NL2005896C2 - Self-installing monopile offshore structure. - Google Patents

Description

Title: Self-installing monopile offshore structure

The present invention is related to an offshore structure, such as semi permanent platform standing on the sea bottom on one raisable leg, which is designed to be installed without the requirement of a crane vessel.

Several platforms are being operated in the North Sea which have 5 been installed which stand on one leg, and are often referred to as “monopile platform” or “monotower platform”. The leg is usually a hollow leg that is built from cylindrical sections of one or more diameters and conical sections for the transition of one diameter to another. These platforms can be fixed to the sea bottom by one or more so called suction cans at the bottom of the leg or by the 10 use of a pipe frame structure at the bottom of the leg which is fixed to the sea bottom by piles which are driven into the sea bottom. An advantage of the monopile is that the hollow leg provides protection for pipelines or cables that are installed in between the sea bottom and the platform. Another advantage of a monopile is that the costs of materials and construction of the leg are 15 lower than for example the alternative of a leg constructed as a pipe frame or “jacket”. A disadvantage of existing monopiles is that the installation is expensive and time consuming. The transport to the location offshore is done on a barge or vessel and the installation in place is done by a crane vessel, which costs per day are high, while the lifting and installation require long 20 periods of good weather.

Publication US 3 921 558 discloses a floating structure for the recovery of escaping oil comprising a superstructure, a column and a ballast chamber. The column has a central section and an upper section with a tapered section inbetween. Around the tapered column section an extension is 25 provided to protect manifolding and to provide a supporting structure for a sliding collar surrounding the upper column section. The superstructure is movable between a towing position in which it is positioned against the ballast chamber and an operating condition in which it is attached at an upper end of the column by means of the collar.

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2005896 2

The object of the present invention is to reduce the installation costs of “monopile platform”. The objective is achieved according to claim 1 by design provisions and an installation method as described in the following.

Thereto, the invention provides an offshore structure for exploitation 5 of energy resources at an offshore location comprising a floatable pontoon with a single leg that is adjustable between a transport position in which the leg extends substantially above the pontoon and an installation position in which the leg extends substantially below the pontoon for anchoring the leg in the sea bottom and raising the pontoon above the water, wherein the leg comprises 10 at least two parts of different diameters with a transition part between each different diameter part, wherein the offshore structure further comprises a guide element for guiding the transition part during adjustment of the leg. Advantageously, the pontoon is provided with a central opening for receiving the leg.

15 The offshore structure according to the invention comprises a pontoon, whereby a tubular vertical hole is made through the pontoon approximately in the centre of the deck from the deck to the bottom of the pontoon, which forms a vertical guide for the leg, the latter also referred to as column. The leg can be moved up and down through this guide by a movement 20 system driven by electrical power or hydraulic power, to move the leg between the transport position and the installation position.

Preferably, the guide element is removable from the offshore structure.

According to the invention, the guide element at least partially 25 surrounds the leg. Further, the guide element preferably comprises an inner side that is shaped complementary to an outer surface of the transition part and/or the guide element comprises an outer side that is arranged to fit in the opening of the pontoon.

The monopile platform according to the invention comprises a leg 30 which consists of two or more parts of different outside diameters with a 3 transition part inbetween. During the movement of the leg relative to the pontoon, the hole in the pontoon, that forms the guide for the leg, should be fit for the different diameters. This is achieved by the use of a ring shaped guide element that fits around the smallest diameter, whereby the internal of the 5 ring forms a guide for a longitudinal movement of the leg, and the outside of the ring is connectable to the inside of the guide in the pontoon. During installation of the platform, said ring shaped guide element is initially at the lowest position of the small diameter section of the leg. At some point during lowering of the leg, the ring piece guide element is connected to the pontoon, 10 such that it provides a longitudinal guide for the small diameter section of the leg. During removal of the platform this sequence is reversed, such that the ring initially is connected to the pontoon, thus providing a guide for the leg while it is moved upward, and at some point the ring piece is disconnected from the pontoon and remains at the lowest position of the small diameter 15 section of the leg.

