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BACKGROUND OF THE INVENTION
Previously, there has been disclosed in the U. S. Pat. No. 3,283,734 of Paul T. Gorman, assigned to as the assignee of the present invention, an externally insulated tanker for transporting liquefied cargoes at temperatures well below ambient. In the case of externally insulated tankers, as compared to conventional tankers wherein the cargo tanks are used alternatively as ballast tanks, the ballasting arrangement necessarily cannot be the same as employed in conventional practice, since externally insulated tankers do not employ cargo tanks. Therefore, it is necessary to provide a suitable ballast arrangement for externally insulated tankers that will provide the vessel with the desired degree of stability in its unloaded condition. Conventionally, bulk oil carriers, such as disclosed in U.S. Pat. No. 3,064,612, having a plurality of cargo tanks employ fixed concrete ballast internally on the bottom of the inner hull. As disclosed in the latter patent, the fluid concrete ballast comprises a single continuous layer of concrete.
SUMMARY OF THE INVENTION
The present invention provides a novel ballast arrangement for an externally insulated tanker wherein preferably the ballast is provided longitudinally both externally and internally throughout the cargo midbody in order to limit any bending and shear stresses in the hull structure. According to a preferred form of the present invention the ballast comprises a solid material of high specific weight, for example, heavy weight concrete containing a heavy aggregate such as barite, limonite, hematite, magnetite, or steel punchings and shot, which is distributed throughout the cargo midbody of the vessel. In its preferred embodiment the concrete ballast comprises preformed slabs of appropriate size and shape. These slabs are distributed across the main deck externally of the insulation, or, as disclosed in an alternate embodiment, within the insulation. Additional preformed slabs are disposed internally of the vessel above the bottom structure. The slabs on the deck are secured in place by means of complementary keyways and keys provided in the slabs and secured to the deck, respectively.
Accordingly, it is a primary object of the present invention to provide a novel permanent ballast arrangement.
A further object of this invention is to provide an externally insulated tanker with a novel permanent ballast arrangement.
Having in mind the above and other objects and advantages that will be apparent from an understanding of this disclosure, the present invention comprises the combinations and arrangements as illustrated in the presently preferred embodiments of the invention, which is hereinafter set forth in such detail as to enable those skilled in the art readily to understand the function, operation, construction and advantages of it when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic top plan view of an externally insulated tanker incorporating the present invention and illustrating the overall arrangement of the external permanent ballast;
FIG. 2 is a cross-sectional view taken substantially along the line 2--2 of FIG. 1 illustrating the external and internal permanent ballast arrangement with the protective outer coating removed;
FIG. 3 is a cross-sectional view taken substantially along the line 3--3 of FIG. 1 illustrating in detail the construction and arrangment of the external permanent ballast;
FIG. 4 is an exploded perspective view of a preformed concrete ballast slab and the deck key for holding the preformed slab in place on the deck of the vessel;
FIG. 5 is a horizontal cross-sectional view taken substantially along the line 5--5 of FIG. 3 illustrating details of the arrangement of the external preformed ballast slabs; and
FIG. 6 is a cross-sectional view similar to FIG. 3 showing an alternate construction and arrangement for the external preformed concrete ballast slabs.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Having reference to the drawings wherein like parts are designated by the same reference numeral throughout the several views, the present invention is illustrated in FIG. 1 as embodied in an externally insulated tanker having a nickel steel plate hull at its midbody of the type disclosed in the aforementioned Gorman U.S. Pat. No. 3,283,734 and to which reference is made for a more complete and detailed explanation thereof. As shown the tanker generally includes a hull 10, a forward hull portion or bow 12 and an after hull portion or stern 14. The bow and stern are fabricated conventionally from mild steel plate and mild steel strengthening members. Typically, the tanker also includes a cargo-carrying midbody 16 operably disposed between the bow and stern. Preferably, the midbody is fabricated from a metal plate possessing physical and mechanical properties compatible with the particular cargo to be carried, which, in this case, would be liquefied natural gas and the like. A typical material might comprise a suitable nickel steel containing substantially about 9 percent nickel. A central longitudinal bulkhead 18 (see FIG. 2) is provided to divide the vessel longitudinally into individual cargo holds. Although not shown, there also may be provided transverse bulkheads throughout the midbody to further divide the port and starboard cargo holds 20 into cargo-carrying portions. The midbody 16 completely is enveloped or clad externally with suitable thermal insulation 22. Copending U.S. Pat. application Ser. No. 144,422 (RM-613), entitled "Grid System for External Insulation," assigned to the same assignee as the present invention, discloses various details for securement of the insulation to the vessel's hull. Typically the insulation 22 is provided with an outer protective coating 24 in order to prevent damage to or deterioration of the insulation 22.
