Ship Structural Analysis and Design

575 Pages·2010·13.334 MB·English

by  Hughes, Owen F., Paik, Jeom Kee

SHIP STRUCTURAL ANALYSIS
AND DESIGN
by Owen F. Hughes and Jeom Kee Paik
with Dominique Béghin, John B. Caldwell, Hans G. Payer
and Thomas E. Schellin
Published by
The Society of Naval Architects
and Marine Engineers
601 Pavonia Avenue
Jersey City, New Jersey 07306
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PREFACE
For a structure as large and as complex as a ship two analyses are by far the dominant part of rationally-
there are three levels of structural design, the second based design, and this is refl ected in this text in which
and most central of which is the subject of this 15 of the 17 chapters are devoted to various aspects of
book. Concept design deals with the topology or analysis. Because of this predominance of analysis,
overall ge ometry of the structure; preliminary design rationally-based design is necessarily computer based
establishes the scantlings (structural dimensions) of and this is the key to many of its benefi ts: speed,
all principal structural members; and detail design accuracy, thoroughness, economy, easy modifi cation,
is concerned with local aspects such as joints, and so forth. Also, as explained in Section 1.3, the
openings, and reinf orcements. Overall structural necessary computer programs are already available and
geometry is generally determined by overall design the hardware and software costs are quite mod erate.
requirements rather than by structural requirements, Of the many different topics and aspects in pre-
while detail design is largely guided and constrained liminary structural design some are an inherent part of
by fabrication methods and requirements. Also, since rationally-based design (e.g., the aspects pertaining to
local structural details are numerous and basically response analysis and limit analysis) while others are
similar among various structures they lend themselves more distinct and external (e.g., the selection of mate-
to standardization and to design from handbooks and rials) or are simply constraints in the optimization pro-
structural codes. Thus, it is in preliminary design cess (e.g., the avoidance of some natural frequency).
where the structural designer has the largest number One of the advantages of the rationally-based approach
of signifi cant decisions and options, and the greatest is that it unifi es and coordinates these many different
scope for optimizing the structure so that it best aspects. Even for the more distinct or external aspects
fulfi lls the objectives and satisfi es all of the various the rationally-based approach provides a framework
constraints and requirements. by which each can be better coordinated with the other
Rationally-based design is design from fi rst prin- aspects.
ciples using the tools of modern engineering science:
computers and the methods of structural analysis and PREREQUISITES, LEVELS OF STUDY, AND
optimization which computers have made possible. TIME REQUIREMENTS
Thus, the rationally-based approach is ideally suited
for preliminary structural design, and it is this ap proach The material in this book is suitable for either graduate
and this level of design that is the subject of this book. or undergraduate study, or a combination of both. The
As shown by some examples in Section 1.3, this type methods and practices presented in this book will
of design offers substantial benefi ts to all parties also be useful for practicing engineers and engineers-
concerned: owner, designer, builder, and operator. in-training. The only prerequisites are knowledge of
Designing from fi rst principles requires two sepa- mechanics of soli ds, strength of materials, and the
rate and very extensive analyses: a response analysis to basic aspects of matrix algebra and of statistics. If
ascertain the true and complete response of the struc- necessary, the latter two could be covered in a few
ture to all loads and load combinations, and a limit introductory classes or in outside reading. The total
state analysis to ascertain all of the possible limit or time required to cover all of the topics in this book is
failure values of these responses. Taken together these about nine semester hours.
