GB2163115A

GB2163115A - Improvements in and relating to vessels

Info

Publication number: GB2163115A
Application number: GB08418776A
Authority: GB
Inventors: Lars-Olof Liberg; David Rowan
Original assignee: Brown & Root Const; GOETAVERKEN ARENDAL AB; Gotaverken Arendal AB
Current assignee: Brown & Root Const; GOETAVERKEN ARENDAL AB; Gotaverken Arendal AB
Priority date: 1984-06-21
Filing date: 1984-07-24
Publication date: 1986-02-19
Also published as: GB2162482A; GB8418776D0; GB8415844D0

Abstract

A vessel with sea-water ballast tanks 11 and 13 at a higher level and 12 and 14 at a lower level, so connected by conduits and valves 15 to 27 that ballast can flow from an upper tank 11 or 13 in one part of the vessel to a lower tank 12 or 14 in another part of the vessel under gravity to produce rapid changes in the heel and/or trim of the vessel to compensate for changes caused by the lifting, moving, or lowering of a heavy load. <IMAGE>

Description

1 GB 2 163 115 A 1

SPECIFICATION

Improvements 5n and relating to vessels The invention relates to vessels which are subject to varying moments as a result of load-handling operations. Lifting vessels, that is to say, vessels which are provided with lifting means for lifting and/or lowering loads, which loads do not form part of the vessel itself, are examples of such ves sels.

In a typical lifting vessel, the means for lifting and/or lowering loads will comprise a crane and, although the invention is not limited to the provi sion of one or more cranes on a vessel, it is conve nient to consider the problems to be overcome in the use of what is commonly known as a crane vessel, that is to say, a vessel provided with one or more cranes designed for lifting a load at one posi tion relative to the vessel and lowering it at the same or another position relative to the vessel.

It is clear that the picking up of an external load by a crane on a vessel, the slewing of the crane when it is supporting the load, and the depositing of the load will each tend to alter the heel and/or the trim of the vessel. Further, the first and last of those operations. will after the mean draught of the vessel.

The changes in heel, trim, and draught can, given time, all be corrected by means of a conven tional ballast system, that is to say, by pumping water into and/or out of tanks suitably situated within the vessel. If the load is large (say 1,000 - tonnes or more) and if the crane being used has a long boom (as will be necessary if the object to be lifted is large), the moment applied to the vessel will.be large and very large transfers of water into and out of ballast tanks will be required if the ves sel is to be maintained level within acceptable lim its. Further, if, for example, a large load is being lifted from a cargo barge, it is desirable that at least the part of the lifting operation that extends from shortly before the load separates from the cargo barge to shortly after separation occurs should be accomplished quickly in order to reduce to an acceptable level the risk of damaging contact between the load and the cargo barge during lift ing resulting from, for example, wave motion. Ac cordingly, if the heel, trim, and draught are to be kept within close tolerances during such a lifting operation, the tendency of those variables to change in response to the moment and load ap plied to the v ' essel must be corrected quickly and it is impracticable to do that using a conventional ballast system.

The invention provides a vessel having tanks for liquid ballast, which tanks are spaced apart verti cally and horizontally, conduit means for permit ting ballast to-flow under gravity from an upper tank to a lower tank spaced apart horizontally from 125 the said upper tank to influence the heel and/or trim of the vessel, and valve means for preventing such flows.

The vessel may be a vessel for moving loads, wherein the said tanks and conduit means are so 130 arranged that the said flows of ballast under gravity can counteract, at least in part, changes in the effect on the heel and trim of the vessel arising from load-handling operations. The vessel may comprise means for lifting and/or lowering loads relative to the vessel, and may be a crane vessel, the or a crane of which is the or a said lifting and/ or lowering means.

The invention is especially useful when the ves- sel is a semi-submersible, that is to say, a vessel which comprises an operating platform or the like, a support structure for the platform and a buoyancy portion from which the support structure extends upwardly, the horizontal cross-sectional area of the support structure being materially less than that of the buoyancy portion. Advantageously, the buoyancy portion comprises a plurality of, preferably, two, elongate hulls which extend generally horizontally and generally parallel to one another, and the support structure then advantageously comprises one or more, preferably, two or more, columns extending upwardly from each hull to the operating platform.

Taking the case where the semi-submersible has two hulls, the vessel when it is being moved from one place to another is caused to function as a catamaran, the hulls having freeboard and the support structure being entirely above the water-line. When the vessel reaches its operating position, water is admitted into the hulls, and if necessary- into hollow parts of the support structure also, un til the hulls are completely submerged so that the waterplane intersects the columns or other support structure rather than the hulls.

Because the horizontal cross-section of the sup port structure is materially less than that of the hulls, the natural period of oscillation of the vessel in rotation about any horizontal axis and also in translation in the vertical direction then becomes longer and the coupling between the vessel and waves on the sea or other water in which it is floating is decreased because the longer natural period of oscillation is less similar to the usual periods of waves in the sort of seas in which these 1 1Q- vessels are most often used. If the decrease in horizontal cross- section on submersion is too great, however, there is a decrease in stability because of the reduction in the restoring force that tends to resist an increase in heel or trim. Thus, as is known to those skilled in the art, the horizontal cross-sectional area of the vessel in the waterplane is chosen to be small enough to give the vessel a satisfactorily small reaction to waves, but large enough to ensure that a given degree of heel or trim gives rise to an acceptably large restoring force, Where the vessel is a crane vessel, the said tanks and conduit means are preferably so arranged and of such dimensions that the said flows of ballast under gravity can produce a change of moments opposite to any change of moments that can be produced by a crane of the vessel taking up a load at a rate substantially equal to the maximum rate of change of moments that can be produced by that crane taking up its maximum load at its maxi- 2 GB 2 163 115 A 2 mum rate, assuming no significant wave motion in the ambient water. Where the vessel has more than one crane, the said flows of ballast under gravity can preferably produce a rate of change of moments opposite and substantially equal to any rate of change of moments that can be produced by all the cranes of the vessel acting together to take up a single load at the maximum rate of operation of the cranes, assuming no significant wave motion in the ambient water.

