GB2110602A - Semi-submersible drilling vessel - Google Patents

Abstract

The vessel has two hulls (5), and three or more pairs of columns (2), one column (2) of each pair mounted on one of the two hulls (5) and the other column (2) of the pair mounted on the other hull (5). The vessel also has a deck unit (1), with a centre moonpool, mounted above the columns (2), and tubular bracings (6) extend between the columns (2) of the forwardmost and aftmost pairs of columns (2), to provide the vessel with a portal structure. Tubular bracings (8) extend from the centre columns to the deck above the moonpool. A drill floor comprising a derrick (21) and storage and machinery housings (22) above the moonpool at a height to allow use of a blowout preventer stack and lower riser assembly. <IMAGE>

Description

SPECIFICATION Improvements to vessels for the exploration for and exploitation of hydro-carbons The present invention relates to improvements for vessels for the exploration for and exploitation of hydro-carbons.

The world demand for hydro-carbons, generally in the form of oil and gas and their condensates has led to the development of methods for exploration (drilling) for the exploitation (production) of hydro-carbon deposits lodged in reservoirs situated underground beneath the oceans and inland seas and lakes.

Drilling of holes into sub-sea reservoirs is frequently carried out by semi-submersible types of vessels, either anchored or dynamically positioned. Mono-hull vessels are also available for drilling. It has been the practice on completion of successful drilling operations to deploy and locate fixed production platforms in a required position being supported on the sea in a manner which maintains vertical and horizontal stabiltiy.

Another type of vessel promulgated for similar operations is a tethered tension leg buoyant platform. Preferred current practice is to deploy semi-submersible vessels for drilling operations in an offshore environment and to replace these with fixed seabed supported platforms for production purposes, although convertible drill/production units are available.

The present invention provides a semisubmersible vessel, having two hulls, three or more pairs of columns, one column of each pair mounted on one of the two hulls, the other column of the pair mounted on the other hull, a deck unit, having a moonpool, mounted on the columns, and tubular bracings extending between the columns of the foremost and aftmost pairs of columns, to provide the vessel with a portal structure as herein defined.

According to a first form of the present invention there is provided a semi-submersible vessel for offshore exploration or for exploitation of hydro-carbon deposits, the vessel having two hulls, three or more pairs of columns, one column of each pair mounted on one of the two hulls, the other column of the pair mounted on the other hull, a deck unit, having a moonpool, mounted above the columns, and tubular tracings extending between the columns of forwardmost and aftmost pairs of columns, and each column of a pair, other than the forwardmost or aftmost pairs, having two tubular bracings extending therefrom upwards and apart to join with the deck unit adjacent the moonpool on the same side of the moonpool as the column.

According to a second form of the present invention there is provided a semi-submersible vessel, having two hulls, three or more pairs of columns, one column of each pair mounted on one of the two hulls, the other column of the pair mounted on the other hull, a deck unit, having a moonpool, mounted above the columns, and tubular bracings extending between the columns of the forwardmost and aftmost pairs of columns, the vessel also having a drill floor above the moonpool at a height sufficient for a blowout preventer stack and a lower riser assembly to be handled as a unit, with a protective housing for the handling area, which protective housing is constructed to provide strength and structural integity for the vessel.

The tubular bracing structure employed in a vessel according to the first form of the present invention can have various advantages. Loads, for example from a drilling derrick mounted around the moonpool can be transferred efficiently to the columns from which bracings extend to the deck unit. This structure is simple, offering easy maintenance and inspection and reduced weight, which are important safety considerations.

The alternative structure employed in a vessel according to the second form of the present invention can also provide various advantages, such as simplicity.

In such a twin-hulled vessel according to either form of the present invention each column may be polygonal in cross section, with eight or more facets, with a flared transition piece between the top of the column and the deck unit.

The use of multi-facetted columns, with flared transition pieces provides a relatively simple structure, easy to manufacture and strong.

In such a vessel according to the present invention, for offshore exploration, a drilling derrick may be mounted on the deck unit around the moonpool.

The structural loading from the weight of the derrick, and the weight of drilling machinery, and drilling loads, are transferred in vessels in accordance with the first form of the present invention to the tubular bracings which join with the deck unit, and then through those bracings to columns of the vessel.

