Oil and gas industry  liquefied natural gas tanker LNG

Approaches to marine environmental regulations

At its 70th session in London in October 2016, the Marine Environment Protection Committee (MEPC) of the International Maritime Organization (IMO) decided that the implementation date for the 0.5% global sulphur cap is set for 2020.  This decision came as no surprise since public opinion and global collaboration have been growing stronger with respect to reducing the marine pollutants such as SOx, NOx, particulate matters, and even green-house gases emitted by ships. Without doubt, this will present formidable global-scale challenges on shipping and energy industries. Far before 2020, ship owners must reconsider their strategy regarding what fuel to use for new builds and possibly also for ship conversions, and should choose one among alternative solutions: low-sulphur fuel, scrubbers, and LNG fuel.

Each alternative has own advantages and disadvantages. To use the low-sulphur fuel is advantageous in that it can minimize the initial investment. However, the fuel price is directly linked to the crude price that is subject to a substantial uncertainty and prospects for significant increase. Considering that the fuel price is the most dominant cost factor for the total ownership cost, most ship owners do not regard the first alternative as a long-term solution.

The scrubber option may look attractive in that it can use conventional heavy fuel oil with high sulphur contents with relatively low initial investment. Sulphur pollutants from the engine exhaust can be removed by spraying seawater containing special chemicals. On the dark side of this option are bulky installation space, chemical management, high operating power consumption, and waste handling. For example, some ports or countries prevent ships from discharging the waste water from the scrubber into their seas.

Challenges for LNG solution

The LNG solution has clearly the best environmental performance since this cryogenic fuel is free from sulphur and particulate matters; it is a low carbon fuel that reduces CO2 emissions to a very large extent. It may look almost like a “silver bullet” to kill the emission problem for ships. However, this clean fuel brings with it some noticeable challenges, most of which are directly related to LNG fuel storage:

Volume efficiency and space fitness: The fuel LNG tank should take as small as possible space and should preferably be using the least valuable space within the ship. This is particularly demanding for to-be-converted ships which have been designed without consideration for a fuel LNG tank.

Need for measures against potential leakage: Unpressurized tanks must be equipped with a secondary barrier against potential leakage of cryogenic inventory. This requirement is accompanied with complicated thermal insulation, gas detection, drip trays, and hull protection for the overflow from the trays; all this increases required space and results in more complicated operations.

High vapor pressure of bunkered LNG: During fueling the tank should be able to receive the normally available LNG which typically has vapor pressure of 3 barg or higher. As LNG bunkering infrastructure develops, LNG with lower vapor pressure will become available; however, it will still be difficult only to be able to receive LNG at the low vapor pressure due to the boil-off problem.

BOG (boil-off gas) return during the bunkering operation: The trickiest problem during bunkering operations is not to supply LNG to the tank, but rather to treat the BOG surge from the tank, fueling lines, and equipment for the operation. If the BOG is to be returned to the bunkering vessel or to port facility, this will prolong the bunkering operation. In the end the bunkering facility may have to burn the returned BOG.

BOG removal during the normal operation: Due to heat ingress into cryogenic tanks (always at boiling temperature) new BOG will continuously be formed. There are only two ways of dealing with this: to remove the BOG or to let the internal pressure of the tank increase. The BOG removal may be no problem with most ships in normal voyage operation when the BOG can be used for fuel gas for boilers or generator engines. However, the removed BOG may have to be burned when the ship is in port or slow steaming with low demand for power.

Methods to supply fuel LNG from tanks: Cryogenic pumps represent first option for discharging the LNG from inside large tanks. However, pumps tend to fail with relatively high failure rate, followed by long and complicated maintenance work. The solution to this problem is to install stand-by pumps or to use a pressure build-up (PBU) unit that is much more reliable. However, the PBU requires the tank to be pressurized and cannot be used for non-pressure tanks.

Several types of tanks are available for fuel LNG storage as shown in Table 1 where disadvantage features are in italic letters. Pressurized cylindrical tanks have so far been used for most LNG-fueled ships due to their advantages with no need of measures against potential leakage, no requirement for BOG return or removal, and choice of PBU for discharge. However, they have the serious disadvantage of low volume efficiency and inability to fit a given space within the ship. Several cylinders can be installed within the available space; however, this will typically still give a very poor volume efficiency of less than 50 %. Very often the ship designer is compelled to utilize some of most valuable space onboard the ship.

