GB2327449A

GB2327449A - Method of transporting and installing a substructure

Info

Publication number: GB2327449A
Application number: GB9807135A
Authority: GB
Inventors: David William Haley; Murray Charles Ferguson
Original assignee: Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner Oil and Gas Ltd
Priority date: 1997-04-02
Filing date: 1998-04-02
Publication date: 1999-01-27
Anticipated expiration: 2018-04-02
Also published as: GB9706679D0; GB2327449B; GB9807135D0

Abstract

A method of transporting and installing a substructure for an offshore plafform, the substructure consisting of a generally flat base 20 and a support 21 upstanding from that base; the method comprising the steps of ensuring that an upper surface of the base 20 is secured directly or indirectly to an under surface of a buoyant body 23 at at least two spaced apart points, floating the body to the intended site of the offshore platform, so to transport the substructure to that site, and then lowering the substructure 20,21 from the body 23, so that the base 21 of the substructure rests on the seabed.

Description

METHOD OF TRANSPORTING AND INSTALLING A SUBSTRUCTURE The invention relates to a method of transporting and installing a substructure for an offshore platform; and to a substructure so transported and installed.

In particular the invention relates to a method of transporting and installing a substructure for a windmill to stand at an offshore location. Such a substructure may have a generally flat base to rest on the seabed, and a slender support arrangement upstanding from that base and extending up through the wave effected zone to cany a nacelle and blades for the windmill.

The invention also relates to the transportation and installation of substructures for other offshore plafforms, e.g. for the drilling andlor production of oil and/or gas.

The transportation and installation of a substructure for an offshore platform gives rise to several problems. The substructure must be constructed on shore or at shore, and then moved to its intended offshore location and set down on the seabed. During construction, it may be convenient for a substructure to be in an upright attitude. At its offshore location it is essential for the substructure to be in an upright attitude. However, during transportation across open water from construction site to the intended offshore location, an upright substructure might have only marginal stability. This marginal stability would be particularly critical in high wind conditions when controlling tugs might have difficulties in manoeuvering.

Moreover, during installation, when the substructure is being set down on the seabed, there may be a point of instability, at which it has its centre of buoyancy co-incident with its centre of gravity.

Heretofore it has been known to tow substructures to their intended offshore locations with their centre of gravity below their centre of buoyancy, and then to submerge them onto the seabed. The Condeep type concrete gravity based plafforms in the northem North Sea were installed by that method. These structures were very stable under tow because the centre of gravity was always below the centre of buoyancy1 and so the submergence process never passed through a point of instability. However, these structures were very massive, and required sheltered deepwater (fjord) sites for their construction.

It has also been known to float a substructure out to its intended site with conventional ship stability, and then to tilt or 'indine' it deliberately, so that one edge of its base moves down towards and then rests upon the sea bed. The substructure is stable with its one edge on the seabed, and can then be ballasted down to eliminate its buoyancy, so that its base is set flat on the seabed. The substructure for the Ravensbum North plafform was installed in this way.

(The installation of that platform was described in Offshore Engineer for September 1989 at page 23.) This installation method was very effective. However, it can only be used in shallow water, and even then, only with particular types of seabed soil.

Clearly, there are still problems with the transportation and installation of substructures which have support arrangements formed of slender columns extending up from flat bases.

The invention provides a method of transporting and installing a substructure for an offshore plafform, the substructure consisting of a generally flat base and a support upstanding from that base; the method comprising the steps of ensuring that an upper surface of the base is secured directly or indirectly to an under surface of a buoyant body at at least two spaced apart points, floating the body to the intended site of the offshore platform, so to transport the substructure to that site, and then lowering the substructure from the body, so that the base of the substructure rests on the seabed.

It is preferred that the means to secure the upper surface of the base to the under surface of the body is the same as the means to lower the substructure from the body to the seabed.

It is further preferred that the base is secured beneath a hull shaped floating body with a cable reeved generally lengthwise of the hull, and in which the cable extends downwardly through a hawsepipe on the centreline of the hull at at least one of the two spaced apart points.

Altematively the base may be secured beneath a hull shaped floating body with ropes, chains or hydraulically powered strand jacks arranged to act vertically downwardly through the centreline of the hull at at least one of the two spaced apart points.