v
CONTENTS
1. Rationally-Based Structural Design, 1-1 10. Deformation and Strength Criteria for
Stiffened Panels Under Impact Pressure, 10-1
2. Loads, Structural Response, Limit States, and
Optimization, 2-1 11. Buckling and Ultimate Strength
of Columns, 11-1
3. Hull Girder Response Analysis—Prismatic
Beam, 3-1 12. Elastic Buckling of Plates, 12-1
4. Wave Loads—Statistical, Dynamic, and 13. Large Defl ection Behavior and Ultimate
Nonlinear Aspects, 4-1 Strength of Plates, 13-1
5. Reliability-Based Structural Design, 5-1 14. Elastic Buckling of Stiffened Panels, 14-1
6. Frame Analysis, 6-1 15. Large Defl ection Behavior and Ultimate
Strength of Stiffened Panels, 15-1
7. Basic Aspects of the Finite Element Method, 7-1
16. Ultimate Strength of Ship Hulls, 16-1
8. Nonlinear Finite Element Analysis, 8-1
17. Fatigue of Ship Structural Details, 17-1
9. Plate Bending, 9-1
Index, I-1
xi
CHAPTER
ONE
RATIONALLY-BASED STRUCTURAL DESIGN
Owen Hughes sioned solely on the basis of prescriptive rules from
Professor, Virginia Tech classification societies, which were themselves largely
Blacksburg, VA, USA
based on experience and feedback from ships in serv-
Hans G. Payer ice; in the final quarter of the last century, rational anal-
Germanischer Lloyd, Hamburg, Germany (ret)
ysis and design methods were introduced. The
development and introduction of the finite element
1.1 INTRODUCTION
method brought completely new possibilities to deal
Throughout history, shipping has played a central role with complex structural tasks. Just as it would not have
in transportation and trade. Even today, about 95% of been possible to design and construct the drastically
internationally traded goods is carried by ships. The new jumbo aeroplane, the Boeing 747, in the 1960s
remarkable expansion of world trade and manufactur- without detailed rational analyses, many of the new
ing over the past 50 years with distributed manufac- ship types introduced during the past 40 or 50 years
turing, just-in-time delivery, and other features of our would not exist without the extensive calculation pro-
modern world was possible only with a reliable and cedures and analysis possibilities mostly based on the
dependable shipping network distributing all kinds of finite element method. This includes liquefied natural
goods throughout the world, from basic commodities gas carriers, modern containerships, large passenger
and semiproducts to finished goods. ships, as well as large fast ferries with catamaran or
Simultaneously, with the growth in demand for trimaran hull forms. The structural design and analysis
ships and an increase in their complexity, ship struc- of modern naval ships, too, is quite different today.
tural design and calculation procedures have advanced The history of the containership is a suitable
considerably. Earlier, ships were designed and dimen- example. Figure 1.1 is an example of a finite ele-
Figure 1.1 Finite element model of a 9200 TEU containership.
1-1
1-2 RATIONALLY-BASED STRUCTURAL DESIGN
ment model of a medium-sized containership. The an object ive, of actively moving toward it, and of
evolution from the first container carriers with large achieving it to the fullest extent possible.
deck openings of the 1960s, with a carrying capacity Third, and most important, these formulas involve
of up to 1000 twenty-foot equivalent units (TEU), to a number of simplifying assumptions. They can be
the ultralarge container carriers of today, with a carry- used only within certain limits. Outside of this
ing capacity of beyond 13,500 TEU, was possible range, they may be inaccurate. The history of struc-
only because of the ever increased analysis possibili- tural design abounds with examples of structural
ties of classification societies and design offices. failures—in ships, bridges, and aircraft—that
Improvements of each new class of this ship type occurred when a standard, time-honored method or
were always worked out close to the technically fea- formula was used, unknowingly, beyond its limits of
sible. Ships of that size are characterized by specific validity.
aspects that need special technical attention. This For these reasons, there has been a general trend
involves their static and fatigue strength, their struc- toward “rationally-based” structural design ever
tural flexibility, as well as their behaviour in waves. since the 1970s or 1980s, which may be defined as a
But it is not the big ships alone that have to be care- “design directly and entirely based on structural the-
fully designed and analyzed. Modern container feeder ory and computer-based methods of structural anal-
ships, too, are optimized to efficiently carry a maxi- ysis and optimization to achieve an op timum
mum number of containers for their individual size, structure based on a designer-selected measure of
and so careful design and analysis is also needed for merit.” Thus, a complete rationally-based design
these smaller vessels. Similar aspects can be observed involves a thorough and accurate analysis of all fac-
for cruise ships, bulk carriers and tankers. tors affecting safety and performance of the struc-
The complexities of modern ships and the demand ture throughout its life and a synthesis of this
for greater reliability, efficiency, and economy information, together with the goal or objective the
require a scientific, powerful, and versatile method structure is intended to achieve. The aim is to pro-
for their structural design. In the past, ship structural duce the design that best achieves this objective and
design was largely empirical, based on accumulated that provides adequate safety. This process involves
experi ence and ship performance and expressed in far more calculation than conventional methods and
the form of structural design codes or “rules” pub- can only be achieved by extensive use of computers.
lished by various ship classification societies. These For this reason, rationally-based structural design is
rules prov ide simplified and easy-to-use formulas necessarily a com puter-based and often semiauto-
for struc tural dimensions, or scantlings,* of a ship. mated design.
