DOUBLE HULLS & CORROSION

Dragos RautaINTERTANKO

The Royal Institue of Naval Architects ConferenceLondon, 25-26 February 2004

People

the huge size of these ships makes it impossible to learn and train by simulation

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

for some 120 years, steel has been the common material of construction for ships

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 the development of anti corrosive methods

corrosion can still be the number one enemy and turn a good ship into a "rust bucket"

DH particularities – Ballast Tanks

• DH/SH areas = 2-2.5 times larger • Ballast tanks always wet• Bottom shell continuously wet, submerged and

muddy• Accumulations of mud generate a greater threat of

MIC• DH inner shell subject to large and frequent

temperature fluctuations• The inner shell coating has a higher cracking

potential due to flexing of the steel, and the impact of heat on the coating

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 particularities – Cargo Tanks

DH particularities – Cargo Tanks

the inner shell is at same temperature as cargo and higher temperature increases 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)

DH particularities – Cargo Tanks

with the introduction of 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 diurnal variations of the gas space temperature

•humidity in both cargo tanks and ballast tanks is the excess electrolyte in DH

•the anode is the iron in the steel structure while

•the cathode is represented by various other elements including sulphur

Corrosion mechanisms at work

Fully formed Sulphur Crystals as found on rust samples from Cargo tanks

And More Sulphur

Corrosion Cell found in the Inert Gas Scrubber Unit

Hydrogen induced cracking - Pyrophoric Iron Sulphide

.Hydrogen Induced Cracking (HIC) and can occur where little or no applied or residual tensile stress exists. It is manifested as blisters or blister cracks oriented parallel to the plate surface.

Source: Intercorr International - Wet H2S Cracking

Hydrogen induced cracking - Pyrophoric Iron Sulphide

Stress Oriented Hydrogen Induced Cracking (SOHIC). SOHIC can have a greater effect of serviceability than HIC since it effectively reduces load carrying capabilities to a greater degree

Source: Intercorr International - Wet H2S Cracking

Carbonic Acid and Wet Rust

the inert gas needed for a VLCC with a cargo carrying capacity of 300,000 m3 can produce as much as 12 tons of carbonic acid during one voyage

The ullage space environment is highly acidic and steel quality should be carefully evaluated

Deck Head Corrosion

Pitting as discovered on new double hull tankers (MIC)

Pitting in the tank bottom plate(MIC)

COAT ON TANK BOTTOM

• COATING APPLIED ON PITTING CORROSION

• IN PERFECT CONDITION 5 YEARS LATER

• NO FURTHER PROBLEM EXPECTED

Coated cargo tank inner bottom

The key issue throughout this analysis is the continuous high humidity in all ballast and cargo tanks as well the preservation of a much higher temperature of the oil cargo

No similar problems for the pre-MARPOL tankers. Limited problem for the post-MARPOL PL/SBT SH tankers. However, the DH design has actually created the conditions that give birth to these problems in all cargo and ballast tanks of the ship

Cargo tank top/bottom coating

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 issued Guidelines for best practices with regard to coating through Resolution A.798(19)

• These guidelines require that:

Current legislation – A.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 colours for

each coat, and the final colour must be light• Application process should consider surface

preparation (including steelwork and secondary preparation), health, safety, environmental & quality control procedures

• The coating maintenance should be based on the documentation provided by the paint manufacturer

Pre-striping of blasted tankBefore 1st coat – excelent job! Proper striping before 2nd coat!

Performing systems require two coats. Additional stripe coats with brush should be applied to welds, cut outs and all sharp edges. Sacrificial anodes should be installed in cooperation with the anode and paint manufacturers to avoid side effects

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 coating of 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

The future

Paint fit for purpose:- Sustain Env. Conditions- Elasticity- Durability

CONCLUSIONS• continuous work on anti-corrosive measures• research on the environmental conditions in the cargo or

ballast tanks• identify the adequacy of the paint• strict control of the application, thickness of the coating

and the ambient temperature• coatings less dependent of the accuracy of the film

thickness• coatings with god properties and flexibility at high

temperatures at which double hull tankers operate • the high price paid by the ship owners should give them

in return a quality product

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 entire coating

process• Ship crews to proper monitoring, maintain and repair the

coating• In return, longer term guarantees• Last but not least, the industry needs to further research and

understand the corrosion mechanisms in double hull tankers (cargo and ballast tanks) and to identify the components that create that corrosion

• The elements we acknowledged today might not be the only factors to be taken into consideration

”I cannot imagine any condition which could cause this ship to

founder. I cannot conceive of any vital disaster happening to the

vessel. Modern ship building has gone beyond that.”

Capt. Edward J. Smith, H.M.S. Titanic

THANK YOU VERY MUCH FOR YOUR ATTENTION!