NL2005415C2

NL2005415C2 - Improvements in manufacturing and installing multiple offshore constructions.

Info

Publication number: NL2005415C2
Application number: NL2005415A
Authority: NL
Inventors: Robert Reinder Narold; Frank Cornelis Lange
Original assignee: Heerema Marine Contractors Nl
Priority date: 2010-09-28
Filing date: 2010-09-28
Publication date: 2012-03-29

Description

P30467NL00/WHA

Title: Improvements in manufacturing and installing multiple offshore constructions

Field of the invention

The present invention relates to offshore constructions and to a method of manufacturing and installing multiple offshore constructions.

5 Background and description of the prior art

In recent years, more and more constructions are built offshore. The constructions may be wind turbines, constructions for production of hydrocarbons, and other types of constructions.

A known problem of many of these constructions at sea is that a time consuming and 10 costly process is involved for construction and installation. Installation vessels can generally only work in good weather conditions in order to limit movements due to wind and waves. This means that only during a part of the year installation work can be performed. Other factors may also play a role.

Many different methods for installing offshore constructions have been conceived.

15 Some methods involve manufacturing the entire construction on land and towing the construction out to sea, followed by the installation of the construction.

Other methods involve constructing an offshore construction at sea with a purpose-built installation vessel. Such a vessel generally is very expensive.

Some methods involve the use of underwater divers, which are also costly and bring 20 along safety issues.

Some methods involve the use of gravity bases, which necessitates transport of these extremely heavy bases, or transport and placement of rock fill or concrete or other building materials.

Some methods involve a combination of the above aspects.

25 GB2378679A1 discloses a floating offshore construction for a wind turbine which uses tendons. This construction is suitable for deep water, but relatively expensive for medium and low water depths.

DE102004042066A1 discloses an offshore construction for wind turbines which uses piles. This construction works but is relatively expensive.

30 US2007269272A1 discloses a method which utilizes a gravity base. The method of US2007269272A1 is also quite expensive and quite cumbersome during installation.

Wind power is environmentally friendly and sustainable and demand for wind power is growing. Since the wind is stronger at sea and more space is available at sea, increasing -2- numbers of wind turbines are constructed at sea. Projects for generation of wind power generally involve large numbers of wind turbines. The cost per MW of power generated by wind energy is in general higher than the cost per generated MW in a more traditional manner, e.g. from coal or gas. To make wind energy more attractive from economical point 5 of view the cost per generated MW must be kept as low as possible. The mentioned examples are too expensive for this purpose.

Object of the invention

It is an object of the invention to provide a reliable and cost-effective alternative to 10 the prior art.

It is an object of the invention to provide a reliable and cost-effective way of manufacturing and installing offshore constructions in large numbers and in varying water depths.

15 Description of the invention

In one aspect, the invention provides a method of manufacturing and installing multiple offshore constructions, the method comprising: a. inserting at least three piles proximate to one another for each offshore construction at a target location into a seabed, such that the upper ends of the 20 piles are located at a distance D2 below the water level, b. manufacturing a jacket on shore and transporting the jacket to the target location, c. connecting the jacket to the upper ends of the piles, wherein the jacket intersects the water level and extends upwardly from the water level over a predetermined third distance D3, 25 d. repeating steps a), b) and c) for multiple offshore constructions at multiple target locations having different water depths, wherein the distance D2 is substantially the same for each offshore construction by variation of a distance D1 of the upper ends of the piles above the sea bed, and wherein a substantially same jacket is provided for each offshore construction such that the third distance D3 is 30 also substantially the same for each offshore construction.

The invention allows a simple and efficient construction of large numbers of offshore constructions through standardization of the design.

A pile is an elongate object suitable to be driven into the seabed. A pile may be hollow or solid. A pile may be - in top view - circular or square or have another form.

35 Generally, a pile is an elongate circular tubular object manufactured from steel.

A jacket or jacket structure is a 3-dimensional frame composed of beams. The beams are generally welded to one another. The jacket has legs which generally extend at a -3- relatively small angle to the vertical. The legs may also extend vertically. The jacket generally has cross-beams which extend substantially horizontally or diagonally. The legs and the cross-beams form a lattice structure.

