WO1995019472A1

WO1995019472A1 - Platform construction

Info

Publication number: WO1995019472A1
Application number: PCT/NO1995/000013
Authority: WO
Inventors: Kurt Egil Gramstad
Original assignee: Kurt Egil Gramstad
Priority date: 1994-01-17
Filing date: 1995-01-16
Publication date: 1995-07-20
Also published as: NO940155L; NO940155D0; AU1546595A

Abstract

The invention relates to a method for building an offshore platform having pressure-loadable legs (8), by preference also allowing jacking up above the sea surface while the substructure (3, 3', 3, 3a', 8) is resting on the seabed. In order to be able to build a part platform structure ready in dock (2) and avoid casting of platform legs at large depth in a fjord, the deck frame (6) is built on top of a bottom section (3, 3', 3, 3a') and connected to this through jointable platform leg sections (8) having a length corresponding to a fraction of intended leg length, this part platform structure being towed in a floating condition to a field position, where further leg sections (8) are jointed to form legs of sufficient length for allowing the bottom section (3, 3', 3, 3a') to be anchored to the seabed, the substructure of the platform structure resting on the same. Also, the invention relates to an offshore platform structure having pressure-loadable legs (8), comprising a bottom section, e.g. in the form of a plurality of storage tanks (3, 3', 3, 3a'), and a deck (6) preferably adapted to be jacked up, wherein the platform legs consist of jointable, longitudinal sections (8), and that the deck (6) is formed with vertically through-going, channel-forming sleeves (7), which are formed for the passage of the platform leg sections (8) from above/downwardly, i.a. upon jointing to underlying leg sections (8), and wherein the latter can be attached to the deck.

Description

PLATFORM CONSTRUCTION

The present invention relates to a method for building an offshore platform having fixed legs. Likewise, the invention relates to an offshore platform having fixed legs, built up in accordance with the method.

Such offshore platforms having fixed legs are of a type resting on the seabed, and wherein the substructure in steel or concrete comprises storage tanks or a base structure only. Platforms of the type with which the invention is concerned, may be jackup platforms, where the legs during operations stand on the seabed, the platform deck being jacked up, extending freely above the sea surface, such that the deck and rig equipment carried by the same are spared from participating in heavy sea way.

The building of the concrete substructure for such platforms is started in dock. If the platform concerned is build for a sea depth of e.g. 200 metres, the dock phase is very limited, the partly finished substructure having to be towed out in the nearest fjord having a depth of minimum 200 metres, where the casting work is continued until full substructure height has been reached.

The platform deck is built in a shipyard, thereafter placed on barges and transported to the interconnection place. Prior to the interconnection, the concrete substructure is lowered down to a position a few metres above the sea surface, whereafter the deck is towed into position above the concrete substructure. Thereafter, platform mounting is carried out. The deck is released from the barges, and the entire structure is lifted about 30-40 metres above the sea surface. Thereafter, the whole platform is ready for towing to the field. Building time approximate 3-4 years.

A disadvantage of conventional way of building for offshore platforms of the kind concerned is the very long building time. Other disadvantages consist in the expensive production methods. Also, the structures become very expensive. This is due to the shaft holding the deck. The shaft is shaped and dimensioned such that it withstands high pressures at large depths. The shaft must have a significant buoyancy in order to contribute to the combined buoyancy, such that the platform is capable of being transported to the field. Within the shaft, there is empty space for installing some equipment which, however, without disadvatages is mountable on the outside.

The present invention is based on the idea that the building time for offshore platforms having fixed legs may be reduced very substantially if one builds the substructure in dock and the deck on top of the same, whereafter the interconnected structure is floated to the field for installation on the seabed.

In accordance with the present invention, this is enabled in that the legs comprise jointable length sections, which are jointed together from above, from the deck, as soon as the platform comprising deck and substructure (minus legs) has arrived at the field.

Characterizing features of the method and the platform according to the invention appear from the following claims. According to the method of the invention, the building of the substructure may start with the basis in a steel shirt, on top of which the bottom section is built in the form of domes, whereafter storage tanks are built to a finished state on top of lower domes.

Instead of storage tanks, the platform structure may comprise a frame base.

Storage tanks as well as frame base may be shaped such that they exhibit a substantial buoyancy, so that the tank assembly and the frame base individually will float through a buoyancy of their own.

