Cruising is a leisure business, and as such, it is very visible and sensitive to public opinions. As time goes by, it is likely this focus will sharpen as ever-larger vessels move into famous cities and marinas around the world through fragile waterways —even into arctic areas.In addition, people booking trips are well aware of the environmental impacts and it is for this reason that those operating in the business look for environmentally friendly solutions when they select vessels. This concern covers the overall services and activities offered onboard the vessel, as well as the equipment used for propulsion and the vessel’s hotel load.
The LNG cruise vessel concept design from Wärtsilä and Aker yard includes a split stern-endsuperstructure.
More importantly, emissions regulations are becoming increasingly strict and new authorities are establishing their own emissions regulations in local waters.Cruise operators, therefore, need to put additional efforts into becoming “greener.” “The current international regulations for ship emissions are not very stringent and can usually be met with standard marine equipment,” said Oskar Levander, general manager, conceptual design – Ship Power, Wärtsilä.
“However, we already know of new and tougher limitations, which will come into force in the near future, and we are improving our engine technology continuously to meet future requirements. We are also looking for less conventional solutions to limit harmful emissions from the marine industry, and have, together with Aker Yards, designed a cruise ship concept using LNG as fuel.” The idea behind this new concept is to find out how LNG systems can be efficiently integrated into the cruise vessel design and to see how the gas can improve the overall performance of the ship and provide a clear reduction of almost all major emission levels.
The main contributors to the greenhouse effect are CO2 emissions formed when burning fossil fuels. The 20% reduction of CO2 emissions committedto by the European Union will also require the marine industry to do their share of the task. The ways for ships to reduce the CO2 emissions are to either reduce the fuel consumption or to switch to a fuel containing less carbon. Reduction of the power demand will lessen the fuel consumption and so will more efficient power sources. “There is no easy way to take big steps in increasing the engine efficiency, and increasing the efficiency is often resulting in an increase of some emissions,” said Levander. “Reduction of power demand offers more options, but also here it is not always easy to find ways to do it.
One way would be to adopt more efficient propulsion solutions, to optimize the hull form and to select a more optimal speed profile and routing.” For a cruise vessel, the hotel side consumes lots of energy — representing a large share of the total energy consumption. Consequently it offers a large potential for savings and is an area the cruise industry must investigate.
The vessel machinery used consists of six, 12-cylinder, Wärtsilä 50DF, dual-fuel engines in Vee configuration. They are divided into two separate compartments, providing a total installed power of 68 400 kW. The LNG tanks are located in the center of the superstructure, inside the outer row of cabins and in front of the engine casing.
However, Wärtsilä considers that the potential CO2 savings from reducing the power demand are still limited. “To achieve a significant reduction of all harmful emissions, a switch from HFO (heavy fuel oil) to natural gas would be required,” said Levander. “This would reduce the CO2 emission by 30%, since the main component of natural gas is methane.” Methane is the most efficient hydrocarbon, when measuring energy content against carbon content. By running engines on liquefied natural gas instead of HFO, the result would be a 30% reduction of the CO2 and 85% lower NOx. Furthermore, no SOx would be emitted, since sulphur has been removed
from the LNG during the liquefaction of the natural gas.
The particulate emissions would also be very low and no visible smoke would be seen. The Wärtsilä/Aker Yard project has been based on a large cruise vessel with the volume consisting of 125 000 gt, offering almost 2800 lower beds. The overall length of the ship is 310 m and its breadth is 40 m. The design includes a split stern-end superstructure. The superstructure of the middle and the forward part of the vessel is in one part and is narrower than the hull.
As gas is a new fuel for passenger vessels it raises many issues, whichneed to be addressed as the various authorities’ regulations develop. At the moment, only DNV and LR have rules applicable to gas-fueled passenger vessels.
