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Common Structural Rules (CSR) for tankers
The presentation, the introduction and the failures
Stavros X. Hatzigrigoris 1), Spyros V. Karapanos 1), Nikiforos Papadakis 2)1)
Maran Takers Management Inc., Greece, [email protected])
Anangel Maritime Services Inc., Greece, Email [email protected]
Abstract
The Common Structural Rules (CSR) were developed at
the request of the shipping industry in an effort to stop
competition between Class Societies on the weight of the
structure of ships. One of the major objectives was alsoto reduce the possibility of Builders negotiating in a
non-transparent way with the selected Society using
arguments like, you have to accept what your competi-
tor is accepting, that resulted in designs that were
following the minimum rules of each and every individ-
ual Society. The CSR were introduced in 2004 and im-plemented for ships contracted after the first of April
2006. Yards could opt between two different approaches
the first being to apply the new rules on existing designs
and the second to start with a blank piece of paper. The
first approach has given satisfactory results whilst the
second has produced questionable structures.
Keywords
IACS; CSR; VLCC; Structure; JTP; JBP.
1. Summary
The preparation of the Common Structural Rules started
somewhere back in 2002 by ABS, DNV and LRS, i.ethe three LAN societies. In 2004 the JTP group was
formed as a result of political negotiations within
IACS when it was decided to introduce Common Rulesfor both Bulk Carriers and Tankers. IACS allocated the
development of the CSR for tankers to the JTP and for
bulk carriers to the JBP which was formed with the
participation of BV, Class NK, GL, RINA, KR, CCS
and the Russian Register.
The Tanker Rules were ready in 2005 and became com-
pulsory for all New Buildings contracted after the 1st of
April 2006.
During the preparation stage and for the first time in thehistory of Classification Societies the new Rules were
presented to the industry in a grandiose way and were
subject to discussion over a period of more than one
year. Several ramification studies were released indicat-
ing that the hull weight of the CSR tankers was to be
increased between 4 % and 7 % to meet the new rules.Similar figures were presented for the CSR-compliant
bulk carriers. Reportedly, the major improvements in-
troduced by the JTP were a fatigue life of 25 years
based on North Atlantic environmental conditions andthe net scantlings concept which as mentioned above
resulted to an increased hull weight i.e. presumably a
stronger structure. Furthermore, IACS argued that the
new rules were to achieve optimum steel distribution
and not just a weight increase.
This paper will discuss the experience of the authors
with the CSR design of a Very Large Crude Carrier(VLCC) developed by a major Far Eastern yard. This
showed that despite the promises given by the JTP and
IACS, skilled design engineers had taken advantage of
loop holes in the first version of the Rules and produced
a structure that was significantly weaker than the pre-
CSR design.The major items in which the CSR have to be improved
can be summarized as follows:
1. Better use of the experience gained after the mid90s with the pre-CSR designs.
2. Introduction of a tapering formula that will pro-duce structures equal or better than before outsidethe 0.4 L area. This is especially critical because
there is no analytical tool to assess the structure in
that region, unless of course a complete ship model
is utilized. This implies strengthening of the for-
ward and the engine room areas and was partially
done by January 2010.3. Enhancing of the maximum allowable bending
moment and shearing force Envelopes.
4. Limitation of the liberty given to designers to skipdetailed strength analysis in case the calculated
stresses do not exceed 95 % of yield. Buckling
analysis should be based on fine mesh stress calcu-
lations and not on coarse FEM results.
5. The slamming effects should apply to the hullgirder and not only to the flat bottom areas of the
ships.
The paper will also discuss the IACS Rules improve-
ment procedures and how same should be modernizedto allow for quicker changes, made more transparent
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and more democratic in allowing acceptance by the
majority of the IACS members instead of asking for
unanimous decisions that make changes almost imposs-
ible to process.
2. IACS Common Structural Rules
The rules for the construction of tankers after the mid
90s improved a lot in comparison to the rules that wereavailable before the introduction of fatigue assessment.
