Developing & Operating Better LNG Ships

George Dimopoulos, DNV GL Maritime presented ways for ‘’Developing & Operating Better LNG Ships’’ during the 2016 GREEN4SEA Conference & Awards. He stated that future and emerging trades, markets, export facilities as well as propulsion technologies call for innovation in LNG carrier ship design. He presented the results of the LNGreen joint development project between DNVGL, Gaslog, GTT and Hyundai Heavy Industries that developed an LNG carrier concept with significantly improved efficiency by more than 8%, within the bounds of today’s technology, ready to be ordered and built.

Since the Methane Pioneer carried the first cargo of LNG from the USA to the UK in 1959 (cargo cap. 4400cu.m.) which led to the first purpose-built LNG ships (Methane Princess and Methane Progress, cargo cap. 27400cu.m.) entering service in 1964, the vessels carrying these cargos have often been considered to be flag bearers for innovation, safety and quality in the merchant shipping fleet.

Traditionally these vessels have run with steam turbines, using the boil of gas (BOG) from the LNG cargo as fuel. More recently medium speed diesel engines have been favored, more so with the development of dual-fuel (gas and diesel). The development of 2 stroke gas engines has now arrived, and the impact this will have on the industry is 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 specified with a GTT membrane containment system.

The LNG market has developed significantly over the recent years, with approximately 30% more supply presently being made than in 2009 (source: IEA). This has resulted in trading patterns which are adjusting.

Historically LNG carriers have operated on long-term charter contracts (often with 20yr duration); however, now we see a growing short-term, or spot, market developing (currently in the region of 25% of the total market (source: GIIGNL).  Some sources currently predict that LNG demand may even double within the next 10 years.

The LNGreen project’s objective is to develop tomorrow’s LNG carrier using latest developed technology, within the bounds of existing shipbuilding methods.

Operational data from GasLog combined with DNVGL data streams from AIS were used in order to construct a realistic baseline operating profile for the LNGreen project.

The chart shows the percentage of total operating time spent at different speeds in terms of Ballast and Laden conditions.

For the purposes of the hydrodynamic optimisation, four different speed/draft conditions have been selected: Laden 15kts, Laden 18kts, Ballast 15kts, Ballast 18kts

For the machinery analysis with COSSMOS all profile legs were considered laden, ballast sailing and the non sailing parts such as anchored/waiting, loading and unloading.

Weighting has been applied to represent the duration spent at that speed. In the design optimisation process, the optimisation has focused on the 4 different conditions (shaded blue in the table) while the performance at 19.5knots at both drafts (hatched shading in table) was sought to be maintained as close to optimal as possible.

• Traditionally the design has been optimised for a single design speed. (laden and ballast draft)
• Recent design practices favour optimisation for the actual trade, with some generalisation included so as not to limit trading flexibility.
• LNGreen considered 3 speeds for each draft, a total of 6 optimisation points for hydrodynamics and complete operational profile / round-trip simulations for COSSMOS machinery analyses.
• Weighting was applied to reflect the actual duration spent at each speed.
• 19.5kt is a ref case. If there is large improvement at the lower speeds (e.g. 18kt), then a penalty at the 19.5kt may be acceptable. The intention was to allow to find larger improvements at thelower speeds if this was possible.

Then through this iterative cooperation between GTT and HHI and optimised tank layout has been developed.

• Tank No 1 we increased cargo capacity and reduced void space by adopting a trapezoidal tank shape which is extended fore without changing overall length and also satisfying all safety regulations and codes.
• Tanks No 2 & 3 remain unchanged
• Tank No 4 we sacrifice a small part of the capacity in order to allow for higher aft form optimisation

Hull optimization was carried out based on the operational profile and applying a new tank shape which can minimize the amount of void space around the No.1 cargo tank. In order to evaluate the performance of the new hull form the performance evaluation was carried out by comparison CFD simulations with HHI and DNV GL.

Different CFD codes were applied for comparison of resistance and self-propulsion performance. In case of added resistance, different codes were applied in order to make sure the required power under agreed environment conditions is sufficient to operate with the installed power.

Also, model scale and full scale CFD was carried out. Model scale is well established, but due to scale effects there are some phenomena which occur. Full scale CFD is newer technique (whi requires greater computation power is being used in the industry but not as developed or as widespread as Model scale.  Improvement from 5 to 4 blade is minimal, to 3 bladed is more significant. Why twisted rudder? You can probable imagine that the flow from the propeller is rotational. By using a twisted rudder you can transform some of the rotational force into axial thrust. Then, we arrived into an optimized asset that can be ordered today featuring:

The views presented hereabove are only those of the author and not necessarily those of  GREEN4SEA and are for information sharing and discussion  purposes only.

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About George Dimopoulos,

Principal Research Engineer, DNV GL Maritime DNV GL

George Dimopoulos is a Naval Architect & Marine Engineer, holding a PhD in Marine Engineering from NTUA. His field of expertise is the modelling and optimization of complex ship machinery systems. His professional experience, both in the academia and in DNVGL’s R&D units, is in the application of computer and process modelling techniques and advanced thermodynamic analysis methodologies in order to optimize ship systems for improved performance, safety, fuel savings, emissions reductions and cost-effectiveness. He is lead researcher or project manager in various R&D and commercial projects fusing forefront research methods and new technologies with the modern shipping industry environment. As a researcher, he has authored or co-authored more than 30 peer-reviewed papers in scientific conferences and journals.