Alternative Propulsions

As a part of our evaluation of transportation solutions, we have studied various propulsion alternatives taking into consideration the following parameters:

• Propulsion flexibility
• Maintenance complexity and cost
• Fuel efficiency and cost
• Redundancy / Availability
• Emission Control
• Manning requirements and availability
• Overall operational cost

The analysis of the different propulsion types shows that there are clear economical and environmental arguments to move away from steam propulsion.

Several propulsion alternatives have been developed with focus on:

• slow speed diesel (with BOG reliquefaction)
• diesel-electric (dual-fuel)

Slow speed diesel with BOG reliquefaction - twin screw
Several shuttle tankers in the North Sea are equipped with twin slow speed engines and twin propellers. This ensures that approximately half of the propulsion power always is available, and that one of the diesel engines can be maintained without immobilising the vessel or compromising safety.

Redundant slow speed engine propulsion concepts ensure that sufficient power is available for safe navigation and, for the twin engine concept with completely separated engine rooms, even an additional margin towards any damage is obtained.

For LNG carriers, a twin slow speed diesel engine configuration has been selected for all newbuildings orders for LNG vessels with a cargo capasity in extent of 200 000 cbm.

Dual-fuel diesel electric (DFDE)
The DFDE engine is fuelled by natural gas (LNG) as main fuel and marine diesel oil (MDO) or heavy fuel oil (HFO) as secondary fuel.

The main benefit by the DFDE propulsion is that the total efficiency of such plants is well above 40% compared to the reported 30% or less of the steam plant. The difference becomes even higher on part loads, providing safety, flexibility and efficient use of the installed power. Furthermore, preventive maintenance at sea and during port calls is possible, which is not the case with the steam plant or with large single slow speed diesel alternatives.

The fuel economics of the DFDE alternative today is depending on the price difference between HFO and MDO/LNG in a combination. While making maximum use of the gas boil-off from the LNG, the high efficiency of these engines means a lower fuel consumption overall, resulting in fuel savings compared with a conventional steam turbine plant.

The DFDE engines have lower stack emissions than a steam plant and ordinary diesel engines. The NOx emissions are about one-tenth of the equivalent powered ordinary diesels. C02 emissions are also lower due to low fuel consumption and maximum use of the available natural gas.

For these reasons, DFDE is the favourite propulsion alternative for LNG newbuildings of standard size.

Reheat steam turbine
The manufacturers of marine steam turbine plants have recently (re)introduced the reheat steam turbine system, in order to increase the efficiency of the system. Compared to the traditional “conventional” steam turbine the reheat type operates at higher steam temperatures and steam pressures and introduces a second intermediate turbine, situated between the high and low pressure turbines. The steam, after leaving the high pressure turbine is reheated in the boiler before entering the intermediate turbine. The reheat steam turbine system is intended to increase the efficiency by up to 15%, compared to the conventional steam turbine plants. The system is intended to be on the market in a near future.