Offshore fluid handling

Abstract

An offshore petroleum production system (100), the system (100) comprising: a subsea production well assembly (10), a riser (20) anchored at a first end part (20a) thereof to a sea floor (1), a tank assembly (30) fixed to a second end part (20b) of the riser (20), the tank assembly (30) being buoyant and supported vertically by the riser (20), wherein the tank assembly (30) comprises a storage tank (31) fluidly connected to the subsea production well assembly (10) and arranged to receive produced petroleum fluids from the subsea production well assembly (10).

Description

[0001] The present disclosure relates to offshore fluid handling, for example for temporary storage of fluids at offshore locations. Examples include offshore petroleum production systems for handling and transport of petroleum products from offshore petroleum fields. Other examples include temporary storage of fluids for injection into wells or subterranean reservoirs, or temporary storage of fluids for other purposes, such as energy carrier fluids or fluids used in subsea fluid processing and transport systems.BACKGROUND

[0002] Production of petroleum products from offshore fields is well-established and contributes a considerable amount of the petroleum production in many regions of the world. Such fields can be established with a fixed or floating platform at the offshore location, and production equipment on the platform to handle extracted petroleum products via a riser structure. In some cases, subsea wells are established and the wellhead and production equipment are located at the sea floor. In both cases, the produced products are transported further via for example pipelines to shore or a loading facility for transport via tanker ships.

[0003] The investments required to establish such offshore fields can be considerable, particularly if the petroleum field in question is located remote from shore or from other fields with an established infrastructure for transport of petroleum products.

[0004] Consequently, even if a discovered petroleum field may contain recoverable hydrocarbons, the investment cost for transport infrastructure may be prohibitive and make the field financially non-viable to develop. Such so-called marginal fields are therefore often left undeveloped.

[0005] In other applications, there may be a need or it may be desirable to inject fluids into a well or subterranean reservoir. Examples of this include chemicals injected into wells for well intervention purposes, or CO2 injected into subterranean reservoirs for enhanced oil recovery and / or permanent disposal of the CO2.

[0006] In yet other applications, there may be a need or it may be desirable to store other types of fluids offshore, such as fluids used in subsea processing and transport systems. For example, in conjunction with subsea petroleum wells there may be processing equipment arranged subsea for handling and further transport of produced oil & gas. Such processing equipment may employ other fluids (like additives) in for example treatment of produced oil & gas to improve transportability, e.g. via pipeline to shore or to a remote platform.

[0007] In yet other applications, there may be a need or it may be desirable to store other types of fluids offshore, such as energy carrier fluids. Such fluids may, for example, comprise synthetic fuels produced by renewable energy installations offshore, which are temporarily stored before being collected for further transport or use.

[0008] As the world's demand for energy continues to rise, there is a need for improved technology relating to handling and transport of petroleum products from offshore fields, and for handling other types of fluids offshore. The present disclosure has the objective to provide such improvements, or at least to provide useful alternatives to the current state of the art.SUMMARY

[0009] In an embodiment, there is provided an offshore petroleum production system, the system comprising: a subsea production well assembly, a riser anchored at a first end part thereof to a sea floor, a tank assembly fixed to a second end part of the riser, the tank assembly being buoyant and supported vertically by the riser, wherein the tank assembly comprises a storage tank fluidly connected to the subsea production well assembly and arranged to receive produced petroleum fluids from the subsea production well assembly.

[0010] The detailed description below and appended claims outline further inventive aspects and embodiments.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The above and other characteristics will become clear from the following description of illustrative, non-restrictive examples, with reference to the attached drawings, in which:

[0012] FIG. 1 is a schematic view of an offshore petroleum production system.

[0013] FIG. 2 is a schematic view of an offshore petroleum production system.

[0014] FIG. 3 is a schematic view of an offshore petroleum production system.

[0015] FIGS. 4 and 5 illustrate schematically tank assemblies for use with an offshore petroleum production system.

[0016] FIG. 6 illustrates a tank assembly according to an example.

[0017] FIG. 7 illustrates a tank assembly having an external buoyancy tank.

[0018] FIG. 8 is a schematic view of an offshore petroleum production system.

[0019] FIG. 9 illustrates a system for receiving and temporarily storing fluids offshore.DETAILED DESCRIPTION

[0020] FIG. 1 illustrates an offshore petroleum production system 100 according to one example. The system 100 comprises a subsea production well assembly 10 arranged at a sea floor 1. The subsea production well assembly 10 comprises a wellbore 11 extending to a subterranean reservoir 12, a wellhead foundation 13 arranged at the sea floor 1, and a valve arrangement 14 (for example a so-called valve tree or Christmas tree) arranged at a wellhead supported by the wellhead foundation 13. The skilled reader will be familiar with this common arrangement of a subsea production well assembly 10. Further components may be part of the subsea production well assembly 10 arrangement and / or connected to it, such as processing equipment, other flow control components, etc. There may also be more than one subsea production well assembly 10 or more than one wellbore 11 and other components associated with the subsea production well assembly 10, for example, a marginal field may comprise 1-3 subsea production well assemblies 10, which may also share local sea floor infrastructure.

[0021] A riser 20 is fixed to the sea floor 1 and extends upwardly from the sea floor 1 in a substantially vertical orientation. The riser 20 is fixed to the sea floor 1 at a first, lower end part 20a of the riser 20. In the illustrated example, the riser 20 is fixed to the sea floor 1 via a foundation 21, in this case shown as a suction anchor foundation. Other types of foundation may equally well be suitable for this purpose. Optionally, the riser 20 may share a common foundation with the subsea production well assembly 10, such as a template structure forming the wellhead foundation 13 and also providing support for the riser 20.

