Patent Publication Number: US-2020277936-A1

Title: Off shore wind energy installation foundation system

Description:
The invention relates to a foundation system for an offshore payload, preferably an offshore wind energy installation, however also applicable to oil or gas applications. The foundation system is provided with three or more suction buckets (hereafter also called “bucket”). The foundation system is in particular designed for the next generation off shore wind energy installations of 9 MW and higher. Particularly the foundation system is designed for supporting a single upright mast (also called pole or post) which supports the payload, preferably at its top end. In case of a wind energy installation the mast preferably comprises an upright monopole and on top of it an upright tower, wherein the tower supports the nacelle at its top. In stead of a nacelle the payload could comprise a platform, e.g. for oil or gas application. The mast, monopole and tower are preferably a single tube and/or made of steel however reinforced mineral cement concrete is also feasible. The payload preferably will be located high above the sea, e.g. at least 10 or 20 metre. Sea depth typically will be at least 10 or 20 or 50 metre. 
     Suction buckets and how to install them are a.o. known from GB-B-2300661 and EP-B-0011894, which are enclosed in here by reference. Briefly, a suction bucket is a thin walled steel sleeve or pipe or cylinder, which cylinder is closed at its longitudinal top end by a bulkhead (also called top plate) or different sealing means and which cylinder is sealingly located on the subsea bottom with the open end opposite the bulkhead since this open end penetrates the subsea bottom due to the weight of the suction bucket. Thus the cavity, also called suction space, delimited by the cylinder and the bulkhead is sealed by the subsea floor such that vacuum or suction can be generated by removing water from within the suction space such that a resulting force tends to force the suction bucket deeper into the subsea floor. The creation of the suction can be with the aid of a suction source, such as a pump, being on, or close to or at a distance from the suction bucket and connected to the suction space. The applied level of the suction can be e.g. at least substantially constant, smoothly increase or decrease or else pulsate, for which there are convenient means. After use, the suction bucket can easily be removed by creating an overpressure within the suction space, e.g. by pumping in (sea) water. 
     A self installing marine structure, e.g. platform applying suction buckets is known from e.g. WO99/51821 (SIP1) or EP-A-1 101 872 (SIP2) of the present inventor. WO 02/088.475 (SIP3) discloses a tower carrying a wind turbine at the top and suction buckets as foundation. 
     Suction buckets are more and more applied as (part of) a foundation of an off shore wind energy turbine. For such application, typically three or more mutually spaced suction buckets are applied, providing a static balanced (in case of three suction buckets) or overbalanced (in case of more than three suction buckets) support. In operation, the suction buckets have at least almost completely penetrated the sea bed, are at equal or substantially equal level and have a mutual horizontal spacing providing a clearance of at least 5 metre, typically in the order of 20 metre, or a clearance of at least 0.5 or 1.0 times the diameter of the suction bucket (clearance means the shortest distance between the facing side walls). This assembly of suction buckets carries a single monopole or a space frame (also called jacket) of steel beams or tubes and on top of it a vertical tower supporting at its upper end the nacelle of the wind energy turbine provided with rotor blades, typically rotating around a horizontal axis and driven by the wind. The wind energy turbine converts wind energy into electrical energy. The wind turbine is typically part of a wind farm of identical wind turbines each provided with its own foundation of three or more suction buckets. A cable brings  30  the electricity from the wind turbine generator to an electricity consumer on shore, e.g. a household. 
     One of the benefits of suction buckets is that a marine structure can be designed to be self bearing and/or self installing by providing it with one or more suction buckets. So the hoisting device and the plant for installing the foundation, e.g. hammering device, can be eliminated. 
     Since the structure is provided with one or more suction buckets, removal (also called decommissioning) after use is made easier in that by pressing out the suction bucket, the anchoring of the structure to the underwater bottom can be removed. The structure is typically at least substantially made from metal, typically steel. 
     Preferably each suction bucket has one or more of: a diameter of at least 5 metres, typically between 10 and 15 metre or even more; a height of at least 5 metres, typically between 10 and 15 metre or even more, subject to the soil conditions; a wall thickness of at least 1 centimetre, typically at least 3 or 5 centimetre; the longitudinal axis of the suction bucket and the relevant supporting leg (of the upper structure to be supported by the suction bucket) are substantially in line or eccentric. 
