NL1044403B1

NL1044403B1 - An effective structure and an adjoining installation method for the foundation of an offshore facility

Info

Publication number: NL1044403B1
Application number: NL1044403A
Authority: NL
Inventors: Van Gilst Remco; Jacobus Gerardus Havik Cornelis; Olaf Haarsma Mark; Van Zandwijk Cornelis; Pieter Van Der Horst Jos
Original assignee: Xprthub B V
Priority date: 2022-08-19
Filing date: 2022-08-19
Publication date: 2024-03-04

Abstract

The invention provides a foundation structure 32 for an offshore facility comprising: - a connecting body 33; - at least three arms 34 connected to the connecting body 33; - at least one tubular foundation pile 35 at the end of each arm 34 distant from the connecting body 33; - for each foundation pile 35 a pile-to-arm connector 36, connecting the foundation pile 35 to the arm 34; - a shaft 37 connected to the connecting body 33 and stretching between the top of the connecting body 33 and the mounting level 6 of the offshore facility. The invention further provides an installation method, wherein said foundation structure 32 is placed on the seabed 2, wherein said foundation piles 35 are penetrated to their design penetration, and wherein said foundation piles 35 are connected to said arms 34 by connecting said pile-to-arm connectors 36.

Description

y 1

AN EFFECTIVE STRUCTURE AND AN ADJOINING INSTALLATION METHOD FOR THE FOUNDATION OF AN

OFFSHORE FACILITY

DESCRIPTION FIELD OF THE INVENTION

The presented invention relates to an effective foundation structure for an offshore facility and an adjoining installation method for such a foundation structure.

BACKGROUND

Over the past decades, monopiles have proved to be an economic foundation concept for offshore facilities such as facilities for the production of oil and gas and for the production of wind energy. Monopiles can be fabricated at relatively low cost as almost all welding work can be done automatically. Moreover, monopiles can be stored very effectively, requiring a relatively small storage area as compared to alternative foundation types. Moreover, monopiles can be transported and installed fast and effectively.

In Figure 1, the monopile 1 is a simple tubular structure with a diameter between 4 and 10 meters supporting an offshore facility 7. The monopile 1 is driven into the seabed 2 by means of a heavy impact hammer until it has reached its design penetration 4. The length of the monopile stretches between the design penetration 4 and a few meters above the sea level 3. As during pile driving with heavy impact hammers the monopile is subjected to severe accelerations, it is not possible to pre-install equipment or attachments in or on the monopile. Sensitive equipment such as electrical units, connectors and converters and attachments such as boat landings and ladders are contained inand on a so-called transition piece 5, which stretches between the top of the monopile 1 and the mounting level 6 of the offshore facility at which the offshore facility 7 is mounted. The connection between the transition piece 5 and the monopile 1 can be by means of, for instance, bolted collars or by means of a grouted annulus between the top of the monopile and the bottom of the transition piece or by means of a so-called slip joint which is formed by a conical top of the monopile that sticks into a conical receptacle in the bottom of the transition piece, both conical sections shaped such that the connection between transition piece and monopile fixates under the combination of gravity and friction when the transition piece is stabbed. Other means of connecting the transition piece to the monopile are possible.

A recent development in the monopile technology is the application of transition-piece-less monopiles as shown in Figure 2. In the transition-piece-less concept, the monopile 1 stretches between design penetration 4 and mounting level 6 of the offshore facility, eliminating the complicated connection between the monopile and the transition piece. Transition-piece-less monopiles are driven by hammering directly on top of the monopile. Sensitive equipment and attachments are post-installed after pile driving in the form of a post-installed module 9 at the top of the transition-piece-less monopile.

Monopiles are relatively easy to fabricate and relatively cheap to fabricate, to store, to transport and to install and in a later stage to inspect and to maintain. this makes monopiles a particularly attractive concept for the foundation of wind turbines. Because of their attractiveness, about 80% of the offshore wind turbines in the world are founded on monopiles. Only where soil conditions prohibit the application of monopiles or where logistic restrictions make monopiles economically unattractive, sometimes the wind turbines are founded on truss type structures, as shown in Figure 3.

Such a truss type structure 10 stretches between the seabed 2 and the mounting level 6 of the offshore facility. Common practice is to found the truss type structure on pre-installed foundation piles 11, one per corner. The foundation piles 11 are driven or drilled to their design penetration 12, which generally is deeper than the design penetration 4 of monopiles. The truss type structure is stabbed into the tops of the foundation piles by means of downward stabbing pins 13, whereupon the annuli between the downward stabbing pins 13 and the tops of the foundation piles 11 are filled with grout for ensuring a firm and permanent connection after curing of the grout. The top of the truss type structure 10 is provided with an interface structure 14, allowing to connect the bottom of the wind turbine assembly 8 to the top of the truss type structure 10.

Over the past ten years, the offshore wind industry has seen wind turbine capacities growing from 3 to more than 10 MW with the adjoining size of the monopiies increasing from diameters of 4 meters to diameters of 10 to 12 meters. The scale of economy is expected to push the size of wind turbines even further upward. Plans have been published already showing future wind turbine capacities of 15 up to 20 MW, to be installed within a couple of years.

The market trend to larger wind turbine capacities of 15 to 20 MW is expected to increase the monopile diameters to 12 to 15 meters. Such large diameters are expected to involve increasing problems for the fabrication, storage, transport and installation of such monopiles.

With respect to large diameter monopiles, a first problem is the cross-sectional integrity of the large diameter tubular. In order to limit the weight and costs of the monopile, the wall thickness does not grow with the diameter. As a result, the diameter over wall thickness ratio (D/t ratio) becomes larger, making the monopile’s circular cross-section more vulnerable to deform into an oval (so-called ovalisation). The risk of ovalisation requires specific equipment and support structures for safeguarding the roundness of the monopile during storage, transport, upending and installation. This risk exists already for the traditional monopile diameters of 6 to 10 meters, but is expected to strongly increase for monopile diameters of 12 to 15 meters.

Figures 4 and 5 illustrate the measures to be taken during storage or transport of large diameter monopiles 1 on a yard or on the deck 15 of a jack-up or a floating vessel. Each monopile must be supported by at least two pile support structures 16. The upper side of the support structure 16 in contact with the monopile must be provided with a circularly shaped supporting bed 20 having a radius 17 closely matching the outside radius of the monopile. For monopiles of 12 to 15 meters diameter, the supporting bed 20 is expected to be required over approximately 40% of the bottom side of the monopile circumference. The supporting bed 20 may be a continuous interface bed 18 having a radius 17, as sketched in Figure 6A. Alternatively, the upper side of the pile support structure 16 may be provided with a series of shaped saddles 19, as sketched in Figure 6B, all saddles having the same radius 17 and placed such that together they form a supporting bed 20 with a radius 17 closely matching the outside radius of the monopile.

