GB2169859A

GB2169859A - Semi-submersible lifting vessels

Info

Publication number: GB2169859A
Application number: GB08529180A
Authority: GB
Inventors: Keith Nicholas Henry; Peter Hayes Britton Mitchell; Richard Oliver Basil French
Original assignee: Brown and Root Inc
Current assignee: Brown and Root Inc
Priority date: 1984-12-03
Filing date: 1985-11-27
Publication date: 1986-07-23
Also published as: GB8529180D0; GB2169859B

Abstract

A semi-submersible lifting vessel has a lower hull 3, an upper hull 1, a support structure 3a and 3b that extends upwardly from the lower hull to supply the upper hull, and means on the upper hull for raising and/or lowering loads. The underside of a portion 1a and 1b of the upper hull 1 near to the lifting means is lower than the underside of the remainder of the hull 1c. In a first semi- submerged position the waterplane, at a level w1, intersects the support structure 3a and 3b and the waterplane area of the vessel is less than the horizontal cross-sectional area of the lower hull 3. In a second semi-submerged position lower than the first the waterplane, at a level w2, intersects the said portion 1a and 1b of the upper hull 1 so that the waterplane area is greater than in the first semi-submerged position. <IMAGE>

Description

SPECIFICATION Improvements in and relating to semi-submersible lifting vessels The invention relates to a semi-submersible lifting vessels, that is to say, a vessel which provides support for means for raising and/or lowering loads, which operates in a semi-submerged state, and which in operation is subject to changes in moment as a result of the lifting and/or setting down of loads by such means.

In a typical lifting vessel, the means for raising and/or lowering loads will comprise a crane located at, for example, a position adjacent to the stern of the vessel. The lifting of an external load by the crane and the setting down of such a load will each tend to alter the heel and/or trim of the vessel in addition to the mean draught of the vessel. These alterations can, given time, be corrected by pumping ballast into or out of suitably located tanks.In the case of heavy loads, however, (say, 1% to 10% of the displacement of the vessel) the moment applied to the vessel will be large and not only will large transfers of ballast be necessary, but they will also generally be required to take place rapidly since it is desirable that at least the part of a lifting operation that extends from shortly before the load separates from, for example, a cargo barge to shortly after separation occurs, and the corresponding part of a setting-down operation, should be accomplished quickly. Socalled "Rapid Ballast Systems" have been proposed to dealt with this problem.

The invention provides a semi-submersible lifting vessel which comprises a lower hull, an upper hull having means for raising and/or lowering loads, the configuration of the underside of the upper hull being such that when the vessel has level heel and trim the underside of a portion of the upper hull in the vicinity of the means for raising and/or lowering loads is lower than the underside of the remainder of the upper hull, and a support structure for the upper hull which extends upwardly from the lower hull, the configuration of the upper hull and the support structure being such that, in a first semi-submerged position in which the waterplane intersects the support structure, the waterplane area of the vessel is less than the horizontal cross-sectional area of the lower hull, and in a second semi-suDmerged position, lower than the first, the waterplane contacts or intersects the said portion of the upper hull so that the waterplane area of the vessel is greater than it is in the first position.

When such a semi-submersible lifting vessel is moved from one place to another, the lower hull has freeboard and the support structure and upper hull are entirely clear of the water line. When the vessel reaches the location at which it is to operate, water is admitted into tanks in the lower hull, and if necessary into tanks in the support structure, until the lower hull is completely submerged, the waterplane intersects the support structure and the vessel is in the first semi-submerged position.In the first semi-submerged position, the waterplane area of the vessel is generally less than the horizontal cross-sectional area of the lower hull to such an extent that the naturai period of oscillation of the vessel in rotation about any horizontal axis and also in translation in the vertical direction becomes longer than the usual periods of the waves in the sort of seas in which these vessels are operated, and the coupling between the motion of the vessel and waves in the ambient water is decreased. That is highly desirable for the efficient and effective operation of the means for raising and/or lowering loads in normal sea conditions, that is to say, when the sea is not completely calm.The waterplane area of the vessel should not, however, be decreased to such an extent that the restoring force on the vessel, which tends to resist an increase in heel and/or trim, is unacceptably low.

When raising and/or lowering an exceptionally heavy load, the vessel can be further submerged to its second position so that the restoring force tending to resist an increase in heel and/or trim is increased. That will entail a corresponding reduction in the natural periods of oscillation of the vessel, but even leaving aside other, favourable factors which are discussed below, since operations involving such loads are usually only undertaken in relatively calm conditions, the advantage gained in operating with the increased restoring force outweighs the disadvantage of the reduction in the natural periods of the vessel.

With the vessel of the invention it is possible to reduce or even substantially eliminate the rapid flows of ballast needed to correct for changes in heel and/or trim in certain circumstances. Thus, the vessel will not as a rule be maintained in its first semi-submerged position when handling exceptionally heavy loads so that very large rapid transfers of ballast will not generally be required when operating in the first semi-submerged position.

When exceptionally heavy loads are being handled the vessel can be submerged to its second semisubmerged position and the additional restoring force then available will reduce the magnitude of rapid transfers of ballast as compared with what would be required if the vessel were in the first semi-submerged position.

The said portion of the upper hull that, in the second semi-submerged position of the vessel, is contacted or intersected by the waterplane may be a continuous portion in the vicinity of the means for raising and/or lowering loads. Alternatively, the said portion of the upper hull may consist of two or more, but, preferably, two, discrete sub-portions, one of which is in the vicinity of the means for raising and/or lowering loads. The configuration of the sub-portions may be such that they provide increases in the waterplane area in the second semi-suhmerged position that are substantially equal to each other.Alternatively, the configuration of the sub-portions may be such that the sub-portion in the vicinity of the means for raising and/or lowering loads provides a greater increase in the waterplane area in the second semi-submerged position than that provided by the other sub-portion.

The fact that the vessel according to the invention has, in its second semi-submerged position, an increased waterplane area can have one or more further important advantages, which can most conveniently be explained by considering the case where the raising and/or lowering means is a crane situated in the vicinity of the stern and of one side of the vessel and it is used to raise a load from a conventional cargo barge situated down-wave of the vessel.

The boom of the crane will extend over the side or stern of the vessel and the tip of the boom (taken as the point from which the hook of the crane depends) will move when the heel and/or trim of the vessel changes, either because of the vessel's response to waves in the ambient water or because of the application of a moment to the vessel result ing from a load-handling operation.

A conventional cargo barge has a much worse motion response than a semi-suhmersible vessel according to the invention, that is to say, for a given sea state the amplitude of the oscillation of the barge resulting from waves is much greater than the amplitude of oscillation of the vessel induced by the waves.

When a load is to be lifted from a barge, the slings connecting the load to the crane hook are initially completely slack. when, by operation of the crane, the slack is partially taken up, relative movement between the boom tip and the barge tends to cause snatching in the slings. Further operation of the crane draws the slings taut and then progressively increases the tension in them with the result that a moment is applied to the vessel. Above a certain value of the tension, which depends on the sea state, there is no longer any risk of snatching, but relative movement between the boom tip and the cargo barge results in variations in the tension in the slings and in consequent variations in the moment applied to the vessel. Immediately after the load has been lifted clear of the cargo barge, such relative movement can cause damaging impacts between the load and the cargo barge.

It will be appreciated from what has been said above that relative movement between the boom tip and the cargo barge is highly undesirable. Further, since the phase relationship between movements of the' boom tip and movements of the cargo barge will vary according to circumstances, it is desirable to reduce the movement of the boom tip and to reduce the movement of the cargo barge.

It has already been explained that, when the vessel is in its second semi-suhmerged position, the increased waterplane area results in an increased restoring force tending to oppose an increase in heel and/or trim resulting from a moment applied to the vessel as a result of a load-handling operation, and that in itself results in a decrease in the movement of the boom tip in response to a given moment.A further advantage of a vessel according to the invention in which at least the greater part of the increase in waterplane area when the vessel is in its second semi-submerged position is in the vicinity of the raising and/or lowering means, is that the point about which the vessel tends to rotate in response to a moment arising from a load-handling operation moves in a direction having a component towards the raising and/or lowering means and so, in the example under consideration, in a direction having a component towards the boom tip.