     OBJECT OF THE INVENTION 
     Particularly for wind energy turbines there are stringent requirements on many topics. Examples of these topics are: verticality of the tower for the complete service life (typically 20 years) of the structure; vibration frequency; low production costs; environmental friendly; efficient recovery of verticality to repair a failure. 
     For verticality, typically, a deviation of more than 1 degree from the vertical will result in a seizure of the wind turbine operation, which could lead to penalty claims. Such deviation can occur at any time during the lifetime of the structure, e.g. caused by settlement of the soil underneath or near the suction buckets, excessive forces from sea waves or the wind. 
     As an example for the vibration frequency topic, the design must be such that vibrations generated during operation may not lead to structural damage to the offshore structure. Natural frequencies play an important role in this respect. Resonance is preferably avoided. 
     The object of the invention is versatile and can be learned from the information disclosed in the application documents. 
     The present inventor has developed, in a preferred embodiment, a solution to this object embodied by a foundation system for an offshore wind energy installation, having one or more of the following: N, being at least three, suction buckets at the corners of an imaginary, preferably regular, polygon, seen in top view; a round or polygonal connector body, seen in top view, which preferably has at least N radially external corners and supports the payload, e.g. the tower of the wind energy installation, preferably at its centre, and which is, at each of N corners, connected to the top end of a relevant suction bucket by rigid connections, such that all N suction buckets are rigidly connected to the connector body; the connector body is provided completely below sea level and/or has a hollow monocoque structure or load bearing skin, possibly providing radial stays and/or being non rube like; the connector body being of closed profile for its complete extension, preferably without any slits and/or having an impermeable skin; the cross section of the connector radially towards the outside narrows in height (i.e. axial direction of suction bucket) and/or width (i.e. tangential direction); the vertical distance between a suction bucket or its top plate and the connector body is less than 5 or 2 or 1 metre, more preferably less than 25 centimetre. The invention is also defined in the claims. 
     Thus, the upper structure comprising the mast or monopole and/or tower is supported by the connector body and the connector body is supported by the suction buckets. In different words, the upper structure rests upon the connector body and the connector body rests upon the suction buckets. 
     Preferably one or more of the following applies to the connector body: comprises a centrally located tower or monopole receiving element; is designed to be or is filled with ballast material, preferably for at least 50% of its enclosed volume; is made of steel or reinforced mineral cement concrete; is thin walled; transfers all the loads (including vertical and horizontal loads, bending moments and torsion) from the monopole and/or tower to the suction buckets; is compact in height, e.g. to allow fabrication in a shop, which preferably does not exceed 1.5 times the outer diameter of the prismatic part of the monopole and/or tower; a width such that, seen in top view, the foundation system provides an envelope having a maximum span measuring at least 3 times the outer diameter of a suction bucket; overlaps, seen in top view, with the suction buckets and preferably does not radially extend beyond the suction buckets; extends substantially horizontal or at an angle of less than 10 or 20 degrees with the horizontal; substantially box shaped; made from flat sheets; one or more of side face, upper face and lower face are substantially flat and/or make corners where they mutually join and/or are locally provided with stiffeners, preferably inside; has an angular cross section, at least for its arms; is present above the top plate of the suction buckets; is free from the sea bed; keeps a gap with the sea bed of at least 25 or 50 centimetre; its arms have a height, preferably measured at their location of maximum height, at least 0.5 or 0.6 or 0.75 and/or less than 1.5 or 2 or 2.5 times the diameter of the monopole at the level of the connector body; its arms have a width, preferably measured at their location of maximum width, at least 0.25 or 0.5 and/or less than 1.0 or 1.5 or 2 times the height, preferably measured at their location of maximum height, of the arms; is substantially star or triangular or square shaped as seen in top view; has a wall thickness at least 2 or 5 or 10 millimetre and/or less than 200 or 100 or 50 millimetre; has a substantially flat lower side. The arm is the member extending from the central part towards a bucket. 
     The invention is based on the discovery, made by the inventor, that one or more or all the stringent requirements can be fully met by keeping the foundation system as deep as possible below the water level, preferably below 10 or 15 meters above the seabed. Thus the mast or monopole or tower must be as long as possible. 
