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Independent tanks for a chemical tanker

The Naval Architect: November 2015

Designers often ask what makes independent tanks so interesting.  For the answer it is best to look at the current designs. These are the well-known standard chemical parcel tankers with duplex tanks and corrugated bulkheads, and, typical for this design, the tanks are adjacent to each other and to the ballast tanks.

This positioning has some major disadvantages, such as major heat loss when a tank is heated ­ if you heat your cargo you are also heating the adjacent cargoes and the ship itself. As a consequence a lot of heating coils and a high temperature are needed to maintain the desired temperature of the cargo. The high temperature of the heating coils can be very disadvantageous for the cargo quality.

Another aspect is that these tanks are difficult to clean, especially when products like fats have to be transported. If the ballast tanks are filled at the same time, the fats will solidify and the cleaning becomes a very laborious and time consuming job. In these cases it is quite normal for the cleaning of tanks to take several days. Other problems with these tankers can include fatigue cracking in the tank bulkheads with the risk of cargo contamination. Furthermore stowage is a complex puzzle because of the temperature differences of different cargoes, as well as incompatible cargos that cannot be stored next to each other due to possible hazardous reactions.

Independent tanks on the other hand can be insulated, bringing some important advantages. First of all the heat loss will be minimal; the temperature of the heating coils can therefore be lowered, which is beneficial for the cargo quality for certain products; contamination of cargoes is not possible anymore; cleaning is improved considerably because the tank temperature can rise during the cleaning process; and stowage becomes very simple because every product can be stored in any tank, as you no longer  have to account for different cargo temperatures or hazardous reactions between cargoes.

But if independent tanks are so beneficial why don’t we use them in chemical tankers today? Independent tankers are widely used in both gas carriers as well as bitumen tankers. This is mainly because of their extremely high or low cargo temperatures which makes insulating the cargo tanks a major design objective. They have independent tanks which are relatively expensive as well as complex and heavy. There is a tendency in shipbuilding to make rectangular tanks which fit nicely into the holds. In order to make the side walls of these rectangular tanks strong enough a considerable number of heavy stiffeners are needed. The duplex weight of such independent tanks will increase making these tanks too expensive.

Looking at shore based storage tanks we always see vertical cylindrical tanks. The reason is obvious: due to the liquid inside we only get tensile membrane stresses in the wall. This means we do not need stiffeners on the outside of the tank, and because we only have tensile stresses, the material needed is minimal.

So can we make use of vertical cylindrical tanks on a ship; how do we fix these tanks onboard a ship; don’t we lose a lot of cargo space; and won’t these tanks become too heavy?

Koole Tanktransport has built two sea going tankers with cylindrical tanks, so it is possible. How did we do this? The story starts with our inland ships where we successfully used cylindrical tanks on more than one occasion. However, these ships do not have to cope with the accelerations of seagoing ships. When we started with Star Bonaire in 1995 we first designed a skirt like construction to fix the tanks to the ship. These skirts were able to accommodate thermal expansion caused by cargo temperatures up to 80°C.This design was sent to class and was approved and the building of the ship and tanks was started.

At that moment one of the owners asked me: what would happen if the ship was bending or twisting? We had not taken these deformations of the ship into account in our calculations and so we had to start again. We had to work around certain restrictions because the ship and tanks were already being built, but after four months of working day and night we designed a flexible mounting for the tank. Now, after more than 18 years in service, Star Bonaire and the cargo tanks have proven themselves and are still performing very well.

When we launched Star Bonaire in 1996 we had the feeling that these cylindrical tanks were very good, but the way we fixed them in the ship was sub-optimal due to the restrictions of both the ship and tanks already being half-built.  As a result, we decided to start a new development from zero and asked ourselves some questions: what would be the best way to support a cylindrical tank in a ship and what would the ship look like?

The most favourable way to support a tank which is subject to horizontal accelerations is not only on the lower side, but also on the upper side of the tank. This way the supporting loads are spread, lowering the buckling stresses in the tank wall, and wall thickness can therefore be minimised.

From Star Bonaire we learned that we have to deal with the deformations of the ship in waves. But how can we predict these deformations? Class rules did not give us any help in this situation so we developed our own method. We made a model of the ship and placed it in a wave field. We then calculated the pressures on the hull and placed these pressures onto a FEM model of the ship and tanks. Combined with the accelerations we were able to calculate the deformations of the ship. After this we had to change the position of the ship in the wave field and calculate the deformations again. In total we made more than 15,000 calculations in order to understand in what situations maximum deformations could occur. From this exercise we derived a simplified method to calculate the deformations.

