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DOUBLE HULLS & CORROSION
Dragos Rauta
INTERTANKO
The Royal Institue of Naval Architects Conference
London, 25-26 February 2004
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People
the huge size of
these ships makes
it impossible to
learn and train bysimulation
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Practical logistical problems!
Height to climb 11 km
Area to survey 300,000 m2
Length of weld 1,200 km
Length longitudinals 58 km
Bottom area 10,700 m2
1 % pitting = 85,000 pits
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for some 120
years, steel hasbeen the
common
mater ial of
construction
for ships
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Ballast tank after more than 20 yearsBallast tank after 16 years
Ballast tank after 28 years
the shipping industry
has done a lot to
progress thedevelopment of anti
corrosive methods
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corrosion can still
be the number one
enemy and turn a
good ship into a
"rust bucket"
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DH particularitiesBallast Tanks
DH/SH areas = 2-2.5 times larger Ballast tanks always wet
Bottom shell continuously wet, submerged andmuddy
Accumulations of mud generate a greater threat ofMIC
DH inner shell subject to large and frequenttemperature fluctuations
The inner shell coating has a higher crackingpotential due to flexing of the steel, and the impact ofheat on the coating
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Ref: A study on hul conditin assessment for aged
double hull tankers- The Shipbuilding Research
Association of Japan Panel #242 Committee
(March 2002)
the insulating effect of
double hull construction
DH particularitiesCargo Tanks
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DH particularitiesCargo Tanks
the inner shell is at
same temperature
as cargo and higher
temperatureincreases the
possibility for
corrosion at that
location
Ref: A study on hul conditin assessment for aged double
hull tankers- The Shipbuilding Research Association of
Japan Panel #242 Committee (March 2002)
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DH particularitiesCargo Tanks
with the introductionof water saturated
inert gas prior to and
throughout the
loaded voyage, the
vapour space in the
cargo tanks remains
humid with the
humidity varying
with the diurnalvariations of the gas
space temperature
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humidity in both
cargo tanks and
ballast tanks isthe excess
electrolyte in DH
the anode is the
iron in the steel
structure while
the cathode is
represented byvarious other
elements
including sulphur
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Corrosion mechanisms at work
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Fully formed Sulphur
Crystals as found on
rust samples from
Cargo tanks
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And More Sulphur
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Corrosion Cell
found in the InertGas Scrubber Unit
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Hydrogen induced cracking -
Pyrophoric Iron Sulphide
.
Hydrogen I nduced Cracking (H IC) and can occur where li ttleor no applied or residual tensi le stress exists. I t is manifested as
blisters or blister cracks oriented parallel to the plate surface.
Source: Intercorr International - Wet H2S Cracking
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Hydrogen induced cracking -
Pyrophoric Iron Sulphide
Stress Oriented Hydrogen I nduced
Cracking (SOHIC). SOHIC can have a
greater effect of serviceabil i ty than H IC
since it effectively reduces load carrying
capabil i ties to a greater degree
Source: Intercorr International - Wet H2S Cracking
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Carbonic Acid and Wet Rust
the inert gas needed for a
VLCC with a cargo
carrying capacity of
300,000 m3 can produceas much as 12 tons of
carbonic acid during one
voyage
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The ullage space
environment is highly
acidic and steel quality
should be carefully
evaluated
Deck Head Corrosion
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Pitting as discovered on new double
hull tankers (MIC)
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Pitting in the tank bottom plate
(MIC)
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COAT ON TANK BOTTOM
COATING APPLIED
ON PITTING
CORROSION
IN PERFECTCONDITION 5
YEARS LATER
NO FURTHER
PROBLEM
EXPECTED
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Coated cargo tank inner bottom
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The key issue throughout this
analysis is the continuous high
humidity in all ballast and cargo
tanks as well the preservation of amuch higher temperature of the
oil cargo
No similar problems for thepre-MARPOL tankers.
Limited problem for the
post-MARPOL PL/SBT SH
tankers. However, the DHdesign has actually created
the conditions that give
birth to these problems in
all cargo and ballast tanks
of the ship
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Cargo tank top/bottom coating
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Current legislation
Since July 1st 1998 all new build oil tankers
and bulk carriers must have a corrosion
prevention system in compliance SOLAS
Ch II/1 Reg. 3
2. IMO has also issuedGuidelines for best practices with regard to
coating through Resolution A.798(19)
These guidelines require that:
C l i l i A 798(19)
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Current legislationA.798(19) Selection/Application/Maintenance of Coatings to
be agreed by the Shipyard/Shipowner/Paint
Manufacturer Classification Society (RO) approve these
measures
Hard coating + multi-coat with different coloursfor each coat, and the final colour must be light
Application process should consider surfacepreparation (including steelwork and secondarypreparation), health, safety, environmental &quality control procedures
The coating maintenance should be based on thedocumentation provided by the paintmanufacturer
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Pre-striping of blasted tank
Before 1st coatexcelent job! Proper striping before 2nd coat!
Performing systems require two coats.
Additional stripe coats with brush
should be applied to welds, cut outs andall sharp edges. Sacrificial anodes
should be installed in cooperation with
the anode and paint manufacturers to
avoid side effects
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IACS/Industry WG on
Corrosion/Coating
A revised IACS Unified Interpretation SC 122 for
the verification of compliance with SLAS II71
Reg. 3-2 and A.798(19)
A draft new IACS Unified Regulation for coatingof bottom and top structure of the cargo tanks of
the oil tankers (new buildings only)
Draft new IACS Guidelines for coating
maintenance and repairs for ballast tanks and
combined cargo/ballast tanks on oil tankers
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The future
Paint fit for purpose:
- Sustain Env. Conditions
- Elasticity
- Durability
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CONCLUSIONS continuous work on anti-corrosive measures
research on the environmental conditions in thecargo or ballast tanks
identify the adequacy of the paint
strict control of the application, thickness of thecoating and the ambient temperature
coatings less dependent of the accuracy of the filmthickness
coatings with god properties and flexibility at hightemperatures at which double hull tankers operate
the high price paid by the ship owners should givethem in return a quality product
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CONCLUSIONS Class Societies to increase expertise & experience
Shipbuilders to control the quality of their subcontractors Companies applying the paint to demonstrate experience
and capability
Shipowners to buy in expertise to monitor the entirecoating process
Ship crews to proper monitoring, maintain and repair thecoating
In return, longer term guarantees
Last but not least, the industry needs to further research
and understand the corrosion mechanisms in double hulltankers (cargo and ballast tanks) and to identify thecomponents that create that corrosion
The elements we acknowledged today might not be theonly factors to be taken into consideration
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I cannot imagine any condition
which could cause this ship tofounder. I cannot conceive of any
vital disaster happening to the
vessel. Modern ship building hasgone beyond that.
Capt. Edward J. Smith, H.M.S. Titanic
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THANK YOU VERY MUCH
FOR YOUR ATTENTION!