WO2022098288A1

WO2022098288A1 - Mooring system comprising buoys and anchors

Info

Publication number: WO2022098288A1
Application number: PCT/SE2021/051105
Authority: WO
Inventors: Bertil Moritz
Original assignee: Josok Ab
Priority date: 2020-11-04
Filing date: 2021-11-04
Publication date: 2022-05-12
Also published as: SE545893C2; EP4240644A1; SE2000207A1

Abstract

A mooring system (18) for a floating platform, comprising a plurality of buoys (19) anchored by anchor lines (24) at sea. The mooring system (18) comprises a plurality of mooring unit (27a, 27b) each comprising a plurality of mooring buoys (19a, 19b, 19c, 19d), and each mooring unit being anchored by an anchor system (30) consisting of four anchor lines (24a, 24b, 24c, 24d) each connected to an anchor (26).

Description

Mooring system comprising buoys and anchors

TECHNICAL FIELD

The present invention concerns a floating wind power plant and a system for anchoring a platform to the sea bed. Especially the invention concerns a mooring system comprising a plurality of anchors for mooring a wind power platform. In particular the invention concerns a mooring system for a semi-submerged platform.

BACKGROUND OF THE INVENTION

A mooring system in deep sea comprises an anchor system containing a plurality of anchors. The anchors are organized in special patterns to make the moored object in a desired geographic position at all wind and weather conditions. The anchors may be traditionally digging anchors, gravity anchors or suction anchors. The anchor lines may comprise catenary lines or taut leg lines.

A floating semi-submerged wind power plant comprises a structural platform containing a plurality of buoyancy element. The platform carries a single or a plurality of rotor towers. The buoyancy elements comprise an inner cavity into which water is pumped to immerse the platform. In its operational state the plant is immersed to a level where only the buoyancy elements penetrate the water surface. Thus in its operational state only the tower and the top of the buoyancy elements can be seen above the sea surface. A plant having a tower with a nacelle for turning the rotor towards the wind the platform needs to be stably oriented by the mooring system. A great plurality of different types of semisubmersible platforms for generating wind power have been suggested. Most of them have one single turbine on top of a tower.

From US2019024635 (Siegfriedsen) an offshore wind farm with a plurality of foundation elements is previously known. The foundation elements are arranged so as to form the corners of a plurality of parquetted hexagons and with a plurality of floating offshore wind turbines. Each floating offshore wind turbine within a hexagon is connected to the foundation elements which form the hexagon. The floating offshore wind turbines are connected to the foundation elements by means of connection means designed as a chain and/or a cable or a combination of a chain and a cable. The connection means have a length which allows the offshore wind turbines to drift within a circular area with a radius of up to 10% of the hexagon circumradii about the respective hexagon centre.

From JP 2004176626 (Takada) an offshore wind power generation facility is preciously known. The object is to provide an offshore wind power generation facility which can be placed on the ocean easily even in a deep sea area, which keeps respective floating bodies moderately apart from one another even under severe weather and ocean conditions, and which prevents the power generating capacity from being reduced when the relative positional relationship between wind power generation equipment is changed. The facility comprises a plurality of floating bodies which respectively support the wind power generation equipment or supports control equipment. The floating bodies are connected to one another by means of mooring chains each having an intermediate sinker in the middle of the chain. The floating bodies positioned at the outermost location are further connected to mooring anchors at their one ends by means of mooring chains each having an intermediate sinker in the middle of the chain. The floating bodies and the mooring anchors are arranged to connect a plurality of element structures of plan view equilateral triangles.