In a vessel according to the present invention, a high drill floor can be provided, so that, for example, a blow-out preventer and a lower riser assembly can be tested and serviced as one without need for subsequent disassembly for deployment. Further the servicing and handling area may be enclosed to provide a sheltered environment. This is very advantageous if the vessel is to be employed in the severe conditions which are found in some offshore locations.

Vessels embodying the second form of the present invention are constructed so that the protective housing of the servicing and handling area contributes to the strength of the vessel. In this way the tubular bracings joining the deck unit which are provided in vessels embodying the first aspect of the invention can be omitted.

A vessel according to either aspect of the present invention can be provided with thruster units on the hulls, for dynamic positioning purposes, and for self-propulsion.

A vessel according to the present invention can be constructed from a plurality of units, each of which units can be built at a different site. For example, the hulls, columns, and tubular bracings (if provided) may constitute one unit and be constructed on one site. The deck unit may be built at a separate site and then the two units finally mated together.

In preferred vessels embodying the present invention, the hulls are longer than the deck unit.

Moreover, the vessel is longer than it is wide.

Preferably, the hulls are generally rectangular in cross-section over a substantial proportion of their length.

A vessel embodying the present invention may be self-propelled and dynamically positionable for drilling or production at seabed depths down to 1500 metres. The vessel may be moored in position and assisted by dynamic positioning for depths between about 300 metres and about 460 metres. The vessel may be moored only, without the need for thruster assistance (dynamic positioning), for depths down to about 300 metres. For depths below about 460 metres, positioning can be effected wholly by means of thrusters.

The deck loading will be such that sufficient tools and equipment with logistic and personnel support will enable drilling to be carried out through a depth of water of up to 1 500 metres, and down to about 7620 metres from the seabed.

The vessel is capable of accommodating flexing at a riser base of 7 2 degrees to the vertical, being kept within this degree of movement by its station keeping ability.

Maintenance and support subsea down to the maximum operating sea depth can be provided by diving bells, air and saturation diving systems and remotely controlled methods deployed from the vessel, depending upon the water depths.

Personnel and logistic support can be implemented using helicopters, provision for which is made in the vessel.

A vessel embodying the present invention is a semi-submersible structure having buoyant twin hulls, several tens of metres in length with an integrai deck structure supported by a plurality of hollow buoyant columns several tens of metres high, the plan section of each of which columns is polygonal in shape with a minimum of eight facets or sides.

When the vessel is in use for exploration or exploitation, the hulls are completely submerged and the columns submerged over a part of their height. For example the vessel may be ballasted down to 23 metres to ieave 13 metres airgap between mean sea level and the underside of the deck structure.

Because the vessel may be self propelled, it can traverse long distances across the oceans using its own power. Normally, when the vessel is being taken to a new location, the hulls are only partly submerged, with the columns clear of the water.

For example the hulls may be 10.5 metres high with the vessel having a transit draught of 10 metres to leave 0.5 metres freeboard on the hulls.

Where a vessel embodying the present invention is for exloration purposes, mounted on the deck will be a drilling rig spaced around a rectangular moonpool. The vertical sides of the moonpool form the inner surfaces of the deck structure of the vessel. The load from the drilling rig is transferred onto the columns through the deck structure and, if they are provided, angled tubular bracings which are each attached to the deck structure at one extremity and onto a column below deck level at the other.

On the main deck there is provided a relatively high drill floor above and over the moonpool which will allow for the blowout preventer stack and lower riser assembly to be assembled onto a test stump and to be fully tested prior to subsea deployment, with provision made for all control cables to remain connected to these units during movements to and from the test stump and the subsequent subsea deployment.

Where a vessel embodying the present invention is for exploitation purposes, a high drill floor with adequately sized moonpool enhancse the vessel for hydrocarbon production purposes, because the production riser top uptakes can be located above the cellar deck and in a sheltered space rather than in the wave affected airspace below the cellar deck as has generally been the case previously.

A vessel embodying the present invention has a substantial sub-structure, so that the high torsional and other loadings associated with a tensioned production riser system can be accommodated. The vessel is suitable for heavy duty use.

It has been established in a semi-submersible twin-hull vessel the effects of wave motions and forces are considerably reduced when compared with the effects manifest with mono-hull vessels.

