GB2163393A - A semi-submersible vessel - Google Patents

Abstract

A semi-submersible vessel has a deck structure (10) supported on columns (11) above two hulls (12) and means to ballast it between a transit condition wherein the hulls have freeboard and an operating condition wherein the hulls and part of the columns are submerged. Two cranes (14) are mounted on the deck above the rear columns (11). A body (19) interconnects the hulls (12) below the rear columns (11), and lies underwater on the vessel's transit condition. The body provides structural strength, extra buoyancy under the cranes, and an increase in the inertia of the vessel in relation to heave and trim motions when the vessel is semi-submerged, and is streamlined to reduce drag in transit. A second body (19) may be provided at the front of the vessel close to a second pair of columns (11). <IMAGE>

Description

SPECIFICATION Improvements in and relating to semi-submersible vessels The present invention relates to a semi-semi-submersible vessel which comprises two substantially parallel hulls and a deck structure resting on at least two support columns on each hull.

Such vessels may be equipped with powerful cranes and are used, for example, for heavy-lifting work in the offshore industry, such as in the erection and dismantling of production-platform equipment and also for the transportation of heavy sections. The vessel may then be provided towards one end thereof with a large pivotable crane, or with two such cranes that are capable of working together when lifting heavy loads and of working individually in the case of lighter loads.

The invention provides a semi-submersible vessel comprising two generally horizontal, parallel hulls, a deck structure, at least two columns extending upwardly from each of the said hulls to support the said deck structure, draught-controlling ballasting means for varying the draught of the said vessel between a transit condition in which the said hulls have freeboard and an operating condition in which the said hulls and lower portions of the said columns are submerged, and a body interconnecting the said hulls at an end portion of the said hulls close to a pair of the said columns, the one extending from one hull and the other extending from the other hull, the said body lying beneath the surface of the water when the said vessel is in the said transit condition.

Such a vessel has in general two usual floating conditions. One is an operating condition, with the hulls fully submerged and the waterplane intersecting the columns. In that condition, the vessel has a very small waterplane area and is thus almost transparent to wave action and the like. The vessel then provides a steady working platform that has a very soft motion response. The other usual condition of the vessel is a transit condition in which the hulls have freeboard, and the vessel has reduced drag as compared with its drag when it is in its operating condition. The vessel then has a much larger waterplane area and, consequently, a much stiffer motion response.In the vessel according to the invention, the body inter-connecting the hulls can provide buoyancy even when the vessel is in the transit condition, without the increase in drag that would be caused by a transverse body so located that it broke the waterplane, when the vessel was at transit draught.

One form of semi-submersible crane vessel constructed in accordance with the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the vessel; and Figure 2 is a starboard side elevation view of the vessel, partly in cross-section.

Referring to the accompanying drawings, a semisubmersible crane vessel comprises a working deck structure 10, which is supported above two hulls 12 via four corner columns 11. Also mounted on each hull 12, between the two corner columns 11, is a reinforcing columns 13, which serves to counteract downward deflection of the deck structure 10 when it is placed under load. The deck structure 10 is a multi-storey structure of box-construction and has two large slewing cranes 14 mounted thereon towards its rear end. Located towards the other end of the deck structure 10, the forward orfoXc'sle end 15, is a deck house 16, in which crew accommodation and a wheel-house are provided. The two hulls 12 are joined together by means of a body 19 located at the stern 21 of the vessel.Although not shown, bracing struts may be suitably provided between the forward ends of the hulls and between the columns resting thereon.

Within the columns 11 and the hulls 12 are tanks (not shown) for sea-water ballast and means (not shown) for filling and emptying those tanks to control the heel, trim, and draught of the vessel. The draught can be altered between a semi-submerged operating condition in which the waterplane 18 is part of the way up the columns 11 and 13 and a shallow-draught transit condition in which the waterplane 18a is below the tops of the hulls.

Both hulls 12 are provided with a plurality of azimuth thruster propellers 20, for positioning, steering and forward propulsion of the vessel. The two hulls 12 are also equipped both fore and aft with a number of anchors (not shown).

A number of thruster propellers 20 (for example six, of which two are shown in Figure 2) are located beneath the bottom line 23 of the vessel and must be withdrawn if, for example the vessel is to be laid up in dry dock. The remaining thruster propellers 20 (of which there may be two, one of them being shown in Figure 2) are located on rear portions 24 of the bottom plating 25, which are raised above the bottom line 23, and these propellers 20 are located completely above the bottom line.

The undersurface 19a of the body 19 is flush with the bottom plating 25 of the hulls 12, including the raised portion 24. The leading edge of the body 19 consists of a curved section 19b which connects the undersurface 19a with a substantially planar and horizontal upper surface 19c.

As is most clearly seen in Figure 2, the upper surface 19c of the body 19 is at a lower level than the tops of the hulls 12 and is below the waterplane 18a in the transit condition of the vessel. As shown in Figure 2, the upper surface 19c and the raised portion 24 of the lower surface 19a of the body 19 converge to meet almost at a sharp edge flush with the rear ends 21 of the hulls 12. Instead, the rear portion of the body 19 may be somewhat truncated, terminating in a vertical face part of the way along the raised portion 24. Such a truncated shape is easier to fabricate and has been found to give a more favourable stress distribution in use without seriously increasing the drag of the body 19 when the vessel is in transit.