In one form the support comprises a single slender column displaced from the centre of the base, and the base is secured beneath a single buoyant body with an upper surface of the base near to the centre of the base secured to the under surface of the body.

In another form the support comprises two slender columns, and the base is secured beneath a single buoyant body with an upper surface of the base between the two columns secured to the under surface of the body.

In a third form the support comprises a single slender column located generally centrally with respect to the base, and the base is secured to the undersides of two buoyant bodies, with upper surfaces of the base on opposed sides of the slender column secured to the under surfaces of the respective two bodies.

In this last mentioned form it is preferred that two hull shaped buoyant bodies are secured to the base in spaced apart side by side parallel relationship with the column upstanding between them, whereby the base and the two hull shaped bodies act together as an inverted catamaran.

Preferably the invention includes the preliminary steps of constructing the substructure in a floodable area, flooding that area so that the upper surface of the base is below water level, floating the buoyant body over the base, and securing the upper surface of the base to the under surface of the buoyant body.

Advantageously, the buoyant body is floated over the base at or near high water and the base is secured to the body at or near low water.

The invention includes substructure for an offshore platform when transported and installed in accordance with method(s) described above.

The invention also includes an offshore platform or windmill incorporating a such substructure.

Four specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1 and 2 are a side elevation and plan of a first embodiment; Figures 3 and 4 are a side elevation and plan of a second embodiment; Figures 5 and 6 are a side elevation and plan of a third embodiment; Figures 7 to 14 are a sequence of diagrams illustrating the construction, transportation and installation of a fourth embodiment, which is in the form of an offshore windmill; Figure 15 is a plan view showing a substructure for the windmill under tow; Figure 16 is a more detailed plan view showing arrangements for securing the substructure for the windmill to a pair of pontoons; and Figure 17 is an enlarged scrap section on the lines XVI-XVI through the pontoons shown in Figure 16, with a securing chain fully lowered.

Figures 1 and 2 show a substructure for an offshore platform. The substructure has a flat base 20 of square planform. Midway along one side of the base 20 there is a deck support comprising a single slender column 21. The column 21 extends vertically upward from the base 20 to a height above that of the highest wave anticipated at the intended offshore location.

The substructure is constructed in a floodable area. When the substructure is complete, that area is flooded and a buoyant body 23 is floated over the upper surface of the base 20.

The body 23 may be a purpose built pontoon, or a converted cargo barge. The body 23 has provisions 26 on its under surface adjacent to its four comers to secure the body 23 to the upper surface of the base 20. (A typical arrangement for effecting the provision 26 will be described with reference to Figures 16 and 17.) The body 23 is moved down so that its under surface rests on an upper surface of the base 20. Downward movement can be effected by ballasting the body, heaving in on chains.

or by using a falling tide. A combination of these methods might be used.

When the base 20 has been secured to the under surface of the body 23, the body 23 can be floated out of the floodable area, and towed to the intended location for the offshore platform. At that location the base 20 can be lowered from beneath the body 23 onto the seabed. The buoyant body 23 can then retum to the or another floodable area to collect, and then transport and install, another substructure.

Figures 1 and 2 illustrate the generality of the method, in which a deck support stands at any convenient position on a base of any appropriate shape. The provisions 26 to secure the base to the buoyant body may be on the centre line, or as shown, or at the edges of the body.

Figures 3 and 4 show a deck support formed of two slender support columns 31A and 31 B disposed on opposite sides of a square base 30. A buoyant body 33 is secured to the base 30 between the columns 31A and 31 B. The body 33 has four points at or near its comers to secure the upper surface of the base 30 to the under surface of the body 33. The body 33 is used to cany the substructure out to its intended offshore location. At that location the substructure is lowered to rest on the seabed.

Figures 5 and 6 show a deck support formed of a single slender column 41 disposed centrally with respect to a square base 40. In this case two buoyant bodies 43A and 43B are secured to the base 40, arranged symmetrically about the single column 41. Each body has two points 46 for securing the upper surface of the base to the under surface of the respective body. The bodies and the substructure can be floated out to the intended offshore location in the form of an inverted catamaran. At that location the substructure is lowered to rest on the seabed.

