Liquefied gas ship tank insulation system

Abstract

A ship having a hold containing a tank, for transporting a cryogenic liquefied gas, which is structurally free-standing and is spaced or separated from the ship hold walls, insulation secured to the ship hold walls and hold bottom, a ship hold cover extending over the tank top portion and joined to the ship, and a layer of loose fill insulation and a resilient blanket layer between the tank top portion and the ship hold cover.

Description

This invention relates to ships used for transporting cryogenic liquefied gases. More particularly, this invention is concerned with improvements in insulation systems used on ships which contain tanks for transporting cryogenic liquefied gases and which tanks are separate, independent, free-standing structures resting on supports attached to the ship hull or hold structure.

Many useful gases are available or are produced at geographical locations far removed from the locations where they are used or needed. Although some such gases can be economically transported under pressure in the form of a gas, it is generally more desirable to liquefy the gas and transport it in that form because of the increased volume of gas which can be transported in liquid form rather than in the form of a gas.

Some gases can be liquefied at moderate pressures and shipped in tanks capable of maintaining the gas under such pressure to keep it in liquid form. Propane and butane are representative of such gases. Because the pressure needed to liquefy such gases at atmospheric temperature is not unduly great, the pressure vessel required for storage of the so liquefied gas can be built economically and of a relatively large size. Cryogenic gases however cannot be readily liquefied even at fairly high pressures unless the temperature of the liquefied gas is also reduced substantially below atmospheric temperature. Because it is difficult and expensive to construct a pressure vessel capable of storing a cryogenic liquefied gas at high pressure in large volume, it has been found more practical and less expensive to cool the cryogenic liquefied gas to a temperature at which it can be stored in a tank designed to withstand a minimum internal pressure plus the hydrostatic and dynamic loads of the liquid. For example, it has been found convenient to store liquefied natural gas, which is essentially methane, at about -260.degree.F. and at about 15 psia, or just slightly above atmospheric pressure. Other cryogenic liquefied gases such as hydrogen, helium and ethylene can similarly be stored at about atmospheric pressure following their refrigeration to a temperature at which they boil at such pressure.

Tanks for transporting cryogenic liquefied gases at about atmospheric pressure in a ship or barge are of two main types. The first type of tank is one in which the tank walls and bottom are contoured to be in essentially continuous contact with, and be supported by, the walls and bottom of a ship hold. Such a tank relies upon this continuous contact with the walls of the hold of the ship for its strength and support. The second type of tank is a structurally free-standing tank which is spaced or separated from the ship hold wall. Such a tank does not rely upon continuous contact with the hold walls for necessary structural strength or support. Such tanks have a support system which transmits the loads of the storage tank to the ship structure through localized support areas, such as would be the case, for example, when using columns or a skirt to support a spherical tank. Such tanks are considered to be safer than the first type of tank because they can maintain their integral strength even if there is a failure of, or damage to, portions of the ship's hull.

Suitable means must be used in ships or barges having cargo tanks for transporting cryogenic liquefied gases to reduce or retard heat leak so that the amount of liquefied gas which is vaporized is minimized or limited to an acceptable value. Not only must the tank be insulated to retard heat leak into the liquefied gas but, in addition, it is necessary that insulation prevent the refrigerated contents of the tank from cooling the ship structure to a temperature low enough to cause structural failure of metal parts not made of metal suitable for low temperature use.

Suitably insulating a cryogenic liquefied storage tank on a ship or barge is not a simple matter. One type of tank suitable for this use has double-curved surfaces which do not lend themselves to ready insulation by the application of insulation boards or sheets because of the cost and expense of fitting the insulation in contact with the tank surface. Furthermore, such tanks often have a top portion which extends partially above the ship deck. The tank top portion furthermore is often domed and usually will have a spherical or ellipsoidal shape. Not only must the tank top portion having such a shape be suitably insulated but also the insulation must be protected against the adverse weather conditions which the ship will meet in service. A hold cover is generally installed for this purpose.

