GB2159468A - An offshore vessel - Google Patents

Abstract

In an offshore vessel adapted to be used where extreme weather conditions are expected, the part of the structure coming into direct contact with the sea water is manufactured from concrete, preferably in the form of pre-stressed elements, while the working deck (12) is made as a steel structure. The underwater body includes a four-sided pontoon-frame (10) composed of cube-shaped corner pieces (14) between which tubular boxes (15) extend. The corner pieces (14) carry cylindrical columns (11), upon which, in turn, the working deck (12) rests. Each corner piece (14), plus halves of the adjoining boxes (15) may be assembled in a dry dock and, after being towed out therefrom, can be connected to other similar units. The corner pieces (1a) and box halves may themselves be constructed from prefabricated elements. <IMAGE>

Description

SPECIFICATION An offshore vessel Offshore vessels used for prospecting and drilling, but also as accommodation for the working staff, are conventionally built as steel structures.

Concrete has been extensively used in diverse marine structures, such as harbours, bridges and offshore structures resting on the bottom of the sea. Certain vessels, mostly barges, have also been built of concrete.

Concrete having a low rate of water/cement and a high content of cement has a satisfactory tightness and strength. It is furthermore important that the concrete is frost-resistant and can withstand corrosive attacks and wear.

Reinforced concrete structures have properties making them suitable for use even in very exposed positions, and great advantages, i.a. with respect to maintenance, can be attained if the parts of an offshore vessel coming into direct contact with sea water are made of concrete.

The object of the present invention is to provide an offshore vessel, especially one for use where extreme weather conditions, including the risk of ice drifts, may be expected, but which can be manufactured at an economically competitive price.

The invention refers to an offshore vessel of the type comprising an underwater pontoon formed as a four-sided ring from the corner of which columns project upwards and carry a working deck and is characterized in that the pontoon-frame is com posed of substantially cube-shaped corner pieces, between which tubular boxes extend, wherein said frame components, which may be composed of prefabricated elements, are so designed, that each corner piece, plus halves of the adjoining boxes may be assembled to a unit in a dry dock, and after being towed out therefrom, can be connected to other similar units.

The vessel is preferably designed so a corner piece plus adjoining box portions have sufficient displacement to carry a column mounted upon the corner piece. Corner pieces, boxes and columns advantageously are manufactured of reinforced concrete.

The boxes may be composed of tubular ele ments having internal strengthening walls. The ele ments may be provided with a tubular core, from which radially directed, strengthening walls extend.

The strengthening walls are preferably provided with openings, so spaces within the element com municate.

For the fitting of longitudinally running reinforc ing bars longitudinally directed passages are provided in the envelope wall of the elements, located where the strengthening walls merge into the envelope wall.

Peripherally running reinforcing bars in the enve lope wall of the box elements may be fitted into channels, which each extends from one reinforcing wall, past the adjacent wall and reaches the next wall, the ends of the bars being accessible from within the element.

To facilitate towing out of the dock and for later use as oil storing tanks each element is provided with an end wall at at least one of its ends, said wall not fully closing the element.

The working deck, is preferably in a manner known per se manufactured from steel plate and comprises fitting portions adapted to be attached to the columns by way of the reinforcing bars thereon.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows an offshore vessel according to the invention, fitted for oil production, Figure 2 schematically shows an elevation of the structure of the vessel according to Figure 1, Figure 3 shows a perspective view of a bottom plate in a corner piece, Figure 4 shows a horizontal section through a corner piece, Figure 5 shows a detail of the connection between two side walls in a corner piece, Figure 6 shows a plan view of a corner piece and parts of two associated boxes during fitting in a dry dock, Figure 7 shows the unit according to Figure 6 being towed out of the dry dock, Figure 8 shows the fitting of a column upon the component according to Figures 6 and 7, Figure 9 shows a perspective view of a box element, Figure 10 shows an end view of a box element, Figure 11 on a larger scale, shows a portion of a strengthening wall in a box element, Figure 12 shows a longitudinal section through a box element, Figure 13 shows the principle for the peripheral reinforcing bars in the box elements, Figure 14 shows a detail at the attachment of the reinforcing bars, and Figure 15 shows a longitudinal section through a portion of a box.

The offshore vessel shown in Figure 1 is of a basically well-known semi-submersible type, but has a larger displacement than hitherto used vessels, and has a reinforced structure making it suitable for use where extreme weather conditions are expected, even in arctic waters.

The parts of the vessel coming into direct contact with the sea water - a ring-shaped underwater pontoon 10 and four columns 11 projecting upwards therefrom - are manufactured from prestressed concrete. The columns carry a working deck 12 designed as a steel structure. Equipment including a derrick 13 for drilling and the handling of pipes, is located upon the deck.

As will be evident from the following description the underwater pontoon 10 comprises cube-shaped corner pieces 14 and tubular boxes connecting the same. The corner pieces and the boxes are composed of prefabricated components, in which the composition of the concrete has been selected with due respect to the field of use, and to expected service stresses.

