GB2170248A - Improvements in and relating to semi-submersible vessels - Google Patents

Abstract

A semi-submersible vessel has a deck portion 1 constructed of trusses 16 to 23 supported by eight columns 3 to 10 on a ring of pontoons 11 to 14. The bottom and the lower parts of the sides of the deck portion 1 are clad with watertight plating 35 and 38 so that the deck portion can be towed out of a construction dock without needing a supporting barge. The deck portion 1 is then mated with the tops of the columns 3 to 10.

Description

SPECIFICATION Improvements in and relating to semisubmersible vessels The invention relates to semi-submersible vessels.

The invention provides a semi-submersible vessel comprising a deck portion of truss construction, a pair of elongate, generally parallel, side-by-side longitudinal pontoons, eight upright columns supporting at their upper ends the deck portion and connected at their lower ends to the longitudinal pontoons, four columns being connected to each longitudinal pontoon, and two transverse pontoons each conhecting respective end portions of the longitudinal pontoons.

The truss deck has two major advantages: it can be significantly lighter than a plated deck, and it allows for easier access to the interior of the deck. Because the plates of a plated deck are load-bearing members, holes cannot be cut in them for, for example, pipes or cables, without careful consideration of the effect of such holes on the strength of the deck, whereas with a truss deck quite large objects can be passed through the spaces between the girders that make up the truss and the strength of the deck is not materially affected.

Although the truss deck does not have the same strength and rigidity as a plated deck, and despite the stringency of present-day standards of performance and safety for deepsea operation, it has been found that it is possible for a vessel with a truss deck to meet those standards while having no more than eight columns sufficientiy rigidly connected together at the base, which is here achieved by the provision of the transverse pontoons The arrangement is preferably such that no bracing between the two sides of the vessel is necessary other than that provided by the deck portion and the transverse pontoons.

The columns may be substantially evenly spaced along the length of the longitudinal pontoons, and four of the columns are advantageously in the vicinity of the meetings of the longitudinal and transverse pontoons.

The deck portion advantageously comprises a plurality of longitudinal trusses and a plurality of transverse trusses, each extending substantially in a vertical plane, which preferably have top and bottom horizontal members defining respective common horizontal planes.

The deck portion advantageously includes plating over the top girders of the trusses forming an upper deck, plating over the bottom girders of the trusses forming a lower deck, and/or plating under the bottom girders of the trusses. A minor proportion of the volume of the truss deck may be in the form of a watertight region. Although in normal use the deck is above water at all times, it may dip into the water if, for example, one corner of the vessel is severely damaged in a collision, and such a watertight region can then provide useful additional buoyancy and stability.If the deck portion includes plating both above and below the bottom girders of the trusses, those girders and the plating above and below those girders preferably define between them a watertight region which is advantageously divided into a plurality of independently watertight compartments, preferably by the bottom girders of trusses passing through it. The plating then contributes to the stiffness of the truss against shear in a horizontal plane, enabling some saving in the number of truss members, and does not obstruct the interior of the truss.

There is advantageously also plating over at least a lower portion of the sides (including the ends) of the deck structure.

At least part of the exterior of the truss deck structure, advantageously, an upper portion of the sides, is preferably covered by splash-resistant cladding so mounted that it can be removed by the blast of an explosion within the deck portion without structural damage to the deck portion so that the blast of the explosion can escape to the exterior.

That is especially advantageous in the case of, for example, an oilfield production vessel, because it means that some equipment that may cause explosions can safely be mounted within the truss where on a vessel with a plated deck portion it would have for safety reasons had to be mounted on top of the deck portion.

The vessel advantageously comprises means for transmitting the weight of the deck portion to the columns substantially on the axes of the columns. The weight transmitting means may comprise members, which advantageously act as arches, spanning the interiors of the columns, supporting the deck portion at their centres, and being supported by the columns at both ends. Preferably, the interior of each column is provided with two such spanning members in the form of a cross supporting the deck portion at its centre.

The deck portion is advantageously connected directly to peripheral portions of the columns as well as to the spanning members.

Members of the deck oortion, which are advantageously bottom girders of the truss deck, may be welded to peripheral members of the columns and the spanning members are then preferably prestressed. The spanning members preferably bear at least a major proportion of the static weight of the deck portion and bear a lower proportion of dynamic loads transmitted between the deck portion and the columns.

