Assessment of Corrosion to Aging Ships Using an Experience Database

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Proceedings of OMAE 200322

ndInternational Conference on Offshore Mechanics and Arctic Engineering

8-13 JUNE 2003, CANCUN, MEXICO

OMAE2003-37299

ASSESSMENT OF CORROSION RISKS TO AGING SHIPS USING ANEXPERIENCE DATABASE

Ge Wang1, John Spencer, Haihong Sun

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

email1: [email protected]

ABSTRACTDamages 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 tothickness measurements of individual structural members, this

database also has information on hull girder’s 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 and severe

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

member’s strength, loss of global hull girder strength, andshortened 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.

INTRODUCTIONFigure 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

buckling repeatedly occurred under the action of the cycli

wave loads. Plastic deformation accumulated and eventually

cracks appeared.

Statistics reveal that corrosion is the number one cause formarine 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-varian

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 engineerin

analysis and design standards are moving toward reliability

based formats.

mailto:[email protected]

mailto:[email protected]




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 developing and

demonstrating the time variant reliability (TVR) approach to

explicitly address the uncertainties due to structura

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

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 inthe 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 knownService 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 1970’s, some 1980’s 1960s ~ 1980s Not known Not known

Ship measured 1992 – 2000 Not known Not known Not known




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. There are very few databases of

corrosion wastage available in the literature. The Tanker Structure Co-operative Forum guidance (TSCF 1992) is based

on thickness 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 representative of 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 DATABASEA 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 years old 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 hul

girder strength, as this is calculated and used for assessing the

ship’s structural adequacy for its intended service. This

Distribution of Vessel Age at the Time of

Gauging (158 Records)

0%

5%

10%

15%

20%

1 2

1 4

1 6

1 8

2 0

2 2

2 4

2 6

2 8

3 0

3 2

Vessel Age at the Time of Gauging

F r e q u e n c y

Figure 2. Profile of ship age at the time of thicknessmeasurement (157 gauging reports, 140 oil tankers)

Distribution of Ship Length

(140 Vessels)

0%

10%

20%

30%

40%

1 5 0

1 8 0

2 1 0

2 4 0

2 7 0

3 0 0

3 3 0

3 6 0

3 9 0

4 2 0

Ship Length (m)

F r e

q u e n c y

Figure 3. Profile of ship length (157 gauging reports, 140

oil tankers)




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 sourcesThe 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 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 thickness

measurements 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 ship’s 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 good condition. 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 to replace 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 o

replaced steel can be less than 1.0% of the hull’s steel weight

The replaced steel plates, if there are some, possibly occupy a

very small percentage of the entire population.

Nevertheless, thickness measurements corresponding t

probably replaced plates have been removed from the database

Plates with very small wastage, say less than 0.01 mm, arescreened out; they are probably plates renewed after the ship’s

delivery.

SOME OBSERVED TRENDSThe wastage measurements are categorized according to

location (structural member) and usage space. The location

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 and5 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 ha

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.

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, whereas cargo 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 ship’s age. This observation is not new, and

has been demonstrated in previous studies. Most oil tankers

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

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 1.50

Long 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




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.

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 to the

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% probabilityfor 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 ship’s age.

The lines of 75 and 95% percentile also increase with

ship’s 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 OFCORROSION 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 Melchers 2001) 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

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 30

Age (Year)

C o r r o s i o n W a s t a g e ( m m

MeasuredAverage

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 30

Age (years)

L o s s o f S M ( % a s - b u i l t )

sectionaverage95%75%

Figure 5. Loss of hull girder section modulus to deck over tim

(599 sections)




wastage from the database accordingly. The values of

corrosion wastage as thus determined would measure 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.

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 less than 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 doubles when

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 highlyvariable problem.

CORROSION RISK TO AGING SHIPSCorrosion 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 ship’s 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 detail

are listed in Table 4.

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 buildLength (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%

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%




The differences in corrosion wastage between single hull

tankers and double hull tankers are not considered, though such

differences are recognized.

Corrosion causes loss of strength of individual structural

members.

Some recent oil tanker incidents took place when shipswere 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 hull tanker

(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 inspection or

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 lateral loads 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 effect

3.0

3.1

3.2

3.3

0 5 10 15 20 25 30 35 40

Slight

Moderate

Severe

Age (Years)

A n n u a l R e l i a b i l i t y I n

d e x

Figure 6. Annual reliability index of a stiffened panel at atanker’s 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

SlightModerate

Severe

Age (Years)

A n n

u a l R e l i a b i l i t y I n d e x

Figure 7. Annual reliability index of a stiffened panel at a

tanker’s 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




of plate renewal. Details of this time-variant reliability

assessment can be found in Sun & Bai (2001) and Sun &

Guedes Soares (2003).

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 corrosionis anticipated, inspections should be conducted at 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 DATABASEThe 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 the 75%

percentile. For renewal criteria, corrosion wastage allowanc

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 tankers is

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 successfu

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 tanker

have exhibited possible strength reduction of about 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 varian

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 o

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)

A n n u a l R e l i a b i l i t y I n d

e x

Figure 8. Annual reliability index of the hull girder strengthof an oil tanker for different corrosion levels




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 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 a separate 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 Smith’s 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 ship’s 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.

CONCLUSIONSThis paper presented a database of corrosion wastage that

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 measurements in

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’ structura

integrity are discussed. The investigated risks are loss of loca

member’s strength, loss of global hull girder strength, and

shortened inspection intervals.

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

oil tankers, (2) design limits to the hull girder strength o

FPSOs, (3) a time variant reliability approach, and (4) risk

based inspection schemes.

ACKNOWLEDGMENTSThe 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 thei

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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Assessment of Corrosion to Aging Ships Using an Experience Database