The surge in liquefied natural gas (LNG) carrier newbuildings and the accompanying development of a new generation of these specialized ships has generated interest in alternative propulsion systems to the traditional gas fired steam plant. These alternatives are addressed in the new ABS Guide for Propulsion Systems for LNG Carriers, the first such comprehensive set of standards to have been issued by a classification society. Currently several type of propulsion configurations are being ordered with others yet being planned. At the heart of the differing propulsion systems debate is economics together with safety concerns over the manner in which boil off gas (BOG) is treated - use it as fuel for propulsion or re-liquefy and return it back to the tanks.

Dual fuel diesel electric and direct drive, slow speed diesel plants, coupled with an on-board re-liquefaction plant to handle the cargo boil off, appear to offer operational efficiencies for new designs. However, there is no single obvious propulsion system of choice, according to ABS. The choice depends on numerous factors such as gas price or trade route among others.

The new ABS Guide provides comprehensive criteria for the various propulsion systems currently on offer from shipyards: dual fuel gas turbine/steam turbine combined cycle with electrical propulsion, dual fuel diesel electrical propulsion and slow speed diesel direct drive as well as conventional steam turbines. It includes detailed criteria for the on-board re-liquefaction plant design, piping, electrical systems, construction materials, compressors, pumps and separators. Additionally, the document offers guidance with regard to the placement of propulsion systems in the overall vessel arrangement scheme as well as refrigeration systems and redundancy systems required if there is an equipment failure.

ABS will offer four different notations for those following the design criteria and construction standards set forth within the guide. They are: RELIQ for re-liquefaction unit design; GCU for gas combustion unit; DFD for dual fuel diesel engine power plant; and DFGT for dual fuel gas turbine power plant. The release of this publication is the culmination of more than two years of work over which time ABS has directed its team of marine, mechanical and electrical engineers through various iterations while carrying out a series of technical discussions with virtually every manufacturer supplying specialized propulsion and associated equipment to these new breed of LNG carriers. ABS has also hosted and participated in a number of risk studies along with owners, builders and equipment suppliers in order to develop this comprehensive propulsion guidance.

The ABS Guide for Propulsion Systems for LNG Carriers is available for free download from the ABS website, publication # 112.