A method for building and transporting parts of a marine structure, a vessel for use in this method and structure built up by this method

Abstract

 The invention relates to a method for building a marine structure by building the upper structure thereof on land or in shallow water, by loading this onto a semisubmersible catamaran-shaped vessel. which may be ballasted and deballasted to have a wide range of height positions while in the water, by bringing this vessel to the lower structure of the marine structure in deep water, so that this vessel straddles said lower structure, and by then lowering the vessel to discharge the upper structure onto the lower structure. The invention also relates to means to take over the upper structure at the building site by a pontoon, from which it is in another area taken over by the catamaran-shaped vessel, and to a method for building up the upper structure from modules and for completing this structure. Moreover, the invention relates to such catamaran-shaped vessel itself and to the structure thus built and handled.

Description

[0001] This invention relates to a method for building a marine structure, to methods for building and transporting the upper structure thereof, to a vessel for use therein and to the structure thus built up.

[0002] The most usual method for building up smaller marine structures, e.g. floating structures or structures resting on the sea bottom and used for exploration and/or exploitation of minerals, is to build up such a structure in a dock or port entirely or for the greater part. For marine structures of corsferable dimensions such a method is not always suited or is even impossible in view of for instance insufficient water-depth at the site where oneiwishes to build the structure, or in view of the necessity to pass shallow and/or narrow passages in waterways with such a structure after building up, and/or in view of the dangers by waves and wind when moving the structure to its final operating site.

[0003] In view thereof attempts have been made to complete such marine structures at the final site by placing onto a lower structure at that site the usually heavy upper structure including engine rooms, pumping rooms, spaces, tanks, etc., to store and treat drilling liquid, oil, water, gas, etc., for which purpose ships with one or more heavy cranes may be used. As the lifting capacity thereof is limited, it is known to split the upper structure up into separate heavy units (modules) and to position these modules one by one to the side of each other and, if.desired, one on top of the other on the lower structure and to connect these modules where necessary.

[0004] Apart from the problems occurring when using heavy floating cranes or derricks with eccentric load and serious ballasting problems, such systems have disadvantages in other respects. Much personnel has to work often far away from the coast at sea, such persons have to be transported over long distances and there are many dangers and non-workable periods by waves and wind. The completion of such a structure takes much time and much personnel , in particular in finishing the duct systems between the modules, where ducts have to be guided through walls from one module into the other, many ducts have to be coupled etc. Also the testing of the structure before operation takes a lot of time. This means that for large structures half a year up to almost one year is necessary for such completion.

[0005] It has also been suggested to make the entire upper structure floating and thus to tug it to the lower structure at sea. At the operating site, where the lower structure is already present, the upper structure may now be jacked up along the lower structure so as to reach its final position, but this is very difficult and often even impossible if there are rather high waves.

[0006] Another known method is that the upper structure is built along a quay on a rigid supporting structure including supporting platforms standing in the water, and then to jack this structure up so that a pontoon may be moved below it, after which the upper structure is lowered by jacking it downwards until it rests on the pontoon, which is then tugged to the site of the lower structure. This has been applied for a floating lower structure which may be moved up and down by ballasting and deballasting and which had legs between which the pontoon could enter and be moved. The floating lower structure is in such a method moved to deep but quiet water, for instance in a bay or fiord, or it is built there and in this area the floating lower structure may take over the upper structure from the pontoon by lifting the lower structure, after which the combined marine structure is tugged to the final operating site. This method is only suited for a floating and ballastable lower structure and in many cases requires quiet but very deep water, so that the possibilities of application thereof are limited. The shape of the lower structure should suffice special requirements to allow the pontoon to be moved to a position between the legs thereof. It is also possible to convey the upper structure to the lower structure while the upper structure rests on two separate pontoons at a distance from each other, which gives more freedom in the design of the lower structure.

[0007] In view of the above difficulties and problems the present invention aims at improving such known methods and to supply a relatively cheap and rapid possibility of completing a marine structure, also in cases in which this has a lower structure resting stationarily on the sea bottom in areas with much wind and high waves.

[0008] In view thereof a method for building a marine structure is first of all according to the invention characterized in that the upper structure thereof in at least for the greater part completed condition is made to be carried by a catamaran-shaped semi-submersible vessel with a great vertical movability by ballasting while floating, that this vessel is thereupon moved to the lower structure of said marine structure and positioned above said lower structure with the catamaran hulls one to each side of the upper part of said lower structure, and that said vessel is then ballasted to discharge said upper structure unto said lower structure.

