GB2169570A

GB2169570A - Improvements in and relating to vessels

Info

Publication number: GB2169570A
Application number: GB08431410A
Authority: GB
Inventors: Lee Franklin Rathbun; David Jack Wiffen; Lars-Olof Liberg
Original assignee: Brown & Root Const; GOETAVERKEN ARENDAL AB; Gotaverken Arendal AB
Current assignee: Brown & Root Const; GOETAVERKEN ARENDAL AB; Gotaverken Arendal AB
Priority date: 1984-12-12
Filing date: 1984-12-12
Publication date: 1986-07-16
Also published as: GB2169570B; GB8431410D0

Abstract

A semi-submersible crane vessel has ballast tanks 9 on both sides of the stern, below cranes 6, containing sea-water ballast and compressed air, and ballast tanks 15 and 16 below the operating water line on both sides of the stern and the bow. Ballast can be expelled from either or both of the tanks 9 by the pressure of the compressed air and/or admitted to any or all of the tanks 15 and 16 to compensate at least in part for changes in the heel, trim, and/or draught of the vessel as the load on the cranes 6 changes. <IMAGE>

Description

SPECIFICATION Improvements in 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 vessels.

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 provision of one or more cranes on a vessel, it is convenient 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 position 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 alter the mean draught of the vessel. The magnitude of those alterations for a given load-handling operation will depend on the size and design of the vessel, but it is in general larger when, as is discussed below, the vessel is a semi-submersible vessel at operating draught, as compared with a conventional vessel of the same size.

The changes in heel, trim, and draught can, given time, all be corrected by means of a conventional 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 vessel is to be maintained level within acceptable limits. Further, if, for example, a large load is being lifted from a cargo barge, it is desirable that the lifting operation 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 lifting resulting from, for example, wave motion.Accordingly, 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 applied to the vessel must be corrected quickly and it is impracticable to do that using a conventional ballast system.

In order to overcome that problem, so-called "rapid ballast systems" have been proposed.

The invention provides a vessel having a tank for water ballast, the said tank having inlet and outlet means through which water can flow between the interior of the tank and the ambient water in which the vessel is operating, valve means for preventing and controlling the outflow of ballast water from the tank, and means for enabling the free surface of the ballast water in the said tank to be exposed to air at a superatmospheric pressure to urge ballast water to flow from the tank to the ambient water when the said valve means is open.

When it is desired to discharge ballast water rapidly from such a tank, the appropriate valve means for the tank in question is opened and the superatmospheric pressure acting on the free surface of the water in the tank can be arranged, with a suitable choice of the relevant parameters, to impart to the ballast water in the tank the acceleration necessary to achieve quickly a desired rate of out'low of water from the tank at the instant in question, and the rate of outflow can thereafter be controlled within certain limits by operation of the valve means.

The vessel may be a vessel for moving loads, wherein the said tank is so situated that an outflow of ballast water from the tank can counteract, at least in part, changes in the heel and trim of the vessel arising from load-handling operations and, at least when the load on the vessel is increasing, changes in the draught of the vessel also, and the invention provides a method of operating such a vessel which comprises so allowing ballast water to flow out of a said tank in which the free surface of the ballast water is exposed to air at a superatmospheric pressure that the resulting reduction in the weight of water in the tank tends to counteract a change in the moment exerted on the vessel by a load being handled.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. In that case, the effects arising from the acceleration of a crane hook when picking up a load may be substantial and precise compensation of the resulting change in the moment applied to the vessel may not be possible, because the acceleration that can be imparted to the water may not be large enough. Precise compensation is, however, desirable rather than necessary.

The plan area of the tank is preferably large in order to increase the volume of water expelled for a given change in the water-level, that is to say, for a given decrease in the hydrostatic head assisting the flow of water out of the tank, or a given increase in a hydrostatic head opposing the flow of water.

The air at a superatmospheric pressure may be contained in an upper part of the tank or, at least in part, in a separate air reservoir which may be adjacent to the tank or connected to it by conduit means. Further, it is possible to use a relatively low volume of air at a relatively high pressure or a relatively large volume of air at a relatively low pressure.

Two possible arrangements are advantageous. In one such arrangement, the air at a superatmospheric pressure is at a relatively low pressure and is contained in an upper part of the tank. In the other such arrangement, a separate air reservoir is provided, the air in the air reservoir is at a rela tively high pressure and there is provided a pressure-reducing valve between the air reservoir and the tank.

The use of a separate reservior for air at a relatively high pressure has the advantage that only a relatively small volume is needed and that the tank can be considerably smaller than would otherwise be the case. If the reservoir is large enough and the maximum operating pressure in it is high enough then enough air can be stored in it to empty the tank of water more than once without the reservoir's having to be recharged, which makes possible a very rapid sequence of operations. The disadvantages of that arrangement are that the reservoir has to be charged by means of air compressors, which is inefficient of power, and that the reservoir is heavy and expensive because of the need to contain the high pressures involved.

In that arrangement there is preferably provided a compressor arranged to deliver air to the reservoir at a superatmospheric pressure, which is advantageously not more than 35, and preferably not more than 20, atmospheres gauge. Pumping means to pump ambient water into the tank through the inlet and outlet means is preferably provided.

Containing the air at superatmospheric pressure in an upper part of the tank has the advantages that the construction is simpler and cheaper, because neither a reservoir nor a reducing valve is needed, and that the air can be compressed, at least to some extent, by sealing the tank and pumping water into it, which is comparatively effi cient of power.

The means for enabling the free surface of ballast water in the tank to be subjected to air at a superatmospheric pressure may comprise a compressor (and the provision of a compressor will in general be necessary to make up for air losses, for example, losses resulting from dissolution of air in the water), but the said means preferably comprises pump means arranged to pump ambient water into the tank through the inlet and outlet means against the pressure of air in the tank and/or in an associated air reservoir. Filling, or partially filling, the tank with water by pumping the water in against the air pressure is more efficient than venting the tank, allowing ambient water to flow into the tank and then compressing the air.

That is because water pumps are more efficient than air compresssors. Accordingly, when operating the vessel, it is advantageous, especially in this case, to stop the de-ballasting of a tank before any substantial escape of air occurs. That can be achieved either by a suitable choice of the initial pressures and volumes or by suitable operation of the valve means.

The invention provides a method of operating such a vessel which comprises sealing the upper part of the tank off from the external atmosphere and pumping water into the tank in order to compress air in the upper part of the tank. The upper part of the tank may contain air at a superatmospheric pressure before the pumping of water begins, and the method may then comprise compressing air into the upper part of the tank by means of a compressor before pumping water into the tank. Where the vessel has more than one such ballast tank the method may comprise permitting air to flow from a tank into which water is being pumped to another tank so that the air in both tanks is compressed. Such a method may be used for charging or recharging a ballast tank from which water is discharged during a load-handling operation.