This app roach saves time in the design process and Rationally-based design was first developed and
is still the basis for the preliminary structural design applied for aircraft and aerospace structures. It con-
of most ships. tinues to have its greatest application in these areas
There are, however, several disadvantages and because of the predominant economic significance
risks to a comp letely “rulebook” approach to design. of structural weight, and hence structural efficiency,
First, the modes of structural failure are numerous, coupled with the obvious need for high structural
complex, and interdependent, and with such simpli- reliability. In land-based structures, the move toward
fied formulas the margin against failure remains this approach was given strong impetus in the 1970s
unknown. Thus, one cannot distinguish between by a series of structural failures of steel box girder
structural adequacy and overcapacity. Therefore, bridges. These failures showed that for larger and
such formulas cannot give a truly efficient design. In more slender bridges, the existing empirically-based
some cases, the extra steel may represent a signifi- design codes were inadeq uate. In the ocean environ-
cant cost penalty throughout the life of the ship. ment, an elementary form of this approach has been
Second, these formulas only aim to avoid struc- used for the design of offs hore structures from the
tural failure, but there are usually several ways of beginning, partly because there was little or no pre-
achieving this, and the particular implied in the for- vious experience on which to rely and partly bec ause
mulas may not be the most suitable regarding spe- of the high economic stakes and risks in case of fail-
cific goals of the ship owner over the life of the ship ure. In this area, as well as in ship structures, the
or its particular purpose or economic environment. classification societies encouraged and contributed
A true design process must be capable of accepting greatly to the development of rationally-based meth-
ods. Since first publication of this book, analysis
*Scantlings is an old but still useful naval architecture
methods of classification societies have changed and
term that refers to all local structural sizes, such as
moved considerably towards what may be called
thicknesses, web heights, flange breadths, bracket
sizes, etc. rationally-based design.
1.1 INTRODUCTION 1-3
Rationally-based ship structural design is defi- 2. Detail design to determine geometry and scant-
nitely not fully auto mated design, that is, a “black lings of local structures (brackets, connections, cut-
box” process, where the designer’s only role is to sup- outs, reinforcements, etc.)
ply the input data and whereupon the process presents
the designer with a fin ished design. This type of Of these two levels, the rationally-based approach has
design would require that all de sign decisions— more relevance and more potential benefits regarding
objectives, criteria, priorities, con straints, and so preliminary design because of the following.
on—must be made before the design commences.
Many of these decisions would have to be built into (cid:115)(cid:0) (cid:52)(cid:72)(cid:73)(cid:83)(cid:0)(cid:76)(cid:69)(cid:86)(cid:69)(cid:76)(cid:0)(cid:72)(cid:65)(cid:83)(cid:0)(cid:84)(cid:72)(cid:69)(cid:0)(cid:71)(cid:82)(cid:69)(cid:65)(cid:84)(cid:69)(cid:83)(cid:84)(cid:0)(cid:73)(cid:78)(cid:109)(cid:85)(cid:69)(cid:78)(cid:67)(cid:69)(cid:0)(cid:79)(cid:78)(cid:0)(cid:83)(cid:84)(cid:82)(cid:85)(cid:67)(cid:0)(cid:84)(cid:85)(cid:82)(cid:65)(cid:76)(cid:0)
the program, making it difficult for the de signer to design and, hence, offers large potential savings.
even be aware of the influence of the objectives, much (cid:115)(cid:0) (cid:52)(cid:72)(cid:73)(cid:83)(cid:0)(cid:76)(cid:69)(cid:86)(cid:69)(cid:76)(cid:0)(cid:80)(cid:82)(cid:79)(cid:86)(cid:73)(cid:68)(cid:69)(cid:83)(cid:0)(cid:84)(cid:72)(cid:69)(cid:0)(cid:73)(cid:78)(cid:80)(cid:85)(cid:84)(cid:0)(cid:84)(cid:79)(cid:0)(cid:68)(cid:69)(cid:84)(cid:65)(cid:73)(cid:76)(cid:0)(cid:68)(cid:69)(cid:83)(cid:73)(cid:71)(cid:78)(cid:14)(cid:0)
less to have control over them. Rather, of its very Benefits of good detail designs are strongly depend-
nature rationa lly-based design must be interactive. ent on the quality of this input.
