The energy industry, shipbuilders and shipping lines will face a host of decisions about the adoption of LNG technologies.
There is a quest for better and more efficient solutions for all parts of the LNG chain. Larger and more effective liquefaction plants are being planned; larger ships with efficient propulsion plants and minimised boil off with on board reliquefaction are being built; novel containment systems that challenge the traditional systems are being proposed; and new receiving terminals based on innovative technology are being developed.
Fatigue considerations and tank sloshing loads are becoming important design parameters. The need for seaborne LNG transportation is increasing rapidly, leading to a doubling of the fleet in just one decade. The consumption of natural gas is projected to increase by nearly 70 per cent between 2002 and 2025.
Offshore receiving/storage terminals and regasification and discharge terminals will in some parts of the world be the preferred future option due to safety considerations and environmental concerns. Floating units for receiving, storage, regasification and export of natural gas (LNG FSRU) as well as units for offshore production (FPSO) are emerging markets. For these new applications, safe operation with partial tank fillings has to be carefully studied on a case-by-case basis.
Sloshing has been a concern from the start. And with larger carriers, terminals and ships operating with partially filled tanks it becomes even more relevant. DNV has a long history of sloshing model experiments, and the past several years have been a logical extension of that development work.
Practical experiments are still favoured over computer simulations.
A sloshing impact is characterised by fluid, which hits the tank wall at high velocity. During these impacts gas is entrapped and mixed in the fluid.
DNV recently issued a new LNG sloshing class note for membrane tank systems.
There are three other class notes under development which will be issued in early 2007. One is focusing on the hull and tank support design of membrane tankers, excluding the containment system, the second is focused on sloshing in membrane tanks and the third is focused on the analysis of the hull and tank system of spherical tank LNG carriers.
A specific DNV research programme, called Operation in Cold Climate, has also been set up to develop class services for strengthening and winterization of ships trading in Arctic waters.
As well as slow speed diesel engines several alternative propulsion arrangements are also being discussed and introduced by the industry. A promising option are dual fuel engines, for which the DNV Class Notation Gas Fuelled provides the necessary rules to cover engine room design, gas storage and gas piping.
After four decades, diesel engines with N-BOG reliquefaction plants and dual fuel engines with electric propulsion systems have managed to break the steam turbine dominance in LNG shipping. During the first half of 2006, 90 per cent of LNG carriers ordered were specified with an alternative machinery installation.
Extracts from a DNV white paper written to assist decision makers
Date: 2007-10-09