Figure 2 schematically shows an elevation of the vessel. Columns 11, corner pieces 14 and boxes 15 enclose spaces, which in a conventional manner will be used for various operational functions. The columns and boxes are composed of annular elements lia and 15a, respectively. The deck 12, which is a steel structure, is provided with built-in portions 12a, adapted to be fitted to the top ends of the columns 11, and to be attached thereto by means of the strengthening bars therein.

The corner pieces 14 and the boxes 15 are, as mentioned above, fitted together from concrete components, which are reinforced and pre-stressed in the conventional manner.

Figures 3 and 4 show a bottom plate for, and a cross-section through a corner piece 14, respectively. The bottom plate 16 has a square configuration and is provided with a ledge 17 just inside its perimeter, forming a support for triangular-shaped strengthening member, together forming a pyra mid-shaped structure adapted to abut similar structures in adjoining wall components 19 (Figure 4).

The corner plate of a corner piece, as well as two of its side walls are shaped in substantially the same manner, but are each provided with a cylindrical opening adapted to receive a lower column element 11a, or a box element.

The side wall components 18 are fitted together by means of profiled corner posts 20, and the com ponents are interlocked in the manner shown in Figure 5.

The ledges 17 are provided with external strengthening brackets 21, and in the corners of each plate 16, 18 there are recesses for the corner posts, in Figure 3 denoted by 22. In each corner piece there are thus vertical as well as horizontal corner posts 20. The shape thereof is selected so they will lock the plates securely.

In mounted position the triangular strengthening member 19 of adjacent plates will support each other, as is indicated in Figures 6 and 7.

Figure 5 shows, on a larger scale, a detail view of the connection between plates 16 and 18, or 18 and 18 respectively, at the strengthening members 19 and at the brackets 21. The latter are intercon nected by means of cross clamps 23, and the strengthening members are interconnected by fur ther cross clamps 24, 25.

The elements 15a forming the boxes 15 will be described more fully below. For explaining the mounting illustrated in Figures 6 - 8 it is sufficient to remember that these elements 15a, as well as elements 11a in the columns are annular compo nents.

The fitting together is made in a dry dock 30, and Figure 6 shows a plan view of a portion thereof, where a corner piece 14, plus halves of as sociated boxes 15 have just been fitted together.

The corner piece 14 can be assembled in the dock, and the box elements 15a may be manufactured in the dock, or elsewhere, and will be joined step by step.

The corner piece is turned with its top plate up wards, and the opening where the first column ele ment 11a is to be fitted is denoted by 31.

At least the outward box elements 15a of each box part are provided with an end wall 32 of sufficient height to prevent water from entering the boxes, when the dock 30 is filled with water.

The corner pieces 14 and the associated parts of the boxes 15 will now have a displacement which is sufficient for the structure to be towed out of the dock 30, as indicated in Figure 7.

When four similar units have been completed and towed out of the dock, the halves of the boxes are joined so the square-ring underwater pontoon 10 of Figure 1 is obtained. The pontoon is then moored to a quay 33, as shown in Figure 8, where a crane 34 will locate the column elements 11a upon the pontoon.

The elements 11a of the columns may be simple tubular components, having longitudinal as well as peripheral reinforcing bars. Abutting edges between the annular elements, and between the lowermost element and the corner piece, respectively, are provided with interengaging ledges, and the joint are sealed in any suitable manner. Longitudinal, preferably pre-stressed reinforcing bars connect the elements mutually, and with the corner piece, as well as with the fitting portions 12a at the deck 12.

The annular elements 11a may be provided with internal strengthening walls, which are located so they do not prevent vertical transport within the column.

The annular elements 15a of the boxes 15 of the underwater pontoon are arranged and reinforced in a special manner, i.a. with respect to the wish to be able to use at least part of the boxes for the storing of oil.

Figures 9 and 10 show a perspective view and a cross-section, respectively of an annular element 15a for a box structure 15. Each element has a tubular core 35, which is attached to transverse end walls (see Figure 12). For the sake of simplicity Figure 9 does not show any end wall. A number of reinforcing walls 36 extend radially from the core 35 to the envelope wall 37 of the element. Axially directed passages 38 for reinforcing bars are provided in the envelope wall.

The reinforcing walls 36 are, as is shown on a larger scale in Figure 11, provided with openings 39, so spaces 40 within the element will communicate.

As was mentioned in connection with Figures 6 and 7, at least the outward element in each box half shall be provided with an end wall 32, preventing ingress of water into the boxes and the corner piece, when the dock is filled with water to permit the unit being towed out.

Preferably one wall 32a, 32b is located at each end of each box element 15a. There is then one opening 41 in the part of wall 32a, which in mounted position will be turned upwards, while there is a corresponding opening 42 at the lower portion of the other wall 32b.

When the box elements 15a are fitted together, as indicated in Figure 15, a passage 44 is formed between end pair of juxtaposed elements, and defined by the walls 32b and 32a. These passages will each have an opening 42 at their lower end, and an opening 41 at their upper end, as counted away from the associated corner piece 14.

