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Ultimate strength of intact ship hulls with diurnal temperature effects: a reliability based approach considering corrosion and load combination

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Title: Ultimate strength of intact ship hulls with diurnal temperature effects: a reliability based approach considering corrosion and load combination


1
Ultimate strength of intact ship hulls with
diurnal temperature effects a reliability based
approach considering corrosion and load
combination
Structures and Reliability Group
RPD 2004 University of Strathclyde Research
Presentation Day
  • Ioannis Moatsos MEng (Hons), GMRINA, MSNAME,
    SMIStructE, SMIMarE
  • ASRANet Ship Group Champion

Supervisor Prof. P.K. Das
2
Overview
Structures and Reliability Group
  • The temperature problem.
  • Thermal stresses in Marine Structures
  • Research aims.
  • FPSO Concept.
  • Ultimate Strength.
  • Thermal Stress Modelling.
  • Corrosion in FPSO structures.
  • Loading.
  • Reliability Analysis.
  • Results.
  • Past-Present-Future Work.

3
The Temperature Problem
Structures and Reliability Group
  • Global Warming and Weather and Temperature
    Anomalies.

4
Thermal Stresses in Marine Structures
Structures and Reliability Group
  • Unless the problem is temperature dependent
    (LNGs/Nuclear), the effects of temperature on
    marine structures are often ignored as non
    significant.
  • ABS in the 50s recorded major hull fractures
    occurring on moored vessels in still water while
    temperature was changing.
  • SSC Reports in the 60s mention records of high
    midship stress on five bulk carriers indicating
    surprisingly high thermal effects.

5
Research Aims
Structures and Reliability Group
  • Investigate the effects of diurnal temperature
    variation and the subsequent effect of thermal
    stresses generated on the ultimate strength and
    reliability of stiffened plate structures.
  • Incorporate the effects of thermal stresses and
    slamming in existing stochastic load combination
    methods.
  • Investigate the effect of corrosion on the
    ultimate strength and reliability of stiffened
    plate structures.
  • Propose a methodology for incorporating
    temperature, corrosion and slamming in ultimate
    strength reliability based design.

6
The FPSO Concept
Structures and Reliability Group
Source Douglas-Westwood 2000 report
Image and video sources BP, Bluewater, SHELL
Source Douglas-Westwood 2000 report
7
Research Background
Structures and Reliability Group
  • Deterministic Procedures
  • Classification Societies.
  • Based on First Yield of the hull girder
    Buckling checks for structural components.
  • Fail to assess safety of ageing or damaged
    structures.
  • Fail to assess safety of novel or unusual
    designs.
  • All factors influencing load are assumed known.
  • Strength-load effects are assumed to be known
    functions of those factors.
  • Uncertainties such as variability of material
    properties or fluctuations in loads or
    uncertainty of analysis models not taken into
    account.
  • High-implied safety margins or load factors
    generate appreciably stronger structures than
    their nominal as-designed ultimate capacity.

8
Deterministic vs. Stochastic
Structures and Reliability Group
  • Structural Reliability (Stochastic
    Procedures/Probabilistic Approaches)
  • Based on Probability Theory to analyse and
    predict the reliability and saefty of structures.
  • Classification Societies have indicated such
    analysis methods some have suggested partial
    factors for direct use in design design
    procedures. (DNV Class.Note 30.6),(Bureau Veritas
    Rules 2000)
  • Each element of the design process treated using
    a probabilistic approach that can accurately
    represent uncertainties.
  • Methodology relatively flexible allowing
    different analysis options depending on
    requirements for strength and load modelling

9
Ultimate Strength Analysis
Structures and Reliability Group
  • Using Advanced Closed Form Analytical
    Formulation. (Paik 2001)
  • Taking into account all possible collapse modes.

10
Thermal Stress Modelling
Structures and Reliability Group
  • s1 preventing long. fibres from thermal
    expansion.
  • s2 making the total longitudinal force on the
    section zero and s2 must vary in proportion with
    E to in order to equalize strain in all fibres.
  • A set of stresses s3 and s4 corresponding to
    pure bending in the xy (long.) and xz planes
    respectively is added so that the net bending in
    the section equals zero.
  • Incorporating transverse restrain modeling effect
    of bulkheads

11
Corrosion in FPSO Structures
Structures and Reliability Group
  • Casualties of merchant ships occurring while
    under operation as a result of structural
    failure of aging ships in rough seas and weather.
  • The need exists to develop corrosion
    mathematical models based on statistical analysis
    of actual measurements and incorporate them in
    structural design procedures.

12
Loading
Structures and Reliability Group
13
Load Combination
Structures and Reliability Group
Vertical Wave Bending Type I Gumbel
Distribution (Guedes Soares 1984)
Still Water Bending Moment Type I Gumbel
Distribution (Guedes Soares 1984)
Ferry Borges-Castanheta Method (1971)
Load Combination Factor
Total Vertical Bending Moment
14
Reliability Analysis
Structures and Reliability Group
  • Using FORM (First Order Reliability Method).
  • Results obtained improved using SORM (Second
    Order Reliability Method).
  • U-X Transformation using Rosenblatt, Nataf,
    Hermite.
  • Computer Codes Used CalRel, PROBAN, STRUREL.
  • All codes converged to similar results.
  • Results levels of safety compared to existing
    published literature. (DNV, Bureau Veritas, ABS,
    SSC, HSE).

