EP2593355A1

EP2593355A1 - System and method for accurate positioning of structures in sea under exposure to waves and currents

Info

Publication number: EP2593355A1
Application number: EP11749264.5A
Authority: EP
Inventors: Harald Wergeland; Morten Dinhoff Pedersen
Original assignee: Seatower AS
Current assignee: Seatower AS
Priority date: 2010-07-16
Filing date: 2011-07-13
Publication date: 2013-05-22
Also published as: WO2012008848A1

Abstract

A system and a method for positioning a structure (1), floating in a body of water, at a desired position (P) comprises at least two anchors (7, 8; 1e, 1f) positioned on the seabed. Each anchor comprises respective connection lines (2, 3) for attachment to the structure, and a tug (5) connected to the structure. Each connection line (2, 3) comprises individual adjustment means (15). The length of the individual positioning lines may be adjusted individually and selectively during the positioning.

Description

System and method for accurate positioning of structures in sea under exposure to waves and currents

Field of the invention

The present invention relates to the installation of structures on a seabed. More specifically, the invention relates the positioning of buoyant structures in a body of water in relation to fixed anchors. In particular, the invention relates to offshore structures provided with buoyancy for towage to the installation site.

Background of the invention

The increasing demand for exploitation of renewable energy sources increases the demand for offshore wind power generation, as offshore wind conditions are more favorable than onshore conditions and the environmental impact is much less. There is an increasing need for wind turbine supporting structures that can be transported to the offshore location in a buoyant state and installed on the seabed by e.g. the addition of a ballast material. International patent publication No. WO 2009/154472 A2 describes one such support structure. It is desirable to be able to install such structures with as low as possible restrictions to the maximum waves and currents, acceptable for execution of the installation and at the same time with tight positioning tolerances.

In the prior art, such buoyant structure is positioned in the water by two or three vessels connected by tow lines to the structure. The vessels are able to maneuver the structure to the target area, keep it positioned and in a desired orientation until the structure is installed on the seabed.

As waves and currents exert dynamic loads on the vessels, which in turn influences the movements of the structure, the achievable positioning accuracy depends on the waves and currents occurring during the positioning operation. Consequently, the positioning and installation of the structure can only be executed in sea conditions which are less severe than the operability of the vessel otherwise would allow.

In particular, for offshore wind turbine foundations, which normally need to be installed with high positioning accuracy, the acceptable seastate is significantly less that the limiting conditions for operating the associated vessel. Hence, installation of multiple foundations is hampered by waiting for low wave and current conditions, which has undesirable impact on duration of the installation and on the rate of installation.

The present applicant has devised and embodied this invention to overcome the shortcomings of the prior art and to obtain further advantages. Summary of the invention

The invention is set forth and characterised in the main claim, while the dependent claims describe other characteristics of the invention.

It is thus provided a system for positioning a structure, floating in a body of water, at a desired position, characterised by at least two anchoring means positioned on the seabed, each anchoring means comprising respective connection means for attachment to the structure, and further comprising manoeuvring means connected to the structure. The respective connection means comprise individual adjustment means.

In one embodiment, the manoeuvring means comprises a propulsive unit by which a pulling force is applied to the structure.

In one embodiment, the adjustment means is adjustable while the propulsive unit is applying the pulling force. The connection means preferably comprise individual lines.

It is also provided a method of positioning a structure, floating in a body of water and connected to a positioning vessel via a line, at a desired position, characterised by:

a) placing two anchoring means on the seabed;

b) determine and set the required lengths of individual positioning lines to be extended between the structure and each anchoring means;

c) connect the individual lines between the structure and each anchoring means;

d) manoeuvre a positioning vessel, connected to the structure via the line, to a required heading, and apply tension to the positioning lines, until the structure is at the desired position.

In one embodiment, the length of the individual positioning lines are adjusted individually and selectively during the application of tension by the vessel.

Brief description of the drawings

These and other characteristics of the invention will be clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached figures wherein:

Figure 1 is a top view of a floating structure attached to the positioning arrangement comprising positioning anchors;

Figure 2 is a side view of the floating structure attached to the positioning arrangement comprising positioning anchors shown in figure 1 ;

Figure 3 is a sketch showing the composition of the positioning line; Figure 4 is a side view of an installed positioning anchor with a lower segment of the positioning line buoyed off and ready for connection of the remaining part of the positioning line;

Figure 5 is a side view of the positioning anchor shown in fig. 4 with lower section of the positioning line exposed to loads from a structure (not shown) being positioned; and

Figure 6 is a top view of the positioning arrangement in combination with a number of already installed structures. Detailed description of a preferential embodiment

Referring to figure 1 , a floating structure 1 is schematically illustrated being positioned at a target position P, attached to two positioning lines 2, 3 and to a line 4 connected to a vessel 5 (e.g. a tug, or similar).

