Method for controlling the trim of a transport ship without seawater ballast

Abstract

The present invention concerns a method for controlling the trim of a transport ship without seawater ballast (1), having a width l considered along a transverse axis (yy) of the ship (1), said ship (1) having an unladen weight P.sub.v between 20% and 60% inclusive of its total weight P.sub.T, allowing for a given maximum load weight capacity P.sub.TC, in accordance with the formula:
P.sub.T=P.sub.v+P.sub.TC
at least one first and one second closed liquid tank (3 or 3) not communicating with the sea, the total weight P.sub.RT of which when entirely filled with a liquid of specific gravity equal to 1 represents between 2% and 8%, preferably between 3% and 6%, of said unladen weight P.sub.V, said tanks (3, 3) being in communication via at least one line to transfer liquid from one to the other and being at a distance d from one another, considering the respective geometric center of each of said tanks (3, 3), at least equal to when the tanks (3, 3) are positioned facing one another essentially along the transverse axis (yy): d.

Claims

1. A method for controlling a trim of a transport ship without seawater ballast, the ship having a length L along the longitudinal axis of the ship and a width l along a transverse axis of the ship and including a lower hull a cross section of which takes the form of a trapezium comprising a part forming a flat bottom of the ship from which respectively extend two flanks of identical inclination, said ship having an unladen weight P.sub.v between 20% and 60% inclusive of its total weight P.sub.T, allowing for a given maximum load weight P.sub.TC, according to the formula:
P.sub.T=P.sub.v+P.sub.TC said ship including at least one bow closed liquid tank and one stern closed liquid tank not communicating with the sea, the total weight P.sub.RT of which, when entirely filled with a liquid of specific gravity equal to 1, represents between 2% and 8% of said unladen weight Pv, said tanks being in communication via at least one line for the transfer of liquid from one to the other, and said tanks being positioned facing one another essentially along the longitudinal axis and at a distance d from one another, considering the respective geometric center of each of said tanks, at least equal to L/4: d>L/4, the method including the step of transferring liquid into the bow tank when the load of the ship has a weight less than PTc/10 to straighten the waterline of the ship, wherein the ship further includes a mooring tank, the mooring tank being independent of the bow tank and of the stern tank, the ship further including a line for supplying liquid to the mooring tank and a line for draining the mooring tank, said mooring tank being at the bow of the ship, the method further including the step of transferring the liquid in the mooring tank via the supply line in order to straighten further the waterline of the ship.

2. The method as claimed in claim 1 for controlling the trim of a ship, in which the step of transferring the liquid into the bow tank is carried out until the bow tank is filled.

3. The method as claimed in claim 1 for controlling the trim of a ship, characterized in that the bow tank is situated in the first bow third of the ship and the stern tank is situated in the last stern third of the ship.

4. A method for controlling a trim of a transport ship without seawater ballast, the ship having a length L along the longitudinal axis of the ship and a width l along a transverse axis of the ship and including a lower hull a cross section of which takes the form of a trapezium comprising a part forming a flat bottom of the ship from which respectively extend two flanks of identical inclination, said ship having an unladen weight P.sub.v between 20% and 60% inclusive of its total weight P.sub.T, allowing for a given maximum load weight P.sub.TC, according to the formula:
P.sub.T=P.sub.v+P.sub.TC said ship including at least one bow closed liquid tank and one stern closed liquid tank not communicating with the sea, the total weight P.sub.RT of which, when entirely filled with a liquid of specific gravity equal to 1, represents between 2% and 8% of said unladen weight Pv, said tanks being in communication via at least one line for the transfer of liquid from one to the other, and said tanks being positioned facing one another essentially along the longitudinal axis and at a distance d from one another, considering the respective geometric center of each of said tanks, at least equal to L/4: d>L/4, the method including the step of transferring liquid into the bow tank when the load of the ship has a weight less than PTc/10 to straighten the waterline of the ship, wherein the ship includes a third tank, situated in a zone between 40% and 60% inclusive of the length L of the ship.

Description

DESCRIPTION OF THE APPENDED FIGURES

(1) The following description is given by way of nonlimiting illustration only with reference to the appended figures, in which:

(2) FIG. 1 illustrates, in schematic section, a ship without ballast in accordance with one embodiment of the present invention:

(3) FIG. 2 illustrates, in schematic section, a ship without ballast in accordance with another embodiment of the present invention;

(4) FIG. 3 illustrates a schematic of the functioning of the circuit for transferring liquid between the four tanks present in the tank in accordance with one embodiment of the invention:

(5) FIG. 4 is a view in cross section of a ship in accordance with one embodiment of the invention;

(6) FIGS. 5a and 5b respective and schematically illustrate a ship in accordance with the invention and that same ship subjected to a high crosswind, this ship being disposed in dry dock:

(7) FIG. 6 is a view in section of a portion of the hull of a ship in accordance with one embodiment of the invention in which are shown the waterlines when the ship contains a partial or complete load and when that same ship is unladen, with no load.