The upper end of the jacket generally has a smaller footprint than the lower end of 5 the jacket, but this is not required. Generally, a jacket has four legs. However, three, five, six or a different number of legs are also possible.

In an embodiment, the water depth at the target locations lies between 10 and 60 meter and varies. The invention is particularly suitable for these water depths, which are found in the North Sea, amongst other places. The same jacket may be used in target 10 locations having a variation in water depth of at least 10 percent. In practice, the invention allows the use of standardized jackets in wind turbine fields of substantial size and with a substantial variation in water depth..

In an embodiment, the first distance D1 is selected in a range from 0 to 30 meter.

In an embodiment, a height of the multiple substantially same jackets, i.e. the second 15 distance D2 plus the third distance D3, lies in a range of 10-40 meter.

In an embodiment, the method comprises placing a wind turbine on top of each jacket, wherein the wind turbines are substantially the same for each offshore construction.

In an embodiment, the method comprises: placing an intermediate frame on the upper ends of the piles, wherein the intermediate 20 frame connects the upper ends of the piles to one another, and - placing the jacket on the intermediate frame.

The invention also relates to a plurality of offshore constructions, each offshore construction comprising: a. at least three piles which are inserted into a seabed proximate to one another at a 25 target location, such that the upper ends of the piles are located at a first distance D1 above the seabed and at a second distance D2 below the water level, b. a jacket being connected to the upper ends of the piles, wherein the jacket intersects the water level and extends upwardly from the water level over a predetermined third distance D3, 30 wherein the first distance D1 is chosen in dependence of the water depth at the respective target locations, such that the second distance D2 is substantially the same for each offshore construction, and wherein a substantially same jacket is provided for each offshore construction such that the third distance D3 is also substantially the same for each offshore construction.

35 In an embodiment, the water depth at the target locations lies between 10-60 meter and varies.

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The invention also relates to a plurality of jackets for use in the method according to claim 1 or for use in the plurality of offshore constructions according to claim 8, the jackets being substantially the same and each jacket comprising connectors at the lower ends for connection to at least three piles.

5

List of figures

The invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawing in which like reference numerals refer to similar elements.

10 Figure 1 shows an offshore construction of the prior art.

Figure 2 shows another offshore construction of the prior art.

Figure 3 shows a side view of an offshore construction according to the invention.

Figure 4 shows a side view of multiple offshore constructions according to the invention.

15 Figure 5 shows a side view of another embodiment of the invention.

Detailed description of the figures

Turning to Figure 1, an offshore construction according to DE102004042066 is shown. The construction has three piles 2 and a tripod construction placed on top of the 20 three piles. The three piles 2 extend upwards from the seabed, intersect the water line and rise above the water over a substantial height. An advantage of this construction is that most installation work can be performed above the water line. A disadvantage of this embodiment is that it is heavy and costly

Turning to Figure 2, an embodiment according to US20070269272 is shown. This 25 construction has a gravity base, which is very heavy and cumbersome to construct.

Turning to Figure 3, the offshore construction 10 according to the invention is shown. The offshore construction 10 is located at a target location 11 and comprises four piles 12. Three piles 13 may also be provided, and a number of piles greater than four may also be provided. The piles 12 are hollow circular tubes, manufactured from steel. The piles 12 are 30 provided at a distance 42 from one another.

The piles 12 extend into the seabed 14 over a distance D4. This distance D4 is determined by the local soil conditions as well as the pile diameter.

The piles may typically have a diameter 50 of between 50 cm - 3 meter, depending on the load to be carried as well as the soil conditions.

35 The piles 12 extend upwards from the seabed 14 over a distance D1. The upper ends 16 of the piles 12 of the offshore construction 10 are provided at a distance (or altitude) D1 above the seabed. The upper ends 16 of the four piles 12 are located at a -5- substantially same distance D1 above the seabed 14. The upper ends 16 are submerged and are provided at a distance D2 beneath the water level. Thus, contrary to the device of DE102004042066, the piles 12 do not intersect the water line.