The movement of the point of rotation of the vessel causes the moment applied to the vessel by a given load handled by the crane to be reduced, and so the increase in heel and/or trim caused by a given load to be reduced. Another, but less important, advantage of the movement of the point of rotation of the vessel is that, for a given change in heel and/or trim, the linear distance through which the boom tip moves is reduced.

Because of the change in natural periods referred to above, it might be thought that, throughout a load-handling operation of the kind under discussion, the movement of the vessel would be materially worse in its second semi-submerged position than in its first semisubmerged position, but it has been found that that is not always the case. It has now been found that, when the slings are taut, so that there is no snatching, but the load is still on the cargo barge, the rolling and/or pitching of the vessel as a result of waves is often determined not primarily by the motion response of the vessel itself, but mainly by the motion response of the cargo barge, the barge "driving" the vessel through the slings and the crane. That is especially likely to be true when the conditions are relatively calm, and it is usually only in those conditions that the lifting of an exceptionally heavy load will be feasible.

When, as postulated above, the cargo barge is situated down-wave of the vessel, the vessel in its second semi-submerged position exerts a materially greater sheltering effect on the barge than it would do if it were in its first semi-submerged position and the resultant reduction in wave amplitude at the cargo barge reduces the movements of the cargo barge, which in turn reduces the motion of the vessel resulting from waveaction. This sheltering effect is obtained irrespective of whether the greater part of the increase in waterplane area in the second semi-submerged position is in the vicinity of the crane or not.Thus, as compared with a conventional semi-submersible vessel (or the vessel of the invention in its first semi-submerged position), the apparent motion response of the vessel (that is to say, the motion response of the vessel as supplemented by the "driving" effect of the cargo barge) is actually considerably improved.

A further factor is that the deterioration in the motion response of the vessel that would be expected to follow from the increase in waterplane area when the vessel is brought to its second semi-submerged position is mitigated by the fact that the draught of the vessel is then deep as compared with the draught of the vessel in its first semi-submerged position. The increased draught improves the motion response for a number of reasons. The increased draught implies an increased displacement which in turn implies an increase in the mass, and hence an increase in the inertia of the vessel. The effective inertia is further increased by an increase in the added mass.

In the case of the vertical component of the added mass, the increase stems from the increase in the depth of the surfaces of the lower hulls that have a non-zero area when projected onto a horizontal plane. Because of the increased-depth of the lower hull, the wave action on the lower hull is reduced.

To sum up, in the example under consideration, when the vessel is in its second semisubmerged position so that its waterplane area is increased, and at least the greater part of that increase is in the vicinity of the crane, movements of the boom tip caused by the moment applied to the vessel as a result of the load-handling operation and caused by waves in the ambient water are reduced.

Movement of the cargo barge is also reduced.

The advantage of having at least the greater part of the increase in waterplane area when the vessel is in its second semi-submerged position in the vicinity of the raising and/or lowering means has been explained above.

There may, however, be other factors that make it preferable to have an arrangement in which the said portion of the upper hull consists of two or more discrete sub-portions.

Thus, for example, if the vessel is assembled at sea in an operation involving mounting the upper hull, on the support structure by carrying the upper hull on a barge and suitably manoeuvering the barge so that the upper hull can be correctly positioned on the support structure, then the configuration of the underside of the upper hull should generally be such that the region that engages the barge is level, when the completed vessel has level heel and/or trim. This consideration precludes an arrangement in which the barge would engage the upper hull in the region of the boundary between the said portion and the remainder of the upper hull. Furthermore, the manoeuvering of the barge is limited by the configuration of the support structure.If, for example, the support structure comprises four columns extending upwardly from the lower hull, and the upper hull is to be positioned on the columns with one of the columns adjacent to each corner, then the reduction in width of access for the barge between the columns by the provision of the said portion is less if part of the said portion is arranged to occupy regions rendered inaccessible to the barge by the columns themselves. In practice, it may not be found possible to confine the extent of the said portion to the extent of those inaccessible regions. Also, the upper hull will have to be carried on the barge with its centre of mass substantially in line with the centre line of the barge. Accordingly, there is an advantage in not having a large restriction on the width of access to the barge caused by the said portion at one side or end of the vessel.

It will in general be the case that there is a range of different draughts at any of which the vessel can be said to be in its first semisubmerged position. Similarly, there will in general be another range of different draughts at any of which the vessel can be said to be in its second semi-submerged position, and with the vessel in its second semi-submerged position, the waterplane area may be different for different draughts.Whenever, throughout the specification (including the claims), the waterplane areas of the vessel in its first and second semi-submerged positions are compared numerically, the waterplane area of the vessel in its second semi-submerged position is to be taken as that obtaining when the draught of the vessel is such that the clearance (in calm water) between the waterplane and the underside of the said remainder of the upper hull is equal to one half of the vertical separation between the lowermost part of the underside of the said portion and the underside of the said remainder of the upper hull.

The waterplane area of the vessel may be arranged to be at least 25% greater in the second semi-submerged position than in the first semi-submerged position, and is, advantageously, at least 50% greater, preferably, at least 75% greater, in the second semi-submerged position than in the first semi-submerged position. Advantageously, the waterplane area of the vessel is not more than 150% greater in the second semi-submerged position than in the first semi-submerged position. It should be pointed out here that when the last-mentioned condition is satisfied the waterplane area of the vessel in its second semi-submerged position will (with a typical support structure) be very materially less (and the motion response very materially better) than that of a conventional, mono-hull vessel of about the same displacement.As an indication of the resuitant increase in the restoring force on the vessel in the second semisubmerged position as compared with that in the first semi-submerged position in response to an increase in trim resulting from a moment applied to the vessel caused by a load-handling operation (assuming the means for raising and/or lowering loads to be in the vicinity of one end of the vessel), when the waterplane area of the vessel is 100% greater in the second semi-submerged position than in the first semi-submerged position, the restoring force on the vessel may be increased by a factor of about 3.

The means for raising and/or lowering loads will generally be located in the vicinity of one end of the vessel, usually the stern, as will be discussed in more detail hereinafter. Accordingly, when the said portion of the upper hull that, in the second semi-submerged position of the vessel, is contacted or intersected by the waterplane is a continuous portion, it will generally extend from that end of the vessel and, preferably, across substantially the entire beam of the vessel. In such an arrangement, the said portion of the upper hull then serves primarily to reduce changes in trim rather than changes in heel and to reduce the consequence of a given change in trim.

When the means for raising and/or lowering loads is located in the vicinity of one end of the vessel, and the said portion of the upper hull consists of two discrete sub-portions, then one of the sub-portions may extend across the vessel in the vicinity of that end of the vessel and the other sub-portion may extend across the vessel in the vicinity of the opposite end. Advantageously, with such an arrangement, the said one of the sub-portions that extends across the vessel in the vicinity of the means for raising and/or lowering loads provides a greater increase in the water-plane area in the second semi-submerged position than that provided by the said other sub-portion at the opposite end. Preferably, the subportions each extend across substantially the entire beam.

The means for raising and/or lowering loads may, be located in the vicinity of one side of the vessel, in which case one of the subportions may extend along the vessel in the vicinity of its port side and the other subportion extend along the vessel in the vicinity of its starboard side. With this arrangement, the configuration of the sub-portions are advantageously such that they provide increases in the waterplane area in the second semisubmerged position that are substantially equal to each other.

When the means for raising and/or lowering loads is located in the vicinity of one end of the vessel and the said portion of the upper hull or one of the subportions thereof extends across that end, then the said portion or the said sub-portion advantageously comprises a first section, the underside of which is at a uniform level, immediately adjacent to the said means for raising and/or lowering loads and a second section between the first section and the remainder of the upper hull (that is to say, the upper hull excluding the said portion or each of the sub-portions thereof) the underside of the second section being inclined upwardly from the first section to the underside of the remainder of the upper hull.

From a structural point of view, the angle of inclination of the underside of the second section to the underside of the remainder of the upper hull should be as small as possible in order to avoid local stress concentrations in the regions where the undersides of the first and second sections meet and in the region where the underside of the second section meets the underside of the remainder of the upper hull. It has been found that the angle of inclination of the underside of the second section to the underside of the remainder of the upper hull advantageously does not exceed 20 and is preferably from 6G to 12".

When the said portion of the upper hull.

consists of two discrete sub-portions extending along the vessel in the vicinity of the sides, the underside of each subportion advantageously includes a surface that is inclined downwardly away from the underside of the remainder of the upper hull towards the side of the vessel. The angle of inclination of the said surface to the underside of the remainder of the upper hull advantageously does not exceed 20" and is preferably from 15" to 18 .