In the operating condition, with the hulls of the twin-hull vessel fully submerged and the columns submerged to a depth sufficient for a particular operational mode, the effects of waveforces are considerably smaller than in the case of a monohull vessel. The contact of the semi-submersible twin-hulled vessel with the water surface where the effects of wave action are most severe is relatively limited since this contact occurs at the vertical columns supporting the deck structure. As a consequence the natural tendency of a semisubmersible to return to a position of equilibrium after displacement by wave induced forces is such that the vessel can operate even in the severe conditions found for example in the Northern North Sea, offshore Eastern Canada and Offshore Alaska.

The depth of water in areas which can have severe weather, and the added hazards experienced in winter, such as ice formation on a vessel, calls for an alternative mode of well drilling and product production to the existing shallow water techniques together with a method of carrying out surveillance and maintenance at the depths usually associated with these conditions. A vessel embodying the present invention can meet these requirements.

One form of vessel embodying the present invention can be used for drilling purposes, and another form for production purposes. A vessel embodying the present invention may be called a Heavy Duty Vessel because of its ability to meet and be used in severe conditions.

Reference will now be made, by way of example, to the accompnaying drawings, in which: Figure 1 is a diagrammatic perspective illustration of one of form of semi-submersible drilling vessel embodying the first form of the present invention, Figure 2A is a schematic plan view of the twin hulls of an embodiment of the present invention, showing a preferred arrangement of six columns for the vessel, Figure 2B is a schematic elevational view, showing a deck unit rigidly attached to the hulls by the columns, of a vessel embodying the present invention. A helicopter facility and landing pad are shown.A drilling derrick, which is schematically illustrated in Figure 2B, and equipment housing, will be attached and secured rigidly to the deck unit over the moonpool area which allows access down through the deck unit for drilling or production operation, Figure 3 is a more detailed schematic elevational view of a vessel embodying the present invention, looking onto the starboard side of the vessel, Figure 4 is a schematic plan view of a vessel embodying the present invention, looking down on to a main deck of the vessel, Figure 5 is a schematic plan view of a lower deck, in which a moonpool can be seen, of a vessel embodying the present invention, and Figure 6 a schematic plan view illustrating the columns and bracings of a vessel embodying the first form of the present invention.

Figure 1 is a schematic perspective representation of a vessel embodying the first form of the present invention.

A platform or deck 1 is supported by a plurality of hollow vertical columns 2 which are polygonal in cross-section and sub-divided into water tights flats and compartments and spaces (see Figure 3) providing buoyancy and some areas for containment of liquids or the installation of machinery and equipment. The columns 2 are several tens of metres in height. In plan section, each column is polygonal, and has at least eight facets 3. Flared transition piecse 4 extend between the columns 2 and the deck structure 1, for transferring loads between the column shells and the deck structure.

Two hollow hulls 5 are provided. The hollow hulls 5 provide buoyancy for the vessel and are sub-divided into watertight compartments, some of which may be used for the storage of liquids, equipment and machinery for operating purposes.

Each hull 5 is generally rectangular in crosssectoin along a substantial part of its length.

Each hull 5 is several tens of metres in length, and the two hulls are parallel to one another in a forward and aft direction, and extend beyond the deck unit or structure at the forward and aft ends of the vessel. The overall length of the hull 5 is substantially greater than the overall width of the hulls or pontoons. This allows the illustrated arrangement of columns 2 to be used for supporting the deck unit 1, which deck unit is generally rectangular in shape having vertical outer sides, and vertical inner sides (around the moonpool). The deck unit is longer in the forward and aft direction than it is transversely or athwartships.

As mentioned above, the deck unit can be constructed at an establishment other than used for the construction of the columns, for example, and this is taken into account in the design of the deck unit.

The deck unit 1 is divided into watertight spaces or compartments with a rectangular shaped open moonpool having vertical sides extending down from the main deck, thereby to allow drilling equipment, strings, risers, casings and tubing, for example to be deployed. The moonpool 17 can be seen in Figure 5.

When the vessel embodying the present invention is for use for hydro-carbon production, the moonpool 1 7 is used for well maintenance and servicing, allowing the deployment of risers, guidelines and other equipment necessary for production purposes.

The arrangement of columns 2 on the hulls 5 can be seen in plan in Figure 2A. In Figure 28 the arrangement of columns 2 on the hulls 5, and the arrangement of the deck unit 1 on the columns 3 can be seen. In Figure 2B a drilling derrick is shown on the deck unit above the moonpool (not shown in Figure 2B).