The body 19 forms a substantially rigid loadbearing connection between the two hulls 12 and is divided internally by vertical bulkheads 28 extending transversely of the vessel from one hull 12 to the other Within the hulls 12 there are vertical bulkheads (not shown) coplanar with the bulkheads 28 and decks (not shown) coplanar with the upper surface 19c of the body 19, and there are vertical bulkheads (not shown) within the columns 11 and the deck structure 10 coplanar with the bulkheads 28. The coplanar bulkheads form continuous load-bearing structures that transmit and distribute forces on the vessel from one hull to the other and from one column to another.

The vertical bulkheads form closed ring-like structures extending through the deck structure 10, the columns 11, the hulls 12, and the body 19. The ring-like structures give good distribution of stresses throughout the vessel, reducing fatigue and stress concentrations, and the fact that the body 19 is so low down makes it possible to achieve especially favourable results.

Because the body 19 has a substantial displacement and is not open to the ambient water, it can provide useful extra buoyancy at the rear of the vessel beneath the cranes 14 both at operating draught and at transit draught of the vessel.

It will be seen from Figure 2 that the longitudintal extent of the body 19 corresponds substantially to the longitudinal extent of the adjacent columns 11. Consequently, substantially all the vertical bulkheads 28 in the body 19 can be continued through the adjacent columns 11, and substantially all the vertical bulkheads (not shown) in the adjacent columns 11 can be continued (as the bulkheads 28) through the body 19.

The body 19 is provided internally with conduits (for example, pipes) 26 through which ballast water can be pumped from one hull to the other when that is necessary to maintain the heel of the vessel within acceptable limits. The body 19 can also be provided with an internal passage 27, through which crew can cross from one hull to the other. Some of the internal volume of the body 19 may be used for ballast tanks if its fully buoyancy is not always needed.

Because the body 19 has, as may be seen from Figure 2, a hydrodynamically favourable, streamlined, shape, and because it is covered by a substantial depth of water even in the transmit condition of the vessel, it does not cause an unacceptably large drag when the vessel is moving from station to station.

At operating draught, although the large horizontal plan area of the body 19 leads to an increased wave loading on the vessel, it has been found that the increase in vessel inertia (taking into account added mass) produced by the body more than compensates for such increased wave loading, at least over a large part of the wave-frequency spectrum, with the result that the effect of waves on heave and changes of trim of the vessel is actually reduced.

Instead of, or in addition to, the body 19 at the stern of the vessel, a similar body may be placed towards the bow of the vessel, beneath and between the forward columns 11. Such a forward body might be similar in shape and dimensions to the body 19, with a rounded front and a tapered rear end. Because the rear end of the forward body would not be adjacent to a raised poriton of the hull bottom plating 25, the rear end portion need not have a horizontal upper surface and a raised lower surface. A taper symmetrical with respect to a horizontal plane, for example, might be used instead.

As an example of suitable dimensions, in a semisubmersible crane vessel with a total displacement of about 150 000 tonnes, the body 19 may have a total displacement of about 6 500 tones and an area, when projected onto a horizontal plane, of about 1 100m2, and its upper surface may be 3 or 4 metres below the waterplane 18a at transit draught and about 20 metres below the waterplane at operating draught.

Claims (10)

1. A semi-submersible vessel comprising two generally horizontal, parallel hulls, a deck structure, at least two columns extending upwardly from each of the said hulls to support the said deck structure, draught-controlling ballasting means for varying the draught of the said vessel between a transit condition in which the said hulls have freeboard and an operating condition in which the said hulls and lower portions of the said columns are submerged, and a body interconnecting the said hulls at an end portion of the said hulls close to a pair of the said columns, the one extending from one hull and the other extending from the other hull, said body lying beneath the surface of the water when the said vessel is in the said transit condition.

2. A vessel according to claim 1, comprising means for lifting and/or lowering loads relative to the vessel, the said lifting and/or lowering means being mounted on an end portion of the said deck structure at the same end of the said vessel as the said body.

3. A vessel according to claim 2, wherein the said lifting and/or lowering means comprises at least one crane.

4. A vessel according to any one of claims 1 to 3, comprising a second body interconnecting the said hulls at the end portion of the said hulls opposite to the first said body, close to a second pair of the said columns, the said second body also lying beneath the surface of the water when the said vessel is in the said transit condition.

5. A vessel according to any one of claims 1 to 4, wherein the or each said body is streamlined in cross section.

6. A vessel according to any one of claims 1 to 5, wherein the bottom of the or each said body is flush with the bottoms of the hulls over at least a major portion of its length.

7. A vessel as claimed in any one of claims 1 to 6, wherein each said hull includes an intermediate deck or bulkhead or other internal reinforcement adjacent to the upper surface of the body.

8. A vessel as claimed in any one of claims 1 to 7, wherein the body, the hulls, the adjacent pair of columns, and the deck structure comprise trans verse vertical bulkheads that provide at least one substantially continuous ring-like load-bearing bulkhead structure.

9. A vessel as claimed in any one of claims 1 to 8, wherein the longitudinal extent of the said body corresponds substantially to the longitudinal extent of the said columns close to it.

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