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CHINA SHIP SCIENTIFIC RESEARCH CENTER
Structura! Analysis and Design of Ships:
A Review
Cui Weicheng
CSSRC Report
October 1995 English version 95005
P. 0. BOX 116, WUXI, JIANGSU
Content s
pase
Abstract
Introduction I
SimplIfied Structural Models of Ship Structures i
3, Developments In Calculation Models I
3.1 SimplIfied Calculation Models 2
3.2 Finite Element Analysis 3
4. Some Current Research york on Ship Structural 5
Analysis
4. 1 Unstiffened Plates 5
4.2 StIffened Panels 6
4.3 UltImate Strength of Ship hulls 7
4.4 Residual Strength of Damaged Ship Structures 8 5. Reliability Analysis and Design of Ship Structures 10
I StateoftheArt of Reliability Analysis 10
5.2 Uncertainty Analysis li
5. 3 Fatigue Strength Assessment Il
5.4 Quality Assurance 12
5.5 !esign PhIlosophy 13
Sumeary 14
Structural Anal sis and Design of Ships : A Review
Cui \VeichengChina Ship Scientific Research Centre, P.O.Box 116. W 214082
Abstract
This paper provides a state-of-the-art review on the structural analysis
methods, current research activities with regard to ultimate strength,
structural reliability analysis and design. Brief introduction to sorne new
developments in quality assurance (QA) and design philosophy is also given. The areas which requires fl.irther research are identified
Introduction
Structural analysis and design of ships cover a very wide range of research areas. In the 12th ISSC proceedings[I], it was divided into sixteen areas. These include: Environmental Conditions, Loads, Quasi-Static Load Effects, Dynamic Load Effects, Ductile Collapse, Fatigue and Fracture, Material and Fabrication Factors, Design Philosophy, Applied Design-Strength Limit States Formulations, Quality Assurance in Building and Operation of Marine Structures, Design of Structures Against
Fire and Blast, Surface-Effect Ships, Applied Computer Aided Design, Structural Design for
Pollution Control, Slender Marine Structures, and Weight Critical Structures. It is not the intention of this paper to review all these chapters. Instead, I would like to concentrate myself on the chapters of Quasi-Static Load Effects, Ductile Collapse and Strength Limit States Formulations. However,
some basic knowledge about Design Philosophy, Quality Assurance and Structural Design for
Pollution Control is also briefly touched upon. The purpose of this paper is to provide a state-of-the-art review on the structural analysis methods, current research activities with regard to ultimate strength, structural reliability analysis and design.
Sirniilified Structural Models of Ship Structures
Actual ships are very complex stn.ictures from the structural analysis point of view. Only in some
vety special conditions, a refined finite element analysis of the complete ship is required. Most
frequently sorne typical parts of the ship structure is chosen for analysis. In tliismethod it is assumed that if the strengths of these parts are adequate, then the strength of the whole ship is also adequate. The typical sub-structures chosen for representing the behaviour f the whole ship from simple to
complex are: bars and frames (including column, beam, beam-column, grillages, frames and
connections), plates, stiffened panels, stiffened girders, hull girders.
Devdopments in Calculation Models
The development of calculation models seems to be twofold. One trend is to develop more powerful
computer programs for large scale FEA. Another trend is to develop simplified but realistic
calculation methods to be used in the initial design stage.
-1-3.1 Si ni pli lied Calculation Mo(lelS
3.1.1 Advantages of simplified calcnlatiou models
The advantages of simplified calculation models can he stinunarized as foilovs:
a. Large fine models can only he used, 'Iìeii the structure is designed at a rather detailed leve] In commercial applications, the time schedule is usually so tight. that large detailed analyses cannot he as design tools, but rather to check the final desi!.m.
h. Simplified methods can be in conforming with the amount of structural data available in earlier
design sages and can be used for parametric studies or for the optimization of main structural p aram et ë rs
c. Another aspect for the use of simplified nietliods is that they can, and should be, used to check the
results of large FE analyses to avoid gross errors. Their undeniable advantage is also an easy
interpretation of results. 3.1.2 Beam models
The bean; model is the basic model that can be used to represent the major bending and torsional
behaviour of a ship hull. Beam models can be elaborated by including several other deformation
patterns up to the so-called beam-shell representation of thin-walled beams. Table I gives ari overall review on classical and generalized beam models [2].