As best seen in Figure 6, the forward pair of columns 2 (C1 and C2) and the after pair of columns 2 (C5 and C6) are joined or cross braced by hollow tubes 6 (respectively B1, B5) which are internally stiffened. As best seen in Figure 1, transition pieces 7 at each extremity of the cross braces B1 and B5 allow for structural strength and integrity to be maintained. The tubular stiffeners 6 each join with one of the facets 3 of the columns 2 at a point above but near to the hulls 5, as best seen in Figure 1.

Four hollow tubular bracings 8 (respectively B31, B32, 833, B34) extend in respective pairs from the respective central columns 2 (C3, C4).

The tubular bracings 8 of the respective pairs extend upwardly and apart from the respective columns to join the deck unit 1. The tubular bracings 8 allow for the transfer of loads, for example from drilling machinery and equipment, from the deck unit to the columns.

The tubular bracings 8 are separately joined to inwardly facing facets 8 of the centre port and starboard columns at points above but near to the hulls 5. The tubular bracings 8 are angled upwards and outwards to join the deck unit 1 at the four corners at the lower edges of the moonpool 1 7 (see Figure 5).

Attached to the hulls 5 are the equipment and appendages 9 necessary for ship-like operations, when the vessel is in transit. In these circumstances the hulls 5 will be only partly submerged.

As seen in Figure 1, a plurality of thrusters 11 are mounted below the hulls 5, free to rotate in a horizontal plane, for use in propelling the vessel in transit, and for manoeuvering and dynamic positioning purposes.

Figure 1 illustrates a vessel embodying the first form of the present invention in which bracings 8 are provided. In a vessel embodying the second form of the present invention the bracings 8 are omitted. The deck structure is constructed to carry the load thereon to the columns. In this case a protective housing for a servicing and handling area can contribute to the strength of the vessel.

In the preferred arrangement of Figure 2A six vertical columns 3 are provided on the hulls 5.

Figure 2B shows a preferred form of deck unit 1. The deck unit 1 has a double bottom 12 and a lower deck 13, a between deck 14, and a main deck 1 5 which together form the deck unit 1. Each deck is sub-divided into machinery rooms, accommodation and public spaces, in conformance with the forward and aft and transverse vertical bulkhead arrangements provided in the deck unit. An example of an arrangement of machinery rooms etc is shown in Figure 5, in which the various compartments into which the deck unit is divided by the bulkhead arrangements are indicated by 1 6. In Figure 5, 1 7 is the moonpool.

Mounted on the main deck 15 of the deck unit 1 are cranes or booms 25, a helideck 19, a control and navigation centre 20, the drilling derrick 21, drilling equipment, storage and drilling machinery housings 22, drill floor 26, subsea maintenance housing 23, for equipment used for the deployment and support of saturation diving, and the deployment of vehicles used subsea such as remote control vehicles and diving bells, for example.

In Figure 4, the equipment shown mounted on the main deck 1 5 is that provided when the vessel is for exploration purposes. Alternatively, the drilling equipment and machinery shown in Figure 4 would be replaced by equipment and machinery for oil production purposes.

Figure 3 illustrates an arrangement in which some of the columns 2 are subdivided into machinery spaces and fluid compartments or tanks 23.

Also shown in Figure 3 is life saving equipment 24, and winch housings and winches 25 which are placed at the four corners of the main deck and which are used for mooring purposes.

Figure 4 is a plan view looking down onto the main deck showing one arrangement of mooring winches and housings, life saving equipment, cranes, bridge and control centre, diving equipment housing, and helipads.

It will be seen from Figures 1 and 3 that the hulls 5 are generally rectangular, with rounded or facetted corners, in cross-section, over a substantial part of their length. With thruster units for self propulsion and dynamic positioning purposes at the extremities of the hulls.

The bracings 6 and 8 are shown to be generally circular in cross-section by way of example. It is possible that the bracings 6 and 8 may have a polygonal, facetted, form.

A vessel embodying the present invention can be fabricated, assembled and outfitted all at one building site. Or, alternatively, the vessel may be constructed in separate units at separate sites.

When the separate units are completed they can be mated together, to form a complete vessel, at a site or offshire. This provides great flexibility with regard to the building of a vessel embodying the present invention.

For example, the vessel may be constructed in three separate units: Unit 1:-the hulls 5 with vertical columns 2 and transition pieces 4, with horizontal bracing 6 and their transition pieces 7 (together with the angled bracings 8 if provided).

