Stronger, lighter, and cheaper; sandwich structures of steel and lightweight concrete core retain their overall carrying capability even after multiple cracks have developed. This new concept can mean a quantum leap for shipbuilding.
DNV’s innovative concept for building ships and floating offshore structures using a sandwich design has recently received much attention. The first scientific paper on this technology, ‘Sandwich Design: A Solution for Marine Structures?’ was published by Pål G. Bergan and Kåre Bakken at the international conference ‘Marine 2005’ held at DNV’s headquarters in Oslo, Norway, on 27–29 June.
The idea of using a sandwich design made of steel surface plates and a lightweight concrete core was conceived by Dr Pål G. Bergan in DNV Research about five years ago. The concept has been further developed during the past three years in a strategic research project together with Aker Yards ASA, in which DNV has focused on technology development and Aker Yards has studied the potential for efficient fabrication and different types of ship applications. Recently a series of successful laboratory tests of component fatigue and strength was completed. The tests showed that the current composite materials are in many ways superior to today’s welded steel structures.
Improved safety performance
A main motivation for DNV working on this ship concept has been to develop a way of building ships that could improve their overall safety performance. Even though further testing and documentation is required, it seems like the sandwich design potentially offers a series of new safety features.
This cellular hull structure means that there are many barriers against water ingress into the hull: the outside skin, the outer concrete core, the inner skin of the outer sandwich, the cell compartments, the first skin of the inner sandwich, the core of the same, and finally the inside skin. In effect there are four steel plates that have to be cracked or penetrated in order to have water leakage into the cargo space as opposed to only two barriers for a standard double hull made of steel. The repetitive cellular walls also prevent water flow into larger areas of the side walls or bottom structure.
The ability to absorb energy, as evidenced by the sandwich beam testing combined with the multiple barrier aspect, implies an exceptional ability to withstand accidental loading, such as explosions, collisions and groundings. Moreover, light-weight concrete is also a good heat insulator and it is non-combustible; this implies better fire performance than for steel ships.
It is further believed that the fact that there are few ‘hot spots’ and stress concentrations will lead to reduced cracking and, thus, less damage that could hamper safety. Similarly, less exposed surface areas and fewer coating areas mean less corrosion and structural degradation.
Entering an exciting phase
The current concept is still in an early phase of development; the full potential of the ideas has not yet been explored. There seem to be many types of ships and floating units for which the present concept could offer significant advantages.
The project is currently in a phase of further testing and qualification of the technology. The practical use of the new sandwich technology may come as early as next year.
“We are now entering a new and exciting phase of the project where we open up for extended cooperation regarding research and development as well as commercial use of the technology,” says Pål Bergan. “The test results so far confirm that we are on to something that could represent an entirely new way of building ships.”
“The concept means that about 30 to 40 per cent of the steel weight of a ship could be replaced by light-weight concrete and implies a substantial potential for saving costs and improving safety,” says Kåre Bakken, DNV’s project manager.
As part of benchmarking the technology DNV has studied a Panamax size bulk ship that has been analysed and compared with regular steel ships of the same type. As opposed to previous concrete ships, the study shows that the current concept comes out with no more weight than steel ships and has an important potential for savings as nearly 40 per cent of the total weight is made up of relatively inexpensive concrete.
Cheap and readily available material
Concrete is a cheap and readily available material. Several dozen concrete ships were built during the two world wars when steel was scarce; several of these ships are still afloat and in a remarkably good condition. Reinforced concrete has also proved to be a strong and durable material for fixed and floating offshore platforms.
Even though concrete ships in many ways have proved to serve well, they have one basic flaw: their weight. Typically, ships built of normal, reinforced concrete carry a weight penalty of more than 50 per cent compared to steel ships. Under normal circumstances this implies that they are not competitive since higher weight means a corresponding reduction in cargo capacity. Moreover, heavy weight also means they are slow and have low fuel efficiency.
Concrete that floats
Very light-weight, strong concrete mixes have been developed as part of the project. In particular, it has been found that light-weight aggregates like Liaver and Liapor work remarkably well in light-weight concretes. The aim of the current study has been to develop good, structural concretes with a mass density of less than 1,000 kg per m3. Although it does not necessarily have any significance as such, this means that the concrete will float in water.
Depending on how the cells are connected, such a ship may be virtually unsinkable because of its resistance to water ingress and water filling.
The concept also implies the use of a cellular structural principle that enables the concept to be ‘scalable’ and applicable to very large structures.
Amazing structural response
A series of sandwich beam specimens have been tested statically and dynamically according to Eurocode 4 requirements. Even though the concrete as such is a brittle material, the overall deformation curve for the sandwich beam displays substantial ductility and capability to absorb energy. This is a most important finding in view of safety and the overall capability of a sandwich ship to absorb energy during a collision, grounding or other type of accidental loading.
Ships and marine structures have to sustain extreme fatigue conditions due to the natural dynamic character of the sea and wave environment. Fatigue testing has been carried out for a series of different conditions to establish fatigue curves. The results were positively surprising with respect to resistance to initial cracking. Equally important, there were no signs of material disintegration at cracks that were exposed to millions of additional cycles.
The photo on page 6 shows a typical shear crack in the light-weight concrete core of a test specimen. The crack itself has a jagged geometry; this implies that extensive shear interlock exists and that shear forces can still be transferred along the crack after it has developed.
The diagram above shows the phenomenon of repeated strength recovery after cracking is clearly evidenced by the Load-deflection curve shown in four different beam specimens. By looking at the loading curve for specimen 3 it can be seen that the strength is rapidly regained after the first crack has developed, and that an even higher carrying capacity is displayed at the stage where the third crack has developed.
The ‘saw-tooth’ type of strength recovery is displayed repeatedly. Typically, the cracks developed alternatively at the two sides of the beam specimen. This behaviour is much different from what is typical for plastic composite sandwich beams, where the structure normally fails completely after the first shear crack.
Full report – see:
http://congress.cimne.upc.es/marine05/frontal/default.asp