A number of spaces within a portion of a box structure 15 will communicate in this manner. The element located remotely with respect to the corner piece is terminated by a full wall 32c, and here an opening 45 is instead provided in the envelope wall, forming a connection to the surrounding water.

Initially the portion of the box structure, referred to (in Figure 15 comprising four elements), is filled with sea water up to a full wall 46, which separates the box structure from the corner piece 14.

The oil produced during operation is forced through a conduit 49 to the box element 15a, adjacent to the corner piece, and expells the sea water therefrom. The water is transferred to a following element and so on, and a volume of water, corresponding to the amount of oil supplied, is expelled through opening 45.

The oil will float on top of the water, and when the space within the first box element is filled with oil, this will continue to flow through the first passage 44 and pass into the following box element, and so on until all elements in the series have been filled.

Withdrawal of oil also occurs through conduit 49, the pressure of the surrounding sea water ensuring a back-filling through opening 45, so the oil is withdrawn A box element is, in conventional manner provided with a cast-in reinforcing structure, in its envelope wall as well as in the partition walls. As mentioned in connection with Figure 10 there are passages for axially running reinforcing bars 38.

These passages will connect the elements mutually, as well as with the associated corner boxes.

In order to improve the strength there are further peripheral reinforcing bars 47. As is indicated in Figure 13, where the unfitted reinforcing bars 47 are shown outside the envelope wall, each bar 47 extends from the point where one reinforcing wall 36 joins the envelope wall 37, past an adjacent reinforcing wall to a following wall.

In a well known manner the reinforcing bars 47 are located in cast channels 48 in the envelope wall and are subjected to straining in the cast structure. As is evident from the enlarged view in Figure 14 the channels 48 open at the inward face of the envelope wall, and the operation of straining the bars will be performed from within the elements.

The embodiment described above and shown in the drawings refer to an actual project, and the design and shape of the components forming part thereof may be varied in many ways within the scope of the appended claims depending upon the desired field of operation.

Instead of using corner posts 20, the corner pieces 14 may be manufactured from plates, where the reinforcing bars are permitted to project beyond the bordering edges. When the bars of abutting plates have been joined, for instance by welding, the plates are cast together by a suitable concrete mixture.

The elements for the boxes 15 may be manufactured in a similar manner as the corner posts, i.e.

they are composed of plane plates which are cast together. The tubular elements will then not obtain a pure cylindrical cross-section, but rather a section similar to a rectangle with deeply rounded corners.

In order to provide actual proportions to the embodiment shown in the drawings it may be mentioned that the cube-shaped corner pieces 14 have a side measure of 18.5m., that the external diameter of the box structures and columns is close to 17m., that the length of the box structures 15 is 48m. and that the height of the column 11 is 24m.

The steel deck structure may be built directly upon the columns, but at least a carrying structure is preferably manufactured separately and is transferred to the columns. This may be performed using conventional technique where the structure is carried by barges to the floating unit of pontoon and columns, which has been ballasted sufficient to permit the barges passing in between the columns. When the ballast is removed from the pontoon the columns will rise and lift the deck structure away from the barges.

Claims (11)

1. An offshore vessel of the type comprising an underwater pontoon formed as a four-sided ring from the corner of which columns project upwards and carry a working deck, characterized in that the pontoon-frame is composed of substantially cubeshaped corner pieces, between which tubular boxes extend, wherein said frame components, which may be composed of prefabricated elements, are so designed, that each corner piece, plus halves of the adjoining boxes may be assembled to a unit in a dry dock, and, after being towed out therefrom, can be connected to other similar units.

2. An offshore vessel as claimed in claim 1, characterized in that a corner piece plus adjoining box portions have sufficient displacement to carry a column mounted upon the corner piece.

3. An offshore vessel as claimed in either claim 1 or claim 2, characterized in that corner pieces, boxes and columns are manufactured of reinforced concrete.

4. An offshore vessel as claimed in claim 3, characterized in that the boxes are composed of tubular elements having internal strengthening walls.

5. An offshore vessel as claimed in claim 4, characterized in that the elements are provided with a tubular core, and radially directed, strengthening walls, extending therefrom.

6. An offshore vessel as claimed in either of claims 5 and 6, characterized in that the strengthening walls are provided with openings, so spaces within the element communicate.

7. An offshore vessel as claimed in any one of the preceding claims, characterized in longitudinally directed passages in the envelope wall of the elements located where the strengthening walls merge into the envelope wall.

8. An offshore vessel as claimed in any one of claims 4 to 7, characterized in peripherally running reinforcing bars in the envelope wall of the box elements fitted into channels, which each extends from one reinforcing wall, past the adjacent wall and reaches the next wall, the ends of the bars being accessible from within the element.

9. An offshore vessel as claimed in any one of claims 3 to 8, characterized in that each element is provided with an end wall at at least one of its ends, said wall not fully closing the element.

10. An offshore vessel as claimed in any one of the preceding claims, characterized in that the working deck, in a manner known verse is manufactured from steel plate and comprises fitting portions adapted to be attached to the columns by way of reinforcing bars thereon.

11. An offshore vessel substantially as hereinbefore described with reference to and as shown in the accompanying drawings.