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Cost Effectiveness of Hull Girder Safety

Rolf Skjong & EM Bitner-GregersenDet Norske Veritas

OMAE, Oslo, June 24-28, 2002

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Content

Acceptance criteria in Structural Reliability Analysis

Acceptance Criteria in Formal Safety Assessment

Common Criteria? Application to Hull Girder Strength Conclusion

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Traditional Approach - SRA

Typical Example:DNV Classification Note 30.6 (1992) on Structural Reliability Analysis of Maritime Structures

SRA is Bayesian theory Explains why SRA does not produce Probabilities with

a frequency interpretation– No gross error– Epistemic uncertainty & model uncertainties

included– SRA talk of “notional” reliabilities

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Traditional Approach - SRA

Target should depend on consequence Calibration against known cases (that are acceptable

good/best practices in the industry) Calibration against similar cases with similar

consequences Based on accepted decision analysis techniques Based on tabular values (presented as a last resort)

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Traditional Approach - SRA

Based on tabular values (presented as a last resort)

Annual Target Probabilities (and Target βT) from DNV Classification Note 30.6Consequence of FailureClass of Failure

Less serious SeriousI- Redundant Structure PF = 10-3, βT = 3.09 PF = 10-4, βT = 3.71II - Significant warning before the occurrence offailure in a non-redundant structure

PF = 10-4, βT = 3.71 PF = 10-5, βT = 4.26

III - No warning before the occurrence of failure ina non-redundant structure

PF = 10-5, β T = 4.26 PF = 10-6, β T = 4.75

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Traditional Approach - RA

Quantitative risk assessment is the basis for regulations in many industries– PSA/PRA - Nuclear– Hazardous Industries (Seveso I/II)– Offshore (Safety Case)– Shipping (FSA)– Etc.

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Traditional Approach - QRA

Present Risk Results in terms of– Individual Risk (Fatalities)– Individual Risk (Health and Injuries)– Societal Risk (Group Risk)– Environmental Risk– Economic risk (not necessarily a regulatory issue)

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Traditional Approach - RA

Example Individual risk

1.00E-07

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

Ind

ivid

ual

risk Intolerable Risk

ALARP

Negligible Risk

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Traditional Approach - QRA

Example Societal Risk

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1 10 100

Fatalities (N)

Fre

qu

en

cy

of

N o

r m

ore

fa

taliti

es

(p

er

sh

ip

ye

ar)

Oil tankers

Chem. tankers

Oil/Chemicaltankers

Gas tanker

Negligible

Intolerable

ALARP

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Traditional Approach - QRA

Low Risk

High Risk Intolerable

ALARP

Negligible

Not acceptable

Acceptable

Acceptable if made ALARP

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Traditional Approach - QRA

The As Low As Reasonably Practicable Area implies that cost effectiveness assessment may be used

Risk is made As Low As Reasonably Practicable, when all cost effective safety measures have been implemented

Implies that a decision criteria for cost effectiveness will be required

It seems to be accepted at IMO that most ship types are in the ALARP area but not ALARP

Cost Effectiveness will be the only criteria

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Common Criteria

Cost effectiveness criteria is easy to use, both for SRA and FSA studies

Advantage to SRA: Only change in risk is used in the decision process, not the absolute numbers

Criteria to use: $ 1.5 - $ 3.0 million Decision at MSC 76 in December 2002

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Previous IMO decisions

Example from IMO: UN Organisation for maritime safety and environmental protection regulations

Cost of averting fatalities in actual decisions

Decision DecisionMaker

Value

Strengthening Bulkheads on Existing BulkCarriers

IACS andIMO (1)

> $ 1.5 million

Helicopter Landing Area on non-Ro/RoPassenger Ships

IMO(2) < $ 37 million

3 bulkheads on car deck Passenger Ro/Ro IMO(3) < $ 5 million3 bulkheads + sponsons IMO(3) < 7.8 millionExtended sponsons only IMO(3) < $ 11 millionExtra Deck Officer IMO(3) < $ 5.5 millionTwo conventional lifeboats BC IMO(4) > $ 1 millionThrow overboard life-raft on BC IMO (4) > $ 3 millionRe: (1) Mathisen et al.(1997), (2) Skjong et al.(1997), (3) DNV(1997), (4) Skjong andWenthworth,

