7/31/2019 Assessment Corrosion Risks
1/12
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.
7/31/2019 Assessment Corrosion Risks
2/12
ABS TECHNICAL PAPERS 2003
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
7/31/2019 Assessment Corrosion Risks
3/12
ABS TECHNICAL PAPERS 2003
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)
7/31/2019 Assessment Corrosion Risks
4/12
ABS TECHNICAL PAPERS 2003
152 Assessment of Corrosion Risks to Aging Ships Using an Experience Database
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.
7/31/2019 Assessment Corrosion Risks
5/12
ABS TECHNICAL PAPERS 2003
Assessment of Corrosion Risks to Aging Ships Using an Experience Database 153e
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
7/31/2019 Assessment Corrosion Risks
6/12
ABS TECHNICAL PAPERS 2003
154 Assessment of Corrosion Risks to Aging Ships Using an Experience Database
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)
7/31/2019 Assessment Corrosion Risks
7/12
ABS TECHNICAL PAPERS 2003
Assessment of Corrosion Risks to Aging Ships Using an Experience Database 155e
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%
7/31/2019 Assessment Corrosion Risks
8/12
ABS TECHNICAL PAPERS 2003
156 Assessment of Corrosion Risks to Aging Ships Using an Experience Database
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
7/31/2019 Assessment Corrosion Risks
9/12
ABS TECHNICAL PAPERS 2003
Assessment of Corrosion Risks to Aging Ships Using an Experience Database 157e
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
7/31/2019 Assessment Corrosion Risks
10/12
ABS TECHNICAL PAPERS 2003
158 Assessment of Corrosion Risks to Aging Ships Using an Experience Database
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.
REFERENCES
ABS, 2000, Guide for building and classing floating
production installations, American Bureau of Shipping.
ABS, 2001, Final report of Investigation into the
damage sustained by the M.V. Castor on 30 December
2000, www.eagle.org, American Bureau of Shipping.
Gardiner C.P., Melchers R.E., 2001, Bulk carrier
corrosion modeling, International Offshore and Polar
Engineering Conference, IV: 609-615, Stavanger,
Norway.
Guedes Soares, C. and Ivanov, L. D., 1989, Time
Dependent Reliability of the Primary Ship Structure,
Reliability Engineering and System Safety , 26, 59-71.
Guedes Soares, C. and Garbatov, Y., 1996,
Reliability of Maintained Ship Hulls Subjected to
Corrosion,Journal of Ship Research, 40(3), 235-243.
Harada S., Yamamoto No., Magaino A., Sone H.,
2001, Corrosion analysis and determination of corrosion
margin, Part 1&2, IACS discussion paper.
Horn G.E., Johnson M.D., 1994, Using the ABSSafeHull Approach to Optimize repairs ABS SafeHull
Condition Assessment Services, Ship Repair & Marine
Maintenance 94, New Orleans, USA.
Ivanov, L. Spencer, J., Wang, G., 2003, Probabilistic
evaluation of hull structure renewals for aging ships,
The Eighth International Marine Design Conference
(IMDC), 5-8 May 2003, Athens, Greece.
JMT, 1997, Report on the investigation of causes of
the casualty of Nakhodka, Japan Ministry of Transport,
The committee for the investigation on causes of the
casualty of Nakhodka.
Mansour A.E., 1997, Assessment of reliability ofship structures, Ship Structure Committee report SSC-
398.
7/31/2019 Assessment Corrosion Risks
11/12
ABS TECHNICAL PAPERS 2003
Assessment of Corrosion Risks to Aging Ships Using an Experience Database 159e
Melchers, R E., 1997, Structural reliability analysis
and prediction, John Wiley & Sons, New York.
Melchers R.E., 2001, Probabilistic models of
corrosion for reliability assessment an maintenance
planning, 20th International Conference on Offshore
Mechanics and Arctic Engineering, Rio de Janeiro,
Brazil, June 3-8, 2001.Paik J.K., Kim S.K., Lee S.K., 1998, Probabilistic
corrosion rate estimation model for longitudinal strength
members of bulk carriers, Journal of Ocean
Engineering, 10, 837-860.
Paik J.K., Wang G., Thayamballi A.K., Lee JM.,
2003, Time-variant risk assessment of aging ships
accounting for general / pit corrosion, fatigue cracking
and local dent damage, SNAME annual meeting, San
Francisco, CA.
Qin S., Cui W., 2003, Effect of corrosion models on
the time-dependent reliability of steel plated elements,
Marine Structures, 16, 15-34.Sun H.H., Bai Y., 2001, Time variant reliability of
FPSO hulls, SNAME annual meeting, Orlando, FL.
Sun H.H., Guedes Soares C. 2003, A corrosion
model and reliability-based inspection for ship-type FPSO
hulls,Journal of Ship Research, submitted.
TSCF (Tanker Structure Co-operative Forum), 1992,
Condition evaluation and maintenance of tanker
structures, Witherby & Co. Ltd, London.
TSCF (Tanker Structure Co-operative Forum), 1997,
Guidance manual for tanker structures, Witherby & Co.
Ltd, London.
Wang G., Tang S., Shin Y., 2002, Direct calculation
approach and design criteria for wave slamming of an
FPSO bow,International Journal of Offshore and Polar
Engineering. 12 (4).Wang G., Chen Y.J., Zhang H., Peng H., 2002,
Longitudinal strength of ships with accidental
damages,Marine Structures, 15, 119 - 138.
Wang G., Spencer J., Elsayed T., 2003, Estimation
of corrosion rates of oil tankers, 22th International
Conference on Offshore Mechanics and Arctic
Engineering, Cancun, Mexico, 8-13 June 2002.
Wang, X., Jiao, G., Moan, T., 1996, Analysis of Oil
Production Ships Considering Load Combination,
Ultimate Strength and Structural Reliability, Trans.
SNAME, 104: 3-30.
Wirsching, P.H., et al., 1997, Reliability withRespect to Ultimate Strength of a Corroding Ship Hull,
Marine Structures, 10: 501-518.
Yamamoto N., Ikegami K., 1998, A study on the
degradation of coating and corrosion of ships hull based
on the probabilistic approach, Journal of Offshore
Mechanics and Arctic Engineering, 120, 121-128.
7/31/2019 Assessment Corrosion Risks
12/12
ABS TECHNICAL PAPERS 2003
160 Assessment of Corrosion Risks to Aging Ships Using an Experience Database