WO2005073069A1

WO2005073069A1 - A lng-carrier with spherical tanks and double bottom

Info

Publication number: WO2005073069A1
Application number: PCT/NO2005/000026
Authority: WO
Inventors: Per Herbert Kristensen; Lars Sannes
Original assignee: Moss Maritime As
Priority date: 2004-01-28
Filing date: 2005-01-21
Publication date: 2005-08-11
Also published as: GB2410471A; EP1711395A1; EP1711395B1; CN1910079B; JP2007519572A; GB2410471B; DE602005021457D1; CN1910079A; JP4774373B2; GB0401847D0; KR20060127958A

Abstract

A carrier arranged with at least one cargo tank preferably spherical or near spherical, for transporting liquefied gas preferably liquefied natural gas (LNG). The carrier has an outer bottom (4) and a inner bottom (3) defining a double bottom and a number of hollows (5) are provided in the inner bottom (3) and spaced in the longitudinal direction of the carrier. Each hollow is designed to accomodate a portion of each cargo tank, and wherein the distance from the inner bottom to the neutral axis of the carrier constitutes about 25 % (15-30 %) of the distance from the outer bottom to the maindeck.

Description

A LNG-carrier with spherical tanks and double bottom

The invention concerns a LNG carrier in accordance with the preamble of claim 1.

In the following the ship transporting liquefied natural gas (LNG) is named a carrier or a LNG-carrier. As the skilled person will understand the expression carrier is to be understood equal to a vessel, a ship etc.

The vessels tonnage is often used to characterize a ships size such as a LNG-carrier. The calculation of a ships tonnage is based on the overall enclosed volumes of the ships hull and most of the utility compartments of the ship. The ships tonnage is made use of, when calculating fees and tolls in harbour and canal transits. Due to various regulations and practice throughout the world of shipping, the method for calculating the ships tonnage may vary from various authorities.

The individual method for calculation the ships tonnage may be crucial to the specific ship, for instance when deciding the fiscal duty to let the ship pass through a canal. This is particularly true for the ships to enter for instance the Suez canal, as the authorities of the Suez canal has established a distinctive method for calculating the ships tonnage. According to this method the calculated volume of the ship may be reduced if the ship is constructed with a double bottom. When applying this method, and include in the design of the ship a true double bottom, the characterizing tonnage of the ship which is defined by subtracting the volume of the double bottom from the gross volume of the ship, will be reduced

The alternative for ships not entering the entrance of the Suez canal, are to make the long journey around Africa, and thereby adding time to total travelling time and increasing the operation costs of the ship.

These specific regulations of the Suez has initiated the construction of a new LNG carrier, which specifically is adapted to fulfil the requirements of the Suez regulations regarding the calculated tonnage of the ship. A further initiative has been to bring about a well defined double bottom which evidently will be accepted within the regulatory definition of a double bottom and which fulfils the safety aspects for double bottoms set by the authorities. However, the double bottom must be adapted to the carrier ensuring that the carriers load capacity is attended to.

The object of the invention is to provide a LNG-carrier where the need for a clearly defined double bottom contributing to reduction of the tonnage of the vessel is balanced against the need for a double bottom which gives the carrier an adequate strength and satisfactory support for the LNG cargo tanks.

Seen purely from a commercial or economical point of view it would be desirable to provide a LNG-carrier with a highest possible double bottom to reduce the fiscal duties through the canal. But this desire has to be balanced against the need for a hull construction which has an adequate subdivision, adequate strength and is stable.

The object of the invention is obtained with a carrier according to claim 1, wherein these aforementioned needs are balanced.

In accordance with the invention a carrier is arranged with at least one cargo tank for transporting liquefied gas preferably liquefied natural gas (LNG). The carrier will preferably be provided with a number of cargo tanks which have a spherical shape or near spherical shape. The carrier is designed with an outer bottom and an inner bottom defining a double bottom. A number of hollows are provided in the inner bottom and spaced in the longitudinal direction of the carrier wherein each hollow is designed to accommodate a lower part of each cargo tank. The distance from the inner bottom to the neutral axis of the carrier constitutes about 25% of the distance from the outer bottom to the main deck (depth of the vessel). The distance from the inner bottom to the neutral axis can vary, and thereby also the percentage which may vary from 15 -to 30%.