So now we knew that we had to fix the lower and upper parts of the tanks and we could calculate the deformations a ship would undergo in waves. The next step was to make a design for the tank which could withstand the accelerations and the deformations of the ship in waves.

We developed various concept designs for the tanks and we made a more detailed FEM model of these tank designs. With these models we made extensive calculations accounting for still water and wave loads, deflection of ship and tanks, thermal expansion, strength, fatigue and buckling. From these analyses we could choose a design which fulfilled all requirements and could be built by a tank builder. The concept of this tank design is quite innovative and we applied for and were granted a patent. The patented design enables the use of extremely thin tank walls, which are relatively even thinner than that of a beer can, while keeping far away from any danger of buckling. Part of the trick is to use the shear strength of the tank, which is quite different from other independent tank concepts. This is just one of the many innovations connected to this design. 

In 2005 we started with the development of Star Curacao and the decision was made to use this new tank design. We could now focus the tank development with this ship project in mind. The vessel was intended to carry 3,500 tonnes of cargo so we first had to determine the cargo volume and thereby the diameter and height of the tanks. The cargo tank plan comprised seven large centre tanks and so the diameter would be limited by the breadth of the ship. From this the height of the tank could be derived. In order to meet the cargo volume requirement we had to increase the depth of the ship by about 25% compared with ordinary tankers.

A nice feature of this concept is that the hull can be built as a single hulled vessel because the tanks are independent. This meant that even building in the Netherlands was possible. However, stability and damage stability issues required close attention and so to meet damage stability requirements we had to divide the hold space into five compartments.

During the whole development we had regular meetings with Class. We kept them informed about the details of the development and had extensive discussions about the calculations. This meant that when we had to approve the design of the ship and tanks for Star Curacao we only received minor remarks on our drawings.

Star Curacao came into service in January 2009. She has an IMO 2 chemical tanker notation, 7 duplex cylindrical tanks, 4 deck tanks and a cargo capacity of 4,400m3. The focus for Star Curacao is on edible oils and fats in the Northwest European short sea market. She makes around 100 trips a year, and is operated by a crew of 8.

One of the things that makes Star Curacao so efficient is the ease of cleaning the cargo tanks. If the tanks have to be swept, this can be achieved within 15 minutes by two people. Washing takes only 20 minutes, even for the difficult fats. The cleaning is going so well because of the insulation around the cargo tanks which ensures that the temperature of the tank wall can rise above 70°C during washing. We have calculated that we save about 60% on energy costs during washing compared with a conventional tanker.

After cleaning, the tank is ventilated for 20 minutes making the tank completely dry. And even when ballast is set the tank stays dry because the cargo tanks are insulated and segregated from the ballast tanks. The cleaning and ventilating is fully automated. Thanks to this efficient cleaning Star Curacao can make 100 trips a year, about 40% more than the competitors.

I have heard numerous stories of tankers who have been cleaning for two days or more on solidifying fats. Or tankers that are refused for loading because of water in the cargo tanks ­ if you put cold ballast water adjacent to a cargo tank it is likely that the water vapour in the air condenses on these walls. All these problems can be avoided with suitable independent tanks.

Another aspect that makes Star Curacao an interesting ship is the saving on energy for cargo heating. We calculated that due to the insulation we save about 85% on energy compared to a conventional tanker. Moreover we use the HT cooling of the main engine for heating the cargo to 60°C. So when sailing and the cargo temperature is below 60°C we don’t use any fuel for heating at all.

Some time ago I told this story to a ship-owner of chemical tankers. He said that it was all very impressive, but he could not believe that we would not lose cargo capacity. We decided to make a comparison between one of their existing ships and the same ship with independent cylindrical tanks. Together with Conoship we made a cargo tank plan, designed and calculated the construction of the midship, and calculated the duplex usage for the independent tanks. We kept all the dimensions, the tank volume and the number of cargo tanks the same except for the depth of the ship. The result was that we had to increase the depth by about 25%, but surprisingly the total lightship weight remained the same. The deadweight therefore also remained the same. We did however save 35% on the usage of duplex!

We also made this comparison for a ship about 8 times the size of Star Curacao and we found the same results: we did not lose any cargo capacity and we saved around 35% on duplex.

Koole Tankstorage, as it is called today, is very satisfied about Star Bonaire and Star Curacao as the ships are performing much better than their competitors. At the moment we are designing the next ship for Koole Tankstorage.

Some years ago the ship development department was separated from the Koole Tankstorage company and is now continuing on as an independent company under the name of Koole Engineering. Today we are working together with several ship owners in designing chemical tankers with independent tanks. Within a few years we might see more chemical tankers with independent tanks sailing around the seas.

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