Subsea foundation structure and method of installation

Abstract

The invention relates to a subsea foundation structure (1) comprising: - at least two suction anchors (21,22,23,24) defining a foundation structure area (FSA) therebetween; - a beam sub-structure (30) interconnecting the suction anchors (21,22,23,24); - a central support (10';10'') arranged within the foundation structure area (FSA) and connected to said beam sub-structure (30), wherein each of the at least two suction anchors (21,22,23,24) supports a skirt roof (31,32,33,34), and the beam sub-structure connects to the at least two suction anchors (21,22,23,24) and the central support (10';10''), such that the skirt roofs (31,32,33,34) are in a first plane (p1) and the central support (10';10'') is in a second plane (p2), wherein the second plane (p2) is at a lower elevation than the first plane (p1). It is further described methods of installing a subsea foundation structure (1).

Description

[0001] SUBSEA FOUNDATION STRUCTURE AND ASSOCIATED METHODS OF INSTALLATION

[0002] Technical Field

[0003] The present disclosure relates to the field of subsea structures, and in particular, the invention is directed to a subsea foundation structure and an associated method of installing the subsea foundation structure.

[0004] It is further described a method of installing a subsea foundation structure with a central support that is movable relative to a bean sub-structure of the central support such that the suction anchors of the subsea foundation structure can be installed prior to installation of the central support.

[0005] Background

[0006] Subsea foundations are used in supporting well templates, manifolds and other subsea equipment on a seabed. The subsea foundation provides a levelled support for the equipment. In some cases it is not required to level but in most cases a maximum inclination is specified. In some cases, even if inclination requirement, there is still no means to level the foundation structure. Especially if hard soil with very little seabed inclination, no leveling equipment is available. Also single can structures (CCFs) often have no leveling equipment. For well templates it is normally an inclination requirement. It is normal to use several suction cans (normally 4) as an integrated leveling system that can rotate the structure by pumping water in / out of typically 2 suction cans while the 2 other are closed off. Dependent on the properties of the soil and seabed at the site of installation, the operator chooses type of foundation. If the soil is soft, a suction anchor with a skirt having a typical length of 4-6 meters, but may be shorter or longer, and supporting a skirt roof on top is normally used, whereas if the soil is hard, a mudmat may be used. As an alternative to suction anchors, piled foundations can also be used if the soil is soft. The mudmat is normally, or at least often, positioned directly on the seabed. The skirt roof normally has a closable opening (e.g. a hatch), and the closable opening is open during the first phase of installation to evacuate air when suction anchor is lifted into the water, then evacuating water during self -penetration (penetration from structure self-weight) of foundation structure. The closable opening is then closed to make a water-tight compartment inside the suction anchor. The skirt roof may have means connectable to a suction pump for removing water from inside the suction anchor during penetration into the soil.

[0007] Dependent on the size and weight of the expected loads that shall be supported during the lifetime of the foundation, the overall size and weight of the foundation is often significant. This is particular an issue with the subsea foundations which have suction anchors.

[0008] With the increased demands for safety and robustness of equipment used offshore, these demands normally adds weight and pushes the operation limits for operation by today’s standard installation vessels.

[0009] When installing the subsea foundations from installation vessels, the subsea foundation is lifted off the vessel and into the water using a lifting arm on a crane. The length of the lifting arm affects the lifting capacity of the crane. Therefore, in order to be able to lift as heavy as possible, it is thus an advantage that the object to be lifted is relatively narrow in at least one direction (i.e. in the X direction and / or in the Y direction) such that the lifting capacity is kept as high as possible.

[0010] Although some of the components of the foundation, such as protective covers, guideposts etc., can be removed during the installation phase (and rather be mounted once the subsea installation has been positioned subsea), the lifting capacity of the crane may still be exceeded and it may be required to use larger installation vessels which are more costly in day rate as well as they have less availability.

[0011] It is an objective to provide a subsea foundation structure which is of less weight.

[0012] Summary of the invention

[0013] The invention for which protection is sought is defined in the attached claims.

[0014] It is described a subsea foundation structure comprising:

[0015] - at least two suction anchors defining a foundation structure area therebetween;

[0016] - a beam sub-structure interconnecting the suction anchors;

[0017] - a central support arranged within the foundation structure area and connected to said beam sub-structure, wherein each of the at least two suction anchors supports a skirt roof, and the beam sub-structure connects to the at least two suction anchors and the central support, such that the skirt roofs are in a first plane and the central support is in a second plane, wherein the second plane is at a lower elevation than the first plane.

[0018] In other words, the central support is arranged within a footprint formed by the at least two suction anchors.

[0019] In other words, the central support is arranged inside an area formed by the at least two suction anchors. There may be one central support within the foundation structure area.

[0020] Alternatively, there may be more than one central support within the foundation structure area. The number of central supports will depend on the requirements for each specific project. For example, there may be two, three, four, seven, eight central supports within the foundation structure area.

[0021] The different central supports may have the same size or be of different size.

[0022] The central supports may be arranged next to each other. Alternatively, one central support may be arranged within another central support.

[0023] In any case, if it is one or more central supports, all of the central supports are arranged within the foundation structure area.

[0024] According to a first aspect, the present disclosure provides a subsea foundation structure comprising:

[0025] - at least three suction anchors defining a foundation structure area therebetween;

[0026] - a beam sub-structure interconnecting the suction anchors;

[0027] - a central support arranged within the foundation structure area and connected to said beam sub-structure.

[0028] The suction anchors are also denoted suction can and suction caisson anchor within the art. In addition, suction anchors can also be denoted suction piles within the art.

[0029] In other words, the central support is arranged within a footprint formed by the at least three suction anchors.

[0030] The suction anchors may be arranged at corner portions or along an outer periphery of the subsea foundation structure.

[0031] In other words, the central support is arranged inside an area formed by the at least three suction anchors. I.e., the central support(s) can be arranged or positioned anywhere inside the foundation structure area, either with the same distance or with different distances from the respective suction anchors.

[0032] The central support(s) is configured to provide a support for the subsea foundation structure against the underlying seabed.

[0033] The beam sub-structure comprises a number of beams which together form the beam sub-structure connecting the suction anchors and the central support. I.e., when the suction anchors and the central support are connected to the beam substructure thereby forming the subsea foundation structure, the subsea foundation structure forms one piece.