The platform leg sections may consist of concrete or steel. In the first case it is advantageous to start the pre- fabrication of the leg sections when the deck mounting is started in dock. The length sections of the legs may possibly be pre-stored and used as standard modules.

Taller substructures need normally a stiffening approximately in the mid-region of the height. Such stiffening- or interconnection frames are known per se. This interconnection frame in steel or concrete are placed on top of storage tanks (according to first alternative) or base frame (according to second alternative) prior to the start of the building of the deck frame.

Now, above the interconnection frame, a deck frame is laid, the latter being shaped in order to effect a substantial buoyancy: A parallelepipedon-shaped hollow body with or without bottom represents an advantageous embodiment which, through suitable height extent, will give a sufficiently large buoyancy in order to make the deck frame floating and, additionally, possibly would be capable of carrying mounted rig equipment (e.g. derrick, cranes, etc., and, possibly, unjointed leg sections) . Alternatively, the deck frame would be capable of receiving extra buoyancy from the underlying subsection/substructure component(s) in the form of storage tanks/base frame or from provisional buoyancy bodies.

In any case, the hitherto partial platform structure built in dock must collectively have a sufficient buoyancy to be floating when having rig equipment mounted thereon and, preferably, transporting with it the necessary number of platform leg sections, so that the partial platform section in this condition may be towed to the place of use at the field.

The deck frame which, preferably, on all sides is covered by means of steel plates, is, according to the invention, formed with a number of vertically through-going guidance channels, the number and positioning thereof corresponding to number and positioning of platform legs subsequently mounted. In an elongate rectangular deck frame in combination wih six legs, said vertically through-going guidance channels are distributed such that three appear in the longitudinal direction of the deck frame and two in the width direction thereof. First leg sections are inserted from above into the vertically through-going guidance channels in the deck frame and anchored to the top of adjacent storage tanks, respectively to the top of adjacent portions of base frame.

In this condition, the partial platform structure is self- floating and suitable to be towed out to the field. (Possibly to adjacent fjord to be completed.) Therefore, the partial platform structure leaves the dock now. The buoyancy is, preferably, secured by means of the deck frame and, possibly, storage tanks/base frame (hollow base frame - later to be filled with water) . If the deck frame is provided with an open bottom, air may, if desired, be supplied so that the deck frame is air-filled at all times.

At the place of use at the field, the necessary number of leg sections are supplied from above through the vertically through-going channels of the deck frame, successively jointing them to the top ends of already anchored leg sections. When the bottom section has reached the seabed, anchoring of the lower ends of the legs is effected. Finally, the deck frame may be lifted e.g. 30 metres above the sea surface in a manner known per se, so that it does not have to participate in heavy seaway (confer jackup platforms) .

The legs/leg sections may be made in concrete or steel. They may e.g. have a trellis work structure. The mutual jointing of the leg sections may be effected by means of any suitable interconnecting/coupling means, by preference non-permanent means, in order to adapt the platform to a possible subsequent removal.

In accordance with the invention, an offshore platform having fixed legs (in opposition to platforms kept in place by means of tension legs, respectively floating platforms attached to the seabed by means of vertical, biased chains or the like) , which e.g. shall be standing on the seabed at a depth of 200 metres, may now be built to a half-finished condition (without the socalled fjord phase) and towed out to the field, where the legs are installed sectionwise. Proceeding according to the method of the invention, respectively building the platform structure according to the invention, enormous building time savings are achieved, simultaneously as the conditions are made suitable for using cheaper production techniques.

Further advantages, features and details of the invention appear from the following specification of examples of preferable embodiments, reference being made to the accompanying drawings, wherein each of the following figures represents an extra building step in relation foregoing figures, and wherein:

Figure 1 shows a perspective view of a floating dock, in which is disposed a steel shirt to be included in the bottom section of a platform structure, in the form of one substructure component;

Figure 2 corresponds to figure 1, but here the bottom section is supplemented through the addition of lower domes for storage tanks;

Figure 3 corresponds to figure 2, but here the storage tanks are assembled;

Figure 4 corresponds to figure 3, but here an interconnecting frame is placed on the top of the storage tanks;

Figure 5 corresponds to figure 4, but here an uncovered deck frame has been laid on top of the interconnecting frame. The deck frame's six vertically through-going guidance pipes for supply of leg sections appear from this figure;

