Assessment Corrosion Risks

7/31/2019 Assessment Corrosion Risks

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ABS TECHNICAL PAPERS 2003

Assessment of Corrosion Risks to Aging Ships Using an Experience Database 149

Proceedings of OMAE 2003

22nd

International Conference on Offshore Mechanics and Arctic Engineering

8-13 JUNE 2003, CANCUN, MEXICO

OMAE2003-37299

ASSESSMENT OF CORROSION RISKS TO AGING SHIPS

USING AN EXPERIENCE DATABASE

Ge Wang1, John Spencer, Haihong Sun

American Bureau of Shipping16855 Northchase Drive, Houston, TX, USA, 77060

email1: [email protected]

ABSTRACT

Damages to ships due to corrosion are very likely,

and the likelihood increases with the aging of ships. Risk

and reliability approaches are more and more frequently

applied in design and maintenance planning. These

advanced approaches require reliable data reflecting the

structural condition of ships in service. Such data is

scarce.

This paper presents a database of corrosion wastage.It is based on over 110,00 thickness measurements

recently collected from 140 trading tankers. This

database is larger than most other corrosion databases in

the public domain. Corrosion wastage exhibits a high

level of variability. In addition to thickness measurements

of individual structural members, this database also has

information on hull girders geometrical properties and

strength of ships in service. Corrosion wastage has an

influence on the hull girder strength.

Statistical interpretations of the database are used to

represent corrosion wastage in oil tankers. The severity of

corrosion is ranked by three levels: slight, moderate andsevere levels corresponding respectively to 50, 75 and

95% cumulative probability on the database.

The risks of corrosion wastage to aging ships

structural integrity are assessed using the observations of

the corrosion wastage database. The investigated risks are

loss of local members strength, loss of global hull girder

strength, and shortened inspection intervals.

The experience database can be used in many

aspects, such as design requirements for corrosion

additions and wastage allowance for plate renewal,

establishment of limits to hull girder strength of FPSOs,

time variant reliability approach and risk based

inspection schemes.

INTRODUCTION

Figure 1 shows the underdeck area of a 22-year-old

tanker (ABS 2001). The deck plates and deck

longitudinals suffered various degrees of corrosion. In

some locations, the web plate of some deck longitudinals

was totally wasted away. This caused loss of support of

deck plates from deck longitudinals. The unsupported

span of the deck plate increased, with a corresponding

decrease in buckling strength. In heavy seas, bucklingrepeatedly occurred under the action of the cyclic wave

loads. Plastic deformation accumulated and eventually

cracks appeared.

Statistics reveal that corrosion is the number one

cause for marine casualties in old ships (Harada et al.

2001). Damages to ships due to corrosion are very likely,

and the likelihood increases with the aging of ships.

The consequences of corrosion wastage can be local

or minor, but also can be very serious in some

circumstances. Severe corrosion has resulted in deck

cracks across almost the entire ship width (ABS 2001),

and has even resulted in the loss of ships (JMT 1997).Structures deteriorate over time due to corrosion.

This causes variability in structural properties and

capability. Traditional engineering and analysis use

simplified deterministic approaches to account for this

time-variant random process; in most cases some

nominal values are predefined for corrosion additions

(e.g., Wang et al. 2002). A more rational and direct

approach is to model the uncertainties probabilistically.

There is a clear trend that engineering analysis and

design standards are moving toward reliability-based

formats.


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150 Assessment of Corrosion Risks to Aging Ships Using an Experience Database

Originally, the structural reliability approach was

introduced for establishing safety factors. Probabilistic

presentations of global and local loads have been

developed, and structural failure modes and limit states

have been extensively studied. As a result, the reliability

approach has been refined and applied to some

engineering problems (Guedes Soares et al. 1989,

Mansour 1997, Wang et al. 1996, Melchers 1999).

Recently, there is an increased interest in developingand demonstrating the time variant reliability (TVR)

approach to explicitly address the uncertainties due to

structural deterioration (e.g., Guedes Soares et al. 1996,

Wirsching et al. 1997, Sun & Bai 2001, Ivanov et al.