(8) FIG. 7 is a view in section of a ship including a mooring tank.

DETAILED DESCRIPTION OF THE INVENTION

(9) FIG. 1 shows an embodiment of a ship 1 in accordance with the invention and the ship 1 chosen to illustrate the invention is not transporting merchandise/cargo or a relatively small quantity thereof.

(10) In this embodiment, this ship 1 includes two liquid tanks 2, 3, one 2 situated in the front (bow) part and the other 3 situated in the rear (stern) part, these two liquid tanks 2, 3 communicating with one another in such a manner as to allow a transfer of liquid from one to the other. To be more precise, the bow tank 2 is placed in the first bow quarter of the ship 1, with reference to the length L of the ship, from the bow end 5 of the ship 1 to the stern end 6 of the ship 1. In the same manner, in this embodiment, the stern tank 3 is situated in the final stern quarter of the ship 1. It may be envisaged that the bow liquid tank 2 is situated in the first bow part representing the first 12.5% () of the length L of the ship 1 and/or that the stern liquid tank 3 is situated in the last stern part representing the last 12.5% () of the length L of the ship 1.

(11) As can be seen in FIG. 1, the ship 1 chosen to illustrate the invention conventionally includes a navigation tower 11, conventionally termed the superstructure, and equipment 10, conventionally termed the smokestack, situated essentially at the stern of the ship 1 so that the ship 1 is inclined toward the stern, along the longitudinal axis xx, in other words the Plimsoll line 20 of the ship 1 has an inclination relative to the surface of the sea 9, here represented along the longitudinal axis xx.

(12) This inclination of the ship 1 is particularly important in the case of very long ships 1 intended to transport a large load, the navigation tower 11 and the equipment 10 being situated at the stern of the ship 1 and the bow of the ship 1 being reserved for the storage of the merchandise. For example, in the context of a methane tanker type ship 1, tanks intended to store LNG are disposed over all the length of the ship 1 forward of the superstructure. Accordingly, when the ship is not transporting LNG, the bow of the ship 1 has a weight significantly less than the weight of the stern of the ship 1 so that there is a high inclination of the ship 1 relative to the level of the sea. This inclination may cause a large portion of the bow portion of the hull to emerge and in particular at least a portion of the bow bulb, thus degrading the navigation conditions of the ship.

(13) In this example, if the choice is made to send all or virtually all of the liquid into the bow tank 2, then the Plimsoll line 20 of the ship 1 has no or little inclination relative to the surface of the sea, as represented by the waterline 109 in FIG. 1 or by the waterline 209 in FIG. 2. In other words, the transfer of the liquid into the bow tank enables the trim of the ship 1 to be corrected by reducing the inclination of the ship 1 relative to the surface of the sea, typically by reducing the inclination between the Plimsoll line 20 and the waterline 109.

(14) In a complementary manner, the ship 1 may include a mooring tank 12 as sketched in dashed line in FIG. 1. This mooring tank 12 is situated at the bow of the ship 1. A mooring tank of this kind is dedicated to correcting the trim of the ship 1 when unladen, in particular to facilitate maneuvering in a port zone, and to ensure a homogeneous distribution of the weight of the ship when the ship 1 is in dry dock. This mooring tank 12 is filled with liquid in order further to increase the weight of the bow of the ship 1 and thus to correct the trim of the ship 1 by balancing the stern of the ship 1 including the equipment 11 and the superstructure 10 and the empty storage zone situated at the bow of the ship 1. This kind of mooring tank 12 is typically filled with seawater when the ship is not transporting any load and enables a waterline 209 to be obtained that is substantially parallel to the Plimsoll line 20. This kind of mooring tank is preferably independent of the bow tank 2 and the stern tank 3, in other words the liquid used for the functioning of the bow tank 2 and the stern tank 3 does not communicate with the liquid enabling the mooring tank 12 to function.

