WO1998051563A1

WO1998051563A1 - Bulk carrier

Info

Publication number: WO1998051563A1
Application number: PCT/SE1997/000800
Authority: WO
Inventors: Eric Bruce
Original assignee: Bruce, May-Britt
Priority date: 1997-05-15
Filing date: 1997-05-15
Publication date: 1998-11-19
Also published as: EP0981477A1

Abstract

A bulk carrier having shell-plating (17), a double bottom (14), a cargo hold divided into sections and at least one column-shaped cargo cell (12) in each section that extends up above the weather deck of the ship, each cargo cell having a convex bottom provided with a central outlet (23). According to the invention the convex bottom of each cargo cell rests on a correspondingly concave bed (13) that is heat-insulated (21) from the double bottom and is at least partially self-sealing. Further, the bed is also provided with a heating arrangement (22) for indirect heating of the cargo in the cargo cell. The space (11) between the cargo cells and the shell-plating, double bottom and column-shaped ballast tanks (18) is arranged to be filled with an inert medium to be kept there permanently.

Description

Bulk carrier

The present invention relates to a bulk carrier having shell-plating, a double bottom, a cargo hold divided into several sections and at least one column-shaped cargo cell in each section that extends up above the weather deck of the ship, each cargo cell having a convex bottom provided with a central outlet .

Shipping accidents and disasters have caused the authorities to set stricter requirements as to design, construction, operation and maintenance of ships. Certain designs developed have proved extremely expensive. Neither have the operating and maintenance conditions for work on board been observed to the desired extent. Bulk carriers have traditionally been double-bottomed but older vessels have usually had single shell -plating. Damage to this type of vessel has resulted in the trafficking of certain channels being called in question, and also to ships being designed with double shell-plating. The outer hull is thus constructionally joined to the inner hull constituting the cargo hold. Damage to the outer hull also affects the inner hull, reproducing the damage, with leakage as a result. This type of vessel has additional problems with a cargo that must be heated, partly because hot internal coils may affect the quality of the cargo negatively, and partly since taking in cold ballast water in the double bottom cools the cargo. Unloading at the final destination is thus made more difficult and in some cases impossible.

Handling the ballast water always involves problems for reasons of stability, for instance.

US patent specification 5,233,937 shows an arrangement in connection with installing a double-hulled vessel and problems involved with the use of inert gas in the ballast tanks of this type of vessel . The bulk carrier according to the invention and its inert gas arrangement is therefore not technically comparable with the embodiment shown in US-5, 233 , 937.

The object of the present invention is to provide a bulk carrier that has high environmental and operational safety, that permits simple and efficient loading and unloading and that facilitates inspection and maintenance. The bulk carrier according to the invention is characterized in that the convex bottom of each cargo cell rests on a correspondingly concave bed on the double bottom, in that the bed is heat-insulted from the double bottom and is at least partially self-sealing, in that the bed is provided with a heating arrangement for indirect heating of the cargo in the cargo cell, and in that the space between the cargo cells and the shell-plating, double bottom and column-shaped ballast tanks is arranged to be filled with an inert medium to be kept there permanently.

Conventional relations between length, breadth and moulded depth (LBD) can be retained for a given load capacity. The ballast water is carried in segregated tanks, well spaced from the cargo compartments, as well as in the double bottom. Oil leaking into the ballast tank can always generate explosive gas. The completely dry space around the tank constitutes a cofferdam which is permanently filled with inert gas, thereby forming an explosion-proof and corrosion-free zone throughout the entire operative life of the ship. In ships using inert gas in the ballast tanks in order to reduce the explosion risk, and where the inert gas is produced by combustion of oil, it must be generated continuously and therefore constitutes an extremely negative environmental factor. Furthermore, residual products in the gas combine with the salt water to increase corrosion, as has been dramatically documented in certain cases. The cargo cell suitably has substantially circular cross section. This allows optimal use of the material since the cell walls are subjected by the pressure of the load to membrane stresses of the same magnitude through the entire thickness of the plate. The circular shape results in the least amount of material, which is accentuated when steel of higher strength than ordinary ship ' s steel is used. Fatigue problems are practically negligible in the membrane-stressed circular construction in comparison with conventional tank designs which have many places for potential cracking. In conventional tankers, which have exclusively longitudinal framing throughout, this means that the frame, the longitudinals, which may be angle irons or bulb irons, must pass through all transverse strengths, sealed or unsealed, and it is well known that these passageways are by their very design subject to both strong corrosion and fatigue loads. In the design according to the invention the number of such passageways can be greatly reduced and can to a great extent be localized to the corrosion-free area.