[0022] A tank assembly 30 is fixed to a second, upper end part 20b of the riser 20. The tank assembly 30 is buoyant, i.e. it has positive buoyancy in water. The length of the riser 20 and the height of the tank assembly 30 are such that the tank assembly 30 is fully submerged, i.e. located below the sea surface 2. The tank assembly 30, by means of its buoyancy, is arranged to maintain or help maintain the riser 20 in a substantially vertical orientation and under tension. The tank assembly 30 thus holds or contributes to hold the riser 20 vertically oriented and prevents the riser 20 from buckling or collapsing.

[0023] The riser 20 may comprise one or more interconnected pipes, such as steel pipes, and may be made up of a plurality of interconnected riser sections. The riser 20 may have one or more fluid pipes therein, which may or may not be under tension. For example, the riser 20 may comprise a central fluid pipe held in tension by the buoyancy force from the tank assembly 30, or it may comprise a fluid pipe and one or more auxiliary tension member (e.g. auxiliary pipe(s) and / or steel wire(s)) which relieve the pipe such that the central pipe is under less or no tension.

[0024] In some examples, the riser 20 may itself comprise buoyancy members arranged thereon, such as to maintain a vertical orientation also without a tension force provided by the tank assembly 30.

[0025] The tank assembly 30 further comprises a storage tank 31 fluidly connected to the subsea production well assembly 10 and arranged to receive produced petroleum fluids from the subsea production well assembly 10. The storage tank 31 provides a temporary storage for petroleum fluids. The storage tank 31 may, for example, be a container structure arranged inside or otherwise in conjunction with the tank assembly 30. The tank assembly 30 and the storage tank 31 may optionally be formed integrally, so that the tank assembly 30 is or makes up the storage tank 31.

[0026] The fluid connection between the subsea production well assembly 10 and the storage tank 31 may preferably be via one or more fluid pipes in (or part of) the riser 20 or, optionally, one or more fluid pipes fixed to the riser 20. Illustrated in FIG. 1, connector pipes (for example so-called jumpers) 15,16 may be arranged to convey produced petroleum fluids from the subsea production well assembly 10 to the riser 20 and to the storage tank 31. Flow control and processing equipment, which may include e.g. valves, throttles, pumps, separators and / or fluid conditioning equipment, may be arranged at the subsea production well assembly 10, in or at the valve arrangement 14, and / or between the subsea production well assembly 10 and the storage tank 31. In FIG. 1, equipment 17 between the subsea production well assembly 10 and the storage tank 31 is illustrated. A pump package may be arranged as part of the flow control and processing equipment, between the subsea production well assembly 10 and the connection to the riser 20. In some examples, the pressure in the reservoir 12 may be sufficiently high that no pump package is required, and that fluid flow to the storage tank 31 is driven by reservoir pressure.

[0027] In the illustrated example, an offloading arrangement 40 comprises an offloading pipe 41 configured to be retrieved by a tanker vessel 42 (such as a shuttle tanker) and for moving petroleum fluids from the storage tank 31 to the tanker vessel 42. In the illustrated example, the offloading pipe 41 is retrievable by the vessel 42 via an offloading buoy 43, and the offloading pipe 41 is arranged in a catenary between the tank assembly 30 and the offloading buoy 43.

[0028] In an alternative configuration, the tanker vessel 42 may have an offloading pipe 41 which is lowered into the water and down to the tank assembly 30 for connection e.g. at a top part of the tank assembly 30. This configuration is illustrated schematically in FIG. 3. The offloading pipe 41 may, for example, comprise an end connector which is landed and locked in a corresponding connector receiver in or at the tank assembly 30. A suitable pump arrangement, for example arranged in the offloading pipe 41 or in the tank assembly 30, may be provided to move the petroleum fluids to the tanker vessel 42.

[0029] Illustrated in FIG. 2, in one example the storage tank 31 may be releasable from the riser 20. In this example, the storage tank 31 may be fluidly connected to the riser 20 and receive petroleum fluids from the subsea production well assembly 10 and subsequently be released from the riser 20 for transporting the petroleum fluids away while contained in the storage tank 31. The storage tank 31 may, for example, be configured to be towed by a vessel 42′, which in this case does not have to be a tanker vessel but can be, for example, an offshore service vessel. The storage tank 31 may, for example, be towed to shore or a quayside for emptying or towed to another offshore location (such as another petroleum field installation) having suitable infrastructure for handling and further transport of the petroleum fluids. The storage tank 31 can then be emptied and returned for re-connection to the riser 20. In the illustrated example, the storage tank 31 is arranged in a first tank assembly part 30a which can be released from the second tank assembly part 30b.

[0030] Alternatively, the storage tank 31 may make up or be part of an own structure which is disconnectable from the tank assembly 30. Yet alternatively, the entire tank assembly 30 (see FIG. 1) may be disconnectable from the riser 20 for this purpose. In the first case, as illustrated in FIG. 2, the second tank assembly part 30b may be designed to have sufficient buoyancy to maintain the required tension in the riser 20 and / or to have a required self-buoyancy also with the first part 30a disconnected. In the latter case, one may provide a second tank assembly 30 which is fixed to the riser 20 as or before a first tank assembly is disconnected. In this manner, save possibly for the short switching process, one tank assembly 30 is always connected to the riser 20, thereby also permitting substantially continuous loading via the riser 20. For example, one may design the system 100 with two tank assemblies 30 having a capacity such that the filling of one tank assembly approximately matches the time required to tow away and empty the other tank assembly.