15
Failure Function and Stochastic Model
Structures and Reliability Group
Variable Distribution COV Mean
lu Ultimate Strength Uncertainty Normal 0.10 1
Mu Ultimate Strength LogNormal 0.10 Calc.
Mse Most Probable Extreme Still Water Bending Moment Gumbel Extreme 0.15 Calc.
fw Load Combination Factor Constant N/A 0.78
lw Uncertainty in Wave Load Prediction Constant N/A 0.1
cnw Non-Linear Effects Constant N/A 1.2 S 0.8 H
Mwe Extreme Vertical Wave Bending Moment Gumbel Extreme 0.15 Calc.
16
Analysis Design
Structures and Reliability Group
17
Past-Present-Future Work
Structures and Reliability Group
  • Review and comparison of thermal modelling
    procedures recommended by the Ship Structure
    Committee (SSC) and based on Thermal Stress
    Theory (Jasper 1956, Corlett 1950). Procedures
    for analysis of ship structures suggested.
  • Review and comparison of ultimate strength
    modelling procedures (analytical, progressive
    collapse FE methods) and incorporation of the
    effects of corrosion (Paik and Mansour 2001, Paik
    2003, Smith 1977). Methodology for analysis of
    ship structures has been suggested.
  • Significant reduction of the hull girder ultimate
    strength (5-10) when thermal stresses where
    taken into account. Extreme diurnal temperature
    variations during the summer months.

18
Past-Present-Future Work
Structures and Reliability Group
  • Review and comparison of existing load
    combination methods and extended to incorporate
    the effects of temperature and slamming (Ferry
    Borges-Castanheta 1971, Ferro and Mansour 1985,
    Friis Hansen 1994). Procedures for analysis of
    ship structures were developed.
  • Using a number of different reliability
    techniques (FORM, SORM, Monte Carlo) the
    reliability of a number of FPSO structures has
    been determined and partial safety factors have
    been obtained for use in design.
  • Future work in modelling the effects of
    temperature on stiffened plate FE model and
    performing reliability analysis using Monte Carlo
    simulation and calibrate partial safety factors
    for use in design.

19
Publications
Structures and Reliability Group
1. MOATSOS, I. and DAS, P.K. (2003), Reliability
based ultimate strength structural design for
safety, Proceedings of the 8th International
Marine Design Conference (IMDC 2003), Athens
Greece, May 2003. 2. MOATSOS, I. and DAS, P.K.
(2003), Temperature dependent FPSO ultimate
strength reliability, Proceedings of the 13th
International Offshore and Polar Engineering
Conference (ISOPE 2003, Honolulu Hawaii USA, May
2003. 3. MOATSOS, I. and DAS, P.K. (2003),
Temperature and corrosion effects on FPSO
ultimate strength a reliability based approach,
Proceedings of the 22nd International Conference
on Offshore Mechanics and Arctic Engineering
(OMAE 2003), Cancun Mexico, June 2003. 4.
MOATSOS, I. and DAS, P.K. (2004), Modelling the
effect of extreme diurnal temperature changes on
ship structures for the assessment of structural
reliability load combination techniques,
2004-JSC-320, Abstract accepted and paper to be
published in the Proceedings of the 14th
International Offshore and Polar Engineering
Conference (ISOPE 2004), Toulon France, May
2004. 5. MOATSOS, I. and DAS, P.K. (2004),
Assessment of structural reliability of ships
under combined loading including extreme diurnal
temperature effects, OMAE2004-51316, Abstract
accepted and paper to be published in the
Proceedings of the 23nd International Conference
on Offshore Mechanics and Arctic Engineering
(OMAE 2004), Vancouver Canada, June 2004. 6.
MOATSOS, I. and DAS, P.K. (2004), Structural
Reliability of Ship Structures with Advanced Hull
Girder Modelling, Abstract accepted and paper to
be published in the Proceedings of the 2nd
ASRANet International Colloquium, Barcelona
Spain, July 2004. 7. MOATSOS, I. and DAS, P.K.
(2004), Implementation of combined loading
techniques modelling the effects of diurnal
thermal loads, slamming and corrosion to
determine the reliability of Tanker/FPSO
structures. Journal Paper to be submitted for
publication in the Marine Structures Journal,
Elsevier Applied Science Publishing.
20
Research Support
Structures and Reliability Group
  • Sponsors and Supporters
  • The UK Engineering and Physical Sciences Research
    Council.
  • UK The Health and Safety Executive.
  • The Alexander S. Onassis Public Benefit
    Foundation.
  • Mr. Michel Huther Bureau Veritas.
  • The UN World Meteorological Organization.
  • Prof. Douglas Faulkner.
  • SHELL UK Exploration and Production.
  • Awards
  • ISOPE Offshore Mechanics Award 2003.
  • Royal Society of Edinburgh Lessels Award 2003.
  • Selection for the 2003 Young Engineers
    Presentations Reception in the House of Commons.
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