From US 8471396 (Roddier) a column-stabilized offshore platform with waterentrapment plates and asymmetric mooring system for support of a single wind turbine is previously known. The floating wind turbine platform includes a floatation frame that includes three columns which are coupled to each other with horizontal main beams. A wind turbine tower is mounted above a tower support column to simplify the system construction and improve the structural strength. The turbine blades are coupled to a nacelle that rotates on top of the tower. The floatation frame includes a water ballasting system that pumps water between the columns to keep the tower in a vertical alignment regardless of the wind speed. Waterentrapment plates are mounted to the bottoms of the columns to minimize the rotational movement of the floatation frame due to waves. The platform is connected to seabed by anchor lines from each column. From EP 2789848B1 (Komatsu) is previously known a floating body wind power generating device and method for mooring the floating body wind power generation device. An objective of the wind power generation device is to provide a floating body wind power generating device with which it is possible to moor the floating body stably with respect to drift force or rotational moment acting on the floating body. The wind power generating device comprises a wind power generator and a floating body. Further the device comprises a first column which is located on the upwind side of a primary wind direction and whereupon the wind power generator is installed. A second column and a third column which are located on the further downwind side of the primary wind direction than the first column is connected to the first column with two rower hulls to the first column. A plurality of mooring cables connects the floating body to anchors. At least two of the plurality of mooring cables are connected to the first column. At least one of the pluralities of mooring cables is respectively connected to the second column and the third column. Each of the plurality of mooring cables is positioned extending in radiating directions from the floating body so as not to intersect in plan view.

The long-term accuracy of positioning the wind power units is important. If drag anchors are used, the accuracy is not sufficient. The anchoring lines are pulled until the anchor catch sufficient grip to the sea bed. Hence the seabed conditions may cause the anchor and thereby the platform to change its position hundreds of meters from a desired position. Therefore, position reliability requires the use of suction anchors and pre-stressed mooring lines.

One of the main advantages of floating platform is that it may be fully fabricated at a shipyard and thus reduce the installation cost. It is then transported to a desired location and permanently moored to an anchor system. It is, however, desirable to be able to tug the platform back to the shipyard for major repair and maintenance. It is therefore preferable to avoid installing a mooring system each time. Already when planning and deploying a wind farm with a plurality of floating platforms, it is desirable to first establish an infrastructure of electric cabling and a mooring system. When several buoys or other floating elements are used to make the platform stable, it is associated with considerable cost to install the mooring system each time the platform need to be repaired and return to shipyard. Thus, there has been a longtime need within the floating wind power industry to provide a permanent anchoring system and yet providing an easy facility to moor the wind power plant.

Since the water depth is limited at a shipyard and the optimal design of a semisubmersible platform requires deeper water, there is also a need to provide a platform capable of easily changing the draft of the platform.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to seek ways to improve a floating wind power plant including the mooring of a floating wind power platform. A second objective of the invention is to provide a permanent mooring system and a method of mooring the platform.

These objects are achieved according to the invention by a mooring system characterized by the features in the independent claim 1 or by a mooring facility characterized by the features in the independent claim 5, or by a method characterized by the steps in the independent claim 7 and 8. Preferred embodiments are described in the dependent claims.

According to the invention the mooring system comprises a pair of mooring buoys anchored at sea. The pair of buoys are separated from each other by a distance wire to keep a desired distance between them. The distance between the pair of buoys is preferably the same as the distance between two adjacent arms of a floating wind power plant. The pair of buoys are anchored by four anchors to receive a geographic stable position and orientation. Preferably suction anchors and prestressed anchor lines are used to anchor the buoys. By an anchor line is meant a catenary element capable of resisting tension forces. The anchors are spread equally and widely on the sea bottom in a square pattern. By providing a mooring system with only two distinctive mooring points any floating platform may be moored in a secure way to receive a desired position and a desired orientation. The pair of buoy concept may be applicable to any self-floating and stabilised platforms.