The invention will be described in the following with reference to the accompanying drawings.

Figure 1 is a lateral view of a bulk carrier according to the invention, with cargo sections and a row of cargo cells. Figure 2 is a view from above of the tank top plating on the vessel according to Figure 1.

Figure 3 is a partial view from above, corresponding to that in Figure 2 but with two rows of cargo cells.

Figure 4 is a cross section through a bulk carrier according to the invention with two rows of cargo cells. ω L to to H F>

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size and general construction of the ship and the desire to keep cargoes of different types separate.

Figure 3 shows a partial view from above through a bulk carrier having two rows of cargo cells 12. Standing ballast tanks 18 are arranged against the side shell-plating 17, both at the transverse bulkheads 10 and between the cargo cells 12 when several cargo cells are located in each section. The cross-sectional shape of these tanks is adapted to the space and strength requirements. These ballast tanks provide a stronger protective zone for the cargo cells along the length of the ship than is provided by a conventional double-hulled arrangement. The space 11 around the cargo cells 12 constitutes a cofferdam which, when filled with inert medium, will constitute an explosion-safe zone. The space is completely dry which, together with the inert medium, completely eliminates corrosion conditions. An arrangement for supplying inert medium is indicated at 20 in Figure 4.

Figure 4 shows a vertical circular shaft 19 constituting pump-room cover and pump room for cargo and ballast pumps. Many tankers today have installations of "immersible" explosion-safe pumps that are lowered down into the cargo tanks. However, these must be lifted up in the event of operational disturbances, with shut-down as a result. The cargo oil pump connected to the four cargo cells in the cargo section allows individual unloading of each cargo cell under full control of the ship's stability conditions, irrespective of segregation requirements and unloading sequence. The number of cargo oil pumps can thus be reduced to one for each group of four cargo cells - an up to 75% reduction in the number of pumps in comparison with today's tankers which have at least one immersible pump installed in each tank. The pump-room shaft 19, circular in cross section, is lifted aboard fully equipped just like the cargo cells and is connected by plug-in assembly to the other equipment.

The concave bed 13 is of honey-comb construction and comprises a heat-insulating layer 21 in the part facing the double bottom. Heating coils 22 or heating cables are cast in the part facing the bottom of the cargo cells 12. The heat transport medium is suitably hot oil. The convex bottom of the cargo cell facilitates loading and unloading through a central drainage well and bottom pipe 23. No heating coils are located in the cargo cell. The embodiment is comparable to a hotplate that indirectly heats the contents of the cargo cell. The structure of the bed is such that it distributes the surface pressure uniformly and also achieves a satisfactory transmission of force between the bed and the steel construction of the double bottom. The cargo cells 12 are provided with hydraulic lifting devices 24 enabling them to be lifted to the necessary height for inspection and overhaul of the bed construction and installations therein. The bed is at least partially self-sealing .

The cargo cells 12, completely smooth on the inner side, are provided with requisite external reinforcements and can be manufactured and equipped separately. They are lifted aboard and connected to installations in the vessel, suitably through plug-in systems. Together with the side shell-plating, lateral ballast tanks and double bottom, the cargo cells form a double, and in certain cases triple barrier against leakage. Safety is thus considerably greater than with a conventional double-hulled ship.

A known method of restricting the discharge of oil if a tanker is damaged is to place the pressure-vacuum valve 25 under a certain negative pressure so that it is released when oil starts to flow out from a damaged tank, e.g. in the event of damage to the bottom. The negative pressure thus limits the magnitude of the oil discharge. If the negative pressure is too great a tank of conventional design can be seriously damaged, resulting in aggravated oil discharge. The circular, free-standing cargo cell construction has considerably greater resistance to internal negative pressure than a traditional tank construction.