The tanks are advantageously sufficiently large that the said flows of ballast under gravity can counteract at least 5%, preferably, at least 10%, of the maximum change in the said moments caused by transferring to the vessel any single load that the vessel is capable of handling at one time.

The said tanks and conduit means are advantageously so arranged and of such dimensions that a change in the moments acting on the vessel that is substantially opposite and substantially equal to a significant part, preferably 5%, of the maximum change in the said moments caused by the transfer to the vessel of any single load.that the vessel is capable of handling at one time can be effected by the said flows of ballast under gravity in a time comparable with the shortest period of time within which the said significant part of the maximum change in the said moments can occur as the result of the transfer of a load to the vessel.

Advantageously, the said flows of ballast under gravity can take place in a period of not more than five minutes, and preferably in a period of not more than one minute.

The arrangement and dimensions of the said tanks and the said conduit means are advantageously such that a quantity of ballast equal to at least 500 tonnes of ballast can be transferred. under gravity from a tank or tanks situated at or towards the stern of the vessel to a tank or tanks situated at or towards the bow of the vessel, or vice versa, without the need for any contemporaneous. pumped transfer of ballast into or out of any of the tanks in question. The tank or tanks situated at or towards the stern of the vessel and the respective tank or tanks situated at or towards the bow of the vessel may of course be on the same or opposite sides of the fore-and-aft centre line of the vessel.

The arrangement and dimensions of the said tanks and the said conduit means are advanta- geously such that a quantity of ballast equal to at least 1 000 tonnes of ballast can be transferred under gravity from a tank or tanks situated at or towards the port side of the vessel to a tank or tanks situated at or towards the starboard side of the vessel, or vice versa, without the need for any contemporaneous pumped transfer of ballast into or out of any of the tanks in question. The tank or tanks situated at or towards the port side of the vessel and the respective tank or tanks situated at or towards the starboard side of the vessel may of 125 course be at or towards the same or opposite ends of the vessel.

The provision of conduit means extending obliquely with respect to the fore-and-aft centre line of the vessel may present constructional diffi130 culties with certain types of vessel, and the arrangement and dimensions of the said tanks and the said conduit,means are then preferably such that a quantity of ballast can be transferred under gravity from a tank or tanks situated on one side of the fore-and-aft centre line of the vessel at or towards the stern of the vessel to a tank or tanks situated on the same side of the said centre line and at or towards the bow oi the vessel or in the re- verse direction, and that an additional quantity of ballast can be transferred under gravity from a tank or tanks situated at or towards the port side of the vessel and at or towards either the, stern or bow of the vessel to a tank or tanks situated at or towards the starboard side of the vessel and at or towards either the stern or the bow of the vessel, respectively, or in the reverse direction, without the need for any contemporaneous pumped.transfer of ballast into or out of any of the tanks in question, the said quantity of ballast and the said additional quantity of ballast each being equal to at least 500 tonnes. The or each said quantity of ballast is preferably equal to at least 1, 000 tonnes. 90 The conduit means is preferably arranged to permit the said flows of ballast under gravity to take place without the flowing ballast passing through any means for pumping ballast. The minimum total cross-sectional area of the said conduit means through which a said flow of ballast under gravity can occur from one or more said upper tanks to one or more said lower tanks spaced apart horizontally from their respective said upper tanks in a given direction may be at least 0.5 square metre, is advantageously at least 1.0 square metre, and is preferably at least 3.0 square metres.

The vessel preferably has at least two pairs of tanks, each pair consisting of an upper tank and a lower tank spaced apart horizontally and connected by said conduit means through said valve means.

The upper tanks of two such pairs of tanks may be spaced apart from their respective lower tanks in substantially opposite directions, so that the two pairs of tanks can be used for changing the heel or trim of the vessel in opposite directions about the same axis, or in generally perpendicular directions, so that they can be used for changing the heel and trim of the vessel about two perpendicular axes. Especially preferred is an arranegment with at least four said pairs of tanks the upper tanks of which, or of four pairs of which, are so spaced apart horizontally from their respective lower tanks in different directions that ballast can be permitted to flow under gravity from one. or more upper tanks to their respective lower tanks in sucha manner as to produce a movement, as seen in plan view, in the position of the centre of mass of the ballast in -any horizontal direction, so that they can be used for changing the heel and trim in both senses about perpendicular axes. The upper tanks of the said four pairs of tanks are advantageously spaced apart from their respective lower tanks towards the bow, towards the stern, to port, and to starboard, respectively. The vessel preferably has said upper and lower tanks on both sides of the longitudinal 3 GB 2 163 115 A 3 centre line of the vessel towards the bow and towards the stern, each said upper tank being connected by said conduit means to a lower tank on the other side of the vessel at the same end and/or to a lower tank on the same side of the vessel at the other end.

Where there are pairs of tanks spaced apart in opposite directions, ballast will not normally be permitted to flow in both directions at once, and advantageously a portion of the conduit means joining the tanks of each pair is common to both pairs. It is also advantageous for the upper tank of each pair to be substantially vertically above the lower tank of the other pair.

Where there are two pairs of tanks spaced apart in generally perpendicular directions, there may be a tank that is a said tank of both of the said two pairs of tanks so that ballast can flow to or from it in either of the said directions.