The need to improve originated from the serious struc-tural problems that kept appearing in ships delivered in
the late 80s and early 90s even during their one-year
guarantee period. SafeHull, Nauticus, Shipright, Veris-
tar and other Class-developed software helped in the
construction of much better ships which have not pre-
sented any serious structural problems until today asthey approach their third special survey. However,
competition between Class Societies and the continued
pressure from the shipyards was causing a downgrading
of the rules. Super optimization was the word of the
day. Moreover, the deterioration of the rules was be-coming an issue and at the same time there was no clearand rational link between the requirements for new
buildings and the Ships in Service requirements. It is
interesting to note that the scantling renewal criteria in
the pre-CSR rules were different for each class society.
Several owners were opting for optional class notationslike the ABS DLA notation in an effort to make sure
that the ships that they were getting from the yards
would be structurally sound. Other societies were intro-
ducing enhanced scantlings notations. In private talks
Class Societies were claiming that the structural rules
had not kept up with design development in terms ofusing advanced design tools (they were simple and
prescriptive). Several Class Societies were supporting
that considerable improvements and more robust ships
could be produced if the rules were rewritten.
The aim of the International Association of Classifica-
tion Societies was to develop a set of unified Rules and
Procedures for the determination of the structural re-
quirements for oil tankers and bulk carriers.
The Joint Tanker Project (JTP) was initiated by ABS,
DNV and LR in January 2002 to develop common
structural Rules for tankers. The Chief Executive Offic-ers of the three classification societies determined that a
joint project should be undertaken to develop a commonset of Rules for the hull structure and a promise was
given that the best parts in the pre-CSR rules were to be
used in creating the new rules.
The major objectives were also to eliminate competition
between class societies with regard to structural re-
quirements and standards, to employ the combined
experience and resources of all three societies (for the
tanker rules) to develop a single common standard, or
set of Rules. They also wanted to ensure that a tanker
meeting this new standard will be recognized by theindustry as being at least as safe and robust as would
have been required by any of the existing Rules and to
fully embrace the intentions of the anticipated IMOrequirements for the goal-based new construction stan-
dards. Another target was to reduce the cost of dealing
with a number of similar but different sets of Rules and
to insure that the resulting rules and procedures are
written in such a way as to result in common scantling
requirements.
For each set of rules, these goals were developed by theproject team and the JTP steering committee in response
to requests from owners and shipyards for standardiza-
tion. Individual working groups were formed to dealwith loads, longitudinal strength, design principles,
fatigue, buckling, local strength, general requirements,
FEM, administration and testing.
The general approach was to adhere to the overall
project objectives, to work under a coherent framework,
to include some new approaches where there was suffi-
cient confidence and justification for their adoption, to
incorporate best and most transparent methods and toensure that Rules are internally consistent and reflect the
service experience. Also the rules had to result in com-
mon scantling requirements and to include the industry
input and the feedback.The basic criteria for the Dynamic Load Model were
based on simple Rule load formulations for basic load
components (ship motions and accelerations, external
pressure, internal pressures, global loads), wave head-
ings applying to the North Atlantic scatter diagram
according to IACS Rec. 34, on existing Rules (sloshing
pressures, bottom slamming pressures, bow impact
pressures) and finally on the existing IACS URs when
available.
2.1 Net Thickness
The CSR are formulated using a net thickness approach.
This approach assumes that various degrees of corrosionwill occur to the structural members during the life of
the vessel. The net scantling approach sets out to deter-
mine and verify the minimum hull scantlings that are to
be maintained from the new building stage throughout
the ships design life to satisfy the structural strength
requirements. The net thickness philosophy aims to
provide a link between the assumed reduction in
strength during new building strength evaluations and
the in-service gauging assessment criteria. Renewal
values were to cover general and local corrosion given
in millimeters and to be indicated in the vessel s draw-
ing. Large areas like the girder cross section do not
corrode uniformly. Any assumptions made during newbuilding evaluation need to be followed up during the
operation of the vessel.
IACS claimed that the actual corrosion values included
in the rules were established based on the extensive
work of the IACS working party on strength that as-
sembled a database of over 600,000 thickness measure-ments. This database covers measurements taken over a
wide variation of corrosion associated with exposure to
the marine environment such as various types of dry
cargo, crude oil and products cargoes, ballast water,
temperature variations, etc. and covers the structural
members to which the margins are applied. Corrosionprocesses from initial occurrence and during propaga-
tion were investigated based on extensive thickness
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measurement data. A corrosion process model was de-
veloped based on probabilistic theories thus estimating
the thickness diminution of structural members.