The vessel may be assembled by constructing the pontoons and columns, constructing the deck separately, floating the two parts, submerging the pontoons and columns until the deck can be floated over them, and then either lowering the deck or raising the pontoons and columns until the two parts mate.

The invention also provides a deck portion for a semi-submersible vessel, so constructed as to be capable of floating unaided in calm water. The deck portion may be at least partially of truss construction, and is then preferably suitable for use as the deck portion of a semi-submersible vessel according to the invention.

The deck portion can then be floated out and mated with the hull without the need for a barge to support the deck portion, and the invention further provides a method of constructing a semi-submersible vessel, which comprises constructing a deck portion according to the invention, floating the deck portion by means of its own buoyancy, and mating the deck portion with a supporting portion.

The deck portion may be constructed in a dry dock, floated by flooding the dry dock, and removed from the dock while floating by means of its own buoyancy.

The deck portion is advantageously lifted out of the water for mating with the supporting portion, and if a submersible or partially submersible barge or other vessel is available then such a vessel is preferably positioned under the deck portion and then raised to lift the deck portion out of the water for mating, but the deck portion can still be floated out of the construction dock on its own, which means that the barge or other vessel is only needed for a short time and that the deck portion can be built on the floor of a dry dock without room under it for the barge, both of which are advantages.

Because the full buoyancy of the deck portion is usually required only when floating out the deck for mating, which can be done in inshore waters in fairly calm weather, the part of the vessel that displaces water in order to produce the necessary buoyancy does not need to be in the form of a region that is completely watertight in the sense that it remains watertight even if fully submerged, and advantageously the bottom and at least lower portions of the sides of the deck portion are provided with watertight plating. Once the vessel has been assembled, any of the olating that is not needed for other purposes may be removed.If it is left in place then, because it is not necessary for the structural strength of the deck, holes may be-cut in it for pipes or cables or other modifications may be made without the calculations and precautions that are necesssary when working on a load-bearing plate.

The invention also provides a semi-submersible vessel constructed by a method according to the invention, and a semi-submersible vessel comprising a deck according to the invention.

One form of semi-submersible vessel constructed in accordance with the invention, and a method of constructing such a vessel, will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic perspective view of the vessel; Fig. 2 is a partly cut-away perspective view of a deck portion of the vessel, to a larger scale than Fig.1; Fig. 3 is a partly cut-away side elevation view of the deck portion; Fig. 4 is a section along the line IV-IV of Fig. 3, to a larger scale than Fig. 3; Figs. 5 to 7 are exploded fragmentary sectional views, partly cut away, of connections between the deck portion and supporting columns; and Figs. 8 to 18 are schematic views of stages in the construction of the vessel.

Referring to the accompanying drawings, and initially to Figs. 1 and 2, the vessel comprises a deck portion which in plan is generally rectangular, indicated generally by the reference numeral 1, on top of a hull portion, indicated generally by the reference numeral 2.

The hull portion 2 comprises eight vertical, substantially right-circular cylindrical, columns 3 to 10 joined at the bottoms by two longitudinal pontoons 11 and 12 and two transverse pontoons 13 and 14 forming a rectangular (but not square) ring. Four columns 3 to 6 are substantially evenly spaced apart along one longitudinal -pontoon 11, and the other four columns 10 to 7 are correspondingiy spaced apart along the other longitudinal pontoon 12.

The transverse pontoons 13 and 14 extend between the columns of the pairs of columns 3 and 10 and 6 and 7, respectively. The construction of the columns 3 to 10 and pontoons 11 to 14 may be conventional and in the interests of conciseness will not be described in detail. The hull portion 2 provides strength, rigidity, and buoyancy and may carry attachment points (not shown) for mooring lines 15 (not shown in detail) to keep the vessel on station in operation at sea.

The deck portion 1 may be provided with a moonpool 16 with a tower over it, mountings 17 for cranes, a superstructure 18, a helicopter landing pad, or any other structures or equipment necessary for the function that the vessel is to perform. If, for example, the vessel is to be used as a production platform in an oilfield, then it may have a flare boom and mountings for pipes connecting the vessel with the seabed. As will be explained below, equipment may also be installed within the deck portion 1.