2003, Qin and Cui 2002, Paik et al. 2003). The TVR

approach is more suitable to the assessment of the

strength of ships in service and new constructions, and

can also be used in maintenance or inspection planning,

and development of new designs.

The success of these state-of-the-art technologies

depends to a large extent on reliable estimates of

corrosion wastage of various structural members. Thereare very few databases of corrosion wastage available in

the literature. The Tanker Structure Co-operative Forum

guidance (TSCF 1992) is based on thickness

Figure 1. Heavily corroded under-deck of a 22 year old oil tanker (ABS 2001)

Table 1. Main details of the corrosion wastage database and comparisons with other database of oil tankers introduced

in the public domain

The present database TSCF (1992) Harada et al. (2001) Paik et al. (2003)

Ship type Single hull oil tankers Single hull tankers Single hull tankers Single hull tankers

Data sources SafeHull Condition Assessment Owner, class Gauging records Gauging reports

Vessels 140 52 197 >100

Gauging reports 157 Not known 346 Not known

Thick. measurements 110,082 Not known > 250,000 33,820

Info. Hull strength Yes, 599 sections No No No

Ship size 168 ~ 401 meters > 150, 000 DWT 100 ~ 400 meters Not known

Service years 12 ~ 26, 32 years ~ 25 years ~ 23 years 12 ~ 26 years

Class ABS, LR, NK, DnV, KR ABS, DnV, LR, NK NK KR, ABS

Ship built Mostly 1970s, some 1980s 1960s ~ 1980s Not known Not known

Ship measured 1992 2000 Not known Not known Not known


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Assessment of Corrosion Risks to Aging Ships Using an Experience Database 151e

measurements of 52 oil tankers. Yamamoto and Ikegama

(1998) introduced a database of 50 bulk carriers. There

was a probabilistic corrosion rate estimation model

developed from and calibrated with the measurements of

44 bulk carriers (Paik et al. 1998), and more than 100 oil

tankers (Paik et al. 2003). These databases are, however,

relatively small in size, and some are not representativeof commercial ships of today. Harada et al. (2001)

collected a database from 197 oil tankers. This database

has been circulated with a working group of the

International Association of Classification Societies

(IACS), and has not been released to the public.

There is a need to develop a sizable database that

reflects, as close as possible, the structural conditions of

ships in service.

This paper presents a database of corrosion wastage

of oil tankers. It is aimed to provide a more realistic

picture of corrosion wastage of oil tankers.

This newly developed database has been analyzed,and general trends of corrosion wastage, which change

over the service life, have been studied.

Discussion is given to some safety issues of tankers

from the standpoints of both local strength of individual

structural members and global hull girder strength.

It is expected that the database will enhance and

update the knowledge about corrosion wastage in oil

tankers, and also provide more realistic estimates of

corrosion for structural members that can form a reliable

basis for a quantitative assessment of structural integrity

of ships in service.

A NEW CORROSION WASTAGE DATABASE

A new corrosion wastage database was built recently

at ABS. It is an integral part of the efforts to develop

reliability based design standards.

Database particulars

The database has more than 110,000 corrosion

wastage measurements of various structural members,

which are collected from 157 gauging reports of 140

tankers. Most of the ships are still in service. Some have

been or will be converted to FPSOs. The ships are classed

with five major classification societies. The ship length

ranges from 170 m to 400 m. They were 12 to 33 yearsold when thickness measurements were taken.

Table 1 summarizes some of the main details of this

database. Table 1 also includes corrosion databases on oil

tankers that have been introduced in the literature.

Obviously, there are only a limited number of databases

on corrosion wastage. The present database is one of the

largest of its kind, second only to Harada et al. (2001). It

provides up-to-date information about corrosion in oil

tankers.

The database also includes information about the

hull girder strength, as this is calculated and used for

assessing the ships structural adequacy for its intendedservice. This database is the only one that has

information about hull girder sectional properties for

ships in service (see Table 1).

Figures 2 and 3 are ship age and length profiles of

the sampled ships. These ships are representative of

modern single hull oil tankers.