[0031] In other examples, illustrated in FIG. 3, the riser 20 may have more than one storage tank 31a, b attached to it, for example two, three or four storage tanks 31a, b, and the storage tanks 31a, b can be disconnected at different times, such that there is always one storage tank 31a, b attached to the riser 20 for receiving petroleum fluids. The storage tanks 31a, b may be arranged as removably connectable to a tank assembly 30, for example where a tank assembly part 30b has connection arrangement to allow each of the storage tanks 31a, b to be connected thereto to disconnected therefrom. Alternatively, the second tank assembly part 30b may be replaced by a feature on or part of the riser 20 which permits releasable connection of the storage tanks 31a, b individually.

[0032] Fluid connection between the one or more releasable storage tank(s) 31a, b and the riser 20 may be provided by a connector arranged for example in a landing feature at the riser 20 or at the tank assembly part 30b. This may, for example, be a vertically arranged connector arrangement where a connector part on the respective storage tank 31a, b lands vertically into a corresponding connector part in the tank assembly part 30b and a fluid connection is subsequently made up. Alternatively, or additionally, the fluid connection may be provided by e.g. a flexible connection 37, illustrated schematically in FIG. 3, which may be connected externally to the storage tank(s) 31a, b. The flexible connection can, for example, be made up with the assistance of a remotely operated underwater vehicle (ROV), a robotic arm, or by other suitable means.

[0033] FIGS. 4 and 5 illustrate an example of an arrangement of multiple storage tanks onto one tank assembly part 30b which is fixed to the riser 20. In FIG. 4, four storage tanks 31a-d are shown, individually arranged as indicated in the top view of the tank assembly 30 shown in the upper part of the figure. In FIG. 5, three storage tanks 31a-c are used. Each of the storage tanks 31a-c may be arranged as part of a tank assembly part 30a, as described in relation to FIG. 2.

[0034] The riser 20, tank assembly 30 (e.g., the tank assembly part 30b) and / or the storage tank(s) 31,31a-d may contain necessary valves and other flow control components to control fluid flow to the individual storage tank(s) 31a-d.

[0035] FIG. 6 illustrates an example of a tank assembly 30 as illustrated in FIG. 1. The tank assembly 30 in this example comprises a storage tank 31 in the form of a compartment 35 in the tank assembly 30. In configurations where the storage tank 31 is releasable, the storage tank 31 may be arranged in a tank assembly part 30a which is releasable from tank assembly part 30b, as described above. The configuration in FIG. 6 may also be used for each of a plurality of storage tanks 31a-d, for example in configurations as shown in FIGS. 4 and 5.

[0036] The compartment 35 is here illustrated partly filled with petroleum fluids 32. The tank assembly 30 in this example further comprises a gas compartment 34 for buoyancy. The gas compartment 34 may contain air or another suitable gas or gas mixture. In some examples, the gas compartment 34 may be a bladder or equivalent container. The tank assembly 30 may have arrangements for varying the buoyancy, for example the tank assembly 30 may have a pump arrangement operable to pump sea water into or out of the gas compartment 34 so as to change the volume available for the gas. FIG. 6 illustrates water 33 having been pumped into the gas compartment 34, whereby the gas contained therein will be compressed, and the buoyancy of the tank assembly 30 is reduced. In this manner, by pumping water into or out of the gas compartment 34, the buoyancy of the tank assembly 30 can be regulated, for example in order to maintain a close to constant buoyancy force on the riser 20 as the storage compartment 35 is gradually filled with petroleum fluids.

[0037] An extra buoyancy tank 39, illustrated schematically in FIG. 7, may be fixed to the tank assembly 30 as required, for example externally to the tank assembly 30 and / or separately but mechanically connected to the tank assembly 30 e.g. via wire ropes or other means.

[0038] In any of the examples and embodiments described herein, the buoyancy of the tank assembly 30 or individual storage tank(s) 31,31a-d may be variable such that the buoyancy is reduced when installing the tank assembly 30 and / or the storage tank(s) 31,31a-d.

[0039] Illustrated in FIG. 2, but applicable to any of the examples or embodiments described or claimed herein, the system 100 may comprise one or more flexible joints 38a, b connecting the riser 20 to the foundation 21 and / or connecting the riser 20 to the storage tank(s) 31,31a-d. A flexible joint 38a may, for example, be arranged at the lower end part 20a of the riser 20 at the interface towards the foundation 21 and / or at the interface between the upper end part 20b of the riser 20 and the tank assembly part 30b. This can assist in limiting bending loads applied to or from the riser 20 during operation.

[0040] FIG. 8 schematically illustrates an example exchange operation for a storage tank 31a-d in a tank assembly 30 comprising a plurality of storage tanks 31a-d. A full storage tank 31a is released from the riser 20. This can be done with the assistance of an ROV 36 which is controlled from the vessel 42′, for example as illustrated in this example by operating the ROV 36 to release the storage tank 31a from the tank assembly part 30b. A replacement, empty storage tank 31d is provided and installed in the place of the removed storage tank 31a. This can similarly be done with the assistance of the ROV 36, for example to make up structural and / or fluid connections to the replacement storage tank 31d.

[0041] The storage tanks 31a-d may be towed by the vessel 42′ between their installation site and another site, such as a quayside or offshore unloading facility. This reduces the requirements for the vessel 42′.

[0042] By temporary storing petroleum fluids in a storage tank 31 being part of a tank assembly 30 as described in the examples herein, the need for sea floor structures for storage can be eliminated or reduced. Such conventional sea floor storage tanks may need to be ballasted down and / or anchored to the sea floor to avoid floatation.

[0043] Access to such a sea floor storage may also be challenging, particularly in deep waters.

