1. DOUBLE HULLS & CORROSION
Dragos Rauta
INTERTANKO
The Royal Institue of Naval Architects Conference
London, February 2004

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

4. 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

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

6. Ballast tank after 16 years
Ballast tank after more than 20 years
the shipping industry has done a lot to progress the development of anti corrosive methods
Ballast tank after 28 years

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

9. DH particularities – Ballast Tanks
DH/SH areas = 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

10. DH particularities – Cargo Tanks
the insulating effect of double hull construction
Ref: ”A study on hul conditin assessment for aged double hull tankers”- The Shipbuilding Research Association of Japan Panel #242 Committee (March 2002)

11. 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)

12. 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

13. 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

14. Corrosion mechanisms at work

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

16. And More Sulphur

17. Corrosion Cell found in the Inert Gas Scrubber Unit

18. 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

19. 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

20. 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

21. Deck Head Corrosion
The ullage space environment is highly acidic and steel quality should be carefully evaluated

22. Pitting as discovered on new double hull tankers (MIC)

23. Pitting in the tank bottom plate (MIC)

24. COAT ON TANK BOTTOM COATING APPLIED ON PITTING CORROSION
IN PERFECT CONDITION 5 YEARS LATER
NO FURTHER PROBLEM EXPECTED

25. Coated cargo tank inner bottom

26. 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

27. Cargo tank top/bottom coating

28. 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:

29. 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

30. Pre-striping of blasted tank
Before 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

31. 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

32. The future Paint fit for purpose: Sustain Env. Conditions Elasticity
Durability

33. 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

34. 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

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

36. THANK YOU VERY MUCH
FOR YOUR ATTENTION!