Table i Review of different beam representations
(from [2])
Kinematic assumption Calculation model
Longitudinal warping negicctcd (plane sections) Rigid contour of cross section Deformnable contour Vithout shear deformation With shear deformation Bcrnoulli-Euler beam Tinioshenko heaiiì noi used Longitudinal warping allowed Rigid contour of cross section Deformable contour
(ivi thout stiffness)
Without shear deformation With shear deformation \Vithoiil shear deformation Viasos' beam Ringed beam-plate model
3.1.3 Plate theory
Beam models can only be applied to bars, for plates and stiffened panels, plate theory is often
employed to derive some simple design equations.
3.1.3 Other simplified models
In addition to various beam models and plate theory, there are several other simplitied models which are developed to avoid the hard work and laborious interpretation of results associated with general FEA or to be used in personal computers. These include module-element model, combined finlie
strip/finite element model, The Idealized Structural Unit Method (ISUM) and the Plastic Node
Method (PNM).
3.1.4 Further work required or suggested for simplified models
Continuous need for the development of simple and reliable calculation methods.
Comparison of existing simplified models with complex meThods such as FE or experiments to
further validate or improve the existing simplified models.
Using sorne existing simplified models to analyze the existing ships or ship design rules to derive more rational design equations or establish new design rules,
3.2 Finite Element Ana'ysis
3.2.1 State-of-the-art of FEMFEM was introduced to analyze ship structures in 1960's. In the last 30 years there lias been a trend towards the use of tiiore refined structural analyses [1]. One reason for this is to answer shipping industry demands to carry more and heavier cargoes, to facilitate loading and unloading operations,
and to reduce costs and weight of the ships through an improved knowledge of their structural behaviour. More recently, this tendency has been driven by owner requests for ship arrangements which allow a large flexibility in goods transportation. The introduction of new international ruies (double hull tankers)
has been another reason, requiring the adoption of new structural
arrangements. Finally, another reason has been the necessity to rationalize knowledge of structural behaviour in order to allow the direct estimation of the fatigue strength of some critical structures. Modern powerful computers and rapidly developin computer programs have made it possible to3
\Vith shear deformation
Delormabic contour \Vithoiit shear \Iaso' beam-shell
viih stifmncss) dc formati ori cnu -nìonìcnticss)
With shear deformation
apply the direct design methods based on numerical simulation techniques.
The FEMs today have reached a certain state of maturit', at least regardint linear analysis. In recent years, tile emphasis has been on the development of pre- and postprocessin programs to increase automation and reduce manual work when preparing FE-models, to improve error checking and put more emphasis on quality assurance aspects.
3.2.2 Current research interests on FEl1
FEA of ship structures has matured as a technology over the last three decades. However, the quality of some analyses remains questionable today, with the principles not properly applied This
problem can be evidenced from the recent comparative FE studies carried out by
technicalcommittee 11.1 of ISSC'91 [2] and ISSC'94 [fl. The naval architect today needs more knowledgeof pat practice than for the simplifications, such as Fixed End Moments, that analysts previously used in structural design. To correct this problem, a guide for FEA of ship structures is needed. Such a document would show the type of analyses that have been performed in the past, comparing them
with improper analyses as well as with substantiating experimental data. There is a need for guidance
in a unified approach to finite element analysis of ship structures.
Analysts differ over the means to go from an overall model to a refined model. ln one case for a SWATH ship, the displacements from an overall model were applied to the boundaries of a detailed
model in four different ways. The computed stresses varied by as much as 200 percent between the
different approaches.
Several organizations have developed guidelines for FEA. The Canadian Department of National Defense has a guide which contains a tutorial on FEA of structures, and a few examples ofanalysis
of ship structures. The American Bureau of Shipping addresses the issue of FEA in the new Dynamic Load Analysis guideline. Tile National Association of Finire Element Methods and Standards(NAFEMS) has published standards for the professional qualifications of persons
performing FEA. Registro Italiano Navale has also produced a guideline for FEA of ship structures including load definition, result acceptance criteria and some examples. These show there is a need for such guidance. However, to become proficient in the art of FEA, an analyst needs the proper experience. This should include comparing the results of several different analyses to experimental data to confirm and validate the analysis.
Panel HS-3, 'Stress Analysis and Strength of Structural Members' of the Society of Naval Architects and Marine Engineers(SNAME) is assembling several analyses of ship structure that
meets this criterion, and will publish them in a report so other analysts may gain from the experience of others. In addition, the U.S. Interagency Ship Structure Committee will sponsor a project in 1994
on 'Guidelines for Evaluation of Finite Element Models and Results". The goal of this study will
he to develop a methodology for efficiently qualif'ing finite element computer codes and models for engineering aniIysis of ship structures. The study will provide guidelines for modeling in typical marine applications, and for rapid assessment of validity of results The work of SNAME Panel
HS-3 and the SSC will be closely coordinated.
-4-3.2.3 Future work re(Juired or st1gesed
for lEA
Comparative FE studies locally, nationally or internationally. Verification of FEA results by experiments.
To establish a more well defined approach or to develop guidelines for FEA of ship structures. To introduce failure criteria to the FEA process for predicting the progressive failure.
4. Some Current Research \Vork on Ship Structurai Analysis
4.1 Unstiffened Plates
For unstiffened plates, some recent researches include (a) statistics of ship plating distortions; (b) effect of ifilperfections and residual stresses on plate ultimate strength (c) effect of large deflections; (d) simple and reliable design equations
statistics of ship plating distortions
Kmiecik et al [3] have summarized the results of many 'ears of measurenients of post-welding
distortions of ship plating mainly from Polish shipyards and also some from Warnow Shipyard in Rostock, Germany. Altogether 1988 plates of merchant ships were examined. For these plates, the mean plate aspect ratio (alb) is 2.29 and COV is 0.477; the mean plate slenderness (bit) is 53.32 and
COV is 0.42. The investigations included the analysis of plate fabrication distortion statistical
distributions and the analysis of correlations and regressions. In particular the absolute values of the
following quantities were examined: relative values of plate maximum deflection, w.,Jt; of the
buckling mode component, wjt (wwlJ), w, w0(3)); of the global component, w,/t; and of
wjw,.,, Examination of the histograms from the measurements and the results of the tests on thegoodness of fit indicated that they can be approximated by means of Weibull distributions. The
results of the statistical analysis have been used to quantify the relationship between plate distortions and structural parameters. The results of the correlation and regression analyses are given in Table
2.
Tabfe 2 Regression Relations and Correlation Coefficients
effect of imperfections and residual stresses on plate ultimate strength
Krniecik [4] analyzed simply supported and clamped ijlates using FEM to study the effect of in-plane
boundary conditions and imperfections (initial deflections and stresses) ori their load carrying
-5-Regression relations Correlation
coefficients w,,,. ¡t 0.0083 b/i - 0. 1989 0.8997
w/t =
0.0062 b/t - 0.1618 0.8946 w It = 0.0018 b/i - 0.0331 0.8374 = -0.O3OSalb+ 0.1336 0.9815 w,, / w = -0.3066 alb + 1.0215 0.8734capacity under uniaxial compression. Guedes Soares and Kiniecil [] used a Monte-Carlo
simulation together with FEM to investiízate the variability of the plate ultimate streneth due to the initial distortions.
effect of large deflections
Kmiecik [6] has compared the load-carrying capacity of plates obtained from large deflection theory
(LD) with that obtained from small deflection theory (SD). The comparison indicates that onlythe
FEM based oil LD is capable of correctly predicting the load-carrying capacity of plates in the
elastic-plastic range. Ueda et al [7] have presented an improved ISUM plate element in which the
effectiveness of the plate after buckling is expressed as a function of the total strain, and a new concept cf strain hardening is introduced in evaluating the post-ultimate strength elastic-plastic
stiffness matrix, in this way, after the element reaches its ultimate strength, the reduction of plaie
strength with the increase of in-plane displacement can be evaluated. simple and reliable design equations
Various authors have been interested in deriving simple and reliable design equations for plates.
Guedes Soares [8] lias derived two design equations for merchant ships and warships. The equations were derived from a full description of tile variables that govern plate strength. Ueda et al [9] have derived buckling, ultimate and fully plastic strength interaction relationships for perfectly flat plates
subjected to inpiane biaxial and shearing forces. Detailed design equations for plate buckling
strength analysis are also given in DnV classification notes [IO].