According to the present invention there is provided an improved system for insulating a ship tank for transporting a cryogenic liquefied gas and which tank is structurally free-standing and is spaced or separated from the ship hold wall. As used herein and in the claims, "ship" includes all types of ships as well as barges and boats. The improved insulation system comprises non-granular insulation secured to the ship hold walls and hold bottom, a ship hold cover extending over the tank top portion and joined to the ship, and a layer of loose fill insulation plus a resilient blanket layer between the tank top portion and the ship hold cover. The insulating system of this invention is particularly useful when the tank top portion extends partially above the ship deck and also when the ship hold cover are both domed and are advisably also approximately equally spaced apart over a substantial part of their areas.

A further important feature of the invention is the inclusion of a splash shield which separates the tank top portion from the loose fill insulation so that in case there is a leak in the tank shell the escaping liquid or vapor will be prevented from penetrating the loose fill insulation and cooling the adjacent portion of the ship hold cover or hold wall.

It is important that the tank be completely surrounded by an insulating system. To do this, the resilient blanket and the loose fill insulation can be terminated adjacent to the non-granular insulation on the ship hold walls. In this way the tank is positioned within a totally insulated space even though only the upper portion of the tank has insulation directly, or essentially, in contact with the outer surface of the tank shell. Thus, since the tank shell lower part is located inside of the insulated ship hold, it is suitable to leave the tank shell non-insulated over that surface area which projects downwardly from the lower edge of the resilient blanket.

The invention will now be described further in conjunction with the attached drawings, in which:

FIG. 1 is an isometric view of a ship containing five spherical storage tanks for cryogenic liquefied gas;

FIG. 2 is a sectional view taken along the line 2--2 of FIG. 1;

FIG. 3 is an enlarged sectional view of the insulation structure used in the top portion of the tank shown in FIG. 2;

FIG. 4 is a vertical sectional view of an expansion joint arrangement which separates the loose fill insulation from the insulation applied to the ship hold walls;

FIG. 5 is a view similar to that shown in FIG. 2 but with a somewhat different splash shield arrangement over the upper part of the tank and with the tank supported by a cylindrical skirt rather than a series of columns; and

FIG. 6 is an enlarged view of the insulation structure used in the top portion of the tank of FIG. 5.

So far as is practical the same numbers will be used in the different views of the drawings to identify the same or identical parts or elements.

With reference to FIGS. 1, 2 and 3, the ship 10 is shown with five spherical tanks 11 for storing cryogenic liquefied gas at a very low temperature and at about atmospheric pressure or at such other pressure as is considered appropriate and suitable for practical transportation of the liquefied gas. Each tank 11 comprises a spherical metal shell 12 positioned in the ship hold 13 so that the major part of the shell is located below the main deck 14 of the ship but with an upper top portion of the tank shell protruding above the deck. As shown in FIG. 2, metal rings 15 and 16 extend around shell 12 in a horizontal manner and the vertical stiffeners 17 extend between the rings. A plurality of spaced apart columns 18 around shell 12 extend from a supporting ledge in the ship hold to ring 16 to support the metal shell 12 securely in position. The lower portion of columns 18 can be insulated (not shown) to prevent the adjacent ship hold at the base of the column from becoming too cold.

The lower portion of the inside surface of the hold 13 is covered by non-granular insulation 19 which extends over the entire ship hold bottom and side walls up to the expansion joint 20. Insulation 19 can be of the foamed-in-place type of insulation or it can be composed of insulation board or sheet stock firmly secured to the hold surfaces. Sheet 21 is positioned over the insulation 19 in the ship hold bottom and functions as a drip pan to collect all liquefied gas which may leak from the spherical shell 12. The drip pan 21 is made of a material which can withstand the low temperature of the liquefied gas without structural failure. The drip pan prevents liquefied gas from penetrating the insulation on the ship hold bottom and thereby keeps it from contacting the metal plates used in the construction of the ship hold. By keeping the cold liquefied gas out of contact with the metal used for the ship hold there is avoided any necessity to construct that part of the ship proper of a metal or other material capable of withstanding the cold temperature of the liquefied gas without failing.

