hull structure assessments of the u.s. coast guard's high ...• reliability of ship structure...

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United StatesCoast Guard

Assistant Commandant forEngineering and Logistics (CG-4)

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Hull Structure Assessments of the U.S. Coast Guard's High Endurance Cutters,

Application of Reliability Methodologyto Aging Ship Structure

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United StatesCoast GuardHull Reliability

• Introduction• Objective• Approach• Required Input• Desired Outcomes


• Desired Outcomes• Discussion on way ahead

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Introduction• Problems of aging ship structure

• General corrosion and local pitting• Thinner material leads to higher stress and lower buckling capacity• Fatigue damage accumulation, cracks and crack growth


• Fatigue damage accumulation, cracks and crack growth

• Leading to uncertainty of condition and reliability• Structural assessment based on original condition does

not address reliability• Reliability methods offer a way to assess probability of

failure, consequences, levels of maintenance and associated costs to support good business decisions

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Objectives • Reliability of ship structure is the probability of

maintaining its ability to safely fulfill its design purpose for some time period.

• The assessment must account for:• Corrosion and fatigue damage accumulation as a


• Corrosion and fatigue damage accumulation as a function of time

• Uncertainties in strength• Uncertainties in operational loading • Current condition• Maintenance levels and associated costs• Safety and consequences of failure

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• Strength (R) needs to be greater than the Loads (L).• R - L = Z• The area where Z is less than 0 is the failure probability.• For reliability and safety the failure probability must be

sufficiently low.

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Load Uncertainty • Variable wave induced bending moment• Slam loads• Lifetime exposure to loads• Function of operations and operating area.


• Function of operations and operating area.• Fatigue Life Assessment Program – research into

uncertainties

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Strength Uncertainty • Current Condition• Strength decreases with time due to corrosion• Crack initiation and growth with time• Quality of construction practices


• Quality of construction practices• Weld distortion• Weld details• Stress concentrations• Material property variability

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Approach• Hull Strength

• Structural Life Assessment of Ship Hulls (SLASH) –assessment of structure reliability.

• Hull girder strength with general corrosion degradation over time.

• Fatigue


• Fatigue• Fatigue Life Assessment of Ship Structures (FLASH) –

assessment of structure fatigue reliability.• Working on adding corrosion and fracture mechanics

approach to FLASH.

• Serviceability of structure and foundations• Assess main engine and reduction gear foundations.

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Required Input (for each ship)• Operational history (primary areas of operation

and associated days at sea)• Structural repair history (significant areas of plating

renewals)


• Current structural condition survey• Rates of corrosion• Planned operations area (and planned days at

sea)

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Corrosion Survey Review Summary

Hull Girder Elements

Cutter Date of Survey 01 Level* Inserts Shell** Inserts 01 Level Shell

Jarvis 4-Jan-2010 2.09% 1 0.45% 9 0 20

Chase 26-Oct-2009 4.78% 3 1.51% 16 1 10

Mellon 9-Sep-2009 3.38% 0 1.64% 1 0 20

Boutwell 20-Jul-2009 3.50% 0 3.40% 0 1 2

Morganthau 12-May-2009 3.60% 1 1.30% 9 1 8

*01 Weather Deck Plate

**Underwater Shell frs 174, 180, 186

Avg Plate t Wastage near midships and

number of insert plate repairs required

Number of locations with

stiffener renewal issues

Hull Reliability


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Sag

Hull Reliability


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Hog

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Wave Only HogWave + Whip HogWave Only SagWave + Whip Sag

Fatigue Load Exceedance - 378 WHEC class

Log10(# Times Moment Is Exceeded)7.576.565.554.543.532.521.510.50

Be

ndi

ng

Mom

ent

, ft-

lton

60,000

40,000

20,000

0

-20,000

-40,000

-60,000

Hull Reliability


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Log10(# Times Moment Is Exceeded)

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Wave Only HogWave + Whip HogWave Only SagWave + Whip Sag

Fatigue Load Histogram - 378 WHEC class

Be

nd

ing

Mo

me

nt,

ft-lto

n

60,000

40,000

20,000

0

-20,000

-40,000

-60,000

Life Distribution

1.0E-06

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

Cu

mu

lati

ve P

rob

abili

ty

Group D

Group C

Group B

Group A

Hull Reliability


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Log10(# of Cycles at Moment)76.565.554.543.532.521.510.5

1.0E-08

1.0E-07

0 5 10 15 20 25 30 35 40 45

Years

• Fatigue Reliability• FLASH currently developed to determine time to crack

initiation.• Working on incorporating the effects of corrosion and

crack growth in the system reliability assessment

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Desired Outcomes• Determine critical failure mechanisms and associated

target reliability / safety indices (probabilities of failure).• Target reliability levels will vary for each type of failure,

consequence, and degree of system redundancy.• Determine the governing structural reliability value for the


• Determine the governing structural reliability value for the system and time frame for safe / reliable operation.

• Address actions needed to maintain reliability and safety• Costs of maintaining reliability can be input to business

case analyses

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