Unit 2:-the complete deck unit 1. The deck unit may be provided bare of the equipment used for exploration or exploitation, for example equipment for vessel upkeep and navigation and the drilling derrick and drilling equipment, or hydro-carbon production equipment.

Unit 3:-the hull of or any part of equipment to be mounted on the main deck can be treated as a separate third unit. This allows the weight of the deck unit to be kept down.

Unit 1 and unit 2 may be mated as follows.

Unit 1 is taken offshore then submerged so that the tops of the columns are only slightly above water level. Unit 2, the deck unit 1, is floated on barges to a position over the columns of unit 1 so that the lower surface of the deck unit 1 can be mated to the tops of the columns.

As mentioned above, the deck unit may be bare of operating equipment for this mating operation, but if the transport barges can take a sufficient load, operating equipment may be provided upon the deck unit before it and unit 1 are brought together.

Thus, an embodiment of the present invention provides a twin hulled type of vessel, with decks rectangular in shapQbeing generally of greater overall dimension forward and aft than athwartships, suitable for offshore hydro-carbon exploration, when provided with the necessary derrick and equipments used offshore for drilling, or suitable for offshore hydro-carbon exploitation, with the vessel either moored, or moored and dynamically assisted, or wholly dynamically positioned, the vessel having twin hulls which are compartmented and generally rectangular, with rounded or facetted corners, in cross-section, for a substantial part of their length, with the extremeities of the hulls supporting a plurality of thruster units for self-propulsion and dynamic positioning purposes, the vessel also having three or more pairs of vertically mounted columns placed opposite each other, each column having a plurality of facets, more than eight in number, with flared transition pieces forming the structure between the columns and the deck unit, and incorporating a method of separating the hulls at a set distance by horizontally placed tubular bracings at each extremity.

A vessel embodying the first form of the present invention has tubular bracings in pairs joined to each of the centre columns at each extremity in way of the column facets being angled apart and upwards to mate with the deck unit in way of the double bottom outer skin and at the bottom edges of the rectangular moonpool with each pair of bracings mating with the moonpool and the double bottom skin at the same side.

In a vessel embodying the second form of the present invention the tubular bracings are omitted.

A protective housing is constructed to maintain strength and structural integrity.

A vessel embodying the present invention can be built as a unit, or as a combination of units such as hulls, columns and bracings.

A vessel embodying the present invention can have a deck unit comprising a series of decks such that the deck unit has a generally rectangular shape with vertical outer sides and with a rectangular opening therethrough, (the moonpool) having vertical sides parallel to the outer sides of the deck unit. The vessel structure is such that a substantial part of, for example, the drilling derrick, drilling machinery and drilling loads are transferred through load bearing members to the angled tubular bracings down to the centre columns. Alternatively the angled tubular bracings can be omitted and a protective housing constructed so as to maintain strength and structural integrity.

The deck unit of a vessel embodying the present invention has forward-to-aft and transverse bulkheads forming a series of box-like compartments to contain the necessary machinery, outfits, equipment, accommodation and offices to support offshore deepwater hydrocarbon exploration activity or offshore deepwater production.

The deck unit of a vessel embodying the present invention can be so structured that it can quickly be put into a condition of watertight integrity in the case of collision damage or in any other case in which flooding of the hulls or part of the hulls, or flooding of the columns or part of the columns, or flooding of the horizontal (or angled bracings if provided) occurs. This is achieved by providing that the bulkheads form an arrangement of integral watertight compartments separated by watertight doors. Only limited communication between different watertight compartments is allowed, and in particular the number of openings accessing the moonpool is restricted. A doubledoor lock-out arrangement can be used to control this access.Such a double-door lock-out arrangement is such that access to the moonpool, for example, can only be gained through two doors, each watertight, each of the two doors being openable only when the other door is closed.

A vessel embodying the present invention can have a series of compartments containing equipments, and equipments mounted on the main deck, to enable the vessel to navigate and to enable operations to be undertaken for the purpose of implementing offshore deepwater drilling for hydro-carbon products, or for the purpose of implementing the production of hydrocarbons.

In a vessel embodying the present invention, for hydrocarbon exploration, preferably, the dimensions of the moonpool are substantially those of the drilling derrick base mounted on the main deck, so that the drilling derrick base is around and immediately adjacent to the moonpool.