     The invention is also based on the teaching, obtained by the inventor, that one or more of the following is possible: tilting correction; ease of transport over water to the final offshore destination; deeper penetration of the suction buckets into the sea bottom; locating ballast on top of the suction buckets; minimizing pumping effect caused by cyclic loading of bucket top plate by the payload. 
     For the mast/monopole/tower one or more of the following applies: the lower part connecting to the connector body has an enlarged diameter, e.g. diameter 10 or 12 metre compared to 6.5 metre at the prismatic part; diameter at water level at least 1 or 2 metre smaller than at the level of the connector body; wall thickness at least 20 or 35 millimetre and/or less than 200 millimetre, e.g about 50 millimetre; hollow; thin walled; cylindrical for substantially its complete height; above the level of the upper face of the suction bucket top plate or the under side of the connector body. 
     The prior art shows many proposals for a foundation system for a mast or monopole or tower. Examples are: WO2012103867A1 (Weserwind) discloses a below sea level extending tripod type foundation for off shore wind energy application using three into the sea bed rammed piles; EP2558648B1 (Siemens) discloses another tripod type for off shore wind energy application using three into the sea bed rammed piles; EP1805414B1 (Bard Engineering) discloses a from the sea bed to above sea level extending tripile type for off shore wind energy application using three into the sea bed rammed piles, and also addresses the need for avoiding the natural frequency of the foundation being equal to the rotor frequency to avoid resonance; U.S. Pat. No. 5,567,086A discloses a below sea level extending tension leg type system for off shore oil drilling; EP1074663A1 discloses an into the ground embedded star type foundation system for wind energy application using ground anchoring rods. 
     The in this application cited documents are inserted in here by reference and each provide technical background for a better understanding of this invention. 
     After installation into the sea bed is completed, a gap (also called “void”) can remain between the top of a soil plug inside the suction space and the closed suction bucket top. For wind turbine applications, such gap needs be filled with filler material or a filler body to prevent settlement of the suction bucket and to transfer the loads, e.g. downward or shear, from the wind turbine and structure in to the seabed. It is feasible that this filler material cures or hardens or becomes rigid after it has entered the gap. This filler material provides a body (hereafter also called “slab”) inside the suction space. Obviously, this slab is typically provided after the suction bucket is sunk to the water bottom and penetrated the water soil to its final depth, by pouring or casting the at that time flowable material of the slab into the sea water filled space between the top bulkhead and the top face of the soil plug within the suction bucket. 
     This slab typically has a height of at least 10 or 20 or 30 centimetres and/or less than 50 or 100 or 150 centimetres. 
     It is noted that the invention is preferably directed to suction buckets for foundations, in other words designed to carry the weight of an upper structure, e.g. wind turbine or platform, placed on top, to avoid that such upper structure sinks into the subsea bottom. Thus a foundation suction bucket bears loads from the associated upper structure which tend to force the suction bucket further into the ground. The slab below the top bulkhead is designed to prevent that the suction bucket moves deeper into the subsea bottom due to the pushing loads generated by the weight of the upper structure. A foundation suction bucket is by the nature of its loading different from a suction bucket for anchoring, which anchoring suction bucket must withstand pulling forces from the anchored object which tries to leave its desires location by trying to pull the anchoring suction bucket out of the subsea bottom. 