The designer must always remain in charge and be
able to make changes and decisions—with regard to In fact, rationally-based preliminary design offers
objectives, criteria, constraints, priorities, and so several kinds of potential benefits. The economic
on—in light of intermediate re sults. Therefore, a benefits are illustrated by the tanker example quoted
rationally-based design process should allow the in Section 1.3, in which the rationally-based
designer to interrupt, go back, make changes, call for approach gives a 6% savings in ship structural cost
more information, skip some steps if they are not rel- compared with current standard designs (which, for
evant at the time, and so forth. a large tanker, represents a savings of over 1 million
Ration ally-based design gives the designer much dollars) and an even greater amount of extra revenue
more scope, capability, and efficiency than ever from in creased cargo capacity arising from weight
before. But it does require a basic knowledge of struc- savings. Naval vessels can obtain greater mission
tures and structural analysis (e.g., fundamentals of capability by a reduction of weight. Ship designers
finite element analysis and basic types of structural gain a large increase in design capabil ity and effi-
failure) together with some experience in structural ciency and are able to concentrate more on the con-
design. Given these requirements, the deciding factor ceptual and creative (and more far-reaching and
in choosing the rationally-based approach is whether rewarding) aspects of design. And finally, there are
and to what extent a product and/or a perfor mance also substantial benefits to be gained in ship struc-
(economic, operational, or both) is desired that goes tural safety and reliability.
beyond what is obtainable from the rule-based This is not meant to imply that detail design is
approach. The latter is simpler, but it may not be opti- less important than preliminary design; it is equally
mal and is nonadaptable. Thus, the two approaches impor tant for obtaining an efficient, safe, and relia-
are com plementary, and a good designer will use ble ship. Also, there are many benefits to be gained
whichever is more appropriate for a given situation. by applying modern methods of engineering sci-
ence, but the applications are different from prelimi-
1.1.1 Preliminary Design and Detail Design nary design and the benefits are likewise different.
Since the items being designed are much smaller, it
As in most structures, the principal dimensions of a is possible to do full-scale testing and, since they are
ship design are usually not determined by structural more repetitive, it is possible to obtain benefits of
considerations, but rather by more general require- mass production, standardization, methods engi-
ments, such as beam and draft limitations, required neering, and so on. In fact, production aspects are of
cargo capacity, and so on. For this reason, structural importance primarily in detail design.
design usually begins with the principal dimensions Also, most of the structural items that come under
already established. The designer must determine detail design are similar from ship to ship, and so
the complete set of scantlings that provide adequate in-service experience provides a sound basis for
strength and safety for least cost (or whatever other their design. In fact, because of the large number of
objective is chosen). Structural design consists of such items, it is inefficient to attempt to design all of
two distinct levels: them from first principles. Instead, it is generally
more efficient to use design codes and standard
1. Preliminary design to determine loca tion, spac- designs proven by experience. In other words, detail
ing, and scantlings of principal structural members* design is an area where a rule-based approach is
appropriate, and rules published by various ship
*For naval vessels, this is termed “contract design.” classification societies contain a great deal of useful
1-4 RATIONALLY-BASED STRUCTURAL DESIGN
information on the design of local structures, struc- able to the designer. Moreover, the method’s breadth
tural connections, and other structural details. of application and the benefits gained from its use
increase in proportion to the knowledge presented
1.1.2 Aims and Scope of the Book here. It is the authors’ hope that the presentation of the
underlying theory and analysis methods in this text
Now that we have defined the term “rationally-based” will assist designers to obtain the maximum possible
and noted the distinction between preliminary benefits from this new approach.