The disadvantage of containing the air at a superatmospheric pressure in an upper part of the tank is that, because the pressure cannot be as high as in a separate reservoir, the tank must be substantially larger than the volume of water, and in operation the volume of compressed air may be at least 25%, and is preferably at least 50%, of the volume of water before ballast water is allowed to flow out of the tank in association with a load-handling operation.

The air pressure to which the free surface of the ballast water in the tank is in use exposed advantageously does not exceed 6 atmospheres gauge, and preferably does not exceed 4 atmospheres gauge. In that way, the cost of construction is reduced as compared with that required when higher pressures are used.

The quantity of air which is under a superatmospheric pressure is advantageously sufficient to fill the whole volume of the tank at zero gauge pressure and ambient pressure and, where the bottom of the tank is below the operating waterline of the vessel, is preferably sufficient to fill the whole volume of the tank at a pressure equal to the ambient pressure in the water at the level of the bottom of the tank. Advantageously the outflow of water stops before the ballast water is entirely expelled from the tank and any associated conduit means communicating between the interior of the tank and the ambient water, so that (dissolved air apart) there is no substantial escape of air from the tank to the ambient water.

If pumping means is provided, the inlet and outlet means may comprise inlet means through which the pumping means is arranged to pump ambient water into the tank and outlet means separate from the inlet means through which ballast water is arranged to flow from the tank to the ambient water when the said valve means is open.

The tank is preferably provided with a relief valve arranged to permit air or water to escape if the pressure in the tank exceeds, say, 110% of its maximum permitted operating pressure. The relief valve may comprise a water-filled column open at the top and exposed to the pressure in the tank at the bottom, the height of the column determining the pressure at which air can escape through it.

Advantageously, in oder to enable moments resulting from a variety of load-handling operations to be at least partly compensated for, the vessel is provided with a plurality of said tanks for liquid ballast, each with associated inlet and outlet means, associated valve means, and associated means for enabling the free surface of ballast water in the additional tank or tanks to be exposed to air at a superatmospheric pressure. When that is done, there is preferably provided conduit means for enabling air to flow from one of the said tanks to another or from an air reservoir associated with one such tank to an air reservoir associated with another such tank.In that way, it is possible to make available for the expulsion of ballast water from a single tank air associated with more than one such tank, with the result that there is a decrease in the rate of fall of the air pressure as water is expelled from the tank. Instead, the total initial volume of air may be reduced provided that the tanks connected together never all need to be emptied in the course of a single sequence of operations too short to allow for recharging of the tanks.

The said tank or tanks and the said inlet and outlet means may be so arranged and of such dimensions that the said flows of ballast from the tank or tanks can produce a change of moments opposite to any change of moments 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 maximum rate, assuming no significant wave motion in the ambient water. If the vessel has more than one crane, the said flows of ballast urged by the air pressure can advantageously 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 maxi mum rate of operation of the cranes, assuming no significant wave motion in the ambient water.

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

The said tanks and inlet and outlet means may be so arranged and of such dimensions that a change in the moments acting on the vessel that is opposite and substantially equal to a significant part, advantageously at least 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 urged by the air pressure 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.

The said flows of ballast urged by the air pres sure can advantageously take place in a period of not more than five minutes, preferably not more than one minute.

The arrangement and dimensions of the said tanks and the said inlet and outlet means may be such that a quantity of ballast equal to at least 500 tonnes, advantageously at least 1,000 tonnes, of ballast can be discharged from a tank or tanks situ ated at or towards the stern of the vessel or from a tank or tanks situated at or towards the bow of the vessel without the need for any contemporaneous pumped transfer of ballast into any of the tanks in question, and may be such that a quantity of ballast equal to at least 1,000 tonnes of ballast can be discharged from a tank or tanks situated at or towards the port side of the vessel or from a tank or tanks situated at or towards the starboard side of the vessel without the need for any contemporaneous pumped transfer of ballast into any of the tanks in question.

The vessel may be a semi-submersible vessel, that is to say, a vessel which comprises an operating platform or the like, a support structure for the platform, a buoyancy portion from which the support structure extends upwardly, and means for introducing liquid ballast into the buoyancy portion to submerge the vessel to such a depth that the waterplane intersects the support structure, when the vessel is in an operating condition, and to expel liquid ballast from the buoyancy portion so that the waterplane intersects the buoyancy portion, when the vessel is in a transit condition to facilitate moving the vessel from one place to another, the horizontal cross-sectional area of the support structure at the water-line when the vessel is in its said operating condition being less than the horizontal cross-sectional area of the buoyancy portion at the water-line when the vessel is in its said transit condition. The buoyancy portion of the vessel then advantageously comprises a plurality of elongate hulls, preferably two such hulls, which extend generally horizontally and generally parallel to one another, and the support structure than advantageously comprises at least two columns extending generally vertically upwards from each hull to the platform.

The inlet and outlet means may comprise conduit means through which ballast water can flow from the or each tank to the ambient water when the valve means is open and which at least when the vessel is in an operating condition opens to the ambient water below the water-line. The conduit means advantageously comprises a pipe extending downwardly from the or a said tank through which ballast water can flow from that tank when the valve means is open. The minimum total crosssectional area of the said conduit means through which a said flow of ballast can occur from one or more said tanks located towards a given end or a given side of the vessel may be at least 2 square metres, and is advantageously at least 10, and preferably at least 15, square metres.

The upper end of the or a said conduit means may comprise an arcuate duct opening downwards into the conduit means at one end and opening out into the tank, advantageously in a direction radially outwards from the conduit means and obliquely downwards, at the other end. A plurality of said arcuate ducts preferably radiate from the axis of a said conduit means. The valve means advantageously comprises a valve, preferably, a gate valve, arranged to close off the or each said arcuate duct. Where the valve means comprises a valve arranged to close off each of a plurality of arcuate ducts radiating from a conduit means, those valves are preferably so arranged that they can be opened and closed independently.

The outlet pipe may open out through the side or the bottom of the vessel. If the vessel is a semisubmersible vessel, the said conduit means advantageously opens to the exterior of the vessel above the water-line when the vessel is in the said transit condition and below the water-line when the vessel is in the said operating condition. In operation, the initial pressure and volume of air are preferably so selected that when the tank is emptied the water comes to rest with the outlet pipe partly filled with water and partly with air, and preferably with the surface of the water in the pipe below the external water-line.

Advantageously, at least when the vessel is in an operating condition at least part of the or at least one said tank is below the water-line.