Figure 7 shows the upending of a large diameter monopile 1 or 8 on the deck 15 of a jack-up or a floating vessel. Also, during upending due attention must be given to safeguarding the roundness and integrity of the monopile against deformation and overstressing. When the monopile is rotated from horizontal to vertical over a hinge 24 by hoisting the top of the monopile by a hoist 21, a specially shaped lifting tool 22 must safeguard the roundness of the monopile top. A specially shaped upending cradle 23 is provided with one or more pile support structures 16 and a plug shoe 25 inside the monopile tip for protecting the lower end of the monopile against deformation and overstressing.

Also, during pile driving, the roundness and integrity of the monopile 1 must be safeguarded.

When during pile driving the pile is supported by a supporting frame 26 mounted on the deck 15 of a jack-up or a floating vessel, as shown in Figure 8, several roller assemblies 27 must be provided along the circumference of the monopile with specially shaped rollers 28, sufficient in number and size, in order to spread the supporting reactions from the frame 26 over a sufficiently wide area of the monopile with the aim to prevent local overstressing of the monopile. When during pile driving the monopile is supported by a so-called supporting template 29 on the seabed 2, as shown in Figure 9, the monopile must be supported at two levels 30 at some vertical distance above each other, in order to be capable of sustaining the overturning moments from the monopile 1 and the pile driving hammer 31 and correcting any out-of-verticality occurring during pile driving. The supporting reactions at levels 30 are transferred from the template 29 to the monopile by means of several roller assemblies 27 on the circumference of the monopile, the roller assemblies 27 being provided with specially shaped rollers 28, sufficient in number and size in order to spread the reaction forces over a sufficiently wide area of the monopile in order to prevent local overstressing the monopile.

All this special equipment needed to safeguard the roundness and integrity of the monopile during fabrication, storage, transport and installation is becoming larger and more expensive as the diameters of the monopiles and the D/t ratios grow, eventually increasing the total costs of the wind turbine.

Another disadvantage of the growing size of monopiles as presently seen in the wind industry is their increasing length and weight. Increasing water depths involve longer monopiles. The combination of longer monopile lengths and larger diameters results in larger pile weights. The increasing lengths and weights of monopiles limit the choice of installation equipment and installation contractors capable of installing such long and heavy monopiles. A limitation of choice involves higher installation costs. In addition, less equipment is available for installing such large monopiles, restricting the number of large monopiles that can be installed in a given period of time.

Other disadvantages of the growing size of monopiles are in the pile driving. Larger and longer monopiles require extremely heavy impact hammers for driving them to their design penetration. In the first place the selection and availability of such heavy hammers is limited, having an upward effect on the installation costs. In the second place, these heavy hammers cause severe stresses and consequently severe fatigue damage in the monopile during pile driving. Wind turbines produce a lot of dynamic loading, imposing severe fatigue loading on the supporting foundation structure. It may happen that extra steel must be provided in the monopile for limiting the fatigue damage during pile driving, in order to have sufficient fatigue life left in the monopile for the phase that the wind turbine is producing wind energy. A third disadvantage of pile driving with such heavy hammers on large monopiles is in the emission of pile driving noise. Pile driving noise is detrimental for marine life, in particular for marine mammals and therefore strict limitations to noise emission during pile driving are imposed by regulating authorities worldwide. The amount of noise emitted is increasing with the output of hammer energy and with the surface of the monopile in contact with the water column between the seabed 2 and the sea level 3. The heavier the hammer and the deeper the water and the larger the diameter of the monopile, the more the noise emitted. Noise emission can be mitigated by several measures, known to a person skilled in the art. All these measures involve certain costs and the more noise is to be mitigated, the higher the costs of the mitigating measures.

Monopiles driven by impact hammers are subjected to high accelerations during pile driving.

These high accelerations prohibit the pre-installation of structural elements such as boat landings, ladders, working platforms, J-tubes, etc. and sensitive equipment such as converters, cable connectors, navigation lights, etc. on the monopile before pile driving. Such structural elements and equipment must be post-installed after pile driving. Often such structural elements and equipment are contained in a separate transitions piece 5, as shown in Figure 1, post-installed in a separate operation on the monopile 1 after pile driving. On transition-piece-less monopiles 8, as shown in Figure 2, the structural and equipment elements are contained in a separate post-installed module 9, placed on the monopile 8 after pile driving. Post-installation of these structural elements and equipment, either in the form of a transition piece 5 or in the form of a post-installed module 9, is costing extra installation time and therefore increasing the installation costs. Once installed, the sensitive equipment must be tested offshore with respect to proper functioning, further increasing the installation costs.

When monopiles are technically or economically impossible, the industry sometimes opts for a truss type structure, also called a ‘jacket’, as a foundation structure for the wind turbine (Figure 3). An advantage of jackets is, that the type of structure is well-known in the industry thanks to more than 70 years of experience in the offshore oil and gas industry where they were widely applied as support 5 structures between the seabed and the oil and gas producing equipment above the sea level. As a result, there is a wide variety of construction yards woridwide capable of building such jackets. The foundation of a jacket is simple, consisting of relatively small piles (much smaller than monopiles), one or a few per corner of the jacket and driven by pile driving hammers of a relatively limited energy output. The installation of a jacket is a well-known and well-controlled process.

Advantages of pile driving of smaller piles with lighter hammers are in limited fatigue damage to the foundation piles and much less noise emission during pile driving as compared to large diameter monopiles.

A disadvantage of truss type structures is their high fabrication costs. The trusses consist of numerous short members, specially shaped at the connecting ends and manually welded together to form the structure.

Another disadvantage of truss type structures is their sensitivity for fatigue damage. Wind turbines produce a high amount of dynamic loading, much more than the traditional oil and gas producing equipment, involving extra steel or complicated details to be required to limit the fatigue damage in the structure. Extra steel and complicated details further increase the fabrication costs of the structure.

Another disadvantage of truss type structures is in the fact that they are more difficult for regulating the eigen period of the total assembly of wind turbine, truss type structure and foundation piles. Wind turbines generate strictly defined frequencies and the eigen period of the total assembly of wind turbine, truss type structure and foundation piles must be sufficiently different from the frequencies produced by the wind turbine, in order to prevent resonance and associated severe fatigue damage in the structure.

Another disadvantage of truss type structures is that they require a larger storage area than monopiles to store them in the period between fabrication and transport followed by installation. A larger storage area involves extra costs and increases the eventual total costs of the wind turbine.

Another disadvantage of truss type structures is the high inspection and maintenance costs during the phase that the wind turbine is producing energy. Truss type structures consist of numerous members welded together in nodes. When such a structure is subjected to severe fatigue loading, cracks may grow at sensitive places. Regular and extensive inspection is required to inspect truss type structures on the occurrence of fatigue damage. Such regular and extensive inspections increase the operational costs of the wind turbine.

SUMMARY OF THE INVENTION it is the object of the invention to provide an effective foundation structure for offshore facilities, in particuiar offshore wind turbines, and an adjoining installation method for such a structure, the foundation structure combining the advantages of a monopile in fabrication with the advantages of truss type structures in the availability of fabrication yards and in transport and installation.