[0034] The subsea foundation structure may have physical protection means such as covers to protect against fishing nets, trawls and / or protected from falling objects etc. Each of the at least two or three suction anchors may support a skirt roof, and the beam sub-structure may connect to the at least two or three suction anchors and the central support, such that the skirt roofs are in a first plane, and the central support is in a second plane. The second plane may be at a lower elevation than the first plane. This ensures that the central support takes up forces in that it is forced to be in contact with an underlying seabed even in the case of seabed inclination and / or seabed unevenness. Another advantage of having the central support to be in contact with the underlying seabed is that, similar as for the suction anchors, the operators may in some cases not allow to account for bearing on skirt roof for permanent loads (in reality or normally weights so vertical loads) as any small leakage over time could make the structure tilt. This may occur since structures are often installed parallel or close to a horizontal plane while seabed inclination could cause 1 or more skirt roofs not to be in contact with the seabed. This would be fine if completely water tight as water can be regarded as incompressible but as large loads generate pressure inside the suction anchor, water will or could slowly leak during the life time of the structure and, as water escapes, the structure may tilt if the skirts do not have sufficient friction capacity. Hence, many operators have historically required that all permanent loads to be taken on skirt friction alone. By using a central support which is ensured to be in contact with the underlying seabed provides for bearing on mudmat or support also for permanent loads.

[0035] Furthermore, it should be understood that the suction anchors and the central support may have other cross-sections than circular, such as polygonal. The polygonal shapes may be triangular, square, pentagon, sextant, hexagon, octagon, dodecagon.

[0036] The central support may be a central suction anchor. If the central support is a central suction anchor, a skirt roof of the central suction anchor defines the second plane.

[0037] The central support may comprise means for connection to a suction pump for suction of water out from a volume inside the central suction anchor.

[0038] The means for connection can be a pipe connection with a closable valve, such that once the central suction anchor is installed, the valve can be closed preventing fluid flow between the volume inside the central suction anchor and the surrounding seawater.

[0039] A diameter of the central support may be larger than a diameter of the suction anchors.

[0040] The central support may be a central mudmat. If the central support is a central mudmat, the central mudmat defines the second plane. In other words, the second plane is an underside of the central mudmat, i.e. the part of the central mudmat which is configured to rest on, or be in contact with, the seabed.

[0041] The central mudmat normally rests directly on the seabed. The central mudmat may have a downwardly extending protrusion penetrating a sufficient depth of the soil of the seabed in order to secure ensure that the central mudmat has vertical bearing capacity. Furthermore, the downwardly extending protrusion may support the central mudmat horizontally, thereby preventing movement of the central mudmat on the seabed. I.e. the mudmat with downwardly extending protrusion may have a form of a skirted can with skirt roof that does not require suction (i.e. suction anchor without suction”). A skirt will also reduce the risk of wash-out under the mudmat for a well template which would (obviously) remove the mudmat bearing. Also, the skirts will reduce foundation settlement. Both these effects are especially important for well templates.

[0042] As an alternative to central suction anchor and central mudmat, the central support may be a hollow cylinder with open ends, i.e. a ring-shape. The hollow cylinder may have an axial extension such that it allows for partial penetration into the soil of the seabed, either by self-penetration or by forced penetration via the suction anchors.

[0043] The central support may be of a different size than the at least two or three suction anchors.

[0044] The at least two or three suction anchors are preferably of the same size. This may simplify manufacturing. However, it is also possible to have different size on one or more of the suction anchors.

[0045] The suction anchors may comprise a skirt of a first length and the central support may comprise a skirt of a second length, and the first length may be longer than the second length. This has the advantage that the foundation structure can be levelled by pumping water in or out of normally 2 neighboring suction anchors (though maybe cases where pump on 1 or 3 suction anchors as well and even diagonally on 2, but base case is 2 neighboring suction anchors) before the central support enters the underlying soil.

[0046] Alternatively, the second length may be longer than the first length.

[0047] The length of the skirt on the central support may be less than the expected self- penetration depth such that the central support is levelled before the central skirt enters the seabed.

[0048] It may be beneficial having the bottom of the skirt of the central suction anchor shorter than the other suction anchors for transportation purposes, that is, to ensure the foundation structure is resting on the suction anchors (i.e. the “corner” suction anchors), and not the central suction anchor. However, there could be situations where the central suction anchor is longer (and possibly of a larger diameter) than the suction anchors where it is more advantageous to transport the foundation structure on the central suction anchor.

[0049] The central support may possibly be forced into the soil when the suction anchors are penetrated, thereby avoiding the need of assisted suction by means of e.g. a suction pump during penetration of the central support into the underlying soil.

[0050] Alternatively, the central support may also require assisted suction during penetration into the underlying soil.

[0051] The central support may be equidistantly arranged in relation to the suction anchors. However, in principle there could be reasons having the central support off center if loads are not symmetric (like heavy module on one side or wells on one side with high loads).

[0052] The central support may be arranged such that a center of gravity of the central support may be coinciding with a center of gravity of the subsea foundation structure.

[0053] The subsea foundation structure may be a well template. The well template may comprise one or more well slots.

[0054] If the subsea foundation structure is a well template, it is a known problem that drilling through a well slot may result in wash-out of the soil around the suction anchor in proximity of the well slot. By having a central support, the wash-out of the suction anchor may be less critical since the central support is also supporting the foundation structure.

[0055] The subsea foundation structure may comprise one, two, three or four well slots.

[0056] The subsea foundation structure may comprise between 5 and 12 well slots.

[0057] The subsea foundation structure may comprise a number of tail pipes equal to the number of well slots, wherein one tail pipe may extend downwardly from each of the well slots. Tail pipes are also often called wash-out sleeves. These may be fixed / locked to the well template or be post installed. Normally they are permanently installed but they may also be temporarily used only during drilling.

[0058] The tail pipes prevents, or at least minimize the risk of, wash-out of the soil around the well being drilled, and thus the soil around any suction anchors and central support (i.e. central suction anchor) in the proximity of the well being drilled.

[0059] Alternatively, the subsea foundation structure may support larger modules like manifold modules or process equipment like subsea pumps, subsea separators, power supply, storage units or other modules. The modules or equipment may be separate structures from the subsea foundation structure or the modules or equipment may be an integrated part of the subsea foundation structure. Each of the suction anchors supports a skirt roof, and, when the subsea foundation structure is levelled in a horizontal plane, the central support may be at a lower elevation relative the skirt roofs supported by the suction anchors.

[0060] As such, the central support may rest against the soil and take up forces. If the central support is a central suction anchor supporting a skirt roof, the skirt roof of the central suction anchor may be at a lower elevation relative the skirt roofs supported by the suction anchors.

[0061] Similarly, if the central support is a central mudmat, the central mudmat may be at a lower elevation relative the skirt roofs supported by the suction anchors.

[0062] The central support may be movable relative to the beam sub-structure.

[0063] The subsea foundation structure may comprise a locking arrangement, and the locking arrangement may be configured to:

[0064] - in a first state, prevent movement of the central support relative to the beam substructure, and

[0065] - in a second state, allow movement of the central support relative to the beam sub - structure.

