DOUBLE HULLS & CORROSION Dragos Rauta INTERTANKO The Royal Institue of Naval Architects Conference...
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Transcript of DOUBLE HULLS & CORROSION Dragos Rauta INTERTANKO The Royal Institue of Naval Architects Conference...
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!