Especially when the vessel has means for lifting 85 and lowering loads mounted on one side thereof at one end thereof, at least one said upper tank or at least one said lower tank or, preferably, both, at that end on that side is or are advantageously con nected to at least one said lower tank or at least one said upper tank, or both, respectively, at the other end on the same side of the vessel and to at least one said lower tank or at least one said upper tank, or both, respectively, on the other side at the same end of the vessel. Especially when, as it then 95 advantageous, the vessel has lifting and lowering means on both sides of the stern, said upper and lower tanks at each side of the stern of the vessel are preferably connected to said lower and upper tanks, respectively, on the opposite side of the stern and the same side of the bow. Especially if the two said lifting and lowering means are sub stantially identical, the said tanks and conduit means are preferably symmetrically arranged about the longitudinal centre line of the vessel.

The vessel preferably comprises means for pumping ballast from at least one said lower tank to an upper tank that is nearer to that lower tank than is any upper tank that is connected to that lower tank by said conduit means. W hen at least 110 so e said upper tanks are substantially vertically above said lower tanks, pumping means is preferably provided for pumping ballast upwards from a said lower tank to a said upper tank vertically above the said lower tank. That enables the lower 115 tanks to be emptied and the upper tanks re charged, to prepare for another load-handling op eration, witho - ut materially affecting the heel or trim of the vessel.

The vessel may comprise pumping means arranged both to transfer ballast from at least one said lower tank to at least one said upper tank and to transfer ballast horizontally between ballast tanks other than the said upper and lower tanks.

At least one said tank may consist of a plurality 125 -of compartments and the conduit means and valve means are then preferably so arranged that ballast can be permitted to flow and prevented from flow ing to or from different compartments independ- ently. Where such a tank consisting of a plurality of 130 compartments is a tank connected by the said conduit means for flows under gravity to or from two other tanks, some compartments may so communicate with the said conduit means that ballast can flow from them to only one of the two other tanks, or to them from only one of the two other tanks, in order to simplify the pipework and valve arrangements.

The vessel is advantageously arranged to be used with sea water as the said ballast.

Where the vessel comprises means for lifting and/or lowering loads it preferably also comprises means for measuring the load on the lifting and/or lowering means, which may be means arranged to measure the tension in the falls of a crane, and means for regulating the said flows of ballast in response to an output from the measuring means.

The invention also provides a method of operating a vessel according to the invention, which comprises influencing the heel and/or trim of the vessel by permitting ballast to flow under gravity from a said upper tank to a said lower tank spaced apart therefrom horizontally.

The said ballast is preferably permitted to flow in a horizontal direction such that the influence of that flow tends to counteract another influence that is tending to change, or has just changed, or is expected to change, the heel and/or trim of the vessel, and the said other influence may then be a change in the moment exerted on the vessel by a load being handled by the vessel.

If the load is being lifted off or set down onto a support by lifting and/or lowering means mounted on the vessel, the ballast is preferably permitted to flow at a rate sufficient to compensate for at least a substantial part of the instantaneous rate of transfer of the weight of the load during the most rapid portion of the transfer. Where the lifting means is on the vessel, the force from the load acting on the lifting means is preferably measured and the flows of ballast regulated in response to the result of the measurement. Where the lifting means is a crane, the force may be measured by measuring the tension in the falls of the crane either directly or indirectly.

If the load is being moved horizontally by means mounted on the vessel, ballast is preferably permitted to flow at a rate sufficient to compensate for at least a substantial part of the instantaneous rate of change of the moment of the weight of the load.

In the operation of the vessel, ballast may be pumped directly from a said lower tank of the vessel to a said upper tank vertically above that said lower tank.

Although it is possible according to the invention to compensate for changes in the heel and trim of a vessel when, for example, lifting a heavy load, it is not possible to compen ate for changes in the mean draught of the vessel caused by the load. Vessels according to the invention are therefore preferably provided with, and methods according to the invention are, where appropriate, preferably carried out in combination with the use of, means for altering the total buoyancy or weight of the 4 GB 2 163 115 A 4 vessel.

One form of semi-submersible crane vessel con structed in accordance with the present invention will now be described by way of example only with reference to the accompanying drawing, in which:

Figure 1 is a diagrammatic side elevation view of the vessel; Figure 2 is a diagrammatic rear elevation view of the vessel; Figure 3 is a fragmentary starboard side eleva tion view showing schematically parts of the bal last system; Figure 4 is a fragmentary port side elevation view showing schematically parts of the ballast 80 system; and Figure 5 is a fragmentary rear elevation view showing schematically parts of the ballast system.

Referring to the drawings, a crane vessel com prises a working platform 1 supported on two hulls 2 and 3. As may be seen from Fig. 1, the platform 1 is supported on the starboard hull 3 by three col umns 4S, 5S, and 6S. The platform 1 is supported on the port hull 2 by corresponding columns 4P, 5P and 6P (see Fig. 4). Cross-braces (not shown) are provided between the hulls and columns where necessary. As has been explained above, when the vessel is in transit it is usually floated until the hulls 2 and 3 have freeboard. When the vessel is being used to lift heavy loads, it is partly sub merged, with the water-line then coming around half-way up the columns 4 to 6r as shown in Figs.

3 and 4.

Above each of the rear columns 4P and 4S is a crane 7P or 7S, respectively, having a boom 8P or 8S, respectively. The cranes are positioned there so that the crane 7P can reach over the port side of the vessel, the crane 7S can reach over the star board side, and both cranes can reach over the stern of the vessel, and so that much of the load on the cranes can be transmitted to the hulls 2 and 3 directly by the columns 4P and 4S. Towards the bow end of the working platform 1 is a superstruc ture 9 that may include crew accommodation, con trol areas, and the like, and has a helicopter landing pad 10. The area of the working platform 1 between the cranes 7 and the superstructure 9 may be used for storing or working on cargo or equip ment and may include hatches (not shown) for lift ing or lowering loads between the hulls 2 and 3.