[0066] These features of the locking arrangement enable that the suction anchors can be fully installed prior to installation of the central support. In addition, operating the locking arrangement to the first state after installing the central support, ensures that the central support, in addition to the suction anchors, supports the subsea foundation structure (and any loads of elements or structures positioned on the subsea foundation structure).

[0067] The beam sub-structure may be configured to receive the central support. For example, the locking arrangement may have means for receiving the central support in case the central support is installed after the subsea foundation structure.

[0068] According to a second aspect, the present disclosure provides a method of installing a subsea foundation structure as defined above, comprising the steps of:

[0069] I) landing the subsea foundation structure on a seabed;

[0070] II) penetrating the suction anchors into the seabed to a first depth.

[0071] The first depth into the seabed may be an estimated depth for self -penetration, i.e. how deep the subsea foundation structure is expected to penetrate the soil of the seabed by its own weight. The term “by its own weight” of the foundation structure including any equipment installed on the foundation structure during the installation. In some cases the foundation may be installed with a module like a well template with a manifold. The actual self-penetration depth can be different from the estimated depth for self-penetration. The foundation structure will stop itself when friction force is balanced by self- weight, regardless of the estimated depth of self-penetration.

[0072] Step II) may comprise self -penetration of the suction anchors into the seabed to the first depth and, the method may comprise a step of:

[0073] III) applying a suction force to the suction anchors thereby providing forced penetration to a second depth.

[0074] When step II) has been fulfilled, i.e. the suction anchors have penetrated into the soil to the first depth, for example by self-penetration upon the weight from the subsea foundation structure, the active steps in step III) of sucking or pumping water and soil out of the suction anchors is performed. Step III) may involve creating an under-pressure relative to the surroundings thereby forcing the suction anchors to penetrate to the second depth.

[0075] The suction anchors may comprise a skirt of a first length and the central support may be a central suction anchor with a skirt of a second length . The first length may be longer than the second length.

[0076] The method may comprise a step of: levelling the subsea foundation structure prior to penetrating the central suction anchor into the seabed.

[0077] The step of levelling the subsea foundation structure may comprise levelling the subsea foundation structure in a horizontal plane. Horizontal plane may be in an X direction of a cartesian coordinate system.

[0078] The step of levelling may be performed after step II) of self -penetration, but prior to the skirt of the central support penetrates the seabed. The step of levelling may be a one-time operation or it may be a step-by-step operation (i.e. level a bit, then apply suction, then level again before applying suction etc.). The step of levelling may even include to level upwards instead of downwards, i.e. applying over pressure to 2 lowest suction cans until the subsea foundation structure is level. In situations using four corner suctions anchors, levelling is normally performed by applying suction on the two highest suction anchors.

[0079] Step III) may comprise applying a suction force to a volume inside the central suction anchor thereby providing forced penetration of the central suction anchor to the second depth. According to a third aspect, the present disclosure provides a subsea foundation structure comprising:

[0080] - at least two suction anchors defining a foundation structure area therebetween;

[0081] - a beam sub-structure interconnecting the suction anchors;

[0082] - a central support arranged within the foundation structure area and connected to said beam sub-structure. According to the third aspect, in the case of only two suction anchors, two suction anchors and the one center support may be arranged on a straight line.

[0083] According to a fourth aspect it is described a method of installing a subsea foundation structure, the subsea foundation structure comprising at least two suction anchors defining a foundation structure area therebetween and a beam sub-structure interconnecting the suction anchors, wherein each of the at least two suction anchors supports a skirt roof, wherein the method comprises the steps of:

[0084] A) landing the subsea foundation structure on a seabed;

[0085] B) penetrating the suction anchors into the seabed to a first depth;

[0086] C) applying a suction force to the suction anchors thereby providing forced penetration to a second depth;

[0087] D) providing a central support;

[0088] E) installing the central support such that the skirt roofs are in a first plane and the central support is in a second plane, wherein the second plane is at a lower elevation than the first plane.

[0089] Step D) may comprise landing the central support on the subsea foundation structure. Consequently, the central support, being either a central suction anchor or a mudmat with skirt, may be post installed. I.e., first the subsea foundation structure is installed and, after the subsea foundation structure has been installed, the central support is installed. The beam sub-structure may be configured to receive the central support. For example, the locking arrangement may have means for receiving the central support in case the central support is installed after the subsea foundation structure.

[0090] Post installation of the central support may allow use of an installation vessel with lower lifting capacity. This may be more cost effective even if installation time increases.

[0091] Alternatively, if a dedicated installation vessel is to be used, but the weight is too high, post installation of the central support may in some cases allow for the dedicated vessel to be used anyways.

[0092] Post installation of the central support may even allow for using a smaller installation for the installation of the central support. The subsea foundation structure may comprise a locking arrangement wherein the subsea foundation structure comprises a locking arrangement, and the locking arrangement may be configured to:

[0093] - in a first state, prevent movement of the central support relative to the beam sub - structure, and

[0094] - in a second state, allow movement of the central support relative to the beam sub - structure.

[0095] Consequently, in the first state, the central support is locked to the beam substructure, and, in the second state the central support is released from the beam substructure such that it can move relative to the beam sub-structure.

[0096] The locking arrangement may e.g. comprise a ROV-operated locking pin which is installed or removed depending on whether the locking arrangement is in the first state or in the second state.

[0097] Alternatively, the locking arrangement may be actuated by an actuator. The actuator may be e.g. electric, hydraulic or electro -hydraulic.

[0098] Step A) may comprise landing the subsea foundation structure and the central support subsea with the locking arrangement in the first state. The central support may for example be locked in a raised position during landing of the subsea foundation structure. When the suction anchors have been installed, the central support may, while the locking arrangement is in the first state, be moved to a lowered position by the locking arrangement. As such, the locking arrangement (and the central support locked thereto) may be movable relative to the beam substructure. This may be done using e.g. a linear actuator, such as an electric, hydraulic or electro-hydraulic actuator. Alternatively, or additionally, there may be a rack-and-pinion connection between the locking arrangement and the beam substructure such that the locking arrangement can move up and down relative to the beam sub-structure along said rack-and-pinion connection.

[0099] The central support may be locked in the raised position during the step E). This will reduce loads and suction pressure during leveling of the suction anchors.

[0100] Step D) may comprise operating the locking arrangement from the first state to the second state. Thereby the central support is allowed to move relative to the beam sub-structure.

[0101] The method may comprise, after step E), a step F) of operating the locking arrangement from the second state to the first state. This is done in order to lock the central support relative to the beam sub-structure such that it is ensured that the central support also supports the subsea foundation structure (and any loads thereon) after installation.