The present invention thus provides a bulk carrier with high environmental and operating safety that permits simple and efficient loading and unloading under all loading and operating conditions, with the emphasis on bulk cargo of various types in both solid and fluid form. Many types of cargo in liquid form, such as most oil products, require heating and tanks or cargo spaces intended for such cargoes are thus conventionally provided with heating coils placed inside said areas. This makes transport and unloading of most types of cargoes in solid form impossible since the heating coils may easily be damaged during manipulation of the cargo and when using known unloading equipment. No such risk of damage exists with the arrangement according to the present invention, which has the heating coils outside the cargo space, in a bed construction situated beneath the cargo cells. It is also known that in the case of certain cargoes, e.g. grain products, health- impairing gases are formed as a result of leakage of the heating medium due to damaged heating coils, and the cargo is ruined. According to the invention several possible combinations of materials are technically possible for the structure of the bed which, if the vessel runs aground thereby damaging the double bottom, will at least partially make the bed self-sealing and thus reduce possible discharge of oil. Decisive in assessing the safety level of an object is the presumed reliability the technical equipment, but particularly its operative > ω NJ N F¹ H

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form a broad carbonizing zone that must be constantly kept under control. In practice this means that the oxygen in the air must be entirely replaced with inert medium. It is well known that in a number of tanker accidents interruption in the supply of inert medium and subsequent attempts to restore the balance between inert medium and the oxygen in the air has resulted locally in explosive gas mixtures which have triggered an explosion in the ballast tank. The double-hulled vessels and alternatives suggested all have ballast tanks immediately adjacent to the cargo tank area and their construction makes filling every nook and cranny with inert medium difficult, the trim and sidesway of the ship during loading and unloading forming inaccessible pockets so that the inert medium simply cannot get at them to replace the explosive gas. It is also well documented that, due to rapidly appearing pitting damage caused by corrosion, oil enters the ballast tank filled with inert medium and, when the tank is subsequently filled with ballast water and when this is being pumped, oil mixed into the water generate explosive gas which triggers an explosion in the pump room, for instance. Another well documented risk of accident is that, as soon as the inert gas is removed, e.g. during maintenance work, oil that has leaked into the ballast tank starts generating explosive and health-impairing gas. The double-hulled construction of tankers required by the International Maritime Organization is based on the assumption that these are safer as regards the risk of direct oil leakage if the ship runs aground or is involved in a collision with another ship. However, in a number of known cases, it was exactly this double hull that, when gas exploded in the ballast tanks due to oil leakage, aggravated the damage to the ship and increased the oil leakage. The high degree of safety aimed at is thus in the cases mentioned and in practice a chimera with consequences difficult to assess unless better alternatives and ) NJ H H

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Claims

C L A I M S

1. A bulk carrier having shell-plating (17), a double bottom (14) , a cargo hold divided into several sections and at least one column-shaped cargo cell (12) in each section that extends up above the weather deck of the ship, each cargo cell (12) having a convex bottom provided with a central outlet (23) , characterized in that the convex bottom of each cargo cell (12) rests on a correspondingly concave bed (13) on the double bottom

(14), that the bed (13) is heat-insulated (21) from the double bottom (14) and is at least partially self-sealing, that the bed (13) is provided with a heating arrangement (22) for indirect heating of the cargo in the cargo cell (12) , and that the space (11) between the cargo cells (12) and the shell-plating (17) , double bottom (14) and column-shaped ballast tanks (18) is arranged to be filled with an inert medium to be kept there permanently.

2. A bulk carrier as claimed in claim 1, characterized in that the cargo cell (12) has circular cross section.

3. A bulk carrier as claimed in claim 2, characterized in that the inner side of the cargo cell (12) is smooth and said cell is provided with external reinforcements.

4. A bulk carrier as claimed in any of the preceding claims, characterized in that the cargo cells (12) are provided with lifting devices (24) arranged to permit lifting of each individual cargo cell for inspection and repair of the structure of the bed (13) .

5. A bulk carrier for fluid cargoes as claimed in any of the preceding claims, characterized in that the cargo cell (12) is provided at the top with a trunk (16) of sufficient capacity to hold approximately 2% of the cargo cell volume, and a pressure vacuum valve (25) arranged to be released at a certain negative pressure and to restrict escape of the cargo in the event of damage.

6. A bulk carrier as claimed in any of the preceding claims, characterized in that each individual cargo cell (12) is so designed and manufactured completely finished, surface treated and with the necessary installations that it can be lifted aboard and connected to installations on board by means of plug-in systems.

7. A bulk carrier as claimed in any of the preceding claims, characterized in that the column-shaped ballast tanks (18) are vertical and are arranged against the shell-plating (17) .

8. A bulk carrier as claimed in any of the preceding claims, characterized in that each cargo hold section contains four or more cargo cells (12) and is provided with a pump room (19) having circular cross section which is placed on the centre line of the ship and is connected to all the cargo cells (12) in the cargo hold section.