The vessel is propelled in transit by waterscrew propellers (not shown) and may be maintained on station when operating by propeller thrusters and/ or anchors (not shown).

Referring now to Figs. 3 to 5, the vessel is provided with upper ballast tanks 11 P, 11 S, 13P, and 13S in the columns 4P, 4S, 6P, and 6S, respectively, and with lower tanks 12P, 12S, 14P, and 14S, respectively, in the hulls 2 and 3 below those columns.

The lower tanks 12P, 12S, 14P, and 14S are substantially at the bottom of the vessel, and the upper tanks 1 1P, 1 1S, 13P, and 13S are spaced above them, so that there is a gravity head available even between an upper tank that is almost empty and a lower tank that is almost full. The tanks 1 1S to 14S are connected by pipes 15S to 18S, respectively, through valves 19S to 22S, respectively, to a starboard longitudinal main pipe 23S which connects the starboard bow ballast tanks 13S and 14S with. the starboard stern ballast tanks 1 1S and 12S. The starboard bow and stern lower tanks 12S and 14S are connected by pipes 24S and 26S, respectively, through pumps 25S and 27S, respectively, to the upper tanks 11 S and 13S, respectively, directly above them, the pipes 24S and 26S and the pumps 25S and 27S serving for the transfer of ballast from the lower to the upper tanks. The pipes 24S and 26S are provided with valve means (not shown) to prevent unwanted flows of ballast downwards through them.

A similar system of pipes 15P to 18P, 23P, 24P, and 26P, valves 19P to 22P, and pumps 25P and 27P connects the port ballast tanks 11 P to 14P, as may be seen from Fig. 4 (in which parts corresponding to those of Fig. 3 have been given the reference numerals corresponding to those of the corresponding parts shown in Fig. 3 with the suffix 'P' instead of 'S'), and the description hereinbelow of the system on the starboard side is to be understood as applying mutatis mutandis to the system on the port side also.

The tanks 1 1P, 12P, 1 1S, and 12S are connected by pipes 28P, 29P, 28S, and 29S, respectively, through valves 30P to 31S, respectively, to a transverse pi pe 32 that connects the port and starboard stern tanks and runs through a cross-brace (not shown) between the hulls 2 and 3.

Although each of the tanks 11 P to 14S is de- scribed for the sake of simplicity as 'a tank', such schematic tanks will usually in practice either be divided into a plurality of smaller compartments or consist of a plurality of smaller, separate tanks, both to reduce structural difficulties and to reduce 1Q5 the effect on the static and dynamic stability of the vessel when the tanks are partly full.

Although each of the pipes 15 to 32 has been described as a single pipe, each of the valves 19 to 31 as a single valve, and each of the pumps 25 and 27 as a single pump, for the sake of simplicity, each of these schematic pipes, valves, and pumps will usually in practice take the form of a plurality of actual pipes, valves, or pumps in order to provide a degree of redundancy, and to avoid the need for unduly large single pumps and valves. Such an arrangement also gives versatility in use, because different ones of a set of pumps, for example, can be used for different purposes at the same time, and easier control at low flow rates can be obtained by using only some of the components of such a set. If, as is mentioned above, any of the schematic tanks 11 P to 14S consists of a plurality of compartments or smaller, separate tanks, then an appropriate plurality of pipes and valves must replace the single schematic pipe and valve shown in order to provide effective control over the flows of ballast. In that case, some of the compartments or smaller, separate tanks that in practice constitute, for example, the schematic starboard stern upper tank 11 S may be connected GB 2 163 115 A 5 to the schematic pipe 15S but not to the schematic pipe 28S, and some to the schematic pipe 28S but not to the schematic pipe 15S.

The vessel includes conventional ballast tanks (not shown) provided with pumps for transferring ballast from one such tank to another and/or between such a tank and the body of water in which the vessel is floating, which may be used for small adjustments to the heel or trim of the vessel or to compensate for slow changes in the moments acting on the vessel, the gravity-operated system shown in the drawings being used primarily when rates of change of moments too high for the pumps to cope with are involved. The ballast tanks 11 P to 14S are connected to the conventional ballast system, and may be used as part of it except when their full capacity is required for their use according to the invention, and the pumps of the conventional ballast system can be used either to transfer ballast between the tanks 11 P to 14S (instead of, or in addition to, using the pumps 25 and 27 for that purpose), or to transfer ballast between two of the tanks 11 to 14 at different corners of the vessel if for any reason that is needed.

lf.ballast is taken in from outside the vessel, then it will usually be sea-water, but where ballast is kept permanently within the vessel it may instead be fresh water treated to reduce corrosion of the tanks and pipework and substantially devoid of marine life.

The general operation of the ballast system shown in Fig. 3 and 4 is as follows:

If an upper tank, for example, the starboard rear upper tank 11 S, is full, and a corresponding lower tank, for example, the starboard front lower tank 148, is empty, and if the valves 19S and 22S are opened while all of the other valves are maintained closed, then ballast will flow under gravity from the tan k 1 'IS to the tank 14S. Because the tank 1 'IS is at the stern of the vessel and the tank 14S is at the bow, there will he a transfer of mass from the stern of the vessel to the bow, and consequently a change in the trim of the vessel, with the bow sinking and the stern rising, unless the effect of the said transfer of mass is neutralised by a change in other moments acting on the vessel. Because the flow of ballast is driven by gravity, if the tanks 11 S and 14S and the pipes 15, 18 and 23 are made sufficiently large, almost any desired change in the ef- fect of the ballast on the trim of the vessel, and almost any desired rate of change in that effect, can in principle be achieved.