With reference to Figures 10 and 11, the invention provides a foundation structure 32 for an offshore facility comprising: - a connecting body 33; - at least three arms 34 connected to the connecting body 33; - at least one tubular foundation pile 35 at the end of each arm 34 distant from the connecting body 33; - for each pile a pile-to-arm connector 36, connecting the foundation pile 35 to the arm 34; 5 - a shaft 37 connected to the connecting body 33 and stretching between the top of the connecting body 33 and the mounting level 6 of the offshore facility.

The design of a foundation structure for an offshore facility, in particular when the offshore facility is an offshore wind turbine, is dominated by two design conditions: ultimate loading and fatigue.

Inthe ultimate loading condition, the foundation structure must be capable of safely supporting the offshore facility in the situation of an extreme storm. Next to extreme storm loading, offshore facilities are subjected to severe dynamic loading due to wind and waves. When the eigen frequency of the foundation structure is close to the frequencies generated by wind and waves, the entire assembly of foundation structure and the offshore facility may become in a state of resonance, resulting in extremely high dynamic loading on the assembly of foundation structure and offshore facility. Offshore wind turbines generate even more aggressive dynamic loading than the more traditional offshore facilities used for the production of oil and gas. Such dynamic loading may cause severe fatigue damage or reduced strength at places of high stress concentration. For avoiding resonance and severe fatigue damage, it is required that the eigen frequency of the assembly of foundation structure and offshore facility is sufficiently different from the dominating frequencies generated by wind and waves. The fatigue condition requires that the integrity of the foundation structure and offshore facility is safely maintained during the entire lifetime of the foundation structure and the offshore facility. The eigen frequency of the assembly of foundation structure and offshore facility is influenced by the stiffness of the foundation structure.

The invented foundation structure allows to easily adapt the foundation structure 32 to meet the two design conditions by varying the diameter of the pile circle 38 on which the piles 35 are placed and the total capacity of the piles placed at the end of each arm 34. The total capacity of the piles is determined by the number of piles per arm end, the diameter 39 and the penetration 40 of the piles 35.

An almost infinite number of variants comprising pile circle 38, number of piles per arm end, pile diameter 39 and pile penetration 40 can be found, all sufficing the two design conditions of ultimate loading and fatigue. Between these variants, the variant can be found which gives the lowest summed costs of fabrication, storage, transport and installation. The invented structure thus offers a great flexibility for minimizing the total costs of the structure.

Another advantage of the invented structure is in the fact that no pile driving will take place on the shaft 37. As no high accelerations are generated in the shaft during pile driving, it is possible to pre- install structural elements such as boat landings, ladders, working platforms, J-tubes, etc. and sensitive equipment such as converters, cable connectors, navigation lights, etc. on the shaft before installation offshore. This saves a lot of installation work and testing offshore and reduces the installation costs.

Another advantage of the invented structure is that during pile driving substantially less noise is produced as compared to the pile driving of a monopile. In the first place, the required energy output of the pile driving hammer 31 needed to drive the piles 35 to penetration 40 is considerably lower than the energy output required for a monopile. A lower energy output involves lower noise emission from the pile into the water during pile driving. In the second place, the diameter 39 of the piles 35 is considerably smaller than the diameter of a monopile, making the surface of pile wall in contact with the water smaller, thus further reducing the amount of noise emitted from the pile into the water during pile driving. In the third place, in the final stage of pile driving, the tops of the piles 35 are driven under water to closely above the sea bed 2. This again further reduces the surface of pile wall in contact with the water and thus the amount of noise emitted from the pile into the water during pile driving.

Wind and wave loading at sea are often much higher in the dominant direction of wind and waves than in the direction perpendicular to the dominant direction. This is shown in Figure 12. Another advantage of the invented structure is that the strength and the stiffness of the structure as required for the ultimate and fatigue loading can be easily adapted to the dominant direction 41 of the wind and wave loading. If the shape of foundation structure 32 is kept symmetrical as shown in Figure 11, steel can be saved in the arms 34 and piles 35 supporting the structure in the direction perpendicular to the dominant direction 41 of the wind and wave loading. Further steel may be reduced in the foundation structure 32 by increasing the distance 42 between the piles parallel to the dominant direction 41 of the wind and wave loading and decreasing the distance 43 between the piles perpendicular to the dominant direction 41 of the wind and wave loading, as sketched in Figure 12.

Offshore facilities are subjected to severe horizontal loading, resulting in lateral loading on the foundation. Foundation piles are much more effective in supporting lateral loading when they are placed under a small inclination. Another advantage of the invented structure is that the piles 35 can easily be designed and installed under a small inclination. This is shown in Figure 13. For installing inclined piles, the pile-to-arm connectors 36 are constructed as pile sleeves 44, the pile sleeves 44 are given a small inclination and the foundation structure 32 is preinstalled before the piles are stabbed and driven. During pile driving, the piles 35 are supported by the inclined pile sleeves 44. The inclination of the pile is restricted by the requirements that the pile driving hammer 31 shall not interfere with the shaft 37 at the initial stage of pile driving and that the underwater weight of the foundation structure 32 shall be sufficient not to topple over under the overturning moment exerted by the inclined assembly of pile and hammer. After the piles have been driven, the pile heads are connected to the pile sleeves by grouting the annuli between the piles 35 and the sieeves 44 or by swaging, which is expanding the heads of piles 35 radially into recesses machined into the inside walis of the pile sleeves 44.

The shaft 37 fulfils the function of the monopile in the present state-of-the-art as shown in

Figures 1 and 2, namely to transfer the loads from the offshore facility, in Figures 1 and 2 being a wind turbine 7, to the seabed 2. As no pile driving takes place on the shaft 37 (as is the case on monopiles 1), the shaft is not subjected to high accelerations during pile driving. This gives more freedom in the design of the shaft.

In a first method of construction, the shaft is constructed as a monopile. This will limit the diameter to the maximum diameter the monopile fabrication yards can make. In addition, a minimum wall thickness will be required as dictated by the stages of fabrication, storage, transport and installation, the monopile is going through.

An alternative method of constructing the shaft is by means of methods applied in the shipbuilding industry. Figure 14 shows such a type of structure. A thin wall shell plate 45 is stiffened against local buckling by stiffeners 46. The stiffeners can be in the form of a flat plate or a T-profile or an

L-profiles as sketched in Figures 15A, 15B and 15C, respectively or other profiles as appropriate in the design. In the shaft, the direction of the stiffeners 46 can be in axial and / or in circumferential direction.

The welds 56 can be welded automatically, suppressing the costs of fabrication.

Alternatively, the shaft 37 may be built as a polygon as shown in Figure 16, multiple fiat panels 47 together forming a prismatic shape. The flat panels 47 are connected by longitudinal welds 48 over the lines at which the panels intersect. These longitudinal welds can be automatically welded, suppressing the costs of fabrication.

The different fabrication methods available for constructing the shaft 37 have the advantage that the fabrication is not limited to a few fabricators in the world as is the case with monopiles, but that a wealth of construction yards is available all over the world for building the shaft. This ample availability of yards is expected to reduce the fabrication and transportation costs of the wind turbine foundation structures.

