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Monday, December 18, 2017

In defence of double-hulls

Gerry Vagliano, Director of London-based Lykiardopulo & Co, wrote the following letter to Lloyd’s List earlier this week.

 

SIR, With regard to your article ‘Double-hulls: oil spills hit the buffers’ (February 12, 2008), to visualise clearly the concept of double-hull tanker design one should read the magnificent study presented in 1991 by the U.S. National Academy of Science, entitled ‘Tanker spills: prevention by design’.

 

I consider myself fortunate working for a company which in 1991 had the courage, foresight and essential dynamics to support the concept of double-hull crude oil tanker design, and to successfully proceed with such a project.

 

At that time, as is obviously the case in the present, there were people in the industry who had reservations about the design of such vessels. These reservations should no longer exist after 15 years of experience in their operations and technical performance.

 

It is not correct to say that the first designs of double-hulls “suffered a myriad of problems from stress fatigue and cracking, which had tarnished their reputation”.

 

In my experience of double-hull tankers, from smaller sizes to VLCCs, none of their cargo containment structures, including the outer hull, ever suffered from any fatigue or cracks, and this includes the first ever double-hull VLCC.

 

Lloyd’s Register was the classification society directly involved with the approval of designs for the first double-hull tankers ever built.

 

Contrary to what has been written in the February 12 article, FEM calculations in determining stresses, fatigue stresses and hot spots in the structure of such vessels were already available and were extensively used for the design of those first vessels, which actually carry the appropriate classification notations of Ship Wright FDA, SDA and CM.

 

In relation to the thicknesses of outer shell plates with their internals, it is not correct to say that single-hulls, due to the nature of their design, are better equipped to sustain “parking contacts” in contrast to the double-hulls. We all know that if we have masses of up to 350,000 tonnes moving at 0.25 knots accidentally making contact with any protrusion, it will get damaged regardless of whether the plate is 18 mm or 22 mm thick. Parking contacts are unavoidable and all types of vessels are liable to such incidents.

 

As for the double-bottom construction, the inner and outer bottom plates might be both slightly thicker than the single-hull design, because each has to withstand either the cargo or buoyancy forces, without being counter-balanced by the opposite force.

 

The construction of double-hull double bottoms is designed to support the weight of the cargo above, but the unsupported panels are of a greater area than anything similar with single-hull tanker bottom structures when in ballast condition.

 

Double-hulls have not experienced cracks and leakages, assuming that all rigorous requirements from class during the building stage were strictly applied. Therefore, any defects are more likely to be human error rather than a design weakness.

 

Reverting to the United States National Academy of Science research and conclusions, on which the double-hull tanker design was conceived and legislated upon, the basis of their findings was that the greatest number and extent of pollution from tankers happened during the approach to, or departure from, a port when the ships’ speed was low.

 

The research found that most incidents were groundings and collisions.

 

Fifteen years of experience has proved that these assumptions were correct, and therefore the industry should be satisfied with such a design.

 

Not only has the concept of double-hull been successful in its original intention of pollution prevention, but it has now been proven to be a most efficient design in its operations capabilities and versatility by not only carrying crude oils, but also carrying pre-heated fuel oils without the necessity of heating coils.

 

Equally, the maintenance of ballast tanks has become a very straightforward procedure, with easy accessibility. Close-up inspections are also easy due to the hull construction characteristics.

 

Finally, no matter how thick a shell plate might be, such a plate will never be able to withstand high-energy collisions, such as the one shown in the photograph of Samco Europe that accompanied your article, nor groundings which propagate the damage into the inner shell of the vessel, with resultant pollution, in cases where impacts are within the cargo area.

 

Thicker plates and scantlings are not the answer. The remedy for reducing even further pollution incidents at sea is proper education, in situ experience and dedication from all those who operate within the shipping industry, both at sea and onshore.

 

G M Vagliano C.Eng, FRINA, FIMarEST