Figure 6 corresponds to figure 5, but here the deck frame and its guidance pipes are covered by steel plates, simultaneously as each of the first leg sections are placed in their respective guidance pipe, some rig equipment being mounted on the deck frame;

Figure 7 shows a perspective detail view, partly with cut¬ away wall portions, of such a vertically through-going guidance pipe, which is assigned gear wheels meshing with vertical, driving/braking (jacking up) toothed bars formed along vertical edge portions of longitudinal leg sections;

Figure 8 shows the part platform structure according to figure 6 moved to a field position, first leg sections being lowered in relation to figure 6 and anchored to top storage tanks;

Figure 9 corresponds to figure 8, but here second leg sections have been added and coupled from above, in order to be fed through the vertically through-going guidance channels of the deck frame;

Figure 10 shows a side elevational view where the platform is resting with the substructure (the storage tanks/legs) on the seabed. This represents a situation where all necessary leg sections have been jointed together from above, the leg sections, additionally to mutual joining, also have been coupled to the interconnecting frame.

Figure 11 shows a side elevational view where the deck frame is jacked up such that it occupy a position at a substantial height above the sea surface;

Figure 12 shows a perspective view of a platform structure where the storage tanks are replaced by a base frame. Otherwise, the structure according to figure 12 is formed substantially in accordance with the preceeding embodiment.

According to figure 1, a shaped steel shirt 1 having a vertical wall has - as an initiating phase of a platform building process - been placed in a dock 2. The dock 2 has an ordinary construction. With conventional platform building, such docks have been used during the first step casting of the substructure, whereafter the legs are casted in full height in a following fjord phase, such as explained introductorily. I.a., according to the invention, this fjord phase is to be made redundant.

In accordance with figure 2, the steel shirt 1 defines a larger number of lower domes 3 from outside, said lower domes 3 being hollow semi-spherical bodies constituting the lower parts of storage tanks 3,3'according to figure 3, of which six, three in the longitudinal direction and two in the lateral direction, have been formed with a plane top. In figure 3, these are denoted with the reference indication 3,3a', i.e. that it is the upper truncated conical part 3a' that distinguishes them from the upper domes 3' of the remaining storage tanks 3,3'. As it will appear later on, these six storage tanks 3,3a¹ having a plane top are positioned in the area of the platform's six legs which, according to the invention, comprise rigid, jointable sections, as closer described in the following.

According to figure 4, on top of the storage tanks 3,3', 3,3a', a horizontal interconnecting frame has been laid. The use of such interconnecting frames is known per se; one, two or more being used according to the length of the platform legs. The interconnecting frame is denoted with the reference numeral 4 and has, in the area of each of the six storage tanks 3,3a' having a plane top, a first guidance sleeve 5, through which the leg sections may pass in a later working operation. The interconnecting frame may consist of steel or concrete.

In accordance with figure 5, a deck frame 6 has been laid on top of the interconnecting frame 4,5. Because of the trusswork structure of the deck frame 6 and the interconnecting frame 4, the less comprehensive interconnecting frame 4 is more or less hidden below the deck frame 6 in the perspective view of figure 5.

It appears from figure 5 that the deck frame 6 has six guidance sleeves 7 for the passage of platform leg sections later to be described. Because of the interconnecting frame's 4 corresponding (first) guidance sleeves 5, the guidance sleeves 7 of the deck frame 6 are allotted the term "second" guidance sleeves. Second guidance sleeves 7 are each placed in alignment directly above the plane tops of the storage tanks 3,3a' and the first guidance sleeves 5 of the interconnecting frame 4.

In accordance with figure 6, the entire deck frame, inclusive second guidance sleeves 7, is externally covered with steel plates. In another embodiment, however, the deck frame 6,7 could have consisted of a steel-reinforced concrete structure. According to figure 6, some rig equipment, such as cranes, has been installed on the deck frame 6. The mounting of such rig equipment does not really constitute any part of the present specification, especially not because such installations have a more optional installation time, either in dock or out on the field. Equipment to be used prior to the installation of the platform at the field, will, of course, be monted during the deck phase, figure 6.

