T.Moan.26.03.2007 1
Development of
Accidental Collapse Limit State Criteria for Offshore Structures
byTorgeir Moan
Norwegian University of Science and Technology
Risk Acceptance and RiskCommunicationStanford, March 26-27, 2007
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Outline
Critical event
• Introduction
• Accident experiences
• Safety management at large in theoffshore industry
• Accidental Collapse Limit State
• Concluding remarks
ULS:RC/γR > γDDC + γLLC + γEEC
FLS:D=Σni/Ni ≤ allowable D
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Oil and gas are dominant sources of energy; 20 % of the oil and gas is produced in the ocean environment
- affects ”world” economy
Introduction
Regulatory requirements: - National Regulatory bodies;
(MMS, HSE, PSA (NPD)- Industry : API, NORSOK,…- Class societies/IACS- IMO/ISO/(CEN)
Safety- oil and gas represents energy withlarge potential accidentconsequences
- operating in a demanding environment- loss of reputation could also be anexpensive consequence of accidents
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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….)
Introduction (continued)
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Technical-physicalpoint of view- Capsizing or total loss of
structural integritycommonly develops in a sequence of events
Human andorganizationalpoint of view- Codes- Design, fabrication and
use of individual structures
Accident Experiences
CriticaleventFault tree Event tree
- Fatalities- Environmental
damage- Property
damage
Safety: absence of accidents or failures
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Failure or accident due to ”natural hazards”
Severe damage caused by hurricane Lilli in the Gulf ofMexico
Technical-physical causes:Wave forces exceeded the structuralresistance
Human – organizational factors:State of engineering practice (codes)
Errors and omissions during the design(fabrication) phases
- relating to assessment of- wave conditions or load calculation- strength formulation- safety factors
Should the platforms have been stengthenedif improved state of the art knowledge becameavailable later ?
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Accident experiences for offshore platforms
Num
ber o
f acc
iden
ts p
er
1000
pla
tform
yea
rs
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Safety management• 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 structuralresistance , monitoring ofloads)
• Event control ofaccidental events
• Evacuation and Escape
• Direct design for damage tolerance (ALS)
ULS:RC/γR > γDDC + γLLC + γEECFLS:D=Σni/Ni ≤ allowable D
P,F P,FA
E
- Leak- Dispersion- Ignition
- Leak- Dispersion- Ignition
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Causes of structural failures and risk reduction measures
None- Research & DevelopmentUnknown phenomena
Quantitative riskanalysis
- Improve skills, competence, self-checking (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)
Gross error or omissionduring life cycle phase:- design (d)- fabrication (f)- operation (o)
Structural reliabilityanalysis
- Increase safety factors or marginsin ULS, FLS;
- Improve inspection of the structure (FLS)
Less than adequate safety margin to cover “normal”inherent uncertainties.
Quantitative method
Risk Reduction MeasureCause
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Safety management – the ALS check
• 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)
• 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
Specifieddamage
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Accidental Collapse Limit State relating to structural strength (NPD,1984, later NORSOK)
• 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
• 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: 10-5 for each type of hazard
P, F
A
P, F
A
P, FP, F
E
A
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Failure modes - Hazards (accidental actions)Instability:
Hazards
Structuraldamage onsubmergedparts, includingexplosion in column
Unintended pressure or ballast/cargo distribution
Structural failure:HazardsFire/Explosion
Fire on sea
Ship impact
Dropped objects
Mooring failure:Hazards
- Hull, mooring, risers- Safety equipment(escape ways and evacuation means)
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Estimating the Accidental Event
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.