design and detail design, we can give a specific Also, the authors emphasize that the design method
statement of the two aims of this book: presented herein is not the only possible method, at
least not regarding the particular component methods
(cid:115)(cid:0) (cid:52)(cid:79)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:69)(cid:78)(cid:84)(cid:0)(cid:83)(cid:84)(cid:82)(cid:85)(cid:67)(cid:84)(cid:85)(cid:82)(cid:65)(cid:76)(cid:0)(cid:65)(cid:78)(cid:65)(cid:76)(cid:89)(cid:83)(cid:73)(cid:83)(cid:0)(cid:84)(cid:72)(cid:69)(cid:79)(cid:82)(cid:89)(cid:0)(cid:82)(cid:69)(cid:81)(cid:85)(cid:73)(cid:82)(cid:69)(cid:68)(cid:0) for achieving the basic tasks, such as structural mod-
for rationally-based preliminary ship structural eling techniques and methods of member limit analy-
design in a complete and unified treatment that sis. The methods presented herein were selected or
assumes only basic engineering subjects, such as developed on the basis of their suitability for ration-
mechanics and strength of materials ally-based design, but this type of design involves so
(cid:115)(cid:0) (cid:52)(cid:79)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:69)(cid:78)(cid:84)(cid:0)(cid:65)(cid:0)(cid:77)(cid:69)(cid:84)(cid:72)(cid:79)(cid:68)(cid:0)(cid:70)(cid:79)(cid:82)(cid:0)(cid:82)(cid:65)(cid:84)(cid:73)(cid:79)(cid:78)(cid:65)(cid:76)(cid:76)(cid:89)(cid:13)(cid:66)(cid:65)(cid:83)(cid:69)(cid:68)(cid:0)(cid:68)(cid:69)(cid:0)(cid:83)(cid:73)(cid:71)(cid:78)(cid:0) many different areas that there are bound to be some
that is practical, efficient, and versatile and that has particular methods and techniques that are as good or
already been implemented in a computer program better than those given here. Moreover, as further
and that has been tested and proven progress is made in such areas as structural theory,
numerical methods, and computer hardware and soft-
This book is entirely self-sufficient and self-con- ware, still better methods will be developed. But the
tained; that is, it covers all basic aspects of ration- important point is that now, as the result of many
ally-based design required by a designer. Even basic years of effort by many persons and organizations
aspects such as the finite element method, column both inside and outside of the field of ship structures,
buckling, and plate buckling are included. This has all of the required ingredients for rationally-based
been done for two reasons. design are available.
First, because this book is intended primarily as a
textbook, and in the field of ship structures such 1.1.3 Applicability to Naval Design
books are few and far between. Because of the
greater complexity and sophistication of ration- The design method presented herein applies equally
ally-based design, lack of a unified and com- well to naval vessels and commercial vessels. Because
prehensive text would constitute a correspondingly basic classification rules are intended for commercial
greater difficulty for students and a serious obstacle vessels and are not suitable for warships, various
to further progress in this field. navies and naval design agencies developed their own
The second reason is that rationally-based design, methods of struc tural design. Like classification rules,
both in general and in the particular method pre- these meth ods evolved over a long period and many
sented here, is radically different from the traditional of them were systematized and codified into some
rule-based method and, although many of its fea- form of design manual—a sort of naval counterpart to
tures are familiar to experienced designers (such as the rules. Recently, some classification societies in
finite element analysis and elastic buckling), other cooperation with a Navy developed rules for naval
features are either relatively new (such as nonlinear vessels. Because of the need for greater structural
finite ele ment theory and statistical prediction of efficiency and other special requirements, naval
wave loads) or totally new (such as new techniques de sign methods are generally more thorough and rig-
for structural modeling and new methods for ulti- orous than rule-based design methods of commercial
mate strength analysis of a stiffened panel and of an ships, and they show a stronger trend toward a ration-
entire hull girder). ally-based approach. Thus, in addition to design man-
For this reason, the book is also intended for prac- uals many current methods of naval design already
ticing designers who wish to become more familiar include some of the basic features of rationally-based
with this alternative method of design. In fact, the design.