The provision of an inclined surface as part of the underside of the said portion or a subportion thereof, which is in any case desirable from a structural point of view, means that it is possible for the waterplane area of the vessel in the second semi-suhmerged position of the vessel to be adjusted when the vessel is in operation.

Advantageously, the arrangement is such that the variation in the level of the underside of the upper hull stems from a corresponding variation in the overall depth of the upper hull.

The difference between the lowest level of the underside of the said portion and the level of the underside of the remainder of the upper hull is, advantageously, at least 2m, and, preferably, at least 3m. with suitable design, it is then possible for the vessel to be in its second semi-submerged position and yet for there to be sufficient clearance between the underside of the remainder of the upper hull and the waterplane when the vessel is in the second semisubmerged position to avoid wave contact with the underside of the remainder of the upper hull in relatively calm conditions. The lowest level of the underside of the said portion should also be such that it is substantially clear of storm wave impact when the vessels at its survival draught.

Advantageously, the lower hull comprises a plurality of, preferably, two, elongate pontoons which extend generally horizontally and generally parallel to one another. The support structure then advantageously comprises one or more, preferably, two or more, columns extending upwardly from each pontoon to the upper hull.

Advantageously, the means for raising and/or lowering loads comprises a crane situated in the vicinity of one side of the vessel and in the vicinity of one end of the vessel.

There is, of course, a limit on the weight of a load that can be lifted by a single crane, and the means for raising and/or lowering loads may also comprise a second crane.

when the portion of the upper hull that, in the second semi-submerged position of the vessel, is contacted or intersected by the waterplane, or one of the sub-portions thereof, extends across the vessel in the vicinity of the said one end, the second crane is preferably situated in the vicinity of the other side of the vessel and in the vicinity of the said one end of the vessel.Then, one crane can reach over the port side of the vessel, and the other can reach over the starboard side of the vessel, and both cranes can reach over one end of the vessel, usually the stern. when the said portion of the upper hull consists of two discrete sub-portions, one of which extends along the vessel in the vicinity of its port side and the other of which extends along the vessel in the vicinity of the starboard side, then the second crane is preferably situated in the vicinity of the same side as the first-mentioned crane and in the vicinity of the other end of the vessel.

Each of the cranes may be located on the upper hull directly over a respective column of the support structure so that much of the load on the cranes can be transmitted to the lower hull directly by the columns. Such cranes are not generally used independently at the same time except for lifting small, light loads, and are otherwise generally only operated together when they are operating in conjunction to raise a single exceptionally heavy or large load over the end or side of the vessel in the vicinity of which the cranes are situated. In such a case, the cranes are generally secured mechanically against rotation and may also have their booms tied back if the load is exceptionally heavy.

Accordingly, there is a tendency for one or other of the cranes to be under-utilised, but the ability of the vessel to raise such exceptional loads is important. For example, platforms of marine drilling rigs or similar installations are often of modular construction, the modules being built on land and assembled at sea, a lifting vessel being used to raise each of the modules into position. It is extremely advantageous for the vessel to have the capacity to lift two or more modules, which have been assembled together on land prior to shipment, into position in a single lifting operation. A considerable saving in man-hours during construction at sea can then be achieved since fewer modules have to be interconnected offshore.Accordingly, since construction costs at sea are much greater than those on land, it is possible to achieve a considerable saving in the overall cost of rig construction. Further, the reduction in the time spent on offshore completion work enables the rig to be brought into operation earlier.

When the said portion of the upper hull is a portion of greater depth than the remainder of the upper hull, the construction of at least a part of the said portion of the upper hull is advantageously such as to provide a range of different sites, each of sufficient strength for an A-frame to be mounted on it, the different sites being spaced apart from each other and so situated that an A-frame mounted on any one of at least some of the said sites can cooperate in the lifting of a single load with the said crane or one of the said cranes. The structure of the upper hull will normally comprise a plurality of bulkheads, and each mounting site is then so situated that the foot of each leg of an A-frame mounted on it is located over such a bulkhead.

Advantageously, the means for raising and/or lowering loads comprises an A-frame mounted on one of the said sites.

The provision of an A-frame eliminates the need for two large-capacity cranes and makes possible a substantial saving in the costs of equipping the vessel.

The disadvantage of using an A-frame in the place of a second crane is that it is unable to rotate about a vertical axis relative to the vessel and, although that is not required during the lifting of a heavy load in conjunction with the crane, it does impose limits on the type and scale of its lifting operations. Thus, were an A-frame to be mounted permanently over a single site and used in conjunction with a crane of which the boom had been tied back, there would be only limited scope for variation of the separation between the pick-up points.

That disadvantage is mitigated by the provision of a range of different sites over which an A-frame can be mounted. Advantageously, adjacent mounting sites for an A-frame are spaced apart by not more than 10m, preferably by not more than 7m. Advantageously, adjacent mounting sites for an A-frame are spaced apart by a sub-multiple, preferably, 1/5, 1/4 or 1/3, of the separation of the legs of the A-frame at its base. The vessel may have, secured to the said portion of the upper hull, a mounting for each leg of the A-frame.

Since there is generally a considerable period of notice of a lifting operation involving an exceptionally heavy and/or bulky load, the Aframe can be dismounted and moved to another site on the said portion of the upper hull suitable for any particular load, even if mountings for each leg of the A-frame are welded to the upper hull for, when it is desired to move the position of the A-frame, the mountings may be cut from the upper hull and re-welded in a different position. Alternatively, a series of mountings may be permanently provided on the said portion of the upper hull.

A large capacity crane will normally have a boom that comprises an A-frame and it is then convenient if the A-frame referred to above has the same dimensions as that of the crane boom. The A-frame may lie on the upper hull when not in use, giving an increased clearance envelope for the crane, and it can be raised into its operating position by means of the crane. Furthermore, if not reguired, the A-frame can be dismounted and left ashore allowing increased deck space for cargo-handling or other operations.

In a vessel having a crane and an A-frame located in the vicinity of one end of the vessel, the crane will be positioned in the vicinity of either the port side or the starboard side of the vessel. Advantageously, the vessel is then provided with a second crane, which may be of much smaller capacity than the first crane, located in the vicinity of the other side of the vessel.

Although, as stated above, it is possible with the vessel of the invention to reduce (as compared with a conventional semi-submersible vessel) or even substantially eliminate the need for rapid flows of ballast needed to correct for changes in trim in certain circumstances, the vessel is advantageously provided with a rapid ballast system especially for use in correcting for changes in heel and trim when the vessel is used to handle a substantial load with the vessel in its first semi-submerged position. Further, a rapid ballast system may be useful when handling an unusually heavy load with the vessel in its second semisubmerged position.

The rapid ballast system may, for example, comprise means for discharging liguid ballast rapidly under gravity to the ambient water, means for allowing ambient water to enter ballast tanks rapidly, or means for allowing liquid ballast to flow between ballast tanks spaced apart from each other both horizontally and vertically, or a combination of more than one of those arrangements.

The invention also provides a method of handling a load using a semi-submersible lifting vessel according to the invention, wherein at least a part of the load handling operation is carried out with the vessel in its second semi-submerged position.

When the means for raising and/or lowering loads is a crane and an A-frame, the method may comprise raising and/or lowering a load with the crane in conjunction with the Aframe. When the means for raising and/or lowering loads comprises two dranes, the method may comprise raising and/or lowering a load with the two cranes operating in conjunction with each other. The method is especially advantageous when it involves raising a load from, and/or lowering a load onto, a conventional cargo barge situated down-wave of the vessel.

Four forms of semi-submersible lifting vessel constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic side elevation of the first form of vessel, some of the eguipment of which has been omitted for clarity; Fig. 2 is a diagrammatic rear elevation of the vessel shown in Fig. 1; Fig. 3 is a diagrammatic plan view of the vessel shown in Fig. 1, some of the eguipment of which has been omitted for clarity; Fig. 4 is a diagrammatic plan view of a portion of the deck of the vessel shown in Fig. 1 on a larger scale than Fig. 1; Fig. 5 is a section along the line V-V of Fig.