Although the provision of such ballast tanks in principle allows the heel and/or trim of the vessel to be kept level or the draught to be kept constant when a load is taken up, it does not in general allow all three of the draught, heel, and trim to be kept completely constant at the same time, and does not allow the vessel to be kept at constant draught when a load is set down.Accordingly, the vessel advantageously further includes at least one ballast tank which is so located within the vessel that, at least when the vessel is in an operating condition, it is situated below the water-line, which is vented to the atmosphere and which is provided with inlet and outlet means through which water can flow between the interior of the tank and the ambient water, valve means for preventing and controlling the flow of ambient water into the tank under its own pressure through the inlet and outlet means, and means for pumping water out of the tank to the ambient water through the inlet and outlet means. The or a said further ballast tank may be the or a first said ballast tank at least part of which is below the water-line at least when the vessel is in an operating condition.

If the vessel comprises means for lifting and/or lowering loads relative to the vessel, the or a said further ballast tank, if the vessel includes one, and/ or at least one first said ballast tank is or are preferably located on the same side of the vessel, at or towards the same end as the or a said lifting and/ or lowering means.

The or a first said ballast tank may be vertically above the or a said further ballast tank, and means is then advantageously provided for pumping ballast upwards from a said further tank to a first said tank vertically above the said further tank.

When the vessel has a first said ballast tank on the same side, at the same end, as lifting and/or lowering means, it advantageously also comprises another said further ballast tank remote from the said lifting and/or lowering means.

The vessel may have said lifting and/or lowering means on both sides of the stern and then advantageously has first said ballast tanks on both sides towards the stern and said further ballast tanks on both sides towards the stern and towards the bow.

Assuming that all of those ballast tanks, and their respective inlet and outlet means, are sufficiently large, it is then usually possible to compensate completely for changes in heel, trim, and draught when picking up a load and to compensate at least partially when setting down a load. Thus, when a load is being picked up by one of the lifting and/or lowering means over the stern or over its respective side of the vessel, ballast may be discharged from the first said ballast tank towards that side of the stern and admitted to one or more of the further ballast tanks towards the other side and/or towards the bow. Provided that the difference between the weights of ballast discharged and admitted is equal to the load taken up, the draught of the vessel is unchanged.With suitable ratios of the weights of ballast discharged and admitted at various positions, the moment exerted by the load can simultaneously be cancelled out and the vessel maintained at level heel and trim.

At least one said ballast tank advantageously comprises a plurality of compartments, and the inlet and outlet means and valve means for that tank are then preferably so arranged that ballast can be permitted to flow and prevented from flowing to or from different compartments independently.

If the vessel comprises means for lifting and/or lowering loads, it may also comprise means for measuring the load on the lifting and/or lowering means, which may be means for measuring 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.

One form of semi-submersible crane vessel constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a starboard side elevation view of the vessel; Figure 2 is a stern elevation view of the vessel; Figure 3 is a top plan view of the vessel taken in cross-section at the operating water-line; Figure 4 is a top plan view of the vessel taken in cross-section at the transit water-line; Figure 5 is a more detailed view of part of the vessel as seen in Figure 3, to a larger scale than Figure 3; Figure 6 is a side elevation view in cross-section through a dump chute and the associated valves; Figure 7 is a more detailed side elevation view of a valve similar to those shown in Figure 6, to a larger scale than Figure 6; Figure 8 is a front elevation view of the valve shown in Figure 7;; Figure 9 is a more detailed view of part of the vessel as seen in Figure 4, to a larger scale than Figure 4; and Figure 10 is a side elevation view of the part of the vessel shown in Figure 9.

Referring to the drawings, and initially to Figures 1 to 4, the vessel comprises a working platform 1 that is above water level in all normal operating conditions of the vessel and is supported on port and starboard hulls 2P and 2S, respectively, by columns 3P and 3S at the stern, 4P and 4S amidships, and 5P and 5S towards the bow. In normal operation, in transit the vessel is floated with the hulls 2P and 2S having freeboard, and when the vessel is being used to lift heavy loads and when a stable working platform is required the vessel is brought to a semi-submerged condition with the waterplane intersecting the columns 3 to 5. Cross-braces (not shown) are provided where necessary between the hulls 2P and 2S and between the columns 3P and 5P and 3S to 5S.

Two cranes 6P and 6S are provided on the working platform 1 over the stern columns 3P and 3S, respectively. The cranes are so positioned there that the crane 6P can reach over the port side of the vessel, the crane 6S can reach over the starboard side of the vessel, and both cranes can reach over the stern of the vessel, and so that much of the load on the cranes is transmitted directly to the columns 3P and 3S and thence to the hulls 2P and 2S. A superstructure 7, containing crew accommodation, control quarters, and the like, and provided with a helicopter landing pad 8, is provided towards the bow end of the platform.

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).

Tanks for sea-water ballast indicated generally by the reference numerals 9P and 9S are provided in the stern columns 3P and 3S, respectively, partly above and partly below the semi-submerged water level of the vessel. The upper parts of the tanks 9P and 9S are connected by compressed-air pipes 10P and 10S through valves 11P and 11S to each other and to compressors 12. The lower parts of the tanks 9P and 9S are connected by water pipes 13P and 13S to pumps 14P and 14S that draw water from outside the vessel. The walls of the tanks 9P and 9S are reinforced to withstand an operating pressure of 3 to 4 atmospheres gauge, and the pumps 14 are capable of delivering water into the tanks against that pressure.

The tanks 9P and 9S are provided with safety valves (not shown) to prevent the pressure within the tanks from rising above a safe level. Each safety valve consists of a vertical column of which the upper end is open to the atmosphere and the lower end is immersed in a bath of water. The surface of the water in the bath is exposed to an airspace that is in free communication with, and thus is at the same pressure as the air in, the associated ballast tank, and as that pressure is raised above ambient pressure it forces water from the bath up the column. When the gauge pressure in the tank exceeds a certain level, determined by the height of the column, the water flows over the top of the column.If the air pressure in the tank remains high, it will expel the water from the bath down to the bottom of the column and the water from the column, and air from the tank will then be able to escape freely to the exterior. If, for example, the pumps 14 delivering water to the tank in question continue to operate, in time all or most of the air will be expelled from the tank, the safety valve will be flooded, and water will then be expelled from the column of the safty valve.The column, the bath, and any conduit means connecting the tank to the air-space above the surface of the bath are sufficiently large that even if the pumps operate at their maximum rate the pressure in the tank will not exceed the maximum pressure that the tank is arranged to contain safely, and in this example the heights of the columns are such that each safety valve will operate at a pressure that is 110% of the maximum operating pressure of its respective tank.

Below the ballast tanks 9P and 9S, and in the bows of the hulls 2P and 2S, are other ballast tanks indicated generally by the reference numerals 15P, 15S, 16P, and 16S, respectively, which are permanently vented to the atmosphere at the top, and are provided with inlets indicated generally by the reference numerals 17P, 17S, 18P, and 18S, respectively, that communicate with the ambient water outside the hulls. The ballast tanks 15 and 16 are also connected by water pipes 19P, 19S, 20P, and 20S to the inlets of the pumps 14P and 14S and/or of pumps 21P, 21S, 22P, and 22S discharging to the exterior.

