Could GTT’s Brick Solve the LNG Fuel Tank Dilemma?
If LNG is to make any big impact as an alternative fuel for shipping, the problem of fuel storage is one that has be solved. It is not so much a problem for small vessels with limited range but to guarantee take up for larger vessels a new approach is needed.
A new fuel tank system developed by GTT promises to improve the take-up of LNG not least because it promises to be more compact and more flexible than the typical Type C tanks mostly used on gas-fuelled vessels.
Most of the gas-fuelled vessels aside from LNG tankers have been built not to a universal set or regulations but under approvals given on a case-by-case basis by flag states. That is because until the IGF Code came into effect on 1 January 2017, there were no standard universal rules. The IGF Code and its defined rules that can apply to all ships regardless of flag are important in building confidence in LNG as a fuel. It is no coincidence that since the IGF Code was adopted, the take-up of LNG appears to have accelerated.
Under the IGF Code there are various options for installing an LNG tank. The first is a membrane tank which is constructed to be attached to and fully enclosed by the ship structure and can have a maximum pressure of 2.0barg. There are also three variants of independent tanks. Type A and B tanks are similar to membrane tanks but not attached to the ship structure. They require a full or partial secondary barrier respectively to prevent potential release of the liquefied gas in the event of a tank failure. Both types have a maximum allowable pressure of 0.7barg.
For smaller LNG-fuelled vessels, prefabricated vacuum-isolated cryogenic type C tanks are available in a wide range of sizes (but mostly small) and with a maximum allowable working pressure of 20 bar. Type C tanks are already in operation on ferries and offshore supply vessels. There are several designs for larger LNG-fuelled ships that propose using type B tanks because they require less space. A number of Membrane tanks for use as fuel tanks have been contracted.
Space requirements for LNG storage are greater than for conventional fuel which is the main reason why operators of long haul ships have mostly rejected LNG as a fuel. Of the independent tank types, Type C pressure tanks require the most space: about 2-4 times more than an HFO tank.
GTT’s new tank is dubbed the LNG Brick and has been designed for ships that require a tank capacity of around 3,000m³ or less. The LNG Brick name is protected as a registered trade mark but the name accurately describes the appearance of the tank which is designed to be cuboid in shape. It includes storage and the gas preparation room.
The LNG Brick is strictly speaking a membrane tank and therefore would be subject to the 2.0barg pressure. Guillaume Gelin, Head of Product Development - LNG as fuel Division at GTT, told ShipInsight that from a technical point of view there is no reason to limit the maximum pressure of membrane at 0.7 barg as it is able to sustain much more. For example, the static pressure on the bottom of typical LNG carrier will be more than double that at 1.57barg although because the height of a fuel tank will never equal that of a cargo tank on an LNG carrier, the hydrostatic pressure would never reach such levels.
For historical reasons, the IGF code limits the maximum setting pressure of a membrane tank to 0.7 barg. “In order to increase the tank maximum setting pressure, the alternative design process will be followed for the projects involving LNG Brick. That will allow the classification society/Flag state, to allow the use of a technology that is not already covered by the IGF code,” explained Gelin. GTT has already demonstrated the feasibility of the membrane and construction method that will be used in the LNG Brick to class societies and has been granted approvals in principle from DNV, ABS, LR, and BV although the LNG Brick itself has not yet been given approval.
Tried and tested construction method
The LNG Brick is based on GTT’s Mark III membrane technology with some slight modifications aimed at simplifying the design and erection. Using tried and tested technology should give ship operators confidence in the robustness of the system. The Mark III membrane system is described as a cryogenic liner directly supported by the ship’s inner hull.
The liner consists of several layers beginning with a corrugated stainless steel 304 L, 1.2mm thick metal membrane which is in direct contact with the LNG. This is supported by and fixed to an insulating layer. Next is the secondary membrane made of a composite laminated material comprising a thin sheet of aluminium between two layers of glass cloth and resin finally there is the second insulation layer which is attached to the structure of the vessel.
The layers are prefabricated in standard sized panels and fixed together in situ. There are different types of panels; flat panels, corner panels covering three or two planes for use at the points where the sides, bottom and top of the tanks meet. These prefabricated allow for the tank to be built to custom sizes for individual ships. Although conceived as best suited to capacities between 1,000m3 and 3,000m3, the LNG Brick can be constructed to smaller sizes if required. The fact that it requires much less space than a Type C tank would appear to make it more suited to retrofit situations where there is limited space and little chance to reconfigure the tanks and machinery spaces.
Constructing the complete membrane and fixing it to the hull is a skilled job. Gelin, explained that since LNG tanks and particularly membrane types, requires investment in tools and well-trained people, shipyards are not always willing to take responsibility for them. The principle of LNG Brick is to give shipyards the possibility to sub-contract the block of the vessel where the membrane tank will be located.
In this way, the membrane erection and tests will be performed by a skilled GTT licensee and the tank will be delivered already tested. Currently a LNG Brick tank could be built in Korea at DongSung Finetec (DSFT) or in Spain at Gabadi, or in any GTT qualified fabrication company.
DSFT built and tested a prototype of the system in 2017 and already provide components for membrane tank construction. Gabadi is an outfitter company already licensed by GTT. They are currently building an LNG Brick for an offshore application. As things stand the present capacity is around ten tanks per year but this could be stepped up reasonably quickly.
No problem with sloshing
LNG tank systems were originally designed for use on LNG carriers when tanks would usually be either full or empty but almost never partially loaded. The question of sloshing damage became an issue some years ago when the beginning of a spot market in LNG as being contemplated and it was thought that sometimes ships would not have full cargoes. Obviously for a fuel tank, the tank would be expected to be partially full most of the time. Gelin said that since the capacity of the LNG Brick is relatively low, the quantity of liquid involved will not be sufficient enough to create a significant sloshing loads. Sloshing will therefore not be an issue and a standard Mark III membrane without reinforcement will be applied on LNG Brick.
With the LNG in the tank intended to be used during the voyage, the issue of low fuel levels and vessel movement need to be considered and addressed. GTT has designed the Brick so that it can be used with any amount of fuel from full to almost completely empty. To accommodate this, the LNG Brick is fitted with a sump to minimise the quantity of unpumpable LNG at sea.
Without a sump, at low filling levels with liquid motions the inlet suction could briefly emerge out of the liquid causing the pump to trip. A sump will ensure that the inlet remains submerged, guarantee good behaviour with low filling level and increase the usable capacity of the tank.
Most shipowners used to operating with oil fuels will be unaware of the fuel requirement for a vessel running on LNG so to put the 3,000m3 capacity of the LNG Brick into perspective, it would provide sufficient fuel for a typical 50,000dwt MR product tanker to make a complete 32-day, 10,300NM round voyage between Rotterdam and Houston at a speed of 13.5kt. As LNG bunkering facilities increase in number, longer voyages with intermediate refuelling could allow for a smaller tank or for longer range.