book’s role is of particular importance because ration- Section 1.2 gives a brief overview of basic features
ally-based design of its very nature requires at least a of rationally-based design, including a dis cussion of
basic knowledge of its underlying theory and meth- the different aims, measures of merit, and design cri-
ods. Once this is acquired, the method’s enormous teria in commercial ships and naval ships. Section 1.3
capability (some of which is demonstrated in Section considers capabilities, applications, and some sample
1.3 and in the references given there) becomes avail- results. Once these aspects are treated, it becomes
1.1 INTRODUCTION 1-5
apparent that the method presented herein applies 1.1.5 Practicality of the Method
equally well to both ship types and that it matches the
needs and challenges of naval designs particularly Rationally-based design is necessarily compu-
well. Because commercial vessels are more numer- ter-based. This raises a number of practical ques-
ous, most of the expla nations and figures in this text tions in re gard to computer implementation,
refer to this type. There fore, it seems desirable at this accuracy, cost-eff ectiveness, availability, ease of
point to briefly consider why the rationally-based use, documentation, and so on. These are impor-
method presented herein is so well suited for naval tant questions, and they are dealt with fully in
design, even though a full appreciation can only be Section 1.3. But, since practicality is so essential
obtained after covering Sections 1.2 and 1.3. in a design method, it is appropriate to mention
Naval ship structures are subject to many special here that this method, ever since its first version in
requirements and constraints. For example, they 1975, has been developed and improved continu-
must be capable of withstanding specified levels of ously, and that during this same period a computer
blast, shock, and other special loads. Also, they must program based on it has likewise been continu-
be damage tolerant, that is, capable of sustaining ously developed and improved. This program,
some structural damage without loss of main func- called MAESTRO†, has now been used for hun-
tions. Since rationally-based design consid ers each dreds of ship structural analyses and designs. In
limit state explicitly, it can accommodate these spe- addition to its use for optimum design, the analy-
cial constraints. As discussed in Section 1.2, mission sis portion of MAESTRO can be used to evaluate
require ments of naval vessels make it extremely a given design, to assess prop osed changes to a
important to minimize the weight and vertical center design or to an existing ship, or to evaluate the
of gravity (VCG) of the structure to the extent seriousness of damage incurred by a ves sel. The
allowed by the various cons traints (such as cost, program is also a valuable tool for ship struc tural
adequate strength and safety, and damage toler- research and for the teaching of ship structural
ance). Hence, there is a paramount need for struc- design. Further details of all these aspects are
tural optimization, which is one of the basic features given in Section 1.3 and in the references cited
of rationally-based design. there.
Finally, it is worth noting that the ability of ration-
ally-based design to deal with both commercial and 1.1.6 International Maritime Organization
naval ships can also help to unify the field of ship Goal-Based Standards and IACS Common
structural design, which until now has been largely Structural Rules
split into two separate areas.
As noted earlier, ships have historically been designed
1.1.4 Applicability to Other Types of and dimensioned on the basis of rules of a ship clas-
Structure sification society. These rules were largely based on
structural mechanics principles as well as on the
In this text, the rationally-based approach is described extensive experience individual classification socie-
purely in terms of ships. However, because this ties gathered over the years with ships in service.
ap proach represents the most fundamental and most With their worldwide network of surveyors, classifi-
gen eral type of engineering design, the material pre- cation societies looked after their classified ships not
sented herein is also applicable to a wide variety of only from the time of initial design to the construction
other steel structures, both fixed and floating.* All of in the shipyards, but also throughout the ship’s life-
the basic principles and most of the analysis methods time up to decommissioning and scrapping. When
for other steel structures are the same as for ships, and weaknesses were found in a ship or in a class of ships
the scope of this text could have been extended to indicating a lack of strength, the rules were adjusted.
include these other structures without requiring fun- This is sound practice followed even today.
damental change of approach. However, this would Competition between classification (or “class”) soci-
have re quired the extension of the consideration to eties was and is a strong driving force to support inno-
the specifics of other structures, such as of additional vation. The International Association of Classification
types of loads and failure modes, plus some new Societies (IACS) looked after a certain degree of
examples to illus trate these other applications. This alignment between rules of member societies and a
would have increased the book’s length unduly. common minimum standard, a situation that was
*For example, in Hughes, Mistree, and Davies (1977),
the method presented herein was used for the structural †Modeling, Analysis, Evaluation and STRuctural
optimization of a large steel box girder bridge. Optimization.
