DECKS FOR MARINE TANKERS

Abstract: The invention relates to a deck structure for marine tankers of the kind which have no centreline bulkhead and which are provided with at least one tank (1) defined by shell plating (2), transverse bulkheads (4) and deck structure (5,6,7). The invention is characterized in that the primary strength joint of the deck comprises a fore-aft beam (5) which extends along the centreline (CL) and which is carried by the transverse bulkheads (4) and is connected to the shell plating (2) by means of downwardly inclined deck portions (6,7). The beam is located at a height above the location (8) at which the deck portions connect with the shell plating such that when the vessel rolls to the dimensioning roll angle, the highest deck portion will lie at least approximately horizontally.

Description: The present invention relates to a deck construction for marine tankers of the kind which have no centreline bulkhead and which are fitted with at least one tank which is defined by shell plating, transverse bulkheads, and deck structure in accordance with the preamble of Claim 1. There are two known kinds of tanker construction, by which is meant essentially marine vessels for transporting mainly liquid products and/or crude oil but which may also transport so-called pourable cargoes, e.g. seed. The first of these vessel types includes marine vessels which are fitted with one or more centreline bulkheads and transverse frames, in order to provide the strength or rigidity required. With the other vessel type, the requisite rigidity is obtained by fitting the vessels with twin bottoms and twin hulls and by installing deck frames across the deck. This type of vessel will often require the provision of a centreline bulkhead for strength purposes. The advantage of this latter vessel construction over the former is that the inner surfaces of the tanks are smooth and therewith easy to clean, present a smaller surface area to be painted, and greatly facilitate emptying of the tanks. One serious drawback with this type of vessel, however, is the heavy weight of steel involved, which can be far greater than the steel weight of the first type of vessel. A further drawback is found in the presence of on-deck frames, which restrict access in the fore-and-aft directions. Still another drawback with known tanker constructions, particularly broad-beam tankers, resides in the powerful stresses to which corners and junctions in the shell plating and deck in particular are subjected when the vessel is at sea, these stresses being due to hammering of the liquid against the structure, and also in the fact that the tank must be dimensioned in correspondence with a dimension-determining roll angle. The main object of the invention is therefore to provide a deck construction which will firstly permit greater tank volumes to be obtained at a given hull width, secondly results in a vessel of lighter construction, and thirdly eliminate the aforesaid drawbacks of known marine vessel constructions. This object is realized completely by means of the invention defined in the claims and described hereinafter with reference to the accompanying drawing, in which Figure 1 is a cross-sectional view of a marine vessel fitted with a deck construction according to the invention and Figure 2 is a cross-sectional view according to Figure 1 illustrating rolling of the vessel in correspondence with a calculated maximum rolling angle, the dimension-determining roll angle, and a section through a conventional tanker at the same angle of roll. Figure 1 is a cross-sectional view of a tanker fitted with one or more transverse tanks 1, each tank being defined by two transverse bulkheads 4 which are mutually spaced in the fore-and-aft direction of the vessel and which are joined to the shell plating 2 and to the bottom 3 of the vessel. Since the illustrated vessel has no fore-and-aft bulkhead, the width of tank 1 will correspond to the width between the hull sides 2. Adjoining the hull sides 2 is a deck construction comprising a beam 5 which is arranged in a vertical plane through the centreline of the vessel and which, e.g., may have the form of a box-beam. The beam 5 is joined to the transverse bulkheads and also to inclined deck portions 6,7, which slope downwardly away from the beam to the hull sides 2 and are connected thereto. The tank 1 of the illustrated embodiment will therefore be defined by the hull sides or shell platings 2, the bottom 3, two mutually sequential transverse bulkheads 4 and the deck construction comprising the beam 5 and the sloping deck portions 6 and 7. The beam 5 is placed at a level which is located far above the position 8 at which the deck joins the sides of the hull or the shell plating 2. Consequently, when using a deck construction according to the invention on a given hull there is obtained the advantage of a considerable increase in tank volume, as will be seen immediately from Figure 2, which illustrates on the right a hull provided with a conventional deck structure and on the left a hull of the same width as the former hull but fitted with a deck constructed in accordance with the invention. Naturally, it is assumed that the transverse hulls are located at the same distance apart in both cases. The inventive deck construction also affords other advantages, which will be made apparent in the following. Normally, the rolling angle of a tanker lies between 20-30°, although other rolling angles may be selected. The rolling angle forms a dimensioning parameter when determining the dimensions of the vessel. The angle V₁ between respective deck