In order to dissipate the kinetic energy in the ballast in a controlled manner at the end of a transfer, the pipe 15S from the upper tank 1 'IS is closed by the valve 19S at a controlled rate. - As may be seen from the drawings, by opening the valves between the correct pair of ballast tanks a change in the moments. affecting heel or trim in either sense may be obtained.

The operation of the vessel when a load is lifted off a cargo-barge on the starboard side of the vessel by the crane 7S and set down again behind the vessel will now be described by way of example.

65. As the load is taken by the crane, the vessel is ini- tially maintained at constant draught by pumping water out of conventional ballast tanks on the starboard quarter of the vessel, and is kept on an even keel by pumping water into tanks on the port bow and/or by pumping ballast between ballast tanks from stern to bow and from starboard to port. The rate at which the load can be taken up is thus limited primarily by the capacity of the pumps. That comparatively slow load-transfer can becontinued until about half of the weight of the load (or rather more than that) is taken by the crane 7S in a typical case, and may last a few tens of minutes.

As has been explained above, the final lift-off must take place rapidly, say, within 20 seconds, in order to limit the risk of impact between the load and the cargo-barge under sea conditions. During that time the valves 19 and 22 are opened to produce a rapid flow of ballast from tanks 11 S and 11 P in the stern to tanks 14S and 14P in the bow and the valves 30S and 31 P are opened to produce a rapid flow from starboard to port. Because the load being lifted is alongside the vessel, the effect of its weight on heel of the vessel will be greater than the effect on trim, and because the transverse distance between the columns 4P and 4S is less than the longitudinal distance between the columns 4S and 6S, a greater mass of ballast must be transferred from side to side than from end to end.

As the crane 7S is slewed carrying the load from the side to the stern, the heeling effect of the load will be reduced by up to a half while the trimming effect will be increased by up to a half, depending on the radius of the load from the axis of the crane. In order to maintain the vessel upright, fur- ther ballast must be transferred from stern to bow and ballast must now be transferred from port to starboard. If there is sufficient ballast left in the tanks 11 P and 11 S, then ballast may again be allowed to flow from 11 P and 11 S to 14P and 14S.

Ballast may also be allowed to f low from tank 11 P through valves 30P and 31S and pipes 28P, 32, and 29S to tank 12S. Because slewing is a comparatively slow operation, taking several minutes for a 90' turn, a useful contribution may be made by pumping ballast between ballast tanks, and indeed pumped flow may completely replace gravity flow, especially for the transverse flow of ballast from port to starboard, if large enough pumps are provided.

In order to set down the load, the procedure for lifting off is substantially reversed. Because the load is being set down astern of the vessel, the transfers of ballast from port to starboard will be smaller and the transfers frow bow to stern will be larger if the same degree of compensation is to be achieved. As when lifting off, the weight of the load must be transferred rapidly until about half of it (or rather less than that) is taken by the support onto which the load is being set down, with any compensating transfers of ballast being substan- tially entirely by gravity from 13P and 13S to 120 and 12S, and from 11 P to 12S, but the subsequent release of the load can be comparatively slow, with ballast being pumped to compensate, The operation of the rapid ballast system may be 6 GB 2163 115 A 6 controlled automatically in response to the load on the hook of the crane 7P or 7S, in combination with the orientation of the crane and its boom 8P or 8S. Instead, the operation of the system may be controlled automatically in response to direct measurements of the heel and trim of the vessel, either alone or in addition to the load on the hook of the crane and, in any case, manual overriding of the automatic control system may be provided for., It will be appreciated that, apart from any eccen- 75 tricity in the weight of the crane 7S itself, the final state of the vessel is substantially the same as its initial state, so that the final horizontal distribution of ballast (including ballast transferred by pumps) may be the same as or equivalent to (in terms of 80 the relevant moments) its initial distribution, al though ballast that has been transferred by gravity - will finish lower than it started.

- In order to reset the ballast system shown in preparation for another lifting operation, it is nec- 85 essary to transfer ballast by pump from the lower tanks to the upper tanks. That can be achieved us ing only the pumps 25 and 27 with all of the M last being transferred vertically to the tank above that to which it has been transferred by gravity, but pumps of the conventional ballast system may if necessary be used to transfer ballast between ones of the said tanks that are spaced from each other horizontally if the final state of the vessel is different from its initial state. It is not necessary to 95 complete a lifting and lowering operation before resetting the system. Ballast may be pumped verti cally from lower to upper tanks at any time when the necessary pumps, pipes, and power are avail able, although if ballast is transferred horizontally 100 in the process then sufficient other pumped ballast capacity should be available to compensate for any unwanted changes in moments.

If an exceptionally heavy load is to be lifted, it may be lifted close to the stern of the vessel by both cranes 7P and 7S acting together. If the two cranes are equally loaded and the load is centred over the stern, then no transfers of ballast from side to side of the vessel are necessary. but a larger amount of ballast is transferred from the stern to the bow of each hull than in the opera tlons just described. If the capacity of the ballast tanks 11 to 14 becomes a limiting, factor, then (as mentioned above) the change in the moment on the vessel arising from a greater proportion of the weight of the load than the half mentioned above may be compensated for using the conventional ballast system before the rapid transfer is begun if the sea is sufficiently calm.

Although the ballast system shown in the draw ings compensates for changes in the heel and trim of the vessel, it does not compensate for changes in the mean draught resulting from the weight of the load. The conventional pumped ballast capacity may be used for discharging water to the sur rounding sea during the slow initial phase of lifting off and taking water in from the surrounding sea during the slow final phase of setting down, and a rapid system for changing the displacement of the vessel during the rapid phases of lifting off and setting down may also be provided.

For a vessel having a displacement of about 150,000 tonnes, and a draught of about 25 metres, in its semi-submerged state, the following dimensions, which are given by way of example, are suitable.

