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

2. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Overview 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day The Temperature Problem Global Warming and Weather and Temperature Anomalies.

4. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Thermal Stresses in Marine Structures 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Research Aims  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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day The FPSO Concept Source: Douglas-Westwood 2000 report Image and video sources: BP, Bluewater, SHELL

7. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Research Background 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Deterministic vs. Stochastic 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Ultimate Strength Analysis Hogging Ultimate Moment CapacitySagging Ultimate Moment Capacity Using Advanced Closed Form Analytical Formulation. (Paik 2001) Taking into account all possible collapse modes.

10. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Thermal Stress Modelling  1 preventing long. fibres from thermal expansion.  2 making the total longitudinal force on the section zero and  2 must vary in proportion with E to in order to equalize strain in all fibres. A set of stresses  3 and  4 corresponding to pure bending in the x y (long.) and x z planes respectively is added so that the net bending in the section equals zero. Incorporating transverse restrain modeling effect of bulkheads

11. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Corrosion in FPSO Structures 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Loading Sea Spectra: ISSC version of the Pierson-Moskowitz spectrum given by Warnsick (1964) RAOs: All headings in increments of 10, 30, 45 o A Weibull distribution was fitted to the resulting distribution to determine scale (k) and shape (b) parameters against Q(Mw>Mi) using: Cumulative long-term distribution: Probability of exceeding amplitude:

13. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Load Combination Total Vertical Bending Moment Ferry Borges-Castanheta Method (1971) Load Combination Factor Vertical Wave Bending Type I Gumbel Distribution (Guedes Soares 1984) Still Water Bending Moment Type I Gumbel Distribution (Guedes Soares 1984)

14. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Reliability Analysis 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Failure Function and Stochastic Model VariableDistributionCOVMean u Ultimate Strength UncertaintyNormal0.101 MuMu Ultimate StrengthLogNormal0.10Calc. M se Most Probable Extreme Still Water Bending Moment Gumbel Extreme0.15Calc.  Load Combination FactorConstantN/A0.78  Uncertainty in Wave Load PredictionConstantN/A0.1 nn Non-Linear EffectsConstantN/A1.2 S 0.8 H MeMe Extreme Vertical Wave Bending Moment Gumbel Extreme0.15Calc.

16. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Analysis & Design

17. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Past-Present-Future Work 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Past-Present-Future Work 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day Publications 1. MOATSOS, I. and DAS, P.K. (2003), “Reliability based ultimate strength structural design for safety”, Proceedings of the 8 th 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 13 th 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 22 nd 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 14 th 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 23 nd 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 2 nd 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. Universities of Glasgow and StrathclydeRPD 2004 Research Presentation Day 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. Research Support