hull structure assessments of the u.s. coast guard's high ...• reliability of ship structure...
TRANSCRIPT
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HomelandSecurity
United StatesCoast Guard
Assistant Commandant forEngineering and Logistics (CG-4)
1
Hull Structure Assessments of the U.S. Coast Guard's High Endurance Cutters,
Application of Reliability Methodologyto Aging Ship Structure
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HomelandSecurity
United StatesCoast GuardHull Reliability
• Introduction• Objective• Approach• Required Input• Desired Outcomes
Assistant Commandant forEngineering and Logistics (CG-4)
• Desired Outcomes• Discussion on way ahead
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HomelandSecurity
United StatesCoast GuardHull Reliability
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
Assistant Commandant forEngineering and Logistics (CG-4)
• 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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HomelandSecurity
United StatesCoast GuardHull Reliability
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
Assistant Commandant forEngineering and Logistics (CG-4)
• 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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HomelandSecurity
United StatesCoast GuardHull Reliability
Assistant Commandant forEngineering and Logistics (CG-4)
• 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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HomelandSecurity
United StatesCoast GuardHull Reliability
Load Uncertainty • Variable wave induced bending moment• Slam loads• Lifetime exposure to loads• Function of operations and operating area.
Assistant Commandant forEngineering and Logistics (CG-4)
• Function of operations and operating area.• Fatigue Life Assessment Program – research into
uncertainties
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HomelandSecurity
United StatesCoast GuardHull Reliability
Strength Uncertainty • Current Condition• Strength decreases with time due to corrosion• Crack initiation and growth with time• Quality of construction practices
Assistant Commandant forEngineering and Logistics (CG-4)
• Quality of construction practices• Weld distortion• Weld details• Stress concentrations• Material property variability
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HomelandSecurity
United StatesCoast GuardHull Reliability
Approach• Hull Strength
• Structural Life Assessment of Ship Hulls (SLASH) –assessment of structure reliability.
• Hull girder strength with general corrosion degradation over time.
• Fatigue
Assistant Commandant forEngineering and Logistics (CG-4)
• 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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HomelandSecurity
United StatesCoast GuardHull Reliability
Required Input (for each ship)• Operational history (primary areas of operation
and associated days at sea)• Structural repair history (significant areas of plating
renewals)
Assistant Commandant forEngineering and Logistics (CG-4)
• Current structural condition survey• Rates of corrosion• Planned operations area (and planned days at
sea)
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HomelandSecurity
United StatesCoast GuardHull Reliability
Assistant Commandant forEngineering and Logistics (CG-4)
10
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HomelandSecurity
United StatesCoast GuardHull Reliability
Assistant Commandant forEngineering and Logistics (CG-4)
11
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HomelandSecurity
United StatesCoast Guard
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
Assistant Commandant forEngineering and Logistics (CG-4)
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HomelandSecurity
United StatesCoast Guard
Sag
Hull Reliability
Assistant Commandant forEngineering and Logistics (CG-4)
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Hog
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HomelandSecurity
United StatesCoast Guard
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
Assistant Commandant forEngineering and Logistics (CG-4)
14
Log10(# Times Moment Is Exceeded)
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HomelandSecurity
United StatesCoast Guard
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
Assistant Commandant forEngineering and Logistics (CG-4)
15
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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HomelandSecurity
United StatesCoast GuardHull Reliability
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
Assistant Commandant forEngineering and Logistics (CG-4)
• 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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