[0044] The riser 20, tank assembly 30 and other components may be re-usable. For example when an existing marginal field is depleted, some or all of the components in the system 100 can be moved to a new field and continue to be used in the same manner as described above.

[0045] In other examples, systems and methods are provided for temporary storage of fluids for injection into wells or subterranean reservoirs, or temporary storage of fluids used in subsea fluid processing and transport systems.

[0046] The arrangement of such systems may be equivalent to that described in relation to FIGS. 1-8, however with the difference that the tank assembly 30 is configured to hold a fluid which is not produced petroleum but which is a fluid for injection or other use. With reference to FIGS. 1-8, such as offshore system 100 may comprise a subsea well assembly 10 with a wellbore 11 extending from a sea floor 1 to a subterranean reservoir 12, a riser 20 anchored at a first end part 20a thereof to the sea floor 1, with a tank assembly 30 fixed to a second end part 20b of the riser 20 and the tank assembly 30 being buoyant and supported vertically by the riser 20. The tank assembly 30 comprises one or more storage tank(s) 31,31a-d.

[0047] The storage tank(s) 31,31a-d may in some examples be fluidly connected to the subsea production well assembly 10 via the riser 20 and arranged to hold fluids for injection into wellbore 11 and / or the subterranean reservoir 12. The connection may be via processing equipment 17, and may be via suitable connector pipes (jumpers) 15,16 at or adjacent the sea floor 1. The well 11 and reservoir 12 may, for example, require well intervention at some points during its operational lifetime, and suitable fluids therefor may be provided from the storage tank(s) 31,31a-d.

[0048] Alternatively, or additionally, the storage tank(s) 31,31a-d may in some examples be fluidly connected to processing equipment 17 and arranged to hold fluids, such as additives, for injection into a petroleum stream from the subsea well assembly 10.

[0049] Such injection can be carried out via the processing equipment 17, which may be part of a subsea production system. The petroleum stream may, for example, need to be provided with additives prior to being led (such as pumped) away from the well assembly site for example via subsea pipelines to a remote location. Suitable fluids therefor may be provided from the storage tank(s) 31,31a-d.

[0050] The processing equipment 17 may comprise flow control equipment, such as valves, dosing equipment, pumping equipment or other relevant flow control equipment, operable to control an injection of the fluids for well or reservoir treatment, and / or fluids for injection into the produced petroleum stream.

[0051] Such fluids may, for example, be acid or other stimulation fluids, scale inhibitors, corrosion inhibitors, MEG, methanol, chemicals for wax / hydrate management, or other fluids used for well / reservoir intervention, flow assurance or other purposes.

[0052] In one example, the system 100 may be used to produce petroleum fluids from the subsea production well assembly 10 and when well intervention is required, the production of petroleum fluids may be temporarily halted and fluids for well or reservoir treatment injected into the wellbore 11 and / or the reservoir 12 from the storage tank(s) 31,31a-d.

[0053] In one example, the system 100 may be used to produce petroleum fluids from the subsea production well assembly 10 while injecting additives into a produced petroleum stream from the subsea production well assembly 10, with the additives being provided from the storage tank(s) 31,31a-d.

[0054] One or more pumps for pressurizing the fluids provided from the storage tank(s) 31,31a-d may, for example, be provided in the processing equipment 17 and / or in the riser 20 or tank assembly 30. Similarly, valves or other flow control equipment can be provided in the processing equipment 17 and / or in the riser 20 or tank assembly 30. The pump(s), valves and flow control equipment may also be operated in conjunction with the valve arrangement (valve tree) 14 to control flow into or out of the wellbore 11 and reservoir 12.

[0055] Advantageously, additives and / or fluids for well or reservoir treatment can be loaded to the storage tank(s) 31,31a-d from a vessel 42 via a loading arrangement 40′, as illustrated in FIGS. 1 and 3. The loading pipe 41′, loading buoy 43′ and associated components of the loading arrangement 40′ may be configured principally similar as for the offloading arrangement 40 described above. Alternatively, or additionally, the additives or fluids may be provided by moving a releasable storage tank 31,31a-d comprising such fluids or additives to the well assembly site with a vessel 42′ and connecting the releasable storage tank 31,31a-d to the riser 20, as illustrated in FIGS. 2 and 8.

[0056] In one example, the system 100 may be used for carbon dioxide (CO2) handling and particularly for temporary storage and subsequent injection of CO2 into the subterranean reservoir 12 for permanent storage. In this example, the storage tank(s) 31,31a-d is (are) configured to temporarily hold CO2 and the processing equipment 17 comprises flow control equipment, such as pumping equipment, operable to control an injection of the CO2 into the reservoir 12.

[0057] CO2 can for this purpose be loaded to the storage tank(s) 31,31a-d from a vessel 42 via a loading pipe 41′. Alternatively, CO2 may be provided by moving a releasable storage tank 31,31a-d comprising CO2 with a vessel 42′, and connecting the releasable storage tank 31,31a-d to the riser 20. Yet alternatively, CO2 may be provided via a pipeline, for example from a shore-based location, fluidly connected to the storage tank(s) 31,31a-d.

[0058] In this manner, the system 100 can provide buffer storage of CO2 at the well assembly site, whereby CO2 injection into the reservoir 12 can be carried out at a stable and efficient rate.

[0059] In any of the examples described above, the offshore system 100 may further comprising a loading arrangement 40′, wherein the loading arrangement 40′ comprises a loading pipe 41′ configured to be operatively engaged by a vessel 42 for providing fluids to the storage tank(s) 31,31a-d, as illustrated in FIG. 1.

[0060] Alternatively, the tank assembly 30 may be configured to receive an loading pipe 41′ for providing fluids from a vessel 42, as illustrated in FIG. 3.

[0061] The storage tank(s) 31,31a-d may be releasable from the riser 20 and operable to be moved by a vessel 42′, as illustrated for example in FIGS. 2 and 8.

[0062] A plurality of storage tanks 31,31a-d may be used, and each of the plurality of storage tanks 31,31a-d may be releasable from the riser 20 and operable to be moved by a vessel 42′.

[0063] The tank assembly 30 and / or at least one, optionally each, of the plurality of releasable storage tanks 31,31a-d may comprise a variable buoyancy arrangement. The variable buoyancy arrangement can, for example, comprise a variable-volume gas compartment 34.

[0064] The storage tank 31 may comprise a plurality of compartments, the compartments configured to store different fluids. The different fluids may, for example, be different types fluids for injection into the well 11 or reservoir 12 for different purposes, and / or different additives for injection into the produced petroleum stream for different purposes. Each of the plurality of compartments may, for this purpose, comprise a separate fluid connection to the subsea production well assembly 10 or the processing equipment 17 such as to allow injection of each (type of) fluid separately. Similarly, if the tank assembly 30 comprises a plurality of storage tanks 31,31a-d, the plurality of storage tanks 31,31a-d may be configured to store different fluids.

[0065] Each of the plurality of storage tanks 31,31a-d may then comprise a separate fluid connection to the subsea production well assembly 10 or the processing equipment 17.

[0066] The following numbered clauses outline further inventive examples and embodiments.

[0067] A1. An offshore system (100), the system (100) comprising:

[0068] a subsea well assembly (10) comprising a wellbore (11) extending from a sea floor (1) to a subterranean reservoir (12),

[0069] a riser (20) anchored at a first end part (20a) thereof to the sea floor (1),

[0070] a tank assembly (30) fixed to a second end part (20b) of the riser (20),

[0071] the tank assembly (30) being buoyant and supported vertically by the riser (20),

[0072] wherein the tank assembly (30) comprises a storage tank (31,31a-d), the storage tank (31,31a-d) being

[0073] fluidly connected, optionally via processing equipment (17), to the subsea production well assembly (10) and arranged to hold fluids for injection into wellbore (11) and / or the subterranean reservoir (12), or

[0074] fluidly connected to processing equipment (17) and arranged to hold fluids for injection into a petroleum stream from the subsea well assembly (10) via the processing equipment (17).

[0075] A2. The offshore system (100) of clause A1, further comprising a loading arrangement (40′), wherein the loading arrangement (40′) comprises a loading pipe (41′) configured to be operatively engaged by a vessel (42,42′) for providing fluids to the storage tank (31,31a-d).

[0076] A3. The offshore system (100) of clause A1, wherein the tank assembly (30) is configured to receive an loading pipe (41′) for providing fluids from a vessel (42,42′).

[0077] A4. The offshore system (100) of any preceding clause, wherein the storage tank (31,31a-d) is releasable from the riser (20) and operable to be moved by a vessel (42,42′).

[0078] A5. The offshore system (100) of any preceding clause, comprising a plurality of storage tanks (31,31a-d).

[0079] A6. The offshore system (100) of any preceding clause, wherein each of the plurality of storage tanks (31,31a-d) is releasable from the riser (20) and operable to be moved by a vessel (42,42′).

[0080] A7. The offshore system (100) of any preceding clause, wherein the tank assembly (30) comprises a variable buoyancy arrangement.

[0081] A8. The offshore system (100) of any preceding clause, wherein at least one or each of the plurality of releasable storage tanks (31,31a-d) comprises a variable buoyancy arrangement.

[0082] A9. The offshore system (100) of any preceding clause, wherein the variable buoyancy arrangement comprises a variable-volume gas compartment (34).

[0083] A10. The offshore system (100) of any preceding clause, wherein the storage tank (31) comprises a plurality of compartments, the compartments configured to store different fluids.

[0084] A11. The offshore system (100) of any preceding clause, wherein each of the plurality of compartments comprises a separate fluid connection to the subsea production well assembly (10) or the processing equipment (17).

[0085] A12. The offshore system (100) of any preceding clause, wherein the plurality of storage tanks (31,31a-d) are configured to store different fluids.

[0086] A13. The offshore system (100) of any preceding clause, wherein each of the plurality of storage tanks (31,31a-d) comprises a separate fluid connection to the subsea production well assembly (10) or the processing equipment (17).

[0087] A14. The offshore system (100) of any preceding clause, wherein the storage tank (31) comprises carbon dioxide (CO2) and the processing equipment (17) comprises flow control equipment, such as pumping equipment, operable to control an injection of the carbon dioxide (CO2) into the reservoir (12).

[0088] A15. The offshore system (100) of any preceding clause, wherein the storage tank (31) comprises fluids for well or reservoir treatment and the processing equipment (17) comprises flow control equipment, such as pumping equipment, operable to control an injection of the fluids for well or reservoir treatment into the wellbore (11) and / or the reservoir (12).

[0089] A16. The offshore system (100) of any preceding clause, wherein the storage tank (31) comprises additives for produced petroleum and the processing equipment (17) comprises flow control or dosing equipment, such as pumping equipment, operable to control an injection of the additives into a produced petroleum stream from the subsea production well assembly (10).

[0090] A17. A method of producing petroleum, the method comprising:

[0091] providing an offshore system (100) according to any preceding clause,

[0092] producing petroleum fluids from the subsea production well assembly (10),

[0093] temporarily halting the production of petroleum fluids and injecting fluids for well or reservoir treatment into the wellbore (11) and / or the reservoir (12), the fluids for well or reservoir treatment being provided from the storage tank (31,31a-d).