A first anchor 6, a second anchor 7, and a third anchor 8 are pre-installed on the seabed. As the positioning lines 2, 3 are attached to respective fixed second and third anchors 7, 8, and to respective points on the structure, the position P and orientation of the structure 1 is determined by the physical parameters involved; i.e. the position of the second and third anchors 7, 8, lengths and stiffness of the positioning lines 2, 3, tension in the positioning lines caused by the vessel 5, and finally by the geometrical shape of the positioning lines in the vertical plane, often resembling a catenary.

Normally, the required lengths of each of the positioning lines 2, 3 may be calculated from known as-installed positions of the second and third anchors 7, 8; accounting for the lines' properties and tension from the positioning vessel 5, such that very accurate position of the installed structure 1 can be achieved.

As the intention is to enable a positioning method and equipment for performing the positioning is as high a seastate as possible, and the liberty of approaching the installation site from an arbitrary direction, it may be advantageous to preinstall the three fixed anchors 6, 7, 8 with an approximate separation of 120° in relation to the structure's target position P. Which pair of the three anchors 6, 7, 8 should be used for the positioning, would be determined from the actual weather and seastate direction. For example, referring to figure 1, if waves are approaching mostly in direction within the sector a, then the second and third anchors 7, 8 are used. If the approach direction is within sectors b, then the first and second anchors 6, 7 are used, and if the approach direction is with sectors c, then the first and third anchors 6, 8 are applicable. With this arrangement, the positioning vessel 5 will not have to operate in quartering sea. Figure 2 is a schematic side view of the positioning arrangement for the structure 1. When floating in waves, the structure moves in six degrees of freedom. The invented attachment of the structure 1 to the anchors 6, 7, 8 ensures that the structure's motions in the water are kept to a minimum, thus allowing accurate positioning in the water, even in large waves. This is achieved by connecting the positioning lines 2, 3 to the structure 1 by means of respective bridles 9, 10, which in turn are connected to the structure at respective points 1 la,b and 12a,b. The elevations on the structure at which the connections points are positioned, and the vertical distance between them, are calculated from a dynamic analysis targeting minimum motions of the structure at acceptable loads induced in the positioning lines 2, 3 and in the towing line 4.

Figure 3 identifies main components in the invented positioning line 2, 3, i.e., sequentially from left: a lower segment 13, a main segment 14, a length adjustment segment 15 and a tension release device 16. The function of the lower segment 13 is explained below, with reference to figure 4.

The length adjustment segment 15 allows rapid adjustments of the positioning line length to reflect as-installed positions of the positioning lines. Typically, the length adjustment segment 15 comprises a chain (indicated as a dotted line in figure 3) that allows any point of the adjustment segment to be connected to an adjacent segment. The length adjustment segment 15 may also comprise a winch unit, or similar (not shown), which for example may be placed on a floating vessel (not shown) tethered to the anchors 6, 7. The length of the individual positioning lines may be adjusted individually and selectively during the positioning operation.

In figure 3, the adjacent segment is the tension release device 16. The tension release device 16 is used upon completion of the positioning and installation procedure of the structure, when the tensioned positioning lines 2, 3 are to be slackened for easy disconnection and subsequent retrieval. In order to ensure minimal motions of the structure 1 connected to the positioning system (as well as acceptable loads), the positioning lines 2, 3 are designed with the appropriate stiffness, determined by dynamic analysis. This stiffness is mainly achieved by the design and composition of the main segment 14. Typically, the main segment 14 comprises fiber ropes that have lower elasticity modules than steel wires.

Figure 4 shows an anchor 7 for use during positioning and installation of the structure once it has been installed. The anchor 7 - which is similar to the anchors 6, 8 described above - is made as a hollow cylinder which is embedded

substantially vertically into seabed S. As costs for retrieving positioning anchors often exceed the reuse value of the anchors, it is advantageous to install the anchors below the mudline 17 and abandon them after the use.