Referring now to Figure 5, it will be seen that the ballast tank 9P in fact consists of four separate compartments 23 to 26. As is shown in Figure 9 and in less detail in Figure 3 and 4, the tanks 9P, 15P, and 15S are similarly subdivided, and the tanks 16P and 16S may also be similarly subdivided, and it will be appreciated that each of the schematic pipes, valves, and pumps 10 to 14, 17, 19, and 21 and where appropriate also the valves 18, pipes 20, and pumps 22 shown in Figures 1 and 4 in fact represents a plurality of pipes or valves, each of the compartments of each tank having its own pipes and valves, in order to provide the greatest possible control over the operation of the tanks.The water pumps 14P and 14S are also duplicated, and are preferably arranged so that each pump can serve any of a plurality of the tanks 23 to 26 and each tank can be served by more than one pump, and the pumps 20 and 22 are similarly duplicated. Dividing the tanks into smaller compartments also simplifies the construction of the vessel and reduces the changes in moments that can arise in operation from the ballast water in a partly-filled tank moving around within the tank.

Referring now to Figure 6, each of the compartments 23 to 26, and each of the compartments of the tank 9S, has extending through its bottom a vertical dump chute 27. Within the compartment the walls 28 of the chute spread out and the chute is capped by a roughly conical cap 29 defining with the walls 28 of the chute a duct that extends radially outwards from the chute 27 and arches over so that its outer end opens downwardly and outwardly into the interior of the compartment. The duct is divided by radially and vertically extending partitions 30 into equal sectorial ducts 31, each occupying an arc of 45 . Above the cap 29 of each chute 27 is a watertight chamber 32 that contains air and is accessible from outside the tank through an access tunnel 33.

Referring now to Figures 6 to 8, each of the sectorial ducts 31 is provided with a gate valve indicated generally by the reference numeral 34. Each gate valve 34 comprises a gate 35 that is generally rectangular in front view and is tapered in side view, with the narrowest part at the top. The duct 31 is provided with corresponding tapering seating 36 on its side walls and slots 37 in its top and bottom walls so arranged that if the gate 35 is urged upwards into a position spanning the duct 31 it seats with a wedge action against the seatings 36 and the edges of the slots 37. Instead, the gate 35 may have shoulders that seat against the edge portions of the walls 28 and 29 of the duct 31 defining the slots 37. In either case the gate 35 must, when closed, form a seal that is watertight under pressure differences across it of several atmospheres.Below the duct 31 there may be provided a housing 38 that contains and protects the gate 35 when it is lowered out of the duct and guide rails 39 for the gate are then provided within the housing 38, as shown in Figures 7 and 8. Instead, as shown in Figure 6, the housing 38 may be omitted and the guide rails 39 are then exposed. The top of the gate 35 is connected to a piston rod 40 that passes through a gland 41 into the watertight chamber 32, where it is the piston rod of a hydraulic cylinder 42 that acts to raise and lower the gate.

A pipe 43 smaller than the ducts 31 and provided with a valve 44 opens at its upper end through the bottom of the tank and at its lower end into the chute 27 below the valves 44. The pipes 43 and valves 44 make it possible to drain the tanks 23 to 26 completely or to discharge water from them at a rate lower than can be accurately controlled by the gate valves 34.

Below the bottom of the tank 9P or 9S, the chute 27 opens out through the side of the vessel through a butterfly valve (not shown). The butterfly valve serves to blank off the mouth of the chute 27, to reduce drag in transit and to prevent weather damage to the chute or the inner valves, and to enable the chute to be drained for inspection and maintenance of the valves and ducts 29 to 44. When the chute 27 is in use, the butterfly valve is permanently open. For ease of maintenance, the butterfly valves are preferably in the sides of the vessel above the transit water level.

Referring now to Figures 9 and 10, the lower stern tanks 15P is divided, as was mentioned above, into separate tanks or compartments 45 to 50, each of which is provided with three water inlets 17P. The pipes 19P and pumps 20P, which may be of conventional construction, have been omitted from Figures 9 and 10 in the interests of clarity.

Each of the inlets 17P consists of a pipe 51 opening horizontally through the side of the hull 2P or of the stern column 4P at, just above, or just below the transit water-line. Inlets above the transit water-line are easier to maintain, but may admit water more slowly because of the smaller head of water outside them, at least when the water surface inside the tank is below the inlets.

Each of the pipes 51 passes into the vessel through an air-filled space which may be a nearby compartment 52 that is not a ballast tank or may be a chamber 53 specially provided. Within the airfilled space 52 or 53 each pipe 51 is provided with two valves 54 either of which can close the pipe off completely, greatly reducing the risk that a valve failure will result in one of the tanks flooding uncontrollably.

The inner ends of the pipes 51 open out into their respective tanks 45 to 50 through frusto-conical diffusers 55 which serve to reduce viscous losses and which may have their axes either horizontal, continuing the general line of the respective pipe 51, or vertical with their mouths downwards and a 90-degree curve in the pipe. It is preferred in general to have the diffusers 55 mouth-downwards if they are sufficiently far above the bottom of the tank and horizontal if they are near the bottom of the tank, but other orientations may be used instead.

In addition to the tanks 23 to 26, other pressurisable ballast tanks 56 may be provided that do not have outlet ducts 31 but do have outlet pipes 43 and can thus be used for the discharge of large quantities of ballast at a fairly slow rate. One tank 56 is shown in Figure 5 with its outlet pipe 43 opening into the chute 27 of a neighbouring tank 9, but the outlet pipe of a tank 56 may lead out to the ambient water by any suitable route. The curved outer wall of tanks 26 and 56 may form part of a right-circular cyldindrical drum bearing the crane 6P, and that drum may pass through tanks 23, 24, and 25 as a perforated wall presenting considerable resistance to the flow of water through it.In that case, the bulk of the water in the parts of tanks 23, 24, and 25 outside the drum may not effectively be available for rapid discharge through the chutes 27 because it cannot pass through the perforations in the drum quickly enough, but the volume of compressed air above that water may be available as a reservoir of air to help expel the water within the drum rapidly. In those circumstances, the water outside the drum will be available for a subsequent, slower discharge under the residual air pressure.

The operation of the ballast system described above is as follows: To prepare the vessel for operation, the lower ballast tanks 15 and 16 are emptied (or kept empty) while the upper tanks 9 are filled about three-quarters full of water by the water pumps 14 and charged to a pressure of from 3 to 4 atmospheres gauge by the air compressors 12, or are charged to a lower pressure when empty and further pressurised by the reduction in the volume of air as the tanks are filled with water. It is preferred to use the water pumps rather than the air compressors as far as possible, because air compressors are less efficient than water pumps.