4; Fig. 6 is a section along the line Vl-Vl of Fig. 4, a part of the deck having been omitted for clarity; Fig. 7 is a diagrammatic side elevation of the second form of vessel; Fig. 8 is a diagrammatic rear elevation of the vessel shown in Fig. 7; Fig. 9 is a diagrammatic plan view of the vessel shown in Fig. 7, some of the eguipment of which has been omitted for clarity; Fig. 10 is a diagrammatic plan view of a portion of the deck of the vessel shown in Fig. 7 on a larger scale than Fig. 7; Fig. 1 11 is a diagrammatic side elevation of the third form of vessel; Fig. 12 is a diagrammatic rear elevation of the vessel shown in Fig. 11; Fig. 13 is a diagrammatic plan view of the vessel shown in Fig. 11; Fig. 14 is a horizontal section taken along the line XlV-XlV of Fig. 11; ; Fig. 15 is a vertical section taken along the line XV-XV of Fig. 13, on a larger scale than Fig. 13, some of the eguipment and parts of the vessel having been omitted for clarity; Fig. 16 is a diagrammatic side elevation of the fourth form of vessel; Fig. 17 is a diagrammatic plan view of the vessel shown in Fig. 16; Fig. 18 is a transverse vertical section taken along the line XVlll-XVlll of Fig. 17; and Fig. 19 is a horizontal section taken along the line XIX-XIX of Fig. 16.

Referring to Figs. 1 to 6 of the drawings, the first form of vessel comprises an upper hull, which is indicated generally by the reference numeral 1, supported on a lower hull comprising two elongate parallel-extending pontoons 2 and 3, respectively, by a support structure comprising two columns, extending upwardly from the bow and the stern of the pontoon 2, only the stern column 2b of which is shown (see Fig. 2), and two columns 3a and 3b, respectively, similarly extending from the pontoon 3. Cross-braces (not shown) are provided between the pontoons 2 and 3, and/or between the port and starboard columns where necessary.

The upper hull 1 is of water-tight construction. The configuration of the underside of the upper hull is such that, when the vessel has level heel and trim, the underside of a portion, which can be regarded as being composed of two sections 1a and 1b, respectively, is lower than the underside of the remainder 1c of the upper hull. Further, since the main deck is level, the portion of the upper hull comprising the sections la and 1b is a portion of increased depth as compared with the depth of the remainder 1c. The portion 1a and 1b extends from the stern of the vessel and across its entire beam.

The upper hull 1 is provided with longitudinal bulkheads 4a and 4b (indicated in broken lines in Fig. 2) which extend over the entire depth and a major part of the length of the upper hull including the deeper portion 1a and 1b. The upper hull 1 is also provided with transverse bulkheads 5a and 5b (the transverse bulkheads being omitted from Figs.

1 and 3 for clarity but being shown in, and described in more detail in connection with, Figs. 4 to 6), which extend over the entire depth and breadth of the upper hull. The greater depth of the transverse bulkheads 5a and 5b and the longitudinal bulkheads 4a and 4b in the section la and (to an extent that isprogressively reduced as the distance from the stern increases) of the longitudinal bulkheads in the section 1 b of the upper hull 1 materially increases the strength of that portion of the upper hull.

The first section la of the deeper portion of the upper hull is of uniform depth as is the shallower remainder 1c. The second section 1 b between the first section 1 a and the remainder 1c has an underside which is inclined upwardly from the first section 1a to the remainder 1 c of the upper hull 1.

A large-capacity crane 6 is mounted on a site 6a on the first section 1a of the deeper portion of the upper hull 1 so that its vertical axis of rotation passes through the stern column 3b and its boom 6b can reach over the starboard side of the vessel and over the stern. An A-frame 7 is mounted adjacent to the stern of the vessel onthe first section 1a of the deeper portion in mountings 8 welded to the upper hull 1. In Fig. 2 the mountings 8 are shown as being so located on the upper hull 1 that the foot of each leg of the A-frame 7 is located over one of the longitudinal bulkheads 4a. The A-frame 7 is so mounted that it can pivot relative to the vessel about an axis extending parallel to the upper hull and at right angles to the longitudinal axis of the vessel.Accordingly, the A-frame 7 can lie, when not in use, on the surface of the upper hull 1 as shown in Fig. 3 or it can be raised by the crane 6 to an operating position as shown in Fig. 1 in which it projects over the stern of the vessel, the A-frame being restrained in that position by a tie 9 secured to the upper hull. The A-frame 7 is provided with a hook 10 on a cable 11 passing through an aperture 7a in the upper portion of the A-frame and controlled by a cable-handling system (not shown) to raise and lower the hook.

A crane 12 of smaller capacity than the crane 6 is positioned on a site 12a on the upper hull 1 on the port side of the vessel so as to extend over the port side.

Towards the bow end of the upper hull 1 is a superstructure 13 that may include crew accomodation, control areas, and the like. The area of the upper hull 1 between the crane 6 and the A-frame 7 on the one hand, and the superstructure 13 on the other hand, may be used for storing or working on cargo or eguipment and includes hatches (not shown).

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

The vessel is provided with a rapid ballast system (not shown) comprising tanks spaced apart from each other both horizontally and vertically, and means for allowing liguid ballast to flow rapidly between the tanks under gravity. The vessel is also provided with a conventional ballast system (not shown) comprising tanks with pumps for transferring ballast from one such tank to another and/or between such a tank and the ambient water.

Referring to Figs. 4 to 6, the upper hull 1 comprises a main deck 14, 'tween decks (only one of which is shown at 15 in Figs. 5 and 6, the others having been omitted for clarity) and a double bottom 16. The decks 14 and 15 are supported by transverse bulkheads (only two of which are shown at 5a and 5b in Figs. 4 to 6, the transverse bulkhead 5a being the aft transverse bulkhead) and longitudinal bulkheads of two kinds, continuous longitudinal bulkheads 4a (see Fig. 5), which extend over a major part of the entire length of the upper hull 1, alternating with partial longitudinal bulkheads 4b (see Fig. 6), which extend only from the aft transverse bulkhead 5a to the adjacent transverse bulkhead 5b, all the bulkheads (except the aft transverse bulkhead 5a) being shown in double broken lines in Fig. 4.The decks 14 and 1 5 are strengthened by transverse and longitudinal girders 1 7 and 18, respectively, (indicated by single broken lines in Fig. 4), a longitudinal girder 18 being positioned between the longitudinal bulkheads of each pair of adjacent longitudinal bulkheads, each such pair being made up o one continuous and one partial longitudinal bulkhead 4a and 4b, respectively.

Girders 18 also extend forward, under the main deck 14 from the forward end of each partial longitudinal bulkhead 4b, and under the 'tween deck 15 beneath each partial longitudinal bulkhead 4b (see Fig. 6). As can also be seen in Figure 6, strengthening web frames 19 forth partial longitudinal bulkheads 4b are provided together with brackets 20. Each of the continuous and partial longitudinal bulkheads 4a and 4b, respectively, is provided with a series of web frames 21 all of which project from one side of the bulkhead and each of which is situated directly below a different one of the transverse girders 17. Secondary stiffening is also provided by angle stiffeners 22 on the aft transverse bulkhead 5a, the decks 14 and 15, and the continuous and partial longitudinal bulkheads 4a and 4b, respectively.

In operation, when the vessel is in transit it is floated until the pontoons 2 and 3 have freeboard, the support structure and the upper hull 1 being entirely clear of the waterline, which is at a level indicated in Fig. 1 by the broken line W. when the vessel reaches the location at which it is to operate, water is admitted into ballast tanks (not shown) until the pontoons are completely submerged and the waterplane is at a level, for example, a level Wl shown in Fig. 1, at which it intersects the columns of the support structure.

The vessel is then in its first semisubmerged position in which the waterplane area of the vessel is much less than that of the vessel when it is at transit draught, that is to say, much less than the total horizontal cross-sectional area of the pontoons 2 and 3, so that the natural period of oscillation of the vessel about any horizontal axis and also in translation in the vertical direction is longer than the usual periods of the waves in the sort of seas in which the vessel is operated, and the coupling between the motion of the vessel and the waves is decreased. The waterplane area of the vessel at the level W1 is not, however, decreased to such an extent that the restoring force on the vessel, which tends to resist an increase in heel and/or trim resulting from a load-handling operation, is unacceptably low.

