Why it is important to lightweight superstructures
Faced with reducing the weight of large coastal cruise ferries, without compromising on safety and environmental requirements, Havila Kystruten and Tersan Shipyard asked: Could we use aluminium to build one of the biggest parts of the ship?
For the Havila Kystruten ships’ superstructures, the challenge for the shipyard was to reduce their weight by more than half. Think about that for a second. Together, we needed to find out how the larger structures could become lighter and still maintain all the stringent requirements such as stiffness and deflection.
Tersan Shipyard asked: Would a lighter superstructure be the answer? Could aluminium do the job?
Environmental and weight reduction challenges for cruise vessels
Let me go back to the beginning, when the company contracted with building the new ferries – Tersan Shipyard in Turkey – approached us at a trade exhibition. They talked about the technical and weight reduction challenges they were facing with the design of two new and innovative coastal cruise ferries for the Norwegian company Havila.
The vessels needed to be battery and LNG-powered. They needed to be able to carry 640 passengers. And they needed to meet strict environmental requirements by a big margin.
Tersan was considering replacing steel with lightweight alternatives in the upper sections of the vessels, also known as the superstructure. Could they replace a large section of the ship with a lightweight solution in aluminium?
Lightweighting with aluminium superstructures
We were confident it could be done. When you work with aluminium, you know that it is a great seaworthy option. It maintains its strength in arctic conditions, has excellent corrosion resistance, it is light yet strong, and its stiffness could also be beneficial.
We told them yes, with the right design, aluminium could definitely solve their technical challenges.
Now, replacing materials does not necessarily mean putting a new material in the same design. In this case, a traditional steel superstructure would add too much to the overall weight of the vessels, which could cause displacement and draught issues. We would have to lightweight the superstructure while maintaining stability and structural rigidity. It would have to meet the strict environmental and safety criteria.
We also had to consider:
- Strength and deflection requirements
- Space restrictions, such as ceiling heights and pillar locations
- Innovative designs to speed up the construction process
With the right design we could solve the weight challenge while maintaining the load requirements, so we spent time looking at how much the decks could deflect between the structure. Because of headspace for passengers, there was a maximum depth for the decks, and this also created limitations for how much bigger the construction could be.
Stability in light and strong superstructures
Our solution was to design extruded aluminium profiles with a very high strength-to-weight ratio. These profiles were tailored to the solution. We joined them, using friction stir welding technology, and created large aluminium panels. Construction would be easier because the solution was more modular than before.
At the end of the day, this design helped us achieve a 55 percent weight reduction, going from steel to aluminium. The design also delivered on the stability of the vessel, lowering the center of gravity to improve the balance of the ship in the water. The concept was incorporated into the design and submitted to DNV-GL, which certifies that the safety and technical requirements are seaworthy.