[0009] This avoids the necessity to have a heavy crane or derrick at the site of completing the structure and moreover not much person - nel has to work at this site at sea during a long period, and besides there is much less disadvantage and danger by wind or waves.

[0010] 1'he present invention further relates to a method for transporting the upper structure of a marine structure, which method according to the invention is characterized in that said upper structure in at least for the greater part completed condition is transmitted from a stationary supporting structure or floating pontoon to a catamaran-shaped semi-submersible vessel with a great vertical movability by ballasting while floating by moving this vessel in deeply ballasted condition while floating to a position in which supporting parts thereof are positioned below parts of said upper structure, said vessel thereupon being deballasted to rise in order to take over said upper structure and to lift it from said stationary structure or floating pontoon to allow shipping of said upper structure on said vessel to the lower structure of said marine structure.

[0011] This has the advantage that the upper structure may either be built at a site where no deep water is available, as this upper structure may be loaded onto a shallow pontoon and may then easily and rapidly, also without a crane or derrick, be taken over by said semi-submersible vessel in deeper water but close to the coast to be moved on such vessel to the lower structure of the marine structure, or may at least be built up near the coast in quiet water and then be taken over directly by the semi-submersible vessel if the water is sufficiently deep for doing this.

[0012] A horizontal displacement of the upper structure onto such a pontoon or semi-submersible vessel is not necessary if the said stationary supporting structure is present above water on poles or partly on a quay, as is known as such, so that this upper structure may be supported by such parts in such a way, thatt_he pontoon or the vessel for taking up and carrying the upper structure may be moved below such a structure and may be raised by deballasting to take over the upper structure to carry it. For composing the upper structure on the supporting structure a crane or derrick is normally needed, but in this case this is a crane which may be supported on the quay or other stationary structure and may thus easily have a large capacity and be used also in bad weather.

[0013] Although for transmitting the upper structure onto the lower structure of the marine structure, the building up of said upper structure from modules is less necessary than in other systems such as known systems indicated above, such building up from modules may also when using the present invention have advantages and be preferred for building up the upper structure on the supporting structure, in particular for large and heavy upper structures.

[0014] Onto said stationary supporting structure or said pontoon a supporting frame may be positioned, on which such modules may be positioned and built up or composed or positioned thereon in finished condition, said supporting frame thereupon being taken up by a pontoon or semi-submersible vessel as indicated above together with the combination of modules positioned thereon.

[0015] Although the upper structure will be finished substantially entirely on the supporting structure, it may be preferred in view of wind forces to assemble protruding and/or lighter parts of the upper structure such as quarters for personnel, a crane or derrick, a helicopter deck, etc., to the main upper structure only after the upper structure has been taken up by the catamaran-shaped vessel, which may take place in deeper water but for instance close to the coast, the conveying of the upper structure on the pontoon or the said vessel being facilitated by lower wind forces.

[0016] A semi-submersible vessel is, as is known to the expert, a vessel of which the greater part of the buoyancy is present rather deeply below the water level to be influenced as little as possible by waves, and neither by the returning water circulation by the waves below the water surface, and preferably with a relatively small surface intersecting the water surface. Such a vessel may reach such a depth by ballasting and if desired it may be deballasted so much that its floats giving the buoyancy are close to the water level to allow sailing or tugging with little resi-tance.

[0017] A catamaran-shaped vessel according to the invention is open at at least one end between the hulls for taking up in the bay thus formed a pontoon or lower structure of a marine structure, with a mainly open space above the bay and with supporting means to the side of said open space above the bay to carry an upper structure for a marine structure. Such a vessel is preferably movable up and down in the water by ballasting and deballasting over more than 10 meters and, if desired, even over a distance of 30 meters with a lower position in which said supporting means are positioned close to the water level.

[0018] The invention will now be explained in more detail with reference to the attached drawings giving by way of example only an embodiment of the parts indicated above. In said drawings :

Fig. 1 is a vertical diagrammatic section perpendicular to a quay through a supporting structure, on which the upper structure of a marine structure may be built up, with a pontoon below it;

Fig. 2 is a section along the line II-II in Fig 1;

Fig. 3 is a view from above of the structure of Figs. 1 and 2;

Fig. 4 is a transverse section through a catamaran-shaped semi- submersible vessel in ballasted condition, with on top thereof the upper structure of a marine structure and between the side-walls thereof the pontoon shown in Figs. 1, 2 and 3;

Fig. 5 gives a view from above of the combination of Fig. 4; and

Fig. 6 shows a side-view of the catamaran-shaped vessel of Figs. 4 and 5 in its position around the top of the lower structure of the marine structure.