Referring to Figs. 2 to 4 of the drawings, the deck portion 1 is of truss girder construction and comprises four main longitudinal trusses 19 to 22 and four main transverse trusses 23 to 26. The outer longitudinal trusses 19 and 22 and all four transverse trusses 23 to 26 extend between respective pairs of the columns 3 to 10, and the inner longitudinal trusses 20 and 21 extend between the outer transverse trusses 23 and 26. Each of the main trusses 19 to 28 consists of an upper horizontal girder 27, a lower horizontal girder 28, a plurality of vertical girders 29, and a plurality of diagonal girders 30.

There is a vertical girder 29 at each intersection of two perpendicular trusses, and one midway between the intersections of each pair of adjacent intersections. The horizontal girders 27 and 28 and the vertical girders 29 thus form a plurality of rectangular cells, and there is a diagonal girder 30 along one diagonal of each cell, so oriented as to be in tension if the deck portion 1 sags between the columns 3 to 10. In addition, the outer trusses 19, 22, 23, and 26 have intermediate horizontal girders 31 between their upper girders 27 and lower girders 28, parts of which have been omitted from Fig. 2 in the interests of clarity.

An additional truss 32 extending from one inner longitudinal truss 20 to the other inner longitudinal truss 21 between the inner transverse trusses 24 and 25, and two more additional trusses 33 and 34 extending from the truss 32 to the truss 25, define with the truss 25 the moonpool 16.

The entire deck portion 1 is provided with plating 25 over the upper surfaces of the upper girders 27 and supported by a framework 36 between the girders to form an upper deck. There is also plating 37 on top of, and plating 38 underneath, the lower girders 28, supported by a framework 39 between them.

The plating 37 provides a lower deck, the plating 38 protects the deck portion against spray and the like thrown up from below, and the two together define with the girders 28 a watertight region, which with the particular construction shown is subdivided by the girders 25 of the inner trusses into a plurality of independently watertight compartments. The sides of the moonpool 16 are formed by plating 40 over their whole height between the plating 35 and 38, and the outer faces of the deck portion 1 are provided with plating 41 up to the intermediate horizontal girders 31 and with lightly secured cladding 42 above that.

The plating 38, 40, and 41 renders the deck portion 1 watertight, or at least enables it to be rendered watertight without difficulty, up to the level of the intermediate horizontal girders 31, giving a displacement sufficient to enable the deck portion 1 to be floated without additional buoyancy in calm water. The cladding 42 is arranged to prevent waves or spray from entering the interior of the deck portion 1, but is so mounted that it can be detached by pressure on the inside. The pressure needed to detach the cladding 42 is sufficiently low that, if an explosion occurs within the deck portion 1 as a result of, for example, the ignition of an accumulation of inflammable gas, the blast will be dissipated through the spaces left by the detached cladding without damaging the structure of the vessel.Because of that, certain equipment (not shown) that may cause such explosions is installed on the lower deck 37 of the deck portion 1 although it could not safely have been put inside a plated structure and would thus on a vessel with a plated deck have had to be outside the deck structure on top of the upper deck 35. Since some of the equipment affected is heavy, there is a significant and advantageous lowering of the vessel's centre of gravity, as a result of transferring that equipment from the upper deck 35 to the lower deck 37. In addition, the more sheltered environment on the lower deck is beneficial both to the equipment and to any crew-members who have to work on it.

The diagonal girders 30 and the plating 37 and 38 provide all the shear stiffness needed for the truss deck, and the upper deck plating 35 and the side plating 41 are not necessary to the structure of the vessel. It is therefore possible, after assembly of the vessel as described below, to cut, drill, remove or alter the plating 35 and 41 comparatively freely which, together with the large spaces between the girders of the trusses, affords easy access to all parts of the deck portion 1.

Referring now also to Figs. 5 to 7, the main trusses 19 to 26 cross the tops of the columns 3 to 10 diametrally, so that the vertical girder 29 at the intersection of two trusses at a column is on the axis of the column. Where a truss ends at a column, the bottom horizontal girder 28 of the truss extends to the outside of the column. A diagonal brace 43 extends from the outer end of the bottom horizontal girder 28 to the top of the vertical girder 29 on the axis of the column.

