Patent Application
20240240753
The present invention is concerned with tanks for storage and transportation of liquefied gases, such as utilized in marine installations. Specifically, the invention is concerned with a novel method for applying insulation to combined cylindrical tanks.
Liquefied gases are normally stored at low to very low temperatures, close to their boiling point, to avoid high pressures during storage. Consequently, there is a need for storage tanks with advanced thermal insulation to keep the gas contained therein cold and liquid. Common tank types to store liquefied gases are cylindrically shaped tanks comprising a single cylindrical section, or lobe, such as International Maritime Organization (IMO) independent type C tanks. These single-lobe cylindrical tanks comply with the IMO International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code).
A cylindrical tank is, due to its form, particularly suitable for withstanding pressurized conditions, as the cylindrical form creates few stress concentrations in the tank structure. Consequently, with increasing pressure stress peaks will be more controllable for a cylindrical tank than for a tank type with a transversal cross-section including sharper corners.
On the other hand, a single-lobe cylindrical tank will normally give a poor volume utilization in the tank space, or hold space, in which the tank is placed. This aspect is particularly important for marine installations, which typically have hold spaces with rectangularly shaped transverse cross-sections.
For rectangularly shaped hold spaces, one solution is therefore to combine two or more cylindrical tank sections. Combined cylindrical tanks therefore include two or more cylindrical tank sections, connected along their longitudinal direction, for instance by welding. This solution increases the volume utilization in a hold space with a rectangularly shaped cross-section, as compared to a single cylindrical tank.
The most common types of combined cylindrical tanks are bi-lobe type tanks. The transverse cross-section of a bi-lobe tank includes two connected cylindrical sections and typically has a binocular type form, with a strength bulkhead connecting the two cylindrical tank sections. The strength bulkhead may be welded to the two cylindrical sections along the longitudinal direction thereof. A bi-lobe tank provides a higher volume utilization than a single-lobe cylindrical tank when placed in a rectangular hold space volume. At the same time, the strength wise benefits of the cylindrical shape are mostly maintained.
Alternatively, three or more cylindrical sections may be connected, for instance by welding, in the same manner as for bi-lobe type tanks. Connecting strength bulkheads are placed between each two neighboring cylindrical sections and may also be used for the storage of liquefied gasses. Tri-lobe tanks include three connected cylindrical sections in their transversal cross-section, whereas multi-lobe tanks include more than three connected cylindrical sections in their transversal cross-section.
For thermal insulation purposes, it is well known to directly apply a polymer foam, such as polyurethane (PU) foam, to the exterior surface of the tank, by spraying. The polymer foam usually comprises two main components, i.e. a premixed polyol and an isocyanate (P-MDI) in case of a PU foam. During application, the main components are mixed to form a polymer foam precursor, which is then dispensed from a spray gun and sprayed onto the tank shell exterior. Upon application the polymer spray foam expands and subsequently cures, forming an insulation layer. The polymer spray foam is usually applied in several layers of 10 to 35 mm thickness onto the tank shell exterior, to obtain the required total insulation layer thickness.
Application of polymer spray foam by spraying directly onto the tank shell exterior is mostly used on single-lobe cylindrical tanks. Once applied to the tank, the polymer spray foam is only kept in position on the tank shell exterior surface through the adhesion between the polymer spray foam and the tank shell exterior surface. This adhesion is created during the curing of the polymer foam on the tank. Consequently, the adhesive strength is limited to the tensile strength in the foam.
In contrast to single-lobe tanks, combined cylindrical tanks of the bi-lobe, tri-lobe or multi-lobe type, present challenges for secure adhesion of polymer spray foam insulation. The transversal cross-sectional geometry of these combined cylindrical tanks in the connection area between the separate cylindrical tank sections is complicated. Consequently, a high concentration of thermal stress occurs in the polymer spray foam insulation in the connection area(s) during temperature changes of the tank. Issues arise due to thermal contractions and stress in the multi-directional surfaces at the joint sections between the strength bulkheads and separate cylindrical tank sections. These stresses occur during cooling down and warming up of the tank and increase the risk of delamination between the spray foam and the tank shell exterior surface. Specifically, the thermal shrinkage of the tank material is different from the thermal shrinkage of the foam insulation material. In addition, the foam material experiences a different shrinkage throughout its thickness due to the temperature gradient from its cold side, near the tank shell, to its warm side, on its outer surface. The combination of these geometry-induced differences in thermal shrinkage causes multi-directional stress within the foam which may result in delamination between the foam and the tank shell surface. The colder the liquefied gas contained in the tank is, the more pronounced these effects become, given the higher temperature gradients involved.