4.2 Stiffened Panels
For stiffened panels, sorne recent researches include: (a) effect of imperfections and residûal stresses on the ultimate strength of stiffened panels; (b) effect of the stiffness characteristics of the supporting members of the grillage structure on the plate panel; (c) ultimate torsional strength; (d) design equations.
(a) effect of imperfections and residual stresses on the ultimate strength of stiffened panels
This was studied in a recent SSC publication [111. It was found that initial plate deflection does not significantly affect the behaviour of stiffened plate structures. The reason for this is explained to be
due to the flexibility of the stiffening members (longitudinals and transverses) since they do not
restrain the edges of the unsupported panel.
(h) effect of the stiffness characteristics of the supporting members of the grillage structure on the plate panel
This effect was also investigated in SSC-3 82 [11]. It was found that stresses induced in the plating
of a stiffened panel due to the deflection of tile stiffeners are significantly higher than those where
the stiffeners are assumed to be fairly rigid and hence unyielding. The differences are as high as 50%. The differences in stresses depend on the virtual aspect ratio of the panel. The differences seem to attain a maximum at virtual aspect ratio of three. They then tend to decrease and become constant at higher values of virtual aspect ratio.
-6-ultimate torsional strenth
Panaiotopoulos Il 2] carried out a pavametric study for the iruation ni the interaci e fl'xur:l-torsional failure behaviour of flat-bar stiffeners in plates unde uniforr axial compression. lt is
shown that remarkable differences exist between the values of these stresses for stiffeners restraind
by plating and simply supported outstands The results of the calculations indicate that torsional
strength calculated ignoring interaction effects does not represent a lower bound for the stiffene(s strength capacity as assumed by classical theory.
design equations
Bonello et al [13] have compared several codes for the predtion of
:he ultimate strength ofstiffened plates under axial compression and bending. Some modifications to the existing design
codes are proposed. Alagusundararnoorthy et al
[14] carried out
arr analytical/experimental investigations on the ultimate strength of stiffened panels with cutouts under uniaxial compression. The original method proposed by Home and Narayanan based on the strut approach to predict the ultimate strength of stiffened panels with initial imperfections and without cutouts is used to predict the ultimate strength of stiffened panels without cutouts The mean value of the modelling parameTer (experimental strengthltheoretical strength) is 1380 and COV is 0.235. This method was modified to include the effects of cutouts and the modified method is used to predict the ultimate strength of stiffened panels tested with square cutouts extending the full width between stifferiers. The resultingmean value of the modelling parameter is 1.183 and COV isO 138. It can also be concluded that the presence of square cutouts extending the full width between stiffeners reduces the strength of the stiffened panels by about 20% for stiffener-initiated compression failure and 24-37% for failures due to yielding of plate between stiffeners. A parametric study conducted using the proposed methodfor
stiffener-initiated failures indicates that the normalized ultimate load (P,/P) expressed as function of
11m is almost the same for panels with and without cutouts. This indicates that the effect of cutouts is to reduce hr and the reduction in hr increases the normalized ultimate load.
Similar to unstiffened plates presented in the previous section,
da et
al [9] have derivedbuckling, ultimate and fully plastic strength interaction relationships for uniaxially stiffened plates subjected to inpiane biaxial and shearing forces. Detailed design equations for the buckling strength analysis of stiffened flat plates are also given in DnV classification notes [10]
4.3 Ultimate Strength of Ship Hulls
Recently there is a growing interest in developing efficient methods to predict the ultimate strength of a ship's hull. Rutherford and Caldwell [15] examined sorne prediction methods of the ultimate
longitudinal
strength of ships based on a
particular failure event-a VLCC (the Eneigy
(..'oncentrat ion) broke its back during discharge of oil. From their analysis it was concluded that (a) the ultimate longitudinal bendirg strength M,, is a significant measure of a ship's hull strength, and should he determined and used in design and in the assessment of ship strength and/or reliability; (b) prediction of Mu appears feasible, within acceptable confidence limits, using the various approaches
discussed
Mansour et al [16] have summarized results of an experimental investigation to determine the
ultimate strength of ship hull girders. In the finsi phase a stiffened steel hull model of approximate
dimensions 128m in length 244m in breadth and 076m in depth was tested
The modelrepresented the niiddle portion of a 75600 DWT tanker and was subjected to a saeeini moment in order to examine the failure behaviour of the deck under compression. In the second pliasc, another model was designed to reflect several possible modes of failure of an open deck ship. lt was tested
with loads simulating a lioeeing bending moment along with lateral pressure un the bottom. The
ratios of the average ultimate failure stress to the yield stress for Model I is 0.58 and for Model 11 is 0.74. These values suggest that in general over-estimation @1 hull strength will result ¡f' the ultimate
moment is estimated as the full elâstic section modulus multiplied by the yield strength of the
material, The effective section modulus concept seems to offer a promising and practical method fòr
estimating the collapse bending moment. Exclusion of the buckled material from the section
modulus calculation provided a fairly good analytical estimate of the collapse moment as compared to tle experimental one.