Figures 7 to 14 (not to scale) show the construction, transportation and installation of a substructure to support an 80m diameter 3MW wind turbine under aerodynamic and hydrodynamic forces expected in the Baltic Sea.

The substructure has a nacelle support (comprising a slender column 121) upstanding from a flat plate like base 120. The substructure is constructed in a dry dock 122, as illustrated in Figure 7.

Figure 8 shows the dry dock 122 flooded.

Figure 9 shows how two pontoons 123A and 123B are floated into the dock and over the base 120 at high water.

In Figure 10, the tide has fallen, and the pontoons have settled on to the base 120. The under surfaces of the pontoons are securely fixed to the upper surface of the base 120 (by means to be described with reference to Figures 16 and 17).

At a subsequent high tide, the pontoons and substructure are floated out of the dock (as shown in Figure 11) in the form of an inverted catamaran.

The pontoons and the substructure are then towed to their intended offshore location by a tug 124 as shown in Figure 12. (An enlarged plan view of this transportation phase is shown in Figure 15.) When the pontoons and substructure have reached the intended offshore location, the pontoons are moored. The substructure is then lowered to the seabed, as illustrated in Figure 13, so that it takes up its final intended position, as shown in Figure 14.

The substructure has been configured to support the nacelle and rotor for a windmill based on site conditions in the Baltic Sea.

Dimensional details for one specific configuration are as follows: Overall Height (from seabed to rotor axis) 66.3m Rotor Diameter 80.0m Foundation Diameter 25.0m Water Depth 15.0m The nacelle support comprises a single slender column of 121 tubular construction. This is either manufactured as a long cone or (as shown in Figures 7 to 14) as three tubular elements which are tapered by the inclusion of two short intermediate cone elements.

The material for the slender column 121 could be steel or pre-stressed concrete. Steel offers the benefits of being some four times stiffer and stronger per unit mass than concrete, and as such it offers the potential for appreciably lighter structures. The combination of stiff and light construction, together with steel's flexibility, makes steel the preferred material. Its reduced weight also gives benefits with respect to the column being lifted more easily or requiring less buoyancy for floating.

The material to be adopted for the circular base 120 is less clear. For the gravity base foundation, either steel or concrete may be appropriate; with sand, rock or iron ore used for ballast material.

Floating the substructure into place offers the possibility of installing the windmill as a complete unit without the necessity of using a major crane. It would need either inherent buoyancy or auxiliary buoyancy or both to float and to have sufficient stability for transportation and during lowering.

The present embodiment makes use of auxiliary buoyancy tanks or pontoons 123A and 123B. Connecting the pontoons to the base offers a feasible arrangement for the substructure to be lowered or raised by a jacking system connecting the pontoons and base 120.

In practice the auxiliary buoyancy tanks or pontoons comprise two fairly standard shaped barges 123A and 123B. Each substructure (comprising base 120 and column 121) requires two barges to float in a stable configuration. The barges could be converted from other uses but would preferably be purpose built. Each would be equipped with suitable lifting equipment so as to be able to lift the complete substructure, transport it as a catamaran, and then lower it safely to the seabed. The lifting equipment may comprise strand jacks, or, if the loads are sufficiently low, a simple block and tackle arrangement as shown in Figures 16 and 17 may suffice.

Figures 16 and 17 show details of one particular arrangement for securing and lowering a substructure. Each barge 123A and 123B has two vertical hawse pipes 125. A chain 126 runs up through each hawse pipe 125, over a chain sheave 127, and then horizontally along the deck of the barge to a pulley block 128. Each pulley block 128 is reeved to a second pulley block 129 with a wire 131, to give a tackle having a sufficient mechanical advantage.

The wire 131 is connected to a 15 tonne winch 132 set on the deck of the barge. (In this instance, a 10 to 1 mechanical advantage is used.) The tackles are angled with respect to the centre line of the barge.

Each barge has two chains 126 controlled by winches 132, and each chain leads down through a hawse pipe 125 on the centreline of the barge.

The barges 123A and 123B are floated over the base 120 of the substructure (as shown in Figure 9) with their chains 126 extending down through the hawse pipes 125. The free ends of the chains are connected to reinforced points on the upper surface of the base. As the tide falls, the barges settle (or are ballasted down) onto the base of the substructure. The wires 131 are tightened, so to heave in on the chains 126, and secure the substructure to the barges. Altematively, the base 120 of the substructure might be hauled up and secured by heaving in on the chains 126.