As shown in FIGS. 2 and 3, resilient blanket 25 is placed in contact with the outer surface of spherical shell 12 over that surface of the shell extending above the expansion joint 20. A liquefied gas and vapor impermeable sheet or covering splash shield 26 is placed over the outer surface of resilient blanket 25 to prevent any liquefied gas which leaks therefrom from penetrating into the loose fill insulation 27 which is placed over the resilient blanket. Rigid metal hold cover 28 is placed over the loose fill insulation and extends to the ship, such as to the ship deck. The top of shell 12 is provided with a cupola 29 in which supporting utility pipes, pumps and various operating control systems can be positioned. The cupola is also shown covered both on its sides and top with the resilient blanket 25, splash shield 26, and loose fill insulation 27. In some instances it is desirable to terminate the resilient blanket 25, splash shield 26, and loose fill insulation at some position along the side wall of the cupola. The upper remaining surface of the cupola can then be covered with another insulation, such as foamed-in-place polyurethane foam.

Conduit 30 can be used to withdraw vapor from the shell 12 or to fill it with a liquefied gas. Furthermore, conduit 31, which can extend to the bottom of the tank shell, can be used to remove liquefied gas from the tank.

FIG. 4 shows the expansion joint 20 in greater detail. Vertical plate 32 constitutes the ship hold wall on which non-granular insulation 19 equipped externally with a spray shield (not shown) is secured. Metal plate 33 extends inwardly in a horizontal direction from hold wall 32. Near the inner edge of plate 33 is positioned upwardly extending metal plate 34 which supports an inwardly directed horizontal plate 35. On the inner edge of plate 35 there is positioned a downwardly directed plate 36. Metal plate 38 is horizontally joined to the outer surface of tank shell 12. Drain holes 37 are placed in metal plate 38 to permit drainage of any leaking liquefied gas into the drip pan 21. Joined to the outer edge of metal plate 38 is metal plate 39 which projects vertically upwardly towards plate 35. Vertically extending plate 39 is positioned approximately midway between the plates 34 and 36. Resilient blanket 40 is doubled over twice to form a W-shape in the space between the plates 34 and 36 with the fold between the two v's of the W locked over the upwardly extending vertical plate 39. The described expansion joint prevents the loose fill insulation 27 from falling downwardly into the ship hold yet it permits the tank shell 12 to expand and contact with temperature change without displacement of the loose fill insulation 27.

The described insulation system accommodates temperature expansion and contraction of the tank shell 12 without displacement of the loose fill insulation 27. When the vessel shell 12 expands the dimensional enlargement of the shell is acccommodated through compression of the resilient blanket 25. When the tank shell 12 cools and contracts, such as when it is filled with liquefied natural gas, the dimensional shrinkage or reduction of tank shell 12 is accompanied by expansion of the resilient blanket 25 without any significant displacement of loose fill insulation 27.

Any suitable resilient blanket can be used in the insulating system of this invention. A particularly useful resilient blanket is one made of fine glass fibers bonded together by a thin film of a phenol-formaldehyde resin. The glass fibers can have a nominal diameter less than 0.00015 inch and the blanket can have a density of about 1-2 pounds per cubic foot. A commercially available resilient blanket of glass fibers which can be used in practicing this invention is marketed under the trade names Ultralite and Textrafine. The thickness of the blanket employed will, of course, depend on the size of the tank shell, the metal of which it is made and the temperature cycle to which the shell will be subjected during use. Further details concerning the qualities desired in a suitable resilient blanket are set forth in Wissmiller U.S. Pat. No. 3,147,878.

Any suitable loose fill insulation can be employed in the insulating system of this invention. It is presently considered advisable however to employ expanded perlite or vermiculite for this purpose since it can withstand the pressures applied against it by the resilient blanket during warm-up and expansion of the tank shell.

Insulation of the ship hold is readily achieved because the bottom of the hold and the hold walls generally are plane surfaces or no more than single-curved surfaces so that they can be readily covered with a minimum of fitting and cutting by use of insulation board or sheet stock. Insulation board stock, however, is not readily adaptable to insulating the surfaces of tank shell 12 because the surface of the shell is double-curved. The system of this invention, however, employing the resilient blanket and loose fill insulation combination is readily adaptable to double-curved surfaces and involves no problem in fitting the insulation to the contour of the tank shell.

The use of the resilient blanket-loose fill insulation combination employed on the top portion of the tank shell surface does not lend itself to use on the lower part of the tank shell surface because the loose fill insulation complicates the provision of adequate drain means for conveying spillage of liquefied gas into a drip pan. Furthermore, the application of the insulation to the lower portion of the tank shell would complicate visual inspection of the vertical support system of the storage tank. By insulating the surface of the ship hold below essentially the equator level of the tank shell, instead of insulating the surface of the shell itself over this area, easy access for visual inspection of the majority of the tank support system is achieved.