Chalmers and Smith [17] have proposed a method for using the normalized load-shortening curves for stiffened plating as part of the design synthesis process for ship hull sections and leading directly
into an ultimate strength assessment. The method is illustrated by an example which shows the
procedure to be adequately conservative in the limited range of scantlings used.
Paik [IS]. Paik [19] and Bai et al [20] have used the Idealized Structural Unit Method (ISU'M) to predict the ultimate strength of various forms of ship hulls. All these papers indicated that ISUM can be used as a practical tool for estimating the ultimate strength of ship hulls.
4.4 Residual Strength of Damaged Ship Structures
The trends toward
light veight ship structures and limit state design requirea thorough
understanding of the reser e and residual strength capacity of ship structures. Reserve strength is the margin between the deman'i imposed by the load and the ultimate capacity of the structure. It is dueto the conservatism in the design of individual components and ultimately the ensemble of
coniponents making up the structure. Residual strength refers to the safety of the structure againstfailure after damage has occurred. If all the components that make up the system are not frilly stressed under a given load, there is a potential for stress redistribution upon the failure of one or more components and prevention of complete failure of the structure The difference between the
two concepts can be seen clearly from their following definitions.
Environmental Load At Collapse (Undamaged) Reserve Resistance Factor =
Design Environmental Load
Environmental Load At Collapse (Damaged) Residual Resistance Factor
Environmental Load At Collapse (Undamaged)
This subject has been dealt with very extensively in a recent SSC publication [21]. lt is not
intended here to review the very technical details presented in this publication. Instead, I would like to introduce the research method adopted in this project because I feel that similar kind of research method should also be used in our research work.
8-ilus work was carried out under a SSC-fìinded project havine the saiiìe name as the report title. The ohectives of the project are clearly defined. i.e.
(I) Introduce the subject of residual strength assessment of damaged marine structures
(7) Summarize the state of the art technology and methods available in the marine and non-niaune
industry For quanti'ing residual strength.
(3) Recommend future work to integrate current engineering procedures in the areas of crack
growth, permanent deformation and global ultimate strength to assess residual strength of damaged
marine structures.
In order to accomplish the objectives of the project, the authors adopted the following approach:
Task I - Literature Review
In order td benchmark the state of the art in residual strength asssment and to identify the key
elements that form the basis for residual strength analysis.
Task 2 - Collection and Evaluation of Marine Structure Damage Data
Various databases and hull survey records maintained by regulatory bodies, classification societies and owners were studied to identi' characteristic forms of damage, extent and shipboard locations. Interviews with owners and inspectors were held, to get practical information on criteria followed during inspection and reaction to damages.
This task is the key success for the research results to be of practical use. Therefore, the authors
spent quite a lot of energy to carry out. the task. For example, the percentage of failure they found for different ships are shown in Table 3.
Table 3 Percentage of failure by failure mode and ship type
However, this task is oRen neglected if the project was carried out by us Instead of basing our
research on actual damage characteristics, sorne arbitrary damages will be assumed which may have not foundation at all.
Task 3 - Evaluation of various methods for assessing residual strength;
Task 4 - Residual strength analysis of a typical tanker;
'-9-Ship Type Failure Mode
Tanker (%) Bulk Carrier (%) Container (%)
Cracking 100 65
Task 5 - Conclusions and recommendations for future work.