Due to these issues, it has been standard practice to use mechanically fixed insulation panels, instead of spray foam, for combined cylindrical tanks of the bi-lobe, tri-lobe or multi-lobe type. Mechanically fixed insulation panels are, however, time-consuming and costly to manufacture. Moreover, mechanically fixed panels are labor-intensive and therefore costly to apply.
Consequently, there is a clear need for an improved insulation arrangement for combined cylindrical tanks, as well as an improved method of applying insulation to combined cylindrical tanks.
WO 2020050515 A1 discloses a plurality of sandwich panels including a first heat insulating panel and a second heat insulating panel fixed to the wall structure of a tank. WO 2018029613 A1 discloses a cryogenic insulation system for ocean going vessels, involving the steps of sequentially applying a plurality of layers to the outer surface of a tank. US 2017101163 A1 discloses a marine vessel cryogenic barrier formed of a plurality of individual panels.
The present invention concerns a method for applying insulation to a combined cylindrical tank for storage of liquefied gas. The method comprises providing a combined cylindrical tank comprising a tank shell, spraying one or more layers of a polymer foam onto the exterior surface of the tank shell and mounting crack barriers on top of certain layers of polymer foam, wherein the crack barriers are anchored to the exterior surface of the tank shell.
The present invention also concerns a combined cylindrical tank for storage of liquefied gas. The combined cylindrical tank comprises a tank shell and one or more layers of a polymer spray foam covering the exterior surface of the tank shell, one or more crack barriers, mounted on top of certain layers of polymer spray foam and anchored to the exterior surface of the tank shell.
Finally, the present invention also concerns the use of a combined cylindrical tank according to the invention, for storing and/or transporting a liquefied gas, such as a liquefied natural gas, a liquefied petroleum gas, a liquefied ethane gas or a liquefied ethylene gas.
The method for applying insulation to a combined cylindrical tank and corresponding insulated combined cylindrical tank according to the present invention are applicable to the field of storage and transportation of liquified gases, such as liquefied petroleum gas (LPG), liquified ethane and/or ethylene gas, liquefied natural gas (LNG) or other cryogenic liquefied gases.
One or more layers of polymer spray foam 2 adhere to the exterior of the tank shell 1, forming insulation on the tank shell 1 wherein each layer of polymer spray foam 2 may consist of several sub layers. Preferably, the polymer spray foam 2 comprises a polyurethane (PU) foam. The polymer spray foam 2 may optionally comprise additives, such as strengthening fibers, intumescent additives, anti-microbial or anti-fungal agents, or volumetric fillers.
Studs 3 extend from the connection area of the cylindrical sections of the combined cylindrical tank. Preferably, the studs 3 extend locally in the normal direction to the tank shell surface. The studs 3 are preferably provided with screw threads. Preferably, the studs 3 are welded to the tank shell 1.
One or more crack barriers 4 are connected to the studs 3 and extend laterally along certain layers of polymer spray foam 2. The crack barriers 4 may be in the form of a mesh or a perforated thin plate. The crack barriers 4 may comprise a plastics material, a glass fiber material, a metal, or a composite material. The crack barriers 4 may be placed equidistantly along the studs 3. Alternatively, the crack barriers 4 may be placed along the studs 3 at intervals of varying length, see
Securing bars 6, washers and nuts 5 secure the one or more crack barriers 4 in position on the studs 3. The securing bars 6 may be made from a sufficiently strong material, such as a metal, reinforced plastics, plywood, a composite material, or any other suitable strength bearing material. Advantageously, the securing bars 6 are configured such that securing forces are effectively transferred from the stud bolts 3 to the crack barriers 4.
Each crack barrier 4 locally fixes the underlying layer(s) of polymer spray foam 2 to the tank shell. Therefore, moving from the tank shell outwards along a stud 3, each one or more layer(s) of polymer spray foam 2 is (are) locally secured in place by a crack barrier 4. Thereby, the crack barriers 4 and securing bars 6 secure the polymer spray foam layers to the tank, preventing the polymer spray foam 2 from loosening from the tank shell surface and preventing insulation delamination.