If the valves 34 of one or more compartments of a tank 9 are opened, the water in that tank will flow out of the vessel through the chute or chutes 27 under the pressure of the compressed air in the top of the tank, rapidly reducing the weight of the part of the vessel where that tank is situated and causing it to tend to rise in the water. If the valves 11 in the compressed air pipes 10 connecting the compartments in question with other compart ments or with another tank are open, air will flow through them and the other tank or compartments wil supply compressed air to the compartments that are being emptied, reducing the rate at which the pressure falls as the tank empties.The fall in pressure is not, however, as serious a problem as might at first be supposed, because it is found that the principal obstacle to rapid emptying of ballast tanks is the inertia of the water, and when that has to be overcome, at the beginning of the emptying operation, the pressure is at its highest. Because the force and energy available from the compressed air increase automatically as the size of the tanks is increased, provided that the proportion of airspace and the heights of the compartments are suitably regulated almost any desired weight of water can be discharged, and almost any desired rate of discharge can be achieved, if the tank 9 and the chutes 27 and ducts 31 are made large enough.

If the maximum flows and flow rates are not required, the flow can be reduced by partially or completely closing some or all of the valves 34 in some or all of the compartments of the tank 9. Be cause of the inefficiency of air compressors, it is preferred always to stop the flow of water by clos ing the valves 34 while they are still underwater, or at least while the chutes 27 are still partly full of water, or so to select the initial volume and pres sure of air that the flow of water will of its own ac cord stop while the chutes 27 are still partly full of water. There is then very little loss of air (although some is always lost by, for example, dissolution into the water) and the system can be reset almost entirely by pumping of water into the tank.Closing the valves 34 while they are still underwater can give a more precise end to the discharge than al lowing the tank to drain completely, and thus im proved control over the system, but allowing the tank to empty and the flow to stop of its own accord can produce a very smooth end to the dis charge. In order to avoid trouble from cavitation and hammer if the valves 34 are closed rapidly, air is allowed into the chute 27 through an air pipe (not shown) at a controlled rate to bring the water in the chute to a halt smoothly by establishing a controlled partial vacuum in the upper part of the chute.

If, on the other hand, some or all of the valves 54 in some or all of the compartments of a tank 15 or 16 are opened, then ambient water will flow into that tank, increasing the weight, or reducing the buoyancy, of the part of the vessel in which that tank is situated and causing it to tend to sink.

If, for example, water is discharged from the tanks 9 at the stern and admitted to the tanks 16 at the bow then the trim of the vessel will be altered, with the stern tending to rise and the bow tending to sink. Depending on the amounts of water dis charged and admitted, the mean draught of the vessel may increase, decrease, or stay the same.

The operation of the ballast system described to regulate the heel and trim of the vessel when a load is lifted off a cargo barge on the starboard side of the vessel by the crane 6S and set down again behind the vessel will now be described by way of example.

As the load is taken up by the crane 6S the weight of the load will tend to cause the heel and trim of the vessel to alter with the stern and the starboard side sinking and the bow and the port side rising. The first part of the transfer of the weight of the load can take place relatively slowly, with the vessel being maintained level and at constant draught by pumping ballast into ballast tanks towards the bow and towards the port side and out of ballast tanks towards the stern and towards the starboard side in a conventional manner. In that way at least half of the weight of the load can be transferred safely, depending on the sea conditions, at a rate that is limited primarily by the capacity of the pumps being used.The final lifting of the load off the vessel must, except in completely calm conditions, be conducted rapidly in order to limit the risk of a damaging collision between the cargo barge and the load, and during that phase of the operation ballast water is discharged rapidly by opening the valves 34 of the tank 9S, permitting ballast water to be expelled from that tank by the air pressure above it, and is taken on rapidly by opening the valves 17P in the tank 15P, optionally the valve 18S in the tank 16S, and perhaps also the valves 18P in the tank 16P, in order to keep the vessel level and at constant draught.

As the crane 6S is slewed to bring the load round from the side to the stern of the vessel, the moment of the load influencing the heel of the vessel will decrease but the moment influencing the trim of the vessel will increase. It is therefore necessary to lighten the stern and the port side, and make the bow and the starboard side heavier, in order to keep the vessel level and at constant draught, and that can be largely achieved by discharging ballast water from the tank 9P and admitting water to the tank 16S, possibly with adjustments by use of the tanks 9S and 1 6P or by use of the tank 1 5P and the discharge of ballast water at the port bow, although it is preferred to provide other means for controlling the heel and trim of the vessel while slewing and, because slewing is an operation that can be conducted comparatively slowly, a useful contribution can be made by, for example, pumping ballast water between conventional ballast tanks at different locations on the vessel. If water is discharged from the tank 9P during slewing, it may be discharged through the pipes 43 and the valves 44 instead of through the larger ducts 31; instead, water may be discharged slowly from tanks 56.

In order to set down the load astern of the vessel, a large compensation for trim, and a smaller compensation for heel, will be needed than when the load was to one side of the vessel. The initial phase of setting down is conducted rapidly, with water being admitted to the tank 15S to compensate for the reduction in the load on the crane 6S.

It will be appreciated that it is not in general possible to maintain both the trim and the draught of the vessel constant solely by admitting water to the tank 15S, and an additional compensation may be provided by discharging ballast from ballast tanks (not shown) towards the bow and/or by transferring ballast from the bow towards the stern. Any necessary compensation to the heel of the vessel may be made by using the tanks 9 and 15 or by other means, for example, in the same way as when slewing, if the corrections needed are small enough. Up to half of the weight of the load may be transferred in that way until the load is securely in contact with the cargo barge, or whatever else it is being set down onto, and the rest of the transfer can then be carried out slowly, with ballast being pumped to compensate.

The operation of the rapid ballast system may be controlled automatically in response to the load on the hook of the crane 6P or 6S, in combination with the orientation of the crane and its boom. 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 over-riding of the automatic control system may be provided for.

In order to reset the ballast system described in preparation for another lifting operation, it is necessary to restore by means of the air compressors 12 any air that has been lost from the tanks 9P and 9S, and to pump water into the tanks 9 and out of the tanks 15 and 16 to reverse any changes made when using the rapid ballast system. It is preferred, when that is practical from an operational point of view, to refill the tanks 9, whether by means of the air compressors 12 or by means of the pumps 14, sufficiently slowly that the air in the tanks is compressed substantially isothermally, as that requires less energy than rapid compression followed by cooling.

It is not necessary to complete a lifting and lowering operation before resetting the ballast system.

If an upper ballast tank 9P or 9S is empty or partly empty and the lower tank 15P or 15S, respectively, directly below it is full or partly full, then water may be pumped into the former and out of the latter whenever the necessary pumps and pipes are available, although in any other case recharging during a load-handling operation will usually be acceptable only if ballast can be pumped into, out of, or between, other ballast tanks to compensate for any undesired effects on the heel, trim, or draught of the vessel.