According to an embodiment of the invention the pair of buoys are anchored with four anchor lines and four anchors in a square pattern. The anchors are widely spread to provide an almost horizonal force on the buoys. In an embodiment the pair of buoys are oriented parallel to a side of the anchor square. Each buoy of the pair of are anchored by two anchors and kept in position by the distance wire to the other buoy of the pair which in turn is anchored by two anchors. In an embodiment the pair of buoys are anchored with six anchor lines to four anchors. In this embodiment each buoy of the pair is anchored with three anchor lines. However, on opposite sides of the pair of buoys separated by the distance wire one anchor line from each buoy may be anchored at the same anchor. Thus two of the anchors are arranged to receive one anchor line from each individual of the pair of buoys. Still the distance between the pair of buoys are defined by the distance wire.

In an embodiment of the invention two sets of buoy pairs are anchored by four anchors. The first and second sets of buoy pairs are arranged in parallel with each other. In this embodiment each sets of buoy pairs are connected to a corresponding sets of buoy pairs by a separating means. In an embodiment the separating means comprise a single cable connected by a span arrangement to each sets of buoy pairs. In an embodiment the separating means comprise two cables crossing each other. Each crossing cable is connected between diagonally opposite buoy of the pair of the first and second set of buoy pairs. In an embodiment each pair of buoys of the first set is connected to a corresponding pair of buoys of the second set by parallel lines. Thus the separating means comprises in this embodiment two parallel lines. With long enough length of the separating means two floating wind power platforms may be moored to the anchoring system with a predefined distance. The embodiment of the two sets of buoy pairs may be called a double pair system. Thus the double pair system is still anchored with four anchor lines only but capable of mooring double number of platforms. Building up a facility of a large numbers of wind power platforms moored at sea by the present mooring system may start with one platform using four anchors. A second platform may be anchored in the vicinity of the first platform. A third and a forth platform may be anchored sideways of the first and second platform. Each of these platforms need four anchoring points thus four platforms would need 16 anchoring points. However the second platform needs only two new anchors since two anchors are shared with the first platform. And the second line of platforms including the third and fourth platform needs only three new anchors since three anchors are common to the first line of platforms. In this embodiment four platforms need nine anchors. By organizing a vast number of platforms side by side in lines and columns will make use of a plurality of anchors being used in common by a plurality of platforms. The limit of the number of anchors needed in a facility with a great number of platforms tends to one anchor for each platform. Hence each anchor receives anchor lines from four platforms.

According to an embodiment of the invention a second facility of platforms are organised diagonally to the first facility of platforms. The position of the second such platform will be just on top of an already existing anchor. Hence by adding the second set of platforms which all uses already existing anchors the efficiency of the use of anchors may be drastically improved. For a wind power plant comprising a vast number of both facilities of anchored platforms each anchor receives anchor lines from eight platforms. The limit of the number of anchors needed of such a facility tends to one anchor for every pair of platforms.

In an embodiment of the invention the mooring system comprises a docking means involving the pair of buoys and two of the secondary floats. Each buoy comprises an upper hollow body and a lower hollow body sharing a common water ballast container. The upper body comprises an elongated hollow structure having a small cross-sectional area. A small cross-sectional area impedes the movement of the buoy in the sea. The lower body is preferable a wider structure than the upper body. Each pair of buoys also comprises a pump facility to pump water into and out from the ballast container. By pumping water into the buoy the ballast increases and the buoy immerse in the sea and vice versa. In an embodiment the docking means comprises a first part containing the pair of buoys and a second part containing a pair of secondary floats connected to the platform to be docked. The docking is performed by a relative movement in the vertical plane of the parts of the docking means. According to the invention a method of docking comprises immersing the pair of buoys, orienting the secondary float on top of the pair of buoys, raising the pair of buoys by emptying the ballast water to interact with the float and lock the two parts together.

When the first docking means has docked the platform is rotated by tugs so that the second docking means are aligned. Then the second docking is made the same way as the first docking. When fully docked the platform is immersed to its semi-submersed operational position. The advantage of letting the upper buoy stay above water is that wave forces and motions are avoided to better facilitate the docking.