During application, first one or more layers of polymer spray foam 2 are applied onto the exterior of the tank shell 1, where each layer of polymer spray foam 2 may comprise one or more sublayers. Application of the one or more layers of polymer spray foam 2 is followed by the mounting of a crack barrier 4. The process is then repeated until the desired number of layers of polymer spray foam 2 is achieved. Each layer of polymer spray foam 2 applied directly on top of a crack barrier 4 is mechanically and chemically anchored to said crack barrier 4 and to the layers of polymer spray foam 2 below.
Mechanical anchoring occurs due to expansion of the overlying foam layer into the gaps of the mesh or perforated plate forming the crack barrier 4. Chemical anchoring occurs due to the bonding of the overlying foam layer to the underlying foam layer, situated below the crack barrier 4, through the gaps in the crack barrier 4. Consequently, each crack barrier 4 situated between layers of polymer spray foam 2 is firmly embedded in the polymer spray foam 2.
A mechanical protection material covers the exterior of the polymer spray foam 2, thereby forming the outer surface of the tank and a barrier to the surrounding environment. The mechanical protection material is preferably a cladding 7, preferably comprising a metal material. Preferably, the cladding 7 is configured to be watertight. The cladding 7 may be fixed in place on studs 3 by securing bars 6, washers and nuts 5, see
Advantageously, the crack barriers keep the layers of polymer spray foam in place and prevent the occurrence of delamination due to thermally induced stresses in the foam. Additionally, the crack barriers advantageously take some of the weight off the cladding.
Alternative configurations are shown in
According to another alternative configuration, shown in
A method for applying insulation to a combined cylindrical tank according to the invention is described next.
A combined cylindrical tank as described previously in connection with
Polymer spray foam 2 is sprayed onto the exterior surface of the tank shell 1 in separate layers. A polymer foam precursor is dispensed from a spray gun, which may be a manually operated or a robotically operated spray gun. Each passing of the spray gun forms a sublayer of polymer spray foam 2. One or more sublayers form a layer of polymer spray foam 2. Upon application to the tank shell 1, the polymer foam precursor expands and adheres to the exterior surface of the tank shell 1. Optionally, the polymer spray foam 2 may be cured upon expansion. Crack barriers 4 are then mounted on studs 3 to locally cover the layer of polymer spray foam 2. Mounting of the crack barriers 4 is followed by the application of a subsequent layer of polymer spray foam 2, onto which further crack barriers 4 are applied. The subsequent layers of polymer spray foam 2 adhere to the underlying layer(s) of polymer spray foam 2. The process continues until the desired number of layers of polymer spray foam is achieved. To the outermost layer of polymer spray foam layer 2 crack barriers 4 may or may not be applied. Once completed, the sprayed-on, expanded and possibly cured polymer spray foam 2 forms an insulation layer surrounding the tank shell 1.
During spraying and expansion of each subsequent layer of polymer spray foam 2, the polymer foam precursor penetrates and expands through the gaps in the mesh or perforated plate forming a crack barrier 4. Consequently, each subsequent foam layer is mechanically anchored into the crack barriers 4 onto which it is applied. Additionally, the polymer foam precursor bonds chemically during expansion through the gaps in the crack barriers 4 to the previous layer of polymer spray foam. By mechanical and chemical anchoring, the polymer spray foam 2 becomes securely attached to the crack barriers 4, thereby preventing delamination due to thermal stresses in the foam.
Upon completion of application of the polymer spray foam 2, a mechanical protection material, such as a cladding 7 as described above in connection with
Alternatively, the cladding may remain not secured to the studs 3, thereby forming a thermal break, as shown in the alternative configuration of
Advantageously, the method of the present invention provides the convenience and associated reduced labor efforts and costs of a polymer spray foam tank insulation process, while simultaneously preventing the risk of delamination of insulation usually associated with spray foam insulation on combined cylindrical tanks.
In use, the combined cylindrical tank according to the invention can be utilized for the storing and/or transport of a liquefied gas. Thereto, the combined cylindrical tank may be mounted in the hold space of a marine structure, such as an LNG or LPG carrier.
The coldest liquefied gas stored in combined cylindrical tanks is currently LNG. Advantageously, the present invention allows using combined cylindrical tanks insulated with polymer spray foam insulation for LNG and for even colder liquified gases, while delamination problems are securely prevented.
The foregoing embodiments and examples are by no means limiting, the scope of the invention being solely defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20200801 | Jul 2020 | NO | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB21/55604 | 6/24/2021 | WO |