The vessel has a length of about 150 metres and a beam of about 100 metres, and the columns 4P to 6S are each about 23 metres tall measured from the tops of the hulls 2 and 3 to the underside of the working platform 1.

The cranes 7P and 7S each have a maximum lifting capacity of about 3,500 tonnes at about 40 metres radius when they are free to revolve and a maximum lifting capacity of about 4,500 tonnes when their booms extend over the stern of the vessel and the cranes are secured against rotation. Thus, when the cranes are used, with their booms extending over the stern of the vessel, to act in concert to lift a single load, they can lift a load of 9,000 tonnes, which is the largest single load that the vessel can lift at any one time.

The four schematic stern tanks 11 P to 12S each have a capacity of 2,500 M3 and the four schematic bow tanks 13P to 14S each have a capacity of 2,100 m3. Thus, assuming an arrangement in which one half of the ballast in any given schematic up per stern tank 1 1P or 1 'IS can be transferred only to a respective schematic lower bow tank 14P or 14S, respectively, and the other half of the ballast in such a given schematic upper stern tank can be transferred only to a respective schematic lower stern tank 12S or 12P, respectively, 2,500 m3 (ap proximately 2,500 tonnes) of ballast water can flow under gravity from one end of the vessel to the other in a single operation and witho ut contempor aneous pumping of ballast into or out of any of the tanks in question (pumping being needed at most before and after the operation to charge_upper tanks and empty lower tanks). Similarly, 1,250 m3 of ballast water (approximately 1,250 tonnes) can flow under gravity from one side of the vessel to the other at the stern in a single operation without contemporaneous pumping into or out of any of the tanks involved.

The individual tanks that make up the schematic tanks 11 to 14 are each connected by suitably dimensioned pipes to the schematic transverse main pipe 32 andlor to the respective longitudinal main pipe 23P or 23S. The schematic transverse main pipe 32 actually comprises three pipes connected in parallel, each such pipe being about 2 metres in diameter to give a total cross-sectional area of about 9.4 square metres. The dimensions of the pipes interconnecting individual tanks to the schematic transverse main pipe 32 are such that the figure 9.4 square metres represents the minimum total cross-sectional area of the conduit means (with all three transverse main pipes in use) through which ballast water can flow under gravity from side to side of the vessel. Similarly, the schematic longitudinal main pipes 23P and 23S cornprise four pipes, two in each of the hulls 2 and 3, each about 1.5 m in diameter to give a total crosssectional area of about 7.1 square metres. The di- 7 GB 2 163 115 A 7 mensions of the pipes interconnecting individual tanks to the schematic longitudinal main pipes 23P and 23S are such that the figure of 7.1 square metres will represent the minimum cross-sectional -area of the conduit means (with all four longitudi nal main pipes in use) through which ballast water can flow under gravity from one end of the vessel to the. other.

It will commonly not be required to effect with out interruption the maximum possible transfer of ballast under gravity between respective schematic tanks at opposite sides or ends of the vessel, but the transfer of 1,250 tonnes of ballast from one side of the vessel to the other can, with the dimen 15. sions of pipes mentioned above and suitable loca tion of the tanks involved, be effected readily in not more than one minute, for example, in thirty seconds.

Although the instantaneous rate of transfer of load to the vessel will depend on the sea state in which the vessel is operating, if that factor is ne glected (or if the vessel is assumed to be operating in calm water), the maximum rate of transfer of load to the vessel by the cranes is well defined be cause in practice cranes have a clearly specified maximum lifting rate. Thus, the minimum time in which a load of 900 tonnes (that is to say, 10% of 9,000,tonnes) can be transferred to the vessel is well defined. Similarly, the maximum change in the moments applied to the vessel as a result of the transfer to the vessel of 900 tonnes in a load handling operation can readily be calculated and it will be found that, with sensible design and given the dimensions mentioned by way of example above ' substantially the whole of such a change in moments can be compensated for by flows of bal last water under gravity between suitably chosen ones of the said schematic tanks in a time no longer than the aforesaid minimum time required to transfer a load of 900 tonnes to the vessel. 105

Claims

1. A vessel having tanks for liquid ballast, which tanks are spaced apart vertically and horizontally, conduit means for permitting ballast to flow under gravity from an upper tank to a lower tank spaced apart horizontally from the said upper tank to influence the heel and/or trim of the vessel, and valve means for preventing such flows.

2. A vessel as claimed in claim 1 for moving loads, wherein the said tanks and conduit means are so arranged that the said flows of ballast under gravity can counteract, at least in part, changes in the effect of moments acting on the vessel on the heel and trim of the vessel arising from load-handling operations.

3. A vessel as claimed in claim 2, comprising means for lifting and/or lowering loads relative to the vessel.

4. A vessel as claimed in claim 3, which is a crane vessel, the or a crane of which is the or a said lifting and/or lowering means.

5. A vessel as claimed in claim 4, wherein the said tanks and conduit means are so arranged and 130 of such dimensions that the said flows of ballast under gravity can produce a change of moments opposite to any change of moments that can be produced by a crane of the vessel taking up a load at a rate substantially equal to the maximum rate of change of moments that can be produced by that crane taking up its maximum load at its maxi mum rate, assuming no significant wave motion in the ambient water.

6. A vessel as claimed in claim 5 that has more than one crane and wherein the said flows of bal last under gravity can produce a rate of change of moments opposite and substantially equal to any rate of change of moments that can be produced by all of the cranes of the vessel acting together to take up a single load at the maximum rate of operation of the cranes, assuming no significant wave motion in the ambient water.

7. A vessel as claimed in any one of claims 2 to 6, wherein the said tanks are sufficiently large that the said flows of ballast under gravity can counteract at least 5% of the maximum change in the said moments caused by transferring to the vessel any single load that the vessel is capable of handling at one time.

