptive T.Moan.26.03.2007 23
CONCLUSIONS The main cause of accidents of offshore structures is human and organizational errors and omissions resulting in - abnormal strength - accidental actions Besides introducing measures to limit occurrence of errors by QA/QC etc, a direct quantitative design for robustness has been introduced in NPD/NORSOK offshore codes The Accidental Collapse Limit state Design check introduced in NORSOK is a two step procedure estimate damage due to accidental actions with annual prob. of 10-4 check survival of damaged structure subjected to relevant functional and environmental actions -the necessary methods for structural analysis to determine loacal damage and system survival, have been developed T.Moan.26.03.2007 24