1-6 RATIONALLY-BASED STRUCTURAL DESIGN
important particularly when ships changed class dur- Safety Assessment. It is expected that over time, GBS
ing their lifetime. will also be developed for other ship types.
One of the areas where it was difficult for classi- The MSC agreed on the basic principles of IMO
fication societies to agree on common standards in GBS in conformity with other GBS to be developed
the past is corrosion. Different societies follow dif- by IMO. A five-tier system was agreed for GBS,
ferent philosophies on how to treat corrosion during comprising goals (Tier I), functional requirements
the lifetime of a ship: some have explicit corrosion (Tier II), verification of compliance (Tier III), regu-
allowances added to scantlings determined by their lations and rules for ships such as classification
rules; others take care of corrosion implicitly within rules, IMO requirements, and relevant national
their rules. This works well as long as ships stay requirements (Tier IV), and applicable industry
within the same class from beginning to end. It does, standards and practices (Tier V). The five tiers are
however, cause confusion and difficulties of inter- shown in Figure 1.2.
pretation when a ship changes from a class follow- The first three tiers basically constitute the IMO
ing one philosophy to a class with another procedure. GBS, whereas Tiers IV and V contain detailed pre-
Such problems arose particularly with tankers and scriptive provisions developed or to be developed by
bulk carriers, ships that by nature of their trade are classification societies (recognized by flag states),
especially prone to corrosion. In the 1980s and the IMO and national administrations, and industry
1990s, some of the more spectacular accidents with organizations. Thus, IMO’s GBS establish rules for
older ships, where heavily-loaded bulkers disap- rules, as opposed to rules for ships.
peared during a storm or where tankers floundered Verification of compliance of ship construction
and broke apart with severe pollution to the sea and rules with GBS will be carried out by an interna-
coast, could at least partly be traced back to this tional Group of Experts established by IMO’s
state of affairs. Secretary General in accordance with Guidelines for
It was agreed in maritime circles that this had to verification of compliance with GBS, which are cur-
change, and this was supported by strong political rently under consideration by the Committee. These
pressure. Therefore, the International Maritime Guidelines foresee that national administrations
Organization (IMO) and IACS set out to improve (i.e., flag states) submit requests for verification of
the situation. their ship construction rules or, in most cases, those
developed by an organization recognized by the
1.1.6.1 Goal-Based Standards administration (in most cases, classification socie-
ties) to the Secretary General of IMO, who will for-
The concept of goal-based ship construction stand- ward these requests to the Group of Experts for a
ards (GBS) was introduced at the IMO in 2002, sug- verification of information submitted through an
gesting that IMO plays a larger role in determining independent review. The final report of the group
overall standards to which new ships are built. The with relevant recommendations will then be for-
IMO agreed to develop the basis for ship construc- warded to the MSC for consideration and approval
tion standards that permit innovation in design but, and circulated to the IMO membership by appropri-
at the same time, ensure that ships are constructed in ate means, such as MSC circulars.
such a manner that, if properly maintained, they At the time of finalizing this book, some further
remain safe throughout their economic life. These developments are necessary before GBS will be
standards should eventually be applied to seagoing implemented. Although the Working Group on GBS
ships of all types worldwide. recommended that amendments to the International
Consequently, the Maritime Safety Committee Convention for the Safety of Life at Sea be approved
(MSC) of IMO developed GBS at first for hull con- and that the GBS be considered for approval, neither
struction of bulk carriers and oil tankers. The proce- of these actions was agreed in IMO plenary. It
dures are based on vast practical experience gained appears that there are still several issues that need
with these ship types over the years, mostly collected resolution before that step can be taken. Particularly,
by classification societies. At the same time, GBSs the GBS verification process is not yet agreed on,
advocate the application of a rational holistic and alternative methods are being considered.
approach, such as presented in this book. This
includes, first, defining a procedure for a risk-based 1.1.6.2 Common Structural Rules
evaluation of the current safety level based on exist-
ing mandatory regulations related to the safety of In the early years of this century, IACS developed
these ships and, second, considering ways forward to two sets of common structural rules (CSR) which
establish future risk acceptance criteria using Formal entered into force on April 1, 2006. They apply to all