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

T.Moan.26.03.2007 2

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

T.Moan.26.03.2007 3

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

T.Moan.26.03.2007 4

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