Another advantage of the invented structure is in the fact that it can at choice be installed in a single structure 49 as shown in Figure 17, the single structure consisting of the connecting body 33, the arms 34, the pile-to-arm connectors 36 and the shaft 37, or in two pieces as shown in Figures 18A and

18B, the two pieces comprising a base structure 50 and the shaft 37, whereby the base structure 50 consists of the connecting body 33, the arms 34 and the pile-to-arm connectors 36 as shown in Figure 18A. The option to install the invented foundation structure in two pieces makes it possible for installation contractors with smaller vessels and lighter cranes to install foundation structures for large wind turbines of 15 to 20 MW, which is not possible for them when large and heavy monopiles are applied. Increasing the number of potential installation contractors is expected to have a suppressing effect on the installation costs. The availability of more installation contractors will also offer the possibility of installing more heavy wind turbine foundation structures in a certain period of time.

Another advantage of the invented structure is in the fact that at choice the piles can be post- installed, after the structure has been placed on the seabed or pre-installed, before placement of the structure on the seabed.

Figure 19 shows the situation that the piles are post-installed. The foundation structure 32 is placed on the seabed 2. The pile-to-arm connectors 36 are carried out as pile sleeves 44. The piles 35 are stabbed into the pile sleeves 44 and driven with a pile driving hammer 31. A short follower 51 may be used to drive the pile heads to closely above the top of the pile sleeve 44. After the piles have been driven and the foundation structure 32 is level within specification, the pile heads are connected to the pile sleeves 44 by grouting or swaging of the annuli between pile heads and pile sleeves. Levelling of the foundation structure 32 may be done either by placing the structure on a level bed or, before pile driving, by levelling provisions 52 in the bottom of the foundation structure 32, consisting of for instance mud mats and levelling jacks, or after pile driving, by means of levelling tools resting on top of the driven piles, these tools gripping the tops of the sleeves and pulling the sleeves up along the tops of the piles until the structure is level within specification.

Figures 20A and 20B show a procedure of pre-installing the piles. First the piles are driven at the exact locations where they later will be engaged with the foundation structure. The exact pile locations are realized using an installation template 53. The installation template 53 is provided with pile support sleeves 54 spaced at exactly the same spacings as the piles in the foundation structure 32 to be placed over or on the foundation piles at a later stage. After placement on the seabed 2, the template 53 is levelled to a level within specification using a levelling system 55, commonly existing of mud mats resting on the seabed and levelling jacks connected to the template. After levelling, the piles 35 are stabbed into the installation template and driven to penetration using a pile driving hammer 31. A short follower 51 may be used to drive the pile heads into the supporting sleeves or even below them (Figure 20A).

After the piles have been driven to their design penetration, the foundation structure 32 is lowered over or on tops of the pile heads, whereupon the connections are made between the pile-to- arm connectors and the pile heads.

A very effective method of ensuring the levelness of the foundation structure 32 on pre- installed piles is by driving the pile heads to an exact level and subsequently landing the foundation structure on the tops of the piles. Engagement is ensured by downward stabbing pins protruding underneath the foundation structure at the locations of the pile heads and constructed to be stabbed into the pile heads, whereupon the annuli between the stabbing pins and the pile heads are filled with grout and the grout is let to cure.

SHORT DESCRIPTION OF THE DRAWINGS

Figure 1 shows a side view of an offshore facility on a monopile with transition piece

Figure 2 shows a side view of an offshore facility on a transition-piece-less monopile

Figure 3 shows a side view of a wind turbine on a truss-type foundation structure

Figure 4 shows the cross-section of the deck of a floating vessel or a jack-up with three monopiles

Figure 5 shows a side view of a monopile on the deck of a floating vessel or a jack-up

Figure 6A shows the cross-section of a monopile support with a continuous supporting bed

Figure 6B shows the cross-section of a monopile support with shaped saddles

Figure 7 shows a monopile being upended on the deck of a floating vessel or a jack-up

Figure 8 shows a monopile in a support frame to a floating vessel or a jack-up during pile driving

Figure 9 shows a monopile in a supporting template on the seabed during pile driving

Figure 10 shows a side view of the invented foundation structure

Figure 11 shows a top view of the invented foundation structure

Figure 12 shows a top view of the foundation structure adapted to the dominant direction of wind and waves

Figure 13 shows a variant of the foundation structure with inclined piles

Figure 14 shows a section from the shaft shell, constructed in the form of thin plate with stiffeners

Figure 15A shows the cross-section over a stiffener carried out as a flat plate

Figure 15B shows the cross-section over a stiffener carried out as a T-profile

Figure 15C shows the cross-section over a stiffener carried out as an L-profile

Figure 16 shows a cross-section over the shaft constructed as a polygon

Figure 17 shows a side view of the foundation structure installed in a single piece

Figure 18A shows a side view of the base structure installed without the shaft

Figure 18B shows a side view of the shaft installed on to the base structure

Figure 19 shows a side view of the foundation structure with a pile being post-installed

Figure 20A shows a side view of an installation template with piles being pre-installed

Figure 20B shows a side view of the foundation structure being installed over or on pre-installed piles

Figure 21 shows a cross-section of the connecting body constructed as a polygon

Figure 22A shows a side view of one arm

Figure 22B shows a top view one arm

Figure 22C shows the cross-section of an arm constructed as a box girder

Figure 22D shows the cross-section of an arm constructed as a tubular

Figure 22E shows a side view of one arm constructed as a trapezium shaped box girder

Figure 22F shows a top view of one arm constructed as a trapezium shaped box girder

Figure 22G shows a side view of one arm constructed as a conical tubular

Figure 22H shows a top view of one arm constructed as a conical tubular

Figure 23A shows a side view of an arm connected to the connecting body using external stiffener plates

Figure 23B shows a top view of an arm connected to the connecting body using external stiffener plates

Figure 23C shows a top view of arms connected to the connecting body using an internal stiffener ring

Figure 23D shows a cross-section over the connection shaft-to-connecting body using internal stiffeners as centralizer plates

Figure 23E shows a top view of arms connected to the connecting body using load spreading plates

Figure 24A shows a cross-section over the shaft and connecting body connected by bolt collars

Figure 24B shows a cross-section over the shaft and connecting body connected by a weld

Figure 24C shows a cross-section over the shaft and connecting body connected by a grouted annulus

Figure 24D shows a cross-section over the shaft and connecting body connected by another grouted annulus

Figure 24E shows a cross-section over the shaft and connecting body connected by a slip joint

Figure 24D shows a cross-section over the shaft and connecting body connected by another slip joint

Figure 25 shows a side view of the foundation structure with connecting body and shaft in one piece

Figure 26A shows a side view of the foundation structure with downward pins into preinstalled piles

Figure 26B shows a side view of another foundation structure with downward pins into preinstalled piles

Figure 27A shows a cross-section over a pile and a pile sleeve lowered over a preinstalled pile

Figure 27B shows a cross-section over a pile and another pile sleeve lowered over a preinstalled pile

Figure 27C shows a cross-section over a pile and a pile sleeve engaged with a post-installed pile

DETAILED DESCRIPTION OF EMBODIMENTS in the preferred embodiment of the foundation structure, the shaft 37 is constructed as a monopile. This limits the diameter of the shaft to the maximum diameter the monopile fabrication yards can make. In addition, a minimum wall thickness will be required as dictated by the stages of fabrication, storage, transport and installation, the monopile-type shaft is going through.