(15) This mooring tank 12 being limited to use in the port zone, it may be filled with seawater to facilitate maneuvers in the port zone and emptied when the ship 1 has to leave the port zone. This kind of mooring tank 12 dedicated to navigation in a port zone therefore does not represent any risk to the ecosystem because the seawater used to fill the mooring tank 12 is drawn up and then discharged in the same geographical area. Furthermore, when the ship 1 enters dry dock, a Plimsoll line 20 that is substantially horizontal (that is to say parallel to the level of the water in the dry dock) enables good distribution of the weight of the ship 1 over all of the length of the hull when the dry dock is emptied of water to cause the ship 1 to rest on the bottom of the dry dock.

(16) FIG. 2 shows another embodiment of the ship 1. In this case, the ship 1 includes three liquid tanks 2, 3, 4, that is to say the bow liquid tank 2 and the stern liquid tank 3 present in the ship 1 represented in FIG. 1 to which has been added a midships tank 4, that midships tank 4 being in communication with the other two tanks 2, 3 to transfer liquid from one to the other. The midships tank 4 is situated substantially in the middle of the ship 1, along its longitudinal axis xx, typically in a zone between 30% and 70% inclusive of the length L of the ship 1, considered from the bow end 5 or the stern end 6 of the ship 1 along the longitudinal axis xx, preferably in a zone between 40% and 60% inclusive of the length L of the ship 1.

(17) In accordance with one possibility offered by the invention, liquid is preferably transferred between the bow tank 2 and the stern tank 3 via this midships tank 4. In accordance with another possibility, liquid is transferred or may be transferred between the bow tank 2 and the stern tank 3 independently of this midships tank 4.

(18) As can be seen in this FIG. 2, the distribution of the liquid between the bow tank 2, the stern tank 3 and the midships tank 4 is such that the Plimsoll line 20 of the ship 1 extends approximately parallel to the plane of the sea/ocean (local water level). In this instance, the Plimsoll line 20 of the ship 1 coincides in FIG. 2 with the waterline 209.

(19) FIG. 3 shows schematically an embodiment of the invention in which the ship has or includes four tanks, a bow tank 2, a midships tank 4 and two stern tanks 3, 3 offset relative to one another along the transverse axis yy. This kind of embodiment, with these two laterally offset stern tanks 3, 3, is represented in a clearer manner in FIG. 4 in which only these two stern tanks 3, 3 are represented.

(20) As can be seen in FIG. 3, each of the tanks 2, 3, 3 and 4 has at least one filling/emptying line 30 and one liquid transfer line 40. The filling/emptying line 30 enables filling or emptying of the tank concerned independently of the other tanks with which they communicate whereas the transfer line 40 enables the liquid to be transported from or to that tank, respectively at least partly to empty that tank and at least partly to fill another tank and at least partly to fill that tank and at least partly to empty another tank. Of course, the network of liquid transfer lines 40 interconnecting the various tanks as shown in FIG. 3 is merely one example of this kind of network and any arrangement or layout of these transfer lines 40 may be employed provided that this network addresses the objective of enabling or authorizing circulation of liquid between at least two tanks 2, 3, 3, 4. The network of liquid transfer lines 40 includes at least one pump 60, preferably a plurality of pumps 60 and possible as many thereof as there are tanks 2, 3, 3, 4, able at least partly to empty a tank 2, 3, 3 or 4 to transfer the liquid that it contains to another tank 2, 3, 3 or 4. Of course, a plurality of valves, remotely controlled like the pump 60, are provided in this liquid transfer line network 40 in order to send the liquid to the appropriate/desired tank.

(21) The plurality of liquid tanks 2, 3, 3, 4 and the possibility of transferring liquid from at least one to another of those tanks 2, 3, 3, 4 are intended firstly to enable the inclination of the ship 1 or of the Plimsoll line 20 of the ship 1 to be varied so that the latter is conventionally parallel to the longitudinal axis xx or to the plane in which the surface of the sea/ocean extends. A second objective of these tanks 2, 3, 3, 4 and of the possibility of transferring liquid between at least two tanks aims to lower the waterline of the ship 1 or to increase its draught but only to the minimum level required to authorize or to facilitate its maneuverability, thus in particular when a captain boards in order to direct the ship when it enters a particular port or port zone.

(22) In the embodiment shown in FIG. 4 the ship 1 includes at least two tanks 3, 3 offset relative to one another along the transverse axis yy. To be more precise, a first tank 3 is situated in the first third, preferably in the first quarter, along the width & of the ship, along the transverse axis yy, and the second tank 3 is situated in the last third, preferably in the last quarter, again along the width & of the ship 1.