Each of the columns 3 to 10 is hollow, and its upper end portion is spanned by a cruciform support indicated generally by the reference numeral 44. As may be seen in Figs. 5 to 7, each cruciform support 44 is formed by two intersecting girders with flat, horizontal top flanges 45 and arcuate bottom flanges 46 forming in effect load-bearing arches. The top flanges are overlain by plating 47 closing the tops of the columns, most of which has been omitted from Figs. 5 to 7 in the interests of clarity. At the centre of each cruciform support 44 is an octagonal platform 48 on which stands a foot 49 fixed to the deck structure 1 below the vertical girder 29 on the axis of the cylinder.

The platform 48 of the column 5 (see Fig.

5) bears an upstanding cylindrical probe 50 with a tapered tip that mates with a corresponding socket in its respective foot 49. The platform 48 of the column 9 (see Fig. 6) has a square probe 51 that mates in a socket in its respective foot 49. The socket for the probe 51 is open-sided in directions towards and away from the centre of the deck portion 1 in order that both probes 50 and 51 shall be able to mate with their respective sockets even if the distance between the centres of the probes is slightly different from the distance between the centres of the sockets. As may be seen from Figs. 1 and 2, the columns 5 and 9 bearing the probes 50 and 51 are on opposite sides of the vessel, and are diagonally opposite one another but not at the corners of the vessel. Fig. 7 shows the mating parts at the top of a typical corner column 3, 6 7, or 10.The remaining two columns, 4 and 8, have a similar arrangement to the corner column shown in Fig. 7, except that the main trusses 19 and 25 or 22 and 24 extend away from the column in three directions instead of two.

Where one of the bottom horizontal girders 28 passes over the rim of a column they are secured together by a shoe 52 which is welded to the girder and to the column. As may be seen in Figs. 5 to 7, the cruciform supports 44 are so oriented that the shoes 52 are at the ends of the flanges 45.

When, as will be described below, the vessel is assembled by mating a deck portion 1 onto a hull portion 2, the probes 50 and 51 serve to guide the deck portion into position on the hull portion. The probe 50 and its socket preferably engage first and centre the foot 49 in question on its platform 48, the probe 51 and its socket then engaging to align their foot 49 and platform 48 only in respect of rotation about the vertical axis of the probe 50, any misalignment in a direction towards and away from the probe 50 being accommodated by the open sides of the socket. If a different method of assembly is used the probes 50 and 51 may be omitted, or they may be replaced by any other suitable means of aligning the deck and columns.

The deck portion 1 is preferably installed on the columns, with the weight of the deck portion passing through the feet 49 and the cruciform supports 44, before the shoes 52 are fitted. The cruciform supports 44 are then pre-stressed with the weight of the deck portion 1 and the shoes 52 carry mainly dynamic forces.

Referring now to Figs. 8 to 18, one method of constructing the vessel is as follows.

The deck portion 1 is constructed in a dry dock 53 (see Fig. 8). Because the deck portion is so constructed as to be capable of floating unaided, it can be built in the bottom of the dry dock, without the need for a space under it for a barge or the like, which can make construction considerably easier than if it had to be built high up. Because the deck portion is built as a single unit, much of the internal wiring, pipework, and the like can be installed at this stage, reducing the amount of fitting out that has to be done afloat.

At the same time, the hull portion 2 is built in another dry dock 54 (see Fig. 9). When the hull portion 2, comprising the columns 3 to 10 and the pontoons 11 to 14, is complete, the dry dock is flooded and the hull portion is floated and towed out by a tug 55 (see Fig.

10). As is shown in Fig. 10, the hull portion is de-ballasted so that the pontoons have freeboard, enabling it to fioat in a shallow dry dock and reducing drag when it is in transit.

If the dry dock 54 is far from the place where the vessel is to be assembled then a partially submersible ship 56, which is known per se, may be submerged, and the hull portion 2 manoeuvered over it (see Fig. 11) and the ship is then raised again to lift the hull portion out of the water (see Fig. 12). The ship 56, carrying the hull portion 2, then proceeds under its own power, or is towed- by one or more tugs 55, to a predetermined location, where the ship is separated from the hull portion by reversing the sequence of operations used in picking the hull portion up.