Docking two objects or bodies at sea may be cumbersome if the water is not calm. This is due to the fact that waves are irregular and the forces acting on any buoyant body are high. Even though the body is partly submersed the forces are severe. Each buoyant body and the platform have their own characteristic natural frequencies in the heave of the sea. To minimize the hazardous portion of docking a set of principles are used to design the platform. One such principle is to minimize the cross-sectional area of sea protruding elements. Another principle is to decrease the natural frequencies of bodies which are arranged to mate. The natural frequencies in different directions may be decreased by increasing the mass of the body, which is achieved by balancing ballast water. According to the invention the two buoys which are permanently moored have small cross-sectional areas. By a small cross section area in this context is meant a ratio between the cross section width and the length of the mooring buoy in the range of 6-20 %. When in the position before docking the motion in horizontal direction is relatively well restricted by ballast water and by the pre-tensioned anchor lines.

In a first aspect the object is achieved by a mooring system for a floating platform, comprising a plurality of buoys anchored by anchor lines at sea, wherein the mooring system comprises a plurality of mooring unit each comprising a plurality of mooring buoys, and each mooring unit being anchored by an anchor system consisting of four anchor lines each connected to an anchor.

In an embodiment a first mooring unit consists of a first mooring buoy, a second mooring buoy, and a first distance wire connecting the first and second mooring buoys, the first and second mooring buoy forming with the first distance wire a first pair of mooring buoys. In an embodiment a second mooring unit further consists of a third mooring buoy, a forth mooring buoy and a second distance wire connecting the pair of buoys, the third and forth mooring buoy forming with the second distance wire a second pair of mooring buoys, and wherein the second mooring unit further comprises a separating wire connecting the first and second pair of mooring buoys.

In a further embodiment each buoy comprises an upper body and a lower body having a common ballast compartment fillable with water or air for adjusting the immersed level of the buoy at sea. In a further embodiment each buoy comprises a docking means arranged to hook or mate with a dockable floating object.

In a second aspect of the object is achieved by a method for providing a mooring system for a floating platform comprising a plurality of buoys anchored by anchor lines at sea, wherein a mooring unit comprising a first and a second buoy, and a first distance wire connecting the first and a second buoy, anchoring the first buoy with a first anchor line to a first anchor and with a second anchor line to a second anchor, anchoring the second buoy with a third anchor line to a third anchor and with a fourth anchor line to a forth anchor.

In a third aspect the object is achieved by a method of mooring a floating platform to a mooring system comprising a plurality of buoys anchored by anchor lines at sea, characterized by providing a mooring unit comprising a first and a second buoy connected by a distance wire, providing in each buoy a common ballast compartment, providing each buoy to include docking means arranged to hook or mate with the platform, lowering the buoys by filling water into the ballast compartment, aligning the platform, raising the buoys by emptying water from the ballast compartment whereby the docking means will interact with an equivalent mooring means of the floating platform. BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become more apparent to a person skilled in the art from the following detailed description in conjunction with the appended drawings in which: fig 1 . is a perspective view of a mooring system, fig 2. is plan view with two embodiments of the mooring system, fig 3. is a plan view of mooring facility comprising a large number of mooring systems,

DESCRIPTION OF PREFERRED EMBODIMENTS

A mooring system 18 according to the invention is shown in fig 1 . The mooring system comprises a mooring unit 27 including a pair of buoys 19 and a distance wire 25. A first buoy 19a and a second buoy 19b are anchored at sea by an anchor system 30 consisting of four anchors 26 in a square pattern. The two buoys are acting as a pair since they are permanently held apart by a distance wire 25. The mooring system is anchored with four anchor lines 24. Each buoy of the pair of buoys is anchored with two anchor lines. By the distance wire 25 each pair of buoys is also anchored with the other two anchors. Thus the distance wire is a common anchoring means to both buoys of the pair. In order to distribute the forces from the anchor lines the connection to the buoys comprises a span arrangement 23. Each buoy comprises an elongated container having an upper body 21 and a lower body 22.