[0094] A18. The method according to any preceding clause, further comprising:

[0095] loading fluids for well or reservoir treatment to the storage tank (31,31a-d) from a vessel (42,42′) via a loading pipe (41′).

[0096] A19. The method according to any preceding clause, further comprising:

[0097] moving a releasable storage tank (31,31a-d) comprising fluids for well or reservoir treatment with a vessel (42,42′), and

[0098] connecting the releasable storage tank (31,31a-d) to the riser (20).

[0099] A20. A method of depositing carbon dioxide (CO2), the method comprising:

[0100] providing an offshore system (100) according to any preceding clause,

[0101] temporarily storing carbon dioxide (CO2) in the storage tank (31,31a-d), and

[0102] injecting carbon dioxide (CO2) from the storage tank (31,31a-d) into the reservoir (12).

[0103] A21. The method according to any preceding clause, the method comprising injecting carbon dioxide (CO2) from the storage tank (31,31a-d) into the reservoir (12) via processing equipment (17) arranged at the sea floor (1).

[0104] A22. The method according to any preceding clause, further comprising:

[0105] loading carbon dioxide (CO2) to the storage tank (31,31a-d) from a vessel (42,42′) via a loading pipe (41′).

[0106] A23. The method according to any preceding clause, further comprising:

[0107] moving a releasable storage tank (31,31a-d) comprising carbon dioxide (CO2) with a vessel (42,42′), and

[0108] connecting the releasable storage tank (31,31a-d) to the riser (20).

[0109] A24. A method of producing petroleum, the method comprising:

[0110] providing an offshore system (100) according to any preceding clause,

[0111] producing petroleum fluids from the subsea production well assembly (10)

[0112] injecting additives into a produced petroleum stream from the subsea production well assembly (10), the additives being provided from the storage tank (31,31a-d).

[0113] A25. The method according to any preceding clause, the method comprising injecting additives from the storage tank (31,31a-d) into the produced petroleum stream via processing equipment (17) arranged at the sea floor (1).

[0114] A26. The method according to any preceding clause, further comprising:

[0115] loading additives to the storage tank (31,31a-d) from a vessel (42,42′) via a loading pipe (41′).

[0116] A27. The method according to any preceding clause, further comprising:

[0117] moving a releasable storage tank (31,31a-d) comprising additives with a vessel (42,42′), and

[0118] connecting the releasable storage tank (31,31a-d) to the riser (20).

[0119] In other examples, systems and methods are provided for receiving and temporarily storing fluids offshore.

[0120] Illustrated in FIG. 9, an offshore system 101 may comprise a riser 20 anchored at a first end part 20a thereof to a sea floor 1, a tank assembly 30 fixed to a second end part 20b of the riser 20, the tank assembly 30 being buoyant and supported vertically by the riser 20, and the tank assembly 30 comprising a storage tank 31,31a-d arranged to receive and temporarily store fluids.

[0121] The offshore system 101 may, similarly as described above, comprise a loading and / or offloading arrangement 40,40′ with a loading and / or offloading pipe 41,41′ configured to be operatively engaged by a vessel 42 for moving fluids between the storage tank 31,31a-d and the vessel 42 via the loading and / or offloading pipe 41,41′. A loading and / or offloading buoy 43,43′ may be used for this purpose, similarly as described above. The tank assembly 30 may, alternatively or additionally, be configured to receive a loading and / or offloading pipe 41,41′ for moving fluids between the storage tank 31,31a-d to a vessel 42, similarly as shown in FIG. 3.

[0122] The storage tank(s) 31,31a-d may optionally be releasable from the riser 20 and operable to be moved by a vessel 42,42′, similarly as described above.

[0123] The offshore system 101 may comprise a plurality of storage tanks 31,31a-d, and each of the plurality of storage tanks 31,31a-d may be releasable from the riser 20 and operable to be moved by a vessel 42′, similarly as described above.

[0124] The tank assembly 30 and / or at least one, optionally each, of the plurality of releasable storage tanks 31,31a-d may comprise a variable buoyancy arrangement. The variable buoyancy arrangement can, for example, comprise a variable-volume gas compartment 34.

[0125] The storage tank31,31a-d may comprise a plurality of compartments configured to store different fluids, or the plurality of storage tanks 31,31a-d may be configured to store different fluids.

[0126] In one example, illustrated in FIG. 9, the offshore system 101 comprises an offshore renewable energy installation 50 having a generator 51 and an energy converter 52 operable to produce an energy carrier fluid using electric energy produced by the generator 51. The generator 51 is in this example a wind turbine generator, but may optionally be other types of generators, particularly any type of offshore renewable energy generator. The offshore renewable energy installation 50 may be a floating installation, as illustrated, or alternatively be a bottom-fixed installation, i.e. an installation which is supported on the sea floor 1. The offshore renewable energy installation 50 may be part of or comprise a number of individual units, e.g. several floating foundations of the type illustrated in FIG. 9 or several bottom-fixed units, such as jacket structures or monopiles supporting wind turbine generators.

[0127] The energy converter 52 is fluidly connected to the storage tank(s) 31,31a-d via a fluid supply pipeline 53. The fluid supply pipeline 53 may extend at least partly along the sea floor 1 and to a lower end part 20a of the riser 20, and from there through and along the riser 20 to the tank assembly 30.

[0128] The generator 51 may be operatively connected to an electric export cable (not shown) for transmitting produced electric energy to an external grid, such as a shore-based grid. Such connection may include an inter array cable which interconnects several such units and a common export cable.

[0129] In these examples, renewable energy (here: wind power) may be used to produce an energy carrier fluid, for example ammonia (NH3) or hydrogen (H2), which may be temporarily stored in the tank assembly 30.

[0130] In one example, the energy carrier fluid may be offloaded to a vessel 42 via the offloading arrangement 40. The energy carrier fluid may thereby be transported for use at other locations, such as at land-based locations or for other offshore / maritime uses.

[0131] Alternatively, or additionally, other offshore consumers, such as oil & gas installations, offshore power plants, or other consumer types, may be fluidly connected to the tank assembly 30 and arranged to use energy carrier fluid temporarily stored in the tank assembly 30.

[0132] Alternatively, or additionally, the fluid supply pipeline 53 may be configured for two-way conveyance of energy carrier fluid between the storage tank(s) 31,31a-d and the offshore renewable energy installation 50, and the energy converter 52 may be operable to produce electric energy from the energy carrier fluid and supply produced electric energy to the export cable. In this manner, the tank assembly 30 with the energy carrier fluid may act as an energy storage system for generated renewable energy, whereby electric energy can be produced from energy carrier fluid in periods where export demand is higher than the production from the generator 51.

[0133] In one example, there is provided a method of providing fuel for vessels by means of an offshore system 101. In this example, an energy carrier fluid is temporarily stored in the storage tank(s) 31,31a-d and offloaded to a vessel 42,42′ on demand, for example via an offloading arrangement 40. The vessels 42,42′ may, for example, be service vessels (e.g. for oil & gas operations) operating offshore near the system 101, or transport vessels passing by near the system 101. In this manner, bunkering of vessels may be done offshore using environmentally friendly fuels, and without the need for such a vessel to go to a quayside refueling site or for dedicated bunkering vessels to be used.

[0134] Optionally, the energy carrier fluid can be provided to the storage tank(s) 31,31a-d from a tanker vessel 42,42′ via the offloading arrangement 40. In this manner, the tank assembly 30 can operate as a temporary offshore storage for fuel at or near an operational area or route for consumer vessels. A tanker vessel 42,42′ can, for example, provide energy carrier fluid to a number of such tank assemblies 30 along a shipping route or distributed across an operational area for consumer vessels, such as areas where offshore oil & gas operations are conducted.

[0135] Advantageously, in any of the examples disclosed or claimed herein, the energy carrier fluid has a density lower than that of sea water. In this manner, the energy carrier fluid contributes to the buoyancy of the tank assembly 30, reducing a need for dedicated buoyancy volume in the tank assembly 30 and associated components.

[0136] The following numbered clauses outline further inventive examples and embodiments.

[0137] B1. An offshore system (101), the system (101) comprising:

[0138] a riser (20) anchored at a first end part (20a) thereof to a sea floor (1),

[0139] a tank assembly (30) fixed to a second end part (20b) of the riser (20),

[0140] the tank assembly (30) being buoyant and supported vertically by the riser (20),

[0141] wherein the tank assembly (30) comprises a storage tank (31,31a-d) arranged to receive and temporarily store fluids.

[0142] B2. The offshore system (101) of clause B1, further comprising an offloading arrangement (40), wherein the offloading arrangement (40) comprises a pipe (41) configured to be operatively engaged by a vessel (42) for moving fluids between the storage tank (31,31a-d) and the vessel (42) via the pipe (41).

[0143] B3. The offshore system (101) of clause B1, wherein the tank assembly (30) is configured to receive a pipe (41) for moving fluids between the storage tank (31,31a-d) and a vessel (42).

[0144] B4. The offshore system (101) of any preceding clause, wherein the storage tank (31,31a-d) is releasable from the riser (20) and operable to be moved by a vessel (42′).

[0145] B5. The offshore system (101) of any preceding clause, comprising a plurality of storage tanks (31,31a-d).

[0146] B6. The offshore system (101) of any preceding clause, wherein each of the plurality of storage tanks (31,31a-d) is releasable from the riser (20) and operable to be moved by a vessel (42′).

[0147] B7. The offshore system (101) of any preceding clause, wherein the tank assembly (30) comprises a variable buoyancy arrangement.

[0148] B8. The offshore system (101) of any preceding clause, wherein at least one or each of the plurality of releasable storage tanks (31,31a-d) comprises a variable buoyancy arrangement.

[0149] B9. The offshore system (101) of any preceding clause, wherein the variable buoyancy arrangement comprises a variable-volume gas compartment (34).

[0150] B10. The offshore system (101) of any preceding clause, wherein the storage tank (31) comprises a plurality of compartments, the compartments configured to store different fluids.

[0151] B11. The offshore system (101) of any preceding clause, wherein the plurality of storage tanks (31,31a-d) are configured to store different fluids.

[0152] B12. The offshore system (101) of any preceding clause, wherein the riser (20) is anchored to the sea floor (1) by means of a suction anchor foundation (21).

[0153] B13. The offshore system (101) of any preceding clause, further comprising an offshore renewable energy installation (50), the offshore renewable energy installation (50) comprising:

[0154] a generator (51), and

[0155] an energy converter (52) operable to produce an energy carrier fluid using electric energy produced by the generator (51),

[0156] wherein the energy converter (52) is fluidly connected to the storage tank(s) (31,31a-d) via a fluid supply pipeline (53).

[0157] B14. The offshore system (101) of any preceding clause, wherein the generator (51) is operatively connected to an electric export cable for transmitting produced electric energy to an external grid.

[0158] B15. The offshore system (101) of any preceding clause, wherein

[0159] the fluid supply pipeline (53) is configured for two-way conveyance of energy carrier fluid between the storage tank(s) (31,31a-d) and the offshore renewable energy installation (50), and

[0160] the energy converter (52) is operable to produce electric energy from the energy carrier fluid and supply produced electric energy to the export cable.