A statistical analysis of the database was performed in
order to extract the margins associated with a probabili-
ty of the margin 95% for 25 years. Pre-CSR strength
evaluations were made based on gross thickness or local
simplified buckling evaluations. In CSR well defined
margins in millimeters are set instead of percentages ofinitial thickness. In addition, field stresses on Pre-CSR
are based on gross scantlings. In CSR, same are based
on hull girder properties with a 10% Diminution. Oneissue that has to be considered in the future is the fact
that for smaller vessels built with thinner members theCSR corrosion margins look excessive.
Fig. 1 Net ThicknessGeneral Corrosion
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2.2 Local Strength
Basic Rule Requirements for local strength are built on
the basis of First Principles consistent with other rule
sections (Net Thickness, FE Analysis, Fatigue), on
Load-based formulas for thickness and a section mod-
ulus based on 25-years life, on minimum thickness,stiffness / minimum depth and in some additional em-
pirical requirements.
2.3 Fat igue Strength Assessment
Basic Rules Requirements for Fatigue strength assess-
ment are the JTP wave-induced loads, two loading con-ditions (full load/normal ballast), the JTP net thickness
concept, the Palmgren-Miners linear damage model,
and the long term stress range distribution (two ap-
proaches, i.e. normal stress in conjunction with a stress
concentration factor and hot spot stress).
2.4 FEM Analysis
Strength analysis by FEM at Midship cargo region isrequired as part of the rules to verify that the ship struc-
ture is within the Class requirements. FEM analysis is
also applied for various Load Cases.
Fine Mesh analysis is applied at high stress areas and
also in openings and main brackets on two typical
transverse web frames.
2.5 Advance Buckl ing Analysis
Buckling analysis is based on non-linear analysis tech-
niques. The JTP rules give general requirements for
advanced buckling analysis and for the application of
structural modeling principles and assessment criteria.
The JTP Rules allow the use of the ultimate strength for
certain structural elements subject to lifetime extreme
loading. Ultimate strength assesses the section as a
whole.
2.6 CSR Advantages
All Classification Societies will use the same structuralrules so if a design is approved by one Classification
then it will automatically meet the requirements of any
other Society. In addition, the technical basis of the
Rules was open to public scrutiny not mentioning the
fact that many comments submitted by the Industry
have been largely ignored by the JTP.
3. Ramification Study for a VLCC Tanker
As per IACS and for the Midship cargo region of a
VLCC tanker the thickness of the deck plating in way ofcargo tanks had to be increased due to buckling. Bottom
and side shell plating had to be also increased in some
locations for the same reason. In addition, plating thick-nesses for the mid and upper part of the inner longitu-
dinal bulkhead were increased due to local require-
ment basically because of yielding and buckling. Plate
thickness was increased in the upper part of the side
longitudinal bulkhead due to prescriptive buckling.
The dimensions of the longitudinal stiffeners were in-
creased for the bottom and the inner bottom, the hopper
area, the inner longitudinal bulkhead and in the bottom
& upper part of the side shell. Plate thickness was in-
creased as well in way of side cargo tank transverse
bulkhead (mid and upper part) due to buckling.
In a typical Web section, in the swash bulkhead and in
the stringers, panel stiffeners have been modified due tobuckling or yielding.
INTERTANKO LATIN AMERICAN PANEL, IACS Common Structural Rules, Rio de Janeiro, 25 April 2006
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4. The discussion on the Common Structural
Rules
During the discussion on the CSR a number of ques-
tions summarized the agony of the technical people in
the industry towards improving the structure of tankers.
We will quote below a number of questions and answers
as published in the JTP documents.