The carrier is preferably provided with side tanks for ballast, which side tanks are separated from the double bottom and supported on the top of the double bottom on each longitudinal side of the carrier. The space between the inner bottom and the outer bottom (the double bottom) is a closed one and preferably void or used for ballast. In one embodiment of the invention the longitudinal bulkheads divides the space of the double bottom in port space(s), center space(s) and starboard space(s).

The carrier will be provided with skirts for supporting the cargo tanks. The skirts are attached to the cargo tanks and to the inner bottom.

Each hollow is a structure arranged in the inner bottom in a way that ensures that the space between the inner and outer bottom is a closed one and each hollow deepens into the space of the double bottom.

The horizontal sections of each hollow may vary with the depth of the hollow. Even so, the surface of the inner bottom is larger than the combined surfaces of a horizontal section of the hollows at the inner bottom level. In a preferred embodiment of the hollow, the horizontal section of the hollow decreases towards the bottom of the hollow. The hollow may be shaped as halved spherical structures or half polygon structures,, and the horizontal section of the hollow may be octagonal, polygonal, spherical etc.

In the following one example of a preferred embodiment of the invention is to be described in detail with reference to attached drawings where Figure 1 is a perspective view of the new carriers double bottom.

Figure 2 is a view of the cross section of the new carrier in the middle of a hollow.

Figure 3 is a view of the cross section of a carrier in accordance with present design.

Figure 4 is a perspective view of a lower section of the carrier in accordance with present design

The carrier shown in figures 1 and 2, is a new LNG-carrier 1 designed with a double bottom 2, which is constituted by a inner bottom 3 and a outer bottom 4. The carriers main deck is shown by 23. The inner bottom 3 is provided with a number of hollows 5 corresponding to the number of cargo tanks 6 to be arranged on the LNG- carrier 1. The LNG-carrier 1 is further provided with skirts 7 to support the cargo tanks.

The horizontal sections of the hollows 5 may vary with the depth of the hollows, both in regard to shape and size of the surface of the horizontal section. In figure 1 it is seen that the surface of the inner bottom 3 is larger than the combined surfaces of an horizontal section of each hollow 5. The horizontal section of the hollows 5 at the level of the inner bottom 3 is shown to be octagonal. Each, hollow 5 is arranged in the inner bottom 3 to deepen into the space between the inner bottom 3 and the outer bottom 4. The hollows 5 are integrated in the inner bottom 3 in a way that ensures that the space between the inner and outer bottom is kept closed.

The hollows 5 may be designed as a part of the inner bottom 3 or may be made as an independent structure which is joined to inner bottom 3. The shape of each hollow 5 may vary as long as it is. capable of accomodating a portion of the cargo tank 6. In figure 1 each hollow 5 comprises plural surfaces which slopes downwards towards the outer bottom 4 and towards the center of the hollow 5. The bottom of the hollow 5 is oriented horizontally in a parallel relationship to the inner bottom 3.

In the following the new inventive design of the LNG-carrier as shown in the figures 1 and 2 is to be explained and compared to a present design shown in the figures 3 and 4.

The present design LNG-carrier as displayed by the cross-section in figure 3 has an outer hull 9 and an inner hull 8. The inner hull 8 is shaped to accommodate a lower portion of the cargo tanks 22. The structures 8a is given a rectangular shape to form the cargo hold. The hull is divided into separated spaces or compartments by longitudinal bulkheads 10, 11 and transverse boxes 12, se figure 4. The spaces 13a, 13b are provided as side deep tanks for ballast water and the space 13c is a tunnel for pipes. Skirts 21 for supporting the cargo tanks are attache to a platform integrated with the side tanks. In figure 2 the LNG-carrier in accordance with the invention is shown. The double bottom 2 is constituted by the inner bottom 3 and the outer bottom 4, and hollows 5 for accommodating a lower portion of the cargo tank 6 is arranged in the inner bottom 3. Each hollow 5 is constructed by inclining elements 5 a and a horizontal element 5b. The longitudinal bulkheads 14, 15 divide the space of the double bottom in a port space 16, a center space 17 and a starboard space 18. Side tanks 19, 20 for ballast water are arranged on the inner bottom 3 and skirts 7 for supporting the cargo tanks are attached to the inner bottom 3.