8. A vessel as claimed in claim 7, wherein the said tanks are sufficiently large that the said flows of ballast under gravity can counteract at least 10% of the maximum change in the said moments caused by transferring to the vessel any single load that the vessel is capable of handling at one time.

9. A vessel as claimed in any one of claims 2 to 8, wherein the said tanks and conduit means are so arranged and of such dimensions that a change in the moments acting on the vessel that is substantially opposite and substantially equal to a significant part of the maximum ch ange in the said moments caused by the transfer to the vessel of any single load that the vessel is capable of handling at one time can be effected by the said flows of ballast under gravity in a time comparable with the shortest period of time within which the said significant part of the maximum change in the said moments can occur as the result of the transfer of a load to the vessel.

10. A vessel as claimed in claim 9 when dependent upon claim 7 or claim 8, wherein the said tanks and conduit means are so arranged and of such dimensions that a change in the moments acting on the vessel that is substantially opposite to and equal to 5% of the maximum change in the moments caused by the maximum single load that the vessel is capable of handling at any one time can be effected in a time less than or substantially equal to the time in which 5% of the maximum load that the vessel is capable of handling at any one time can be transferred to the vessel in normal use. 125

11. A vessel as claimed in any one of claims 1 to 10, wherein the said flows of ballast under gravity can take place in a period of not more than five minutes.

12. A vessel as claimed in claim 11, wherein the said flows of ballast under gravity can take place in 8 GB 2 163 115 A 8 a period of not more than one minute.

13. A vessel as claimed in any one of claims 1 to 12, wherein the arrangement and dimensions of the said tanks and the said conduit means are such that a quantity of ballast equal to at least 500 tonnes of ballast can be transferred under gravity from a tank or tanks situated at or towards the stern of the vessel to a tank or tanks situated at or towards the bow of the vessel, or vice versa, with _ out the need for any contemporaneous pumped transfer of ballast into or out of any of the tanks in question.

14. A vessel as claimed in any one of claims 1 to 13, wherein the arrangement and dimensions of the said tanks and the said conduit means are such 80 that a quantity of ballast equal to at least 1,000 tonnes of ballast can be transferred under gravity from a tank or tanks situuated at or towards the port side of the vessel to a tank or tanks situated at or towards the starboard side of the vessel, or vice versa, without the need for any contemporaneous pumped transfer of ballast into or out of any of the tanks in question.

15. Avessel as claimed in anyone of claims 1 to 12, wherein the arrangement and dimensions of 90 the said tanks and the said conduit means are such that a quantity of ballast can be transferred under gravity from a tank or tanks situated on one side of the fore-and-aft centre line of the vessel at or to wards the stern of the vessel to a tank or tanks sit95 uated on the same side of the said centre line and at or towards the bow of the vessel or in the re verse direction, and that an additional quantity of ballast can be transferred under gravity from a tank or tanks situated at or towards the port side of 100 the vessel and at or towards either the stern or bow of the vessel to a tank or tanks situated at or towards the starboard side of the vessel and at or towards either the stern or bow of the vessel, re spectively, or in the reverse direction without the 105 need for any contemporaneous pumped transfer of ballast into or out of any of the tanks in question, the said quantity of ballast and the said additional quantity of ballast each being equal to at least 500 tonnes.

16. Avessel as claimed in anyone of claims 13 to 15, wherein the or each said quantity of ballast is equal to at least 1,000 tonnes.

17. A vessel as claimed in any one of claims 1 to 16, wherein the said conduit means is arranged to permit the said flows of ballast under gravity to take place without the flowing ballast passing through any means for pumping ballast.

18 A vessel as claimed in any one of claims 1 to 17, wherein the minimum total cross-sectional area of the said conduit means through which a said flow of ballast under gravity can occur from one or more said upper tanks to one or more said lower tanks spaced apart horizontally from their re spective said upper tanks in a given direction is at least 0.5 square metre.

19. A vessel as claimed in claim 18, wherein the said minimum total cross-sectional area is at least 1.0 square metre.

20. A vessel as claimed in claim 19, wherein the 130 said minimum total cross-sectional area is at least 3.0 square metres.

21. A vessel as claimed in any one of claims 1 to 20, which has at least two pairs of tanks, each pair consisting of an upper tank and a lower tank spaced apart horizontally and connected by said conduit means through said valve means.

22. A vessel as claimpd in claim 21, wherein the said upper tanks of the said two pairs of tanks or of two of the said pairs of tanks are spaced apart horizontally in substantially opposite directions from their respective lower tanks.

23. A vessel as claimed in claim 22, wherein the upper tank of each of the two said pairs of tanks is substantially vertically above the lower tank of the other of the two said pairs.

24. A vessel as claimed in claim 22 or claim 23, wherein a portion of the conduit means connecting the tanks of each of the two said pairs of tanks is common to both of those pairs.

25. A vessel as claimed in any one of claims 21 to 24, wherein the said upper tanks of the said two pairs of tanks or of two of the said pairs of tanks are spaced apart horizontally in generally perpendicular directions from their respective lower tanks.

26. A vessel as claimed in claim 25, including a tank that is a said tank of both of the said two pairs of tanks so that ballast can flow to or from it in either of the said directions.

27. A vessel as claimed both in any one of claims 22 to 24 and in claim 25 or claim 26, which comprises at least four said pairs of tanks the upper tanks of which or of four pairs of which are so spaced apart horizontally from their respective lower tanks in different directions that ballast can be permitted to flow under gravity from one or more upper tanks to their respective lower tanks in such a manner as to produce a movement, as seen in plan view, ill the position of the centre of mass of the ballast in any horizontal direction. -

28. A vessel as claimed in claim 27, wherein the upper tanks of the said four pairs of tanks are spaced apart from their respective lower tanks towards the bow, towards the stern, to port, and to starboard, respectively.