The minimum embedment depth for the anchor 7 is given by rules applicable for the particular location. The embedment is in this embodiment achieved by an embedment device 18, in the figure shown suspended in the water (dotted line indicating lifting wires connected to installation vessel (not shown)) under repositioning to next preinstalled anchor or under retrieval to surface. The embedment device 18 is designed to provide sufficient weight for embedding the anchor into the seabed. The embedment device is shaped so that it does not produce unnecessary penetration resistance, and is provided with mating guides 19 that facilitates subsea docking onto the pre-installed anchor 7. The embedment device is adapted for addition of weight, by e.g. clump weights or ballast chain (not shown) onto an upper surface.

Before installation of the anchor 7, the respective lower segment 13 of each positioning lines is attached to its respective anchor in air (before it is lowered into the water) in order to avoid subsea work. The lower segment 13 shown in figure 4 is buoyed off to a surface buoy 20, thus allowing connection to the remaining part (e.g. main segment 14) of the positioning line above water.

Figure 5 shows an installed positioning anchor 7 and positioning line 2 exposed to tension loads from the floating structure (not shown) during positioning.

Oftentimes, offshore wind towers with turbines are installed in clusters, in a regular pattern. The invention is able to utilize already installed structures in such pattern in the positioning and installation as described above. This is shown schematically in figure 6, where reference numbers la-f indicate structures 1 already installed to fixed positions on the seabed. Two of these already installed structures (le, If) are being used for placing a floating structure lg at a desired position P. Thus, the two installed structures (le and If in figure 6) serve the similar functions as the fixed anchors 6, 7 as described above with reference to figure 1. When already installed structures le,f are used for positioning in this fashion, they are preferably furnished with individual lower segments 13 (not shown in figure 6) before installation, as described above with reference to figure 4. In figure 6, the floating structure lg (to be positioned and installed) is connected to positioning lines 2, 3 via respective bridles 9, 10 to the already installed structures le, If. The vessel 5 exerts the required tension in the positioning system via the tow line 4, as described above.

The positioning of a floating structure may typically be performed by:

1. Perform dynamic analysis in order to:

a) optimize attachment points of positioning lines on the structure, b) define stiffness of the positioning lines,

c) calculate design loads in the positioning lines,

d) define optimum average tension in the lines to avoid slack and

unreasonably large loads in the positioning lines, and

e) calculate tensioning history from zero tension to the target average tension that avoids resonance like effects. Establish two fixed points on the seabed, i.e. either two tailor made anchors 7, 8, or other two structures le,f already installed.

Attach the positioning lines 2, 3.

Find exact position of the connecting points for the positioning lines and calculate the final length of the positioning lines required for accurate positioning of next structure.

Adjust lengths of the positioning lines 2, 3.

Tow the structure to the installation site and connect it to the preinstalled positioning lines.

Maneuver the positioning vessel (towing tug) to a required heading and tension up the positioning lines with a prescribed rate of the tension increase. Monitor position of the floating structure moving with amplitudes reduced by the positioning lines and compare with predicted acceptable motion.

Perform deployment of the structure on the seabed.

Claims

1. A system for positioning a structure (1), floating in a body of water, at a desired position (P), characterised by at least two anchoring means (7, 8; le, If) positioned on the seabed, each anchoring means comprising respective connection means (2, 3) for attachment to the structure, and further comprising manoeuvring means (4, 5) connected to the structure.

2. The system of claim 1 , wherein the respective connection means (2, 3) comprise individual adjustment means (15).

3. The system of claim 2, wherein the manoeuvring means comprises a propulsive unit (5) by which a pulling force is applied to the structure.

4. The system of claim 3, wherein the adjustment means is adjustable while the propulsive unit is applying the pulling force.

5. The system of any one of the preceding claims, wherein the connection means (2, 3) comprise individual lines (2, 3).

6. A method of positioning a structure (1), floating in a body of water and connected to a positioning vessel (5) via a line (4), at a desired position (P), characterised by:

a) placing two anchoring means (7, 8; l e, I f) on the seabed;

b) determine and set the required lengths of individual positioning lines (2, 3) to be extended between the structure and each anchoring means;

c) connect the individual lines (2, 3) between the structure and each anchoring means;

d) manoeuvre a positioning vessel (5), connected to the structure via the line (4), to a required heading, and apply tension to the positioning lines, until the structure is at the desired position (P).

7. The method of claim 6, wherein the length of the individual positioning lines are adjusted individually and selectively during step d.