In the embodiment shown the distance wire 25 and the anchor lines 24 are cut to indicate that they may comprise considerably longer lines at an actual site. The anchor lines may comprise hundreds of meters depending on the sea bed condition and the dept. Preferably suction anchors are used. The distance wire may be in the range of 50 to 100 meters. Each of the pair of buoys comprises an upper body 21 and a lower body 22 which are structurally connected to form a common ballast compartment. The ratio of the cross section and the length of the buoy is in the range of 6-20 %.

The upper body and the lower body form together a common ballast compartment fillable with water. By pumping water in or out of the compartment the pair of buoys may be adjusted in the sea to keep a predetermined float position. In an embodiment the pair of buoys comprise docking means 20 arranged to hook or mate with a dockable object to be moored. By pumping water into and out from these compartments the floating height of each of the pair of buoys is balanced in the sea. Thus each buoy of the pair may be lowered to be able to dock with a dockable platform. In the embodiment shown there is a structure line 28 of a floating platform to be moored to the mooring system. Any kind of mooring method may be used. According to the invention such a moored platform will be held in a stable position and orientation by the mooring system comprising a pair of buys only.

According to the invention the mooring system may comprise a plurality of mooring units 27. In fig 2 two such mooring units are shown. A first mooring unit 27a is shown in the upper part of fig 2. The first mooring unit comprises a pair of mooring buoys 19a, 19b connected to each other by a first distance wire 25a. A second mooring unit 27b is shown in the lower part of fig 2. The second mooring unit comprises a first pair of mooring buoys 19a, 19b connected to each other by a first distance wire 25a and a second pair of mooring buoys 19c, 19d connected to each other by a second distance wire 25b. Further the second mooring unit comprises a separating wire 29 connecting the first and second pair of buoys at a distance where two adjacently moored wind power platforms do not affect each other.

The first mooring unit 27a has been described above. The pair of buoys are anchored in parallel with the square pattern of anchors. A first anchor line 24a and a second anchor line 24b are anchoring the first buoy at the lefthand side of the figure. A third anchor line 24c and a forth anchor line 24b are anchoring the second buoy at the righthand side of the figure. The distance wire 25a is common to both buoys and thus comprising the third anchoring means to hold each of the pair of buoys firmly anchored. The length of the distance wire 25a is preferably the same as the distance between two adjacent arms of a floating platform. This embodiment of the mooring unit may be known as a single pair type mooring unit.

The second mooring unit 27b according to the invention is also shown in fig 2. In this embodiment the mooring unit comprises a second pair of buoys 19c and 19d, each having a second distance wire 25b between them. The second pair is arranged in parallel with the first pair of buoys 19a and 19b. The first and second pair of buoys are connected with each other by a separating wire 29. Thus the second mooring unit 27b of the mooring system according to the invention comprises two pairs of mooring buoys 19 with distance wires 25 and a separating wire 29. In the embodiment shown the separating wire 29 is connected to opposite pairs of buoys with a span. The aim of the separating wire is to keep the two pairs of buoys in a stable location at all weather conditions. Hence there may be a plurality of possibilities of designing such distance means known to a person skilled in the art. This embodiment of the mooring unit may be known as a double pair type mooring unit.

A vast facility of a mooring system 18 comprising a large number of mooring units 27 according to the invention is shown in fig 3. The mooring units may comprise either the single pair type or the double pair type. Since every mooring unit 27 only needs four anchors 26 the anchors are organized in a square pattern. As shown in the upper part of fig 3 a first mooring unit needs four anchors. An adjacent mooring unit anchored nearby needs only two extra anchors since the other two may be shared with the first mooring unit. The next three also needs two extra anchors but further units need only one new anchor. Building up a large facility thus ends up in a limit of one anchor for every mooring unit.