In another embodiment of the foundation structure, the shaft 37 is constructed as a thin wall shell plate 45, stiffened against local buckling by stiffeners 46, as shown in Figures 14 and 15. The shell can be constructed as a column with a cylindrical shape or with a diameter reducing from a wide diameter at the top of the connecting body 33 to a smaller diameter at the mounting level 6 of the offshore facility. The welds of the shell and the welds 56 between the stiffeners and the shell can be welded automatically, suppressing the costs of fabrication.

In another embodiment of the foundation structure, the shaft 37 is constructed as a polygon as shown in Figure 16, consisting of multiple flat panels 47 together forming a column with a prismatic shape or a shape reducing from a wide enveloping diameter at the top of the connecting body 33 to a smaller enveloping diameter at the mounting level 6 of the offshore facility. The flat panels 47 are connected by longitudinal welds 48 over the lines at which the panels intersect. These longitudinal welds can be welded automatically, suppressing the costs of fabrication. The flat panels 47 may be further strengthened by stiffeners in axial or circumferential direction, similar to the stiffeners shown in

Figure 14. The welds 56 between the stiffeners and the flat panels can be welded automatically, suppressing the costs of fabrication.

In the preferred embodiment of the foundation structure, the connecting body 33 is constructed as a large diameter tubular as sketched in Figure 11 and the shaft is constructed as a tubular of the same diameter as the connecting body whereby the shaft is connected to the top of the connecting body.

In another embodiment of the foundation structure, the connecting body 33 is constructed as a large diameter tubular as sketched in Figure 11 and the shaft is constructed as a tubular of a smaller diameter than the connecting body whereby the shaft is inserted into the tubular of the connecting body and the connection between connecting body and shaft is formed via the annulus between the inner diameter of the connecting body and the outer diameter of the shaft, in the form of a grouted annulus or a slip joint as will be described further on. in another embodiment of the foundation structure, the connecting body 33 is constructed as a polygon of a shape as shown in Figure 16. Such a connecting body couid be applied when the shaft 37 is also constructed as a polygon as shown in Figure 16. The diameter of the connecting body polygon may be the same as the diameter of the polygon of the shaft 37 whereby the shaft polygon is connected to the top of the connecting body polygon, or a larger diameter, whereby the shaft polygon is inserted into the polygon of the connecting body and the connection between connecting body and shaft is formed via the annulus between the inner diameter of the connecting body polygon and the outer diameter of the shaft polygon.

In another embodiment of the foundation structure, the connecting body 33 is constructed as a polygon as sketched in Figure 21. A number of flat plate panels 61, equal to the number of arms, engage the arms 34 and another number of flat plate panels 62, also equal to the number of arms, are welded between the flat plate panels 61. For connecting the shaft 37 to the connecting body shown in Figure 21, an interface element is welded to the top of the connecting body 33. In the case that the shaft is a tubular, the interface element is also a tubular, in the case that the shaft is a polygon as shown in Figure 16, the interface element is also a polygon of the same shape.

Figure 22 shows a number of embodiments of the arms 34 of the foundation structure. In the preferred embodiment of the foundation structure, the cross-section A-A of the arms 34 as shown in

Figures 22A and 22B are constructed as prismatic box girders 63 as shown in Figure 22C. In the case that the local stresses in the wall of the connecting body 33 are too high in the zones where the top and bottom flanges of the prismatic box girder are welded to the outer wall of the connecting body, stiffener plates 67 may be welded between the flanges and the outer wall of the connecting body as shown in

Figures 23A and 23B, in order to better spread the loading from the flanges into the wali of the connecting body. in an alternative embodiment, instead of the external stiffener plates 67, internal stiffener rings 68 may be welded to the inside wall of the connecting body at the elevations of the box girder top and bottom flanges 57, as shown in Figures 23C and 23D. The internal stiffener rings 68 may at the same time act as centralizer plates for centralizing the bottom part of the shaft 37 in the connecting body 33 as shown in Figure 23D, when the smaller outside diameter of the bottom part of the shaft is inserted into the larger inside diameter of the connecting body and the connection between the shaft and the connecting body is made by means of grouting the annulus between both diameters and letting the groutcure.

In another embodiment of the foundation structure, the arms 34 as shown in Figures 22A and 22B are constructed as prismatic tubulars 64 as shown in Figure 22D. In the case that the local stresses in the wall of the connecting body 33 are too high in the zones where the tubular arms are welded to the outer wall of the connecting body, load spreading plates 69 may be welded between the tubular arms and the outer wall of the connecting body as shown in Figure 23E, in order to better spread the loading from the tubulars into the wall of the connecting body

In another embodiment of the foundation structure, the arms 34 are constructed as trapezium shaped box girders 65 as shown in Figures 22E and 22F, the large side of the trapezium pointing to the connecting body 33. As an alternative to this embodiment, the arms 34 may be constructed as an assembly of a prismatic box girder at the side of the pile-to-arm connector 36 connected to a trapezium shaped box girder at the side of the connecting body 33, the large side of the trapezium pointing to the connecting body. in the case that the local stresses in the wall of the connecting body 33 are too high in the zones where the top and bottom flanges of the trapezium shaped box girder are welded to the outer wall of the connecting body, stiffener plates may be welded between the flanges and the outer wall of the connecting body similar to the stiffener plates 67 shown in Figures 23A and 238, in order to better spread the loading from the flanges into the wail of the connecting body. In an alternative embodiment,

instead of external stiffener plates 67, internal stiffener rings may be welded to the inside wall of the connecting body at the elevations of the box girder top and bottom flanges 57, similar to the stiffener rings 68 shown in Figure 23C and 23D. The internal stiffener rings 68 may at the same time act as centralizer plates for centralizing the bottom part of the shaft 37 in the connecting body 33 as shown in

Figure 23D, when the smaller outside diameter of the bottom part of the shaft is inserted into the larger inside diameter of the connecting body and the connection between the shaft and the connecting body is made by means of grouting the annulus between both diameters and letting the grout cure.

In another embodiment of the foundation structure, the arms 34 are constructed as a conical tubular 66 as shown in Figures 22G and 22H, the wide side of the cone pointing to the connecting body 33. As an alternative to this embodiment, the arms 34 may be constructed as an assembly of a prismatic tubular at the side of the pile-to-arm connector 36 connected to a conical tubular at the side of the connecting body 33, the wide side of the cone pointing to the connecting body. In the case that the local stresses in the wall of the connecting body 33 are too high in the zones where the conical tubular arms are welded to the outer wall of the connecting body, load spreading plates may be welded between the tubular arms and the outer wai! of the connecting body similar to the load spreading plates 69 shown in

Figure 23E, in order to better spread the loading from the tubulars into the wall of the connecting body.