portions 6,7 and a horizontal plane through other points on the hull sides 2 is so selected in dependence on the chosen maximum rolling angle V₂ between the centreline plane CL of the vessel and a vertical plane as to obtain the greatest possible increase in cargo volume while decreasing the head or pressure height in the dimensioning roll position (the angle V₂) of the vessel. By head or pressure height is meant the distance, in the rolling position of the vessel, between the highest level N₁ of the liquid or the pourable cargo and the level N₂ of the joints in the tank 1 to be dimensioned. Because the beam 5 is located at a more elevated position than the conventionally constructed deck 9, Figure 2, the rigidity or stiffness of the total ship's beam will increase considerably, which in turn enables the height of the hull sides 2 to be decreased without reducing the tank volume in comparison with the tank volume of a conventional construction. Thus, Figure 2 illustrates a section which has lower hull sides 2 than the conventional construction shown to the right of the Figure. A vessel which is fitted with a deck constructed according to the invention will be lighter in weight, since the hull sides can be made shorter in height and because the transverse frames 10 required in conventional constructions are not required in the inventive construction. Furthermore, the smaller deck portions 6 and 7 can be given a very light construction, due to the short distance between the beam 5 and the location 8 at which the deck portions are connected to the hull sides or shell plating. Because the hull sides can be made shorter in height, the aforesaid pressure height can be decreased, this pressure height, or head, being designated H₁ in Figure 2 for a vessel according to the invention and H₂ for a conventional vessel. In the Figure 2 embodiment the cross-sectional area of the tank in the vessel fitted with a deck structure according to the invention is greater than the cross-sectional area of the tank of the conventional vessel, with both vessels having the same width, i.e. a greater tank volume is obtained despite a decrease in the height of the hull sides and despite a smaller pressure height or head. Naturally, this decrease in pressure heights will also result in less stresses on the hull and deck. Stresses caused by hammering of the cargo, as mentioned in the introduction, are greatly reduced in a deck construction according to the invention, since angles of the order of 90° or thereabove are completely lacking in the construction according to the invention, wherewith it is also possible to reduce the dimensions of the plating. In the case of the preferred embodiment, the angle V₁ is selected so that when the vessel rolls to the maximum dimensioning roll angle V₂, the uppermost deck portion 7 in Figure 1 will lie at least approximately in the horizontal plane. Although the inventive deck construction has been described with reference to a hull fitted with double bo ttoms and double shell plating, it will be understood that the invention can also be applied to other types of hull. The described and illustrated embodiment has been chosen solely by way of an example, and modifications are possible within the scope of the claims. For example, the angle V₁ can be increased to at least 30° even when the dimensioning roll angle is about 25°. In this case the tank, or tanks, is (are) particularly suited for transporting grain or cereal products, which have an angle of repose of about 30°. The cargo space is therewith self-stowing. When the vessel rolls to the maximum roll angle, the deck portions will not lie fully horizontally, but will approach an essentially horizontal position. When the beam 5 has the form of a box-beam, preferably a fully closed box-beam, the beam can be used advantageously as a tunnel system through which conduits, pipes etc. can be passed, therewith relieving the deck portions 6 and 7 of such conduits to a great extent. When the beam 5 comprises a box-beam, reinforcements can be arranged in the interior of the beam. The upper surface of the box-beam is preferably smooth, so as to facilitate access, and the undersurface of the beam which forms part of the tank wall will also be completely smooth.

Claims: 1. A deck construction for marine tankers of the kind which have no centreline bulkhead and which are provided with at least one tank (1) defined by shell plating (2), transverse bulkheads (4) and a deck structure (5,6,7), said deck structure comprising a central part (5) having arranged on both sides thereof inclined deck portions (6,7) which adjoin the hull of the tanker, the inclined deck portions being so formed that when the tanker rolls to the dimensioning rolling angle the highest of the two inclined deck portions will take an at least approximately horizontal position, characterized in that said central part forms the primary strength part of the deck structure and has the form of a beam (5) which extends along the fore-and-aft centreline (CL) of the tanker and which is carried by the transverse bulkheads (4). 2. A deck structure according to Claim 1, characterized in that the beam is a box-beam. 3. A deck structure according to Claim 1 or 2, characterized in that the box-beam (5) has a smooth upper surface and a smooth undersurface; and in that reinforcements are arranged in the interior of the beam. 4. A deck structure according to Claim 3, characterized in that the box-beam is fully closed and forms a tunnel system.

Inventors: BYSTEDT, STIG
Applicants: STENA AB
Application year: 1987
Main classification: B63B3
IPC codes: B63B25, B63B3