LNG Ships and there Future

An LNG carrier is a ship designed for transporting liquefied natural gas. As the LNG market is growing rapidly in the present decade, the fleet of LNG carriers is also growing rapidly.

It is widely known that LNG shipping and LNG ship financing have seen an unprecedented boom in the last few years. More than 80 LNG ships have been ordered in the last two years alone, against a current fleet size of approximately 176. The competition among ship owners for this rapidly growing market is intense. The LNG ship financing market has faced similar trends – it has grown significantly and is extremely competitive. Given current market conditions, both LNG shipping companies and financiers must wonder whether this momentum can hold and, if so, for how long. The question is whether the market will follow a cyclical trend so common to the shipping sector at large or evolve in a different way.

History

In 1914, Godfrey Cabot patented a barge to carry liquid gas, demonstrating that waterborne transportation was technically feasible. It was not until 1959, however that the Methane Pioneer, a converted cargo ship, was used to carry LNG between Lake Charles, Louisiana and the UK .
The first purpose-built ship, called the Methane Princess, went into operation in 1964 and remained in operation until 1998 when it was scrapped. To the end of 2005 a total of 203 vessels had been built and only 10 of them had yet been scrapped.
Mid decade, there is a boom in the size of the LNG fleet. The Gas Carrier Register indicates that there were more than 140 vessels on order at the world's ship yards in late 2005. Today the majority of the new vessels are in the size range of 120,000 m3 to 140,000 m3, but there are orders for ships with capacities near 270,000 m3

Containment systems

In order to transport natural gas, it is cooled to approximately -163 degrees Celsius where it condenses to a liquid at atmospheric pressure shrinking to approximately 1/600 of its original volume with a density of 420 to 490 kg/m3. The tanks onboard LNG carriers function, in effect, as big thermos containers wherein the liquid remains boiling for the duration of voyage. Some gas is removed to prevent a gradual buildup in pressure; this is known as Boil Off Gas (BOG). The latent heat of vapourization required to turn a small amount of LNG from a liquid to a gas is what keeps the remaining liquid cooled.
Recently, designs have been developed for pressurized transport systems as well, to be called pressurized natural gas (PNG) carriers, although none have yet been constructed .
At present, there are four containment systems in use for new ships. Two of the designs are of the self-supporting type. The other two are of the membrane type which are patented designs owned by Gaz Transport and Technigaz (GT&T). The trend is toward the membrane instead of the self-supporting types, most likely due to lower construction costs.

Moss tanks

This design is owned by the Norwegian company Moss Maritime and it is a spherical aluminum tank. It was developed in 1971 by Kvaerner. This is a self supporting type.

IHI prismatic

Ishikawajima-Harima Heavy Industries has developed a self supporting tank type. This tank type is very similar to the ones used on the first ship, Methane Princess. The tank is made of aluminum.

TGZ Mark III

This design was developed by Technigaz and it is of the membrane type. The first membrane consists of stainless steel with 'waffles' to absorb the thermal contraction when the tank is cooled down. Then below that there is a plywood skin which has anchor strips fixed to it and the primary membrane is welded to this. Below the plywood there is expanded foam insulation which then is stuck to the secondary membrane which in known as Triplex (Two layers of Fiberglass with a layer of tinfoil in the middle. Then below the triplex is another layer of insulation which is boned to the final layer of plywood. This plywood is then bonded to the ships inner hull.
The space between the primary and secondary membranes is called the interbarrier space and is kept filled with nitrogen at low pressure. Then the space between the secondary and the inner hull is called the insulation space and this is also kept nitrogen filled at a slightly higher pressure then the interbarrier space.
While loading these spaces cooldown and the nitrogen contracts meaning that more nitrogen must be added to maintain pressure. Once the ship has discharged and the tank begins to warm up the nitrogen expands and the pressure increases meaning that the nitrogen must be vented.
Both the interbarrier and insulation space are constantly monitored for both hydrocarbon levels and pressure of N2 to maintain a safe and healthy tank

GT96

This is Gaz Transport's tank design. The tanks consists of a primary and secondary thin membrane made of the material Invar, which has almost no thermal contraction. The insulation is constructed of plywood boxes filled with Perlite, a lightweight insulating material.

