Prismatic Tanks of Type “B” are an integral part of the shipping industry, specifically for the transport of Liquefied Natural Gas (LNG). As suggested by the name, these tanks display a prismatic or box-like structure, unlike the spherical design of some other tank types. The design of these prismatic tanks allows for a more efficient use of space within the ship’s cargo area. The “B” in Type “B” signifies the classification according to the International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk.
Engineering specifications for these Type “B” Prismatic Tanks are stringently defined to ensure maximum safety and efficiency. The tanks are usually constructed using materials that can endure low temperatures and high-strength, making them ideal for LNG storage. The size and volume of these tanks are decided based on the specific cargo requirements, and their pressure-handling capacity is designed to meet the necessary standards for The business of LNG and historical involvement in maritime transportation of gasLNG transportation. All these factors contribute to making the Type “B” Prismatic Tanks a reliable and efficient solution for LNG transport within the shipping industry.
Type “B” Prismatic Tanks
Features of SPB Ships
The SPB (Self-supporting Prismatic shape IMO Type B Tank) containment system consists of a freestanding, self-supporting independent prismatic tank(s). The liquid natural gas is contained in the tank near atmospheric pressure. The cargo tank rests within the hold space and is supported and restrained by the hull structure in a manner that prevents the movement of the tank due to ship motions; while allowing contraction and expansion of the tank under variation of temperatures during cargo loading/unloading, cooldown and warm-up, as well as hull deflections. Suitable keys are provided to withstand forces deriving from hits of the freestanding tanks against the hull structures due to roll and pitch motions. Anti-flotation keys are also fitted to resist uplifting forces caused in case the tank is empty and the hold becomes flooded. Figure 1 is the photograph of one of the two SPB ships built at this date.
Figure 2 shows a typical arrangement of a SPB tanker.
Figure 3 shows a typical midship section.
The SPB Tank
All stiffening members are fitted inside the tank. The tank is transversally framed on the sides and longitudinally framed on the top and bottom. These members are supported by a combination of vertical webs and horizontal stringers.
The tank is divided internally into two compartments by a longitudinal bulkhead, which is tight below the tank dome and non-tight in way of dome, so that the port and starboard compartments have a common vapor space. A swash bulkhead is fitted transversally at the mid-length of the tank.
A pump sump well is provided at the centerline in the aft part of the tank. The sump is divided into two equal compartments by the longitudinal bulkhead. The risers from the cargo and stripping pumps and the filling lines are supported by the longitudinal and aft bulkhead of the cargo tank, therefore a cargo pipe tower is not needed for this type of containment system.
The internal tank subdivision prevents the occurrence of liquid resonance in the tank and eliminates sloshing problems. As a result, the tank may be filled to any level.
The tank dome with all necessary piping penetrations is arranged near the aft end of the cargo tank. Tank material is A5083-0 aluminum alloy.
The tanks protrude through the deck only in way of the dome giving a clean, flush deck appearance (See Figure 4).
Inspection of the supports and keys as well of the insulation in the hold space is possible since sufficient clearance is provided between the inner hull and the ship insulation.
Figure 5 shows the installation of the pre-assembled tanks in the ship.
Tank Constructions
Figures 6 through 18 show pictures of the workshop where the SPB tanks were built and assembled and various phases of their construction.
Tank Insulation
The tank insulation consists of rigid polyurethane foam panels, which are designed to enclose leakage of cargo, to act as a splash barrier, and to direct any leakage of liquid flow downwards to the partial secondary barriers made of drip trays. The insulation panels are secured to the tank surface by an extension rod connected to aluminum studs welded onto the tank. The joints formed by adjacent panels are packed with glass wool in order to be capable to absorb the relative movements of the tank insulation due to LNGC’s & Tankers Hull Construction and Temperaturetemperature variation. In way of each stud a thin pad is fitted on the tank surface providing a space for the passage of leaked liquid in case of tank damage as well as allowing the continuous monitoring for gas leaks of the space between tank and insulation. Figure 19 shows how the insulation panels are installed on a side of a tank.
Figures 20 through 23 show the installation or the insulation on an SPB tank.
Figure 24 shows how the insulation looks after the installation and gives an idea of the space between the insulated tank and the inner hull.
Drip Trays
Drip trays form the required partial secondary barrier. The drip trays are arranged in the inner bottom at the four corners of the tanks, as well as under the pump sump well. They are separated from the tank top by insulating supports. The drip trays are made of the same material as the tanks, that is A5083-0 aluminum, and are provided with flanged coamings and baffle plates to restrict the movement of any liquid, resulting from heel, trim and motions of the ship. The drip trays are so designed that each is capable of containing the maximum assumed leakage for period of 15 days.
Figure 25 shows the arrangement of the bottom of the SPB tanks including the supports and the drip trays.
Advanced Analysis
The modern techniques for analyzing structures, Advanced ABS Engineering Analysis for LNGCadvanced fatigue analysis, fracture mechanics and quality control enable these tanks to be qualified and accepted as type “B” tanks; accordingly only a partial secondary barrier is required. The analyses carried out are similar to the ones described for spherical tanks. Figure 26 shows a typical mesh of a prismatic tank.
Future Development
Even though only two ships have been built up to now with this system, which license is owned by IHI, it is possible that in the future this system will used for large LNG ships, up to 250 000 m3 capacity.
The reasons why this design appears promising for largest LNGC’s are mainly the followings:
- In general, the prismatic tank design would appear more scalable than the other designs.
- The internal structures of the tanks will eliminate or at least reduce the problems due to sloshing. Partial filling, and multi-port service will be available.
- The internal structure of the tank provides support to the pumps, risers, filling lines, etc. Pump tower stresses continue to pose engineering challenges on increases size of cargo tanks.
The anticipated improvements to the design of the SPB tanks are:
- Use of stainless steel 304 material, instead of aluminum. Stainless steel is a ductile material with good behavior at lower temperatures and possesses greater strength than aluminum.
- The tanks will have a total weight around 3 500 tons each. They will be built in four six pieces adopting a modular system.
- More and more refined computer analysis will be adopted.
Figures 27 and 28 show the concept Midship Section and General Arrangement plan of a large SPB ship.