According to the invention, the deck frame 6 is to have such a large buoyancy that it, preferably, will keep itself floating, possibly with rig equipment mounted thereon. Possibly, additional buoyancy from storage tanks is added. A partial platform structure consisting of a deck frame having equipment mounted thereon, storage tanks and an interconnecting frame (one or more) should, preferably, be capable of floating through its own buoyancy during the towing out to the field.

Instead of storage tanks 3,3' and 3,3a', such a part platform structure may have a base frame 3b in the form of an endless circumferential frame, such as shown in the perspective view of figure 12. In order to possibly secure a buoyancy equal to the empty storage tanks 3,3', 3,3a', the base frame 3b may be formed as an air-filled hollow body during the towing out to the field, the cavities thereof being adapted to be water- filled subsequently to the anchoring of the platform to the seabed.

An important feature of figure 6 is that it shows the six first platform leg sections 8 brought into position within the associated guidance sleeves 7, resting on top of the storage tanks 3,3a' and attached thereto.

Each leg section 8 consists of a lattice structure having four longitudinal, parallel rails 8' interconnected through inclined braces 8 ' • ' , where the leg section has a rectangular/square cross-sectional shape. Cross-sectional shape and design in general may unergo various modifications. The leg sections 8 may be made in steel or in concrete. For their mutually rigid interconnection is chosen the way of intercoupling and the coupling means that are found to be best suited, based on the shape and design of the leg sections themselves. It would be an advantage to use non-permanent connecting means; this with a view of arranging the conditions optimally for a possible removal of the platform structure from the first place of installation to be moved to another place of installation.

A longitudinal portion of a platform leg section 8 is shown on a larger scale in figure 7, which represents a perspective partial view, portions being cut away, of a second guidance sleeve and the adjacent part of the upper wall of the deck frame 6.

Into each longitudinal rail 8' is casted two toothed bars 8", the angular distance therebetween being 90°. The toothed bars 8" are meshing with a gear wheel 9 rotatably mounted on the deck by means of bearing blocks 10. The gear wheels 9 are interconnected in pairs by means of shafts 11. The gear wheels 9 are coupled to a drive/brake device (not shown) . This transmission device 8",9,11 may advantageously be used during jacking-up of the platform deck after the substructure has been rigidly installed on the seabed. Such a jacking-up possibility is previously known in connection with platforms of the kind concerned.

Figure 7 indicates a jointing possibility for aligned, adjacent platform leg sections 8. At the top ends thereof, the longitudinal rails 8' are formed with a narrower pin portion 8a, which may be formed for insertion into a corresponding sleeve portion (not shown) at the bottom ends of overlying leg sections. However, the figures of the drawings do show only one possible design of the platform leg sections and it is, thus, not possible to define more general jointing ways or means for any leg section design and shape .

In relation to the final dock phase of figure 6, the platform leg sections 8 of figure 8 are pushed a distance downwardly and, possibly, rigidly attached to the tops of the underlying storage tanks 3,3a' (or, alternatively, to the base frame 3b in figure 12) , in which condition the partial platform structure (deck frame, the interconnecting frame hidden by the former, as well as the storage tanks/the base frame minus legs) jointly has a sufficient buoyancy to float by means of its own abilities. Figure 8 shows the partial platform structure in a position adapted to subsequent rigid installation on the seabed 12.

In relation to figure 8, figure 9 distinguishes itself by the fact that further leg sections have been supplied from above. Each section 8 may extend over one or more squares. The building of the leg length 8 from above involves an advantageous jointing above water.

According to figure 10, the platform leg sections have been built to a finished length, and the platform substructure has been rigidly anchored on the seabed 12. Here, it appears that the interconnecting frame 4 is positioned somewhat closer to the sea surface 13 than to the seabed 12. At large depths and correspondingly long platform legs 8, it is suitable to use two or more such interconnecting frames 4, which in a way, not shown in detail, are anchored to the platform legs 8 in the area of first guidance sleeves 5.

In accordance with figure 11, the platform deck is jacked up 20-30 metres above the sea surface 13. Such a jacking-up is known in connection with platforms of the kind concerned, and may advantageously be effected by means of the toothed bar sections 8" and the driven gear wheels 9 of figure 7. Generally, the advantage is that platform deck carrying equipment does not have to participate in seaway.