Norway has seen the gasfueled passenger ships develop early. There are already six LNG-fueled ferries running in Norwegian waters, as well as four supply ships. The availability of worldwide governing rules is likely to improve — one reason being that IMO is currently working on some draft rule proposals. An important issue to address has been the location of the LNG tanks. They are neither allowed to be close
An emission comparison is shown here between a conventional cruise vessel and the new vessel with dual-fuel engines.
to the side of the vessel, nor close to the bottom of the vessel. In order to produce the same amount of energy, the volume of LNG needs to be 1.8 times the volume of diesel fuel — so
clearly the large size of the fuel storage is one challenge. LNG is stored at -162°C for it to
remain in a liquid state. The special tanks — with very efficient insulation — require thick walls, which again consume additional space.
A cylindrical tank type is commonly used in the existing LNG ferries and supply vessels. These cylindrical pressure vessels are made in stainless steel and are double walled. The actual volume needed for the LNG storage compartment is about four times that of diesel tanks, taking into account the lost space around the cylindrical tanks. This poses a challenge for the naval architect. The end result is that the vessel needs to be larger to accommodate the same number of passengers. “Within our new concept we have developed a new location for the LNG tanks and located them in the center of the superstructure, inside the outer row of cabins and in front of the enginecasing,” said Levander.
“The tanks are located above the public space decks, so as not to obstruct passenger flows. It forms a logical continuation of the technical areas, such as AC room, which are located in the ‘dark center’ of the superstructure.” Wärtsilä has also looked into using low-pressure steel tanks of International Gas Carrier, code B-type, but for the moment the class rules only recognize this type of tank for LNG carriers. Such tanks would offer a lower cost and take up less space, as they can be rectangular in shape and be conventionally placed on the tank top down in the hull.
The vessel machinery used consists of six, 12-cylinder Wärtsilä 50DF dualfuel engines in a Vee configuration. They are divided into two separate compartments, providing a total installed power of 68 400 kW. The propulsion is by twin fixed propellers on shaft lines, each driven by low-speed, electric motors. The 50DF engine runs on either gas or diesel, and burns gas in a lean mode with the air-gas mix ignited by a small pilot, diesel fuel injection. The LNG is fed via an evaporator to a gas valve unit for each engine. From the valve the gas is led through double-wall pipes directly to the engine. There are no compressors or pumps involved to run the process, the pressure in the LNG tank is enough, making the system simple and reliable.
“The LNG cruise ship concept represents one of the safest and most reliable cruise ship designs,” said Levander, “with the machinery fully redundant, with all systems divided into two or more compartments. Not only arethe power generation and propulsion divided into two parts, but also the comfort systems are. The ventilation systems are increased to avoid buildup of any large quantities of gas — one of the more important features of gas safety principles.” Any gas detection will result in an automatic shutdown of the gas supply and the ship will switch over to diesel operation to allow for an uninterrupted power supply.
In the normal case, only the gas that is consumed will be evaporated and brought to the engines.The rest will be in the form of LNG in the tanks, which in liquid form cannot ignite because of its low temperature. The gas fuel will not have the same availability as diesel fuel, so bunkering arrangements need to be thought of in advance. Today there is no existing infrastructure for supply of LNG supply to vessels, but it can be arranged with trucks, small LNG carriers and barges. “Since the cruise ship will be such a big LNG customer, most gas suppliers would certainly be ready to provide the gas needed,” said Oskar Levander. “The cruise operator would most likely make a long-term contract with the gas supplier, hereby safeguarding the gas supply.
“Compared with a conventional cruise ship the performance and economics of the LNG vessel shows similar cost levels,” said Levander. “The larger vessel and the LNG system increase investment costs, but on the other hand the LNG cruise ship has lower fuel costs, partly due to a lower energy consumption.” The LNG cruise ship concept developed by Wärtsilä and Aker yard shows that LNG is a very interesting option for the cruise business in the future. The introduction of LNG as a marine fuel would be a big step toward a cleaner and more sustainable cruise business.