Preferably, in a vessel embodying the present invention, for hydro-carbon exploration, a drill floor is provided which is situated above the main deck at such a height as will allow a blow-out preventer stack and a lower riser assembly to be tested and serviced as one unit without the need for subsequent disassembly for deployment during drilling operations. Preferably also, in a vessel embodying the present invention for hydro-carbon production, the drill floor is situated above the main deck to a height such that the location of production riser top offtakes is in a sheltered environment.

A vessel embodying the present invention can be so structured as to be able to cope with the severe icing conditions which can prevail in some hostile offshore locations. This can be provided by appropriate strengthening of the structure of the vessel, and the provision of de-icing arrangements. Particularly, the height of the drill floor, allowing operations to take place in a sheltered environment, facilitates operation of the vessel in hostile waters.

A vessel embodying the present invention can be anchored or catenary moored with or without dynamic positioning assistance, in dependence upon the depth of water in which the vessel is operating, for hydro-carbon exploration or production.

Furthermore, a vessel embodying the present invention can operate without resource to catenary mooring or anchoring, using dynamic positioning as the sole means of station keeping.

A vessel embodying this invention, as will be appreciated from Figure 1 , viewed from forward or aft has a portal structure. That is, between the columns rising from the hulls, and between the deck unit atop the columns and the tubular bracing between column (6), a 'portal' is formed, which is characteristic of the structure of the vessel.

The provision of a relatively high drill deck, offering a sheltered working environment facilitates operation of the vessel in severe weather conditions.

Incidentally, in Figure 4, TESC stands for "Totally Enclosed Survival Craft".

Claims (12)

1. A semi-submersible vessel, having two hulls, three or more pairs of columns, one column of each pair mounted on one of the two hulls, the other column of the pair mounted on the other hull, a deck unit, having a moonpool, mounted on the columns, and tubular bracings extending between the columns of the foremost and aftmost pairs of columns, to provide the vessel with a portal structure as herein defined.

2. A semi-submersible vessel, having two hulls, three or more pairs of columns, one column of each pair mounted on one of the two hulls, the other column of the pair mounted on the other hull, a deck unit, having a moonpool, mounted above the columns, and tubular bracings extending between the columns of the forwardmost and aftmost pairs of columns, and each column of a pair, other than the forwardmost or aftmost pairs, having two tubular bracings extending therefrom upwards and apart to join with the deck unit adjacent the moonpool on the same side of the moonpool as the column.

3. A semi-submersible vessel, having two hulls, three or more pairs of columns, one column of each pair mounted on one of the two hulls, the other column of the pair mounted on the other hull, a deck unit, having a moonpool, mounted above the columns, and tubular bracings extending between the columns of the forwardmost and aftmost pairs of columns, the vessel also having a drill floor above the moonpool at a height sufficient for a blowout preventer stack and a lower riser assembly to be handled as a unit, with a protective housing for the handling area, which protective housing is constructed to provide strength and structural integrity for the vessel.

4. A semi-submersible vessel according to claim 1 or 2, having a drill floor above the moonpool at a height sufficient for a blowout preventer stack and a lower riser assembly to be handled as a unit.

5. A vessel as claimed in claim 1, 2, 3, or 4, wherein each column is polygonal in crosssection, having at least eight facets.

6. A vessel as claimed in claim 5, wherein each column is provided with a flared transition piece between the top of the column and the deck unit.

7. A vessel as claimed in any preceding claim, wherein each hull is generally rectangular in crosssection over a substantial portion of its length.

8. A vessel as claimed in any preceding claim, in which the deck unit is of generally greater overall dimension forward and aft than athwartships.

9. A vessel as claimed in any preceding claim, wherein the hulls are longer than the general forward and aft extent of the deck unit.

10. A vessel as claimed in any preceding claim, having a plurality of thrusters mounted below the hulls.

11. A vessel as claimed in any preceding claim, having a first, transit, disposition in which the hulls are only partly submerged, and a second, operational, disposition in which the hulls are submerged and the tubular bracings extending between the columns of the forwardmost and aftmost pairs of columns are at least partially submerged.

12. A vessel as claimed in any preceding claim, constructed by joining together the following separately constructed units: (1) a unit comprising the hulls, the columns and the tubular bracings extending between the columns of the forwardmost and aftmost pairs of columns; (2) the deck unit.

1 3. A semi-submersible vessel, substantially as hereinbefore described with reference to the accompanying drawings.