As shown in FIGS. 1 and 2, there is provided an upper permanent ballast arrangement, generally designated 26, disposed above the deck of the vessel and a lower permanent ballast arrangement 30 located above the bottom hull structure. To limit bending and shear stresses from occurring in a hull structure of acceptable design level, an upper permanent ballast is provided longitudinally for the full extent of the cargo midbody 16. The upper permanent ballast 26 comprises a plurality of preformed concrete slabs or blocks 28 distributed across the main deck external of the insulation 22. These slabs 28 may or may not extend continuously transverse of the deck depending on the particular vessel design. The lower permanent ballast 30 also comprises a plurality of similar preformed concrete slabs 28 disposed internally of the vessel on flats above the bottom hull structure. The slabs 28 are made from a solid material of high specific weight. A suitable material comprises a heavy weight concrete (e.g. 250 lbs./cu. ft.) employing a heavy aggregate such as barite, limonite, hematite, magnetite, or steel punchings and shot. By preforming the ballast into concrete slabs of the proper size and shape, installation and distribution throughout the vessel during construction is facilitated. Depending on the vessel stability required, the specific weight of each preformed slab may be varied by altering the aggregate used. The extent of longitudinal distribution of the slabs will depend upon the distribution of bending and shear loads within the hull girder and vertical placement of these slabs should be such so as not to restrict the vessel's stability. The transverse disposition of the slabs, as shown, is symmetrical bout the vessel's centerline. In the embodiment disclosed in FIGS. 1 through 5, wherein the outer protective covering 24 is disposed between the preformed upper slabs 28 and the insulation 22, it is desirable to employ a protective coating 32 (see FIG. 3 disposed) externally of the concrete ballast slabs 28. The coating 32 may comprise an epoxy that is suitable to protect the concrete from the salt environment. An alternate arrangement would be to omit the protective epoxy coating 32 and to place the upper slabs 28 directly contiguous with the external insulation 22 such that the outer protective covering 24 would be extended over the ballast slabs instead of as shown. To accommodate contraction of the hull there are provided longitudinal and transverse expansion joints 34 between adjacent slabs. These joints may comprise a material such as urethane foam or other cellular insulation. The internal or lower concrete slabs 28 disposed within the cargo hold also may be coated with a suitable material such as epoxy in order to protect them from deleterious effects resulting from contact with the cargo. The internal slabs are secured in place to allow for differential contraction of the concrete and the supporting structure. A suitable means for securing the internal slabs in place might comprise stainless steel hold down bolts. The internal slabs are disposed and constructed so as to allow the cryogenic cargo contained in the hold to flow completely around them which makes for maximum use of the cargo holds for cargo-carrying purposes while still providing the necessary ballast.
FIG. 3 illustrates details of a preferred construction and arrangement for the external ballast slabs 28. The slabs are fitted over steel keys 36 having a cross-like configuration and secured to the deck by means of welding. Each of the slabs 28 is provided with a centrally-disposed keyway 38, also having a configuration complemental to that of the deck keys 36. Each keyway 38 preferably is made from a material such as 9 percent nickel steel and is surrounded on all sides (including the top) by a suitable rigid insulation material 40 such as balsa or high density polyurethane foam. The keyways extended from the bottom of each slab for a predetermined distance upward, terminating in spaced relation to the top of each slab. Although not shown, when the slab keyways 38 mate with corresponding ones of the deck keys 36 a suitable sealant such as silicone can be provided between them for purposes of preventing seawater or water vapor seepage into the keyway. As shown in FIG. 5, within each of the concrete slabs there is provided with an inner reinforcement layer 42 comprising reinforcement bars located between the keyway and the periphery of the slab. In addition o strengthening each slab, it provides means as shown by schematically represented lifting pads 44 to facilitate manipulation of the slabs and a suitable connection for tie-down bolts, base plates for support and the like.
FIG. 6 describes an alternate arrangement of the upper ballast blocks 28. In this embodiment the permanent ballast comprises preformed slabs 28 located beneath the insulation, or, in other words, disposed directly on the deck of the vessel. The deck keys 36 extend between the top and bottom of the slabs 28. The advantage obtained by this arrangement is reduction of the shear forces in the foam insulation at the interface with the steel hull and with the concrete ballast blocks.
It will be appreciated from the foregoing description that there has been provided a novel permanent ballast arrangement for an externally insulated cryogenic tanker. It should be understood that the specific construction and arrangement herein illustrated and described is intended to be representative of a preferred embodiment only and that certain changes may be made therein without departing from the clear teachings of the present invention. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention.