TECHNICAL FIELD

The invention relates to offshore structures for offshore oil and gas drilling and production operations. In particular, the invention relates to offshore towers suitable for releasable attachment to the sea floor. More particularly, the invention relates to a self-installing floating tower for use in off-shore drilling and production operations.

BACKGROUND TO THE INVENTION

Various types of offshore structures may be employed to drill and/or produce subsea oil and gas wells. The type of structure selected for a particular offshore application will depend on factors including the depth of water at the well location.

Fabrication and installation of fixed platform requires a particular infrastructure and types of skilled labour, which may not be available at all desired locations and at the desired times.

Compliant towers offer an alternative for offshore applications with water depths up to about 190m. Compliant towers include a truss structure anchored directly to the sea floor, and a deck positioned above the sea surface and mounted to the upper end of the truss structure. Although the lower end of the truss structure is rigidly secured to the sea floor, the truss structure is designed to flex over its length in response to environmental loads. Compliant towers are not, however useful in all water depths and sea bed types and so alternative structures for different sea bed type, water depths and sea conditions are needed.

US 2012/0093587 Al describes an offshore structure arranged to be installed such that it can pivot relative to the ocean floor to address particular limitations at certain water depths, and also describes a related installation method. SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided an off-shore structure, comprising:

a plurality of tubular legs;

each of the legs having an upper end arranged to be connected to a topside structure, to support the topside structure, and a lower end connected to a foundation;

the interior of each tubular leg being subdivided into a plurality of closed compartments;

the compartments in each leg preferably comprising one or more of:

one or more fixed voids;

one or more fixed ballast tanks;

one or more variable ballast tanks; and

one or more storage tanks configured for storing oil over water; the structure preferably further comprising at least one tubular storage vessel mounted within the array of tubular legs, the tubular storage vessel preferably comprising at least one storage tank configured for storing oil over water.

The structure is configured to be easily redeployable from one location to another. The off-shore structure of the present invention therefore provides an improved self- installing structure, which can be easily transported to its location of installation and which has improved stability and improved storage facilities for the storage of oil or gas produced in a production process. The tower can be considered self-installing, since it can be floated to its location and righted by appropriately sequenced flooding of its storage tanks, without the need for heavy lifting equipment or large cranes. The structure can be used in all soil conditions, and suitable foundation optimised for that soil can be selected. The structure can be used in various weather conditions depending on the choice of structural features and foundation type used. The structure has minimal decommissioning costs due to its modularity and ease of redeployment and decommissioning. This results in a reduced environment impact. The structure can be easily built in modular sections during construction on shore and in deployment. The structure is particularly suited to being fixedly installed on a sea bed in a more rigid and stable manner than previously used designs which are arranged to be pivotable about their base.

The fixed ballast tanks of the legs are preferably located at a lower end of the legs, the lower end being adjacent the foundation.

The fixed ballast tank or tanks of the storage vessel or vessels are preferably located at or near to a lower end of the storage vessel or vessels, the lower end being adjacent the foundation.

The plurality of legs are preferably arranged in an array, spaced from one another, and the storage vessel or vessels are preferably arranged in spaces located between the array of legs. The storage vessels may alternatively be arranged around outer edges of the legs, particularly if the topside is relatively small and so spacing apart of the legs is not necessary.

The storage vessel or vessels are preferably attached to sides of the legs.

The storage vessel or vessels have a length from a lower end to an upper end of each storage vessel which is shorter than a length from the lower end of the legs to the upper end of the legs. Adjacent legs are preferably connected to one another via the storage vessel or vessels along a first portion of the length of the legs and the structure may further comprise a plurality of cross members connecting adjacent legs directly to one another above the upper end of the storage vessel or vessels. The legs may be arranged in a substantially circular or rectangular array and the storage vessel or vessels may be arranged within the outer envelope of the array of legs. The foundation for the structure, which may be integrated with the structure or separately formed and connectable to the structure, comprises a plurality of suction cans. Each suction can preferably has a tubular outer wall and a closed upper end, adjacent the legs, and preferably further comprises a fluid connection to the suction can for extracting fluid from or adding fluid to, the interior of the suction can.

Each of the suction cans preferably has a longitudinal central axis aligned parallel to longitudinal axes of the legs. The suction cans are preferably arranged with their longitudinal central axes spaced outwardly of the array of central axes of the legs.

Each of the suction cans may have an outer width or diameter less than or the same as the diameter of a leg of the structure.

Each leg preferably has a suction can attached to its lower end.

A ratio of the outer width or diameter of one or more of the suction cans to the diameter of the leg to which it is attached is greater than 1, more preferably between 1 and 2, more preferably between 1.2 and 2, more preferably between 1.4 and 2, more preferably between 1.6 and 2.

One or more of the suction cans preferably has a length along its respective longitudinal axis of between 0.5 and 3 times its diameter, preferably between 0.5 and 2.5 times its diameter, more preferably between 0.5 and 2 times its diameter, more preferably between 0.8 and 1.5 times its diameter, more preferably between 0.9 and 1 times its diameter.

The overall width of the foundation is preferably between around 1 and 2 times the width of the array of legs, optionally between 1.2 and 1.6 times, and optionally between 1.4 and 1.6 times the width of the array of legs.

The foundation is optionally anchored to a sea bed by a plurality of micro piles, each having a diameter of less than 0.5m, preferably less than 0.3m, more preferably less than 0.15m. The foundation may be a gravity based foundation structure, comprising a hollow core at least partially filled with magnetite to anchor the foundation to the sea bed. This type of foundation can be easily floated to its destination and magnetite has a preferred high density described in more detail in relation to the figures.

The foundation is optionally removably attached to the legs via repeatably removable and reconnectable connection means. The fixed ballast tank of one or more of the legs and/or storage vessels is preferably at least partially, or fully, filled with magnetite or a magnetite slurry made from particulate magnetite mixed with water.

Wherein at least one of the legs comprises at least one fixed void, located above a variable ballast tank, which is located above a storage tank, which is preferably located above a fixed ballast tank.

The offshore structure may comprise at least one oil over water storage tank in one or more of the legs of the structure.

The offshore structure may comprise at least one oil over water storage tank in one or more of the storage vessels.

At least one of the storage tanks located in the legs and/or the storage vessel or vessels may be connected to a source of water for adjusting an amount of water in the storage tank and to a source of oil for adjusting an amount of oil stored in the storage tank.

At least one of the storage tanks may be provided with a water delivery pipe having an opening disposed toward a lower end of the storage tank, and an oil delivery pipe having an open end disposed toward an upper end of the storage tank.

The structure may further comprise at least one water delivery pipe for delivering water between a storage tank located in at least one of the storage vessels and water pumping or processing equipment located in an upper part of a leg or in topside equipment mounted on the structure, the pipe preferably passing between the topside equipment and the storage tank via a path passing through, or being attached to, a leg of the structure.

The offshore structure may further comprise at least one oil delivery pipe for delivering oil between a storage tank located in at least one of the storage vessels and a supply of oil, the pipe preferably passing between the source of oil and the storage tank via a path passing through, or being attached to, a leg of the structure.

An oil pump may be connected to the oil delivery pipe. The oil pump may be located in a leg of the structure. The oil pump may be a submersible pump, preferably located in a storage tank of the structure or on top of a leg of the structure. The offshore structure may comprise a water delivery circuit having a related water delivery control valve for one or more storage tanks located in one or more of the legs of the structure.

The offshore structure may comprise a water delivery circuit provided with a related water delivery control valve for one or more storage tanks located in one or more of the storage vessel or storage vessels of the structure.

The water delivery circuit may be connected to a main water ring arranged at the upper end of the legs or on the cellar deck of the topside structure.

The offshore structure may further comprise oil and water distribution circuits configured so that when oil is delivered to one or more storage tanks located in the legs or in the storage vessels of the structure, water is removed from the one or more storage tanks to which oil is delivered.

The offshore structure may further comprise an oil distribution system, comprising a filling ring selectively fluidly connectable to a loading pump of at least one of the legs of the structure. The offshore structure may further comprise an oil distribution system comprising a filling ring selectively fluidly connectable to a loading pump of at least one of the storage vessels of the structure.

The filling ring may be connected, via a valve, to a feed of crude oil from an oil production process. The filling ring may be further connected, via a valve, to a tanker loading circuit. The oil distribution system may further comprise an offloading ring selectively connectable to one or more of the loading pumps of one or more of the legs or of the storage vessels.

Either or both of the filling ring and the offloading ring may be operably connectable to a tanker loading circuit via respective filling ring and offloading ring valves to deliver oil thereto from the storage tanks and/or from the source of crude oil.

The offloading ring may be connected, via a valve, to a slop pump or a slop tank and the filling ring may be connected, via a valve, to a slop tank or a slop pump.

The offloading ring and/or the filling ring may be disposed in the upper end of the legs or on the cellar deck of the topside structure for ease of maintenance.

The slop tank and/or the slop pump may be located in a leg of the structure or in a cellar deck of the topside structure.

A water delivery pipe for a storage tank of a storage vessel of the structure may be connected to shared water inlet and outlet pipes, which are configured to communicate water to and from a storage tank of a leg of the structure. Inlet and outlet pipes are configured for delivering fluid to or from a location and can generally be termed delivery pipes. The water delivery circuit may be connected to a separator for separating oil or oil products or residues from water.

The offshore structure may be configured so that at least one leg and/or at least one storage vessel of the structure can be used for the storage of oil during production and for the delivery of oil from the structure via an oil distribution system when required.

According to a second aspect of the present invention, there is provided a foundation for an off-shore structure, comprising any of the features of the various foundations of the structure described herein.

A further aspect of the invention provides a method of installing an offshore structure, comprising any or all of the steps of:

a) providing an offshore structure comprising any of the features of a structure described herein;

b) floating the offshore structure to an offshore location;

c) introducing water into at least one leg and at least one storage vessel of the structure to sink the lower end of the structure to right the structure in the water;

d) introducing further water into the storage vessels and/or the legs to sink the structure toward the sea bed;

e) introducing fixed ballast having a density greater than water into the fixed ballast tank or tanks; and

f) anchoring the structure the sea bed via the foundation. The fixed ballast preferably comprises magnetite.

A method of storing oil extracted from a sub-sea well is further provided comprising any or all of the steps of:

a) installing an offshore structure in accordance with the method described above or in the following description; and

b) storing oil in at least one of the legs and/or storage vessels of the offshore structure. The oil is preferably stored over water in the storage tanks of the structure.

Water is preferably introduced into at least one of the legs or at least one of the storage vessels to displace oil therefrom.

Any or all of the above features of the invention can be combined, in any combination, to provide advantages which will become further apparent on reading the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows an off-shore structure in accordance with an embodiment of the present invention;

Figure 2 shows a view onto an upper end of the structure of Figure 1 with the top side removed;

Figure 3 shows a section through the structure of Figure 1 at a lower end of the structure adjacent the foundation;

Figure 4 shows a structure of Figure 1 with an alternative foundation configuration;

Figure 5 shows a plan view of the foundation configuration of the structure of Figure

4;

Figure 6A shows internal tanks of the structure of the present invention.

Figure 6B shows a distribution system for distributing water and oil between tanks of the structure of the present invention; Figure 7 shows further details of a water and oil distribution system for the structure of the present invention;

Figure 8 shows further details of an oil distribution system for the structure of the present invention;

Figure 9 shows details of a water distribution system for the structure of the present invention; Figures 10 to 14 show steps in a method of installing a structure of the present invention;

DETAILED DESCRIPTION OF THE INVENTION As will be appreciated on reading the following description, each aspect of the invention as described in relation to each of the figures is described in relation to a same or similar structure, and so any feature or aspect of the invention described herein can be beneficially combined in a single structure, or implemented independently of other features or aspects of the invention.

Figure 1 shows an off-shore structure 1 in accordance with an embodiment of the present invention. The main upright section of the structure includes a plurality of tubular legs 11 and 12. Only two legs 11 and 12 can be seen in the side view of the structure, but in the present embodiment, four legs are included, arranged in a circular or square array around a central axis extending from a lower end 101 of the structure to an upper end 102 of the structure. Located between the legs 11 and 12 of the structure, there is provided at least one storage vessel 301. A topside portion 103 is provided on the structure and this can be provided with a variety of optional features which are generally found on offshore structures in the oil producing industry, such as a helipad 104, a crane 105, a weather deck 106, a production deck 107 and a cellar deck 108, all supported by a plurality of uprights 109. A plurality of releasable connections 10 can be provided to allow the topside 103 to be releasably connected to the off-shore structure 1 of the invention. When used in the production of oil, gas or other subsea hydrocarbons, there may be provided a plurality of conductors 1 10 for transporting gases or fluids between the topside 103 and the seabed 1 1 1, to which the structure 1 is anchored via a foundation 30. A conductor guard 1 12 may be provided surrounding the conductors 1 10, to protect them from damage by floating debris or waterborne craft passing near the structure at the level of the water's surface 1 13. The conductors 1 10 may be formed in separate sections, each section connected to another together via fatigue resistant connectors 1 14. The structure may be optionally releasably anchored to the foundation 30 via releasable connections 1 15. Figure 2 shows a view on an upper end 102 of the leg structure 1 of the off-shore structure of the present invention as shown in Figure 1. Here, a plurality of legs 1 1, 12, 13 and 14 can be seen arranged in a substantially square or circular array. At this upper end 102, the legs are connected to one another by a plurality of cross members 21, 22, 23 and 24, each of which extends longitudinally between the upper ends of the legs of the structure. These cross members may be tubular and are preferably located at, or substantially at, or near to the upper end 102 of the legs. One or more of the cross members 21 to 24 may be configured with a walk way to allow personnel to walk between legs along the cross member(s), either within the cross members or on top of the cross member(s), or on a walkway provided below or beside the cross member(s) and generally supported by the cross member(s).

The legs are spaced apart by a distance 20, which may be approximately equal to the diameter of each leg, or may alternatively be between a half of a diameter of each leg and a full diameter of each leg, or any value therebetween. The spacing may also be greater than this distance. Other useful distances may be up to 2 time or up to 3 times the diameter of each leg. Distances up to 4 times, 5 times the diameter or more may be beneficial. In one particular arrangement, the diameter of the legs is 8 metres and the distance 20 between the legs is 7 metres. A distance 200 between the central axes 13', 14', I V and 12' in a particular embodiment is approximately 15 metres. In Figure 2 the array of conductors 1 10 can be seen in plan-view, passing through conductor guard 1 12 which surrounds the conductors as described in relation to Figure 1. Conductors can be set at a non zero angle to the structure, diagonally displaced from a corner of the structure, so as to be spaced from the legs, or close to the structure depending on the topside arrangement. The conductors may alternatively be located in the centre of the structure between the legs.