4.1 Dennis Ar nett, Chevron Texaco (ref . No. 947):
Going into this project, it was stated that the tanker that
would result from JTP Rules will be at least as good as
one from the current Rules of any other three class so-
cieties involved. From the limited consequence calcula-tions provided, it is difficult to readily see that this aim
in fact has been achieved, and this is not to say that such
result is necessarily the realistic one to the expert. How-
ever, it would be fair to say that related clarifications
and education / communication are needed to facilitate
more ready acceptance of the proposed rules.
Reply from JTP: The consequence assessments to date
have demonstrated that scantlings increase in most cas-
es. The critical areas targeted for increases have yielded
these increases. Of course, the change of approach also
shows some areas as reducing in scantlings although the
overall safety level increases. The ship designs that have
been heavily optimized see the largest increases. The
new Rules should therefore create a ship with higher
gross scantlings overall. It is fair comment that more
feedback is needed from consequence studies. The cur-
rent consequence document is dated 23rd July. Further
work is in process to update this.
4.2 S. Hatzigr igori s, Kr isten Navigation (ref. No. 157):
Ref:#703 - The JTP common Rules should be at least as
safe and durable, equivalent or better than the current
Rules. To avoid misunderstandings and miss-
implementations this should be clearly stated in the
preamble of the Rules.
Reply from JTP: JTP agrees with the statement and
considers that sufficient documentation exists in the
background documents. The foreword to the Rules con-
tains the sentence "It is to be particularly noted that in
developing these Rules due cognizance has been takenof ensuring that the overall safety of the hull structure is
equivalent or better than existing design criteria, has
sufficient durability in terms of corrosion margin and
fatigue strength, and the need to transparently define the
relationship between the required scantlings for design
and those for renewal".
4.3 Dennis Ar nett, Chevron Texaco (ref . No. 821):
Do the new joint tanker Rules increase or decrease the
existing deck local design head, such as for green water
shipping effect? Please provide related comparisons for
review.
Reply from JTP: The JTP Rules increase the existing
deck local design head, e.g. green water head. As thedeck scantlings required by the present class Rules are
mainly dominated by the buckling requirements. The
impact of the increased green sea pressure is not sub-
stantial.
4.4 S. Hatzigr igori s, Kr isten Navigation (ref. No. 298):
In the proposed Rules the maximum allowable still
water bending moments are extracted from vesselsLoading Manual. We have seen Aframax designs (basi-
cally sister ships) with maximum allowable BM varying
from 250000tm to 340000tm. The minimum allowable
values should be determined by the Rules and not left to
the skilful shipyard engineer.
Reply from JTP: Your comment is noted and we agree
to include a Rule minimum still water bending moment.
This is presently being worked on and will be included
in the rules prior to publication.
4.5 Hideaki Naoi, Kawasaki Shipbui lding (ref. No 249):
Please explain the detail of feedback from serviceexperience and measurement.
Reply from JTP: Feedback from service experiencerefers to experience obtained from structural damage
and is embodied with current classification Rule re-
quirements and hence incorporated into the JTP Rules.
4.6 Ou Kitamura MHI (ref No 385):
JTP states that draft JTP-CSR brings about increased
scantlings above those given by the existing Rules of
either ABS, DNV or LRS. From technical point of view,
the lower bound scantlings among the three or more
Class Rules can be most likely to give the minimum
figures, provided that feedback from success-
ful/unsuccessful experience in service has been contin-ued. Otherwise the structural safety level cannot be
controlled consistently. It is obvious that the thorough
review of the consequent impact given by the draft CSR
has not yet been completed by JTP. This means that nocalibration has been made systematically. In some cases
overestimation of the safety level may be found as well
as underestimation as the results of a series of calibra-
tion. Reasoning or technical background must be pre-
sented for Each Rule revision based on not impractica-
ble theory but actual results in principle.
Reply from JTP: JTP is continuing to calibrate and fine
tune the Rule requirements to ensure the structural safe-
ty level is controlled consistently.
The preamble of the June 2004 edition of the CSR
stated:
It is to be particularly noted that in developing these
Rules due cognizance has been taken of ensuring that
overall safety of the hull structure is equivalent or better
than existing design criteria, has sufficient durability in
terms of corrosion margin and fatigue strength.