In this position, the crane 6 can be used to raise and/or lower loads over the starboard side of the vessel or over the stern, the conventional ballast system, and the rapid ballast system, being used as necessary to correct for changes in heel and/or trim of the vessel.

The A-frame 7 can lie on the upper hull 1 so that the crane 6 has greater room for manoeuvring loads and an increased clearance envelope. The crane 12 can be used independently to raise and lower smaller loads on the port side of the vessel.

When it is desired to use the vessel to raise an exceptionally heavy load over the stern of the vessel, the crane 6'is used to lift the A-frame 7 into the position shown in Fig.

1 restrained by the tie 9. The crane 6 is then secured mechanically against rotation, and also tied back if necessary.

The conventional ballast system is used to bring the vessel to a starting draught which is relatively deep but at which the vessel is nevertheless in its first semi-submerged position, that is to say, the waterplane intersects the four columns, but no part of the upper hull.

Then, the crane 6 and the A-frame 7 are so operated as to take up a substantial part, say 50% to 75%, of the load to be lifted, but without causing the load to separate from its support, for example, a cargo barge. During that phase of the lifting operation, which can be carried out relatively slowly, level heel and trim are maintained by operation of the conventional ballast system. At the same time, because of the transfer of weight to the vessel, the draught of the vessel increases until the deeper portion 1 a and 1 b of the upper hull. 1 is in contact with, or intersected by, the waterplane, the starting draught having been so chosen that that result is achieved. The vessel is then in its second semi-submerged position, which may be, for example, with the waterplane at a level W2 as shown in Fig. 1.

For the next phase of the operation, the crane 6 and the A-frame 7 are operated rapidly to effect a clean separation of the load from its support and to raise the load at least to a height at which there is no danger of impacts occurring between the load and its.

support. If, during that phase of operation, it is found necessary to change the ballasting of the vessel in order to keep the heel and trim of the vessel within acceptable limits (say plus or minus half a degree), despite the increased restoring force with the vessel in its second semisubmerged position, then such a change in ballasting must be effected by means of the rapid ballast system. Because the whole of the increase in waterplane area when the vessel is in its second semi-submerged position is beneath the crane 6 and the A-frame 7, however, as explained above, the point about which the vessel tends to rotate in response to a moment arising from the lifting operation moves in a direction having a component to-wards the crane 6 and A-frame 7. The moment applied to the vessel by the load is reduced and conseguently the increase in the heel and/or trim caused by the load is also reduced. In addition, for a given heel and/or trim, the linear distances through which the boom tip of the crane 6 and of the uppermost portion of the A-frame 7 move are reduced.

Furthermore, the vessel in its second semisuhmerged position exerts a materially greater sheitering effect on the cargo barge than in its first semi-submerged position and, as also explained above, this results in a reduction in wave amplitude at the cargo barge and a con sequent reduction in the motion of the vessel from wave-action.

The above description of the method employed to lift an exceptionally heavy load over the stern of the vessel with the crane 6 and the A-frame 7 operating together assumes that, as will usually be the case, the operation is carried out in relatively calm conditions. If, on the other hand, it were to be desired to lift a rather less heavy load in less calm conditions, it might be found preferable to carry out the entire operation with the vessel in its first semi-submerged position in order to take advantage of the better motion response of the vessel in that position as compared with the motion response of the vessel in its second semi-submerged position.In that case, however, a rapid ballast system would generally (depending on the weight of the load) have to be used and the flows of ballast required would be larger than if the final phase of the operation were to be carried out with the vessel in its second semisubmerged position.

That is principally because of the lower restoring force on the vessel in its first semisubmerged position and because, as is explained above, the moment applied to the vessel by a given load will be larger. In addition, in order to avoid impacts between the load and the support, a larger proportion (say about 50%, instead of about 25%) of the total weight of the load may have to be taken up rapidly in view of the less calm conditions.

It will be appreciated that corresponding considerations apply when an exceptionally heavy load is to be lowered over the stern of the vessel using the crane 6 and the A-frame 7 together.

Typically, there is a considerable period of notice of a raising and/or lowering operation involving an exceptionally heavy load to be undertaken by the vessel. In preparation for the operation, if required, the mountings 8 of the A-frame 7 can be cut from the upper hull 1 and re-welded in another position on the section 1 a of the upper hull with the foot of each leg of the A-frame again located over either a continuous or a partial longitudinal bulkhead 4a and 4b, respectively.

As an alternative arrangement, a series of mountings may be permanently arranged on the upper hull 1, each mounting located over either a continuous or a partial longitudinal bulkhead 4a and 4b, respectively, spaced so as to provide a range of mounting sites for the A-frame 7. These mounting sites are spaced apart by a sub-multiple of the separation of the legs of the A-frame 7 at its base.

If the A-frame 7 is not required, it can be dismounted and left ashore allowing increased deck space for cargo-handling or other operations.

The broken line Ws in Fig. 1 indicates the level of the waterplane when the vessel is at its survival draught.

For a vessel having a displacement of about 110,000 tonnes in the first semi-submerged position, the following dimensions, which are given by way of example, are suitable.

The upper hull 1 has a length of about 130m. The section 1a has a depth of about 12m and the remainder 1c of the upper hull 1 a depth of 8m. Thus, the vertical separation between the underside of the section la of the upper hull 1 and the underside of the remainder 1c of the upper hull 1 is 4m and, for the purpose of comparing the waterplane area of the vessel in its second semi-submerged position with the waterplane area of the vessel in its first semi-submerged position, the second semi-submerged position is to be taken as the position of the vessel when its draught is such that the lower surface of the section 1a of the upper hull is immersed to a depth of 2m.

The section 1a has a length of about 32m and the section 1 b a length of about 25m, its underside being inclined upwardly from the section 1a to the remainder 1c of the upper hull 1 at an angle of about 9 . Such dimensions lead to an increase of about 100% in the waterplane area of the vessel between the first and second semi-submerged positions, but will usually leave the waterplane area of the vessel in its second semi-submerged position less than the waterplane area of the vessel when it is at transit draught. The displacement of the vessel in the second semi-submerged position is about 130,000 tonnes.

The crane 6 has a capacity of about 5,000 tonnes, the A-frame 7 a capacity of about 3,000 tonnes, and the crane 12 a capacity of about 800 tonnes.

The crane 6 has a boom that comprises an A-frame, and the A-frame 7 is of the same dimensions as that of the boom of the crane 6. The legs of the A-frame 7 are spaced apart by about 20m at its base, and mounting sites for the A-frame are provided spaced apart by about 5m, because that is the separation between each continuous longitudinal bulkhead 4a and its adjacent partial longitudinal bulkhead 4b.

The level W is equivalent to a draught of 10.5m, and the level W3 (the waterplane level at the survival draught) to a draught of 17m.

The level W1 may lie within a range equivalent to a range of draughts of from 22m to 27.5m, and the level W2 is eguivalent to a draught of 32m. It should be noted that the range of draughts of from 22m to 27.5m is a range of draughts which would be used when the vessel is operated in its first semi-submerged position throughout a load-handling operation. The "starting draught" referred to above would normally lie within the range of from 27.5m to just less than, say, 30m (if 30m is the smallest draught at which the vessel is in its second semi-submerged position).

Referring to Figs. 5 and 7 to 10 of the drawings, the second form of vessel is generally similar to the first form of vessel, and the same reference numerals are used to refer to corresponding parts of the two vessels.

The principal differences between the two forms of vessel lie in the nature of the means for raising and/or lowering loads, and in the structure of the section 1 a of the upper hull 1.

In the second form of vessel, the large crane 6, the A-frame 7 and the small crane 12 are replaced by two large cranes 23 and 24 which are situated at the stern of the vessel and are arranged symmetrically with respect to the fore-and-aft axis of the vessel.

The crane 23 can reach over the starboard side of the vessel and the crane 24 can reach over the port side.

Because the two cranes 23 and 24 are situated above the stern columns 3b and 2b, respectively, and there is no requirement for mounting sites for an A-frame, the section la of the upper hull 1 does not need to be of such stiff construction as it is in the first form of vessel. Thus, the partial bulkheads 4b (see Figs. 4 and 6) in the first form of vessel are omitted in the second form of vessel, the corresponding longitudinal girders 18 extending to the aft transverse bulkhead 5a.

Further, although this does not appear from the drawings, components of the section 1a of the upper hull 1, such as the girders 17 and 18 and the bulkheads 4a, 5a and 5b, are, in the second form of vessel, of lighter construction than those in the first form of vessel.