The at least three piles 12 provide a solid substructure for a jacket 20. The piles 12 are 5 relatively inexpensive to produce and install in varying lengths.

The jacket 20 comprises four legs 22 which extend upwards at a slight angle to the vertical. The jacket 20 further comprises diagonal cross-beams 24 which extend between the legs 22. The jacket has a height 26, a width and a length.

The jacket 20 has a base 30 which comprises four connectors 32 for connection of 10 the jacket 20 to the piles 12. Alternatively, connectors 32 are provided in a frame that is placed on top of the piles prior to installation of the jacket. The base 30 further comprises a number of horizontal cross beams 35 extending between the connectors 32. The base 30 has a footprint with a width and length which corresponds to the mutual distance 42 between the piles 12. The connectors 32 may be tubular sections with an open bottom and 15 a closed top, which fit over the upper end of the piles. The closed top of the connectors 32 acts as a bearing surface for transferring the weight of the jacket 20 onto the upper ends of the piles 12. The connectors may flare outwardly, to provide a guiding surface for the piles 12 during installation of the jacket 20.

The jacket 20 has an upper platform 36 which connects the four legs 24 and provides 20 further strength and stiffness to the jacket 20. The platform also serves as the support for a wind turbine 40, or any other construction which may be placed on the jacket 20. The width and length 37 of the platform 36 is smaller than the width and length 42 of the base 30 of the jacket 20. The jacket may have the same width and length but may (in top view) also be elongate, i.e. have a greater length than width.

25 The upper platform is provided at a distance D3 above the mean water level 33. This distance D3 is the same for all jackets in a certain field. The distance D3 may be dependent on the expected wave height in a certain area. The upper platform comprises a base for the installation of a wind turbine.

Turning to figure 4, three offshore constructions 10 at target locations 11 are shown. 30 Each target location 11 has a different water depth 28 (= D1 + D2), due to the varying level of the seabed 14.

For each target location, the distance D1 is different, i.e. the piles 12 extend above the seabed 14 over a different distance D1. The piles 12 may be of the same length for each target location 11, but may also have different lengths if the soil conditions allow.

35 The upper ends 16 of the piles 12 are all provided at substantially the same distance D2 from the water level 33. The jackets 20 are all the same, or substantially the same. This implies that the jackets 20 rise above the water over equal distances D3.

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On the right hand side in Figure 4 it can be seen that the distance D1 may become zero or substantially zero. The upper ends 16 of the piles 12 of the offshore construction on the right are located at the level of the seabed 14 or are slightly above the level of the seabed 14. The connectors 32 are also located at or just above the level of the seabed 14.

5 Figure 5 shows an alternative embodiment. An intermediate frame 60 is placed on top of the piles 12 after these are installed. This intermediate frame 60 may form an interface between the piles 12 and the jacket 20. The connectors 61 may be adjustable in height and used to provide guidance and a level landing area for the connectors 32 of the jacket 20.

10

Method of construction and installation

During construction, the piles 12 are driven into the ground. This may be performed with an installation vessel as is known in the field of the art.

The piles may be guided by a framework which ensures that the piles 12 are 15 positioned in a correct position relative to one another. This framework may be laid on the seabed or may be attached to - or defined by - the installation vessel itself. Also, an auxiliary vessel holding an auxiliary frame may be provided.

An intermediate frame may be placed on top of the piles after installation. This intermediate frame may be adjustable and used to provide guidance and a level landing 20 area for the jacket.

The jacket 20 is constructed on land. The jacket may be constructed prior to installation of the piles 12, during the installation of the piles 12 or after the installation of the piles 12.

The jacket 20 is constructed in series in a controlled environment in a factory. This 25 allows mass-production of a large number of the same jackets 20 at relatively limited costs.

After construction, the jacket is transported to the target location 11. The jacket is positioned upright and lowered onto the upper ends 16 of the piles 12. The connectors 32 of the jacket 20 are constructed to form a connection with the upper ends 12 of the piles. The connection is made under water, at a distance D1 from the seabed and a distance D2 from 30 the water level.

In an embodiment, a locking device is provided to lock the jacket in position 20. However, the jacket 20 may also rest on the piles 12 under the influence of gravity.