In a special embodiment of the invention the said ring shaped guide element is not closed over its full circumference, but is open over a small segment, such that appurtenances attached on the leg, such as a ladder or boat landing structure, can freely pass through the opening.

20 In a preferred embodiment of the invention, one or more suction cans, also referred to as suction anchors, are welded at the base of the leg, which form the fixation means on the sea bottom. The advantage of suction cans is that the installation is fast and can be done by relatively small systems, comprising pumps and hoses, which are operated from the platform 25 or from a nearby vessel.

In another embodiment of the invention, the suction cans are replaced by a tubular frame structure that is connected at the base of the leg. In the installed condition, the base frame is fixed to the sea bottom by piles which are driven into the sea bottom.

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In a preferred embodiment of the invention, hydraulic lifting systems known in the offshore industry as “strand jacks” are used to move the leg relative to the pontoon during the installation. The advantage of these strand jacks is that they are small units which are multi-purpose tools, which 5 can be rented on the market and which can be removed after the installation. This is more economic than permanent systems for the movement of the legs.

In another special embodiment of the invention the draft of the floating platform is reduced as the bottom of the pontoon has a hollow shape such that a cavity is created that fits the suction can shape, such that the 10 suction can is lifted in the cavity.

Further advantageous embodiments are represented in the subclaims.

The invention also relates to a method for installing an offshore structure for exploitation of energy resources at an offshore location, 15 comprising the application of a floatable pontoon with a single leg that is adjustable between a transport position in which the leg extends substantially above the pontoon and an installation position in which the leg extends substantially below the pontoon for anchoring the leg in the sea bottom and raising the pontoon above the water, transporting the floatable pontoon by 20 towing the pontoon in a floating position to the offshore location with the leg in the transport position; at the offshore location adjusting the leg from the transport position to the installation position and adjusting the pontoon from a floating position to a position raised above the water, further providing a guide element for accommodating a difference in diameters between a first part and 25 a second part of the leg during adjustment of the leg.

Further explanation of the invention will be given by means of some application examples represented in figures below, which will give a practical embodiment of the invention, but may not be considered in a limiting sense, wherein: 5

Figure 1 shows a side view of the offshore structure according to the invention in a floating condition;

Figure 2 shows a side view of the offshore structure of figure 1 at a stage during installation; 5 Figure 3 shows a side view of the structure of figure 1 in an installed condition;

Figure 4 shows a side view of the invention in floating condition shown in figure 1, whereby the suction can is replaced by a tubular frame structure.

10 Figure 5 shows a side view of the structure of figure 7 in an installed condition;

Figure 6 shows a cross section of a side view of the leg of the platform according to the invention, and a ring shaped guide element which is placed around the leg section of smaller diameter, and the pontoon, whereby 15 the leg is in its highest position relative to the pontoon;

Figure 7 shows a cross section of figure 9, whereby the leg is lowered relative to the pontoon;

Figure 8 shows a top view of the invention shown in figure 10, whereby the ring shaped guide element is open over part of its circumference; 20 Figure 9 shows a cross section of a side view of a platform according to the invention, whereby the bottom of the pontoon has a hollow shape that matches the shape of the top of the suction can;

Figure 1 shows a side view of the offshore structure according to the invention in a floating condition comprising the main components: pontoon 1, 25 leg 2, in this embodiment having two parts with different diameters, a first part and a second part, which are connected by a transition part 3 and suction anchor 4. The figure further shows modules 5 and pre-tensioning strand jacks 6 and strand bundles 7 which are connected at the other end to the suction anchor 4 and tensioned by the pre-tensioning strand jacks 6. Two sets of 30 double acting strand jacks 8 are mounted on the deck of the pontoon 1 holding 6 the strand bundles 7, which can move the strand bundles by force in both upward and downward direction, such that the leg is moved relative to the pontoon. The strand bundles 7 are bent under their weight and guided over a support 9 into the leg 2.

5 Figure 2 shows a side view of the offshore structure according to the invention at one stage during installation whereby the leg with the suction anchor has been lowered on the sea bottom 10.