The preamble of the January 2006 final edition of the
CSR stated:
The objectives of the Rules are to establish require-
ments to reduce the risks of structural failure in order to
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help improve the safety of life, environment and proper-
ty and to provide adequate durability of the hull struc-
ture for the design life.
5. Structural Drawing Comparison between a
pre-CSR and a CSR VLCC design
In this part of the paper we will compare the structure of
a pre-CSR and a CSR design developed by the same FarEastern yard. The Frame spacing on the CSR design
was increased from 5,110 mm to 5680 mm. This will
normally produce thicker plating requirements. Given
that the scantlings are roughly proportional to the square
of the span, the geometric properties such as sectionmodulus for example need to be increased by about
23%. The percentage of the high tensile steel on the
CSR design was increased to 45 % up from 30 %. This
is an issue to be discussed since one of the effects of the
CSR seems to be the tendency to increase HTS in an
effort to reduce cost to the yard.
5.1 Midship Section
In the hopper area the lower plate thickness was de-
creased by as much as 4.5 mm and the upper plate by
3.5 mm. This thickness difference reduces the total
sectional area of the hopper by 17%. Size of longitu-
dinal stiffeners in the hopper area has been reduced in
the CSR design but the material has been upgraded to
high tensile steel so the strength is similar. This thick-
ness reduction is probably acceptable by the analysis but
it is contrary to all rules of shipbuilding where we try to
keep uniform strength in critical areas such as the hop-
per and the bilge areas.
In addition for the CSR design the cross ties height was
reduced from 2,120 mm to 1,920 mm. whilst the plate
thickness was increased by as much as 2 mm. Given
that the section modulus of the cross tie is proportional
to the square of the height, the thickness needs to be
increased by more than (2.12/1.92)2 = 1.22 or 22% tohave equivalent structure (1.22x18 = 22 mm). Lack of
attention to structural details was evident and scallops
were allowed in high stress areas.
Reduction in breadth and slight increase of plate thick-
ness appear also in the forward transverse web frame
section, where the frame breadth is reduced by 730 mm.
and the plate thickness increased by 2 mm. In addition
the radius of the lower bracket in way of the wing cargo
tanks has been reduced from R 4,000 to R 3, 375.
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The main deck plate thickness is reduced at the center
tank by 1.5 mm at wing cargo tanks by 1.0 mm and in
way of the ballast tanks by 2.0 mm. The sectional areafor the under deck longitudinal increased by 25% in
center tanks and by 15% in wing cargo tanks.
The bottom plate thickness was reduced by 1.0 to 1.5
mm but the sectional area for the bottom longitudinal
was increased by 15%. The inner bottom plate thickness
was reduced by 0.5 to 1.0 mm and the sectional area for
the inner bottom longitudinals increased by 2%. The
double bottom is an area in which the CSR produced abetter structure than before.
5.2 Typical transverse bulkhead
Significant differences appeared in the design of the
stringers. For the No 1 stringer in way of the center
tanks the length was reduced by 300 mm, the plate
thickness reduced by 3mm. and the shear area for the
center part of stringer reduced by 21%. The number of
the transverse stiffeners was reduced from three to two.
Similar reductions appeared for the No 2 and the No 3
stringers in the center tanks.
In addition for the No 1 stringer in way of wing tank the
stringer the breadth was reduced by 300 mm, the plate
thickness reduced by 2mm and the shear area reduced
by 18%. The number of transverse stiffeners below theplate was reduced from three to two. Breadth and plate
thickness reductions were applied in way of No 2 strin-
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ger of Swash Bulkhead.
5.3 Construction profi les and deck plans
The main deck plate thickness in way of No 1 tank has
been decreased for the CSR design. At the forward part
of the tank the thickness was reduced from 17.0 mm to14.5mm, at the mid part from 18.5mm to 14.5mm and at
the aft part from 18.0mm AH to 15.5mm
Similar dramatic reductions were also effected on the
stringers of the swash bulkhead.
The main deck plate thickness in way of No 1 tank has been decreased for the CSR design. At the forward part of the
tank the thickness was reduced from 17.0 mm to 14.5mm, at the mid part from 18.5mm to 14.5mm and at the aft part
from 18.0mm AH to 15.5mm.