When comparing the configuration of the hull in figure 3 to the configuration of the hull of the inventive LNG-carrier in figure 2, it will be apparent to the skilled person that the LNG carrier in figure 3 is not constructed with a double bottom, whereas the inventive LNG-carrier in figure 2 has a clearly defined double bottom. In figure 3 the tanks 13a, 13b for ballast water are so-called hopper tanks, and each tank fills both part of the spaces at the bottom and at the sides of the LNG-carrier. Whereas in figure 2 the double bottom 2 is separated from the spaces 19, 20 at the sides of the LNG-carrier by the inner bottom 3. The spaces 19, 20 at the sides function as tanks for ballast water while the spaces 16, 17, 18 of the double bottom may be for ballast water or are void spaces.

In figure 3 the structure 8a of the top of the hopper tank is given a rectangular shape to form the cargo hold, as illustrated in figure 4. The inner bottom 3 of the new LNG carrier in figure 2 is provided with hollows to accommodate the cargo tanks 6, in such a way that the surface of the inner bottom 3 is larger than the combined surfaces of an horizontal section of each hollow. This proportion is not applicable to the top of the hopper tank, see figure 4, where a predominant portion is rectangular shaped to accommodate the lower portion of the cargo tanks 22. This construction is not applicable as a double bottom.

The neutral axis is illustrated in figures 2 and 3 by NA. To minimize the deflections in the lower part of the supporting skirt the inner bottom 3 should be positioned close to the neutral axis. When the distance from the inner bottom 3 to the neutral axis NA of the carrier constitutes about 25% of the distance from the outer bottom 4 to the main deck 23, the invention fulfils both aspects of the object of the invention; The carrier and the cargo tank support are given an adequate strength and the double bottom is clearly defined and fulfilling the regulatory definition of the double bottom and the set safety aspects. Further, it is clear that the volume of the double bottom is to be subtracted when calculating the tonnage of the carrier to enter for instance the Suez canal.

Claims

CLAIMS 1. A carrier arranged with at least one cargo tank preferably spherical or near spherical, for transporting liquefied gas preferably liquefied natural gas (LNG), wherein the carrier has an outer bottom and a inner bottom de fining a double bottom and a number of hollows are provided in the inner bottom and spaced in the longitudinal direction of the carrier, wherein each hollow is designed to accommodate a portion of each cargo tank, and wherein the distance from the inner bottom to the neutral axis of the carrier constitutes about 25% (15-30%) of the distance from the outer bottom to the main deck. 2. A carrier according to claim 1, wherein the side tanks preferably for ballast are separated from the d xible bottom and supported on the top of the double bottom. 3. A carrier according to claim lor 2, wherein the skirts for supporting the cargo tanks are attached to the inner bottom of the double bottom. 4. A carrier according to one of the claims 1-3, wherein the space between the inner bottom and the outer bottom is a closed one and constitutes voids and ballast tanks. 5. A carrier according to one of the claims 1-4, wherein longitudinal bulkheads divides the space of the double bottom, in port space(s), center space(s) and starboard space(s). 6. A carrier according to one of the claims 1-5, wherein each hollow is a structure arranged in the inner bottom in a way that ensures that the space between the inner and outer bottom is a closed one and each hollow deepens into the space of the double bottom. 7. A carrier according to one of the claims 1-6, wherein the horizontal sections of the hollows may vary with the depth of the hollows and the surface of the inner bottom is larger than the combined surfaces of an horizontal section of the hollows. 8. A carrier according to one of the claims 1-7, wherein the hollow is shaped as half spherical structures or half polygon structures, and the horizontal section of the hollow may be octagonal, polygonal, spheric. A carrier according to one of the claims 1-8, wherein the horizontal section of the hollow decreases towards the bottom of the hollow.