If any exceptionally heavy large load is to be handled, it may be lifted by the two cranes 6P and 6S acting together over the stern of the vessel. If the centre of gravity of the load is on the fore-andaft center-line of the vessel then there will be no effect on the heel of the vessel but large trim compensations will be needed and if the capacity of the tanks 9, 15, and 16 becomes a limiting factor than the change in the effect on the vessel of a larger proportion of the weight of the load than the half mentioned above may be compensated for by pumping ballast using the conventional ballast system provided that the sea is sufficiently calm.

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 3 to 5 are each about 23 metres tall measured from the tops of the hulls 2 to the underside of the platform 1. The operating draught may be varied between about 22 metres and about 28 metres according to circumstances. Each of the cranes 6P and 6S has a lifting capacity of about 3,500 tonnes at 40 metres radius when it is free to revolve and a maximum lifting capacity of about 4,500 tonnes when its boom extends over the stern and the crane is tied back.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 schematic tanks 15P and 15S may each have a capacity of about 10,000m3, and the schematic tanks 16P and 16S of 2,000m3 each. The chutes 27 may each have a diameter of about 2.75 metres, giving a total cross-sectional area of about 6m2 for each of the chutes, and each of the gates 30 may be about 1 metre square. Thus, with the arrangement shown in Figure 5, three of the chutes 27 will have effective cross-sectional areas of about 6m2 while the chute in tank 23 will have an effective cross-sectional area of only 5m2 because it has only five gates 30, giving a total effective crosssectional area of rather over 20m2 for each tank 9.

The schematic tanks 9P and 9S may each have a capacity of about 12,000m3, and the compartments of the tanks 9 may be about 17.5 metres deep. The tanks 9 may in operation be filled up to about 11 metres deep with water each of them and then contains about 7,500m3 of water, the rest of the tank being filled with about 4,500m3 of air at a pressure of about 3.5 atmospheres gauge. Thus, initially, the volume of air in the tank is about 60% of the volume of water in the tank. About 5,300m3 of the water in each of the tanks 9 may be available for rapid discharge through the chutes 27, and a total of about 4,600m3 on each side of the vessel, including water in the tanks 9 that cannot be discharged rapidly through the chutes 27 and water in other tanks that do not have chutes 27, may be available for slower discharge only.The pressure will then drop to about 1/2 atmosphere gauge if the compartment is completely emptied of water without any air being supplied from other tanks or compartments, which is no more than is needed to match the pressure-head that arises because the bottoms of the tanks 9 are below the ambient water-level when the vessel is in its semi-submerged condition. The initial pressure may be varied to adjust the final pressure to take into account the exact draught at which the vessel is operating on any particular occasion.

If lifting a load of 3,500 tonnes over the stern, about 2,700 tonnes of ballast may be discharged from the tanks 9 during the rapid phase of the lift, depending on the exact position of the load, compensating substantially completely for a change in the moments acting on the vessel corresponding to 50% of the weight of the load. In lifting a load of 9,000 tonnes, one third of the weight of the load might be taken up during the rapid phase of the lift, with a total of about 4600 tonnes of ballast being discharged rapidly from the tanks 9P and 9S to compensate. Each of the cranes 6P and 6S would require about 20 seconds to take up the last 1,750 tonnes of the weight of the load (half of 3,500 tonnes for one crane or about a third of 9,000 tonnes for both cranes together) and 2,700 tonnes of water could be discharged from each tank 9 within that period of time.In slewing a crane carrying the 3,500 tonne load through 245 (the practical maximum), up to about 9,200 tonnes of ballast may be discharged, compensating for all of the changes in moments caused by movement of the load, although not all of that would be from the tanks 9.

As a variation on the arrangement described above, some or all of the compartments of the schematic tanks 9 may be replaced by tanks that when full do not have a large volume of air above them but are in communication through pressurereducing valves with reservoirs that are arranged to contain air at a pressure higher than the tanks 9 themselves could safely contain. For the vessel having a displacement of about 150,000 tonnes mentioned above, for example, the reservoirs might contain air at a pressure of about 17 atmospheres gauge, which the reducing valves would supply to the tanks at about 3.5 atmospheres gauge. For a tank 9 containing about 7,500m3 of water a reservoir volume of about 1,100m3 would be sufficient, compared with the 4,500m3 of air needed for the other arrangement.

As a variation on the arrangements described above, instead of separate tanks 9 and 15 being provided, some or all of the compartments of the tanks 9 may be positioned wholly or largely below the operating waterline and may in operation be vented to the atmosphere and allowed to flood with water flowing up through the chutes 27. The sectorial ducts 31 then act in the same way as the diffusers 55 described above, although their shape may be modified if that is desired. The advantage of this variation is that the total ballast-tank volume and the number of valves and the like is reduced, with a corresponding saving in capital costs. The principal disadvantage is that when the tanks are flooded the air above them cannot be compressed because it would quickly attain, if it did not initially have, a pressure that would seriously hinder the flow of water into the tanks, so that the system is less efficient in energy than those previously described and has to be reset completely with pumps and compressors after being emptied and then flooded once. That means in practice that the vessel can pick up one load and set that load down again and must then spend a comparatively long time resetting its ballast system. This variation is therefore preferred for smaller vessels than the one shown in the drawings, for which such a sequence of operations is more often acceptable.

Claims

1. A vessel having a tank for water ballast, the said tank having inlet and outlet means through which water can flow between the interior of the tank and the ambient water in which the vessel is operating, valve means for preventing and control ling the outflow of ballast water from the tank, and means for enabling the free surface of the ballast water in the said tank to be exposed to air at a superatmospheric pressure to urge ballast water to flow from the tank to the ambient water when the said valve means is open.

2. A vessel as claimed in claim 1 for moving loads, wherein the said tank is so situated that an outflow of ballast water from the tank can counteract, at least in part, changes in 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 any one of claims 1 to 4, wherein the arrangement is such that the air at a superamospheric pressure is contained in an upper part of the tank.

6. A vessel as claimed in claim 5, wherein the means enabling the free surface of ballast water in the tank to be subjected to air at a superatmosphere pressure comprises a compressor.

7. A vessel as claimed in claim 6, wherein the compressor is arranged to deliver compressed air to the tank at a pressure not exceeding 6 atmosphere gauge.

8. A vessel as claimed in claim 7, wherein the compressor is arranged to deliver compressed air to the tank at a pressure not exceeding 4 atmospheres gauge.

9. A vessel as claimed in any one of claims 5 to 8, wherein the means enabling the free surface of ballast water in the tank to be subjected to air at a superatmospheric pressure comprises pump means arranged to pump ambient water into the tank through the inlet and outlet means against the pressure of air in the tank.

10. A vessel as claimed in claim 9, wherein the pump means is arranged to pump water into the tank at a pressure not exceeding 6 atmospheres gauge.

11. A vessel as claimed in claim 10, wherein the pump means is arranged to pump water into the tank at a pressure not exceeding 4 atmospheres gauge.