29. A vessel as claimed in claim 28, which has said upper and lower tanks on both sides of the longitudinal centre line of the vessel towards the bow and towards the stern each said upper tank being connected by said conduit means to a lower tank on the other side of the vessel at the same end andlor to a lower tank on the same side of the vessel at the other end.

30. A vessel as claimed in claim 29, wherein at least one said upper tank is connected by-said con- duit means to at least one said lower tank at the other end on the same side of the vessel and to at least one said lower tank on the other side at the same end of the vessel. 125

31. A vessel as claimed both in claim 30 and in claim 3 or claim 4, wherein the or a said lifting andlor lowering means is on the same side of the vessel at the same end as the said one upper tank.

32. A vessel as claimed in any one of claims 29 to 31, wherein at least one said lower tank is con- 9 GB 2 163 115 A 9 nected by said conduit means to at least one said upper tank at the other end on the same side of the vessel and to at least one said upper tank on the other side at the same end of the vessel.

33. A vessel as claimed both in claim 32 and in claim 3 or claim 4, wherein the or a said lifting and/or lowering means is on the same side of the vessel at the same end as the said one lower tank.

34. A vessel as claimed both in claim 30 or claim 31 and in claim 32 or claim 33, wherein the said upper and lower tanks on each side at the stern of the vessel are connected to the said lower and upper tanks, respectively, at the other side of the stern and at the same side of the bow.

35. A vessel as claimed both in claims 34 and in claim 3 or claim 4, which comprises said means for lifting and/or lowering loads on both sides of the stern of the vessel.

36. A vessel as claimed in any one of claims 21 to 35, which comprises means for pumping ballast from at least one said lower tank to an upper tank that is nearer to that lower tank than is any upper tank that is connected to that lower tank by said conduit means.

37. A vessel as claimed in claim 36, wherein at 90 least some said upper tanks are substantially verti cally above said lower tanks and wherein pumping means is provided for pumping ballast upwards from a said lower tank to a said upper tank verti- cally above the said lower tank.

38. A vessel as claimed in any one of claims 1 to 37, which comprises pumping means arranged both to transfer ballast from at least one said lower tank to at least one said upper tank and to transfer ballast horizontally between ballast tanks other than the said upper and lower tanks.

39. A vessel as claimed in any one of claims 1 to 38, wherein at least one said tank comprises a plurality of compartments and the conduit means and valve means are so arranged that ballast can 105 be permitted to flow and prevented from flowing to or from different compartments independently.

40. A vessel as claimed both in claim 39 and in claim 26 or any one of claims 30 to 35, wherein at least one said tank that is a tank of two pairs of tanks comprises a plurality of compartments some of which so communicate with the said conduit means that ballast can flow to or from them in only one of thesaid directions.

41. A vessel as claimed in any one of claims 1 115 to 40, arranged to be used with sea water as the said ballast.

42. A vessel as claimed in any one of claims 1 to 41, comprising means for lifting and/or lowering loads, means for measuring the load of the lifting 120 and/or lowering means, and means for regulating the said flows of ballast in response to an output from the measuring means.

43. A vessel as claimed in claim 42, wherein the measuring means is arranged to measure the ten- 125 sion in the falls of a crane.

44. A vessel as claimed in any one of claims 1 to 42, which is a semi-submersible vessel.

45. A vessel as claimed in claim 44, which com prises a plurality of elongate hulls that extend gen- erally horizontally and generally parallel to one another to provide buoyancy, and that in an operating condition of the vessel are submerged.

46. A vessel as claimed in claim 45, which com- prises two such hulls.

47. A vessel as claimed in claim 45 or claim 46, which comprises at least two columns extending substantially vertically upwardly from each hull to support an operating platform above the water- plane.

48. A vessel substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

49. A method of operating a vessel as claimed in any one of claims 1 to 48, which comprises in- fluencing the heel and/or trim of the vessel by permitting ballast to flow under gravity from a said upper tank to a said lower tank spaced apart therefrom horizontally. 85

50. A method as claimed in claim 49, which comprises permitting the said ballast to flow in a horizontal direction such that the influence of that flow tends to counteract another influence that is tending to change, or has just changed, or is expected to change, the heel and/or trim of the vessel.

51. A method as claimed in claim 50, wherein the said other influence is a change in the moment exerted on the vessel by a load being handled. 95

52. A method as claimed in claim 51, wherein the said load is being lifted off or set down onto a support by lifting means mounted on the vessel, which comprises permitting ballast to flow at a rate sufficient to compensate for at least a substan- tial part of the instantaneous rate of transfer of the weight of the load during the most rapid portion of the transfer.

53. A method as claimed in claim 52, wherein the load is being lifted off, or set down on, a support by lifting and/or lowering means mounted on the vessel, which comprises measuring the force from the load acting on the lifting and/or lowering means and regulating the said flows of ballast in response to the measurement.

54. A method as claimed in claim 53, wherein the lifting and/or lowering means is a crane and which comprises measuring directly or indirectly the tension in the falls of the crane.

55. A method as claimed in claim 51, wherein the load is being moved horizontally by means mounted on the vessel, which comprises permitting the ballast to flow at a rate sufficient to compensate for at least a substantial part of the instantaneous rate of change of the moment of the weight of the load.

56. A method as claimed in any one of claims 49 to 55, of operating a vessel as claimed in any one of claims 1 to 48, which includes pumping ballast directly from a said lower tank to said upper tank vertically above the said lower tank.

57. Any method as claimed in any one of claims 49 to 56 and substantially as hereinbefore described with reference to the accompanying drawings.

Printed in the UK for HMSO, D8818935, 12185, 7102. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.