After the jacket is installed, the wind turbine 40 may be installed. This may be performed in a separate operation with a separate vessel, at a later time.

35 A skilled person will understand that the average water depth of a group of target locations 11 may vary. Thus, it is possible that for one field of wind turbines, multiple first -7- jackets 20 having a first height 26 are used, whereas for another field of wind turbines, multiple second jackets with a different height are used.

It is also possible that multiple first as well as second or even third or more standardized jackets are used within a single field. This way an optimum between cost of 5 pile and jacket fabrication can be achieved.

As long as multiple jackets of the same dimensions are produced for varying water depths, the benefit of the invention is enjoyed.

The invention provides a lighter and more cost-effective offshore construction than the device of DE102004042066, in particular when large numbers are produced.

10 The invention is typically suitable for depths between 10 and 60 meter. A distance D1 of between 0 and 30 meter was found to provide satisfactory results, with a height 26 of the jacket of between 10 and 40 meter.

A skilled person will readily understand that the invention can be varied in multiple ways without departing from the scope of protection as is defined by the claims.

Claims

Method for manufacturing and installing multiple offshore constructions, the method comprising: a. Inserting at least three piles at a target location close to each other in a seabed for each offshore construction, so that the upper ends of the piles are positioned on a distance D2 below the water surface, b. producing a jacket construction on land and transporting the jacket construction to the target location, c. connecting the jacket structure to the upper ends of the posts, wherein the jacket structure intersects the water line and extends upwards over a predetermined third distance D3 above the water surface, d. repeating steps a, b and c for a plurality of offshore structures at a plurality of target locations with different water depths, wherein the distance D2 is substantially the same for each offshore construction by varying a distance D1 from the upper ends of the posts to the seabed, and wherein substantially the same jacket construction is used for each offshore construction, such that the third distance D3 is also substantially the same for each offshore construction.

2. Method as claimed in claim 1, wherein the water depth at the target locations is between 10 and 60 meters and varies.

Method according to claim 1 or 2, wherein the water depth at the target locations with a substantially the same jacket construction varies by at least 10% between a target location with a minimum value depth and a target location with a maximum water depth.

Method according to any of the preceding claims, wherein the first distance D1 is selected from a range of 0 to 30 meters.

A method according to any one of the preceding claims, the method comprising placing a wind turbine on each jacket construction, wherein the wind turbines are substantially the same for each offshore construction.

Method as claimed in any of the foregoing claims, comprising: - placing an intermediate frame on the upper ends of the posts, the intermediate frame connecting the upper ends of the posts to each other, and - placing the jacket construction on the intermediate frame. -9-

7. Multiple number of offshore structures, each offshore structure comprising: a. At least three piles that are inserted close to each other at a target location in a seabed, such that the upper ends of the piles are at a first distance D1 above the seabed and at a second distance D2 below the water surface, b. a jacket construction connected to the upper ends of the posts, the jacket construction intersecting the water line and extending over a predetermined third distance D3 above the water surface, the first distance D1 depending on the water depth on the respective target locations is chosen such that the second distance D2 is substantially the same for each offshore construction, and wherein a substantially the same jacket construction is provided for each offshore construction such that the third distance D3 is also substantially the same for each offshore construction.

A plurality of offshore structures according to claim 7, wherein the water depth at the target locations is between 10 and 60 meters and varies.

9. Multiple number of offshore structures according to claims 7 or 8, wherein the first distance D1 is selected from a range of 0 to 30 meters.

A plurality of offshore structures according to any of claims 7-9, wherein each offshore structure comprises an intermediate frame that rests on the posts, and a jacket structure that rests on the intermediate frame.

11. Multiple number of jacket structures for use in the method of claim 1 or for use in the multiple number of offshore structures according to claim 7, wherein the jacket structures are substantially the same and each jacket structure comprises connectors near the lower ends for connection to the at least three posts.

12. A plurality of jacket constructions according to claim 11, wherein the jacket constructions are substantially the same and wherein each jacket construction comprises connectors near the lower ends for connection to a frame mounted on the at least three posts or for direct connection to the upper ends of the posts.