An Innovative LNG Carrier Concept

Concurrent with marine industry consolidation on the ship owning, building and supply fronts, increasingly innovative vessels concepts are originating from in-house design teams sitting with the major equipment manufacturers. Single-source supplier is today’s mantra on the commercial and military fronts, with companies such as Wärtsilä leading the way. The company recently published details of an innovative new LNG carrier concept, the Wärtsilä DF-electric propulsion concept for liquefied natural gas (LNG) carriers, bringing together two technologies: electric propulsion and dual-fuel engines.

Outline of the DF-Electric LNG carrier

The Wärtsilä DF-electric LNG carrier concept is designed for a single-screw vessel with four cargo tanks and a capacity of about 138,000 cu. m. The hull has a transom stern, a single-skeg aft body and a bulbous bow. The propulsion machinery and accommodation spaces are arranged in the stern part. The cargo machinery room is arranged separate from the accommodation space on the upper deck.

Two cargo tank system variants can be applied: membrane and spherical types. Both variants have a length, (bp) of 902 ft. (275 m), breadth of 141 ft. (43 m) and 157 ft. (48 m) respectively, and design draft of 36 ft. (11 m). The main machinery consists of four nine-cylinder in-line Wärtsilä 50DF dual-fuel engines, each driving an alternator. Each main engine develops 8,550 kW at 514 rpm, giving a total output of 34.2 MW. The main generators feed the ship's electrical network and, through a variable-speed drive system, the propulsion motors. A 500 kW emergency diesel generator set is also installed. The single, five-bladed fixed pitch propeller is driven by two 13.5 MW AC propulsion motors through couplings and a twin-input/single-output reduction gear. There are also two 1,000 kW bow thrusters. To enhance the redundancy of the propulsion plant, the main engine rooms and casings are divided with a fire-resistant bulkhead. The main engine rooms are under diminished air pressure. A back-up arrangement of a thermal oxidizer is provided to dispose of boil-off gas during long periods of low-load operation. The service speed of the ship is about 19.5 knots at the design draft of 36 ft. (11 m) and with 15 percent sea margin, which corresponds to 27 MW shaft power. The power for accommodation and machinery ancillary consumers is about 1 MW.

Duel Fuel Developments

Wärtsilä has eagerly developed dual fuel engines, with the creation of the 32DF and 50DF (see related story on page 41). In fact, the company recently logged an order for a Wärtsilä 50DF on the first LNG carrier to be using DF-electric propulsion.

Steam turbine propulsion dominates today's global LNG carrier fleet, as the availability of high power output and the possibility of using low-grade fuels as well as cargo boil-off gas. Whatever propulsion plant is chosen, there has to be some way of handling this boil-off gas either by utilizing it as fuel, or reliquefying it. Safety is of utmost importance in gas shipping, and LNG carriers have an excellent safety record. The reliability of steam turbine propulsion has helped to achieve this together with strict terminal regulations and procedures, and robust ship designs.

However, times are changing. Short-term contracts and even spot cargoes are becoming more common, owing to the increasing LNG demand and supply.

Some LNG carriers have even been ordered without any shipment contract or route, a previously unheard of practice.

Alternative Concepts

There are effectively four types of prime movers available for LNG carrier power plants: steam turbines, diesel engines (two- and four-stroke), dual-fuel engines and gas turbines. Steam plant development has virtually stood still for many years as there has been practically no market for marine applications other than LNG carriers since the 1973 Oil Crisis.

Diesel engines, by contrast, have come to dominate merchant ship propulsion, except for LNG carriers. The accumulated experience of thousands of diesel engine installations has helped to ensure the successful development of this technology. However, employing diesel engines for LNG carriers calls for total reliquefaction of the boil-off gas. The recent development of dual-fuel engines (liquid and gas fuels), derived from heavy fuel diesel engines, has made it possible to use the boil-off gas efficiently. Therefore propulsion based on dual-fuel engines is a strong option for modern LNG carriers today. When specifying propulsion machinery options for LNG carriers it is essential to consider the differences in operating profiles, fleet configurations and shipping routes. The basic case today is an approx. 138,000 cu. m. vessel with an operating speed of around 19.5 knots and the corresponding power required at the propeller of about 27 MW. However, future operating profiles of LNG carriers will require more flexibility from the power plant. Already there have been inquiries about ships that would normally operate at about 15 knots, but have to be capable of doing 19 knots on spot cargo trades. It is then very important that the power plant is efficient also in part-load operation. The maximum required electrical power for cargo pumping and other consumers is roughly 6 MW whereas the minimum can be less than 1 MW.

Electric Propulsion Based on Medium-Speed Engines

Electric propulsion offers by far the most flexible alternatives for machinery arrangement. It is also easy to build the required level of redundancy into the system with divided engine rooms and ancillary systems. Medium-speed generator sets can be either heavy fuel burning diesel engines or low-pressure dual-fuel engines. A propulsion system based on heavy-fuel engines will naturally require reliquefaction plant to take care of the boil-off gas. Dual-fuel engines can use either boil-off gas or MDO as fuel. With electric propulsion, there is no need for separate auxiliary generator sets, so the total installed power can be reduced. With electric cargo pumps, one generator set should be sufficient to handle the power demand for cargo operations. Single-screw electric propulsion with a fixed pitch (FP) propeller may be selected if appropriate redundancy is built into the electric drive system. More than one electric motor can be used for the propeller shaft either in tandem, or coupled separately through a gearbox. The electric motors can also be double wound for additional redundancy. Twin-screw propulsion can be configured either with podded drives or FP propellers driven by electric motors. Another possibility would be to use a single FP propeller complemented by a podded drive replacing the rudder. Using the 'contra-rotating propeller principle' with the podded drive immediately abaft the main propeller, a considerable propulsive efficiency gain is possible. The main propeller would cater for approximately half the required propulsive power. The other half would be provided by the podded drive. An electric propulsion system based on a combination of heavy fuel burning generator sets and gas burning generator sets has been proposed as well. This is based on the desire not to install any reliquefaction plant and instead to use natural boil-off gas only as fuel (without forced boil-off gas) and to top up the remaining energy requirement with heavy fuel. However, to cater for the wide variation in boil-off gas energy available, the total installed engine power would be high, perhaps up to 65 percent greater than with a single type of engine. Furthermore, since the energy price of LNG compared to heavy fuel is about the same, it is logical to use gas-burning engines if only to keep the propulsion power plant simple.

The preceding was excerpted, in part, from an article by Janne Kosomaa, Product and Application Development, Wärtsilä.