Figure 3 shows a plan- view of the lower end 101 of the structure 1 of Figure 1. The array of conductors 110 can also be seen in this figure as also illustrated in Figure 2. In Figure 3 storages vessels 301, 302, 303 and 304 can be seen arranged in a square or circular array, the array of storage vessels being contained within and in between the legs 11, 12, 13 and 14. The storage vessels in the arrangement shown have a diameter 310 which is smaller than the diameter 320 of the legs of the structure. In the particular arrangement illustrated, the diameter 310 is the same distance as the spacing 20 between the legs of the structure. However, the storage vessels are located with their central axes 30Γ, 302', 303' and 304' located inwardly of the array of axis 1 Γ to 14' of the legs. This can allow connectors 330 to be located between the storage containers and the legs, to connect the storage vessels and the legs to one another. The connectors 330 may be made from a type of connection incorporating a resilient material, or a connection permitting a degree of movement between storage vessels 301 to 304 and to legs 11 to 14. This can help to avoid movement between the different parts of the structure 1 from causing fatigue in the joints or in parts of the legs and storage vessels connected to the connectors 330. In Figure 3, a plan view of a foundation 340 can also be seen. The foundation 340 has a generally square or rectanguloid outer form and is essentially formed as a hollow box. Other useful outer forms may be oval, circular or any polygonal form, such as hexagonal structures. To act as a gravity based foundation, the foundation 340 can be floated out to its desired installation position when filled with a bouyant gas or bouyant material, filled with a non-bouyant material to sink it to the sea bed and optionally subsequently filled with a material of greater density than water. Such materials may include stone, gravel, concrete and the like. The inventors have found that a particularly useful material for use in such a gravity based foundation is magnetite, which can be introduced as a slurry mixed with water. It has a high density, of the order of 5100 kilograms per cubic metre. Further, since a slurry is essentially flowable, the magnetite can also be pumped both into, and out of, the foundation if necessary, for example if the foundation needs to be refloated and moved to a new location on decommissioning or redeployment of the structure 1 to a new location. The legs 11 to 14 can be secured the foundation 340 via releasable connections 31 1, 312, 313 and 314. These can be provided in the form of sleeves on the foundation 340, into which tubes attached to the legs of the structure can be inserted and then secured by releasable fixing rings. For example, a hydra-lock swaging system, distributed by oil states MSC Limited can be used for such a purpose. This system forms a connection between concentric tubes by expanding an inner tubular member to connect with the outer tubular member by means of hydraulic pressure. This system was traditionally developed to make structural connections between off-shore jackets and their driven foundation pile templates. However, this new use of the connection to connect legs of an off-shore structure to its foundations can be advantageous. Other releasable connection means as generally known in the art can be also be used at this connection between the foundation and the legs of the structure. An alternative arrangement of a foundation 340is to use micro piles, which generally have a diameter of around 4" or 10cm. However larger diameter micro piles may be used, having diameters of up to 0.5m, preferably less than 0.3m, more preferably less than 0.15m. An array of 40 micro piles arranged in four groups of 10 piles can be advantageously used. The micro piles may be around 30m in length and installed to that depth in the sea bed. Other advantageous lengths can be between 10m and 50m, preferably 20m and 40m, more preferably between 25m and 35m. The micro-piles can be drilled into the seabed with seawater/ water-based mud using a ROV drill. Once target depth has been achieved, high pressure grout can be circulated through the micro-pile and through the foundation structure, until the annulus is full and compacted with grout or cement.

Figure 4 illustrates a further alternative arrangement of a foundation for the off-shore structure of the present invention. The structure is generally as illustrated in Figure 1, with the exception of the foundation at the lower end of the legs. For the remaining features, no further detailed explanation is given in this section since they are the same as in figure 1. The foundation of the structure in Figure 4 comprises a plurality of suction cans 40, each suction can being attached to the bottom of a respective leg 11, 12 of the structure 1. Reinforcing elements 41, shown in the form of triangular buttress elements may be provided between upper end plate 42 of each suction cans 40 and its respective leg. Other forms of reinforcing element may be beneficial here to provide support to transfer forces between the end plate of the suction can and the side wall of the leg. The illustrated embodiment comprises four suctions cans 40, although other numbers of suction cans may be useful in different types of soil. Any whole number between 2 and 12 suction cans may be beneficial, although between 4 and 8 is particularly practical from construction and installation terms, including 5, 6 or 7 suction cans. Each suction can therefore has an open end 43 at its lower end and an upper end which is closed by an end plate 42. A typical diameter for suction can 40 is in the region of 15 meters. This is found to be useful for legs of diameter of 8 metres as shown in more detail in the following Figure, Figure 5. A typical depth of the suction cans 40 is in the region of 14 meters.

Suction cans 40 can therefore be used to anchor the structure 1 to the seabed 111 by using a fluid connection to an upper end plate 42 of the suction cans to evacuate fluid therefrom. The resulting vacuum causes the hydrostatic pressure in the water surrounding the suction cans to draw the structure downward and drive the suction cans 40 into the seabed 111. The process of locating the structure at its eventual location for installation will be described in relation to later figures. However, when the structure has been towed in an upright configuration to its location, the suction cans 40 can be located on the seabed by filling legs 11 to 14 and storage vessels 301 to 304 with fluid, as will be described in more detail in relation to later figures. Fixed ballast, preferably magnetite, can be added to the structure to further improve its stability as will also be described in more detail later in the description.

Once the suction cans 40 have engaged the upper surface of the seabed 111, fluid can be pumped out from the cans, generally by pumping out the sea water which will be present in the cans, and any residual air which may have been trapped during the rotation of the rig to an upright position. An initial stage of penetration of the cans into the sea bed, due to the mere weight of the structure, will occur when the legs and storage vessels are sufficiently filled with liquid. Once a sufficient degree of initial weight-driven penetration of the cans into the sea bed has occurred, then the evacuation of fluid from the suction cans 40 can begin. In this way, the risk of hydraulic failure of the soil and ingress of water into the suction cans is reduced by that initial weight-driven penetration of the suction cans in to the sea bed. The rate at which fluid is removed from the suction cans 40 will also be carefully controlled to ensure that the structure is driven downwardly onto the seabed by hydrostatic pressure, rather than material from the seabed simply being drawn up into the suction cans. Figure 5 shows a plan view at the lower end 101 of the structure 1 shown in Figure 4. A plurality of suction cans 401, 402, 403 and 404 are attached to the bottoms of respective legs 11, 12, 13 and 14. The suction cans can each have a diameter 400 which is greater than the diameter 210 of the respective leg to which each is attached. The ratio of the diameter of the suction can to that of the leg to which it is attached is preferably greater than 1. Preferably this ratio is between 1 and 2. The width of the overall foundation must be sufficient to provide adequate stability to the structure 1 as a whole and to the topside equipment mounted to it. The ratio is more preferably therefore between 1.2 and 2, more preferably between 1.4 and 2 and even more preferably between 1.6 and 2, or in some cases more than 2. In this way, an adequate degree of stability can be achieved by the suction cans attached to each of the legs. However, if the suction cans are of too great a diameter, then secure attachment of them to each leg can become difficult and the attachment of the legs to the upper end plate 42 of each suction can may create excessive stress concentrations and could ultimately provide a weak point in the structure. For this reason, ratios in the ranges described above are advantageous.

Figure 6A shows how the legs and storage vessels of the structure of the invention are divided into separate tanks. In the preferred embodiment, each leg is divided as will be described in the following in relation to leg 11 of Figure 6A, and each storage vessel is divided as will be described in relation to storage vessel 301 in Figure 6A. However, one, some or all of the legs or storage vessels may be provided with other combinations of the tanks individually described in relation to Figure 6A. Starting at the bottom of leg 11, a fixed ballast tank 601 is provided. This fixed ballast tank is separated from the storage tank located above it and is fluidically isolated therefrom, to prevent contamination of oil and/or water stored in the storage tank 602 by the fixed ballast stored in the fixed ballast tank 601. The content of the fixed ballast tank 601 is, during the operation of the structure 1, generally not changed after installation of the structure at its chosen site. Therefore, a temporary connection may be used to install a fixed ballast during the installation of the structure at its chosen site. Alternatively, a permanent fluid connection (not shown) may be provided to connect the fixed ballast tank to equipment on the topside structure 103 for filling or emptying or changing the contents of the fixed ballast tank or tanks.

A void 601a may be provided below the fixed ballast tank, to effectively provide a double wall structure at the bottom of the legs, to provide improved strength and resilience to damage or corrosion at the lower end of the legs of the structure. A storage tank 602 is provided above the fixed ballast tank 601 in the leg 11. This storage tank 602 is provided with connections to both an oil feed and a water feed, so that when no oil is to be stored in the legs, the legs can be filled with water to provide suitable ballast to keep the structure at the desired buoyancy level. However, when oil needs to be stored in the storage tank 602, water can be pumped out or displaced from the tank by the oil introduced, as will be described more detail in relation to later figures.

Above the storage tank 602 is a variable ballast tank 603. This tank is separated from storage tank 602, and can be used to hold variable amounts water as ballast. If necessary, it can also be used for the storage of oil over water in addition to storage tank 602. When used for oil storage, this variable ballast tank may be put in fluid communication with the main storage tank 602.

Tanks 604 and 605, located above the storage tank and variable ballast tank, may be fixed voids, which are generally filled with a buoyant material, or a gas, such as air or a combination or the two. Being filled with air or another gas helps to keep the centre of gravity of the structure 1 low when in its upright position on the seabed. It is also advantageous to keep the centre of buoyancy higher than the centre of gravity to make the structure naturally self-righting. Therefore, at least a portion of one or both of tanks 604 and 605 is below the water level indicated at line 60, which can also help to improve this self-righting effect by keeping the centre of buoyancy high. Storage vessels, as indicated by 301 in Figure 6A also have a fixed ballast tank 61 1, under which a void 61 1 a may be provided as described for the leg 1 1. A storage tank 612 is also provided above the fixed ballast tank, which is connected to oil and water supplies as described for leg 1 1, so that oil or water or a combination thereof can be stored in the tank 612 as necessary. A void may also optionally be provided at an upper end of storage vessel 301 as indicated at 614.

As can be seen, the storage vessel 301 has a shorter length than the legs of the structure. In order to cope with wave energy coming from rough sea conditions, it is preferable to allow such wave energy to pass between legs 1 1 to 14 of the structure at the surface 60 of the water in which the structure 1 is installed. Keeping the storage vessels located between the legs entirely below the water level 60 helps to reduce the impact of wave motion on the general stability of the structure 1, since wave energy can more easily pass through the legs of the structure rather than being dissipated into the storage vessels located between the legs. In locations where weather conditions are more benign and therefore wave energy is less of a concern, then the height of the storage vessels may be increased and so may be higher than illustrated. Useful ratios for the height of the storage vessel(s) relative to the legs may be between a quarter to a full height of the legs. Ratios between a quarter and three quarters may also be useful in certain arrangements. Other useful ratios may be between 0.2 and 0.8, between 0.3 and 0.7, between 0.4 and 0.6, 0.5 and any combination of these mentioned ratios as upper and lower limits of a range.

Preferably longitudinal dimensions, measured from a lower end of the legs or storage vessels for a particular example are as follows. All dimensions can be varied as necessary and are beneficially within +/- 20%, or +/-10%, or +1-5%, or less, of these values:

For a particular example of a storage vessel, distances covered by each separate tank, measured from a bottom wall of the storage vessel are: Bottom wall: 0m; upper wall of double bottom cavity: 2.5m; fixed ballast tank: 2.5m to 10.5m (=8m high); main storage for oil over water: 10.5m to 60.64m (volume = 1886m ³ ); double top walls: 60.64m to 62.64m (=8m high).

For a particular example of a leg of the structure, distances covered by each separate tank, measured from a bottom wall of a leg of the structure are:

Bottom wall: 0m; upper wall of double bottom cavity: 2.5m; fixed ballast tank: 2.5m to 10.5m (=8m high); main storage for oil over water: 10.5m to 60.64m (volume = 1886m ³ ); variable ballast tank: 60.64m to 69m (volume = 609m ³ ); first void: 69m to 84.5m (i.e. 15.5m high); second void: 84.5m to 93.44m.