Having an anchor pattern like the top view a further set of platforms may be anchored to the already existing anchors. A new set of mooring units may be rotated diagonally and end up in a pattern like the mid view. Thus when the first set of mooring units 27a is added to the second set of mooring units 27b the result is shown in the lower view of fig 3. In this view every new mooring system is positioned over an existing anchor. The effect of the capability to anchor the second set of platforms results in the limit of one anchor for every two mooring units. Although favourable the scope of the invention must not be limited by the embodiments presented but contain also embodiments obvious to a person skilled in the art. The anchor lines may comprise any kind of material with good tensile properties. The anchor lines may comprise chains at one or two of its ends. The buoys may comprise stationary or temporary means for pre-tensioning the lines.

Claims

1 . Mooring system (18) for a floating platform, comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e r i z e d i n that the mooring system (18) comprises a plurality of mooring unit (27a, 27b) each comprising a plurality of mooring buoys (19a, 19b, 19c, 19d), and each mooring unit being anchored by an anchor system (30) consisting of four anchor lines (24a, 24b, 24c, 24d) each connected to an anchor (26).

2. Mooring system according to claim 1 , wherein a first mooring unit (27a) consists of a first mooring buoy (19a), a second mooring buoy (19b), and a first distance wire (25a) connecting the first and second mooring buoys, the first and second mooring buoy forming with the first distance wire a first pair of mooring buoys.

3. Mooring system according to claim 2, wherein a second mooring unit (27b) further consists of a third mooring buoy (19c), a forth mooring buoy (19d) and a second distance wire (25b) connecting the pair of buoys, the third and forth mooring buoy forming with the second distance wire a second pair of mooring buoys, and wherein the second mooring unit further comprises a separating wire (29) connecting the first and second pair of mooring buoys.

4. Mooring system according to any of the preceding claims, wherein each buoy (19) comprises an upper body (21) and a lower body (22) having a common ballast compartment fillable with water or air for adjusting the immersed level of the buoy at sea.

5. Mooring system according to any of the preceding claims, wherein each buoy (19) comprises a docking means (21 ) arranged to hook or mate with a dockable floating object (28).

6. Mooring facility comprising a large number of mooring systems (18) according to any of the preceding claims, c h a r a c t e r i z e d i n that the large number of mooring systems (18) consisting of four anchor lines (24) are anchored adjacently in a column and line like pattern of anchors (26), all using adjacent anchors in common, whereby the limit of necessary anchors tends to one anchor per platform. Mooring facility according to claim 6, wherein a large number of second set of mooring systems (18) consisting of four anchor lines (24) is anchored diagonally to the existing square pattern of anchors, whereby most anchors will receive eight anchor lines (24) whereby the limit of necessary anchors tends to one anchor for every pair of mooring systems. Method for providing a mooring system (18) for a floating platform (28) comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e r i z e d b y providing a mooring unit (27a) comprising a first (19a) and a second (19b) buoy, and a first distance wire (25a) connecting the first (19a) and a second (19b) buoy, anchoring the first buoy (19a) with a first anchor line (24a) to a first anchor (26a) and with a second anchor line (24b) to a second anchor (26b), anchoring the second buoy (19b) with a third anchor line (24c) to a third anchor (26c) and with a fourth anchor line (24d) to a forth anchor (26d). Method of mooring a floating platform (28) to a mooring system (18) comprising a plurality of buoys (19) anchored by anchor lines (24) at sea, c h a r a c t e r i z e d b y providing a mooring unit (27) comprising a first (19a) and a second (19b) buoy connected by a distance wire (25), providing in each buoy a common ballast compartment (21 , 22), providing each buoy to include docking means (20) arranged to hook or mate with the platform, lowering the buoys by filling water into the ballast compartment, aligning the platform, raising the buoys by emptying water from the ballast compartment whereby the docking means will interact with an equivalent mooring means of the floating platform.