Table-1Due to the poor volume efficiency of cylinders it may be natural to consider prismatic, non-pressured tanks or membrane tanks. Traditionally, such alternative solutions create other problems. An underlying design principle for unpressurized tanks is to account for partial leakage of the cryogenic inventory, requiring installment of a secondary leakage barrier. The inability to take pressure load results in that complex BOG-related operations must be provided for. For example, the BOG should be returned during the bunkering operation. Further, the BOG must also be removed from the tanks during normal operation. The PBU (pressure build-up unit) cannot be used to discharge LNG from these tanks. For these reasons, many experts are very skeptical about using prismatic, non-pressurized LNG fuel tanks.

Lattice pressure vessel as prismatic pressure vessel

Recognizing the advantages and disadvantages of the conventional cryogenic tanks, it seems clear that the industry will needs a new type of tank solution that simultaneously can satisfy all the challenges mentioned above; this will clearly have to be a prismatic pressure vessel that can fit into any suitable space. To this end KAIST and LATTICE Technology have developed an entirely new pressure vessel technology termed “lattice pressure vessel” (LPV). The essence of the LPV is an internal, modular structure that carries load by balancing pressure on opposite walls. The concept is fully scalable in all three spatial dimensions. Figure 1 shows an example of such an internal structure.  The shape of the LPV can be modified to suit the given installation space, as shown in Figure 2. The feasibility and flexibility of this technology been successfully demonstrated by building and testing four proto-type tanks and securing approval from DNVGL, LR, ABS, BV, KR, and NK, as well as ASME U2 stamp from ABS Consulting.

Fig1-2

Due to the modular design principle, the LPV has significant advantages over conventional, shell type pressure vessels; in fact, the thickness of the structural parts including the outer shell remains independent of tank size for a given design pressure. Furthermore, fluid-induced dynamic loads become insignificant due to the internal load bearing structure; this in turn leads to long fatigue life and strong resistance to crack propagation. The internal structural redundancy with force redistribution capabilities makes it very unlikely for the tank to fail; this of course means excellent safety performance. These properties were duly considered when the LPV was approved as Type C or Type C equivalent by the classification societies.

Figure 3 shows a case study comparing two Type C solutions, one using multiple cylinders and the other a single LPV. The first solution comprises 6 cylinders with 51 m3 storage capacity each while the second case has only one rounded LPV. The total storage capacity is nearly doubled for the LPV solution. In addition to the improved volume efficiency, the LPV single tank solution provides significant saving for instruments, equipment, and piping, as well as much simplified tank operation. The overall surface to volume ratio is much reduced by the single tank solution meaning savings for insulation and less boil off.

Fig3

In addition, Figure 4 contrasts two LNG bunkering shuttles, one with bi-lobe tanks and the other with Round-wall LPVs (RW-LPVs). Note that all the dimensions for the two ships are the same. However, the total weight of the tanks increases by 9% from the first case to the second while the total volume is increased by 20 %. This means that the specific weight per volume is lighter for the RW-LPV than for the bi-lobes. As mentioned, the LPV has very efficient load-bearing internal structure, leading to better weight efficiency (material weight per unit volume) for the RW-LPVs compared with conventional cylindrical pressure vessels.

Fig-4

A final comment to this case is that a LPV can easily be designed for what is considered an optimal ship geometry whereas the bi-lobe design introduces a given ratio between tank width and height for the ship beam. In other words; the LPV can be designed for the ship rather than the other way around.

The LPV technology has been developed and tested for a period of five years. For a large part of this period the development has been supported by the steel manufacturer POSCO that has regarded the LPV as a very promising route for introducing their new cryogenic material, high-manganese steel, to the commercial market. LATTICE Technology has signed license agreement with two major tank producers to make its patented technology broadly available for use for ships as well as a being offered as a key component for the development of the global LNG infrastructure. The company is also cooperating with ship yards, fuel system suppliers, engine manufacturers, and local agents to provide total system solutions in which the LPV is clearly a key component. Notably, the LPV can be utilized for a great variety of applications: cargo tanks for LNG, LPG, and other liquefied gases, fuel tanks for liquid hydrogen, storage tanks for CNG, and so forth. All of this means that the future potential for this technology is very extensive.