(23) In this figure there has been shown the starboard tank 3 filled, to approximately two-thirds () of its maximum volume/mass capacity, whereas the port tank 3 is empty. Because of this weight difference or gradient, the ship 1 therefore lists to one side, in other words the Plimsoll line 20 of the ship 1, here extending parallel to the transverse axis yy, has a (non-zero) inclination or angle relative to the plane of the surface of the sea/ocean 50 (local water level). This being so, transferring liquid between these two tanks 3, 3 results in the Plimsoll line 20 of the ship 1 here being flush with the level of the the sea/ocean 50 on the starboard side so that a shuttle or the like, not shown in the appended figures, can come to be positioned contiguously with the ship 1 to deliver a captain able to direct the ship for its approach to and its penetration into a difficult port or port zone, without that shuttle or the like risking being crushed or damaged by the flanks 21 of the hull of the ship 1 when the sea conditions are unpredictable. Indeed, thanks to the invention, the possible transfer of liquid from two tanks 3, 3 positioned offset or at a distance along a transverse axis yy of the ship 1 (that is to say along its width) enable the ship to be caused to heel as required, in particular when a smaller boat comes alongside so as not to risk it being crushed/damaged by its inclined flanks 21 situated clearly above the level of the sea/ocean 50 (because of the absence of cargo/load on the ship 1).

(24) FIGS. 5a and 5b show one of the design choices for the ship 1 in accordance with the invention that have led to its particular characteristics and dimensions. Thus when the ship 1 is in dry dock, in particular to undergo refurbishment and possibly repairs, it is imperative that there is no risk of it capsizing in a strong crosswind (along the transverse axis yy), as shown in FIG. 5b. Indeed, because of the absence of seawater ballast, the ship 1 in accordance with the invention has a lower hull of trapezoidal shape, that is to say in particular a plane lower part 22 at the two ends of which respectively extend two inclined flanks 21. Given the length L and the height of the ship 1, the plane lower part 22 of the ship 1 is designed sufficiently wide for the ship 1 to be able to resist, given its unladen weight, a lateral crosswind exerting a maximum force (the value of which is determined by international standards or regulations). The width of the plane part 22 of the ship 1 in accordance with the invention is therefore a function of its length L, of its height and of its unladen weight so that said ship 1 is able to resist an extreme force (quantified by regulations for safe maintenance operations in dry dock) directed laterally, along the axis yy or parallel to that axis, so that the ship 1 does not tilt when in dry dock, resting on the plane part 22 of its lower hull.

(25) FIG. 6 shows a complementary aspect of the ship 1 in accordance with the invention. In this figure there has been represented in vertical section a half-hull of the ship 1 (of width ). Here the design of a ship 1 in accordance with the invention with no seawater ballast aims firstly to ensure that, when the ship 1 is unladen (with no cargo/merchandise), the waterline 44 of the ship 1 is close to the Plimsoll line 20 of the ship 1, that is to say the zone from which the inclined flanks 21 extending from the lower hull terminate. The difference between the waterline 44 of the ship 1 unladen and the Plimsoll line 20 of the ship 1 must be at most equal to 1 meter, preferably less than 50 cm (centimeter), or even very preferably less than 30 cm. Note that, in this FIG. 6, there has also been represented a waterline 45 of the ship 1 when the latter is laden, that is to say when it is transporting cargo and/or merchandise. Moreover, the two flanks have an inclination by angle between 10 and 45 inclusive, preferably between 15 and 35 inclusive.

(26) These requirements for production of the V-shaped lower hull of the ship 1 with no seawater ballast concerning the low height h are provided in particular, but not exclusively, so as not to damage or break a small boat coming alongside the ship when the sea or the ocean is agitated.

(27) FIG. 7 shows a functional schematic view in section of a ship including a mooring tank 12 as shown in FIG. 1. As explained hereinabove, this kind of mooring tank 12 is situated at the bow of the ship in order to balance the ship 1 and to enable it to have a horizontal trim, that is to say a Plimsoll line 20 parallel to the level of the water.

(28) In this FIG. 7 the waterline of the ship when unladen, i.e. when transporting no load, is shown by the number 109 when the mooring tank 12 is empty and by the number 209 when it is filled. The mooring tank 12 is connected on the one hand to a supply line 13 and, on the other hand, to a drain line 14. The supply line discharges into the upper part of the mooring tank 12 in order to fill said mooring tank 12, for example by means of a pump (not shown) drawing up seawater from the port zone to fill the mooring tank 12. The drain line 14 is arranged in the bottom of the mooring tank 12 in order to enable emptying of the mooring tank 12. This drain line 14 discharges directly on a flank of the ship 1, for example above the Plimsoll line 20, in order to decant the content of the mooring tank 12 into the sea.