Data sources

The data comes from the ABS SafeHull Condition

Assessment Program (CAP). CAP is a service separate

from and a supplement to classification (Horn et al.1994). The CAP offers an evaluation of ship structure

Distribution of Vessel Age at the Time of

Gauging (158 Records)

0%

5%

10%

15%

20%

12

14

16

18

20

22

24

26

28

30

32

Vessel Age at the Time of Gauging

Frequency

Figure 2. Profile of ship age at the time of thickness

measurement (157 gauging reports, 140 oil tankers)

Distribution of Ship Length

(140 Vessels)

0%

10%

20%

30%

40%

150

180

210

240

270

300

330

360

390

420

Ship Length (m)

Frequency

Figure 3. Profile of ship length (157 gauging reports,

140 oil tankers)


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recognizing the effects of corrosion with respect to

yielding, buckling and fatigue. Based on extensive

surveys, the CAP database provides a wealth of

information regarding the structural condition of ships in

service.

The database reflects the condition of single hull oil

tankers in service. For ships in CAP, plate thicknessmeasurements of heavily wasted structural members are

recorded and are not excluded from the thickness

measurement reports. They are recorded as is, and repair

work, if necessary, is recommended after the ships

condition is assessed. Traditional gauging reports for

ships in service as required by classification societies,

which almost all the available databases are based upon,

may not include thickness measurements below the

wastage limits. Thickness measurements obtained as part

of CAP evaluations may give a more realistic picture of

the actual corrosion wastage trends. On the other hand,

the vessels assessed in CAP may be in relatively goodcondition. The ship owner probably believes that his ship

can be used for service for a few more years.

Substandard ships, though a small percentage of the fleet,

may not be found in CAP. In this sense, data gained from

CAP may not include the worst cases. Nevertheless,

thickness measurement data from ships in CAP are very

good records of the condition of ships in service.

Corrosion wastage

Wastage due to corrosion is calculated as the

difference between the as-built thickness and the

measured residual thickness.

Thickness measurements are relevant to general

corrosion, where the plates are assumed to be uniformly

wasted. Pitting and grooving are generally not fully

reflected in gauging reports.

Replaced plates

As usual with databases based on gauging reports,

the data may include plates that have been replaced.

However, such plates occupy only a small percentage of

the total. They do not have a prominent influence that

would skew the statistical characteristics of the database.

During the 3rd or 4th special survey, oil tankers in the

range of 150,000 to 300,000 deadweight tons may have toreplace up to 380 tons of steel (TSCF 1992). The hull of a

137,000 deadweight ton tanker weighs about 22,000 tons.

The steel renewal in the 3rd or 4th special survey

accounts for, at the maximum, about 1.7% of the total

steel weight. If it is a VLCC, the maximum percentage of

replaced steel can be less than 1.0% of the hulls steel

weight. The replaced steel plates, if there are some,

possibly occupy a very small percentage of the entire

population.

Nevertheless, thickness measurements corresponding

to probably replaced plates have been removed from the

database. Plates with very small wastage, say less than

0.01 mm, are screened out; they are probably plates

renewed after the ships delivery.

SOME OBSERVED TRENDS

The wastage measurements are categorized

according to location (structural member) and usage

space. The locations are deck, side, bottom and

longitudinal bulkheads. Both plates, and web and flanges

of longitudinals are investigated. In line with

classification rules for new construction designs, two

usage spaces are considered, i.e., cargo tanks and ballast

tanks.

The database provides a lot of information about the

trends of corrosion wastage in oil tankers. Table 2 and

Figures 4 and 5 are snapshots of the database.

Table 2 summarizes the mean values, standard

deviations and maximum values of corrosion

wastage measurements of various structural

members for 20 years of service.

Figure 4 shows the wastage measurements for

deck plates in cargo tanks in millimeters for

ships of 12 to 32 years old. One diamond mark

represents one measurement.

Figure 5 shows the loss of hull girder section

modulus at the deck over the past year. One

diamond mark represents one section of a ship.