12. A vessel as claimed in any one of claims 1 to 4, wherein the means enabling the free surface of the ballast water in the tank to be subjected to air at a superatmospheric pressure comprises a reservoir for air at a superatmospheric pressure, which reservoir is in communication with the interior of the upper part of the tank, and a pressureregulating valve interposed between the reservoir and the tank.

13. A vessel as claimed in claim 12, wherein there is provided a compressor arranged to deliver air to the reservoir at a superatmospheric pressure.

14. A vessel as claimed in claim 13, wherein the compressor is arranged to deliver compressed air at a pressure not exceeding 35 atmospheres gauge.

15. A vessel as claimed in claim 14, wherein the compressor is arranged to deliver compressed air at a pressure not exceeding 20 atmospheres gauge.

16. A vessel as claimed in any one of claims 12 to 15, wherein the pressure-regulating valve is arranged to deliver air from the reservoir to the tank at a pressure not exceeding 6 atmospheres gauge.

17. A vessel as claimed in claim 16, wherein the pressure-regulating valve is arranged to deliver air from the reservoir to the tank at a pressure not exceeding 4 atmospheres gauge.

18. A vessel as claimed in any one of claims 12 to 17, wherein there is provided pumping means arranged to pump ambient water into the tank through the inlet and outlet means.

19. A vessel as claimed in claim 9 or claim 18, wherein the inlet and outlet means comprises inlet means through which the pumping means is arranged to pump ambient water into the tank and outlet means separate from the inlet means through which ballast water is arranged to flow from the tank to the ambient water when the said valve means is open.

20. A vessel as claimed in any one of claims 1 to 19, wherein there are provided a plurality of said tanks for liquid ballast, each with associated inlet and outlet means, associated valve means, and associated means for enabling the free surface of ballast water in the additional tank or tanks to be exposed to air at a superatmospheric pressure.

21. A vessel as claimed in claim 20, wherein there is provided conduit means enabling air to flow from one of the said tanks to another or from an air reservoir associated with one such tank to an air reservoir associated with another such tank.

22. A vessel as claimed in claim 4 or in any one of claims 5 to 21 when dependent upon claim 4 wherein the said tank or tanks and inlet and outlet means are so arranged and of such dimensions that the said flows of ballast from the tank or tanks 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 maximum rate, assuming no significant wave motion in the ambient water.

23. A vessel as claimed in claim 22 that has more than one crane and wherein the said flows of ballast urged by the air pressure 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.

24. A vessel as claimed in claim 2 or in any one of claims 3 to 23 when dependent upon claim 2, wherein the said tank is or tanks are sufficiently large that the said flows of ballast 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.

25. A vessel as claimed in claim 24, wherein the said tank is or tanks are sufficiently large that the said flows of ballast 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.

26. A vessel as claimed in claim 2 or in any one of claims 3 to 25 when dependent upon claim 2, wherein the said tank or tanks and inlet and outlet 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 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 urged by the air pressure 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.

27. A vessel as claimed in claim 26 when dependent upon claim 25 or claim 24, wherein the said tank or tanks and inlet and outlet 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.

28. A vessel as claimed in any one of claims 1 to 27, wherein the said flows of ballast urged by the air pressure can take place in a period of not more than five minutes.

29. A vessel as claimed in claim 28, wherein the said flows of ballast urged by the air pressure can take place in a period of not more than one minute.

30. A vessel as claimed in any one of claims 1 to 12, wherein the arrangement and dimensions of the said tank or tanks and the said inlet and outlet means are such that a quantity of ballast equal to at least 500 tonnes of ballast can be discharged from a tank or tanks situated at or towards the stern of the vessel or from a tank or tanks situated at or towards the bow of the vessel without the need for any contemporaneous pumped transfer of ballast into the tank or any of the tanks in question.

31. A vessel as claimed in claim 30, wherein the said quantity of ballast is equal to at least 1,000 tonnes.

32. A vessel as claimed in any one of claims 1 to 31, wherein the arrangement and dimensions of the said tank or tanks and the said conduit means are such that a quantity of ballast equal to at least 1,000 tonnes of ballast can be discharged from a tank or tanks situated at or towards the port side of the vessel or from a tank or tanks situated at or towards the starboard side of the vessel without the need for any contemporaneous pumped transfer of ballast into the tank or any of the tanks in question.

33. A vessel as claimed in any one of claims 1 to 32, wherein the inlet and outlet means comprises conduit means extending downwardly from the or a said tank through which ballast water can flow from the tank to the ambient water when the valve means is open.

34. A vessel as claimed in claim 33, wherein the minimum total cross-sectional area of the said conduit means through which a said flow of ballast can occur from a tank or tanks located towards a given end or a given side of the vessel is at least 2 square metres.

35. A vessel as claimed in claim 34, wherein the said minimum total cross-sectional area is at least 10 square metres.

36. A vessel as claimed in claim 35, wherein the said minimum total cross-sectional area is at least 15 square metres.

37. A vessel as claimed in any one of claims 33 to 36, wherein the upper end of the or a said conduit means comprises an arcuate duct opening downwards into the conduit means at one end and opening into the tank at the other end.

38. A vessel as claimed in claim 37, wherein the said other end of the arcuate duct opens out in a direction radially outwards from the conduit means and obliquely downwards.

39. A vessel as claimed in claim 37 or claim 38, wherein a plurality of said arcuate ducts radiate from the axis of a said conduit means.

40. A vessel as claimed in any one of claims 37 to 39, wherein the valve means comprises a valve arranged to close off the or each said arcuate duct.

41. A vessel as claimed in claim 40, wherein the or each said valve is a gate valve.

42. A vessel as claimed in claim 40 or claim 41 and in claim 39, wherein the valves arranged to close off the ducts radiating from a conduit means are so arranged that they can be opened and closed independently.

43. A vessel as claimed in any one of claims 1 to 42, which comprises an operating platform or the like, a support structure for the platform, a buoyancy portion from which the support structure extends upwardly, and means for introducing liquid ballast into the buoyancy portion to submerge the vessel to such a depth that the waterplane intersects the support structure, when the vessel is in an operating condition, and to expel liquid ballast from the buoyancy portion so that the waterplane intersects the buoyancy portion, when the vessel is in a transit condition to facilitate moving the vessel from one place to another, the horizontal cross-sectional area of the support structure at the water-line when the vessel is in its said operating condition being less than the horizontal crosssectional area of the buoyancy portion at the water-line when the vessel is in its said transit condition.

44. A vessel as claimed in claim 43, wherein the buoyancy portion of the vessel comprises a plurality of elongate hulls which extend generally horizontally and generally parallel to one another.

45. A vessel as claimed in claim 44, wherein the buoyancy portion of the vessel comprises two such hulls.

46. A vessel as claimed in claim 44 or claim 45, wherein the support structure comprises at least two columns extending generally vertically upwards from each hull to the platform.

