Patrick Janssens, Vice President, Global Gas Solutions, ABS, reviews the latest trends in LNG carrier designs.
The last decade has seen a continuous stream of innovation in LNG carrier design and operations; vessel designs have continuously evolved to meet changing trade demands and this evolution is set to continue.
An owner ordering a vessel 15 years ago would have had very little choice in what they could specify. LNG carriers were almost all large ships of around 140 - 145 m3 with standard designs featuring steam propulsion and little choice of containment systems.
Today, the drive for higher performance vessels has seen a choice of propulsion systems come onto the market. Common options include dual-fuel diesel engines and even slow speed diesel engines, with more options being developed. Owners now have options in types of containment systems ranging from independent tanks to membrane systems.
For large LNG carriers, membrane systems remain the standard, with the choice driven by the need to balance the cost of better performing systems versus the required flexibility.
There have been some design developments in MOSS tanks, mainly optimising the tank shape and recently a number of ships with SPB containment systems were delivered, but at present there are no large LNG carriers on order with independent tanks.
Generally, the choice of membrane type is dictated by the shipyard, but there are nowadays a variety of membrane solutions available including No.96 GW, No.96 LO3, No. 96 LO3+, Mark III Flex, Mark III Flex+, with different degrees of boil off gas (BOG) performance. New membrane types are emerging including the KC-1 and GTT is developing a new system which is a hybrid between No.96 and Mark III.
Where we see the use of independent tanks emerging is in the small and medium size sector, driven by the developing LNG bunkering market as well as a need for local redistribution. Traditionally, LNG carriers could trade their entire lives on fixed routes with little need for flexibility. The need to respond to changing patterns of trade means that it is becoming more common for new vessels to include a reliquefaction system.
In fact, there are a number of factors prompting vessels to install reliquefaction plant. A key driver is the much better energy efficiency of modern power plants such as slow speed dual fuel engines).
As a result, the vessels are only capable of burning the full natural boil-off when sailing at near the design speed. In addition, vessels are on average operating at lower speeds and at times may store the LNG to deliver at the most opportune time commercially.
With the emergence of something akin to a spot market in LNG, ships may load for different destinations depending on the state of the market and the season, potentially spending more time on the water, so requiring greater use of reliquefaction.
Some of the world’s largest LNG exporters are scaling up of their trading operations to find new markets and supply existing ones with greater flexibility, which has increased the interest in different ship types to serve emerging trades.
These changes in demand have also seen the emergence of mid scale and small scale LNG shipping, with new trades requiring discharge flexibility to parcel level where a large LNG carrier may discharge to the shore or other vessels for final delivery to small, draft-restricted ports.
This has driven further innovation in new vessel design with concepts such as the LNT A-Box which can serve the small and mid-size cargo demand directly. Its simplified design also means it can be constructed at shipyards that otherwise would not be able to compete for LNG carrier tonnage.
It is not uncommon for new designs to take time to gain commercial acceptance. ABS granted AIP in 2013 to Lattice Technology for its Lattice Pressure Vessel (LPV) tank, which features a flexible, space-saving shape for carrying fuel or cargo.
The tank is a Type C-equivalent pressure vessel based on a proprietary design which provides a solution for prismatic pressure vessels by combining the space efficiency of the prismatic shape of non-pressure tanks with the load-carrying capability of cylindrical pressure vessels.
It took until 2017 for Lattice Technology to secure its first order, for a 15 m3 LNG fuel tank installed onboard a port clean-up vessel in South Korea. In terms of what comes next, one of the major trends is likely to be more mid scale LNG for energy supply and LNG bunkering. Development of these ships has only scratched the surface and designs will continue to adapt to the need for greater market flexibility.
Of course, the LNG carrier market is not immune to industry-wide drivers and the key one will be the efforts to comply with the IMO’s 2030 and 2050 carbon emission reduction objectives. The need for greater energy efficiency and lower fuel consumption has prompted interest from owners in technologies including air lubrication of the hull and auxiliary wind assistance.
Digitalisation too, will influence the way that ships are designed and operated, in particular how systems are managed and how the data collected. LNG carriers already lead the industry in featuring a very high level of shipboard automation so the innovation will be applied to how the vessel is operated with greatest efficiency in terms of routeing and voyage execution.
All innovation creates technologies with different levels of success. The class society approval in principle (AIP) process is designed to encourage innovation and to recognise technology that can be applied safely and sustainably.
It is in the nature of the design process to have a third party evaluate new concepts for compliance. Not all the concepts that result come fully into the mainstream, not all prove commercially acceptable to owners, but AIPs demonstrate to buyers that creative minds are trying to find new solutions and the best will survive the process of natural selection.
The last decade and a half has demonstrated beyond doubt the depth of creativity in LNG carrier design and operations. The AIP process will continue to give owners confidence across a range of options from which to select the technologies appropriate for the next generation of vessels.