[0019] Figs. 1-3 inclusive show a quay 1 and, in front of it, in the water, supports 2 for two supporting platforms 3. A frame is positioned on the one hand on the quay and on the other hand on the platform 3. This frame consists of two longitudinal beams 4 parallel to the quay and two transverse beams 5. The greater part 9 of the upper structure of the marine structure is built up from separate modules 6 on this frame. In this case there are eight modules in two layers of four. Such modules are intact boxes with in them engines and other machinery, storing rooms, separators, etc., for use in the marine structure. For clarity these modules have been shown with some mutual distances but in fact they will normally contact each other. They may, if desired, be connected by suitable means such as by welding or bolt-connections, and ducts for fluid and electric connectors may be guided through the walls of said modules from one module to the other.

[0020] After thus assembling the main and greater part of the upper structure for the marine structure to be made, a pontoon 7 is moved through the water to a position below the frame of beams 4, 5, said pontoon being provided with suitable supports 8, which may be fixed to the pontoon or loosely supported thereon as to vary their position and height easily. During this movement of the pontoon it is ballasted so as to take up a position rather deep in the water, lower than the position shown. Upon reaching the position shown in Figs. 1-3, the pontoon is now deballasted, so that it rises to take over the beam-frame 4, 5 with the upper structure composed of the modules 6 from the quay 1 and the platforms 3, these being now supported by the supports 8 on the pontoon.

[0021] The pontoon 7 with this frame and upper structure 9 is now moved to a site where this upper structure can be transmitted to the vessel 10 shown in Figs. 4-6 inclusive. This vessel 10 is a catamaran-shaped semi-submersible vessel. It has a deck 11 of a shape which, as seen from above (Fig. 5), is mainly U-shaped and between the two parallel hulls or floats 12 and legs 13 it leaves a free space as a bay 14, open upwardly. The floats or hulls 12 are connected by a float-or hull-part 15 immediately below the connection of the legs of the U of the deck-structure 11.

[0022] This vessel is made so as to be adapted to be moved up and down in the water over a considerable height by ballasting and deballasting. For instance it may be moved over a height of 13 meters, from a draught of about 38 meters to about 25 meters. The deck 11 has a flat load-supporting upper surface or a number of separate supporting platforms 16 at a distance corresponding to the distance between the transverse beams 5 of frame 4, 5. In ballasted condition the top surfaces of such supporting platforms 16 are position ed only a short distance above or about at the water-line. In this condition the pontoon 7 with the upper structure 9 as described on it, is moved into bay 14. As the beams 5 of the frame 4, 5 protrude outside the pontoon 7 to both sides and as the parallel legs of the deck of vessel 10 with the supporting platforms 16 are closer together than the length of the beams 5, the pontoon 7 can be moved with the parts of the beams 5 protruding outside the pontoon immediately above such supporting platforms.

[0023] Deballasting of vessel 10 and/or ballasting of pontoon 7 will now cause the upper structure 9 with frame 4,5 to be discharged from pontoon 7 to vesse110, the supports 8 (Fig. 1) remaining on the pontoon 7.

[0024] It is now possible to further complete the upper structure 9 for instance close to the coast such as in a bay and this is pan- icularly preferred for light protruding parts subject to high wind forces, to personnel quarters, a helicoptrer-deck, a crane or derrick and the like, which parts have been indicated diagrammatically by 17 in Fig. 4 and 6.It depends on circumstances and field of application to choose the parts assembled in this stage. For instance, a drilling derrick, a light beacon or seamark or a lighthouse superstructure may be applied in this way. In doing so, such wind- catching parts are not yet present on the narrower pontoon 7 so that they do not give difficulties when towing this pontoon. The vessel 10 is much wider and so on vessel 10 such parts will not give difficulties by wind as thevessel is much less sensitive to wind-loads than pontoon 7.

[0025] Thereafter vessel 10 is moved by its own propulsion means or by being towed to the site where the upper structure 9 should be mounted onto the lower structure of the marine structure. For this movement the vessel 10 is deballasted as far as is desirable as a compromise between resistance, wind- and wave-loads.

