double hull tankers corrosion

7/30/2019 DOUBLE HULL TANKERS CORROSION

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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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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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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!

double hull tankers corrosion

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