Usually, a ship has about three girth belts

(transverse sections) gauged in one thickness

measurement survey.

Corrosion wastage exhibits a high level of variability

The maximum corrosion wastage is much higher

than the average. For example, for 20 years old

ships (Table 2 and Fig. 4), the maximum

observed wastage in deck plate in cargo tanks is

8.70 mm, while the average wastage is 1.1 mm.

Corrosion wastage measurements spread over

wide ranges. Some structural members exhibit

standard deviations higher than the averages,e.g., deck plates, bottom shell plates, and bottom

longitudinal flanges in cargo tanks (Table 2 and

Fig.4).

The maximum corrosion wastage seems to

be higher in cargo tanks than in ballast tanks

(Table 2).

The average corrosion wastage does not seem to

depend on the usage spaces (cargo or ballast

tank). See Table 2.


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One factor that may have influenced the data is

whether or not the space has been coated. Ballast tanks

generally have a corrosion protection system, whereascargo tanks may not. The presence or absence of a

coating is not noted in the database.

With the aging of ships, more steel is wasted.

The average corrosion wastage exhibits an

increasing trend with the passage of time (Fig.

4).

With the aging of ships, the spread of wastage

measurements becomes more prominent. The

standard deviations tend to increase with the

passage of time.

Figure 4 shows that corrosion wastage does not always

increase with the ships age. This observation is not new,

and has been demonstrated in previous studies. Most oiltankers are scraped at about 22-23 years old and older

(Harada et al. 2001). This database does not include

scraped ships, nor do any other databases. Therefore, the

worst conditions of ships much older than 23 years are

not covered in the database.

There are fluctuations in the average values and

standard deviations of corrosion wastage (Fig. 4). The

measurements come from a fleet of ships, and do not

represent a trend of a single plate in a specific ship. The

variability may be attributed to measurements not being

taken from a single ship, or at the same location. The

different maintenance of ships may also contribute.

Table 2 Corrosion wastage of various structural members at 20 years old (unit: mm)

Structure Tank Mean value Deviation Maximum 50 percentile 75 percentile 95 percentile

Cargo 1.096 1.564 8.70 0.60 1.10 3.50Dk pl

Ballast 1.020 0.771 4.15 0.80 1.40 2.40

Cargo 0.703 0.636 11.00 0.60 0.90 1.40Dk long web

Ballast 0.845 0.678 4.00 0.70 1.10 2.20

Cargo 0.561 0.197 1.20 0.60 0.70 0.90Dk long fl

Ballast 0.331 0.431 2.00 0.15 0.28 0.95

Cargo 0.789 1.048 8.20 0.50 0.82 2.00Side shell

Ballast 0.662 0.504 2.80 0.50 0.90 1.60

Cargo 0.640 0.437 5.00 0.60 0.83 1.30Side long web

Ballast 0.611 0.509 4.00 0.50 0.80 1.60

Cargo 0.543 0.353 2.30 0.50 0.70 1.20Side long fl

Ballast 0.551 0.500 4.50 0.50 0.70 1.43

Cargo 1.678 1.795 10.45 1.00 2.16 5.60Btm shell

Ballast 1.099 0.984 4.80 0.70 1.50 3.56

Cargo 0.547 0.481 3.10 0.42 0.70 1.30Btm long web

Ballast 0.440 0.332 1.40 0.30 0.60 1.15

Cargo 1.014 1.841 11.00 0.60 1.00 1.90Btm long fl

Ballast 1.138 2.118 10.55 0.50 1.00 2.73

Btw cargo 0.704 0.623 7.75 0.60 0.95 1.50Long bhd pl

Others 0.701 0.564 3.65 0.60 0.90 1.10

Cargo 0.589 0.426 3.45 0.50 0.75 1.40Bhd long web

Ballast - - - - - -

Cargo 0.683 0.583 8.60 0.60 0.85 1.30Bhd long fl

Ballast - - - - - -

Abbreviations: btw between, bhd bulkhead, dk deck, fl flange, long longitudinal, pl plate


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Corrosion wastage has an influence on the hull

girder strength

The information about the hull girder sectional

properties is extracted from the calculation results of the

ABS SafeHull Condition Assessment program. Ships in

CAP are evaluated for their local and global strength.