Figure 6B schematically illustrates a fluid distribution arrangement in the form of a piping arrangement which can be used to transfer oil or water to and from the storage tanks 602, 612 and 622 of any of leg 11, storage vessel 301, leg 12 or other legs or storage vessels provided on the structure in the manner generally described herein. In the figure, oil pipes appear as solid lines and water pipes appear as solid lines with a thinner dashed line alongside them.

Starting with leg 11, a water delivery and extraction pipe 661 is provided for delivering water to the bottom of tank 602 when connected to a source of water. When connected to a pump for pumping water out of the tank, then the pipe 661 can be used to extract water from tank 602 as necessary. Such connection refers to a connection providing a fluid communication path to or from the water source or to the pump. As is common in hydraulic systems, components can be constantly mechanically connected to one another, but only placed in fluid communication with one another on opening of a valve placed between the two, to open one component to the other component in the hydraulic sense.

Pipe 661 can be connected, preferably via a suitable control valve, to a water storage tank found on the topside structure, via outlet 664, and can alternatively be connected to a discharge to adjacent waters (i.e. the sea or external environment) via outlet 662 or via a sea chest 663 located below water level 600. Sea chest 663 can also be used to draw new sea, preferably into the topside water storage tank, via pipe 664 for filtering and/or for return into tank 602, when oil is being extracted from the storage tank 602. A second pipe 665 also extends into tank 602, towards an upper end of tank 602. Pipe 665 is connected to an oil inlet pipe 666, through which extracted oil from the production or extraction process can be introduced to the top of tank 602. This is generally the case when oil is being extracted from the seabed, but no pipeline or tanker is available to receive the extracted or produced oil and deliver it immediately to shore, and so intermediate storage is required at the structure 1.

When oil is being introduced into the top of the tank 665, water is removed via pipe 661. Removal of the water will either be passive, due to the pressure increase brought about by the oil being introduced by pipe 665, or alternatively the water may be actively pumped out, by pumping equipment located on the topside structure, via pipe 664.

A deep well type pump 667 is connected to oil outlet pipe 668. When a transport tanker is available, oil can be pumped out of storage tank 602 via pump 667 and outlet pipe 668 to the tanker, or to a pipeline if eventually available. When oil is being removed from the storage tank 602 via pipe 665 and pump 667, water may be reintroduced to the bottom of the storage tank 602 by either introducing sea water directly via sea chest 663, or more preferably by introducing filtered water from the topside structure via the pipe 664. In this way, the storage tank 602 is provided with a water inlet and outlet pipe having an opening disposed toward the bottom of the storage tank 602, and an oil inlet and outlet pipe 665 having an open end disposed towards the upper end of storage tank 602. This allows oil to be stored over water in the same tank 602. To maintain the required ballast level in the leg regardless of the amount of oil to be stored, water can be introduced to the storage tank when oil is being removed and, conversely, water can be removed from the tank when oil is being introduced to upper parts of the tank. A corresponding arrangement is provided for leg 12, so that water can be introduced into storage tank 622 and oil may be stored over water held in that storage tank as ballast. To enable this, a water inlet and outlet pipe 671 is provided, having an opening disposed to towards the bottom of the tank 622. In the same manner as for tank 602, the water inlet and outlet pipe 671 is connected to a sea chest 673 and to an inlet and outlet pipe 674. A further outlet pipe 672 is provided in corresponding manner to pipe 662 for tank 602. As is the case for for tank 602, tank 622 is provided with an oil inlet and outlet pipe 675, which is connected to an oil inlet pipe 676, and also to a pump 677 and related oil extraction pipe 678.

As can be seen in the figures, the tank 612 located in the storage vessel 301 does not extend to the upper ends of the legs 11 and 12 for the reasons set out in the description above. For this reason, it is necessary for the required water and oil inlet and outlet pipes to pass via an adjacent leg, leg 12 in the illustrated example to reach the topside equipment. The related inlet and outlet pipes are therefore, at least in part, located in the upper portion of leg 12, as shown in the figure. For tank 612, water inlet and outlet pipe 681 passes from the tank 612 in the storage vessel 301 into the leg 12 and then is attached to the sea chest 673, which is shared with the tank 622 in leg 12. The water inlet and outlet pipe for the tank 612 may then also be connected to water inlet and outlet pipes 672 and 674, which may therefore be shared with tank 622 in leg 12, although separate connections for tank 612 could alternatively be provided. Tank 612 in storage vessel 301 may also have an oil inlet pipe 686 for the inlet of oil to the tank 612, and an oil outlet pipe 688, for the outlet of oil, which is connected via a pump 687, in a similar manner to the oil extraction pipes 668 and 678 of the tanks 602 and 622 located in the legs 11 and 12.

The structure can therefore be provided with oil over water storage tanks in one or more of its legs and also in one or more storage vessels located between the legs. The storage vessels can have a height lower than the height of the legs. Oil and water can be delivered to and removed from the storage vessel or vessels via oil and water distribution pipes, which pass from the storage vessel via the leg or legs of the structure and on to the topside equipment. Pumping equipment for the storage vessel or vessels may also be located in the leg or legs of the structure.

A skimmer hose 680 may be connected to the slop tank in one or more of the legs, for extracting any spilt oil from the water's surface and delivering it to the slop tank for forwarding to separation and/or storage or treatment equipment in the legs or in the topside structure.