In the preferred embodiment of the foundation structure, the connection between the connecting body 33 and the shaft 37 is made by means of boited collars as shown in Figure 24A. The upper end of the connecting body 33 contains a connecting ring 75 and the lower end of the shaft 37 contains a connecting ring 76. Bolt collars 77 are welded to the connecting rings 75 and 76 by means of welds 78. Both bolt collars 77 are provided with holes 79 drilled on identical pitch circles and spaced such that the holes 79 on both bolt collars 77 correspond exactly with each other when the shaft 37 is aligned with the connecting body 33. Subsequently, a bolt 80 is put into each hole 79 through both bolt collars 77 and tightened.

In the preferred embodiment of the foundation structure, the connecting body 33 and the shaft 37 have a tubular shape. In that case, the connecting ring 75 may be the upper end of the connecting body tubular and the connecting ring 76 may be the bottom end of the shaft tubular. The bolt collars 77 have a ring shape and are welded directly to the upper end of the connecting body and the bottom end of the shaft.

In another embodiment of the foundation structure, the connecting body 33 and the shaft 37 have a polygon shape as shown in Figure 16. In that case, the connecting ring 75 may be the upper end of the connecting body polygon wall and the connecting ring 76 may be the bottom end of the shaft polygon. The bolt collars 77 have a polygon shape as well, matching the polygon shape of the connecting body and the shaft and are welded directly to the upper end of the connecting body and the bottom end of the shaft.

In another embodiment of the foundation structure, the connecting body has a polygon shape as shown in Figure 21 and the shaft 37 has a tubular or polygon shape as shown in Figure 16. In that case, the connecting ring 75 is a separate ring welded to the top of the connecting body 33. The connecting ring 76 may be the bottom end of the shaft. The shape of the connecting ring 75 and the corresponding ring 76 at the bottom end of the shaft are determined by the cross-sectional shape of the shaft and can be circular or polygonal.

In another embodiment of the foundation structure, the connection between the connecting body 33 and the shaft 37 is made by means of welding as shown in Figure 24B. The upper end of the connecting body 33 contains a connecting ring 75 and the lower end of the shaft 37 contains a connecting ring 76. The upper end of ring 75 and the lower end of ring 76 are prepared and shaped to be welded. After alignment of the shaft 37 with the connecting body 33, welds 81 are welded between both rings.

When the connecting body 33 and the shaft 37 have a tubular or polygonal shape, the connecting ring 75 may be the upper end of the connecting body and the connecting ring 76 the bottom end of the shaft. When the connecting body has a cross-sectional shape deviating from the cross- sectional shape of the shaft, a separate connecting ring 75 must be welded to the upper end of the connecting body, the shape of the connecting ring 75 matching the shape of the connecting ring 76 at the bottom end of the shaft. in another embodiment of the foundation structure as shown in Figure 24C, the connection between the connecting body 33 and the shaft 37 is made by means of a grouted annulus. The connecting body 33 contains a tubular section 82 at its top end. The shaft 37 contains a tubular section 83 at its bottom end. The inside diameter 84 of the tubular section 82 is so much wider than the outside diameter 85 of the tubular 83 that an annulus 86 is formed between both diameters 84 and 85. After engagement and alignment of the shaft 37 with the connecting body 33, the annulus is filled with grout.

After curing of the grout, a solid connection between shaft 37 and connecting body 33 has been established. For further improvement of the grout connection, corrugations 87 may be provided on the engaging surfaces of tubular sections 82 and 83. When the connecting body 33 and the shaft 37 have a tubular or polygonal shape, the tubular section 82 may be the upper end of the connecting body and the tubular section 83 the bottom end of the shaft. When the connecting body has a cross-sectional shape deviating from the cross-sectional shape of the shaft, a separate tubular section 82 must be welded to the upper end of the connecting body, the shape of the tubular section 82 matching the shape of the tubular section 83 at the bottom end of the shaft.

In another embodiment of the foundation structure as shown in Figure 24D, a grouted connection is realized between the connecting body 33 and the shaft 37 by making the outside diameter of tubular 82 of the connecting body 33 smaller than the inside diameter of the tubular 83 of the shaft 37. During engagement and alignment of the shaft 37 and connecting body 33, the tubular 83 of the shaft is stabbed over the upward pointing tubular 82 of the connecting body. A seal 88 must be provided in the bottom of the annulus in order to contain the grout in the annulus during filling of the annulus and curing of the grout.

In another embodiment of the foundation structure as shown in Figure 24E, the connection between the connecting body 33 and the shaft 37 is made by means of a so-called slip joint. The connecting body 33 contains a tubular section 82 at its top end. The shaft 37 contains a tubular section 83 at its bottom end. Over the engaging length 89, the inside diameter of tubular 82 is machined to a conical surface 91 forming a receiving bucket. Over the engaging length 90, the outside diameter of tubular 83 is machined to a conical surface 92 forming a stabbing pin. The conical surfaces 91 and 92 are shaped in such a way that under the action of gravity a tight and firm connection is formed between the shaft 37 and the connecting body 33 when both structures are engaged and aligned. in another embodiment of the foundation structure as shown in Figure 24F, the slip joint shown in Figure 24E is applied upside-down. Over the engaging length 89, the outside diameter of tubular 82 is machined to a conical surface 91 forming a stabbing pin. Over the engaging length 90, the inside diameter of tubular 83 is machined to a conical surface 92 forming a receiving bucket. The conical surfaces 91 and 92 are shaped in such a way that under the action of gravity a tight and firm connection is formed between the shaft 37 and the connecting body 33 when both structures are engaged and aligned.

In another embodiment of the foundation structure, as shown in Figure 25, the shaft 37 and the connecting body 33 are fabricated in one piece, as a monopile or as a stiffened tubular as sketched in

Figure 14 or as a polygon as sketched in Figure 16. At a later stage, the arms 34 are welded to the bottom end of this single piece assembly. In such a single piece assembly no specific division line can be appointed separating the connecting body and the shaft. in that case, the zone in the bottom of the single piece assembly where the arms are connected to the assembly is defined as the connecting body 33 and the section there above as the shaft 37.

In the preferred embodiment of the foundation structure, the piles are pre-installed, using an installation template 53 as shown in Figure 20A. The heads of the piles 35 are driven to a level within specification, in order to end up with the foundation structure at a level within specification after it has been landed on the tops of the piles. In that embodiment, the pile-to-arm connectors 36 are carried out as shown in Figure 26A or 26B. Downward pins 101 are welded to the ends of the arms 34 distant from the connecting body 33. The foundation structure 32 is lowered to the seabed and the downward pins 101 are inserted into the heads of the pre-installed piles 35. A smali cone tip 102 is provided at the lower end of each pin for being easier inserted into the top of the pile. Before lowering the downward pins into the pile heads, the soil plugs 103 must be removed to a sufficient level to allow the downward pins to be fully inserted into the pile heads. After engagement of the downward pins with the pile heads, the annuli between the downward pins and the pile heads are filled with grout. After curing of the grout, the foundation structure 32 is firmly connected to the piles 35.