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Girder Collapse/Sagging

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PROCODE

Use limit states formulation with target beta– Optimisation of partial safety factors– Control Variables: Partial Safety Factors– Minimum Scatter around a target reliability by

minimising the penalty function– Target to vary to produce cost effectiveness ratios

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PROCODE

)),,,((min ,1

,mode,

kTijjik

M

kk pw

ij

DEX

},...1,,...1,0),,,,( ,, kjijink NnMkh dexSubjected to:

With one of the inequalities turning into equality

One design Case:

This is generalised to Multiple design cases in PROCODE

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PROCODE

Programmable functions– Limit States– Code Checks– Penalty functions

Defined by Data (additional to PROBAN)– Scope – Safety Factor – Design Parameter

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PROCODE RESULTS

Code Evaluation (before optimisation starts) Optimised partial safety factors Resulting reliabilities Resulting design parameters (input to cost analysis)

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PROCODE Results

Plate and web thickness before and after rule calibration, flat bar stiffener profilesBefore Calibration Calibrated:T=2.5 Calibrated:T=3.09 Calibrated:T=3.5 Calibrated:T=3.72

Ex.tmm

tw

mmtmm

tw

mmtmm

tw

mmtmm

tw

mmtmm

tw

mm1 18.5 20.0 20.8 15.7 21.9 20.4 21.9 25.1 22.2 27.02 18.5 18.6 20.3 17.8 21.3 22.3 21.4 26.5 21.6 28.23 18.5 16.3 19.4 21.0 20.3 25.1 20.4 28.6 20.7 30.04 22.0 10.2 20.4 19.5 21.4 25.1 21.6 30.0 22.8 30.05 22.0 9.5 20.1 23.6 21.0 30.1 21.0 36.1 21.2 38.56 22.0 8.4 19.5 32.3 20.3 40.6 20.2 47.9 20.4 50.97 16.0 27.0 20.8 15.7 21.9 20.4 21.9 25.1 22.2 27.08 16.0 25.0 20.3 17.8 21.3 22.3 21.4 26.5 21.6 28.29 16.0 22.0 19.4 21.0 20.3 25.1 20.4 28.6 20.7 30.010 13.0 35.5 20.8 15.7 21.9 20.4 21.9 25.1 22.2 27.011 13.0 28.7 19.5 21.0 20.3 25.1 20.4 28.6 20.7 30.012 13.0 23.0 17.9 25.4 18.6 29.1 18.7 32.0 18.9 33.1

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PROCODE Results

Plate and web thickness before and after rule calibration, L-stiffener profilesBefore Calibration Calibrated:T=2.5 Calibrated:T=3.09 Calibrated:T=3.5 Calibrated:T=3.72

Ex.tmm

tw

mmtmm

tw

mmtmm

tw

mmtmm

tw

mmtmm

tw

mm1 18.5 14.0 17.0 7.2 17.4 11.9 18.3 16.5 19.6 19.52 18.5 14.0 17.0 8.4 17.5 12.1 18.3 16.1 19.7 17.83 18.5 13.0 17.0 9.3 17.3 13.0 18.4 16.2 19.4 18.24 21.5 7.5 16.9 10.6 17.4 14.8 18.2 18.8 19.5 21.55 21.5 7.0 17.1 8.5 17.5 12.3 18.3 16.5 19.8 18.16 21.5 5.0 16.7 9.2 17.1 12.8 17.9 16.2 19.3 17.77 16.0 18.7 16.8 8.0 17.3 11.4 18.2 15.1 19.6 16.78 16.0 20.5 16.9 7.2 17.5 10.0 18.3 15.0 19.7 18.09 16.0 17.7 16.9 9.2 17.2 12.8 17.9 16.3 19.3 17.810 13.0 24.5 16.9 9.2 17.2 12.8 17.9 16.3 19.3 17.811 13.0 21.5 17.2 9.9 17.5 13.0 17.8 16.3 19.2 17.512 13.0 19.0 17.6 10.5 18.1 13.2 18.3 16.0 19.3 17.3

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Resulting Steel Weight

Cross section area and weight of ship decks per meter length and per stiffer before and after rule calibration, flatbar profiles.

