Title: Ultimate strength of intact ship hulls with diurnal temperature effects: a reliability based approach considering corrosion and load combination
1Ultimate 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
2Overview
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.
3The Temperature Problem
Structures and Reliability Group
- Global Warming and Weather and Temperature
Anomalies.
4Thermal 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.
5Research 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.
6The FPSO Concept
Structures and Reliability Group
Source Douglas-Westwood 2000 report
Image and video sources BP, Bluewater, SHELL
Source Douglas-Westwood 2000 report
7Research 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.
8Deterministic 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
9Ultimate Strength Analysis
Structures and Reliability Group
- Using Advanced Closed Form Analytical
Formulation. (Paik 2001) - Taking into account all possible collapse modes.
10Thermal 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
11Corrosion 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.
12Loading
Structures and Reliability Group
13Load 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
14Reliability 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).
15Failure 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.
16Analysis Design
Structures and Reliability Group
17Past-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.
18Past-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.
19Publications
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.
20Research 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.