The method of raising and/or lowering an exceptionally heavy load over the stern of the second form of vessel using the cranes 23 and 24 is in effect the same as that described above for raising and/or lowering an exceptionally heavy load over the stern of the first form of vessel using the crane 6 and A-frame 7 of that form of vessel.

The dimensions stated to be suitable for the first form of vessel are also suitable for the second form of vessel, with the cranes 23 and 24 each having a lifting capacity of about 4,500 tonnes.

Referring to Figs. 11 to 15 of the drawings, the third form of vessel comprises an upper hull, which is indicated generally by the reference numeral 25, supported on a lower hull comprising two elongate parallel-extending pontoons 26 and 27, respectively, by a support structure comprising two columns 26a and 26b extending upwardly from the bow and the stern of the pontoon 26, respectively, and two columns 27a and 27b, respectively, similarly extending from the pontoon 27.

As can be seen from Fig. 14, the columns 26a, 27a and 27b are of substantially equal cross-sectional area and are substantially rectangular in cross-section except that sides 26at, 27razz and 27by, respectively, of each column, instead of meeting adjacent sides at angles of 90 , are joined to adjacent sides by means of intermediate members 26a2, 27a2, and 27b2, respectively, inclined at angles 45" to adjacent sides. Such a configuration while relatively easy to fabricate, reduces stress concentrations which would occur at the corners of a strictly rectangular column.For reasons of structural strength and buoyancy, the fourth column 26b at the stern of the vessel is of larger cross-sectional area than the other columns, a large-capacity crane 28 being mounted on the upper hull 25 so that its vertical axis of rotation passes through the column. In section, the column 26b has four sides 26b, arranged at 90" to each other, each side being joined to its adjacent sides by intermediate members 26b2 and 26b3, respectively, inclined at angles of 45" to the sides 26b1, the intermediate members 26b2 being longer than the intermediate members 26b3.

Each of the columns 26a, 26b, 27a and 27b, respectively, flares outwardly at its foot where it meets one of the pontoons 26 and 27.

Braces 29 and 30 (shown in broken lines in Fig. 14) extend between the bow columns 26a and 27a and the stern columns 26b and 27b, respectively.

The upper hull 25, like the upper hull 1 of the first form of vessel, is of water-tight construction, and has a main deck 25a. The configuration of the underside of the upper hull 25, as can be seen from Fig. 15, is such that, when the vessel has level heel and trim, the underside of a portion is lower than the underside of the remainder, but, unlike the upper hull 1, the portion is not a continuous one but is composed of two discrete sub-portions indicated generally by the reference numerals 31 and 32, respectively. Since the main deck 25a is level, the sub-portions 31 and 32 together constitute a portion of increased depth as compared with the depth of the remainder of the upper hull 25, which comprises a portion 33a, between the subportions 31 and 32, and a. portion 33b at the bow.The sub-portion 31 extends from the stern of the vessel and across its entire beam except where the stern columns 26b and 27b meet the upper hull 25, and the sub-portion 32 is situated adjacent to the bow, extending across the entire beam except where the bow columns 26a and 27a meet the upper hull.

The upper hull 25 is provided with longitudinal and transverse bulkheads (not shown) similar to those of the upper hull 1.

The sub-portion 31 of the upper hull 25 can be regarded as being composed of three sections 31a, 31b, and 31c, respectively. The section 31b is of uniform depth. The section Sic between the section 31b and the remainder 33a of the upper hull 25 has an underside which is inclined upwardly from the section 31b to the underside of the remainder 33a.

The underside of the section 31a has two surfaces 31a, and 31a2, respectively, inclined upwardly from the underside of the section 31b at different angles.

The sub-portion 32 of the upper hull 25 can also be regarded as being composed of three sections 32a, 32b and 32c, respectively. The section 32b is of uniform depth, and the sections 32a and 32c each have undersides which are inclined upwardly from the section 32b to the underside of the remainder 33a and 33b, respectively, of the upper hull 25.

The undersides of the sections 32a and 32c, similarly to the underside of the section 31a of the sub-portion 31, have two surfaces inclined upwardly at different angles. The sections 32a and 32c are of the same length as the section 31a, but considerably shorter than the section 31c, the underside of which is inclined upwardly at a more gentle angle.

A crane 28 is mounted on the upper hull 25 in the vicinity of the sub-portion 31 and directly over the stern column 26b, as referred to above, and its boom 28a can reach out over the stern and starboard side of the vessel and even to a limited extent over the port side. A boom rest 28b is provided on the main deck 25a for supporting the boom when not in use. Towards the bow end of the upper hull 25 is a super-structure 34 that may include crew accomodation, control areas, and the like, and may also support a landing-pad 35 for helicopters. Three lifeboat platforms 36 are provided at the bow end of the upper hull 25, and the vessel is also provided with propellers and ballast systems (not shown) in a similar manner to the first form of vessel.

The method of operating the third form of vessel is essentially the same as that described for the first form of vessel (the water levels at the different draughts of the vessel being indicated by the same reference numerals) except that the third form of vessel is not provided with an Frame, the crane 28 being used to raise and/or lower loads over the stern, starboard or port sides of the vessel on its own.

Also, because the portion of the upper hull 25 that provides the increase in waterplane area in the second semi-submerged position is not continuous as in the first form of vessel, but is composed of two discrete sub-portions 31 and 32, there are some differences in operation when the vessel is in its second semisubmerged position.

With this form of vessel, when the vessel is submerged until it has just reached its second semisubmerged position and the waterplane is just in contact with the undersides of the subportions 31 and 32 at a level W2,, then the increase in waterplane area provided by each of the sub-portions is substantially equal. On further submerging the vessel to a deeper second semi-submerged position at the waterplane level W2, the waterplane area continues to increase because of the inclined surfaces of the sections Sia, 31c, 32a, and 32c, the increase then being a gradual one'. Because the underside of the section Sic is more gently inclined than the others, however, the increase in waterplane area is greater at the stern than at the bow.Thus, although the point about which the vessel tends to rotate in response to a moment arising from a lifting operation is not shifted towards the crane 28 when the vessel is at the waterplane level W2' as it is with the first form of vessel immediately it reaches the second semi-submerged position, it will tend to move in that direction as the draught is increased towards level W2. The sheltering effect on a cargo barge will, however, be exerted irrespective of whether the vessel is at the waterplane level W2, or W2 although it will be slightly greater when the vessel is at the waterplane level W2 because of the increase in waterplane area.

The configuration of the upper hull 25 with two discrete sub-portions 3 1 and 32, respectively, of increased depth may be preferred over that of the first form of vessel, if the vessel is to be constructed by mounting the upper hull 25 onto the columns 26a, 26b, 27a, and 27b at sea, since it is necessary for there to be a large enough free central region between the columns to accommodate a barge carrying the upper hull 25 to allow correct positioning of the upper hull on the columns.

For a vessel having displacement of about 62,000 tonnes in the first semi-submerged position, the following dimensions, which are given by way of example, are suitable for the third form of vessel.

The upper hull 25 has a length of about 100m. The sub-portion 31 has a length of about 36m and the subportion 32 has a length of about 22.5m, the remaining portion 33a of the upper hull separating the two subportions having a length of about 28m. The sub-portions 31 and 32 have a maximum depth of about ii m and the remainder 33a, 33b of the upper hull 25 a depth of 7m.

Thus, the vertical separation between the undersides of the sub-portions 31 and 32 of the upper hull 25 and the underside of the remainder 33a, 33b of the upper hull is 4m, and, for the purpose of comparing the waterplane area of the vessel in its second semi-submerged position with the waterplane area of the vessel in its first semi-submerged position, is to be taken as the position of the vessel when its draught is such that the lower surface of the sub-portions 31 and 32 of the upper hull 25 are immersed to a depth of 2m, that is to say, at the water level W2.

Of the sub-portion 31, the section 31b has a length of about 7.5m. The section 31a has an overall length of about 7.5m, the surface 31a, being inclined upwardly from the section 31b at an angle of about 20 , and the surface 31 a2 being inclined upwardly from the surface 31a,, at an angle of 45'. The section Sic has an overall length of about 21m and is inclined upwardly from the section 31b to the remainder 33a of the upper hull 25 at an angle of about 11".