[0102] The suction anchors may comprise a skirt of a first length and the central support may be a central suction anchor with a skirt of a second length, wherein the first length is longer than the second length.

[0103] Step C) may comprise levelling the subsea foundation structure.

[0104] The central support may be a central suction anchor and step E) may comprise applying a suction force to a volume inside the central suction anchor thereby providing forced penetration of the central suction anchor to the second depth.

[0105] The method may comprise, after step E), a step of:

[0106] - pumping cement into at least one of the suction anchors.

[0107] The method may comprise, after step E), a step of:

[0108] - pumping cement into the central support.

[0109] This can be done both in situations where the central support is a central suction anchor and when the central support is a mudmat with skirt.

[0110] Consequently, after installation of the subsea foundation structure, the method may comprise a step of pumping cement into at least one of the suction anchors and / or the central support. The pumping of cement ensures contact with the seabed.

[0111] Advantages of the subsea foundation structure described herein may include: reduced foundation installation weight may allow use of installation vessel with lower lifting capacity (such installation vessels normally have lower day rates) and more vessels are available in the market, reducing installation cost and increasing flexibility, use of standard installation vessels having a lifting capacity of -400 tons for installation of the subsea foundation structure, especially when used for a typical 4 slot ITS, reduced steel weight may also reduce cost on hardware.

[0112] Some operators require that the size of the skirts (i.e. diameter and length) is dimensioned to take up all of the permanent loads experienced by the structure. It is thus advantageous to have long enough skirts that skirt friction takes all permanent loads even with soil with lowest shear capacity in order to ensure sufficient friction capacity against the soil. Above-discussed preferred and / or optional features of each aspect of the invention / disclosure may be used, alone or in appropriate combination, in the other aspects of the invention / disclosure.

[0113] The claimed invention is specified in the independent claims of this application. Advantageous adaptations and versions of the claimed invention are specified in the independent claims.

[0114] Description of the drawings

[0115] Following drawings are appended to facilitate the understanding of the claimed invention:

[0116] Fig. 1 is a perspective view from below of a subsea foundation structure with four suction anchors and one central support in the form of either a central suction anchor or a central mudmat with skirt, the central suction anchor or central mudmat with skirt having a larger diameter and a shorter length than the suction anchors;

[0117] Fig. 2 is a side view of the subsea foundation structure of Fig. 1;

[0118] Fig. 3 is a side view of the subsea foundation structure of Figs. 1 and 2, where the subsea foundation structure is inclined relative to a seabed which is parallel to a horizontal plane and where the suction anchors have partly penetrated the seabed;

[0119] Fig. 4 is a side view of the subsea foundation structure in Fig. 3, however in Fig. 4 the subsea foundation structure has been levelled relative the horizontal plane and the suction anchors have penetrated the seabed a first depth;

[0120] Fig. 5 is a side view of the same subsea foundation structure as in Figs. 2-4, where the subsea foundation structure has fully penetrated the seabed, i.e. to a second depth where a skirt roof supported by the central suction anchor, or an underside of the central mudmat, is in contact with the seabed since it is at a lower elevation than the skirt roofs of the suction anchors;

[0121] Fig. 6 is a side view of a subsea foundation structure which has fully penetrated the seabed, where the seabed is inclined forming an angle relative a horizontal plane, and where the subsea foundation structure is parallel with the horizontal plane and the skirt roof supported by the central suction anchor, or the underside of the central mudmat, is in contact with the seabed;

[0122] Fig. 7 is a top view of the subsea foundation structure in Figs. 1-6;

[0123] Fig. 8 is a top view of a subsea foundation structure with three suction anchors and one central support in the form of a central suction anchor; Fig. 9 is a side view of a subsea foundation structure with four suction anchors and one central support in the form of either a central suction anchor in contact with the seabed or a central mudmat with skirt, the central suction anchor or the central mudmat with skirt being identical to the suction anchors, i.e. it has the same length and the same diameter as the suction anchors;

[0124] Fig. 10 is a side view of a subsea foundation structure with four suction anchors and one central support in the form of either a central suction anchor or central mudmat with skirt, the central suction anchor or the central mudmat with skirt having the same length and a larger diameter as the suction anchors;

[0125] Fig. 11 is a side view of a subsea foundation structure with four suction anchors and a central support in the form of a central mudmat in contact with the seabed since it is at a lower elevation than the skirt roofs of the suction anchors;

[0126] Fig. 12 is a side view of a foundation structure similar to the one in Figs. 1-7, with four suction anchors and one central support in the form of either a central suction anchor or a central mudmat with skirt, the central suction anchor or central mudmat with skirt having a larger diameter and a shorter length than the suction anchors and with tail pipes extending downwardly from each well slot in the subsea foundation structure;

[0127] Fig. 13 is a top view of a subsea foundation structure comprising two suction anchors and a central support;

[0128] Figs. 14A-14F show steps of a method where the central support is raised during landing of the subsea installation structure, and where the central support is installed after the suction anchors have been installed;

[0129] Figs. 15A-15F show steps of a method where the central support is installed separately from the subsea foundation structure, and after the suction anchors have been installed;

[0130] It should be understood, however, that the drawings are not intended to limit the claimed invention to the subject-matter depicted in the drawings.

[0131] In the drawings, like reference numerals have been used to indicate common parts, elements or features unless otherwise explicitly stated or implicitly understood by the context.

[0132] Detailed description

[0133] In the following, one or more specific embodiments of the invention will be described in more detail with reference to the drawings. However, it is specifically intended that the invention is not limited to the embodiments and illustrations contained herein but includes modified forms of the embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementationspecific decisions must be made to achieve the developer’s specific goals, such as compliance with system and / or business-related constraints, which may vary from one implementation of the invention to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication and manufacture for the skilled person having the benefit of this disclosure. In the following, it is clear that features of the invention which have only been described in relation to the subsea foundation structure may also be valid for the method of installing the subsea foundation structure, and vice versa.

[0134] Fig. 1 is a perspective view from below of a subsea foundation structure 1 with four suction anchors 21,22,23,24 and one central support 10’ in the form of either a central suction anchor 10’ or a central mudmat with skirt, the central suction anchor 10’ or central mudmat with skirt having a larger diameter and a shorter length than the suction anchors 21,22,23,24. The first suction anchor 21, the second suction anchor 22, the third suction anchor 23 and the fourth suction anchor 24 are arranged at corner portions of the subsea foundation structure 1. The subsea foundation structure 1 comprises a beam substructure 30 formed by a number of beams interconnecting the second suction anchor 22, the third suction anchor 23 and the fourth suction anchor 24 and the central suction anchor 10’. The dimensions, strength and shapes of the beams forming the beam substructure 30 may vary and will be dependent on the specific project.

