In this discussion I concentrate on the science behind steel production, the effect of this material choice on the sustainability of vessels, and the role of the structural engineer.
The decision on which material to use for the hull and superstructure of yachts has often seemed quite straightforward based on size of vessel, speed requirement and how much you want to spend. For displacement yachts over 75 metres, it is typical to have a steel hull and aluminium superstructure. As the length decreases and speed increases, aluminium and fibre reinforced plastic become more common. The decision on which material to use has, in the past, been based on a balance of weight and cost. Shipyard preferences may also override these factors. Quite often this decision is made early in the design process based on previous experience and an engineering study is not carried out. The emphasis on sustainability and lower energy consumption has changed this decision process. It may be that aluminium or composites become attractive at greater vessel lengths where reduced weight resulting in lower installed power overrides the cost. If we consider vessels over 100m in length, steel will remain the practical solution for hulls.
Steel has been around for a long time and we sometimes forget what a useful material it is. It is a unique material which can have a huge range of properties depending on chemical composition and heat treatment. The grades of steel used for ships are ductile, weldable and of high strength. It is the most widely used structural material in the hulls of superyachts and is the best choice for large vessels where global loads start to dominate the design.
In worldwide terms across all industries, the manufacture of steel is currently a major contributor to greenhouse gases and is not viewed as being environmentally friendly. We need to address this as there is no other material which can satisfy the structural requirements at the scale and quantity required for the industry, so we need to understand the environmental implications and how we can improve these. When assessing sustainability, we need to consider both the lifecycle of the material and the lifecycle of the yacht. The energy expended in producing the steel and fabricating the hull together with re-use and recycling all play a part in assessing the carbon footprint.
It's worth taking a few moments to understand the way steel is produced. When making steel from iron ore there are a few different methods. The traditional blast furnace process uses coking coal for generating the heat and as an agent to remove oxygen from the iron ore. A large amount of energy is used and very large amounts of CO2 are released into the atmosphere. In Europe around 60% of steel is produced this way. Fortunately, there is a proven technical solution to make this a more sustainable process. This involves the use of an electric arc furnace powered by green electricity together with green hydrogen as the reduction agent. The carbon footprint is reduced by around 95%. A halfway house is to use the electric arc method for the heat and if this uses green electricity the carbon footprint is reduced by around 60%.
This is good news: steel can be sustainably produced. The bad news is that the quantity of green hydrogen required and the facilities to do this are only just being developed. Moving most of the steel onto this sustainable footing is still a long way off.
Apart from the manufacturing of the steel we should also consider the long life of yachts and the ability to re-fit and recycle. Steel has no limit on how many times it can be recycled. Approximately 85% of steel is recycled and re-used. This is not generally into structural quality steel but processes to make this viable are starting to be developed.
The end goal of fully green steel is highly dependent on the availability of green hydrogen and the building of new steel plants. How quickly this happens largely relies on factors outside of the engineer’s sphere of influence. We are currently in a transition period where we can make incremental improvements to sustainability with a clear aim to reach zero carbon in the future. The technical know-how is in place.
The question for naval architects and structural engineers is how do we contribute to sustainability? This requires a mindset where sustainability is considered in the products we design and in the way we work. We need to understand the science behind sustainability and make decisions based on that knowledge. We need to use and encourage emerging technologies and innovation. In this way we will exert an influence on both the material choices and the overall design of the vessels.
Building towards green steel will bring real environmental benefits. Clients are increasingly aware of the environmental implications of their yachts and, as engineers, we can inform the end customers on the effects of material choice on sustainability. The demand for sustainability will influence the speed at which green steel becomes widely available.
Written by Andy Douglas, Chief Structural Engineer.