Damage or accidental load with annual probability of occurrence: P = 10-4
Need homogeneous empirical data of the order 2/p = 20 000 yearsto estimate events by empirical approach Accumulated platform years world wide:- fixed platforms: ~ 180 000- mobile units: ~ 20 000- FPSO: ~ 2 000
Account of all measures to reduce the probability and consequences of the hazards
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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 againstlarge scale accidents- e.g lack of detection, deluge
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Relevant Accidental Actions (Hazards)1 Explosion actions
(pressure, duration - impulse)scenariosexplosion 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 ballast6 Unintended pressure7 Abnormal Environmental actions8 Environmental actions on platform
in abnormal position
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Release of Gas and/or Liquid
Release of Gas and/or Liquid
No IgnitionNo Ignition
Immediate Ignition
Immediate Ignition FireFire
Formation of Combustible
Fuel-Air Cloud(Pre-mixed)
Formation of Combustible
Fuel-Air Cloud(Pre-mixed)
Ignition(delayed)Ignition
(delayed)Gas
ExplosionGas
Explosion
No damageNo damage
Damage toPersonnel
and Material
Damage toPersonnel
and Material
FireFire
Fireand
BLEVE
Fireand
BLEVE
Explosions & FiresExplosion is a process where combustion of premixed gas cloud is causing rapid increase of pressure
Fires is a slower combustion process
Implication of simultanous occurence of explosion and fire: Explosion can occur first and damage the fire protection beforethe fire occurs
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Probabilistic SimulationsFor example: explosion overpressure
(given type, I of acidental action)
FLACS PROBLAST
Dispersion AnalysisGas leak location and directionGas leak rateEnvironmental conditions
Explosion AnalysisIgnition locationGas cloud location and size
Monte Carlo SimulationProbabilistic scenario definitionOverpressure definition
OVERPRESSURE EXCEEDANCE DATA
1 .0 0 E-0 6
1 .0 0 E-0 5
1 .0 0 E-0 4
1 .0 0 E-0 3
1 .0 0 E-0 2
0 .0 0 .5 1 .0 1 .5 2 .0
P r e s s u r e [ b a r ]
St o ichio met r ic maxInho mo g eneo us maxR isk analys isSt o ichio met r ic averag e
Histogram of overpressure at each location (j)based upon different scenarios (k)
(i)jkA
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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 line3) Tether in tension-leg platforms
Abnormal degradation due to fatigue or corrosion
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FE Analysis of damage and survival of the damaged platformValidation• To be based on full
scale experiments, laboratory tests
• For the use in a design project, the accuracy of predictions is to be identified.
Application of Methodology• Topside structure
on a jacket platform
0 20 40 60 80 1000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
DISPLACEMENT [mm]
PRE
SSU
RE
[N
/mm
2]
Experiment Analysis
Laboratory experiments with corrugated panels
Courtesy:J. Czujko, 2001
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Framework for Risk-based Design against Accidental actions
][]|[]|[)( )(
,
)( ijk
kj
ijkFSYS APADPDFSYSPiP ∑ ⋅⋅=
probability of damaged system failure under relevant F&E actions
probability of damage, D given Ajk
(i) (decreased by designing against large Aj
(i))
probability of accidental action at location (j) and intensity (k)
(i)jkP A⎡ ⎤⎣ ⎦ is determined by risk analysis while the other probabilities
are determined by structural reliability analysis.
[ ]P FSYS | D Is determined by due consideration of relevant action and their correlation with the haazard causing the damage
For each type ofaccidental action
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[ ] (i) (i)FSYS jk jk
jk
P (i) P FSYS| D P D| A P A⎡ ⎤ ⎡ ⎤= ⋅ ⋅⎣ ⎦ ⎣ ⎦∑
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.
-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 PFSYS(i)
- the exceedance probability level relevant for each location (k) is taken to be a portion of 10-4 for each location.
Characteristic value of the accidental action at each relevant location
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- 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
From Prescriptive to Goal-basedto Prescriptive Approach
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Typical Explosion Loads for Design
Explosion scenario
Structural component
Overpressure (barg)
Duration Impulse (kPa s)
Deck girder (30%) 0.3-0.5 0.1 <1.4-2.0 Process area Process roof 0.2 0.3 1;3
Export riser area
0.5
Central blast wall 0.3-0.7 0.2-0.4 1.5-2.5 Wellbay area Upper deck 0.2 0.3 1.7
Note: goal-setting approach tends to become prescriptive
Significant experiences about Explosion Actions for design for commonly occuring cases
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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 introducedin 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