Double Hull and Corrosion

London, February 2004

Dragos Rauta, INTERTANKO

Introduction

Double Hull represents an evolution in tanker design. Safe, solid and quality double hulls can be, have been, and will be built. Double Hull is a complex structure which has positive aspects but also brings its challenges. These challenges need to be further addressed by industry, but some of them can only be addressed through a significant change in design and construction practices. Operators also need to change their own practices.

Unfortunately, the huge size of these ships makes it impossible to learn and train by simulation. So, our industry will continue to improve through practical experience. This may involve the risk of unforeseeable incidents or even accidents. It is therefore imperative that we all fully understand and evaluate the few years of experience in operating a larger number of double hulls and thoroughly review the continuous feedback from different industry stakeholders. I therefore congratulate the Royal Institute of Naval Architects for the initiative taken by organising this Conference which I hope will give further opportunities to share information. I would conclude my initial remarks by also thanking RINA for giving me the opportunity to address this distinguished audience.

I trust that the discussions over these two days will address the many aspects related to the double hull design which are relevant to the subject of this Seminar. I would limit my presentation to one item, corrosion, which in my opinion, at present, does not receive the necessary attention it deserves and needs to be assessed as a high priority.

The issue

In an ideal world with a well built, perfectly operated and maintained tanker there is one single element which could create problems; this is corrosion. However, what is new? Any steel structure is exposed to corrosion. For some 120 years, steel has been the common material of construction for ships. Since oil has been transported in bulk at sea, tankers have been built with steel. So, why do we need to talk about corrosion of tankers, especially double hull tankers?

The shipping industry has done a lot to progress the development of anti corrosive methods. Coating is mandatory for ballast tanks on bulk carriers and tankers. However, the coating in ballast tanks does not always provide the expected protection and high corrosion rates, sometimes with dramatic consequences, have been experienced. In addition, severe corrosion has been experienced in the cargo tanks of oil tankers, including double hull tankers. In this context it appears that today’s choice and application of anti corrosion methods are still open to improvement and, I would add, in need of fast track attention.

Do not misunderstand me: I am not criticising the measures we have in place today. Not at all. They have provided a good background for better safety and operations. However, I think the time has come to move ahead and explore better methods within the field of protection against corrosion. In my opinion, even in an ideal world with a well built, perfectly operated and maintained tanker, corrosion can still be the number one enemy and turn a good ship into a "rust bucket". This is not a purely theoretical assumption but a statement based on experience and factual evidence direct from tanker operators.

My presentation today will give a brief review of what the specific challenges are for a double hull structure with respect to coating; what needs to be done, and what should be the roles of the different stake holders.

Double Hull particularities that impact corrosion

Before addressing the specifics of this issue, please let us bear in mind that a tanker's primary purpose is to transport liquids in bulk, safely and efficiently. To reach such an efficiency, they need to spend as little time as possible in dry-dock for repairs. So, the basic criterion which should be laid down when constructing a new tanker is to get a ship with an inbuilt robustness that, in addition, needs an efficient protection against the only enemy that could be beyond the operator’s control during the ship’s life span; corrosion.

The change from single to double hull construction has brought about some new operating conditions, leading to additional challenges that make corrosion on double hull tankers a different issue to that in a single hull vessel. What are these differences?

Ballast tanks

  • The surface area in Double Hulls is twice or almost three times larger and has to be coated and maintained as compared to a single-hull. Many of these surfaces are not easily accessible due to the hazardous nature of the cargo these ships are transporting
  • The surface area of tanks exclusively dedicated to ballast is greater and always wet therefore generating a greater corrosion potential along the length of the vessel.
  • Coating on the inside of the bottom shell is continuously wet, coated with mud or submerged, depending on vessel trim, particularly in aft locations.
  • Accumulations of mud generate a greater threat of MIC (microbial induced corrosion).
  • The inner shell of a double hull is subject to large and frequent temperature fluctuations which create a greater potential for corrosion due to the impact of temperature upon the corrosion process.
  • The inner shell coating has a higher cracking potential due to flexing of the steel, and the impact of heat on the coating morphology.

Cargo tanks

  • the insulating effect of double hull construction which maintains the cargo oil temperature at relatively high level for a longer period.
  • the inner shell is at same temperature as cargo and higher temperature increases the possibility for corrosion at that location
  • 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.

