Hull Structure Assessments of the U.S. Coast Guard's High Endurance Cutters, Application of Reliability Methodology to Aging Ship Structure

Size: px

Start display at page:

Download "Hull Structure Assessments of the U.S. Coast Guard's High Endurance Cutters, Application of Reliability Methodology to Aging Ship Structure"

Lawrence Lester
5 years ago
Views:

1 Hull Structure Assessments of the U.S. 's High Endurance Cutters, Application of Reliability Methodology to Aging Ship Structure 1

2 Introduction Objective Approach Required Input Desired Outcomes Discussion on way ahead 2

3 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 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 3

4 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 function of time Uncertainties in strength Uncertainties in operational loading Current condition Maintenance levels and associated costs Safety and consequences of failure 4

5 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. 5

6 Load Uncertainty Variable wave induced bending moment Slam loads Lifetime exposure to loads Function of operations and operating area. Fatigue Life Assessment Program research into uncertainties 6

7 Strength Uncertainty Current Condition Strength decreases with time due to corrosion Crack initiation and growth with time Quality of construction practices Weld distortion Weld details Stress concentrations Material property variability 7

8 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 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. 8

9 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) 9

10 10

11 11

12 Corrosion Survey Review Summary Hull Girder Elements Avg Plate t Wastage near midships and Number of locations with number of insert plate repairs required stiffener renewal issues Cutter Date of Survey 01 Level* Inserts Shell** Inserts 01 Level Shell Jarvis 4-Jan % % Chase 26-Oct % % Mellon 9-Sep % % Boutwell 20-Jul % % Morganthau 12-May % % *01 Weather Deck Plate **Underwater Shell frs 174, 180,

13 Sag Hog 13

Fatigue Load Exceedance - 378 WHEC class Bending Moment, ft-lton 60,000 40,000 20,000 0-20,000-40,000-60,000 Wave Only Hog

14 Fatigue Load Exceedance WHEC class Bending Moment, ft-lton 60,000 40,000 20, ,000-40,000-60,000 Wave Only Hog Wave + Whip Hog Wave Only Sag Wave + Whip Sag Log10(# Times Moment Is Exceeded)

15 Life Distribution Fatigue Load Histogram WHEC class 1.0E+00 B e n d in g M o m e n t, ft-lto n 60,000 40,000 20, ,000-40,000-60,000 Wave Only Hog Wave + Whip Hog Wave Only Sag Wave + Whip Sag Cumulative Probability 1.0E E E E E E-06 Group D Group C Group B Group A Log10(# of Cycles at Moment) Fatigue Reliability E E FLASH currently developed to determine time to crack initiation. Working on incorporating the effects of corrosion and crack growth in the system reliability assessment Years 15

16 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 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 16

Hull Structural Safety Assessment of Aged Non-ice Class Container Vessels in an Arctic Operation

Yun-Tzu Huang 6 th EMship cycle: October 2015 February 2017 Master Thesis Hull Structural Safety Assessment of Aged Non-ice Class Container Vessels in an Arctic Operation Supervisor: Professor Zbigniew