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In 2004, world trade volume of LNG reached over 130 million tonnes per year. On the import side, the LNG volume imported by North America has increased significantly, with a volume growth of 1.3 times between 2003 and 2004. India started to import LNG last year, and the UK restarted importing in July 2005 (the UK last imported LNG in 1964). China started importing LNG in 2006, and the markets of Spain, South Korea and Taiwan are showing steady growth. All these factors indicate an expanding demand for LNG for the foreseeable future.
On the LNG supply side, a number of existing projects (Rasgas and Qatargas II in Qatar, North West Shelf Project in Australia, a Nigerian project, etc.) are scheduled for expansion, and many newprojects in, for example, Russia, Australia, Nigeria and Egypt are due to start their operation soon.
The number of LNG vessels required for projects now under construction will be approximately 70, and the number of vessels needed to meet the demands of the projects scheduled to start by the end of 2010 is expected to be around 130 to 140 vessels. Assuming all projects start as scheduled, the demand for new vessels will reach between 200 and 210 by the end of 2010. Some of these projects have already made arrangements for the supply of vessels they will need, but the majority of the projects scheduled to start in 2009 and 2010 have yet to do so. By the end of 2010, the demand for vessels is expected to peak at approximately 85 per year (see Figure 2 & Chart 1).
Regarding the supply of LNG vessels, there were a total of 187 LNG vessels worldwide as of August 2005. Of the vessels scheduled for delivery by the end of 2010, approximately 30 have yet to be tied to any project. Shipyards have the capacity to build around 35 further vessels by the end of 2010, resulting in a possible maximum availability of only 75 vessels by 2010, a shortfall of approximately 10 to meet the demand. Although the long-term demand for new LNG carriers is so vast, this situation will work to the disadvantage of LNG shipping companies in the medium term, because they will be required to operate many uncommitted vessels in the period leading up to 2009, when specific projects are set to start.
There are two type structures of LNG ships, the shelf supporting tanks (free standing) and the integral tank construction. The first has cargo tanks which are independent to the ships hull. They have free thermal expansion and contraction and have easy inspection for leakage. But we have a waste of cargo capacity and space. The second structure performs a better utilisation of space. There is less dead space to be monitored and purged. We have a significant saving in alloy metals and it uses the same construction technique for all tanker sizes. However there is a difficulty in insulation methods and application.
LNG Carriers are classified by their cargo containment designs. The types of cargo containment systems are:
Kvaerner-Moss spherical tank (see figure 1)
Gaz Transport and Technigaz (GTT)
Mark III, No 96, Cs1
Each of the above systems conquer a piece of the entire LNG carriers fleet (see Figure 3).
The Kvaerner Moss spherical system utilises spherical tanks made from aliminum alloy or 9% nickel steel which is heavily insulated (see Figures 4 & 5).
The Moss spherical tank concept was initially developed during 1969-1972 using aluminium as the cryogenic material. The design is an independent tank with a partial secondary barrier. The insulation is normally plastic foam applied to the outer surface of the tank wall. For ships and offshore facilities the spherical tank concept has relative low utilising of a restricted volume and it is not suited for having the possibility to have a flat deck on offshore facilities.
GTT No 96 Membrane Containment System
The latest of the membrane tank containment system developed by GTT, known as 'Combined System 1' (CS1), comprises a primary membrane made from a nickel steel alloy known as Invar, a Triplex secondary membrane and a reinforced polyurethane foam insulation. During construction the flexible composite Triplex secondary membrane is bonded to the rigid secondary insulation panels using epoxy type glue. The CS1 system can be extensively pre-fabricated and the assembly process has been rationalized, enabling shipyards to achieve significant savings of time in what is considered to be one of the most complex parts of the vessel construction (see Figure 6).
GTT Mark III Membrane Containment System
Newer and more radical designs are emerging. Korea Gas Corp (Kogas) is reported to have almost completed the development of its home grown containment system known as KC-1. This is reported to be similar to the GTT membrane system but based on a much simpler insulation system and fabrication process using high density polyurethane foam to provide a single layer of insulation (see Figure 7).
IHI Self Supporting Type B Prismatic Tank
It is an independent prismatic tank with a partial secondary barrier designed as a traditional orthogonally stiffened plate and frame system. The system consists of plates and a stiffening system consisting of stiffeners, frames, girders, stringers and bulkheads as in a traditionally designed ship structure. Due to these structural elements, sloshing is not considered to be a problem. Fatigue may have been considered to be a problem for this tank system due to the significant amount of details and local stress concentrations. Insulation is attached to the outer surface of the tank and the tank rests on a system of wooden block supports.
In March 2007 Flex LNG announced that it had finalised orders worth more than US$400 million with Samsung Heavy Industries for two 90,000m3 M-Flex LNG carriers whose design incorporates the flexibility for liquefaction and regasification facilities. The M-Flex vessels incorporate the self supporting, prismatic, IMO Type B (SPB) containment system first developed in 1985 by IHI in Japan. Although this design is currently in use on only two vessels, Samsung are reported to have been working closely with IHI to refine and improve the original design with a view to it being used more widely (see Figure 8).
The multi-billion dollar investment intensive LNG business is generally governed by risk sharing consortia as well as by 20-30 years long-term supply and ship charter contracts. A predictable and stable LNG supply from producing to consuming country is an essential requirement in this business but also the strive for increased profitability in the transport chain. A variety of modern propulsion technologies for LNG carriers with potential for increased profits has emerged recently and is offered in the market today.
We may divide the different propulsion systems into three main categories with their subdivisions which are as following:
Category I: GAS/ HFO fuel flexibility
diesel- electric system
slow-speed dual fuel diesel engines
steam turbine system
Category II: Pure HFO Burning System
slow-speed dual-fuel diesel engines with Natural Boil-Off Gas (NBOG) reliquefaction on board
Category III: Pure GAS Burning System
gas turbine combined cycle electric
Except form the above categories we may also use a Gas Turbine Propulsion system, a Steam Propulsion System and a Combined Cycle of Gas Turbine and Steam Turbine (COGAS). The first requires a small diesel tank for ballast voyage and has a low efficiency (30-35%). The second presents also a low efficiency (25-30%). The third makes use of the exhaust gas of gas turbine to produce steam for extra power generation, while efficiency can go up to 55%. COGAS has been recently gain increasing market in cruise liners, due to its advantages of high efficiency, low emission, low noise, high power density, low maintenance, etc.
A lot of studies on the economical benefits of modern LNG carrier propulsion systems have already been published. Their results, however, are usually based on certain assumptions regarding oil and gas prices (HFO, LNG purchasing and selling price). In fact, by an appropriate choice of oil- and gas price combination one can produce almost any result needed - either in favour towards the one or the other propulsion solution. It is not without any reason, therefore, that such kind of studies are acknowledged with reservations and caution by the specialized readers.
Figure 1. LNG carriers designs
Figure 2. Historical LNG Fleet Growth
Figure 3. Part of each systems from the total LNG fleet
Figure 4. Kvaerner-Moss spherical tank.
Figure 5. Kvaerner-Moss spherical tank.
Figure 6. GTT No 96 Membrane Containment System