[0161] B16. The offshore system (101) of any preceding clause, wherein the storage tank(s) (31,31a-d) comprises an energy carrier fluid.

[0162] B17. The offshore system (101) of any preceding clause, wherein the energy carrier fluid has a density lower than that of sea water.

[0163] B18. The offshore system (101) of any preceding clause, wherein the energy carrier fluid is ammonia (NH3) or hydrogen (H2).

[0164] B19. A method for producing electric energy, the method comprising, by means of an offshore system (101) according to any preceding claim:

[0165] generating electric energy by means of a generator (51), and

[0166] supplying electric energy generated by the generator (51) to an external grid, such as a land-based grid, via an export cable.

[0167] B20. The method of any preceding clause, comprising

[0168] operating an energy converter (52) at the offshore renewable energy installation (50) to produce an energy carrier fluid using electric energy produced by the generator (51),

[0169] temporarily storing the energy carrier fluid in the storage tank(s) (31,31a-d), and

[0170] subsequently operating the energy converter (52) to produce electric energy from the energy carrier fluid and supply produced electric energy to the export cable.

[0171] B21. A method of providing fuel for vessels, the method comprising, by means of an offshore system (101) according to any preceding claim:

[0172] temporarily storing an energy carrier fluid in the storage tank(s) (31,31a-d), and

[0173] offloading energy carrier fluid to a vessel (42,42′) via an offloading arrangement (40).

[0174] B22. The method of any preceding clause, comprising:

[0175] operating an energy converter (52) at an offshore renewable energy installation (50) to produce energy carrier fluid using electric energy produced by a generator (51),

[0176] providing the energy carrier fluid to the storage tank(s) (31,31a-d) via a fluid supply pipeline (53) arranged between the offshore renewable energy installation (50) and the tank assembly (30).

[0177] B23. The method of any preceding clause, comprising:

[0178] providing energy carrier fluid to the storage tank(s) (31,31a-d) from a tanker vessel (42,42′) via the offloading arrangement (40).

[0179] B24. A system or method according to any preceding clause, wherein the generator (51) is a wind turbine generator.

[0180] In any of the examples or embodiments described or claimed herein, the riser 20 may be anchored to the sea floor 1 by means of a suction anchor foundation 21.

[0181] Advantageously, a suction anchor foundation 21 provides good anchoring support for the riser 20 at a variety of sea floor conditions, particularly in soft soil conditions where the establishment of for example sea floor based tank arrangements can be more challenging. Examples and embodiments according to the present disclosure may be particularly useful in areas having such conditions.

[0182] The invention is not limited by the embodiments described above; reference should be had to the appended claims.

Claims

1. An offshore petroleum production system (100), the system (100) comprising:a subsea production well assembly (10),a riser (20) anchored at a first end part (20a) thereof to a sea floor (1),a tank assembly (30) fixed to a second end part (20b) of the riser (20),the tank assembly (30) being buoyant and supported vertically by the riser (20),wherein the tank assembly (30) comprises a storage tank (31) fluidly connected to the subsea production well assembly (10) and arranged to receive produced petroleum fluids from the subsea production well assembly (10).

2. The offshore petroleum production system (100) of claim 1, further comprising an offloading arrangement (40), wherein the offloading arrangement (40) comprises an offloading pipe (41) configured to be operatively engaged by a tanker vessel (42) for moving petroleum fluids from the storage tank (31,31a-d) to the tanker vessel (42) via the offloading pipe (41).

3. The offshore petroleum production system (100) of claim 1, wherein the tank assembly (30) is configured to receive an offloading pipe (41) for moving petroleum fluids from the storage tank (31,31a-d) to a tanker vessel (42).

4. The offshore petroleum production system (100) of any preceding claim, wherein the storage tank (31,31a-d) is releasable from the riser (20) and operable to be moved by a vessel (42′).

5. The offshore petroleum production system (100) of any preceding claim, comprising a plurality of storage tanks (31,31a-d).

6. The offshore petroleum production system (100) of claim 5, wherein each of the plurality of storage tanks (31,31a-d) is releasable from the riser (20) and operable to be moved by a vessel (42′).

7. The offshore petroleum production system (100) of any preceding claim, wherein the tank assembly (30) comprises a variable buoyancy arrangement.

8. The offshore petroleum production system (100) of claim 5 or 6, wherein at least one or each of the plurality of releasable storage tanks (31,31a-d) comprises a variable buoyancy arrangement.

9. The offshore petroleum production system (100) of claim 7 or 8, wherein the variable buoyancy arrangement comprises a variable-volume gas compartment (34).

10. The offshore petroleum production system (100) of any preceding claim, wherein the storage tank (31) comprises a plurality of compartments, the compartments configured to store different fluids, such as wherein at least one of the plurality of compartments is configured for storing a fuel other than produced petroleum fluids.

11. The offshore petroleum production system (100) of any preceding claim, wherein the plurality of storage tanks (31,31a-d) are configured to store different fluids, such as wherein at least one of the plurality of storage tanks (31,31a-d) is configured for storing a fuel other than produced petroleum fluids.

12. A method of producing petroleum, the method comprising:providing an offshore petroleum production system (100) according to any preceding claim,producing petroleum fluids from the subsea production well assembly (10),temporarily storing produced petroleum fluids in the tank assembly (30).

13. The method according to claim 12, further comprising:offloading produced petroleum fluids from the tank assembly (30) to a tanker vessel (42) via an offloading pipe (41).

14. The method according to claim 12, further comprising:releasing the storage tank (31,31a-d) from the riser (20), andmoving the released storage tank (31,31a-d) by a vessel (42′).

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