The hull portion 2 is then moored in position and ballasted down until only the top end portions of the columns 3 to 10 are above water level, as shown in Fig. 13.

Meanwhile, the construction of the deck portion 1 is completed, the dry dock 53 is flooded, and the deck portion is floated and towed out by a tug 55 (see Fig. 14) to the mooring site of the hull portion 2. The deck portion 1 and the hull portion 2 of the vessel are preferably brought into position at the mooring site substantially simultaneously, and the amount of fitting out that is done on each before it is floated may be adjusted accordingly.

Depending on the distances from the mooring site to the dry docks 53 and 54, neither, either, or both of the portions 1 and 2 of the vessel may be brought to the mooring site by ship 56 as has been described above for the hull portion 2. The deck portion 1, in particular, has a substantial drag when being towed and is not fully seaworthy and should preferably be towed only for fairly short distances and only in sheltered waters. Preferably, however, a mooring site is chosen that is near enough to at least one of the dry docks 53 and 54 that at least one said portion of the vessel can be towed by a tug 55 from its dry dock to the mooring site, or that one portion of the vessel can be left at the mooring site while the ship 56 goes and fetches the other portion, so that only one ship is needed.

Once both portions of the vessel are in position, the ship 56 is submerged, the deck portion 1 is manoeuvred over it (see Fig. 15) and the ship is raised, lifting the deck portion out of the water (see Fig. 16). The ship 56 must be floated to a level at which the underside of the deck portion 1 is higher than the tops of the columns 3 to 10 of the hull portion 2. The ship 56 is then brought between the two rows of columns 3 to 6 and 7 to 10 and so positioned that the feet 49 of the deck portion 1 are over the platforms 48 of the cruciform supports 44 in the tops of the columns. The ship 56 is then submerged, and/or the hull portion 2 is raised, slowly by alterations to ballast until the feet 49 engage the platforms 48.If the vessel is provided with probes 50 and 51 and their sockets, or any equivalent means, then as mentioned above those will guide the portions 1 and 2 of the vessel into exact alignment, and the deck portion 1 must be manoeuvred before mating into close enough alignment with the hull portion 2 that the probe 50 will engage its socket correctly.

When the feet 49 and the cruciform supports 44 have taken the weight of the deck portion 1, the hull portion 2 can be de-ballasted until the deck portion is clear of the ship 56 and the ship can be moved out from between the columns 3 to 10. The vessel can then be raised until the pontoons have freeboard and towed away. The shoes 52 can be, and preferably are, welded into place between the columns 3 to 10 and the lower horizontal girders 28 of the trusses 19 to 26 as soon as the two portions of the vessel have been mated, and any necessary pipework and cabling connecting the deck portion 1 to the hull portion 2 can then be installed. The fitting out of the vessel can be completed in inshore waters, and the completed vessel can then be towed by one or more tugs 55 to its operating position, as shown in Fig. 18.

The following dimensions, which are given by way of example, are suitable for the vessel shown in the drawings.

The deck portion 1 may be about 75 metres long and 55 metres wide, between the centres of the columns 3, 6, 7, and 10. The columns 3 to 10 may be about 12 metres in diameter. The deck portion 1 may be about 10 metres high between the top plating 35 and the bottom plating 38, with the upper and lower horizontal girders 27 and 28, and thus also the watertight region between the plating 37 and 38, about 1 metre deep and the intermediate horizontal girders 31 midway between the upper and lower girders. The pontoons 11 to 14 may be about 10 metres wide and about 10 metres deep. As shown in the drawings the pontoons are rather narrower and deeper than that, but they may instead be broader when they can be less deep while still maintaining sufficient volume to provide the required total buoyancy.It will be appreciated that increasing the width of the pontoons decreases the minimum draught at which the vessel can float in transit, and it has been found also to improve the motion response of the vessel in some conditions of use. The pontoons may then be about 12.5 metres wide and about 6.5 metres deep.

Although one particular form of truss deck has been described above with reference to the drawings, it is to be understood that changes may be made to the number, position, and construction of the trusses according to the requirements of a particular use. In particular, the vertical girders 29 may be omitted if the number, position, and size of the diagonal girders 30 are such as to give the necessary strength and rigidity to the trusses, and the intermediate horizonal girders 31, which serve at least partially as supports for the side plating 41, may also be omitted if the necessary strength is provided by other means. It should further be understood that the structural members of the truss deck need not be arranged into distinct trusses, and need not be the conventional I-section girders used in the vessel shown in the drawings.