Similar thickness reductions were observed also in way of the No 5 Cargo Tanks where the thickness was reduced
from 22.0 AH to 20,0 AH and in way of the Slop Tanks from 18.5 mm to 15.0 mm
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5.4 Shell expansion
The side shell plate thickness was reduced in several
locations. For example in upper part of Engine Room
the plate thickness at Pre-CSR design was 20.0 mm. For
the same area the thickness in the CSR design was re-duced to 16.5 mm and in some locations to 15.0mm.
5.5 Fore body constructi on
The under deck stiffening for Main Deck as well as forbelow platforms was substantially weaker. Also the
Inner Bottom structure (3,000 A/B) that had been in-
stalled at Pre-CSR design was removed in the new de-
sign.
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6. Corrosion Margins
One of the promises given during the presentation of the
CSR was that the structure of a ship, based on the net
scantlings approach was to be more robust than the pre-
CSR ships. This implies that if renewal thicknesses are
to be compared between the two sets of rules the CSR
net scantlings should be higher. In the table prepared
below the pre-CSR corrosion margins of 20 % are com-
pared with the CSR renewal thicknesses. The conclu-
sions are that the CSR rules allow for lower renewal
thicknesses than the pre-CSR rules for several areas of
the ship. This is something that should be studied by the
JTP and it will be difficult to prove how a more robust
ship is a ship with thinner minimum scantlings.
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7. The shortcomings of the CSR
There is an urgent need for harmonization. The two sets
of rules (Bulk Carriers and Tankers) are different in
fundamental aspects of the scantling assessment such as
buckling, fatigue, loads and direct strength analysis.
IACS have initiated the harmonization process two
years ago but there are no progress reports so far onwhat has been achieved or on what is in the pipeline. An
honest harmonization has to be done that will take into
account the different loading and operating conditions
between tankers and bulk carriers. If there are differenc-
es and harmonization in a set of formulae is impossible,
then different calculations should apply to the two dif-
ferent types of ships. Harmonization should not result in
any downgrading of the tankers or the bulk carriersrules.
The problem with the CSR is that they do not specify
basic structural dimensions such as for example depth of
bulkhead girders or stringers, sizes of brackets, number
of tripping brackets and stiffening patterns. The Yardhas freedom to optimize the design and reduce theaforementioned stiffening and sizes accordingly. Our
experience is lowering of the pre-CSR standards as far
steel distribution is concerned. This type of design is
passing through the approval process, since the direct
stress analysis is lenient and forgiving. Zooming analy-sis with mesh refinement would probably have reflected
the problems. One solution may be to ask for zooming
analysis for certain critical details. It is reminded that
these members (also called primary supporting mem-
bers) are estimated in the tanker CSR rules primarily on
the basis of direct strength analysis. The prescriptiveformulation is also applied and a 15% reduction in the
resulting scantlings is acceptable if allowed by the direct
strength analysis. This CSR requirement was fiercely
opposed by the Industry but without any action from the
JTP group. Even worst, in bulk carriers there are not
even prescriptive rules for the primary support mem-bers.
The rules are problematic towards the ends of the ves-
sel. A known deficiency of the three hold model is that
it cannot be applied to the first and last cargo holds. In
that case the only way is the conservative way, i.e. toprescribe the same scantlings as the ones in the mid-ship
region. Even more importantly, there is no specific andclear method to assess the structural integrity of the
stern and the bow. Bow is subject to high frequency
slamming loads as well as increased inertia loads. Stern
is where the propulsion of the vessel is located and issubject to a spectrum of loads at a range of frequencies.
Neither bow nor stern area of the ship are addressed in
the CSR in a clear cut way. The recent common inter-
pretation procedure (published on 01 January 2010)
improves the situation but the ends of the ship are far
weaker than the pre-CSR rules. The problem is aggra-vated by the lack of transparency on the tapering of the
scantlings and associated bending moments and shear
forces at the ends.The lack of common software to assess the CSR scan-
tlings creates discrepancies in various designs. IACS
has abandoned the practice followed in the first days of
CSR application. In the beginning of CSR application,
the MOU between Class Societies dictated that the scan-
tling evaluation will be performed by all. Nowadays, the
MOU dictates that only the initial scantling assessmentis necessary and the next classification society accepts
the results of the first. Two class societies merged their
man-power to develop common software. Two ques-tions result immediately from this news. Why not a
common software for all class societies? What aboutthe validity of the scantlings of the designs approved by
these two class societies before the merging of their
software?