Figure 7 shows a distribution control circuit for use in delivering and receiving water and oil to and from the storage tanks in the one or more legs and the one or more storage vessels of the structure. The illustrated example comprises four legs and four storage vessels, but other numbers of legs and storage vessels can be envisaged and suitable numbers of inputs and outputs configured accordingly. The system comprises a water distribution circuit 711, which is connected at a first inlet point, 710 to a water mains supply located remotely from the storage tanks, preferably at the topside structure. The circuit 711 comprises an oil content monitor 712, and an overboard valve 713 for releasing water overboard on condition that the oil content monitor 712 detects a suitably low level of oil content, which is sufficiently low to be acceptably discharged overboard when the water is no longer required. Such acceptable levels are generally defined by the local or international environmental standards and specifications. Water delivery circuits 701 to 708 are provided with control valves 711 to 718. These are for controlling the delivery of water to the eight individual storage tanks located in the legs and the storage vessels of the structure. A water delivery circuit with a related control valve is therefore provided for each of the legs and for each of the storage vessels. The water distribution circuit 711 is also provided with a corrugated plate interceptor (CPI) 720, which is selectively connected to, or disconnected from, the water distribution circuit 711 via valve 719. Clean water output from the corrugated plate interceptor (CPI) is then delivered to an overflow box 721 via a second oil content monitor. Water from the overflow box 721 can then be vented overboard board via a further control valve 723 as required. Oil is delivered from the CPI to a separate part of the overflow box 721, or to a separately provided overflow box from the overflow box 721, via an oil pipe 724. A further oil pipe 725 connects the overflow box to a slop tank 726. A pump 727, of the deep well kind, is provide in slop tank 726 and can extract the oil delivered to the slop tank 726 from the overflow box to an oil return delivery line 730. The returned oil on line 730 can be delivered either to a filling circuit 740 via a valve 731, or to a loading circuit 750 via a valve 732. The loading circuit can be selectively opened to loading hoses 760 when loading to a tanker is desired. Oil distribution circuits 71 to 78 are configured to be connected to each of the eight storage tanks located in the four legs and four storage vessels of the structure in the illustrated example. These can be selectively opened to the loading circuit 750 by valves 721 to 728 and loading valve 729. This allows oil from the storages tanks to be communicated to loading hoses 760. A crude circuit 770 can also be opened to the oil delivery circuits 71 to 78 via crude delivery valve 771. The crude circuit can also be directly connected to the loading hoses by crude loading connection valve 772. This allows crude oil to be delivered to the oil distribution circuits 71 to 78 to store crude oil in the storage tanks, or optionally allows the crude oil to be delivered to the loading hoses.

As will be appreciated, when crude is delivered to the storage tanks via oil distribution circuits 71 to 78, water will be vented from the storage tanks via water distribution circuits 701 to 708. The water removed from the tanks can be delivered back to the water main via circuit 711, and/or out of circuit 711 via the CPI and overboard if the water is suitably clean.

Figure 8 shows a schematic representation of the eight storage tanks in the legs and storage vessels of the structure 1, labelled in the diagram as cells 1 to 8, cells 1 to 4 indicating the storage tanks of the legs, and cells 5 to 8 indicating the storage tanks of the storage vessels. Each cell has associated with it a loading pump numbered in the diagram as 81 to 88. These are connected to the oil distribution circuitry illustrated in Figure 8 and each of pumps 81 to 88 corresponds to the pumps 667, 677 and 687 illustrated in Figure 6B.

Each of the pumps 81 to 88 is connected via a respective first filling connection 811 to 818 to a filling ring 810. An offloading ring 820 is also provided and each pump is also connected to this via connections 821 to 828. Each of the connections is provided with a valve as illustrate in the figure. Each of the rings 810 and 820 is provided with a series of connections to external systems as follows. A first connection 801 connects the offloading ring to a tanker loading circuit for offloading oil to a tanker, preferably via hoses 760. A second connection 802 connects the slop pump to the offloading ring. A third connection 803 connects the product process delivering crude oil to the filling circuit. A fourth connection 804 connects the slop tank to the filling ring. A fifth connection 805 connects both rings to the tanker loading circuit for offloading oil to tankers. Therefore, the oil distribution system shown in Figure 8 can comprise a filling ring selectively connectable to a loading pump of each of the legs and/or storage vessels of the structure. It may further comprise an offloading ring selectively connectable to one or more of the loading pumps of one or more of the legs and/or storage vessels. The filling ring and the offloading ring may both be connectable to a tanker loading circuit. The offloading ring may be connected to a slop pump and the filling ring may be connected to a slop tank. The filling ring may be connected to a feed of crude oil from the product process.