Propulsion

LNG carriers are unique in that the large majority of them are propelled by steam turbines, with new ships still being built with this propulsion method. This is because the simplest way of handling the boil off gas (BOG) is to burn it in the ships' boilers, creating enough steam to propel the ship when supplemented with additional gas from the cargo tanks. Diesel engines have largely replaced steam turbines in all other ship types, but until recently diesel engines adapted to run on BOG have not been widely utilised, even though the technology has been around since the early 1980's. However, the rapid expansion of the LNG fleet has meant that in the first decade of the 21st century there is a shortage of sea going personnel qualified to operate steam turbine ships. High prices for LNG are also driving the quest to maximise the yield from the transported cargo. Modified diesel engines burn less gas than steam turbines due to greater fuel efficiency. Combined cycle systems have also been implemented, with COGAS (COmbined Gas And Steam) electric propulsion arrangements having thermal efficiencies close to or greater than diesel engine systems. In this arrangement, the gas is burnt in a gas turbine and the waste heat from the gas turbine used to generate steam to run a supplementary steam turbine. However, recent developments have enabled the boil off gas to be re-liquified and returned to the cargo tanks, allowing conventional diesel engine propulsion systems to be utilised. All this has meant that coming into the 21st century the last refuge for steam ships could eventually disappear.

LNG Ship Propulsion: Is Diesel the Future?

The recent resurgence in LNG shipbuilding has injected new life into the debate between steam versus diesel propulsion. As the industry strives to lower costs and generate the additional power required by the new generation of super-large tankers, companies are re-evaluating these critical systems. That the steam turbine has enjoyed such success is no accident. The safe, environmentally acceptable combustion of low-pressure boil-off gas in ship boilers coupled with the low maintenance and high reliability of steam turbine machinery has provided the industry with highly dependable transportation for more than 40 years. But steam’s Achilles heel is its low fuel efficiency, which becomes more significant as fuel costs rise and tanker sizes increase.

In an initial break from steam turbine domination, France’s Chantiers de L’ Atlantique is utilizing a combination of its cruise ship propulsion and LNG containment experience to construct two diesel electric powered carriers for Gaz de France. These propulsion systems are being employed in vessels of varying sizes – the first order is for a 74,000 m3 vessel while the second is for a 153,000 m3 vessel. GdF, together with Japan’s Nippon Yusen Kaisha, are also expected to finalize an order for another 153,000 m3 ship with Chantiers soon. While more conventionally-sized carriers may be ordered with diesel engine propulsion, the shift is likely to accelerate with the advent of large vessels of 200,000 m3 and above. These ships will provide increased carrying capacity through broader beams and greater length, rather than in any significant increase in draft. The relatively shallow draft hulls will likely utilize twin screw and twin skeg design in order to deliver the required increase in propulsive power. This use of dual propellers also provides satisfactory maneuverability characteristics. For reasons related to costs and efficiency, it is unlikely that twin screw steam turbine propulsion will be employed in these larger LNG carriers, making diesel engine power the only viable alternative. Engine manufacturers and ship builders are promoting a variety of innovative arrangements, each offering different combinations of fuel efficiency, environmental benefit, maintainability and operational redundancy. Indeed, the machinery choice now available to both LNG project sponsors and prospective ship owners has never been wider. The single, slow speed two-stroke diesel engine, burning heavy fuel oil and connected directly to a fixed pitch propeller, is the marine industry’s established benchmark for optimum fuel efficiency. Coupled with re-liquefaction equipment, the slow speed engine is the standard installation for LPG vessels. While seagoing LNG re-liquefaction equipment remains unproven, suitable technology is available today and many in the industry are now ready to adopt it. However, the industry seems reluctant to adopt the single diesel engine installation with its routine maintenance requirements. Although the single engine arrangement may be acceptable in certain trades, twin slow speed engine arrangements seem more likely. Current proposals couple the slow speed engine with two full capacity re-liquefaction units to ensure that boil-off can always be accommodated. Moreover, this configuration is ideally suited to benefit from improved cargo tank insulation, thus reducing boil-off and the resulting power requirements for re-liquefaction.

Irrespective of measures to reduce boil-off, the power, and therefore the electrical generating capacity, required for re-liquefaction will remain high. In order to eliminate the need for large auxiliary diesel engine powered generators, one manufacturer is proposing twin screw slow speed diesel propulsion with twin main engine driven generators. The system includes power take off gear drives and clutches between the main engines and generators. Additional clutches on the main propeller shafts allow the generators to be utilized in port and controllable pitch propellers permit constant shaft speeds for 60 Hz power generation while at sea. Although the system sounds complicated, it provides an attractive combination of flexibility, redundancy and efficiency in propulsion and power generation. It is similar to the proven machinery arrangement in some North Sea oil shuttle and Alaskan trade tankers with the electrical power driving LNG re-liquefaction and cargo pumping equipment rather than dynamic positioning and oil pumping hardware. This arrangement would also have two full capacity re-liquefaction units. A third slow speed two-stroke diesel option involves a single engine converted to operate on high-pressure injected gas together with a small quantity of pilot diesel fuel. This arrangement also requires a very large auxiliary electrical capacity to power the high-pressure gas compressor required for gas injection. A trial engine has been operating ashore for several years, but it has not been tested under true marine environment conditions. The engine would be directly coupled to a fixed pitch propeller and therefore unable to consume boil-off gas in port. This would be handled either by a single full capacity re-liquefaction unit or a gas combustor, sometimes referred to as a rapid oxidizer. Both would be installed as part of the machinery package. All of these options are likely to incorporate electronically controlled, camshaft-less 2-stroke engines that are now entering the market and offering considerable advances in performance especially in controlling emissions when burning heavy fuel oil.