5. Reliability Analysis and Design of Ship Structures
5.1 State-of-the-Art of Reliability Analysis
Reliability Analysis was first suggested in 1920's in Russian(not very well-known) and in 1940's in
USA (well-known). However, until 1970's was it first applied to ship structures. Through the
efforts of many pioneering researchers such as Faukner, Mansour, Moan, Guedes Soares and many
people in the leading classification societies such as LR., DnV, ABS, (Ji. and BV, the reliability calculation methods have matured as a technology. Several commercial cotputer programs are
available for the reliability assessment of structures. Table 4 provides a summary information on the cost, reliability methods, and limitations of each program [22].
Table 4 Summary of Commercial Computer Programs [22]
10
-Program Cost Methods and Features Limitations and
shortcomings
PROBAN
DNV. Nonvay
S 20,000 per year FORM, SORM, Directional simulation. Latin Hypercube. Sensitivity factors. Systems
reliability based on simple bounds. Special purpose modules, User friendly interface
No access to source code, High cost, Module for marine (mostly offshore) structure
NESSUS Southwest Research
Institute. USA
$20,000 per year FORM. SORM. Importance sampling, Adaptive importance sampling. Sensitivity factors, Fault tree analysis. SFEM. Random Fields, Interface willi finite element analysis (FRA) packages. (e.g. NASTRAN)
No access to source code. High cost. Limited usage in marine stnicturcs
STRUREL RCP Gmbh,
Germany
$10.000 each huportance sampling. Sensitivity factors. Systems reliahililv index, Inicrfacc with FEA packages
Noaccess to source code. Moderate cost. Limted Usage in marine stnicturcs COMPASS
Marlec Limiiiicd.
Cunada
unknown Currently being developed for naval vessels of Canada
Unknown
PRA_DS
Marice LiBlitcd
Canada
unknown Currently being developed lör naval vessels
of Caimada
Unknown
ISPUD
tjniversitv of Innsbruck. Austria
$2.2t)0 each Importance sampling using design point.
Adapativc sarnphng. Sensitivity factors. S stem reliability index
No access to source code. Limited usage in marine stnicturcs. Not permitted
l'or mimilitan' applications
CALREL
University of California at t3crkelev. USA
$1.10)) c;mcli FORtvI.SORM. Directional sinmilation. Sensitivity lactors. S stcmmis reliability.
Interface nh FRA packages
No access to source code. Limited usage iii umarinc
Some recent publications in the area of reliability analysis of ship structures include: (a) reliability analysis of existing or new design ships: Lee and Yang [231 and Paik et al [24] on new double-hull
tanker; Wang, Moan and Jiao [25] on production ships; Mansour and Hovern [26j on an existing tanker; (b) code calibration: ee [27], Beghin et al [28] and DnV classification notes No30.6 [29]; (c) sensitivity analysis: Mansour and Wirsching [30]; (d) systems reliability approach: Murotsu et al [31]. Ayyub et al [22] provided a very good review of the subject and also pointed out the ftiture directions of development.
5.2 t.Jncertaint'v AnalySiS
Uncertainty analysis is an important part of reliability analysis and this itself is worth some
independent researches. Guedes Soares and Moan [32] have studied the model uncertainty in the
long-term distribution of wave-induced bending moments for fatigue design of ship structures.
Nikolaidis and Kaplan [33] have investigated the uncertainties in stress analyses on marine structures
in very detail. Bitner-Gregersen and Cramer [34] have assessed the uncertainty in ship response
characteristics and fatigue damage accumulation arising from the use of Global Wave Statistics data. In the 12th ISSC proceedings [1], some information on ongoing projects with regard to uncertainty analysis lias been given. These include: (a) Project SR 1338 on "Uncertainty in Strength Models for
Marine Structures". This project has used Monte Carlo simulation to determine the uncertainties
with formulae for inelastic collapse of stiffened panels. Sensitivities of the various parameters used in the formulae are also studied; (b) Project SSC 337 on "Probability Based Ship Design---Loads
and Load Combinations" covers a literature review, load types and combinations for hull girder,
local and fatigue loading and sections on modelling errors and practical applications to design. This work is a good review of past work and proposes methods for inìplementing loading algorithms into
practical reliability analysis of ship structures; (c) Project SR 1353 is underway addressing the
human error aspect of reliability in ship structures. This is an initia! look at this topic to outline
possible methods for addressing this problem within a reliability analysis.