It has become evident that the change to the double hull configuration has provided an increased potential for corrosion.

Sources of Corrosion

In general, aqueous corrosion is the result of an electrolytic process. An electrolytic process needs an electrolyte. The double hulls have an excess of such electrolyte as compared with the single hulls due to higher humidity in both cargo tanks and ballast tanks. The anode is the iron in the steel structure while the cathode is represented by various other elements including sulphur.

If one of these three basic elements is missing, we can limit the corrosion activity. One could aim for the removal of the electrolyte. This is basically provided by the presence of water vapours on the cargo tanks ullage space and the continuous humidity on the ballast tanks. It is obvious that there is nothing one can do to remove humidity from ballast tanks. In cargo tanks, water vapours are introduced via the inert gas system. The inert gas, which is produced by flue gas from, for example a tanker's boiler, is both water scrubbed and passes through the water column in the IG deck seal before entering the tanks. The water vapor content of the inert gas will depend on the temperature of the flue gas on entry into the scrubber and the extent of condensation of water vapour prior to entry into the cargo tanks. A traditional inert gas system on a tanker has no inert gas dryer or dehumidifier to reduce the water content of the inert gas between the scrubber and the cargo tank. Thus, it appears almost impossible to control or stop water vapour ingress into the cargo tanks while in the ballast tanks the percentage of humidity can be as high as 95% depending upon the temperature of the surrounding seawater and cargo temperature.

The other option is to remove the anode. This means no bare steel, or, in other words, an efficient coating in both ballast and cargo tanks. Easy to say difficult to achieve. However, not impossible. I will come back to the subject in the second half of my presentation.

This leave us the “cathode” or the main source of the corrosion; the element that wastes the ship’s steel structure. The cathode, which from now on I will blame as a “source of corrosion”, has multiple causes and yet surprises experts.

Sources for corrosion

There are many sources of corrosions onboard tankers but I will talk only of three main types which have been identified. These can exist in both cargo and ballast tanks during both laden and ballast voyages. It is noteworthy that these types of corrosion seem to be more aggressive on double hull tankers for reasons that I already explained but I will underline again.

1. “Sour cargoes” – cargoes with high H2S concentration

The crude oil cargo tank under-deck area or head space as it is also called is exposed to the various gases present in the ullage space. When a cargo tank is loaded the ullage space volume will vary according to the volume of cargo placed in the tank... This ullage volume is filled with both volatile organic compound (VOC) gases released from the crude oil cargo loaded and inert gas. One of the gases that can also be released from the crude oil is hydrogen sulphide, or H2S. More and more crude oils have been found to have high concentration of H2S. In presence of high humidity, the H2S breaks down and, as a result of a succession of chemical reactions, ends up as sulphur crystals which have been seen adhering to the under deck structure. These sulphur crystals (observations onboard tankers have revealed tons of such crystals) act as a cathode. So, with acid/water droplets fromthe inert gas one gets the basis for aqueous corrosion the ship’s steel structure which gets “eaten” away to get what we see as rust.

Hydrogen induced cracking - Pyrophoric Iron Sulphide

In addition to this generalised corrosion attack, H2S will react with Iron Oxide (rust) to create Iron Sulphide (FeS). This process will release some free hydrogen during the reaction whereas the remainder will create water (H2O). These reaction processes are exothermic, which means that it gives off substantial amounts of heat and thus the term pyrophor, but the whole process is described in some detail within the ISGOTT. A further reduction occurs when Iron Sulphide comes in contact with oxygen in that it reduces further to pure Sulphur (as has been found adhering to the under deck of tanks) and rust again. The free or released hydrogen from the first reaction creates another corrosion problem for the deck head structures of the cargo tanks. The deck head structures of a tanker are under the largest loadings for tension and compression being furthest from the longitudinal “neutral axis” of the vessel. Under these circumstances hydrogen will “absorb” into the steel and remain between the crystal boundaries of the steel. This will cause a lamination decay to occur to the exposed steel which is more correctly termed Hydrogen Cracking. Examples of Hydrogen Induced Cracking (HIC) and Stress Oriented Hydrogen Induced Cracking (SOHIC) have been observed and are illustrated below together with the relevant explanation.


Wet H2S cracking can occur in susceptible steels exposed to aqueous environments containing hydrogen sulphide. It is a form of hydrogen-related cracking and can have two distinct morphologies: The first type is commonly referred to as 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.