There are no clear criteria for the analysis of high stress
areas. It seems that in areas of high stresses (which are
somewhat arbitrarily defined), there is a need for meshrefinement and further analysis. The stresses in the
refined models are allowed to exceed the yield strength.
This is a physical impossibility. Of course it is unders-
tood that it is not feasible to perform an elastic-plastic
analysis for each critical structural detail. On the other
hand it appears that IACS has set a series of acceptancelimits on the maximum stress which are not based on
clear grounds and analysis.
There are areas which are not handled in the CSR, as for
example the problem of ship response to torsional loads.
Torsional loads might be critical for bulk carriers. Even
more so, since IACS accepts the so called hybrid bulk
carriers which have hopper and top side tanks at either
the first and/or the last cargo holds. The remaining car-
go holds do not have hopper and top side tanks, resem-
bling a typical containership and suffering from the
effects of torsion in the same way. To the best of ourknowledge, the strength of the scantlings of the inner
bottom and hopper plate to withstand steel coils is de-
fined in error and need to be re-evaluated. Potentially
there are numerous errors and carry-over material which
is never implemented like for example Section 5, Chap-
ter 8 of the CSR for Bulk Carriers regarding the stressassessment of hatch corners. So far the Rules dictate
that if the thickness of the hatch corners is increased by
up to 60% related to the thickness of the surrounding
deck plate, then there is no need for further analysis. It
remains to be seen if this simplistic consideration will
result in crack-free hatch corners. It is also a common
secret that IACS recommends weld grinding at the in-tersection of the tank top with lower stool. The latter
appears to be the most critical part of the vessel and
impossible to comply with the required fatigue life of 25
years. IACS was quick to reduce this number to below
20 years in case grinding is applied on the weld as de-
manded by the Shipyards of Korea and Japan where
labor cost is high.
Results of fatigue analysis in tankers are somewhat
unrealistic due to prediction of critical areas in the deck
and bottom. It is well known that cracks in tankers aremore prevalent in the side shell longitudinal stiffener-
web-bulkhead crossings for the area between the full
load and normal ballast waterlines.
Shear strength for tankers and bulk carriers is based on
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semi-empirical formulations instead of the well estab-
lished shear flow distribution techniques. In other cases,
very advanced techniques like nonlinear buckling analy-
sis are employed in the rules but the method utilized is
lacking solid theoretical foundation and it represents a
black box for the practicing naval architect.
It appears that IACS has still to agree on the size of the
insert plates required at the critical stress areas as well
as on the type of analysis required in these areas. Forexample the lower and upper hopper connections with
the inner shell are highly stressed areas and need to be
analyzed by refined mesh and evaluated by finite ele-
ment based stresses in fatigue strength. Resulting insert
plate thicknesses are unrealistically high compared to
the surrounding plate.
CSR revisions and corrigenda (?) are invariably reduc-
ing the scantlings. This is done without first gettingexperience/operational feedback from the first version
of the rules.
This practice should be stopped unless there are sound
technical reasons proving that scantlings could get ligh-ter.
The reaction of the shipyards to the CSR was to increase
the HTS portion in order to get a similar steel weight for
designs of standard dimensions. This, coupled with the
problems mentioned above, is now resulting in lighter
hull weights. Buyers (based on the information provided
on the erroneous ramification studies published before
the introduction of the CSR) have more or less acceptedto pay a premium for CSR designs. Now it is common
knowledge that they are not getting any value for the
extra money that they paid.
For certain areas of the ships the new corrosion margins
result in smaller net scantlings than that in the pre-CSR
era. This has been done without any justification from
the experience gained from the operation of the ships.
The introduction of the PSPC requirements will im-
prove the situation as far as ballast tanks corrosion is
concerned but it will be interesting to monitor the net
scantlings approach of the CSR on large bulk carrierscarrying corrosive cargoes.