Figure 5 shows the reduction of section modulus tothe deck as a function of the vessel age. The mean values,

75 and 95 percentile curves are also shown.

The maximum SM reduction is close to 16% of the

as-built condition, which is for ships about 20 years old.

This may be the minimum strength that the present

design standards expect of a tanker.

The majority of ship sections, say at 95%

probability for a given age, have a maximum

reduction of about 10%. This is in line with the

IACS UR S7 requirement that ships in service

be at least 90% of the section modulus required

for new construction. The average SM reduction increases with ships

age. The lines of 75 and 95% percentile also

increase with ships age.

The drop at 24 years old is because most tankers are

scraped at 22 to 23 years, and the corrosion wastage

database does not include scraped ships. As expected, as

ships become older, the hull girder section modulus

reduces further.

SLIGHT, MODERATE AND SEVERE LEVELS OF

CORROSION WASTAGE

Because of the shown high variability, it appears that

the mean values and standard deviations are not

sufficient for presenting corrosion wastage. Statistical

interpretations of a large volume of records, such as the

present database, give more information, and should be

used to provide a more realistic picture of corrosion

wastage in commercial ships.

Despite continuous efforts on corrosion protection,

the mechanisms of corrosion in tankers are still not fully

understood. The inherent complexity casts questions

about the attempts to develop physical models for

predicting corrosion wastage, because the physical

models (e.g., Melchers 2001, Gardiner and Melchers2001) are usually limited to some well-defined

conditions, while it is recognized that there are a vast

variety of possible situations and causal factors.

There is a need to develop a more reliable, yet easy to

use, scheme to quantitatively describe corrosion wastage

in commercial ships.

Cumulative probability

One way to present this highly variable problem is to

assign cumulative probability values, and derive

corrosion wastage from the database accordingly. The

values of corrosion wastage as thus determined wouldmeasure the extent of structural deterioration in a

probabilistic manner.

Table 2 includes values of corrosion wastage

corresponding to 50, 75 and 95% cumulative probability

at 20 years.

Deck Plates in Cargo Tanks

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

10 15 20 25 30Age (Year)

CorrosionWastage(mm)

Measured

Average

95%

75%

50%

Figure 4. Corrosion wastage of deck plate in cargo

tanks (4665 thickness readings, 157 gauging reports,

140 oil tankers)

Loss of Section Modulus to Deck

0%

5%

10%

15%

20%

10 15 20 25

Age (years)

LossofSM(%as-built)

section

average95%75%

Figure 5. Loss of hull girder section modulus to deck

over time (599 sections)


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Figure 4 also includes wastage of deck plates in

cargo tanks for 50, 75 and 95% cumulative probability

values. The lines of 50, 75 and 95% percentile

demonstrate an increasing trend over time. They fluctuate

also because of the sampling, etc.

For deck plates in cargo tanks after 20 years of

service, a 1.10 mm corrosion wastage corresponds to a

75% cumulative probability. This means that the

cumulative probability of wastage measurements lessthan 1.10 mm is 75%, or, the wastage measurements

below 1.10 mm occupies 75% of all deck plate

measurements taken at 20 years.

Slight, moderate and severe levels of corrosion

It seems reasonable to categorize the corrosion

wastage based on the cumulative probability as follows:

Slight corresponds to a 50% percentile.

Moderate corresponds to a 75% percentile.

Severe corresponds to a 95% percentile.

The corrosion wastage approximately doubleswhen the cumulative probability is changed

from 50% to 75%, and roughly triples at 95%.

Most of the structural members have about 0.5

mm wastage for a 50% probability,

approximately 1.0 mm for a 75% probability,

and roughly 1.5 mm for a 95% probability.

Exceptions are deck plates and bottom shell

plates, which have much higher corrosion

wastage than other structural members.