[0135] Either if using a central suction anchor 10’ or a central mudmat with skirt, both of these alternatives look the same in figures. Thus, in Fig. 1, as well as in Figs. 2-7, 9-10 and 12, the element with reference sign 10’ could either be a central suction anchor, i.e. a suction anchor with suction / pumping, or a central mudmat with skirt, i.e. without suction / pumping.

[0136] The subsea foundation structure 1 in Fig. 1 is in the form of a well template. The well template comprising four well slots 55 (only three shown in Fig. 1 since one is behind suction anchor 22).

[0137] As is known in the art, the subsea foundation structure 1 may comprise physical protection means such that it is e.g. over-trawlable and / or protected from falling objects.

[0138] Fig. 2 is a side view of the subsea foundation structure 1 of Fig. 1. As disclosed in Fig. 2, the beam sub-structure 30 connects to the four suction anchors 21,22,23,24 in a first plane pl, and to the central support 10’ in a second plane p2. The second plane p2 is at a lower elevation than the first plane pl. This ensures that the central support / suction anchor 10’ takes up forces in that it is forced to be in contact with an underlying seabed SB (seabed not shown in Fig. 2, see e.g. Figs. 3-6). The required (minimum) height difference between first plane pl and second plane p2 is decided based on maximum seabed inclination for each project. The suction anchors 21,22,23,24 comprise a skirt of a first length hl and the central support 10’ comprises a skirt of a second length h2. The first length hl is longer than the second length h2. The difference in length of the skirt has the technical effect that the suction anchors 21,22,23,24 can be levelled before the central support 10’ enters underlying soil, as will be described in greater detail below with reference to Figs. 3-5.

[0139] Fig. 3 is a side view of the subsea foundation structure 1 of Figs. 1 and 2, where the subsea foundation structure 1 is inclined. This may e.g. be the result after self-weight penetration and may typically happen if different soil resistance for the different suction anchors but also inaccuracy in lifting arrangement, CoG deviation and current may affect inclination. The seabed SB is parallel to a horizontal plane hp.

[0140] Fig. 4 is a side view of the subsea foundation structure 1 in Fig. 3, however in Fig. 4 the subsea foundation structure 1 has been levelled relative to the horizontal plane hp. The suction anchors 21,22,23,24 have penetrated the seabed SB to a first depth dl.

[0141] In Fig. 4, the length of the skirt on the central support 10’ is less than the self-penetration depth (i.e. the first depth dl), however this may vary.

[0142] In order to penetrate or install the subsea foundation structure from the state in Fig. 4, to the state in Fig. 5, a suction pump (not shown) has been connected to the respective first, second, third and fourth suction anchors 21,22,23,24 sucking or pumping the subsea foundation structure 1 into the soil of the seabed SB by removing water from inside the suction anchors 21,22,23,24 until the central support 10’ rests against the seabed SB. The penetration of the central suction anchor 10’ may possibly be as a result of suction of the respective first, second, third and fourth suction anchors 21,22,23,24, thereby avoiding the need of assisted suction by means of e.g. a suction pump (not shown) during installation of the central suction anchor 10’.

[0143] However, it may also be possible to provide the central suction anchor 10’ with means for connection 70 to a suction pump for suction of fluids out from a volume inside the central suction anchor 10’. The means for connection 70 can be a pipe connection with a closable valve, such that once the central suction anchor 10’ is installed, the valve can be closed preventing fluid flow between the volume inside the central suction anchor 10’ and the surrounding seawater. The valve may thus be closed or opened during installation if any reason for this.

[0144] The suction anchors 21,22,23,24 may also have a similar closable valve (not shown) which can be opened and closed as required during installation to control e.g. inclination and structure deformation

[0145] Fig. 5 is a side view of the same subsea foundation structure 1 as in Figs. 2-4, where the subsea foundation structure 1 has fully penetrated the seabed SB, i.e. to a second depth d2 where a skirt roof supported by the central suction anchor 10’ is in contact with the seabed SB since it is at a lower elevation than the skirt roofs of the suction anchors 21,22,23,24.

[0146] Fig. 6 is a side view of a subsea foundation structure 1 which has fully penetrated the seabed SB, where the seabed SB is inclined forming an angle a relative a horizontal plane, and where the subsea foundation structure 1 is parallel with the horizontal plane hp and a skirt roof supported by the central suction anchor 10’ is in contact with the seabed SB.

[0147] Fig. 7 is a top view of the subsea foundation structure 1 in Figs. 1-6. The first, second, third and fourth suction anchor 21,22,23,24 defining a foundation structure area FSA therebetween. As shown in Fig. 7, the central suction anchor 10’ is equidistantly arranged in relation to the four suction anchors 21,22,23,24 and is arranged such that a center of gravity of the central suction anchor 10’ is coinciding with a center of gravity of the subsea foundation structure 1. As seen in Fig. 7, each of the suction anchors 21,22,23,24 supports a skirt roof 31,32,33,34, and the central suction anchor 10’ supports a skirt roof 35. When the subsea foundation structure 1 is levelled in a horizontal plane hp (see Fig. 6), the skirt roof 35 of the central suction anchor 10’ is at a lower elevation relative the skirt roofs 31,32,33,34 supported by the suction anchors 21,22,23,24.

[0148] Fig. 8 is a top view of a subsea foundation structure 1 with three suction anchors 21,22,23 and one central support 10’ in the form of a central suction anchor 10’. The first, second and third suction anchor 21,22,23 defining a foundation structure area FSA therebetween. As shown in Fig. 8, the central suction anchor 10’ is equidistantly arranged in relation to the three suction anchors 21,22,23 and is arranged such that a center of gravity of the central suction anchor 10’ is coinciding with a center of gravity of the subsea foundation structure 1. The skilled person will understand that a central mudmat 10” may be used instead of the central suction anchor 10’ in the subsea foundation structure 1 in Fig. 8.

[0149] Fig. 9 is a side view of a subsea foundation structure 1 with four suction anchors 21,22,23,24 and one central support 10’ in the form of a central suction anchor 10’, the central suction anchor 10’ being identical to the suction anchors 21,22,23,24, i.e. it has the same length and the same diameter.

[0150] Fig. 10 is a side view of a subsea foundation structure 1 with four suction anchors 21,22,23,24 and one central support 10’ in the form of a central suction anchor 10’, the central suction anchor 10’ having the same length and a larger diameter as the suction anchors 21,22,23,24.