There are also several alternative arrangements available utilizing medium speed 4-stroke diesel engines, which offer improved fuel efficiency when compared with steam turbine machinery, but not quite so much as the slow speed 2-strokes. They are well suited for burning low-pressure gas, thus combining efficiency with environmental benefits. Some operators believe that it is preferable to deal with the boil-off by low-pressure combustion in a 4-stroke engine rather than engaging in the high gas pressures required for re-liquefaction or injection in a 2-stroke engine. The 4-stroke system favored by many involves a number (typically four) of identical dual fuel engines driving high voltage generators in a central power station concept.

This is the system currently being installed by Chantiers de l’Atlantique in the vessels being built for GdF and, except for the gas burning capability, it is similar to the power plant installations in most of today’s new cruise liners. Vessel propulsion is typically by two electric motors either geared to a single fixed pitch propeller or driving two such propellers in a twin screw arrangement. A rapid oxidizer is provided to dispose of any excess boil-off, thereby minimizing environmental pollution. The fuel required to provide pilot ignition in the engines, or to supplement boil-off gas, is marine diesel (MDO) rather than HFO diesel. This eliminates the need for fuel heating, heavy oil purifying and other handling systems. Although not yielding the highest possible fuel efficiency, this arrangement combines relatively safe low-pressure gas handling, operational and maintenance flexibility, simplified fuel oil requirements and low environmental emissions. Proponents also suggest that it may provide shipbuilders with opportunity to reduce building dock time due to elimination of the need to install and align heavy diesel engine machinery bedplates with propeller shafting. The multi-engine 4-stroke dual fuel diesel electric power station concept could also be coupled with electric podded propulsors with commercial names such as Azipods or Mermaid pods. These are already widely used in the cruise ship industry where the ability to remove machinery from the hull allows more revenue earning cabins to be installed. However, placing large main propulsion electric motors outside the hull (i.e., underwater) has its own challenges and their reliability to date has been less than that required by the LNG trade. The diesel electric power station concept has a further advantage in that LNG vessels are now very much all-electric. Most of the machinery which was originally powered by steam – such as mooring equipment, gas compressors and ballast pumps – is now all driven by electric motors yielding further fuel efficiencies.

Irrespective of any move to diesel engines, the fact remains that there are more than 60 steam turbine powered LNG ships currently on order. Each one of these vessels has an anticipated operational life of at least 40 years. This guarantees that steam turbine propulsion will dominate LNG shipping for many years to come and provide employment for another generation of steam engineers. The diesel engine will most certainly proliferate as vessel sizes grow. However, this propulsion system may never achieve the same dominance now enjoyed by steam turbines since the continuing development of gas turbines could create a wider range of choices for LNG ship builders.

Floating factory could change future of LNG

Called Excelsior, the massive red-hulled ship -- almost two football fields in length and 15 stories high -- is a signature piece in the world's first offshore liquefied natural gas port, which began operating here last year.
The ship holds enough super-cooled gas in its tanks and labyrinth of gray pipes to heat about 30,000 homes for a year. But the ship's critical attribute is the ability to turn that liquid back into a vapor at sea and pump it into an underwater pipe that carries the gas to shore.
As fears swirl over the risk of a catastrophic accident or terrorist attack at LNG terminals on land, many see such floating factories as a safer alternative for meeting surging energy demand in the United States. Two offshore LNG ports have been proposed off Gloucester, including one by Texas-based Excelerate Energy LLC, the company that charters Excelsior and owns the port in the Gulf.
There is significant opposition to the Gulf terminal and other proposed LNG deepwater ports nearby because they can take in more than 135 million gallons of the warm Gulf water a day to vaporize the liquefied gas, killing billions of fish eggs and larvae in the process. But the companies proposing the terminals off Massachusetts say they have modified the onboard vaporization process to dramatically decrease the water used, and some Bay State environmentalists and politicians are giving tempered support to the proposals.

Nandkishore Gitte