This ranking, summarized in Table 3, provides a

convenient and practical vehicle for presenting a

highly variable problem

CORROSION RISK TO AGING SHIPS

Corrosion causes change in the thickness of

structures. With the aging of a ship, more and more steel

is wasted away, increasing the risks to the ships safety.

The majority of marine casualties involving ships older

than about 22 years is found to be due to corrosion

wastage.

Two sample ships will be used for following

discussions on some aspects of corrosion risks to aging

ships. Their details are listed in Table 4.

The differences in corrosion wastage between single

hull tankers and double hull tankers are not considered,

though such differences are recognized.

Table 3. Slight, moderate and severe corrosion levels

based on the cumulative probability of corrosion wastage

in the database

Levels Slight Moderate Severe

Cumulative probability 50% 75% 95%

Table 4. Particulars of a single hull and a

double hull tanker

Ship SHT DHT

Ship type Single hull Double hull

Construction Conversion to FPSO New build

Length (m) 346.0 315.82

Breadth (m) 60.0 58.0

Depth (m) 28.32 31.0

Ship built 1970 2001

Section modulus 103.2% required 103.6% required

Deck plate (mm) 24.0 19.0

Material HT36 HT32

Long. Sp. (mm) 966 913

Table 5. Buckling strength of deck plates for

different levels of corrosion wastage

Ship Corrosion Thick (mm) Buckling/yield

As-built 24.0 0.832

Slight 23.4 0.824

Moderate 22.9 0.816

SHT

Severe 20.5 0.770

As-built 19.0 0.788

Slight 18.4 0.774

Moderate 17.9 0.761

DHT

Severe 15.5 0.677

Table 6. Hull girder section strength for

different levels of corrosion wastage

Ship SHT DHT

As-built 100.0% 100.0%

Slight 97.0% 96.7%

Moderate 94.5% 94.0%

Severe 88.5% 87.3%


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Corrosion causes loss of strength of individual

structural members.

Some recent oil tanker incidents took place when

ships were loaded in a sagging condition. Deck plates

were under compression, and buckling and ultimate

strength were reduced due to wastage, which led to

catastrophic failure (ABS 2001).Table 5 shows the loss of buckling strength of deck

plates assuming that they are 20 years old and have

different levels of corrosion wastage. The plates are

compressed at the shorter edges from longitudinal

bending of the hull girder. The slight, moderate and

severe corrosion levels, corresponding to the 50, 75 and

95% percentiles, are based on Table 2 (for ships 20 years

old). They may be regarded as the results of different

maintenance practices, though other factors such as

coating condition may also play a role.

In the case of severe corrosion, the buckling strength

of deck plate is reduced by about 7% for the single hulltanker (SHT in Table 4), and by 14% for the double hull

tanker (DHT). Combined with the reduced hull girder

strength, the deck plates may buckle under heavy seas.

Corrosion causes loss of hull girder strength.

Hull girder section modulus is a well-accepted

parameter measuring the longitudinal bending strength of

ships. This is perhaps the single most important design

parameter describing hull girder strength. Hull girder

section modulus to the deck often determines the bending

strength of the entire hull girder.

Table 6 shows the loss of hull girder section modulus

to deck as a result of different levels of corrosion wastage.

When every structural member is severely corroded, the

single hull tanker (SHT) has a 11.5% reduction in hull

girder strength, and the double hull tanker (DHT) has a

12.3% reduction.

It is assumed that every member at the same location

(e.g., every strake at deck) has the same level of

corrosion. This assumption may not be realistic, but is

used here for convenience and demonstration purposes.

Figure 5 is a realistic picture of hull girder strength of

corroded ships.

Severe corrosion requires more frequent inspectionor maintenance.

Figure 6 is the estimated time-dependent annual

reliability index of a stiffened panel. Details of the

structural dimensions are in Table 7. This panel is at the

bottom of a cargo hold of a single hull tanker 232 meters

in length. The three corrosion levels specified in Tables 3

and 2 are assumed. The corresponding corrosion rates

obtained from Table 2 are assumed to remain constant

beyond 20 years old. Discussions on corrosion rates are

detailed in Wang et al. (2003).