8. The IACS procedure for Rule Changes
As already mentioned above there have been serious
concerns about the way that was followed during thedevelopment of the CSR, the methodology used and the
political power games that influenced the develop-
ment process.
It has also been shown that the theoretical approach
followed and the subsequent calibration of the structural
models (used to reduce CSR scantlings to values close
to the pre-CSR rules) have resulted in scantlings that
could be lower than the pre-CSR rules. Shipyards have
followed two different approaches:
Using the CSR in order to improve pre-CSR designs
and starting for a blank piece of paper and taking advan-tage of the CSR loopholes.
The designs produced by shipyards that followed the
first approach seem to be satisfactory although there is
no guarantee given that the same calculation methods
will be followed specially after comparing their work
with the work done by builders that followed the blank
paper solution.
The increase of frame spacing and HTS percentage are
issues that may cause problems in the future and willhave to be studied further together with the general
trend seen to avoid the use of tripping brackets, reduce
the size of primary and secondary supporting membersand other similar issues. Furthermore, buckling prevent-
ing devices such as large tripping brackets and supports
should be examined with fine mesh analysis and re-
duced buckling coefficients in particular in areas where
unstable buckling is to be expected.
IACS in their effort to maintain the political stability in
their organization have issued Procedural Requirement
No 32 according to which a very complex organizationhas been set up for further developing the CSR. This
organization includes the Council, the Steering Group
for the CSR, the CSR Secretariat, the Hull Panel, the
CSR Maintenance Project Teams, the CSR Harmoniza-tion Management Team and the CSR Harmonization
Project Teams.
If we consider that most of the CSR changes (with the
exception of the UI on tapering) that have been pub-
lished until today have mainly reduced the scantlings
required by the original version of the CSR, it is reason-
able to assume that either no improvement proposals
have been submitted or that improvement proposals are
impossible to pass through the complex mechanism
discussed above.
If this is coupled with the existence of numerous loop
holes in the CSR and the ingenuity of shipyard design
engineers, it will become obvious that a large number ofships will be delivered before any improvement of the
CSR can be achieved. Unfortunately if this comes true,
it will bring the industry back to the late 1980s early 90s
where a number of serious structural damages that were
attributed to inadequate building rules rocked the ship-
ping industry. It was during that period that the image of
Class Societies reached its nadir and prompted under-
writers to ask for their own structural surveys on ships.
The only way to save the day for IACS is for them to
revise their procedures and create a more flexible, dem-
ocratic and transparent way for improving the rules.
Accidents, today, receive a lot of publicity and Societiesshould consider seriously the liabilities associated with
the possible structural failures of ships.
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9. Conclusions
The new CSR rules leave space for a lot of substantial
improvements.
Class Societies are invited to decide if the new CSRrules are better than the rules that they replaced and in
particular if:
a. We have seen a real substantial improvement.b. If, within the near future, a common interpretation
will prevail to the individual ones that we are cur-
rently getting from the Societies.
c. If and how they will control the innovative yardengineers.
d. If changes will improve or water down the rules.e. If harmonization between the JTP and the JBP will
work to the detriment of the tankers rules.
f. If the higher use of HTS that the yards are using asan antidote to the CSR will enhance tanker safety.
This is of particular importance for yards that do not
have extensive experience with the use of HTS.
It has to be said at this point that the opinion of the
authors is that changes are needed on an urgent basis
and that we do not have the time to wait to get expe-
rience from the application of the new rules.
We do not see any radical changes in the methodology
used for the development of the rules and we do not
want to end up with a generation of structurally proble-
matic ships as happened during the late 80s and the
early 90s.
.
10. Acknowledgements
Our special thanks to all the Class Societies people that
were involved in the development of the CSR. The
discussion that will follow the first pure CSR designswill give the industry the opportunity to deepen and
enrich the understanding of the structural problemsassociated with the design of large tanker structures.
Our appreciation to the Far Eastern yard VLCC design
engineers that gave us the opportunity to spend a num-
ber of late evenings evaluating their work instead of
watching Greek TV channels of questionable quality(this goes both for the news / political and the enter-
tainment broadcasts).