Of the sub-portion 32, the sections 32a, 32b and 32c each have a length of about 7.5m, the surfaces 32a, and 32a2 of the section 32a, and the surfaces 32c and 32c2 of the section 32c being inclined upwardly from the section 32b at angles of 20 and 45 , respectively.

Such dimensions lead to an increase of about 135% in the waterplane area of the vessel between the first and second semi-submerged positions, but will usually leave the waterplane area of the vessel in its second semi-submerged position less than the waterplane area of the vessel when it is at transit draught. The displacement of the vessel in the second semi-submerged position at the water level W2 is about 74,000 tonnes.

The crane 28 has a main hoist capacity of about 2,000 to 2,400 tonnes, an auxiliary hoist capacity of about 750 tonnes, and a whip hoist capacity of about 90 tonnes.

The waterplane level W is equivalent to a draught of 6.5m, and the level W3 (the waterplane level at the survival draught) to a draught of 14 m. The level W, may lie within a range equivalent to a range of draughts of from 22m to 25.5m, and the level W2 is equivalent to a draught of 28m, the level W2, being equivalent to a draught of 26m.

Referring to Figs. 16 to 19 of the drawings, the fourth form of vessel similarly to the first, second and third forms of vessel, comprises an upper hull, which is indicated generally by the reference numeral 37, supported on a lower hull comprising two elongate parallel-extending pontoons 38 and 39, respectively, by a support structure comprising two columns 38a and 38b extending upwardly from the bow and the stern of the pontoon 38, respectively, and two columns 39a and 39b, respectively, similarly extending from the bow and the stern of the pontoon 39.

All four columns 38a, 38b, 39a and 39b of the fourth form of vessel are of substantially equal cross-sectional area, as can be seen from Fig. 19, and are substantially rectangular in section. Braces 40 and 41 (shown in broken lines) extend between the bow columns 38a and 39a and the stern columns 38b and 39b respectively.

The configuration of the underside of the upper hull 37, as can be seen from Fig. 18, is such that, when the vessel has level heel and trim, the underside of a portion is lower than the remainder, and, like the upper hull 25 of the third form of vessel, the portion is also made up of two discrete sub-portions 42 and 43, respectively. These two sub-portions 42 and 43 together constitute a portion of increase depth as compared with remainder 44 of the upper hull. The sub-portions 42 and 43 extend fore and aft along the starboard and port sides of the upper hull 37, respectively, between the bow and stern columns 38a, 38b and 39a, 39b, respectively. The width of each of the sub-portions 42 and 43 (in a transverse direction) is substantially equal to the width of the columns 38a, 38b and 39a, 39b~~in the same direction.

The greater part of the underside of each of the sub-portions 42 and 43 is inclined downwardly away from the underside of the remainder 44 of the upper hull towards the starboard and port sides of the vessel, respectively, but it curves upwardly for a short distance before it reaches the side in order to avoid a sharp edge.

The upper hull 37 is provided with longitudinal and transverse bulkheads (not shown) similar to the upper hull 1.

Twin masthead type cranes 45 and 46, respectively, are mounted on the upper hull 37 in the vicinity of the sub-portion 43 so that their vertical axes of rotation pass through the bow and stern columns 39a and 39b, respectively. The boom of the crane 45 can reach out over the port side and the bow of the vessel, and the boom of the crane 46 can reach out over the the port side and the stern of the vessel, and to a limited extent, over the bow of the vessel. Traditional cranes may be used in place of the masthead-type cranes, if desired.

Towards the stern end of the upper hull 37 is a superstructure 47 that may include crew accommodation, control areas and the like, and which provides-support for a helicopter landing pad 48. A lifeboat platform is provided at the stern end of the upper hull 37, and a moonpool 50 is also provided in the upper hull. The vessel is further provided with waterscrew propellers 51 and ballast systems (not shown) in a similar manner to the first form of vessel.

The method of operating the fourth form of vessel is similar to that described for the third form of vessel, the cranes 45 and 46 being used to raise and/or lower loads over the bow, stern or the port side of the vessel when working on their own or over the port side when they are working in conjunction. In this form of vessel, however, the increase in waterplane area in the second semi-submerged position of the vessel is provided equally by the sub-portions 42 and 43 irrespective of the exact draught of the vessel in the second semisubmerged position.

For a vessel having a displacement of about 27,000 tonnes to 29,300 tonnes in the first semi-submerged position, the following dimensions, which are given by way of example, are suitable for the fourth form of vessel.

The upper hull 37 has a length of about 85m and is about 65m across its beam. The sub-portions 42 and 43, respectively, have a length of about 40m and a width of 15m, the same width as that of the columns 38a, 38b and 39a, 39b which are each about 13m in length. The pontoons 38 and 39 each have a length of 80m and a width of 15m. The subportions 42 and 43 have a maximum depth of about 11 m and the remainder 44 of the upper hull 37 a depth of 7m.Thus, the maximum vertical separation between the undersides of the sub-portions 42 and 43 of the upper hull 37 and the underside of the remainder 44 of the upper hull is 4m, and, for the purpose of comparing the waterplane area of the vessel in its second semi-submerged position with the waterplane area of the vessel in its first semi-submerged position is to be taken as the position of the vessel when its draught is such that the lower surface of the sub-portions 42 and 43 of the upper hull 37 are immersed to a depth of 2m, that is to say, at the waterplane level W2.

The greater part of the undersides of the subportions 42 and 43 are inclined downwardly away from the underside of the remainder 44 of the upper hull 37 at an angle of about 17 . The curved surface at the outermost edge of the underside of each of the subportions 42 and 43, respectively, has a radius of curvature of about 2m.

Such dimensions lead to an increase of about 87% in the waterplane area of the vessel between the first and second semi-submerged positions, but will usually leave the waterplane area of the vessel in its second semisubmerged position less than the waterplane area of the vessel when it is at transit draught. The displacement of the vessel in the second semi-submerged position at the waterplane level W2 is about 34,000 tonnes.

Each of the cranes 45 and 46 has a main hoist capacity of about 750 tonnes, and a whip hoist capacity of about 125 tonnes.

The waterplane level W (the waterplane level at the transit draught) is equivalent to a draught of 7m, and the wateplane level W3 (the waterplane level at the survival draught) to a draught of 12m. The level Wl may lie within a range equivalent to a range of draughts of from 15m to 18m, and the level W2 is equivalent to a draught of 23m, the level W2, being equivalent to a draught of 21m.

Claims

1. A semi-submersible lifting vessel which comprises a lower hull, an upper hull having means for raising and/or lowering loads, the configuration of the underside of the upper hull being such that when the vessel has level heel and trim the underside of a portion of the upper hull in the vicinity of the means for raising and/or lowering loads is lower than the underside of the remainder of the upper hull, and a support structure for the upper hull which extends upwardly from the lower hull, the configuration of the upper hull and the support structure being such that, in a first semi-submerged position in which the waterplane intersects the support structure, the waterplane area of the vessel is less than the horizontal cross-sectional area of the lower hull, and in a second semi-submerged position, lower than the first, the waterplane contacts or intersects the said portion of the upper hull so that the waterplane area of the vessel is greater than it is in the first position.

2. A vessel as claimed in claim 1, wherein the said portion of the upper hull that, in the second semisubmerged position of the vessel, is contacted or intersected by the waterplane is a continuous portion in the vicinity of the means for raising and/or lowering loads.

3. A vessel as claimed in claim 1, wherein the said portion of the upper hull which, in the second semisubmerged position of the vessel, is contacted or intersected by the waterplane consists of two or more discrete sub-portions, one of which is in the vicinity of the means for raising and/or lowering loads.

4. A vessel as claimed in claim 3, wherein the said portion of the upper hull consists of two discrete subportions.

5. A vessel as claimed in claim 4, wherein the configuration of the sub-portions is such that they provide increases in the waterplane area in the second semi-submerged position that are substantially equal to each other.

6. A vessel as claimed in claim 4, wherein the configuration of the sub-portions is such that the subportion in the vicinity of the means for raising and/or lowering loads provides a greater increase in the waterplane area in the second semi-submerged position than that provided by the other sub-portion.

7. A vessel as claimed in any one of claims 1 to 6, wherein the waterplane area of the vessel is arranged to be at least 25% greater in the second semi-suhmerged position than in the first semi-submerged position.