Figure 9 shows a water distribution circuit for delivering water to one or more of the legs and/or storage vessels of the structure. As in Figure 8, each of the storage tanks in the respective legs and storage vessels are numbered cells 1 to 8, cells 1 to 4 indicating the storage tanks of the legs, and cells 5 to 8 indicating the storage tanks of the storage vessels. The water main ring 90 is connected to each of cells 1 to 8 via water main delivery lines 91 to 98. Each water main delivery line 91 to 98 is provided with a valve to selectively open or close its fluid connection to the water main ring 90. The water main ring is connected at a first point 901, via a valve, to a separator for separating oil from water, such as the CPI 720 of Figure 7. The water main ring 90 is also connected at a second point 902 to an overboard, via a valve such as valve 713 of Figure 7. One or more of the tanks comprises a sea chest as indicated at 911 to 918, corresponding to sea chest 663 and 673 of Figure 6B. In practice a sea chest may not be necessary for each tank. Depending upon the capacity and fill rate required for the tanks, one or two or more sea chests may be sufficient, or any number up to the number of tanks in the system. If large flow rates or redundancy are required, more than one sea chest may be provide in one or more of the tanks. The water main delivery lines 91 to 98 deliver water to or from the storage tanks and to or from the sea chests shown in the figure and as numbered 663 and 673 in Figure 6B. Control valves can be provided to selectively draw water in via the sea chest as water is introduced in to the tanks. When water being removed, then rather than being pumped out of the sea chest, the water is pumped out via separation and/or filtration equipment to clean the water before being returned overboard to the surrounding waters. One or more of the water or oil distribution rings may be located in the topside equipment, on its cellar deck or in the tops of the legs of the structure for ease of access and maintenance.

Figures 10 to 14 illustrate a method by which the structure of the invention can be transported to and installed at a chosen location. It will be understood that the reverse of this process can be carried out for decommissioning and relocation of the structure as and when necessary.

The structure can be delivered to the vicinity of its ultimate destination via a heavy lift vessel (HLV). Such a delivery method is known in the industry and is described in relation to a previous version of the platform described in Patent Application US 2012/0093587 Al, application number 13/275,648, filed on 18 October 2011, which is hereby incorporated herein by reference. The transporting and floating-off steps may be carried out as described in that document, although since the structure of the present invention has differences from the structure described therein, the method can be adapted to the present invention as set out in the following description. As alternatives to these methods, the structure cam also be towed to its location from a slipway, obviating the need for a HLV. If a sufficiently sized crane is available, the structure can also be lifted into place by a suitably sized crane. The following method may therefore be carried out replacing one or more of the earlier steps with towing or lifting into place by a crane.

Figure 10 shows the structure 1 of the present invention when it has been floated free from the HLV and is lying on its side, buoyant in the water. The water level is indicated by line 1000. As described in relation to earlier Figures, the structure comprises legs 1010 and 1020, which are provided with fixed ballast tanks 1011 and 1021. Each leg is also provided with a storage tank 1012 and 1022, respectively. Each leg is also provided with a variable ballast and/or storage tank 1013 and 1023, respectively, and each is provided with one or more voids 1014 and 1015 and 1024 and 1025, respectively. Storage vessel 1030 is, as described in relation to earlier Figures, also provided with a fixed ballast tank 1031 and a storage tank 1032. As illustrated in Figure 11, in order to begin to rotate the structure 1 toward an upright position, in a direction of arrow 1100, one or more, of fixed ballast tanks 1021, 1031 and 1011 can begin to be flooded, either partially or completely, with water, or with other heavy materials preferably having a density equal to or greater than that of water. Similarly, storage tanks 1022 to 1032 and 1012 can be gradually flooded, for example to a level indicated by line 1101. This acts to move a centre of gravity G of the structure 1 towards a lower end 101 of the structure. This causes the structure to tend towards the orientation illustrated in Figure 12.

As shown in Figure 12, the structure 1 will eventually reach an upright position, with the fixed ballast tanks 1021, 1031 and 1011 all full of water or other material, and the storage tanks 1012, 1032 and 1022 all at least partially filled with water. A preferred sequence is to at least partially fill the storage tanks with water to upend the structure and to then further ballast the structure to lower the structure in the water. Once in its upright position, and with the appropriate ballast in place, as long as the centre of gravity G remains below the centre of buoyancy B, then the structure 1 will tend towards the upright position indicated in Figure 12. Once this equilibrium has been reached, the structure can be towed by tug boats more precisely towards its eventual position of installation. In order to install the lower end 101 of the structure 1 at the sea bed, it is necessary to lower the structure 1 in the water. This can be done by filling additional water in the storage tanks 1012, 1022 and 1032 with water and also by at least partially filling variable ballast tanks 1013 and 1023 with water, for example, to the line indicated 1300 in Figure 13. Fixed ballast may also be introduced into the fixed ballast tanks 1011, 1021 and 1031. Such fixed ballast may be material having a greater density than water and a particularly effective form of fixed ballast has been found to be magnetite. The magnetite can be introduced in the form of a slurry mixed with water, which also makes it convenient for flowing into the fixed ballast tanks and in pipe work (not shown), which can be provided either internally or externally to the legs and/or storage vessel or vessels of the structure 1 for delivering fixed ballast to the fixed ballast tanks.

As illustrated in Figure 14, once in the appropriate location, the structure can be lowered into its final resting position by filling the fixed ballast tanks and variable ballast tanks to the desired final levels (not shown) to give the desired buoyancy and stability at its final location. Where a removable foundation 140 is used, the structure 1 can be attached to the foundation its final position. Alternatively, where the foundation 140 is provided in the form of suction cans 40 as illustrated in Figure 4, then the structure can be sucked down onto the seabed as described in relation to Figures 4 and 5. The topside structure 103 can be floated over the structure 1 by the HLV and attached to the topside of the structure 1 as appropriate, either before or after installation of the structure 1 in its operational position. The topside structure can also be lifted onto the structure 1 by a crane depending upon the size or weight of the topside structure. The structure 1 can then be used in oil exploration and/or oil production operations and the legs and storage vessels can be used for the storage of oil during production and for the delivery to tankers as and when necessary.