In the embodiment as shown in Figure 26A, the pins protrude over several meters below the structure. During transportation of the foundation structure on the deck 15 of a floating vessel or a jack- up, the downward pins 101 must stay above the deck 15, thus increasing the lift height of the hoist 21 by the length of the pins 101. This may reduce the number of installation vessels or jack-ups capable of lifting and installing the foundation structure 32. Figure 26B shows an embodiment which provides a solution for reducing the lift height of the hoist 21. The arms 34 are shaped upward such that the downward pins 101 no longer protrude below the base of the connecting body 33. For the embodiment of Figure 26B, the pile heads are driven to an elevation far enough above the seabed 2 such that the bottom of the connecting body does not interfere with the seabed during engagement of the downward pins with the pile heads. If needed, the soil plugs 103 are removed over some depth, to allow the downward pins to be fully inserted into the pile heads.

Another embodiment of the foundation structure is shown in Figure 27A. The piles are pre- installed and the pile-to-arm connectors 36 are carried out as pile sleeves 44. The pile tops are driven to a level to protrude over some distance above the tops of the sleeves. A small downward cone 104 is provided at the bottom of the sleeve in order to easier stab the foundation structure 32 over the piles 35. The foundation structure is landed on the seabed and levelled using a levelling system 105. Grout seals 106 are provided in the bottoms of the sleeves in order to enable the annuli between the piles and the sleeves to be filled with grout. After curing of the grout, the foundation structure 32 is firmly connected to the piles 35.

Figure 27B shows a variant to the embodiment of Figure 27A. The pile heads are driven to a level within specification. The sleeves 44 are provided with a bearing plate 107 in the top of the sieeve.

The foundation structure 32 is lowered over the piles until the bearing plates 107 are landed on the pile tops. By driving the pile tops to a level within specification, the levelness of the foundation structure is ensured and the levelling system 105 shown in Figure 27A can be eliminated.

Figure 27C shows an embodiment of the foundation structure suitable for post-installing the foundation piles. The pile-to-arm connectors 36 again are constructed as pile sleeves 44 welded to the ends of the arms 34. First the foundation structure 32 is landed on the seabed, resting on a levelling system 105. After the foundation structure has been levelled, the piles 35 are stabbed into the pile sleeves 44. For easier stabbing of the piles into the sleeves, the upper ends of the sleeves are provided with upward cones 108.

During pile driving, the piles 35 are supported by the sleeves 44. After the piles have been driven, the annuli between the piles and the sleeves are filled with grout. Grout seals 106 prevent the grout from flowing out of the annuli. After curing of the grout, the foundation structure 32 is firmly connected to the piles 35.

i8

Claims

CONCLUSIONS

1. A foundation structure for an offshore facility, consisting of - a connecting body 33; - at least three arms 34, connected to the connecting body 33; - at least one tubular foundation pile 35 at the end of each arm 34 on the side remote from the connecting body 33; - for each foundation pile 35, a pile-arm coupling 36 that connects the foundation pile 35 to the arm 34; - a column 37 connected to the connecting body 33 and extending between the top of the connecting body 33 and the level 6 on which the offshore facility is mounted.

The foundation structure of claim 1, wherein said column 37 is constructed as a monopile.

The foundation structure of claim 1, wherein said column 37 is constructed as a stiffened tubular shell.

A4. The foundation structure of claim 3, wherein the stiffened tubular shell is formed in the shape of a cylinder of constant diameter.

The foundation structure of claim 3, wherein the stiffened tubular shell has a tapered shape, with the largest diameter at the top of said connecting body 33 and the smallest diameter at the level 6 on which the offshore facility is mounted.

The foundation structure of claim 1, wherein said column 37 is constructed in the shape of a polygonal cross-section.

The foundation structure of claim 6, wherein the polygonal column is prismatic.

The foundation structure of claim 6, wherein the polygonal column has a tapered shape, with the largest envelope diameter at the top of said connecting body 33 and the smallest envelope diameter at the level 6 on which the offshore facility is mounted.

The foundation structure of claim 1, wherein said connecting body 33 is constructed as a large diameter tube.

The foundation structure of claim 1, wherein said connecting body 33 is constructed in the shape of a polygonal tube.

The foundation structure of claim 1, wherein said arms 34 are constructed as prismatic box girders.

The foundation structure of claim 1, wherein said arms 34 are constructed as trapezoidal box girders, with the wide base of the trapezium pointing towards said column 37.

The foundation structure of claim 1, wherein said arms 34 are constructed as tubes of constant cross-section.

The foundation structure of claim 1, wherein said arms 34 are constructed as conical tubes, with the wide base of the cone pointing toward said column 37.

The foundation structure of claim 1, wherein said pile-arm couplings 36 are constructed as down pins 101 with an outer diameter smaller than the inner diameter of said foundation piles 35.

The foundation structure of claim 1, wherein said pile-arm couplings 36 are constructed as a casing pipe 44 with an inner diameter larger than the outer diameter of the foundation piles 35.

The foundation structure of claim 16, wherein the axis of the casing pipe 44 has a vertical orientation that does not deviate more than one degree from the vertical.

The foundation structure of claim 17, wherein the top of said casing pipe 44 is provided with a supporting plate 107.

The foundation structure of claim 16, wherein the axis of the casing pipe 44 has an oblique orientation that deviates more than one degree from the vertical.

The foundation structure of claim 2, wherein said connecting body 33 has the same diameter as said column 37 and wherein the connection between said connecting body 33 and column 37 is made by means of a bolt ring 77.

The foundation structure of claim 2, wherein said connecting body 33 has the same diameter as said column 37 and wherein the connection between said connecting body 33 and column 37 is made by means of a weld 81.

The foundation structure of claim 2, wherein said connecting body 33 has a diameter different from the diameter of said column 37 and wherein the connection between said connecting body 33 and column 37 is made by the smaller diameter construction element in the construction element with the larger diameter, fill the annular gap between the two diameters with a cement-water mixture and allow this mixture to harden.

The foundation structure of claim 2, wherein said connecting body 33 has a diameter different from the diameter of said column 37 and wherein the connection between said connecting body 33 and column 37 is made by means of a so-called slip-joint.

The foundation structure of claim 2, wherein said connecting body 33 and said column 37 are constructed as a one-piece monopile.

The foundation structure of claim 3, wherein said connecting body 33 has the same diameter as said column 37 and wherein the connection between said connecting body 33 and column 37 is made by means of a bolt ring 77.

The foundation structure of claim 3, wherein said connecting body 33 has the same diameter as said column 37 and wherein the connection between said connecting body 33 and column 37 is made by means of a weld 81.

The foundation structure of claim 3, wherein said connecting body 33 has a diameter different from the diameter of said column 37 and wherein the connection between said connecting body 33 and column 37 is made by the smaller diameter construction element in the construction element with the larger diameter, fill the annular gap between the two diameters with a cement-water mixture and allow this mixture to harden.