[0151] Fig. 11 is a side view of a subsea foundation structure 1 with four suction anchors 21,22,23,24 (only two of the suction anchors 21,22 shown in the figure) and a central support 10’ in the form of a central mudmat 10” in contact with the seabed SB since it is at a lower elevation than the skirt roofs of the suction anchors 21,22,23,24. When the subsea foundation structure 1 is levelled in a horizontal plane hp (see Fig. 6), the central mudmat 10” is at a lower elevation relative the skirt roofs 31,32,33,34 supported by the suction anchors 21,22,23,24. This ensures that the central mudmat 10” rests against the seabed SB taking up loads. The first, second, third and fourth suction anchor 21,22,23,24 defining a foundation structure area FSA therebetween. As shown in Fig. 11, the central mudmat 10” is equidistantly arranged in relation to the four suction anchors 21,22,23,24 and is arranged such that a center of gravity of the central mudmat 10” is coinciding with a center of gravity of the subsea foundation structure 1.

[0152] Fig. 12 is a side view of a foundation structure 1 similar to the one in Figs. 1-7, with four suction anchors 21,22,23,24 and one central support 10’ in the form of a central suction anchor 10’, the central suction anchor 10’ having a larger diameter and a shorter length than the suction anchors 21,22.23,24 and with tail pipes 60 extending downwardly from each well slot 55 in the subsea foundation structure 1. The number of tail pipes 60 are equal to the number of well slots 55. The tail pipes 60 prevents wash-out of the soil around the well being drilled, and thus the soil around any suction anchors 21,22,23,24 and central suction anchor 10’ in the proximity of the well being drilled.

[0153] Fig. 13 is a top view of a subsea foundation structure 1 comprising two suction anchors 21,22 and a central support 10’. The two suction anchors 21,22 defining a foundation structure area FSA therebetween. A beam sub-structure 30 interconnecting the suction anchors 21,22 and the central support 10’. The central support 10’ is arranged within the foundation structure area FSA. The two suction anchors 21,22 and the central support 10’ are arranged on a straight line.

[0154] Referring to Figs. 1-6, a method of installing a subsea foundation structure 1 is described, which method may comprise the steps of:

[0155] I) landing the subsea foundation structure 1 on a seabed SB;

[0156] II) penetrating the suction anchors 21,22,23,24 into the seabed SB to a first depth dl. The first depth dl into the seabed SB may be an estimated depth for self-penetration, i.e. how deep the subsea foundation structure 1 is expected to penetrate the soil of the seabed SB by its own weight. The actual self-penetration depth can be different from the estimated depth for self-penetration. The foundation structure will stop itself when friction force is balanced by self-weight, regardless of the estimated depth of self-penetration.

[0157] When the suction anchors 21,22,23,24 have reached the first depth dl, the method may comprise a step of:

[0158] III) applying a suction force to the suction anchors 21,22,23,24 thereby providing forced penetration to a second depth d2. When step II) has been fulfilled, i.e. the suction anchors 21,22,23,24 have penetrated into the soil to the first depth dl, for example upon the weight from the subsea foundation structure 1, the active steps in step III) of sucking or pumping water and soil out of the suction anchors 21,22,23,24 is performed. Step III) may involve creating an under-pressure relative to the surroundings thereby forcing the suction anchors 21,22,23,24 to penetrate to the second depth d2.

[0159] The method may comprise a step of: levelling the subsea foundation structure 1 prior to penetrating the central suction anchor 10’ into the seabed SB. Alternatively, the method may comprise a step of: levelling the subsea foundation structure 1 even if the central suction anchor 10’ has penetrated the seabed SB.

[0160] The step of levelling the subsea foundation structure 1 may comprise levelling the subsea foundation structure 1 in a horizontal plane hp. The horizontal plane hp being in an X direction of a cartesian coordinate system.

[0161] Step III) may comprise a step of: applying a suction force to a volume inside the central suction anchor 10’ thereby providing forced penetration of the central suction anchor 10’ to the second depth d2.

[0162] Figs. 14A-14F show steps of a method where the central support 10’ ; 10” is raised during landing of the subsea installation structure 1, and where the central support 10’; 10” is installed after the suction anchors 21,22,23,24 have been installed. The central support 10’; 10” can be a central suction anchor 10’ or a mudmat 10” with skirt. The subsea foundation structure 1 is in the form of a well template comprising four well slots 55 (only two shown in Figs. 14A-14F).

[0163] The subsea foundation structure 1 features a locking arrangement 80 which is configured to, in a first state, prevent movement of the central support 10’; 10” relative to the beam sub-structure 30, and in a second state, allow movement of the central support 10’; 10” relative to the beam sub-structure 30.

[0164] In Fig,.14 A, the subsea foundation structure 1 has been landed on the seabed SB with the locking arrangement 80 in the first state, i.e. the central support 10’; 10” is locked to the beam sub-structure 30. The central support 10’; 10” is thus prevented from movement relative to the beam sub-structure 30. In Fig. 14A, the locking arrangement holds the central support 10’; 10” in a raised position while the suction anchors 21,22,23,24 selfpenetrate into the seabed SB to a first depth dl. The locking arrangement 80 may be moved up and down relative the beam sub-structure 30 using e.g. a linear actuator, such as an electric, hydraulic or electro -hydraulic actuator (not shown). Alternatively, or additionally, there may be a rack-and-pinion connection (not shown) between the locking arrangement and the beam sub-structure such that the locking arrangement can move up and down relative to the beam sub-structure along said rack-and-pinion connection.

[0165] Referring to Fig. 14B, after self-penetration of the suction anchors 21,22,23,24, the suction anchors 21,22,23,24 are forced into the seabed SB by e.g. pumping fluid out from the volume inside the suction anchors 21,22,23,24 until at least one of the suction anchors 21,22,23,24 are fully penetrated, i.e. the skirt roofs 31,32 of the suction anchors 21,22 are in contact with the seabed SB (see Fig. 14C). Required levelling of the subsea foundation structure 1 is performed during this step.

[0166] In Fig. 14C the suction anchors 21,22,23,24 are fully penetrated to a second depth d2.

[0167] In Fig. 14D, the locking arrangement 80 is in the second state where the central support 10’; 10” is released from the beam sub-structure 30 such that it can move relative to the beam sub-structure 30. The central support 10’; 10” will self-penetrate into the seabed SB.

[0168] In case the central support 10’ is a central suction anchor 10’, suction pressure may be applied to penetrate the central suction anchor 10’ to its installation depth, see Figs. 14E and 14F indicating further penetration of the central suction anchor 10’.

[0169] In case the central support 10” is a mudmat 10” with skirt, the mudmat 10’ may be fully installed by self-penetration to the position of the mudmat 10’ with skirt as shown in Fig. 14F. However, if the mudmat 10’ is not in contact with the seabed SB by selfpenetration, cement may be pumped into the mudmat 10’ with skirt in order to ensure contact with the seabed SB.

