A revolution in ship building?

Steel and light weight concrete sandwich construction may
represent new opportunities for the ship builders and ship owners as well as being a step forward for everyone concerned with safety
at sea.

The strategic research department of DNV and Aker Yards are exploring the potential for building ships using a novel cellular construction method. Stiffeners and secondary structural elements are to a large extent eliminated by way of using a “sandwich” of thin, double steel plates with light weight concrete as a core material in between. New concretes lighter than water have been developed and tested. A prototype Panamax bulk ship has been studied; a particularly interesting result was that the overall weight came out to be equal to or less than that of a conventional steel ship. The proposed construction method has potentially many interesting safety features, such as improved integrity of the ship in collision and grounding, increased fatigue life, less corrosion, improved fire resistance, enhanced environmental performance etc. These aspects will be studied further in the future.

Historical retrospect
The first concrete boat was built for the World Exhibition in Paris in 1854 and the first ship was built in America in 1892 as a schooner which was soon forgotten. The first practical use of concrete ships started in Italy in 1896 and was soon followed by Germany (1908), England and Holland (1910) and Norway (1913).

Several barges were built in this period. The first patent for design of concrete ship capable of deep-sea trading came from Fougner (Norway) in 1912. Unable to find a ship owner willing to place a firm order, the ship was built on own risk and launched in 1917. With the success of the ship, several small ships were built and some are still afloat. The casting of the first ships, when the frame is completed, took only two days. A 1,000-ton ship was completed in six weeks.

In the following years (1917–1921), 12 large concrete ships were built. A typical cost for a 5,000 ton concrete freighter was USD 750,000. During WWII, several large concrete ships were built due to steel supply shortage. Innovations in cement mixing and composition made these ships stronger than the previous fleet. In 1942, the United States Maritime Commission built a new fleet of 24 concrete ships. The ships were built at an incredible rate, with one being launched every month. Concrete hulls were particularly well suited for carrying dry cargo as the condensation problems that plagued steel ships did not occur. These vessels worked well but were only capable of a maximum of ten knots due to the high light weight. Of all the American concrete ships built during WW I and II, only ten are known to today be afloat. These ships form a massive floating breakwater on the Malaspina Strait in the City of Powell River in British Columbia, Canada.

	A typical view of a tank ship applying the Steel-Concrete Cellular Sandwich Concept.

About the Steel-Concrete Cellular Sandwich Concept
The state of the art merchant vessels is based on old technology and design with no major technical breakthrough during the last 50 years. The Steel-Concrete Cellular Sandwich concept for merchant ships may represent a significant break through for ship design and construction by introducing new technologies and production methods.

The concept of using steel and light-weight concrete sandwich construction aims at developing a new generation of merchant ships with enhanced properties with regard to safety, economy, environmental performance and overall operational efficiency. In particular, the proposed cellular design implies that the concept will be scalable and applied to ships of any size and still maintain its intended properties of weight and price competitiveness.

The innovative concept may represent new opportunities for the ship builders and the ship owners as well as being a step forward for everyone concerned with safety at sea.

It is necessary to make ships safer
The long term trend for European shipyards is that new orders are in rapid decline and the market share is steadily being reduced, going from 19 per cent in 2000 to 8.3 per cent at the end of 2002 and further down to 5.4 per cent in June 2003. Demand for specialised ships, which comprises a large part of European production (cruise ships, ro-ro ships, ferries, chemical carriers, LNG carriers) has remained weak or has shifted towards yards in the Far East. Innovative ideas and new technologies, particularly within new materials and joining methods, is hoped to provide an opportunity to turn the negative market trend for the European ship building industry.

Making ships safer is necessary and is demanded by the society at large. The Erika and Prestige catastrophes have in a dramatic way illustrated the risks of pollution linked to maritime transportation, particularly the risks of old ships and single skin design. Also the recent Rocknes catastrophe definitely leads to increased attention to grounding accidents and integrity performance of the ship hulls. The steel-concrete sandwich concept may introduce an important step forward in terms of hull performance and ship safety.

Lower material and maintenance cost
The project will utilise new technologies for sandwich structures, laser or plasma welding, and modular production methods.

	The casting of the first ships, when the frame was completed, took only two days.

Risk based design (Quantitative Risk Assessment) complying with the principles for Formal Safety Assessment according to IMO circulars will be applied to demonstrate that the design is as safe as or safer than existing designs and that the risk level is “As Low As Reasonably Practicable”.

Risk based design offers freedom to the designer to choose optimal solutions to meet safety targets and opens the door for innovation, as radically novel and inventive design solutions may become feasible.

Generally it is foreseen that the concept may first of all be of interest for chemical carriers and gas carriers because different steel qualities may be applied in the sandwich concept with no/reduced thermal bridge between the steel plates.

Stainless steel or other materials for low or high temperature applications may be used on the surface towards the load and normal structural steel can be used on the other surface. The low density concrete core has very good insulation and fire properties, and the properties of the concrete can be varied for different application areas and purposes throughout the ship. The specific weight of the concrete which will be applied will typically vary between 350 and 1,200 kg/cbm.
It is expected that the life cycle cost of a steel–concrete sandwich ship is less than for a traditional steel ship. The material cost will be lower than for a conventional steel ship, the coating and maintenance cost will also be lower than for conventional ships.

An initial feasibility study of a new and alternative way to design dry bulk carriers by using the ideas of the steel–concrete sandwich concept has been performed.
The project was sponsored by Aker Yards and Det Norske Veritas Research. Through the project it was concluded that the concept is indeed feasible with respect to weights, capacities, structural strength, stability, ballast water management, cost and safety. Further considerations have indicated that the concept could be even more interesting to other types of ships.

Further work will thus concentrate on other ship types that may have stronger safety and cost saving potential than the somewhat “simple” bulk carriers. Two UK patent applications and a PCT application have been submitted in connection with the first phase of the project.

When considering the main benefits of the Cellular Sandwich Concept several application areas seem very promising. The technology may also open new possibilities within maritime transport giving feasibility to new conceptual ideas.

The main benefits of the concept are believed to be:
• Improved safety performance
• Reduced corrosion and fatigue problems giving potential for extended life time
• Multiple barrier and extensive compartmentization improving water ingress resistance
• Enhanced performance and structural integrity during collisions, grounding and accidental loads
• Improved thermal insulation properties and fire resistance
• Reduced noise and vibration problems
• No secondary stiffeners
• Lightship weight same as for ordinary steel ships
• 40 per cent of steel weight replaced by cheaper concrete core material
• Potential for efficiency gains through standardized element production
• Reduced coating cost.

A technical qualification program has been started involving Aker Ostsee Werft, DNV Research, Universität der Bundeswehr München and Liapor Gmbh & Co. Laboratory testing, technological investigations and manufacturing studies will be the main activities of this phase.

Sources: ConcreteShips. Org and Maritime Research of Uddevalla

By Pål G. Bergan, DNV Research
Kåre Bakken, DNV Research
Oddvar Slettevold, Aker Yards

Latest update 18-10-2006 8:49