5.3 Fatigue Strength Assessment
Eatigue strength assessment of ship structures lias received a considerable attention recently. One reason is that from the accumulated damage databases of ship structures it is realized that fatigue damage is the most frequent failure mode occurred in the existing ships. Another reason is that as higher tensile steels are extensively applied to hull construction for reducing ship steel weight, the fatigue problem will become more severe if it is still designed according to thé current design ruhe. This is due to the well-known fact that welded HTS structures have a higher operational stress leve! but no significant improvement in fatigue strength compared with that of mild steel structures. Prof Charm [35] in a review paper about reliability analysis claimed that ifa reliability analysis does riot have an explicit consideration of fatigue then it is incomplete. This may also be evidenced from the above-mentioned refèrences on reliability ana!vsis 122-29. 32-34 1. All the papers have carried out the fatigue strength assessment. There are also many papers dedicated specifically for fatigue. Xue et al [36J have calculated the fatigue damage for oil tanker and container ship structures.
However, since fatigue damage is not rzenera!lv respons!i!e for the loss of ships. the attempt to
-eliminate the possibility of fatigue damage during the design stage may not be economically
worthwhii'. What we really need to do is to fully understand the fatigue failure mechanism arid make the event predictable and be under control of human beings. The first utility of this understanding is that the various methods to improve the fatigue strength of welds may be established without any significant sacrifice on costs and weight. The second utility of tbk research is to set out a scientific procedure for inspection and maintenance. The third utility is that efficient repair methods of fatigue damage can be established. Bohlmann [371 have proposed some practical fatigue daniage repair
methods.
5.4
Qualit Assurance
5.4. L Brief introduction to quality assurance
The role of quality assurance (QA) has become more and more important. It has long been
incorporated into manufacturing through tolerance requirements. The QA methodologies are now
expanding o cover practically every aspect from design to operation and maintenance. The purpose
ofA methodologies is to keep the human errors to minimum. QA can be regarded as an extension
of reliability analysis in which the human factor is incorporated irr the analysis. A detailed introduction on the recent development of rational procedures for QA and quality control (QC) of ships is given in the 12th ISSC proceedings [I]. Here only sorne basic knowledge is given.
Quality is defined as freedom from unanticipated defects. Quality is to nieet the requirements of
serviceability, safety, compatibility and durability.
Serviceability is suitability for the proposed purposes, i.e. functionality. Serviceability is intended to guarantee the use of the system for the agreed purpose and under the agreed conditions of use.
Safety is the freedom froni excessive danger to human life, the environment, and property damage. Safety is the state of being free of undesirable and hazardous situations.
Compatibility assures that the system does not have unnecessary or excessive negative impacts on the environment and society during its life-cycle. Compatibility is the ability of the system to meet economic and time requirements.
Durability assures that serviceability, safety, and environment compatibility are maintained during
the intended life of the marine system. Durability is freedom from unanticipated maintenance
problems and costs.
There aie three primary aspects that should be addressed in achieving quality in marine structures: the front-line designers. constructors, and operators of the structure (humans), the groups that are responsible for the management of the sYstems (organizations), and the physical elements (system
5.4.2 Brief introduction to i\larine Structural I iltegrity Programs
Marine StructuraJ Integrity Programs (MSIP) is a large and continuous program sponsored by SSC which belongs to the category of QA. The leader of the project is Profi R.G.Bea from University of
California, Berkeley. The idea canìe from Airframe Structural Integrity Program (ASIP) carried out in aircraft industry from I 9O's to now. In the first phase of the work, they only intended to improve
the durability of commercial-ship stmctures to enable the design, construction, operation, and
maintenance of a ship structure that will be free from unexpected durahility problems. This work was summarized in flea [38]. In this paper, the meaning of durability is also limited to the degree of
resistance of the hull structure to degradation in strength and integrity (capacity) with tinic.
Therefore, he only considers time combined effects of corrosion and fatigue which leads to ship hull degradation. Life-cycle costs models for fatigue durability and corrosion durability were established. i'hese models combines the reliability analysis with economic value analysis.