Claims

C l i m s

1. A method for building an offshore platform having pressure-loadable legs (8) , preferably with deck jacking-up possibility up above the sea surface (13) while the substructure (3,3 ' ,3,3a' ,8 or 3b,8, respectively) is resting on the seabed (12), c h a r a c t e r i z e d i n that a deck frame (6) is built on top of a bottom section (3,3',3,3a' or 3b, respectively) and connected with the latter through jointable platform leg sections (8) having a length corresponding to a fraction of the intended leg length; that this partial platform structure is towed floatingly preferably to a field position, where further leg sections (8) are jointed to form legs having a sufficient length for allowing the bottom section

(3,3 ',3,3a' or 3b, respectively) to be anchored to the seabed

(12).

2. A method as set forth in claim 1, wherein the platform structure comprises at least one interconnecting frame (4) to be connected to the platform leges (8) between the deck frame (6) and said bottom section (3,3 ',3,3a' or 3b, respectively) , c h a r a c t e r i z e d i n that said interconnecting frame(s) (4) which has/have sleeves (5) with through-going bores for leg sections (8) , is/are placed on top of said bottom section (3,3',3,3a* or 3b, respectively) prior to the start of the building of the deck.

3. An offshore platform structure having pressure-loadable legs (8) , comprising a bottom section, e.g. in the form of a number of storage tanks (3,3', 3,3a') or a base frame (3b), and a deck (6) preferably adapted to be jacked up, c h a r a c t e r i z e d i n that the platform legs consist of jointable, longitudinal sections (8) , and that the deck (6) is formed with vertically through-going, channel- forming sleeves (7) , which are formed for the passage of the platform leg sections (8) from above/downwardly, i.a. upon jointing to underlying leg sections (8) , and wherein the latter can be attached to the deck.

4. An offshore platform structure as set forth in claim 3, comprising at least one interconnecting frame (4) , c h a r a c t e r i z e d i n that said interconnecting frame (4) or each such frame is formed with vertically through-going, channel-forming sleeves (5) , which are formed for the passage of the leg sections (8) from above/- downwardly, and wherein the platform legs (8) can be connected to said frame.

5. An offshore platform structure as set forth in claim 3 or 4, wherein the platform legs (8) have substantially straight, vertical edges, c h a r a c t e r i z e d i n that, along at least one substantially straight edge, a vertical toothed bar (8") is attached, the latter cooperating with at least one driven gear wheel (9) , in order to i.a. enable the jacking-up of the deck (6) after the substructure (3,3',3,3a',8 or 3b,8, respectively) has been brought to rest on the seabed (12) , said gear wheel (9) being rotatably mounted on the deck frame (6) .

6. An offshore platform structure as set forth in claim 5, c h a r a c t e r i z e d i n that each platform leg section (8) has a substantially square (horizontal) cross-sectional shape, where four longitudinal, mutually parallel rod-like members (8') each is provided with two angularly displaced toothed bars (8") each meshing with a driven gear wheel (9) .

7. An offshore platform structure as set forth in claim 6, c h a r a c t e r i z e d i n that said gear wheels (9) are connected in pairs by means of driven shafts (11) .

8. An offshore platform stucture as set forth in claim 6, c h a r a c t e r i z e d i n that the free end portions of said rod-like members (8') comprise either a pin portion (8a) having a reduced diameter or a sleeve portion having an internal diameter insignificantly exceeding said pin portion's (8a) diameter.

9. An offshore partial platform structure comprising a bottom section and therewith connected deck frame having possibly installed thereon rig equipment, but without jointed platform legs (8), c h a r a c t e r i z e d i n that the deck frame (6,7) is formed such that it in sea water gets a buoyancy that makes it floating, possibly with rig equipment installed thereon.

10. An offshore partial platform structure comprising a bottom section and therewih connected deck frame having possibly installed rig equipment thereon, but without jointed platform legs (8), c h a r a c t e r i z e d i n that the deck frame (6,7) and the bottom section (3, 3 ',3,3a' or 3b, respectively) are formed such that they in sea water get a common buoyancy, providing means to keep the partial structure floating, the deck frame extending partly above the sea surface (13) .

11. An offshore partial structure as set forth in claim 9 or 10, c h a r a c t e r i z e d i n that the deck frame (6) substantially has the form of a parallelepipedon- shaped hollow body, possibly having an open bottom portion.