Figure 3 shows a side view of the invention in installed condition, whereby the suction anchor 4 has been sucked into the sea bottom 10 and the 10 pontoon 1 with the modules 5 has been lifted above sea level 11. The pontoon is fixed to the leg, after which the strand jacks and strand bundles are removed. In an alternative arrangement the double acting strand jacks are not mounted on deck but are mounted inside the pontoon structure, such that the pontoon can be lifted higher.

15 Figure 4 shows a side view of the invention in floating condition shown in figure 1, whereby the suction can is replaced by a base frame 12.

The installed condition is shown in Figure 5. The installation is done by lowering the leg with the frame on the sea bottom and lifting the pontoon out of the water, after which the frame is fixed to the sea bottom by piles 14 20 which inserted in pile guides 13 and are driven into the sea bottom.

Figure 6 shows a cross section of a side view of the pontoon with the leg 2 according to the invention, whereby the leg 2 has two parts with two different diameters which are connected by a tapered transition part 3, whereby the leg is in its highest position relative to the pontoon. A cross 25 section of a ring shaped guide element 15 is also shown and indicated by hatching, which is placed around the leg section of smaller diameter. During installation the leg will be lowered relative to the pontoon, as indicated by the arrow, and the guide element 15 is moving with the leg until it is inside the pontoon 1. At this position the guide element 15 is connected to the pontoon, 7 such that the leg is guided through the inside of the guide element 15 when it is moved further down as shown in Figure 7.

Figure 8 shows a top view of the invention shown in figure 7, whereby the ring shaped guide element 15, indicated by the hatching pattern, 5 is open over part of its circumference. The opening enables the guide element 15 to pass along fixed appurtenances attached at the leg, such as a ladder or a bumper structure for a boat landing.

Figure 9 shows a cross section of a side view of pontoon 1, whereby the bottom of the pontoon has a hollow shape in order to provide a cavity 16 for 10 the suction anchor 4 in its highest position. In transport condition the suction anchor 4 is raised inside the cavity 16, such that the draft of the floating structure is reduced, which enables transportation in shallower water. As in another embodiment of the invention, the suction anchor 4 is replaced by a tubular frame 12, as shown in figure 7, in which case the hollow shape of the 15 bottom is such that it matches with the tubular frame.

Many variants will be apparent to the person skilled in the art. All variants are understood to be comprised within the scope of the invention as defined in the following claims.

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Claims (8)

An offshore construction for exploiting energy sources at an offshore location comprising a floating pontoon with a single leg that is adjustable between a transport position, wherein the leg protrudes mainly above the pontoon, and an installation position, wherein the leg 5 protrudes mainly below the pontoon for anchoring the leg into the seabed and lifting the pontoon above the water, the leg comprising at least two sections of different diameter with a transition section between the sections of different diameter, the offshore structure further comprising a guide element for 10 guiding the transition part during the adjustment of the leg. 2. Offshore construction as claimed in claim 1, wherein the pontoon is provided with a central opening for receiving the leg. 3. Offshore construction according to claim 1 or 2, wherein the guide element is removable from the offshore construction. An offshore structure according to any one of claims 1-3, wherein the guide element at least partially encloses the leg. The offshore structure according to any of claims 1-4, wherein the guide element comprises an inner side which is complementarily formed on an outer surface of the transition portion. An offshore structure according to any one of claims 2-5, wherein the guide element comprises an outside which is adapted to fit into the opening of the pontoon. 7. Method for installing an offshore construction for the exploitation of energy sources at an offshore location, comprising - the use of a single-leg floating pontoon that is adjustable between a transport position, the leg protruding mainly above the pontoon, and an installation position, wherein the leg protrudes mainly below the pontoon for anchoring the leg in the seabed and lifting the pontoon above the water, - transporting the floating pontoon by dragging the pontoon in floating position to an offshore location with the leg in the transport position; - adjusting the leg from the transport position to the installation position at the offshore location and adjusting the pontoon from a floating position to a position lifted above the water, further provided with a guide element for accommodating a difference in diameters between a first part and a second part of the leg during the adjustment of the leg. The method of claim 7, wherein the guide element is removed from the offshore structure after the leg has been adjusted.