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Focusing on lighter, more efficient and safer hulls

20 February 2020

An optimized hull design can reduce greenhouse gas emissions by up to 20 percent. But how is it best done? In a research project, Jonny Nisbet and his colleagues at SSPA have developed a model that measures how wave loads affects the structural response.

It began with the Lighthouse Pre-study Dynamic Design of ships which was completed in 2017. Through this, a collaboration was established between SSPA, Chalmers, KTH and Stena to look at how to conduct joint research on methods to optimize a ship's hull construction - i.e. make it easier, more efficient and cheaper to build.

“The vessels are getting bigger and bigger and more steel is being used. Therefore, there is the desire to reduce weight, either to be able to load more or to save fuel. But it must be done safely. There are examples of container ships that have been wrongly sized and broken”, Jonny Nisbet, project manager at SSPA, says.

The Lighthouse Pre-study turned into a three-year research project (financed with SEK 3.2 million by the Swedish Transport Administration) with the goal of developing new methods for optimized hull construction. Among other things, a ship model would be created and equipped to measure how wave loads affects the structural response. And that's exactly what Jonny Nisbet and his colleagues accomplished.

“We have built a segmented lightweight model that divides the ship into four parts. In each cut we put an instrument that can measure all forces and torques. What is important when testing in waves is not only that the model has the same shape as a full-scale vessel, but also that the weight is distributed in the same way as in reality.”

“In order to achieve the correct weight distribution on a model scale, large parts of the model needed to be built in carbon fiber. Then the right weight distribution could be achieved by placing small weights in the right place in the model”.

When in it comes to dynamic sizing, all dynamic loads are taken into account when designing the lighter and more energy efficient hulls of the future. So how well does the model developed at SSPA work?

“Part of the project has been to compare how well our model matches with calculations made at Chalmers. And in normal waves, the results are very well matched. In slightly higher waves, the deviations are slightly larger. The idea is that we should be able to validate calculations and that we in symbiosis with calculation tools and methods ensure the results. So we have to work on that. But we have an experimental set-up that works, which was our main goal at SSPA”.

There are also other challenges. Today's regulations do not fully account for the interaction between the wave loads and the structural response, and therefore there is uncertainty about whether the safety margins are sufficient enough or unnecessarily conservative. The latter can cause the vessels to become oversized and have an unnecessarily high energy consumption and environmental impact.

“In the next step, we will also work with the development of regulations”, Johnny Nisbet says.

In the project, Stena Line's concept ship Stena Elektra has been used as a model ship. The planned Ropax vessel is intended to run on electricity and will be built in lightweight materials. But if the new methods are to help in the design of them, both them and the rules have to be refined.
"We are currently looking for money to get a continuation of the project. The is a lot of interes so it looks positive".

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