The Green Answer

The marine business is facing new challenges as public opinion becomes ever more focused on ship emissions. Since onshore industries have been able to cut certain emissions, the share of pollutants contributed by shipping has grown proportionally to become a significant part of total emissions – this despite the fact that transport by ship remains the most efficient means of transportation. The marine industry therefore needs to look at new ways of reducing the environmental impact from ships.

Wärtsilä is continuously seeking to find better propulsion machinery solutions for ships. This involves looking at the entire shipping concept, not only the engines and propellers. One area of special interest is the environment. As part of its development work, Wärtsilä has developed a new coastal cruise ferry using liquefied natural gas (LNG) as fuel.

LNG is an efficient way to cut emissions. All SOX emissions are eliminated and the NOX and CO2 emissions are reduced by about 80% and 20% respectively (see figure 1). LNG is not only an environmentally sound solution, but also economically interesting at today’s oil prices.

The new ferry is designed for cruising along the Norwegian coast between small coastal communities. As this still represents a relatively untouched natural landscape, an environmentally friendly ship solution is required. The same ship and machinery concept could also be applied easily to other passengership operations, such as short-route ferries and expedition cruise vessels.

LNG AS MARINE FUEL

During the next few decades natural gas (NG) is expected to be the world’s fastest growing major energy source. The driving forces behind this development are the depleting known oil reserves, increasing environmental care and the continuous tightening of emission restrictions.

NG is one of the largest sources of energy worldwide and at today’s rate of consumption it is expected to last about 150 years. On an energy basis, known NG resources even exceed known oil resources. Furthermore, the resources are widely spread around the world. Interest has recently grown in the use of LNG in marine applications.

NG PROPERTIES

NG consists of methane (CH4) with minor concentrations of heavier hydrocarbons like ethane and propane. In normal ambient conditions NG is a gas, but it can be liquefied by cooling down to -162°C. In liquid form the specific volume is reduced significantly, which allows a reasonable size of storage tanks relative to energy content.

The burning process of NG is clean. Its high hydrogen-to-coal ratio (the highest among the fossil fuels) means lower CO2 emissions compared with oil based fuels. When NG is liquefied all the sulphur is removed, which means zero SOX emissions. The clean burning properties of NG also significantly reduce NOX and particle emissions compared with oil-based fuels.

STORAGE SOLUTIONS

The most feasible way to store NG in ships is in liquid form. In existing ship installations LNG is stored in cylindrical, double-wall, insulated stainless steel tanks. The tank pressure is defined by the requirement of the engines burning the gas and is usually less than 5 bar. A higher (typically 9 bar) tank design pressure is selected due to the natural boil-off phenomenon.

This means that the heat flow through the tank insulation boils the LNG, which increases the pressure in the tank. In the case of long lay-up periods, some boil-off gas must be released or burned (see figure 2).

The main problem with using LNG in ships is the large amount of space required for the LNG tanks. Compared with marine diesel oil (MDO), an equal energy content of LNG requires about 1.8 times more volume than MDO. When adding the tank insulation, noting the maximum filling ratio of 95%, the required volume is increased to about 2.3 times.

The practical space required in the ship becomes about four times higher when taking into account the squared space around the cylindrical LNG tank. If compared with an MDO tank located above a double bottom, the total volume difference is smaller, about 3.0.

The weight of LNG is marginally lower than for MDO when considering the fuel itself. However, the special tank and tank room steel structure increases the total weight for LNG storage to about 1.5 times higher than for MDO.

SAFETY PRECAUTIONS

Many people would think that gas is highly explosive and dangerous used as a ship fuel. When examining the properties closer, however, this proves to be a hasty conclusion. NG is a safe fuel when the right precautions are taken.