The resilient blanket-loose fill insulation system also provides an advantage in that no seams or gaps can develop in the insulation over the top portion of the tank shell even during thermal movement of the storage tank or through movement of the tank caused by motion of the ship. If an insulating board system was used over the top portion of the tank, gaps or seams could develop through such movement and this would lead to a decrease in insulating capacity because of the discontinuity in the insulation and the flow of air through the gaps.

Any suitable material can be used for the splash shield 26. One suitable material is made of a sheet of a polymeric material bonded to an aluminum sheet. The polymeric material can be Mylar or some equivalent material which can withstand the low temperatures to which it will be subjected. A commercially available laminate of Mylar and aluminum foil is available under the trade name Zero perm.

FIGS. 5 and 6 illustrate a second embodiment of the invention; however, the features, except those which will be subsequently described, are the same as those illustrated in FIGS. 1 to 4. The tank shell 12 in FIGS. 5 and 6 is supported by a cylindrical metal skirt 50 which is mounted on its lower edge on a ledge of the ship hold 13. Ring 51 is mounted on the shell 12 and extends around the spherical shell. The ring is joined to the top edge of cylindrical skirt 50. The top portion of the shell 12 shown in FIGS. 5 and 6 has a splash shield 55 made of metal, such as aluminum sheet, spaced outwardly from the surface of shell 12 but joined thereto by metal tabs 56. The space between the splash shield 55 and shell 12 provides clearance for any liquefied gas which leaks from the tank to flow downwardly and to thereby avoid penetrating into any of the adjacent insulation. The space also provides an accessible space for leak detection by the passage of a carrier gas which can then be analyzed for the presence of hydrocarbons. The illustrated splash shield configuration does not depend on the use of columns or a skirt support system. Of course, suitable drain holes 37 are positioned in the expansion joint 20 to provide conduit means for escaped liquefied gas to flow downwardly along the tank into the drip pan 21.

Although the tank shown in the drawings is spherical, it should be understood that the invention is not limited to a tank of this shape or to the illustrated systems used to secure the tank to the ship.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art.

Claims

What is claimed is:

1. In a ship having a hold containing at least one tank, for transporting a cryogenic liquefied gas, which is structurally free-standing and is spaced or separated from the ship hold walls, the improvement comprising:

insulation secured to the ship hold walls and hold bottom,

a ship hold cover extending over the tank top portion and joined to the ship,

a layer of loose fill insulation and a resilient blanket layer between the tank top portion and the ship hold cover,

said resilient blanket and the loose fill insulation terminating at a lower edge adjacent to the insulation on the ship hold walls, and

said loose fill insulation is supported at the bottom against downward flow by an expansion joint between the tank and ship hold walls which permits horizontal and vertical displacements of the tank relative to the ship hold walls.

2. In a ship having a hold containing at least one tank, for transporting a cryogenic liquefied gas, which is structurally free-standing and is spaced or separated from the ship hold walls, the improvement comprising:

insulation secured to the ship hold walls and hold bottom,

a ship hold cover extending over the tank top portion and joined to the ship,

a layer of loose fill insulation and a resilient blanket layer between the tank top portion and the ship hold cover,

said resilient blanket and the loose fill insulation terminating at a lower edge adjacent to the insulation on the ship hold walls, and

the tank is a metal spherical shell and that part of the shell which is in the insulated ship hold is uninsulated below the resilient blanket, and the hold contains no other insulation.

3. The improvement according to claim 1 which the tank top portion and the ship hold cover are both domed.

4. The improvement according to claim 1 in which a splash shield separates the tank top portion from the loose fill insulation.

5. The improvement according to claim 3 in which the splash shield is between the resilient blanket and the loose fill insulation.

6. The improvement according to claim 1 in which the tank is a metal spherical shell and that part of the shell which is in the insulated ship hold is uninsulated below the resilient blanket.

7. The improvement according to claim 1 in which the loose fill insulation is expanded perlite or vermiculite.

8. The improvement according to claim 5 in which the splash shield is fabricated from bonded sheets of polymeric material and aluminum sheet.