Composite marine structures – economic and environmental benefits

PhD student Luis Sanchez´s doctoral thesis about opportunities for weight reduction in composite marine structures suggests that there is much to gain by improving structural design.

Thank you for letting me read your interesting thesis! Can you please explain what your thesis is about for those who have little time to spare?

- I have been trying to find ways to use composite structures, namely carbon and glass fibre reinforced plastics, in an economical favourable way through weight reduction. Weight is very important because it is directly related to the acquisition cost of a composite structure and its– long-term economic benefits. 

- One of the reasons for using composite structures is that they can reduce the fuel consumption of a vehicle. One famous example is the Boeing 787 commercial airplane. A large fraction of its structure is built with composites, making it, along with other improvements, 20% more fuel efficient than the plane it is replacing. Despite their benefits, composites are not widely used in marine structures. There are two main reasons for this: Fire regulations and cost. My thesis addresses the latter problem.

But what are the benefits compared to for instance a steel structure?

- Comparison with steel structure has shown that it is case dependent. However, the question is no longer whether composite structure is good or not – it is a question of making it more accessible for more companies.
Are there any differences in regards to security aspects?
- It is not a question of reducing safety levels to an unsecure level; the question is how to optimize the safety level so that you do not have an unnecessary security level that only cost money. I am trying to reduce the unnecessary conservatism, so that the design can be as light as possible and as safe as it should be, not more, not less. 

In your thesis you investigate opportunities for weight reduction in three areas, operational limits, material characterization and structural design. What results did you get by investigating them?

- For the operational limits I shown that some aspects in their estimation through reliability analyses [the overall consistency of a measure] are really important. The thesis indicates that  reliability analyses of composite structures, namely fibre-reinforced plastic structures, are frail and uncertain. . This does not mean that structures out there are not safe. It means that current prescriptive rules for the design of composite structures cannot be challenged with reliability analyses.

- For material characterization, I show that easy improvements in the characterization methodology can lead to more accurate material properties, which result in a better use of the material and therefore cheaper and lighter structures.
For structural design, I compared the potential for weight reduction of structural design improvements against the two previous approaches: operational limits and material characterization. I found that improvements in structural design haves the largest potential. This means that the most beneficial approach for reducing the weight of a composite structure is structural design optimization.

- If you were a company in the naval architect business and you asked me how to invest 1 million I would tell suggest you that you to put it into structural design optimization.

 

The image shows how different acceptable structure designs can have very different weights, as well that the reasons for rejecting other designs greatly.

 

It is quite clear by reading your thesis that there are advantages for companies with composite marine structures. However, could this also be beneficial for the rest of the society?

- The lighter a ship structure is, the lower the fuel consumption or the larger its cargo capacity. Lower fuel consumption is obviously good for the environment, since among other things, leads to a reduction of C02 emissions. Increasing cargo capacity is good because it reduces the environmental footprint of product. From a social and economic perspective, the widespread adoption of composite marine structures can lead to increased job opportunities and possibilities within the marine technology sector.

So how long do we have left until most ships have composite marine structures?
- This year the first composite structure in a SOLAS vessel was approved. This is a huge step forward, as the SOLAS fire regulations has been the biggest hurdle for the widespread adoption of composites in ships. I would say that we are close.

Are you planning on any future research with regards to this subject?
- I will continue to work with structural design of composite marine structures. Mainly for strengthening some of the arguments presented in the thesis, but also to provide good practices for the optimization of this structures through computer models.

Luis Sanchez doctoral thesis will be presented May 22 at Chalmers, more information here.

 

Text and photo: Carolina Kihlström