The second phase of work extends the scope of MSIP following the idea of í\SIP. They call it
Advanced MSLP. The purpose of this work is to develop a procedure for defInition of AMSIP or commercial ships that would include more efficient inspection, more economical and safer operation,
and more effective maintenance. A fundamental objective of an AMSJP is to improve the
serviceability --- durability, reliability and economy (initial and long-term) --- of the shipstructure-systeni. A balance must be achieved between the costs to impróve durability and the benefits of
these investments. The detailed components that an AMSI1 should possess were given in Bea [39]. After the completion of the above-mentioned two-year project in which Profi Bea introduced the need to tie together all of the failure information in order to identify developing problems earlier and
address them before they manifest themselves as a severe catastrophe, a second project "Ship
Structural Integrity Information System" vas awarded. This project develops the concept further by evaluating databases currently in use by ship operators to monitor their vessels. It then proposes a
system to address the data capture needs for design, construction, inspection, maintenance, and operations. As one possible application of the program, it
is used to develop Critical Area
Inspections Plans (CAIPs)..as required by the U.S. Coast Guard for some vessels. The detailedcontents of the ship structural integrity information system were documented in SSC-3S0 [40]. And
the research in this line is still going on.
5.5 IJesion Pli ilosophy
Enì the last several decades, some significant changes have been seen with regard to the design philosophy in designing a ship. These changes are briefly summarizedas follows:
concern moving from structural "scantlings" to structural "architecture". concern moving from simple initial cost to complex life-cycle cost.
(e) concern moving horn single strength consideration to emivironnimental protection.
(d) concern moving horn deterministic design codes to probabilistic design codes.
-(e) niore and niore advanced materials are adopted for reducing weight (O emphasis made on design by first principles instead of design by codes.
The change of the design philosophy marked the progress of human civilization. In relies heavily on the achievements of scientific investigations and accumulations, of past experience. However Lhe change itself could bring niany new problems to be investigated. For example, the adoption of iI'A (Oil Pollution Act) resulted in many research projects about double hull structures. Naval S'it hce
Warfare Centre of DTMB carried out the "Advanced Double Hull Technology Project The
objective of this project is to demonstrate the feasibility of the advanced double hull (ADH) ncept
for its reliable and effective application to U.S. Naval ships and commercial tankers. Adarnchak et al [411 presented some research results within this project.
ABS proposed a Dynamic Load Approach (DLA) in tanker design in order to meet OPA
requirement. The DLA approach uses explicitly determined dynamic loads, and the results of thestructural analysis are used as the basis to increase scantlings where indicated, but allows no
decreases inscantlings from those obtained from the direct application of the ABS rules scantling euations. Liu et al [42] provided a detailed introduction to the DLA approach.
6. Sniiiiiiary
This paper has provided a state-of-the-art review on the structural analysis methods, current
research activities with regard to ultimate strength, structural reliability analysis and design. Brief introduction to sorne new developments in QA arid Design Philosophy has also been given. Based on this discussion, the following conclusions may be drawn:
The simplified structural models used in ship structural analysis are: bars, unstiffened and
stiffened 'plates, and hull girders.
Calculation models are developed following two lines. One is to develop more powerful
computer programs for large scale FEA. The other is to develop simplified but realistic calculation methods to be used in the initial design stage.
Within the simplified models, the required future researches include: (a) further development of simple and reliable calculation methods; (b) comparison of existing simplified models with complex methods such as FE or experiments to further validate or improve the existing simplified tiiodels; (c) using sonic existing simplified models to analyze the existing ships or ship design rules to derive more rational design equations or establish new design rules.
For FEA required future work includes: (a) comparative FE studies: (bJ verification of FEA
results by experiments; (e) to establish a niore well defined approach or to develop guidelines for
FEA of ship stri.ictures; (d) to introduce failure criteria to the FEA pi ocess for predicting the
Current researches on stiffened and unstiffened Plates are tiìainly concentrate on the effects of boundary conditions, initial imperfections and residual stresses. The objective of these researches is to develop simple and reliable design equations. The residual strength assessment of damaged ship
structures has also been received considerable attention.
Classical reliability analysis has matured as a technology, its immediate future development is the
code calibration and puts it into practice. However, classical reliability analysis can only provide a very limited information on the safety of actual ships. Another important factor which affects the safety of ships is due to the human involvement. This factor should be taken into account in a
modern reliability analysis and its influence on safety,or more generally integrity, should be limited to minimum through QA.
Many changes in design philosophy have been observed in the past and with these changes new scientific research fields are promoted.
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