This bottom panel is acted upon by in-plane

compression due to longitudinal bending and lateralloads due to water pressure. The ultimate strength of the

panel is calculated and compared with the external loads.

It is assumed that plates are replaced at special surveys

when failing the requirements of classification societies.

The spikes in Fig. 6 reflect the effects of plate renewal.

Details of this time-variant reliability assessment can be

found in Sun & Bai (2001) and Sun & Guedes Soares(2003).

3.0

3.1

3.2

3.3

0 5 10 15 20 25 30 35 40

Slight

Moderate

Severe

Age (Years )

AnnualReliabilityInd

ex

Figure 6. Annual reliability index of a stiffened panel at

a tankers bottom for different corrosion levels

2.1

2.2

2.3

2.4

2.5

2.6

0 5 10 15 20 25 30 35 40

Slight

Moderate

Severe

Age (Years)

AnnualR

eliabilityIndex

Figure 7. Annual reliability index of a stiffened panel at

a tankers deck for different corrosion levels

Table 7. Dimension of analyzed stiffened panels (mm)

Plate Web Flange

b t hw tw bf tf

Fig. 6 952 25.0 350 30.0 0.0 0.0

Fig. 7 950 28.0 595 14.0 180 25.0


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The renewal criteria in ABS Steel Vessel Rules were

used. Plate components that are wasted by 20% were

assumed to be renewed.

If corrosion remains slight, inspections at five-year

intervals will be sufficient, and no plate renewals are

needed for more than 30 years.

When experiencing moderate level of corrosion,inspections at five-year intervals seem sufficient for

maintaining the reliability index at reasonable level,

though plate renewals are expected after 30 years in

service.

When experiencing severe level of corrosion,

inspections at five-year intervals can not prevent the

reliability index from becoming too low. The curve of the

reliability index declines quickly. Within 5 years, the

reliability index decreases from 3.28 to 3.12, and plate

renewals are necessary at every special survey.

In order to maintain enough margin when severe

corrosion is anticipated, inspections should be conductedat intervals shorter than 5 years.

Similar conclusions can be drawn from the analyses

on a deck panel (Fig. 7) in the cargo hold and on the hull

girder (Fig. 8) of the same tanker. Details of structural

dimensions are also listed in Table 7.

APPLICATIONS OF THE DATABASE

The database can be used in some other applications,

in addition to those described in the previous section.

A sizable database is the key to the development of

corrosion wastage allowance in design standards.

Classification Societies have set safety standards

requiring that structural scantlings of ships be designed

with a certain allowance for corrosion wastage. This

allowance is often referred to as corrosion addition

(TSCF 1992). Ships in service are periodically surveyed

and inspected. While deemed necessary according to

defined criteria, i.e., the wastage allowances (TSCF

1992), wasted plates are recommended to be replaced.

To a large extent, the relevant requirements for

corrosion addition and wastage allowance were

empirically derived from experience. One of the key

issues is that there is very limited data, and a quantitative

assessment is nearly impossible.The corrosion wastage database in this paper has

extensive data, which makes it possible to quantitatively

evaluate corrosion in oil tankers.

A more refined approach for developing standards

regarding corrosion wastage should be based on thickness

measurement data, and use probabilistic interpretations of

the data. The approach includes: constructing a database

of corrosion wastage measurements, properly assigning

the level of confidence for these records, and obtaining

the corresponding values from the experience database.

For structural design purposes, corrosion additions

may be based on a moderate corrosion level, at about the75% percentile. For renewal criteria, corrosion wastage

allowance may be based on a severe corrosion level, at

approximately the 95% percentile. This study is ongoing

and will be reported in a future paper.

The experience gained in trading tanker designs

provides useful information for establishing limits to

strength of FPSOs.

Because of the limited experience of designing and

operating FPSOs, experience gained from trading tankersis often considered.

FPSOs are generally designed based on site-specific

environments. It is necessary to introduce limits to keep

design parameters from going too low. These limits

reflect successful experience, not to inadvertently create a

re-ordering of the dominant structural failure modes, and

to avoid the introduction of new controlling limit states

(ABS 2000).

