The protection of the environment is surely the final frontier for Naval Architects, as new materials allow us to dare to use complex shapes, open spaces and very appealing lines but the challenge is to match the increase in size of modern mega yachts, which are often ships of 90-100-120 metres and more, with greater environmental protection and to permit these enormous ‘toys’ to travel everywhere – especially in the most evocative corners of the planet – while respecting the air, sea and land. So we have had to deal with new rules for pollution caused by engines, and contamination caused by the transfer of ballast water from one sea to another, In this article we will examine the effect of the rules concerning pollution caused by engine NOx on yacht design.
The IMO, the International Maritime Organization, has published new rules, which hold for yachts whose keels were laid after 1 January 2016, defining them as the new IMO TIER III standard.
These new Rules will reduce NOx pollution by over 70% compared with the previous TIER II, and this is certainly a very important goal, but as yacht designers we have to tackle a problem: engines have basically been the same since the end of the 19th century; as for the diesel cycle, we can make changes, adopt new devices and increase combustion efficiency, but the engines remain the same.
So, naval architects have co-operated with mechanical engineers to install the most modern engines on board, in order to respect the new rules, but this effort has not been without a cost: engines and engine rooms have become more and more complicated, in other words there is less space in the engine room and much more weight to carry.
Exhaust treatment machinery is often comparable in size with the engine itself and in some situations this has obliged naval architects to make important changes. Furthermore, it is obviously impossible to avoid effects on cost, stability and so on simply by increasing the dimension of the engine areas.
The International Maritime Organization (IMO) is an agency of the United Nations for the promotion of safety at sea. The IMO ship pollution rules are contained in the “International Convention on the Prevention of Pollution from Ships”, known as MARPOL 73/78. MARPOL Annex VI (1997) deals with limits on NOx and SOx emissions from ship exhausts, and prohibits deliberate emissions of ozone depleting substances. The IMO emission standards are commonly referred to as Tier I…III standards.
Tier I and Tier II limits are global, while the Tier III standards apply only in NOx Emission Control Areas.
Fig.1 ECA Areas in the world
Yacht limits: according to the previous examples, the situation for yachts is as follows.
IMO TIER III applied for yachts with keel laid after 1/1/2016 , if over 500 GT.
From 1/1/2021 the IMO TIER III will be requested also for all yachts over 24 metres in length and weighing less than 500 GT.
How to meet requirements:
Describing how to meet the standards would require a complete examination of the theory of engines, involving a description of the combustion processes, but we can summarize and describe the division of the system in 2 main sections:
The control of combustion processes with an impact on the mechanical characteristics of engines, heavy redesign of pistons, crankshaft, injection system etc.
The use of additives, such as an SCR system to post treat the exhausts. The main one is the Selective Catalytic Reduction system, which uses urea to produce nitrogen, water and CO2 .
Fig.2 General example of SCR system principles
Fig.3 General example of an SCR system
So we will consider the situation of a 70-m yacht, where the shipyard decided to install an SCR system to comply with IMO TIER III Rules.
Actually, it is important to say that most yachts over 500 GT use high/medium speed diesel engines, which means engines with an operational range between 1600 and 2300 rpm and an average power output (depending on the speed and performance requested) ranging from 800 to 2500 kW. The SCR system is still used on such engines, and now we will see how the size of the treatment system can impact the arrangement.
The average size of engines in this range can be summed up as follows:
Length = 3-6 m.
Width = 1-2 m.
Height =2-2.5 m
Weight = 3-12 t.
The weight and size of a system for a 1800-kW engine, therefore in the middle of the range above described, can be:
L = 2800 mm
B = 700 mm
H = 700 mm
Weight = 400 kg
In addition we have to install a tank for the additive liquid with a volume of about 1500 litres for each engine, in order to guarantee a range of about 2000 nm.
The layout of the engine room will basically change as follows:
Fig. 4 : Engine room before post treatment
Fig. 5 : Engine room after SCR post treatment
We can see that the Engine Room becomes even more crowded, and the system with the annexed piping makes it very difficult to carry out maintenance on the upper part of the engines.
The solution adopted on new ships basically consists of increasing the height of the hull to have enough space for the systems:
Fig.7 Engine room transversal section with increased height
The ship’s height has been increased by 150 mm, but on a 70-m yacht, this means 8-10 tons of extra steel, and a reduction in GMT stability of about 5-6 cm, because the entire weight of the superstructure is shifted up by 150 mm.
The impact of the IMO TIER III Rules on the design and arrangement of the engine room of a modern yacht of over 60 metres in length is significant. In same cases, apart from the sheer cost of the system, which can be estimated at around 110 euros per installed kW, the layout of the project can also require changes.
In the future, probably, we will see a growth in the space reserved for machinery, reducing the area available for crew/accommodation or requiring an increase in the size of the yacht.
Ing. Valerio Ruggiero