• Liquid form. In a liquid state LNG is not explosive, nor is it corrosive or toxic. Thus any possible spillage does not cause any lasting contamination, as the liquid will boil to gas. The low temperature, however, is an issue when considering normal ship steel, but this problem is avoided by using appropriate materials in LNG systems.
• Gaseous form. Gaseous NG is lighter than air, which means that in case of leakage the gas will disperse upwards and not build up in the ship’s bilge. The ignition temperature of NG is relatively high (600°C) compared with diesel oil (250°C), and NG is flammable only in a small concentration range between 5% and 15% of air.

CRUISE FERRY CONCEPT

Few ships are currently using LNG as fuel other than LNG carriers. Norway has been the forerunner for LNG-fuelled ships. The country has two offshore supply vessels and one double-end ferry in operation, and a further five double-end ferries and one supply vessel are on order.

The cruise ferry is designed for cruises sailing up and down the Norwegian coastline. It offers cruiseship comfort for 300 passengers in 137 cabins and features a wide range of public spaces, such as bars, an à la carte restaurant, buffet, lounges and a spa.

Most of the public spaces are located on deck 5 with some lounges and the spa on the top deck. The ferry has a small cargo space for private cars and some refrigerated cargo holds on the bulkhead deck.

The ship can be used either as a ferry or as an expedition cruise vessel. It has a range that is suited to cruises lasting over one week without refuelling. This means that large LNG tanks with a capacity of 520m³ are needed. It is a clear challenge to find space for them inside the ship and they affect the entire arrangement as well as the ship dimensions.

In this case, the two large LNG tanks are located down on the tank-top in the centre of the vessel inside the B/5 lines as the classification rules state. A small recess in the bulkhead deck has even been made to make space for the forward tank.

The ship is divided into two main fire zones to allow for an efficient arrangement. The machinery is located in the centre of the vessel with the engines divided on both sides of the main fire bulkhead. The engine casing and funnel are quite far forward.

MACHINERY CONCEPT

The ferry features a machinery of the hybrid dual fuel (H-DF) type, which consists of two Wärtsilä 6R32DF propulsion engines and two 9R32DF generating sets. These give a total installed power of 10.5MW. The engines use LNG as primary fuel and marine diesel oil as pilot and back-up fuel.

A conventional ferry propulsion set-up with two open shaft lines has been used in this concept. Two W6R32DF propulsion engines drive controllable pitch propellers (CPP) each via a reduction gear, which constitutes the H-DF machinery’s mechanical propulsion power. Two 2MW generator/ motors are connected to the PTO/PTIs on the gears.

With the two large W9R32DF generating sets, they form the ship’s electrical power plant. The electric generators/motors can drive the propellers and are equipped with frequency converters for variable speed operation. The total propulsion efficiency has been improved by applying Wärtsilä efficiency rudders. The normal tunnel thrusters in the stern has been replaced with a new steerable thruster integrated into the skeg.

The machinery is designed according to an Emergency Shut Down philosophy – if there is a gas leak in one engine room, the gas supply will be cut off and the engines are switch to diesel mode. If the leak continues, the entire room is shut down, so fully redundant machinery is needed.

The propulsion engines and generating sets are therefore divided into two different compartments with the reduction gears in a third separate compartment. This allows the propellers to be driven with either the PTO/ PTI propulsion motors or with the mechanical engines in the case of loss of either engine compartment.

Electric power can also be maintained with either generating sets or shaft generators. The LNG storage tanks are situated between the B/5 zones and the engines are located to the sides in areas where gas storage is not allowed. This is a compact arrangement that complies with the stringent rules for gas-fuelled machineries.

The H-DF machinery combines the lower investment cost of mechanical propulsion with the good characteristics of diesel-electric machinery. The machinery principle offers extra flexibility and allows a smooth switch between the different operation modes.

Furthermore, all engines can usually be run at constant speed, close to the optimum load to achieve lower fuel consumption. Low engine loads can be avoided by switching between the various power generation options. Hybrid machinery is ideal for ferries when designing the vessel for cost-effective operation in various operation modes.