     Preferably one or more of the following applies: the suction required to penetrate the suction bucket into the subsea bottom during installation and/or the overpressure applied during settlement correction or to extract the suction bucket from the sea bed is generated within the suction bucket above the slab or above the top bulkhead of the suction bucket, preferably since the suction side of a suction pump means or the pressure side of a pressure pump means is connected to the suction bucket at a location above the slab, e.g. the top bulkhead is provided with a nozzle or different sealable port for fluid connection of the suction space with a suction or pressure pump means; the diameter of the suction bucket is constant over its height (the height is the direction from the top bulkhead towards the opposite open end); from the top bulkhead the cylinder walls of the suction bucket extend parallel; the open end of the suction bucket, designed to be located on the sea floor first is completely open, in other words, its aperture is merely bordered by the cylinder walls; the water depth is such that the suction bucket is completely below the water surface when its lower end contacts the sea floor, in other words when its lower end has not penetrated the sea floor yet; the foundation comprises three, four or more mutually spaced suction buckets; the slab completely fills the gap; with the penetration of the suction bucket into the sea floor completed, the top bulkhead is spaced from the sea floor and/or the lower side of the slab bears onto the sea floor which is possibly at elevated level within the suction bucket, compared to the seafloor level external from the suction bucket, due to raising of the seabed plug within the suction space caused by penetration of the suction bucket into the seabed; the by releasable sealing means, e.g. a valve, selectively closable port in the top bulkhead to allow water entering and/or exiting the suction bucket is provided with a coupling means designed for temporary engagement of a suction and/or pressure pump at the time of installing, settlement correction and removing, respectively, of the suction bucket into and from, respectively, the seafloor soil, which port is associated with the fluid flow channel. 
     Preferably, the design of the suction bucket is such that fluid from a source, e.g. pressure pump, flows from the source through a sealed channel, terminating below the bulkhead and within the suction space. During sucking in the pressure is typically at least 0.1 or 0.25 or 0.5 or 1 bars below the local water pressure external from the suction bucket. During pressing out (correction operation or decommissioning) the pressure is typically at least 0.25 or 0.5 or 1 or 2 bars above the local water pressure external from the suction bucket. 
     The suction bucket is also preferably provided with known as such valves and/or hatches adjacent or at its top bulkhead for selectively allowing water and air to enter or exit the suction space through the top side of the suction bucket. 
     Preferably the invention is directed to an offshore foundation system or a suction bucket of said system, the suction bucket preferably provided by an open bottom and closed top, advantageously cylindrical, elongate shell providing a suction compartment or suction space, said closed top having an externally facing upper face and an opposite, toward the suction space facing lower face and preferably provided with one or more valves selectively allowing fluid communication between the suction space and the environment, the suction space being provided with a fixedly located slab and wherein, in use, the slab bottom bears onto atop of a soil plug inside the suction space, the top bulkhead of the suction bucket bears onto the slab. 
     A possible procedure is as follows: the foundation system provided with at least three suction buckets is installed and when the buckets have arrived at their final penetration depth into the sea bed, e.g. of sand or clay, the slab, if applied, is provided by introducing the flowable filler material such that the gap is completely or substantially filled. Subsequently the upper structure to be supported by the foundation system is installed. First, the monopole is located on top of the foundation system, followed by installing the tower on top of the monopole. The tower carries the wind energy turbine nacelle at its top end. The tower is completely or partly above water level. 
     The ballast material applied preferably has a specific weight of at least 1,400 (e.g. sand) or 2,000 (e.g. rock) kg per cubic metre, thus at least 1.4 times or twice the specific gravity of water. In an embodiment the ballast is concentrated near the suction buckets, e.g. located on top of the suction buckets. The ballast can have a thickness of at least 1 or 1.5 or 2 metres. Application of ballast to the connector body and/or the suction buckets is also feasible. 
     The connection between connector body and monopole can be provided by grouting or welding or mechanical fastening means, e.g. riveting or bolting. Use of a quick coupling is preferred, e.g. of so called slip joint type, such as disclosed in EP 2 910 686 (KCI the engineers, disclosed in here by reference) and to which claim  14  is directed. 
     A quick coupling of slip joint type is preferably provided (see also  FIG. 15 ) by wedging walls inclined at a sharp angle relative to the axial direction of the tower and located at the tower and/or connector body at locations where the tower penetrates into the connector body, or vice versa, and oriented such that said wedging walls extend outward from the tower, as viewed in upward direction of the tower in its final vertical attitude as installed, such that the wedging walls provide a conical shaped circumferential or peripheral, e.g. ring like, means, a first one at the tower, a second one at the connector body and configured such that if the tower and connector body are mutually penetrated or inserted, the wedging walls of the first and second one mutually engage and contact, retaining the tower against further lowering by gravity action and also generating radially inward directed clamping forces between these wedging walls, keeping the tower clamped to the connector body. The first one and the second one make a pair and preferably there are two pairs, mutually spaced axially of the tower, at least 0.5 meter. 