8. A vessel as claimed in any one of claims 1 to 7, wherein the waterplane area of the vessel is arranged to be at least 50% greater in the second semi-submerged position than in the first semi-submerged position.

9. A vessel as claimed in any one of claims 1 to 8, wherein the waterplane area of the vessel is arranged to be at least 75% greater in the second semi-submerged position than in the first semi-submerged position.

10. A vessel as claimed in any one of claims 1 to 9, wherein the waterplane area of the vessel is arranged to be not more than 150% greater in the second semi-submerged position than in the first semi-submerged position.

11. A vessel as claimed in claim 2 or any one of claims 7 to 10, wherein the means for raising and/or lowering loads is located in the vicinity of one end of the vessel and the said portion of the upper hull that, in the second semi-submerged position of the vessel, is con tacted or intersected by the waterplane is a continuous portion and extends from that end of the vessel.

12. A vessel as claimed in claim 11, wherein the means for raising and/or lowering loads is located in the vicinity of the stern of the vessel and the said portion of the upper hull that, in the second semi-submerged position of the vessel is contacted or intersected by the waterplane extends from the stern of the vessel.

IS. A vessel as claimed in claim 11 or claim 12, wherein the said portion of the hull that, in the second semi-submerged position of the vessel, is contacted or intersected by the waterplane extends across substantially the entire beam of the vessel.

14. A vessel as claimed in any one of claims 4 to 10, wherein the means for raising and/or lowering loads is located in the vicinity of one end of the vessel, and the said portion of the upper hull consists of two discrete subportions, one of which extends across the vessel in the vicinity of that end of the vessel and the other of which extends across the vessel in the vicinity of the opposite end.

15. A vessel as claimed in claim 14, wherein the subportions each extend across substantially the entire beam.

16. A vessel as claimed in any one of claims 4 to 10, wherein the means for raising and/or lowering loads is located in the vicinity of one side of the vessel, and the said portion of the upper hull consists of two discrete subportions one of which extends along the vessel in the vicinity of its port side, and the other of which extends along the vessel in the vicinity of its starboard side.

17. A vessel as claimed in any one of claims 1 to 15, wherein the means for raising and/or lowering loads is located in the vicinity of one end of the vessel, the said portion of the upper hull or one of the subportions thereof extends across that end, and the said portion or the said sub-portion comprises a first section, the underside of which is at a uniform level, immediately adjacent to the said means for raising and/or lowering loads and a second section between the first section and the remainder of the upper hull (that is to say, the upper hull excluding the said portion or each of the sub-portions thereof) the underside of the second section being inclined upwardly from the first section to the underside of the remainder of the upper hull.

18. A vessel as claimed in claim 17, wherein the angle of inclination of the underside of the second section to the underside of the remainder of the upper hull does not exceed 20 .

19. A vessel as claimed in claim 18, wherein the angle of inclination of the underside of the second section to the underside of the remainder of the upper hull lies within the range of from 6" to 12".

20. A vessel as claimed in claim 16, wherein the underside of each sub-portion in cludes a surface that is inclined downwardly away from the underside of the remainder of the upper hull towards the side of the vessel.

21. A vessel as claimed in claim 20, wherein the angle of inclination of the said surface to the underside of the remainder of the upper hull does not exceed 200.

22. A vessel as claimed in claim 21, wherein the angle of inclination of the said surface to the underside of the remainder of the upper hull lies within the range of from 15 to 180.

23. A vessel as claimed in any one of claims 1 to 22, wherein the arrangement is such that the variation in level of the under side of the upper hull stems from a corre sponding variation in the overall depth of the upper hull.

24. A vessel as claimed in any one of claims 1 to 23, wherein the difference be tween the lowest level of the underside of the said portion and the level of the underside of the remainder of the upper hull is at least 2m.

25. A vessel as claimed in claim 24, wherein the difference between the lowest level of the underside of the said portion and the level of the underside of the remainder of the upper hull is at least 3m.

26. A vessel as claimed in any one of claims 1 to 25, wherein the lower hull com prises a plurality of elongate pontoons which extend generally horizontally and generally par allel to one another.

27. A vessel as claimed in claim 26, which comprises two such pontoons.

28. A vessel as claimed in claim 26 or -claim 27, wherein the support structure com prises one or more columns extending up wardly from each pontoon to the upper hull.

29. A vessel as claimed in claim 28, wherein the support structure comprises at least two columns extending upwardly from each pontoon to the upper hull.

30. A vessel as claimed in any one of claims 1 to 29, wherein the means for raising and/or lowering loads comprises a crane situ ated in the vicinity of one side of the vessel and in the vicinity of one end of the vessel.

31. A vessel as claimed in claim 30, wherein the means for raising and/or lowering loads also comprises a second crane.

32. A vessel as claimed in claim 31, wherein the said portion of the upper hull, or one of the sub-portions thereof extends across the vessel in the vicinity of the said one end and the second crane is situated in the vicinity of the other side of the vessel and in the vicinity of the said one end of the ves sel.

33. A vessel as claimed in claim 32, wherein the said one end of the vessel is its stern.

34. A vessel as claimed in claim 31, wherein the said portion of the upper hull consists of two discrete subportions, one of which extends along the vessel in the vicinity of its port side and the other of which extends along the vessel in the vicinity of its starboard side, and the second crane is situated in the vicinity of the same side as the first-mentioned crane and in the vicinity of the other end of the vessel.

35. A vessel as claimed in any one of claims 30 to 34 when dependent on claim 23, wherein the construction of at least a part of the said portion of the upper hull is such as to provide a range of different sites, each of sufficient strength for an A-frame to be mounted on it, the different sites being spaced apart from each other and so situated that an A-frame mounted on any one of at least some of the said sites can co-operate in the lifting of a single load with the said crane or one of the said cranes.

36. A vessel as claimed in claim 35, wherein the structure of the upper hull comprises a plurality of bulkheads and each mounting site is so situated that the foot of each leg of an A-frame mounted on it is located over such a bulkhead.

37. A vessel as claimed in claim 35 or claim 36, wherein adjacent mounting sites for an A-frame are spaced apart by not more than 1 Om.

38. A vessel as claimed in claim 37, wherein adjacent mounting sites for an Aframe are spaced apart by not more than 7m.

39. A vessel as claimed in any one of claims 35 to 38, wherein adjacent mounting sites for an A-frame are spaced apart by a sub-multiple of the separation of the legs of the A-frame at its base.

40. A vessel as claimed in claim 39, wherein the sub-multiple is 1/5, 1/4, or 1/3.

41. A vessel as claimed in any one of claims 35 to 40, which has, secured to the said portion of the upper hull, a mounting for each leg of the A-frame.

42. A vessel as claimed in any one of claims 35 to 41, wherein the arrangement is such that an A-frame mounted on one of the said sites can lie on the upper hull when not in use.

43. A vessel as claimed in any one of claims 35 to 42, wherein the means for raising and/or lowering loads comprises an Aframe mounted on one of the said sites.

44. A vessel as claimed in any one of claims 1 to 43, which is provided with a rapid ballast system.

45. A semi-submersible lifting vessel substantially as hereinbefore described with reference to, and as shown in, Figs. 1 to 6 of the accompanying drawings.

46. A semi-submersible lifting vessel substantially as hereinbefore described with reference to, and as shown in, Figs. 1 to 6 of the accompanying drawings, modified substantially as hereinbefore described with reference to, and as shown in, Figs. 7 to 10 of the accompanying drawings.

47. A semi-submersible lifting vessel substantially as hereinbefore described with reference to, and as shown in Figs. 11 to 15 of the accompanying drawings.

48. A semi-submersible lifting vessel substantially as hereinbefore described with reference to, and as shown in Figs. 16 to 19 of the drawings.

49. A method of handling a load using a semi-submersible lifting vessel as claimed in any one of claims 1 to 48, wherein at least a part of the load-handling operation is carried out with the vessel in its second semi-submerged position.

50. A method as claimed in claim 49, when dependent on claim 43, which comprises raising and/or lowering a load with the said crane or one of the said cranes operating in conjunction with the A-frame.

51. A method as claimed in claim 49 when dependent on any one of claims 31 to 34, which comprises raising and/or lowering a load with the two cranes operating in conjunction with each other.

52. A method as claimed in any one of claims 49 to 51, which comprises raising a load from, and/or lowering a load onto, a conventional cargo barge situated down-wave of the vessel.