OPERATION PHILOSOPHY

The ferry operates as a diesel-electric vessel when manoeuvring and at low speeds. The mechanical propulsion engines are engaged at higher speeds to boost the shaft power in combination with the electric motors. This offers more economical transit than pure electric propulsion. The propulsion engines can also be used alone, but this is not intended as a normal operation mode.

Four main operation modes have been considered when designing the machinery: port, manoeuvring, slow speed and cruising speed. The two sea modes use the twin CPPs as the primary propelling device. The integrated skeg thruster is used only in the
manoeuvring mode as a steering device alongside the propellers and the rudders. However, it can also be started in the case of failure as a back-up propulsion device.

The CPPs are never operated at low pitch and high rpm because at low speeds (manoeuvring and slow speed modes) the CPPs are driven by frequency-controlled electric motors, which enable variable speed operation. At higher speeds, when the mechanical engines are engaged, the propellers are operated at constant rpm by adjusting the pitch, but the power is so high that low pitch is not needed.

One of the propulsion engines (smaller than the generating set engines( is used in port when the electric load is low to drive the shaft generator. This results in a more optimum engine load.

INTEGRATED SKEG THRUSTER

The integrated skeg thruster is a new product that offers excellent manoeuvring characteristics and replaces the tunnel thrusters that are normally used in the stern. A steerable skeg thruster also allows large side thrust when moving forward, which is not possible with tunnel thrusters.

The turnaround time in port can be reduced compared with conventional twin-shaft vessels owing to better manoeuvring. The steering redundancy is also enhanced by the three independent steering devices. The skeg thruster is designed to be a primary steering device and a secondary propulsion device.

The Z-geared skeg thruster is driven by a horizontally mounted electric motor placed above the bulkhead deck. The steerable thruster can be turned more than 90º to both sides to direct thrust in the desired direction. It is integrated into the skeg to give minimal increase in resistance; its strut is hydro-dynamically shaped to continue the skeg shape and allow for smooth water flow.

The strut of the steerable thruster extends well forward of its centre vertical turning axis. This means that a large opening is created between the skeg and the thruster when it is turned to the side. The water flow from the thrusters propeller can therefore easily pass between the skeg and the thruster’s strut, allowing greater side thrust.

The skeg thruster is provided with a propeller of the feathering type to further minimise the extra resistance when not in use. The blades are able to turn until they are parallel to the longitudinal axis of the vessel. The steerable thruster can therefore be resting at the feathering position when the ship is running forward at service speed.

WÄRTSILÄ 32DF

The Wärtsilä 32DF engine (based on the well proven Wärtsilä Vasa 32 diesel engine) is a 4-stroke dual-fuel engine that can be run on either gas or fuel oil. The maximum cylinder output is either 335kW or 350kW. In-line engines are available in 6- and 9- cylinder configurations and V-engines with 12 and 18 cylinders. In gas mode it runs as a lean-burn engine according to the Otto cycle, using diesel oil as pilot fuel.

The ‘micro-pilot’ injection system uses less than 1% of nominal fuel energy input. In liquid fuel mode the DF engine works just like any diesel engine, using a traditional fuel injection system. Transfers between the two operating modes take place without interruption in power supply.

Electronic control of the gas admission and pilot injection is employed to regulate the combustion process individually for each cylinder. Accurate control ensures that combustion stays within the optimum operating window, giving the best performance for all cylinders under all conditions. Today, the NG-fuelled, lean-burn, medium-speed engine has proved to be a reliable, clean, high-efficiency power source.

ALTERNATIVE ENERGY SOURCE

Calculations made for the new concept shows a remarkable difference compared with diesel-fuelled machinery. All the major emissions can be significantly reduced to truly form an environmentally sound solution, in particular, the reduction in CO2 is hard to achieve with conventional oil-based fuels.

LNG offers an alternative energy source for ships. The new ferry concept shows how it can be efficiently integrated into the ship design and the environmental benefits it offers. Some extra space is required for LNG storage, which has to be taken into account at the outset of the design process. However, this is a small price to pay when considering the enormous reduction in emissions that can be achieved with this concept.