It has been recognized that limits to the minimum

allowable hull girder strength should be established for

FPSOs to take into account the inevitable corrosion risks.

Oil tankers have exhibited possible strength reduction ofabout 10 to 16%, see Figure 5. The same level of strength

reduction may also need to be taken into account at the

design stage for FPSOs.

The database can be incorporated into a time

variant reliability approach.

One of main advantages in structural reliability

analysis is the recognition of the inherent uncertain

nature of various random variables. There is a need to

estimate the reliability of a structure over its lifetime to

take account of inspection and repairs.

Time variant reliability explicitly addresses the

effects of corrosion wastage on the structural integrity of

ships. This is a more refined reliability approach. One of

2.0

2.2

2.4

2.6

2.8

3.0

0 5 10 15 20 25 30 35 40

Slight

Moderate

Severe

Age (Years)

AnnualReliabilityInde

x

Figure 8. Annual reliability index of the hull girder

strength of an oil tanker for different corrosion levels


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the keys to the successful application of the time variant

reliability approach is the prediction of corrosion wastage

of structures over time.

In addition to Figs. 6 and 7, Figure 8 illustrates an

application of the time variant reliability approach to the

hull girder strength of a single hull tanker 232 meters in

length. Estimation of corrosion rates is detailed in aseparate paper (Wang et al. 2003). Plate renewals are

assumed to be conducted at special surveys when the

wastage exceeds the limits specified by classification

societies. The ultimate strength of the hull girder is

calculated using a program based on the Smiths method

(Sun and Bai 2001). Hull girder failure is defined as the

total bending moment exceeding the maximum hull

girder bending capacity, both of which are expressed in

probabilistic terms.

The database can be incorporated into a risk based

inspection planning scheme.

One of the major objectives of inspections is to detect

defects of any kind, and remedy the situation before the

defect develops into an unwanted event, for example, loss

of containment or failure of structures.

Inspections can possibly be conducted in a smarter

way if the likely situations can be predicted in advance,

and the associated risks can be properly assessed.

Corrosion wastage is the number one causes for marine

casualties in old ships. Predictions of corrosion wastage

over a ships life are very important.

Risk is often defined as the product of failure

consequence and probability of failure. According to the

failure consequence and failure type, the lower limit of

safety level of a component or structural system can be

defined in order to keep the component or structural

system free from failure. The likelihood of failure can be

determined by statistical studies, analytical solutions, or

both. The database can provide the foundation to evaluate

the risk due to corrosion damage and help to determine

inspection planning.

CONCLUSIONS

This paper presented a database of corrosion wastagethat contains more than 110,000 wastage measurements

collected from 140 oil tankers. This database also has

information about the hull girder strength of corroded

ships.

The following conclusions are reached:

Corrosion wastage exhibits high variability.

Corrosion wastage exhibits an increasing trend

with the passage of time.

Corrosion wastage has an influence on the hull

girder strength.

Based on the cumulative probability of measurementsin the database, corrosion wastage may be ranked in three

levels, slight, moderate and severe. This ranking scheme

provides a convenient vehicle to represent a highly

variable problem.

The risks of corrosion wastage to aging ships

structural integrity are discussed. The investigated risks

are loss of local members strength, loss of global hull

girder strength, and shortened inspection intervals.

The experience database can be used to develop (1)design requirements for corrosion additions and wastage

allowance for oil tankers, (2) design limits to the hull

girder strength of FPSOs, (3) a time variant reliability

approach, and (4) risk based inspection schemes.

ACKNOWLEDGMENTS

The authors appreciate very much the contributions

of Yongjun Chen, Tarek Elsayed and Sara Irwin in

building up the database. The authors wish to thank

many colleagues for their valuable comments and

reviews, especially those from J. Card, D. Diettrich, L.Ivanov, J. Baxter, Y. Shin, P. Rynn and K. Tamura. The

authors are indebted to Jo Feuerbacher for editing the

manuscript.

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