     The words “mast”, “monopole” and “tower” have individual meaning, however also identical meaning, e.g. more general, such as: each being an elongated tube or pole like object. Thus, if any of these three words is used in this disclosure, it can also have a meaning identical to any of the two other of these three words and/or the more general meaning. 
    
    
     
       The invention is further illustrated by way of non-limiting, presently preferred embodiments providing the best way of carrying out the invention and shown in the drawings. 
         FIG. 1A-C  show a first embodiment from three different angles;  FIG. 2-4  show a perspective view of a second, third and fourth embodiment, respectively;  FIG. 5A-C  show a perspective view, of exploded type, of three alternative ways of mounting the monopole to the foundation system;  FIG. 6-8  show in side view the three main phases during a possible manner of installing the off shore wind energy installation;  FIG. 9-14  show prior art foundation systems;  FIG. 15  shows a double slip joint in section from the side. 
     
    
    
       FIG. 1  shows three suction buckets, on top of it a star schaped connector body having three arms, each radially outward converging, and there above a single upright tube providing a mast and/or tower and/or monopole. The lower part of the mast has a conical shape. 
       FIG. 2  shows three suction buckets, there above a triangular shaped connector body and there above a prismatic mast. The water level  100  is also illustrated.  FIG. 3  shows four suction buckets, a star shaped connector body having tour arms and above it a prismatic mast.  FIG. 4  shows a scar shaped connector body having three arms and a prismatic mast. 
     In  FIG. 5A  the lower end of the mast penetrates the connector body. In  FIG. 5B  the lower end of the mast penetrates a from the connector body upwards projecting transition piece. In  FIG. 5C  the transition piece penetrates the lower end of the mast. In all three cases the slip joint can be applied. 
     According to  FIG. 6 , the suction buckets and connector body provide a sub assembly separate from the mast or tower or monopole. This subassembly was sailed to its final offshore location and there the suction buckets were penetrated into the sea bed. After that part of the mast was added ( FIG. 7 ) and after that a further part of the mast was added ( FIG. 8 ). 
     Different from  FIG. 6-8 , an alternative manner of installation is to sail the subassembly shown in  FIG. 7  (buckets, connector body and monopole mutually assembled at a remote location) to the final offshore location and install it there, after which the payload is added. 
       FIG. 9  shows a jacket type,  FIG. 10  a tripod type,  FIG. 11  a tripod type,  FIG. 12  a tri pile type,  FIG. 13  a buried type and  FIG. 14  a tension leg type offshore structure. 
     Typically, there are three stages during penetration of the suction bucket into the sea floor by suction within the suction space. In the initial stage the open bottom of the suction bucket has penetrated the seabed by gravity, such that the suction space is sealed. The second stage is obtained by removing water from the suction space by pumping, such that suction is created within the suction space such that the suction bucket penetrates deeper into the seabed, thus its top comes closer to the seabed. In the third stage the suction bucket is penetrated to its final depth, providing its design load bearing capacity for a weight resting on top of it. Typically, the top bulkhead is spaced from the sea floor. Within the suction space internal from the side wall of the bucket, the surface of the sea floor material rises due to penetration of the suction bucket. Such seabed part captive within the suction space is also called soil plug. Typically the void between the bulkhead and the soil plug is tilled by a slab or body. The suction space is bounded by the top bulkhead, the cylindrical side wall and the open end opposite the top bulkhead. 
       FIG. 15  shows an inner tube, e.g. the monopole, and an outer tube, e.g. the wall of the central hole of the connector body to receive the monopole. Each tube is provided with two axially spaced conical rings, providing two pairs of each an inner ring of the inner tube and an outer ring of the outer tube. Due to the downward directed force Fv, oriented according to the gravity force, the radially inward directed clamping forces are generated (only shown for the upper pair). 
     The invention is not limited to the above described and in the drawings illustrated embodiments. E.g. the marine structure can have a different number of suction buckets. The drawing, the specification and claims contain many features in combination. The skilled person will consider these also individually and combine them to further embodiments. Features of different in here disclosed embodiments can in different manners be combined and different aspects of some features are regarded mutually exchangeable. All described or in the drawing disclosed features provide as such or in arbitrary combination the subject matter of the invention, also independent from their arrangement in the claims or their referral.