The substructure is set down to rest on the seabed by veering on chains. Pneumatic fenders 133 are used to cushion any accidental contact between the column 121 and the barges 123A and 123B.

The same two barges could be used repeatedly to install substructures for an offshore wind farm having many offshore windmills.

Claims

1. A method of transporting and installing a substructure for an offshore plafform, the substructure consisting of a generally flat base and a support upstanding from that base; the method comprising the steps of ensuring that an upper surface of the base is secured directly or indirectly to an under surface of a buoyant body at at least two spaced apart points, floating the body to the intended site of the offshore platform, so to transport the substructure to that site, and then lowering the substructure from the body, so that the base of the substructure rests on the seabed.

2. A method as claimed in Claim 1 in which the means to secure the upper surface of the base to the under surface of the body is the same as the means to lower the substructure from the body to the seabed.

3. A method as claimed in Claim 1 or Claim 2 in which the base is secured beneath a hull shaped floating body with a cable reeved generally lengthwise of the hull, and in which the cable extends downwardly through a hawsepipe on the centreline of the hull at at least one of the two spaced apart points.

4. A method as claimed in Claim 1 or Claim 2 in which the base is secured beneath a hull shaped floating body with ropes, chains or hydraulically powered strand jacks arranged to act vertically downwardly through the centreline of the hull at at least one of the two spaced apart points.

5. A method as claimed in any one of the preceding claims in which the support comprises a single slender column displaced from the centre of the base, and the base is secured beneath a single buoyant body with an upper surface of the base near to the centre of the base secured to the under surface of the body.

6. A method as claimed in any one of Claims 1 to 4 in which the support comprises two slender columns, and the base is secured beneath a single buoyant body with an upper surface of the base between the two columns secured to the under surface of the body.

7. A method as claimed in any one of Claims 1 to 4 in which the support comprises a single slender column located generally centrally with respect to the base, and the base is secured to the undersides of two buoyant bodies, with upper surfaces of the base on opposed sides of the slender column secured to the under surfaces of the respective two bodies.

8. A method as claimed in Claim 7 in which two hull shaped buoyant bodies are secured to the base in spaced apart side by side parallel relationship with the column upstanding between them, whereby the base and the two hull shaped bodies act together as an inverted catamaran.

9. A method as claimed in any one of the preceding claims and including the preliminary steps of constructing the substructure in a floodable area, flooding that area so that the upper surface of the base is below water level, floating the buoyant body over the base, and securing the upper surface of the base to the under surface of the buoyant body.

10. A method of construction, transportation and installation as claimed in Claim 9 in which the buoyant body is floated over the base at or near high water and the base is secured to the body at or near low water.

11. A method substantially as hereinbefore described with reference to Figures 1 and 2, or 3 and 4, or 5 and 6, or 7 to 17 of the accompanying drawings.

12. A substructure for an offshore platform when transported and installed in accordance with the method(s) described above.

13. A substructure for an offshore windmill when constructed, transported and installed in accordance with Claim 9 or Claim 10, or Claim 11 insofar as it relates to Figures 7 to 17 of the accompanying drawings.

14. An offshore plafform or windmill incorporating a substructure according to Claim 12 or Claim 13.

Amendments to the claims have been faed as follows 1. A method of transporting and installing a substructure for an offshore platform, the substructure consisting of a generally flat base and a support upstanding from that base; the method comprising the steps of ensuring that an upper surface of the base is secured directly or indirectly to an undersurface of a removable buoyant body at at least two spaced apart points, floating the body to the intended site of the offshore platform, so to transport the substructure to that site, lowering the substructure from the body, so that the base of the substructure rests on the seabed, and then freeing the body from the base and floating the body away from the base so that the body can be used for other purposes (e.g. the installation of another substructure).

7. A method as daimed in any one of Claims 1 to 4 in which the support comprises a single slender column located generally centrally with respect to the base, and the base is secured to the undersides of two buoyant bodies, with upper surfaces of the base on opposed sides of the slender column secured to the under surfaces of the respective two bodies.