Traditionally, these vessels have run on steam turbines, using the boil-off gas from the LNG cargo as fuel. More recently, medium-speed diesel engines have been favoured, even more so with the development of dual-fuel (gas and diesel) engines. Now, two-stroke gas engines have been introduced and the impact they will have on the industry has yet to be seen.
In terms of cargo containment, the membrane-type has become the most prevalent in recent years, with over 90% of the world LNG carrier fleet presently on order being built with a GTT membrane containment system.
The LNG market has developed significantly over recent years, and the amount of LNG supplied is now approximately 30% more than in 2009 (source: IEA).
This has resulted in new trading patterns and requires new thinking when planning the LNG carrier of tomorrow. Historically, LNG carriers have operated on long-term charter contracts (often with a 20-year duration); however, we are now seeing a growing short term, or spot, market developing. This is currently in the region of 25% of the total market (source: GIIGNL).
The world’s energy supply and demand are undergoing turbulent times at the moment. Shale gas from the USA has been a game changer, and the full impact of this development is yet to be seen. At the same time, traditional LNG importers are considering reducing their volumes. Large LNG export projects are coming online in the USA and Australia. Some sources currently predict that LNG demand may even double within the next 10 years. Some claim there are not enough LNG carriers in the fleet, others claim the opposite.
What is certain is that modern, more environmentally friendly and economical vessels will be just as attractive in the future as they have been in the past.
The expanding spot market requires LNG carriers that are flexible in their operation and not restricted by specific design features that optimize them for one particular trade.
With this in mind, DNV GL initiated a Joint Development Project with HHI, GasLog and GTT to develop tomorrow’s LNG carrier using the latest developed technology and within the bounds of existing shipbuilding methods – “LNGreen”.
Future trading patterns and scenarios
LNGreen has investigated how the efficiency and performance of an LNG carrier may be improved by considering actual operational conditions and optimising the ship in terms of hydrodynamics, machinery and system configuration. LNG carrier machinery systems are highly complex configurations featuring a number of tightly integrated sub-systems and components, including but not limited to the BOG compression trains, gas management system, reliquefaction (if any), propulsion and/or generating engines, economizers and boilers. The primary fuel, i.e. boil-off gas, has variable properties (heating value and composition) depending on the cargo type and in-voyage boil-off rate conditions.
In addition, the ships usually operate in a number of trading routes and with varying operating profiles in terms of speed, propulsion and electricity and heat demand. The above features have been analysed by GasLog, whose operational experience helped not only to identify a number of operating parameters and restrictions that should be included in the design but also to define realistic future trading patterns and scenarios.
DNV GL COSSMOS has taken account of this versatility in a rigorous model-based approach that allows the integrated machinery system to be assessed under realistic operating conditions and then evaluates the resulting performance and efficiency.
The hydrodynamic performance evaluation was carried out by comparing CFD simulations by HHI and DNV GL. Different CFD codes were applied to compare the resistance and self-propulsion performance but various scale effects were also considered.
In the case of added resistance, different CFD codes were applied to make sure the required power was sufficient for the ship to operate in the target environmental conditions.
GTT considered the cargo containment aspects, including tank shapes, necessary reinforcements and boil-off rate calculation.
Increased energy-efficiency and larger cargo volume capacity
The LNGreen project identified the importance of designing a vessel for its intended trading operations and of considering it as an integrated unit. Traditional optimization has focused mainly on the hull form, propeller and rudder design – which LNGreen has too – but the inclusion of a system evaluation in the development process has produced significant benefits.
The project developed an LNG carrier vessel concept which is about 8% more energy-efficient than conventional designs and has a 5% larger cargo volume capacity. The LNGreen concept ship delivered here is more suited to the trading operations now envisaged and is optimized to be more efficient than current state-of-the art vessels whilst making use of currently (or soon to be) available technology.
The LNGreen concept provides the content necessary for an owner or shipyard to develop its specification and/or detailed design. It is important to note that each owner and shipyard will have their own requirements which must be considered too. In addition, the vessel’s proposed trade will have to be considered in each specific case –as what has been developed in LNGreen highlights further the importance of considering the vessel’s actual purpose and intended operation.