47. A vessel as claimed in any one of claims 1 to 46, wherein the inlet and outlet means comprises conduit means through which ballast water can flow from the or each tank to the ambient water when the valve means is open and which at least when the vessel is in an operating condition opens to the ambient water below the water iine.

48. A vessel as claimed in claim 47, wherein the conduit means comprises a pipe extending downwardly from the or a said tank through which ballast water can flow from that tank when the valve means is open.

49. A vessel as claimed both in claim 47 or claim 48 and in any one of claims 43 to 46, wherein the said conduit means opens to the exterior of the vessel above the water-line when the vessel is in the said transit condition and below the water-line when the vessel is in the said operating condition.

50. A vessel as claimed in any one of claims 1 to 49, wherein at least when the vessel is in an operating condition at least part of the or at least one said tank is below the water-line.

51. A vessel as claimed in any one of claims 1 to 50, which further includes a ballast tank which is so located within the vessel that, at least when the vessel is in an operating condition, it is situated below the water-line, which is vented to the atmosphere, and which is provided with inlet and outlet means through which water can flow between the interior of the tank and the ambient water, valve means for preventing and controlling the flow of ambient water into the tank under its own pressure through the inlet and outlet means, and pump means for pumping water out of the tank to the ambient water through the inlet and outlet means.

52. A vessel as claimed in claim 51, wherein the or a said further ballast tank is the or a first said ballast tank.

53. A vessel as claimed both in claim 51 or claim 52 and in claim 3 or claim 4, wherein the or a said further ballast tank is located on the same side of the vessel, at the same end, as the or a said lifting and/or lowering means.

54. A vessel as claimed in claim 3 or claim 4 or in any one of claims 5 to 53 when dependent upon claim 3 or claim 4, wherein the or a first said ballast tank is located on or towards the same side of the vessel, at the same end, as the or a said lifting and/or lowering means.

55. A vessel as claimed both in claim 53 and in claim 54, wherein the or a first said ballast tank is vertically above the or a said further ballast tank.

56. A vessel as claimed in claim 55, wherein pumping means is provided for pumping ballast upwards from a said further tank to a first said tank vertically above the said further tank.

57. A vessel as claimed in any one of claims 53 to 56, comprising another said further ballast tank remote from the said lifting and/or lowering means.

58. A vessel as claimed in any one of claims 53 to 57, having lifting and/or lowering means on both sides of the stern of the vessel.

59. A vessel as claimed in both claim 57 and claim 58, having first said ballast tanks on both sides towards the stern and said further ballast tanks on both sides towards the stern and towards the bow.

60. A vessel as claimed in any one of claims 1 to 59, wherein at least one said ballast tank comprises a plurality of compartments and the inlet and outlet means and valve means are so arranged that ballast can be permitted to flow and prevented from flowing to or from different compartments independently.

61. A vessel as claimed in any one of claims 1 to 60, comprising means for lifting and/or lowering loads, means for measuring the load on the lifting and/or lowering means, and means for regulating the said flows of ballast in response to an output from the measuring means.

62. A vessel as claimed in claim 61, wherein the measuring means is arranged to measure the tension in the falls of a crane.

63. A semi-submersible crane vessel substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

64. A method of operating a vessel as claimed in claim 2 or in any one of claims 3 to 63 when dependent upon claim 2, which comprises so allowing ballast water to flow out of a first said ballast tank in which the free surface of the ballast is exposed to air at a superatmospheric pressure that the resulting reduction in the weight of water in the tank tends to counteract a change in the moment exerted on the vessel by a load being handled.

65. A method as claimed in claim 64, wherein the pressure of the air to which the free surface of ballast water in the tank is exposed does not exceed 6 atmospheres gauge.

66. A method as claimed in claim 65, wherein the said pressure does not exceed 4 atmospheres gauge.

67. A method as claimed in any one of claims 64 to 66, wherein the vessel is as claimed in any one of claims 5 to 12, and the volume of the air which is under a superatmospheric pressure and to the pressure of which the free surface of ballast water in the tank is exposed is at least 25% of the volume of water in the tank before ballast water is allowed to flow out of the tank in association with a load handling operation.

68. A method as claimed in claim 67, wherein the said volume of air in the tank is at least 50% of the said volume of water in the tank before ballast water is allowed to flow out of the tank in association with a load handling operation.

69. A method as claimed in any one of claims 64 to 68, wherein the quantity of the air which is under a superatmospheric pressure is at least sufficient to fill the whole volume of the tank at zero gauge pressure and ambient temperature.

70. A method as claimed in any one of claims 64 to 69, wherein the outflow of water stops before the ballast water is entirely expelled from the tank and any associated conduit means communicating between the interior of the tank and the ambient water, so that (dissolved air apart) there is no substantial escape of air from the tank to the ambient water.

71. A method as claimed in any one of claims 64 to 70, which comprises causing and/or permitting the said ballast so to flow into and/or out of such a tank or such tanks that the influence of that flow or those flows tends to counteract another influence that is tending to change, or has just changed, or is expected to change, the draught, heel, and/or trim of the vessel.

72. A method as claimed in claim 71, wherein the said other influence is a change in the moment and/or force exerted on the vessel by a load being handled.

73. A method as claimed in claim 72, wherein the said load is being lifted off or set down onto a support by lifting means mounted on the vessel, which comprises permitting the ballast to flow at a rate sufficient to compensate for at least substantial part of the instantaneous rate of transfer of the weight of the load during the most rapid portion of the transfer.

74. A method as claimed in claim 73, 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.

75. A method as claimed in claim 74, 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.

76. A method as claimed in claim 75, 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.

77. A method as claimed in any one of claims 64 to 76, of operating a vessel as claimed in claim 51, or in any one of claims 52 to 62 when dependent upon claim 51, or in claim 63, which includes pumping ballast directly from a said further tank to a said first tank vertically above the said further tank.

78. A method of operating a vessel as claimed in any one of claims 9 to 11, or in any one of claims 19 to 62 when dependent upon any one of claims 9 to 11, or in claim 63, which comprises sealing the upper part of the tank off from the external atmosphere and pumping water into the tank in order to compress air in the upper part of the tank.

79. A method as claimed in claim 78, wherein the upper part of the tank contains air at a superatmospheric pressure before the pumping of water beg ins.

80. A method as claimed in claim 79, which comprises compressing air into the upper part of the tank by means of a compressor before pumping water into the tank.

81. A method as claimed in any one of claims 78 to 80 of operating a vessel as claim 21, which comprises permitting air to flow from a tank into which water is being pumped to another tank so that the air in both tanks is compressed.

82. A method as claimed in any one of claims 64 to 77, which comprises charging or recharging a first said ballast tank by a method as claimed in any one of claims 78 to 81.

83. Any method as claimed in any one of claims 64 to 82 and substantially as hereinbefore described with reference to the accompanying drawings.