[0170] Referring to Fig. 14F, the central suction anchor 10’ is fully penetrated and the skirt roof 31,32 is in contact with the seabed SB. The locking arrangement 80 is operated to the first state such that the central suction anchor 10’ (or the mudmat 10” with skirt) prevented from relative to the beam sub-structure 30.

[0171] Figs. 15A-15F show steps of a method where the central support 10’, 10” is installed separately from the subsea foundation structure 1 and after the suction anchors

[0172] 21,22,23,24 have been installed. The central support 10’; 10” can be a central suction anchor 10’ or a mudmat 10” with skirt. The subsea foundation structure 1 is in the form of a well template comprising four well slots 55 (only two shown in Figs. 15A-15F).

[0173] In Fig. 15 A, the subsea foundation structure 1 has been landed on the seabed with the central support 10’; 10”, and the suction anchors 21,22,23,24 have self-penetrated into the seabed SB to a first depth dl.

[0174] The method in Figs 15A-15F may comprise a locking arrangement 80 similar to the one described in relation to Figs. 14A-14F.

[0175] In Fig. 15B, after self-penetration of the suction anchors 21,22,23,24, the suction anchors

[0176] 21,22,23,24 are forced into the seabed SB by e.g. pumping fluid out from the volume inside the suction anchors 21,22,23,24 until at least one of the suction anchors

[0177] 21,22,23,24 are fully penetrated, i.e. the skirt roofs 31,32 of the suction anchors 21,22 are in contact with the seabed SB (see Fig. 15C). Required levelling of the subsea foundation structure 1 is performed during this step.

[0178] In Fig. 15C the suction anchors 21,22,23,24 are fully penetrated to a second depth d2. In Fig. 15D the central support 10’; 10” is received by the subsea foundation structure 1.

[0179] This may e.g. be performed using the locking arrangement 80.

[0180] In Fig. 15E the central support 10’;’ 10” moves downwardly by self-penetration. In the case the central support 10’ is a central suction anchor 10’, suction pressure may be applied (in addition to the self-penetration) to penetrate the central suction anchor 10’ to its installation depth.

[0181] Referring to Fig. 15F, the central suction anchor 10’ is fully penetrated and the skirt roof 31,32 is in contact with the seabed SB. The locking arrangement 80 is operated to the first state such that the central suction anchor 10’ (or the mudmat 10” with skirt) prevented from relative to the beam sub-structure 30.

[0182] Further referring to Figs. 14A-14F and Figs. 15A-15F, it is described a method of installing a subsea foundation structure 1, the subsea foundation structure 1 comprising at least two suction anchors 21,22,23,24 defining a foundation structure area FSA therebetween and a beam sub-structure 30 interconnecting the suction anchors 21,22,23,24, wherein each of the at least two suction anchors 21,22,23,24 supports a skirt roof 31,32,33,34, wherein the method comprises the steps of:

[0183] A) landing the subsea foundation structure 1 on a seabed SB;

[0184] B) penetrating the suction anchors 21,22,23,24 into the seabed SB to a first depth dl;

[0185] C) applying a suction force to the suction anchors 21,22,23,24 thereby providing forced penetration to a second depth d2;

[0186] D) providing a central support 10’; 10”;

[0187] E) installing the central support 10’; 10” such that the skirt roofs 31,32,33,34 are in a first plane pl and the central support 10’; 10” is in a second plane p2, wherein the second plane p2 is at a lower elevation than the first plane pl.

[0188] Referring to Figs. 15A-15F, step D) may comprise landing the central support 10’; 10” on the subsea foundation structure 1.

[0189] Referring to Figs. 14A-14D, the subsea foundation structure 1 may comprise a locking arrangement, wherein the locking arrangement is configured to:

[0190] - in a first state, prevent movement of the central support 10’; 10” relative to the beam sub-structure 30, and

[0191] - in a second state, allow movement of the central support 10’; 10” relative to the beam sub-structure 30. In all of the examples in Figs. 1-13 and in Figs 14A-14F and 15A-15F, the presence of the central support 10’; 10” which is configured to take up forces, renders it possible to achieve one or more of the following advantages, such as:

[0192] Bearing on mudmat or skirt roof (of central support 10’ if suction anchor) for permanent loads may allow shorter skirts and / or smaller diameter of suction anchors, Increased structure integrity and interfaces between equipment installed on foundation since the structure will potentially experience less deformations of main structure as smaller suction anchors give less loads into structure during levelling, and the central support will contribute to stiffer main structure reducing deformations,

[0193] Possibly thinner skirt walls if lower suction pressure as shorter skirts have less penetration resistance and hence, require less pressure,

[0194] Central support 10’; 10” will be part of main structure, allow slimming / lighter beam sub-structure,

[0195] Slimmer structure of beam sub-structure 30 to suction anchors 21,22,23,24 (as central support 10’; 10” take part up loads or forces),

[0196] Less overall weight of the subsea foundation structure 1 (despite one additional central support 10’; 10” in center and additional beams to central support 10’; 10” since the suction anchors 21,22,23,24 can be made smaller and lighter and the main structure may consequently be slimmed).

[0197] Although example subsea foundation structures with two, three and four suction anchors have been disclosed, it is clear the subsea foundation structure may have more suction anchors, such as five, six, seven eight, . . . , twelve.

[0198] Similarly, although the example subsea foundation structures are disclosed with one central support, there may be more than one central support, such as two, three, four, . . . , seven, eight central supports within the foundation structure area.

[0199] In the preceding description, various aspects of the subsea equipment and the subsea receptacle according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the apparatus and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the subsea equipment and the subsea receptacle, which are apparent to person skilled in the art to which the disclosed subject-matter pertains, are deemed to lie within the scope of the present invention as defined by the following claims. LIST OF REFERENCE NUMBERS

[0200] 1 Subsea foundation structure

[0201] 10’ Central support / central suction anchor

[0202] 10” Central support / central mudmat

[0203] 21 First suction anchor

[0204] 22 Second suction anchor

[0205] 23 Third suction anchor

[0206] 24 Fourth suction anchor

[0207] 30 Beam sub-structure

[0208] 31 First skirt roof

[0209] 32 Second skirt roof

[0210] 33 Third skirt roof

[0211] 34 Fourth skirt roof

[0212] 35 Skirt roof of central suction anchor 10’

[0213] 55 Well slot

[0214] 60 Tail pipe

[0215] 70 Means for connection

[0216] 80 locking arrangement dl first depth d2 second depth

[0217] FSA Foundation structure area hp Horizontal plane hl First length h2 Second length pl First plane p2 Second plane

Claims

CLAIMS1. A subsea foundation structure (1) comprising:- at least two suction anchors (21,22,23,24) defining a foundation structure area (FSA) therebetween;- a beam sub-structure (30) interconnecting the suction anchors(21.22.23.24);- a central support (10’;10”) arranged within the foundation structure area (FSA) and connected to said beam sub-structure (30), wherein each of the at least two suction anchors (21,22,23,24) supports a skirt roof (31,32,33,34), and the beam sub-structure connects to the at least two suction anchors(21.22.23.24) and the central support (10’ ; 10’ ’), such that the skirt roofs (31,32,33,34) are in a first plane (pl) and the central support (10’;10”) is in a second plane (p2), wherein the second plane (p2) is at a lower elevation than the first plane (pl).

