Hatzigrigoris.common Structura.2010.SYMP

7/29/2019 Hatzigrigoris.common Structura.2010.SYMP

1/13

1

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


2/13

2

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


3/13

3

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


4/13

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


5/13

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


6/13

6

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.


7/13

7

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-


8/13

8

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


9/13

9

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.


10/13

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.


11/13

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


12/13

12

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.


13/13

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).

Hatzigrigoris.common Structura.2010.SYMP

Documents

Transcript of Hatzigrigoris.common Structura.2010.SYMP