Claims (29)

1. A semi-submersible vessel comprising a deck portion of truss construction, a pair of elongate, generally parallel, side-by-side longitudinal pontoons, eight upright columns supporting at their upper ends the deck portion and connected at their lower ends to the longitudinal pontoons, four columns being connected to each longitudinal pontoon, and two transverse pontoons each connecting respective end portions of the longitudinal pontoons.

2. A vessel as claimed in claim 1, wherein the columns are substantially evenly spaced along the length of the longitudinal pontoons.

3. A vessel as claimed claims 1 or claim 2, wherein four of the columns are in the vicinity of the meetings of the longitudinal and transverse pontoons.

4. A vessel as claimed in any one of claims 1 to 3, wherein the deck portion comprises a plurality of longitudinal trusses and a plurality of transverse trusses, each extending substantially in a vertical plane.

5. A vessel as claimed in claim 4, wherein the trusses have top and bottom horizontal members defining respective common horizontal planes.

6. A vessel as claimed in claim 5, which includes plating over the top girders of the trusses forming an upper deck.

7. A vessel as claimed in claim 5 or claim 6, which includes plating over the bottom girders of the trusses forming a lower deck.

8. A vessel as claimed in any one of claims 5 to 7, wherein there is provided plating under the bottom girders of the trusses.

9. A vessel as claimed in both claim 7 and claim 8, wherein the bottom girders of the trusses and the plating above and below those girders define between them a watertight region.

10. A vessel as claimed in any one of claims 7 to 9, wherein there is provided plating over at least a lower portion of the out side of the deck structure.

11. A vessel as claimed in any one of claims 1 to 10, wherein at least part of the exterior of the truss deck structure is covered by splash-resistant cladding that can be removed by the blast of an explosion within the deck portion without damage to the load-bearing structure of the deck portion.

12. A vessel as claimed in any one of claims 1 to 11 comprising means for transmitting the weight of the deck portion to the columns substantially on the axes of the columns.

13. A vessel as claimed in claim 12, which comprises members spanning the interiors of the columns, supporting the deck portion at their centres, and being supported by the columns at both ends.

14. A vessel as claimed in claim 13, wherein the spanning members act as arches.

15. A vessel as claimed in claim 13 or claim 14, wherein the interior of each column is spanned by spanning members in the form of a cross supporting the deck portion at its centre.

16. A vessel as claimed in any one of claims 12 to 15, wherein the deck portion is connected directly to peripheral portions of the columns as well as to the spanning members.

17. A vessel as claimed in claim 16, wherein members of the deck portion are welded to peripheral members of the columns and wherein the spanning members are prestressed.

18. A deck portion for a semi-submersible vessel, so constructed as to be capable of floating unaided in calm water.

19. A deck portion as claimed in claim 18 that is at least partially of trussed construction.

20. A deck portion as claimed in claim 19, the bottom and at least lower portions of the sides of which are provided with watertight plating.

21. A deck portion for a semi-submersible vessel substantially as hereinbefore described with reference to, and as shown in Figs. 1 to 7 of, the accompanying drawings.

22. A method of constructing a semi-submersible vessel, which comprises constructing a deck portion as claimed in any one of claims 18 to 21, floating the deck portion by means of its own buoyancy, and mating the deck portion with a supporting portion.

23. A method as claimed in claim 22, wherein the deck portion is constructed in a dry dock, floated by flooding the dry dock, and removed from the dock while floating by means of its own buoyancy.

24. A method as claimed in claim 22 or claim 23 wherein the deck portion is lifted out of the water for mating with the supporting portion.

25. A method as claimed in claim 24, wherein a submersible or partially submersible vessel is positioned under the deck portion and then raised to lift the deck portion out of the water.

26. A method of constructing a semi-submersible vessel, substantially as hereinbefore described with reference to, and as shown in, Figs. 8 to 18 of the accompanying drawings.

27. A semi-submersible vessel constructed by a method as claimed in any one of claims 22 to 26.

28. A semi-submersible vessel comprising a deck as claimed in any one of claims 18 to 21.

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