[0026] In Fig. 6 the lower structure of the marine structure, for instance a jacket of a marine platform, is indicated diagrammatically by dot-and-dash-lines 18. In this case the structure 18 is considered to be a structure resting on the seabottom and being a frame-structure, but this structure may also be floating mainly below the water-level. Such structure may have any shape suitable for its purpose, for instance an upright cylinder. When vessel 10 reaches this structure it is, if desired, deballasted further and thereupon it is moved so as to be in a position straddling structure 18 as shown in Fig. 6 but not yet contacting it, so that the upper part of structure 18 is taken up in bay 14. Thereupon vessel 10 is ballasted until the upper part of structure 18 is made to carry the upper structure 9 with the beam-frame 4, 5 by taking it over from vessel 10 which can now be tugged away or moved away by its own propulsion means. It is now possible to connect the upper structure 9 to the structure 18 and the entire marine structure thus formed can now easily and rapidly be completed and put into operation.

[0027] In this case it was assumed that the beam-frame 4, 5 is taken up in the marine structure. If this is not the case, several measures may be taken to avoid this. One of the possibilities is that the beams 5 are shorter than the modules 6, so that, when balasting vessel 10, the structure 18 takes up the upper structure 9 by contacting the lower surfaces of modules 6without contacting beams 5. In this way the frame-structure 4, 5 remains on the vessel 10.

[0028] It is also possible, if desired, to leave the beam-frame 4, 5 back on the pontoon 7 when vessel 10 takes over the upper structure 9 from the pontoon 7 as shown in Fig. 4 and described above. It is also possible to have the structure 18 contact and takeup the forces from the upper structure 9 without contacting the beams 4, 5 after which this frame 4, 5 is taken apart.

[0029] 3y applying the invention it is also possible to build up, examine and test the entire upper structure close to the coast, which means an enormous saving as compared with the case in which many persons have to be transported to a site far off the coast, a saving both in costs and in time.

Claims

1) A method for building a marine structure, characterized in that the upper structure thereof in at least for the greater part completed condition is made to be carried by a catamaran-shaped semi-submersible vessel with a great vertical movability by ballasting while floating, that this vessel is thereupon moved to the lower structure of said marine structure and positioned above said lower structure with the catamaran hulls one to each side of the upper part of said lower structure, and that said vessel is then ballasted to discharge said upper structure unto said lower structure.

2) A method for transporting the upper structure of a marine structure, characterized in that said upper structure in at least for the greater part completed condition is transmitted from a stationary supporting structure or floating pontoon to a catamaran-shaped semi-submersible vessel with a great vertical movability by ballasting while floating by moving this vessel in deeply ballasted condition while floating to a position in which supporting parts thereof are positioned below parts of said upper structure, said vessel thereupon being deballasted to rise in order to take over said upper structure and to lift it from said stationary structure or floating pontoon to allow shipping of said upper structure on said vessel to the lower structure of said marine structure.

3) A method for manufacturing the upper structure of a marine structure, characterized in that this upper structure is built up from modules, which are mutually connected while present on a stationary supporting structure or pontoon before being taken over from said stationary structure or pontoon by a vessel such as according to claim 2.

4) A method according to claim 3, characterised in that on said stationary supporting structure or pontoon a supporting frame is mounted, onto which the modules are positioned and connected, which supporting frame is thereupon taken over by said vessel while the upper structure is present thereon.

5) A method for completing the upper structure of a marine structure, characterized in that the upper structure after transmitting it to said catamaran-shaped vessel is completed by mounting protruding and/or lighter parts thereto, such as personnel quarters a crane or derrick, a helicopter deck etc.

6) A catamaran-shaped semi-submersible vessel of great vertical movability by ballasting while floating, for use in the method according to any of the preceding claims, characterized in that it is open at at least one end between the catamaran hulls for taking up in the bay thus formed between the hulls a pontoon or upper part of a lower structure of a marine structure, said vessel being substantially open above the bay and having supporting means to the side of said open space above the bay to carry an upper structure for a marine structure.

7) A vessel according to claim 6, characterized in that this is movable vertically while a float over more that ten meters by ballasting and deballasting, with the top of said supporting means close to the water level in its lowest position.

8) A vessel according to claim 6 or claim 7, characterized in that it is U-shaped as seen in horizontal planes both in its deck structure and in its hull structure, with the connections between the legs of the U of both structures one on top of the other.

9) An upper structure for a marine structure, built up from mutually connected modules and resting on a supporting structure or pontoon or on a catamaran-shaped vessel according to preceding claims.

10) A marine structure built up by applying the method according to any of claims 1 to 5 inclusive.

Drawing