The second type produces an array of blister cracks linked in the through thickness direction by transgranular, cleavage cracks. The latter type of cracking is referred to as 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”[1]

H2S

The option to fight this type of corrosion would be to remove the hydrogen sulphide from the crude oils prior to loading. Another measure would be to remove the sulfur deposits from under the main deck. COW of the under deck area would leave a layer of fatty oil film on the steel surface as well as remove the sulphurous acid and corrosion products. The purpose of regular tank head space COW would be for corrosion prevention by applying an interim protective coating to the head space steel. Such a proposal would necessitate the requirement for a cost benefit evaluations to be made. The cost of additional COW machines, maintenance, and additional cowing time (if tank head space cowing was to be made during discharge) will have to be weighed against the possible benefits. For new crude oil carriers suitable coatings should be cost benefit evaluated.

2. Carbonic acid

The ullage space will also contain inert gas from the flue gas produced by burning fuel oil in the boilers. The flue gas contains N2, CO2, CO and traces of sulphur dioxide. CO2, in presence of water can form carbonic acid with a pH of about 4.0 (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). This carbonic acid will also remain in the cargo tanks after cargo is discharged as a result of the use of Inert Gas during the discharge programme. We have seen clear evidence of cargo tank bottom corrosion due to carbonic acid. This carbonic acid can also be found in ballast tanks when wind conditions allow. During loading and the laden voyage, released VOCs and inert gas could find their way through the deck vents into the empty and very humid ballast tanks. The worrying fact is that, according to consultations we have had with paint manufacturers, we understand that the most used tar epoxy paints used for coating of ballast an cargo tanks might be impacted by environments with a pH as low as 4.

The options for protection against this type of corrosion are either a clean inert medium, such as pure N2, or again an adequate and quality type coating that it designed to meet this environment.

More acidic environment

Water enters the ullage space with the inert gas as vapor. If the crude oil in the tanks need frequent topping up during the loaded passage to maintain the cargo tank overpressure there is steady flow of inert gas and water to the cargo tank ullage space. The temperature in the ullage space will vary with the ambient conditions and day and night changes in temperature. Given the dew point of the vapour, water will condense on the steel surface during the cooler night time periods... The water droplets will contain sulfurous elements and carbon dioxide and will be acidic. pH as low as one (pH 1) has been reported.

Depending on the tanker’s deck paint color, light or dark it is not unusual to measure a steel surface temperature of 65 Centigrade or more for trade routes frequently traveled by Crude Oil Tankers. This high temperature will accelerate the corrosion rate significantly. Cargo temperature in double hull tankers is some 20˚C higher than in the single hull tankers.

The ullage space environment is highly acidic and steel quality should be carefully evaluated. To apply coating of the cargo tank headspace will affect the capital cost of a tanker. Should Crude Oils be categorized according to, for example, their Acid Number (AN) reflecting their content of naphthenic acid - an organic acid found in crude oils, VOC and H2S content? The extra cost of cargo tank coating, or the accelerated corrosion rate could then be compensated for through the World Scale rates? These are issues to be further assessed but as a priority matter.

3. Microbial Induced Corrosion (MIC) - Cargo Tank Bottom Plate Pitting Corrosion

Pitting in the bottom plating and on other horizontal surfaces in a crude oil cargo tank is a well-known condition normally encountered when the single hull tanker reached its 3rd and 4th special survey, 15 to 20 years old. Repairs were made by a combination of steel replacement, filling the pits with weld metal, grit blast the tank bottom plate and apply coating.

Some operators have, however. had a unexpected surprise to find, at the first Special Survey of their double hull tankers, a very high rate and intensity of cargo tank bottom pitting. The corrosion rates of some of the pitting have been, in the more severe cases, exceeding 1.0 mm per year. If this pitting development were allowed to go un-noticed it would allow crude oil to leak into the ballast tank space. Such a leakage will pose both a potential safety problem and a pollution risk with the crude oil and combustible gases entering a ballast tank.

There are several theorems concerning this question but one of the most popular is that of the corrosion being caused by Sulphate Reducing Bacteria (SRB). These bacteria cause what is known to be Microbial Induced Corrosion (MIC) to the tank bottom plating. This type of corrosion is normally found as deep pitting.