The foundation structure of claim 3, wherein said connecting body 33 has a diameter different from the diameter of said column 37 and wherein the connection between said connecting body 33 and column 37 is made by means of a so-called slip-joint.

The foundation structure of claim 3, wherein said connecting body 33 and said column 37 are constructed as a one-piece stiffened tubular shell.

The foundation structure of claim 6, wherein said connecting body 33 and said column 37 are constructed as polygonal tubes of the same envelope diameter and wherein the connection between said connecting body 33 and column 37 is made by means of a bolt ring 77.

The foundation structure of claim 6, wherein said connecting body 33 and said column 37 are constructed as polygonal tubes of the same envelope diameter and wherein the connection between said connecting body 33 and column 37 is made by means of a weld 81.

The foundation structure of claim 6, wherein said connecting body 33 and said column 37 are constructed as polygonal tubes of different envelope diameter and wherein the connection between said connecting body 33 and column 37 is made by the structural element with the smaller envelope diameter into the construction element with the larger surrounding diameter, fill the annular gap between the two diameters with a cement-water mixture and allow this mixture to harden.

The foundation structure of claim 6, wherein said connecting body 33 and said column 37 are constructed as polygonal tubes of different envelope diameter and wherein the connection between said connecting body 33 and column 37 is made by means of a so-called slip-joint.

The foundation structure of claim 6, wherein said connecting body 33 and said column 37 are constructed as a one-piece polygonal tube.

35. The foundation structure of claim 9, wherein said arms 34 are constructed as box girders and wherein the connection between said arms 34 and said connecting body 33 is reinforced with stiffening plates 67 between the top and bottom flanges of the box girders and the outer wall of the connecting body 33.

36. The foundation structure of claim 9, wherein said arms 34 are constructed as box girders and wherein the connections between said arms 34 and said connecting body 33 are reinforced with stiffening rings 68 against the inner wall of connecting body 33 opposite the levels at which the upper and lower flanges are located. of the box girders are connected to the connecting body 33.

The foundation structure of claims 22 and 36, wherein said stiffening rings 68 are designed to centralize the smaller diameter of the column 37 in the larger diameter of the connecting body 33, so as to form a regular gap for the cement-water mixture and to to support the column 37 until the cement-water mixture has hardened.

The foundation structure of claims 27 and 36, wherein said stiffening rings 68 are designed to centralize the smaller diameter of the column 37 in the larger diameter of the connecting body 33, so as to form a regular gap for the cement-water mixture and to to support the column 37 until the cement-water mixture has hardened.

The foundation structure of claim 9, wherein the connection between said arms 34 and said connecting body 33 are reinforced with a reinforcing plate 69 between said arm 34 and the outer wall of the connecting body 33.

40. The foundation structure of claim 10, wherein said arms 34 are constructed as box girders and wherein the connection between said arms 34 and said connecting body 33 is reinforced with stiffening plates 67 between the top and bottom flanges of the box girders and the outer wall of the connecting body 33.

The foundation structure of claim 10, wherein said arms 34 are constructed as box girders and wherein the connection between said arms 34 and said connecting body 33 are reinforced with stiffening rings 68 against the inner wall of connecting body 33 opposite the levels at which the top and bottom flanges of the box girders are connected to the connecting body 33.

The foundation structure of claims 32 and 41, wherein said stiffening rings 68 are designed to centralize the smaller envelope diameter of the column 37 in the larger envelope diameter of the connecting body 33, so as to form a regular gap for the cement-water mixture and to support the column 37 until the cement-water mixture has hardened.

The foundation structure of claim 10, wherein the connection between said arms 34 and said connecting body 33 are reinforced with a reinforcing plate 69 between said arm 34 and the outer wall of the connecting body 33.

The foundation structure of claim 15, wherein the connection between said arm 34 and said down pin 101 is made by welding said tubular pin 101 to the bottom of said arm 34.

45. A method of installing a foundation structure for an offshore facility, consisting of - a connecting body 33; - at least three arms 34, connected to the connecting body 33; - at least one tubular foundation pile 35 at the end of each arm 34 on the side remote from the connecting body 33; - for each foundation pile 35, a pile-arm coupling 36 that connects the foundation pile 35 to the arm 34; - a column 37 connected to the connecting body 33 and extending between the top of the connecting body 33 and the level 6 on which the offshore facility is mounted, in which said foundation structure 32 is placed on the seabed, and in which said foundation piles 35 are placed according to their design penetration are penetrated, and wherein said foundation piles 35 are connected to said arms 34 by connecting said spawning arm couplings 36.

The installation method of claim 45, wherein a single structure consisting of said connecting body 33, said column 37, said arms 34 and said pole-arm couplings 36 is installed in one piece.

The installation method of claim 45, wherein said connecting body 33, said arms 34 and said pole-arm couplings 36 are installed in one piece as a first structure and said column 37 as a second structure, wherein the connection between said column and the first structure at sea is built.

The installation method of claim 47, wherein the connection between said column 37 and said connecting body 33 is made by filling a gap between said column 37 and said connecting body 33 with a cement-water mixture and allowing this mixture to harden.

The installation method of claim 47, wherein the connection between said column 37 and said connecting body 33 is made by means of a so-called slip joint.

The installation method of claims 46 or 47, wherein said foundation piles 35 are penetrated to their design penetration before said foundation structure 32 is placed on the seabed.

The installation method of claim 50, wherein after penetration the pile heads of said foundation piles 35 form an exact horizontal plane for said foundation structure 32 when said foundation structure 32 is placed on the pile heads, wherein said pile-arm couplings 36 consist of downward pins 101 designed to to fit into said tubular foundation piles 35 and in which said foundation piles 35 are connected to said arms 34 by filling the gap between the outside of the downward pins 101 and the inside of said foundation piles 35 with a cement-water mixture and this mixture! to harden.

The installation method of claim 50, wherein after penetration the pile heads of said foundation piles 35 form an exactly horizontal plane for said foundation structure 32 when said foundation structure 32 is placed on the pile heads, wherein said pile-arm couplings 36 consist of tubular casing pipes 44 with a bearing plate 107 in the top of the casing pipe, the casing pipes being designed to fit around said tubular foundation piles 35 and the bearing plates 107 to rest on the pile heads, wherein said foundation piles 35 are connected to said arms 34 through the gap between the inside of the casing pipes 44 and filling the outside of said foundation piles 35 with a cement-water mixture and allowing this mixture to harden.

The installation method of claims 46 or 47, wherein said foundation piles 35 are penetrated to their design penetration after said foundation structure 32 has been placed on the seabed.

54. The installation method of claim 53, wherein said pile-arm couplings 36 consist of tubular casing pipes 44 designed to fit around said tubular foundation piles 35 and wherein the axes of said casing pipes and foundation piles have an angle of inclination that deviates more than one degree from the vertical and in which an inclined pole is installed through the casing pipe.