2. The subsea foundation structure (1) according to claim 1, wherein the subsea foundation structure (1) comprises at least three suction anchors (21,22,23,24) defining the foundation structure area (FSA) therebetween.

3. The subsea foundation structure (1) according to any of the preceding claims 1, wherein the central support (10’) is a central suction anchor (10’) supporting a skirt roof (35).

4. The subsea foundation structure (1) according to claim 1 or 2, wherein the central support (10”) is a central mudmat (10’).

5. The subsea foundation structure (1) according to any of the preceding claims, wherein the central support ( 10’ ; 10”) is of a different size than the at least two suction anchors (21,22,23,24).

6. The subsea foundation structure (1) according to any of the preceding claims, wherein the suction anchors (21,22,23,24) comprise a skirt of a first length (hl) and the central support (10’) comprises a skirt of a second length (h2), wherein the first length (hl) is longer than the second length (h2).

7. The subsea foundation structure (1) according to any of the preceding claims, wherein the central support (10’; 10”) is equidistantly arranged in relation to the suction anchors (21,22,23,24).

8. The subsea foundation structure (1) according to any of the preceding claims, wherein the central support (10’; 10”) is arranged such that a center of gravity of the central support (10’; 10”) is coinciding with a center of gravity of the subsea foundation structure (1).

9. The subsea foundation structure (1) according to any of the preceding claims, wherein each of the suction anchors (21,22,23,24) supports a skirt roof (31,32,33,34), and wherein, when the subsea foundation structure (1) is levelled in a horizontal plane (hp), the central support (10’ ; 10”) is at a lower elevation relative the skirt roofs (31,32,33,34) supported by the suction anchors (21,22,23,24).

10. The subsea foundation structure (1) according to any of the preceding claims, wherein the central support (1 O’; 10”) is movable relative to the beam sub-structure (30).

11. The subsea foundation structure (1) according to claim 10, wherein the subsea foundation structure (1) comprises a locking arrangement, wherein the locking arrangement is configured to:- in a first state, prevent movement of the central support (10’; 10”) relative to the beam sub-structure (30), and- in a second state, allow movement of the central support (10’; 10”) relative to the beam sub-structure (30).

12. The subsea foundation structure (1) according to any of the preceding claims, wherein the beam sub-structure (30) is configured to receive the central support (10’, 10”).

13. A method of installing a subsea foundation structure (1) according to any one of claims 1-12, comprising the steps of:I) landing the subsea foundation structure (1) on a seabed (SB);II) penetrating the suction anchors (21,22,23,24) into the seabed (SB) to a first depth (dl).

14. The method according to claim 13, wherein step II) comprises selfpenetration of the suction anchors (21,22,23,24) into the seabed (SB) to the first depth (dl), and the method further comprises a step of:III) applying a suction force to the suction anchors (21,22,23,24) thereby providing forced penetration to a second depth (d2).

15. The method according to claim 13 or 14, wherein the suction anchors (21,22,23,24) comprise a skirt of a first length (hl) and the central support (10’) is a central suction anchor (10’) with a skirt of a second length (h2), wherein the first length (hl) is longer than the second length (h2).

16. The method according to claim 15, wherein the method comprises a step of: levelling the subsea foundation structure (1) prior to penetrating the central suction anchor into the seabed (SB).

17. The method according to claim 16, wherein the step of levelling is performed after step II).

18. The method according to claim 14, or any one of claims 15-17 when dependent upon claim 14, wherein step III) comprises applying a suction force to a volume inside the central suction anchor (10’) thereby providing forced penetration of the central suction anchor (10’) to the second depth (d2).

19. A method of installing a subsea foundation structure (1), the subsea foundation structure (1) comprising at least two suction anchors (21,22,23,24) defining a foundation structure area (FSA) therebetween and a beam sub-structure (30) interconnecting the suction anchors (21,22,23,24), wherein each of the at least two suction anchors (21,22,23,24) supports a skirt roof (31,32,33,34), wherein the method comprises the steps of:A) landing the subsea foundation structure (1) on a seabed (SB);B) penetrating the suction anchors (21,22,23,24) into the seabed (SB) to a first depth (dl);C) applying a suction force to the suction anchors (21,22,23,24) thereby providing forced penetration to a second depth (d2);D) providing a central support (10’ ; 10”);E) installing the central support (10’; 10”) such that the skirt roofs (31,32,33,34) are in a first plane (pl) and the central support (10’; 10”) is in a second plane (p2), wherein the second plane (p2) is at a lower elevation than the first plane (pl).

20. The method according to claim 19, wherein step D) comprises landing the central support (10’; 10”) on the subsea foundation structure (1).

21. The method according to claim 19, wherein the subsea foundation structure (1) comprises a locking arrangement, wherein the locking arrangement is configured to:- in a first state, prevent movement of the central support (10’; 10”) relative to the beam sub-structure (30), and- in a second state, allow movement of the central support (10’; 10”) relative to the beam sub-structure (30).

22. The method according to claim 21, wherein step A) comprises landing the subsea foundation structure (1) and the central support (10’; 10”) subsea with the locking arrangement in the first state.

23. The method according to claim 22, wherein step D) comprises operating the locking arrangement from the first state to the second state.

24. The method according to claim 23, wherein the method comprises, after step E), a step F) of operating the locking arrangement from the second state to the first state.

25. The method according to any one of claims 19-24, wherein the suction anchors (21,22,23,24) comprise a skirt of a first length (hl) and the central support (10’) is a central suction anchor (10’) with a skirt of a second length (h2), wherein the first length (hl) is longer than the second length (h2).

26. The method according to any one of claims 19-25, wherein step C) comprises levelling the subsea foundation structure (1).

27. The method according to any one of claims 19-26, wherein the central support is a central suction anchor (10’) and wherein step E) comprises applying a suction force to a volume inside the central suction anchor (10’) thereby providing forced penetration of the central suction anchor (10’) to the second depth (d2).

28. The method according to any of the preceding claims 19-27, wherein the method comprises, after step E), a step of:- pumping cement into at least one of the suction anchors (21,22,23,24).

29. The method according to any of the preceding claims 19-27, wherein the method comprises, after step E), a step of:- pumping cement into the central support (10’; 10”).

Images