BeforeCalibration

Calibrated:T=2.5 Calibrated:T=3.09 Calibrated:T=3.5 Calibrated:T=3.72

Ex.Area

( 2mm)

Weight(t)

Area

( 2mm)

Weight(t)

=3.09( 2mm)

Weight(t)

Area

( 2mm)

Weight(t)

Area

( 2mm)

Weight(t)

MeanValue

23301 0.1829 25875 0.2031 28546 0.2241 30230 0.2373 31086 0.2440

Cross section area and weight of ship decks per meter length and per stiffer before and after rule calibration, L-profiles.

Before Calibration Calibrated:T=2.5 Calibrated:T=3.09 Calibrated:T=3.5 Calibrated:T=3.72

Ex.Area

( 2mm)

Weight(t)

Area

( 2mm)

Weight(t)

=3.09( 2mm)

Weight(t)

Area

( 2mm)

Weight(t)

Area

( 2mm)

Weight(t)

MeanValue

23355 0.1833 21281 0.1671 22143 0.1738 24481 0.1922 26399 0.2072

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Costs

Total weight and cost for the average ship deck before and after Calibration, Flat bar and L- stiffener profiles.Stiffener profile Before

CalibrationAfter=2.50

After.=3.09

After=3.50

After=3.72

Flatbar Weight (T) 1,829 2,031 2,241 2,373 2,440

FlatbarCost (US$) 3,658,000 4,062,000 4,482,000 4,746,000 4,880,000L-profileWeight (T) 1,833 1,671 1,738 1,922 2,072L-profileCost (US$) 3,812,640 3,475,680 3,615,040 3,997,760 4,309,760

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Costs

Increase of the ship deck cost for different target beta,

Flat bar stiffener profiles.Cost increase relativeto

=2.50 =3.09 =3.50 =3.72

=2.50 - 10.3% 16.8% 20.1%

Past practice* =2.40

11.0% 22.5% 29.7% 33.4%

* before calibration

Increase of the ship deck cost for different target beta,

L-stiffener profiles.Cost increase relative to =2.50 =3.09 =3.50 =3.72

=2.50 - 4.0% 15.0% 24.0%

Past practice* =3.40

-8.5% -5.5% 4.9% 13.0%

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GCAF

Calculation of GCAF (L Profiles)

GCAF = (Cost of RCO)/( PLL)

-> 3.50 to 3.72 (Pf=2.41 10-4 to 10-4)

P 20 persons 20 years

PLL = (2.41 10-4 - 1.0 10-4 ) •20 •20 •0.5 = 0.0302

50% survives

GCAF = (4,309,760 -3,997,769)/0.0302 = $ 10.3 million

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NCAF

Calculation of NCAF (L Profiles)

NCAF = (Cost of RCO -Economic Benefits)/( PLL)

-> 3.50 to 3.72 (Pf=2.41 10-4 to 10-4)

P 20 years

Benefits = (2.41 10-4 - 1.0 10-4 ) •20• ($11 + $ 21 million ) = $ 90,240

Old Ship

NCAF = (4,309,760 -3,997,769-90,240)/0.0302 = $ 7.13 million

Cargo

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Results

CAF in US$ million for increases of the reliability index Flat Bar L-Profile

GCAF NCAF GCAF NCAF2.53.09 0.402 -2.80 0.133 -3.073.093.50 1.76 -1.74 2.55 -0.6453.503.72 4.44 1.24 10.3 7.13

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Conclusion

In reliability based code calibration CEA (NCAF) may be used as an alternative to target

Consistency with RA The decision is based on the derivative of PF with

respect to design variables only Not reliant on probability (the absolute number) Mid ship bending moment (ship deck/sagging)

– NCAF at about $ 3 million corresponds to 10-4 target– Compares well to last line of defence, lifeboats at $

1 million More studies necessary for other limit states

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One criteria HE, QRA, SRA

Human error, navigation

Collision

Flooding

Evacuation Failure

Fire

?

Risk

Cost ?

Risk

Cost?

Risk

Cost?

Risk

Cost

Optimising risk control options according to their cost effectiveness: