The present invention relates to unmanned offshore oil and/or gas production platforms, and in particular, to the local storage of produced hydrocarbons and the provision of utilities in relation to such platforms.

Conventional offshore platforms used for producing hydrocarbons are relatively massive structures that are designed for operation by a large crew. In addition to the production equipment itself, there is typically also processing apparatus, such as separators, compressors, etc. and of course the accommodation and support services required to allow the crew to live and work on board the platform. In addition, various utilities have to be provided. This term includes power production, chemical storage, TEG regeneration, etc.

Such platforms are well suited for the exploitation of large hydrocarbon reserves, but there is a growing need to be able to exploit smaller and/or more remote reserves in an economical manner. This may be done by means of satellite platforms that are located remotely from a host platform but which are operated in conjunction with the host to which they may be ‘tied back’ by means of submarine pipelines to allow transportation of the produced fluids. Alternatively, shuttle tankers and other vessels may be used for this purpose and to otherwise facilitate operation of the satellite platform.

In order to further reduce costs, unmanned production platforms may be used at remote locations. By “unmanned” it is meant that the platform has no permanent personnel and may only be occupied for particular operations such as maintenance and/or installation of equipment. The unmanned platform may be a platform where no personnel are required to be present for the platform to carry out its normal function, for example day-to-day functions relating to handling of oil and/or gas products at the platform. Examples of known unmanned platforms are disclosed in the applicant’s earlier patent applications WO 2018/139939 and GB 2566502.

It will be appreciated that the use of unmanned production platforms (UPP) enables costs to be reduced when exploiting remote/marginal reserves. With such reserves, there is particular focus on reducing the operational (opex) and capital (capex) expenditure associated with these platforms and the recovery of hydrocarbons therefrom. There are typically two approaches to producing hydrocarbons at a UPP at a remote/marginal oil reserve. A first approach is to have all of the production equipment (e.g. processing separators, compressors, scrubbers, pumps), utilities equipment (e.g. power production, chemical storage, TEG regeneration), offloading equipment (i.e. equipment allowing for offload to a transportation tanker), water processing and reinjection equipment, and storage equipment required for operation of the UPP situated permanently on the platform or nearby (e.g. there may be seabed storage). Using such an approach, the production operation at the UPP is essentially ‘self-contained’ and no external equipment is required for the production operation. All that is required for the retrieval of the produced hydrocarbon fluid is a shuttle type tanker to travel to a site of the UPP to receive the produced hydrocarbon fluid therefrom as required.

However, such an approach is disadvantageous in that the amount of equipment at the site of the UPP is inconsistent with the ‘unmanned’ nature of the platform. This is because this equipment will require personnel to be present, at least intermittently, for maintenance, inspection and performance tuning thereof. Furthermore, if the UPP is situated in a harsh environment that may be hard to reach in severe weather conditions, personnel may not be able to get to the UPP as often as desired to allow for tuning and maintenance of the equipment thereon. This can result in a lower than desired production efficiency at the UPP.

As a more commonplace and alternative approach to the above, floating production storage and offloading vessel (FPSO) solutions have been used. In such scenarios, the equipment on the UPP is stripped back such that it comprises no (or a bare minimum of) production equipment, utilities equipment, offloading equipment and storage facilities. Instead, such equipment and utilities required for operation of the platform are provided on a FPSO. In such a scenario, production requires that the FPSO travel to a site of the UPP and connect thereto as appropriate to begin the production of fluid. For the retrieval of hydrocarbons using an FPSO approach, a shuttle tanker travels and connects to the FPSO and receives the hydrocarbons received and stored on the FPSO for subsequent transportation.

It will be appreciated that the FPSO approach reduces the demand for personnel on the UPP for maintenance, inspection, etc. as compared to the first approach described above, since the maintenance and control of equipment may be carried out by the crew at the FPSO. This in turn leads to an improved production efficiency since the crew is readily at the site of the equipment. However, whilst this is beneficial in terms of production efficiency, it is simultaneously disadvantageous in terms of high capex and opex. The opex associated with having a large production crew on the FPSO and transporting such a crew and vessel to a site of the UPP is significant. Moreover, the opex and capex associated with the FPSO in itself is significant, most notably because it comprises equipment configured to handle/process hydrocarbons (compressors, separators scrubbers and the like). Such equipment is both expensive to provide and operate, with such expense being even further increased due to the safety regulations that this equipment must adhere to being on board a personnel-carrying vessel.

Another exemplary prior art unmanned production platform is disclosed in US 2018/141625. Here, produced oil is pumped to a nearby Floating Storage and Offloading unit (FSO), e.g. a converted oil tanker, for storage and offloading by another tanker (i.e. a shuttle tanker). Gas may be used as fuel, flared or transported. In contrast to the FPSO solutions discussed above, the FSO does not comprise any of the production or utilities equipment for the platform. This is instead provided at the platform.

A further prior art arrangement is disclosed in US 2006/004593 A. The primary embodiment of this document comprises a floating oil storage and offloading vessel with gas utilisation capabilities (FGSO) in communication with a plurality of manned production platforms situated at different hydrocarbon fields. Thus, the FGSO is situated tens of kilometres away from each of the platforms to which it is connected. The FGSO is configured to receive produced fluid (both oil and gas) from each platform. The produced oil is stored on the FGSO until such a time as it is collected by a shuttle tanker. The gas transferred to the FGSO can be utilised in a number of ways, one of which involves using the gas to generate electricity which can be subsequently sold back for use at the production platforms.

According to a first aspect of the invention, there is provided an unmanned production platform in combination with, and typically proximate to, a storage vessel, wherein the platform comprises apparatus for producing and processing hydrocarbons from a subsea reservoir and the storage vessel provides storage for hydrocarbons produced thereby and further provides a utility for use at the platform.

Thus, the invention addresses the drawbacks of the prior art approaches to the operation of a UPP discussed above by providing hydrocarbon-processing facilities at the UPP, whilst providing utilities at the storage vessel. The first aspect of the invention is also advantageous over the manned platform arrangement of US 2006/004593 A. The platforms of US 2006/004593 A are not unmanned, and the only utility disclosed as being provided from the FGSO to the various platforms is electricity. As such all other utilities would necessarily be provided at each respective platform. Moreover, given that the platforms of US 2006/004593 A are manned it is highly unlikely that some electricity generation is not also provided on each platform. Furthermore, as the FGSO of US 2006/004593 A is situated tens of kilometres away from each of the connected platforms it would be entirely unfeasible to provide all of the requisite utilities from the vessel to the platform.

Thus, the proximity of the vessel of the first aspect of the invention with the platform can be seen to provide advantages with regard to the provision of utilities to the platform that is not available from the arrangement of US 2006/004593 A.

The proximity of the platform and vessel of the first aspect can thus be seen to form a ‘pair’. That is, the platform and vessel are located geographically close to one another, particularly with reference to the distances between different hydrocarbon fields.

The storage vessel of the invention may not be in connection or communication with any other platform. As such, the storage vessel may only receive hydrocarbons from the platform and may only provide a utility to that platform. Similarly the platform may not be in connection or communication with any other storage vessel. As such, the platform may only receive a utility from the storage vessel and may only provide hydrocarbons to that vessel. Thus, the platform and the vessel of the first aspect of the invention may be only connected to one another.

As used above, the term proximate in the context of the invention is to be interpreted as meaning a distance much smaller than the distances between neighbouring hydrocarbon fields. Thus, the distance between the storage vessel and the platform in the current invention is also far smaller than the distance between the platform and FGSO as disclosed in US 2006/004593 A.

The storage vessel may be situated less than 2 km away from the platform, and optionally less than 1km. The storage vessel of the first aspect may be situated as close as feasibly possible to the platform. Thus, the storage vessel may be situated just outside the ‘safety zone’ of the platform, which is a zone typically extending 500 m around the platform. The unmanned production platform may comprise all of the apparatus for producing and processing hydrocarbons. Alternatively, at least some apparatus for producing and processing hydrocarbons may be provided at a site of the unmanned production platform, for example at a subsea location proximate the production platform. The storage vessel may comprise no apparatus for producing and processing hydrocarbons.

The storage vessel used in the invention may be termed a Floating Utility, Storage and Offloading Vessel (FUSO).

The unmanned production platform of the invention may be termed a UPP.

The storage provided on the vessel will typically comprise an oil storage tank or tanks and in use, this is preferably in fluid communication with the platform. However, in some embodiments, it may additionally be in fluid communication with a source of oil located at the sea bed, for example, where subsea separation apparatus is provided in addition to the production and processing apparatus on the platform. The fluid communication between platform and vessel is preferably direct, but may be indirect (i.e. via some other apparatus or vessel).

In a typical embodiment, produced hydrocarbons will flow from wellheads at the sea bed via riser(s) to the platform. There may be provided a stage of separation apparatus at the sea bed, or the separation apparatus may be entirely provided on board the platform itself. This apparatus for producing and processing hydrocarbons preferably comprises separation apparatus for separating produced hydrocarbon into oil and gas components and may additionally separate water from produced hydrocarbons. Separated water may be used for water injection (i.e. for enhanced oil recovery) and this may be in addition to injection water provided by the storage vessel (see below).

The separation apparatus preferably comprises multiple separation stages and it is particularly preferred for multiple stages (e.g. 2 or 3 such stages) to be provided on board the platform. (As noted above, the separation apparatus may further comprise subsea separation apparatus.)

The storage vessel may provide one or more of a variety of utilities. Thus, it may provide seawater treatment for seawater injection; storage for chemicals used at the production platform such as storage for emulsion inhibitors, hydrate inhibitors, corrosion inhibitors and/or hydrogen sulphide scavengers; treatment for chemicals used at the production platform such as treatment for emulsion inhibitors, hydrate inhibitors, corrosion inhibitors and/or hydrogen sulphide scavengers; a supply of liquid desiccant and desiccant regeneration; and/or sulphate removal for support re-injection purposes or otherwise.

The storage vessel of the first aspect may additionally or alternatively comprise an electricity generator for supplying power to the platform. This may be driven by a prime mover powered by a supply of produced hydrocarbon. For example, a gas motor may be used.

In a preferred embodiment, the vessel (FUSO) comprises all of the necessary utilities (other than electricity generation), storage, and/or offloading equipment for the UPP to allow for operation and production to be carried out at the UPP; however there is no hydrocarbon handling equipment (processing/production equipment) provided on board the FUSO. This production/processing equipment (including the separators, compressors and the like) may instead be positioned on or at the UPP. In a further preferred embodiment, electricity generations is also provided on the FUSO.

The UPP may comprise none of the utilities, storage and/or offloading equipment to allow for operation and production to be carried out at the UPP, or may only comprise the utilities, storage and/or offloading equipment not provided by the vessel.

Although by its nature, the storage vessel is mobile, in order to maintain the availability of its utilities for use at the platform, the invention preferably further comprises a shuttle tanker for collecting and transporting hydrocarbons stored on the storage vessel. The shuttle tanker may visit the storage vessel as required, e.g. to perform side-by-side unloading of stored hydrocarbon.

As noted above, the hydrocarbon stored on the vessel will preferably be oil. Whilst, it is possible for gas to be (additionally or alternatively) stored (e.g. liquefied), in many cases, it may not be economic to transport produced gas and in others it may be more economic to provide a transport pipeline. In the former case, gas may be used to provide power to the platform and/or for reinjection.

It will be appreciated that the invention extends to the provision of a vessel for use in such a combination. Accordingly, viewed from another aspect, the invention provides a hydrocarbon storage and utility vessel comprising an oil storage tank and providing one or more of the following utilities for use by a hydrocarbon production platform: chemical storage; liquid desiccant and desiccant regeneration; seawater treatment; and/or sulphate removal. The vessel of the second aspect may additionally provide electricity generation for the platform.

The vessel may have no hydrocarbon handling equipment (processing/production equipment) provided thereon.

The vessel may have any of the optional features discussed above with which it is compatible.

The invention also extends to a corresponding method. Accordingly, viewed from a still further aspect, there is provided a method of producing hydrocarbons comprising the use of the combination discussed above and preferably comprising the use of the preferred and/or optional features thereof.

In particularly preferred forms of such methods, the start of production requires that the FUSO travel to a site of the UPP and connect thereto. Once connected, the UPP is provided with utilities from the FUSO to allow for hydrocarbon production to commence. The hydrocarbons are produced at the UPP and are then sent to the FUSO, via the connection therebetween, for storage. Later retrieval of hydrocarbons then preferably uses a shuttle tanker to travel and connect to the FUSO such that it can receive the produced fluid stored thereon for subsequent transportation.

By providing the utilities and storage equipment on board the FUSO, the maintenance hours required at the UPP may be kept to a minimum whilst the production efficiency of the UPP may be kept relatively high. However, unlike the FPSO, the FUSO may comprise no hydrocarbon production equipment so the significant capex and opex associated with this equipment as described above is avoided. Thus, the approach proposed avoids the drawbacks of the FPSO approach, whilst simultaneously benefitting from its advantages.

As discussed above, the platform is an unmanned production platform (as defined herein) and the system preferably further comprises a remote host to which hydrocarbons may be transported via a shuttle tanker.

An “unmanned platform” as used herein is a platform that has no permanent personnel and may only be occupied for particular operations such as maintenance and/or installation of equipment. The unmanned platform may be a platform where no personnel are required to be present for the platform to carry out its normal function, for example day-to-day functions relating to handling of oil and/or gas products at the platform. As such, the unmanned platform can be considered as a normally unmanned platform. An unmanned platform may be a platform with no provision of facilities for personnel to stay on the platform, for example there may be no shelters for personnel, no toilet facilities, no drinking water and/or no personnel operated communications equipment. The unmanned platform may also include no heli-deck and/or no lifeboat, and advantageously may be accessed in normal use solely by a gangway to a vessel or bridge a bridge to another platform, for example via a Walk to Work (W2W) system.

An unmanned platform may alternatively or additionally be defined based on the relative amount of time that personnel are needed to be present on the platform during operation. This relative amount of time may be defined as maintenance hours needed per annum, for example, an unmanned platform may be a platform requiring fewer than 10,000 maintenance hours per year, optionally fewer than 5000 maintenance hours per year, perhaps fewer than 3000 maintenance hours per year.

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a perspective overview of an unmanned production platform and FUSO vessel according to an embodiment of the invention;

Figure 2 is a view corresponding to Figure 1 in which a shuttle tanker is additionally shown; and

Figure 3 is a schematic view of the production and processing apparatus of the platform and FUSO vessel of Figure 1.

As may be seen from Figure 1 , an unmanned production platform 1 is provided in the form of a floating spar buoy 2 with a superstructure 3 mounted thereon in a region of the sea 4 remote from a host platform (not shown). The platform is moored to the sea bed 5 by means of suitable catenary moorings 6. A Floating Utility, Storage and Offloading vessel (FUSO) 7 is located at the surface 8 nearby and is typically situated just outside of the safety zone of the platform 1.

A subsea separator 9 connected to a wellhead (not shown) via conduit 11 is provided at the seabed. The subsea separator 9 is connected to the platform 1 by means of two risers 10a, 10b. A further riser 10c connects the platform 1 to an additional wellhead (not shown). Utilities conduit 12 and hydrocarbon fluid conduit 13 interconnect the platform 1 with the FUSO vessel 7. A water conduit 14a also connects to the FUSO vessel 7 and interconnects the vessel 7 with injection wells (not shown). A similar water conduit 14b interconnects the platform 1 with the injection wells. The FUSO vessel 7 includes storage tanks (not shown) for receiving produced hydrocarbons from the platform 1 via the hydrocarbon fluid conduit 13 and provides various utilities to the platform 1 via the utilities conduit 12, as will be discussed further below. The FUSO vessel 7 further comprises a water treatment facility as described further below for providing water via the water conduit 14a to the injection wells.

Figure 2 corresponds exactly to Figure 1 except that it additionally shows a shuttle tanker 15 connected to FUSO vessel 7 by means of a flexible offloading conduit 16.

During operation, produced hydrocarbons pass from the wellhead, via the subsea separator, 9 and flow via risers 10a, 10b to the platform 1 for processing. Produced hydrocarbons similarly pass from the other wellhead via riser 10c to the platform 1 for processing. Processed hydrocarbons (e.g. separated and stabilised or semi-stabilised oil and/or gas) flow from the platform 1 to the FUSO vessel 7 via hydrocarbon fluid conduit 13 where they are stored pending transportation by a shuttle tanker.

Once the tanks on the FUSO vessel 7 approach being filled, a shuttle tanker 15 sails to the site of the FUSO vessel 7. Here, as shown in Figure 2, it is connected via conduit 16 to the FUSO vessel 7 and hydrocarbon fluids are then transferred to the shuttle tanker 15. Once it is fully loaded, the shuttle tanker 15 sails to the host with its cargo of hydrocarbon fluid where it is unloaded. The shuttle tanker 15 may then return to the FUSO vessel 7 when required.

The FUSO vessel 7 also serves to provide various utilities to the production platform.

In the present embodiment, a first utility is the provision of electrical power. This is generated on board the FUSO vessel 7 using a gas motor powered by gas supplied by the platform. The electrical power is transmitted to the platform via a cable provided within the utilities conduit 12.

A second utility provided at the FUSO vessel 7 is the storage and processing of chemicals, such as glycols e.g. MEG and TEG used to dehydrate produced fluids. The processing of these fluids comprises regeneration of them to remove water absorbed from the hydrocarbon fluids, in the known manner. The FUSO vessel 7 may additionally provide storage and processing of emulsion inhibitors, other hydrate inhibitors, corrosion inhibitors and hydrogen sulphide scavengers. These fluids are transported via respective lines within the utilities conduit 12 to/from the platform 1.

A third utility provided at the FUSO vessel 7 is the provision of treated sea water for injection. It is known that sulphates found in sea water can cause problematic scale formation and so it is known to remove sulphates from the water before it is injected.

The FUSO vessel 7 draws sea water through an inlet port provided in its hull (not shown) and uses treatment apparatus known in the art to remove sulphates. The treated sea water is then transported via water conduit 14a to the injection wells at the sea bed.

The operation of the platform 1 and FUSO vessel 7 will now be described in more detail with reference to Figure 3, which shows a schematic flow diagram. This diagram has four regions demarcated by dashed lines. These are platform superstructure 3, FUSO vessel 7, shuttle tanker 15 and subsea components 17.

Considering first the subsea components, a plurality of production wells 20 are connected via conduits to manifold 21, which in turn feeds subsea first-stage separator 9. The gas outlet from this is connected to gas riser 10a. The oil outlet is connected to oil pumps 24a feeding oil riser 10b and the water outlet connected to water pumps 24b feeds water riser 26. Each of these risers is connected to apparatus at the superstructure 3 of the platform 1 as will be described further below.

In addition, water injection wells 27 (not shown in Figure 1) are provided at the seabed. These wells 27 are fed by the water conduits 14a, 14b from the FUSO vessel 7 and the UPP 1.

As discussed above, the FUSO vessel 7 provides both hydrocarbon (here oil) storage and various utilities.

Oil storage is provided by means of conventional storage tanks 30 on board the vessel. These are supplied by means of hydrocarbon conduit 13 from the platform 1.

TEG regeneration is provided using conventional apparatus at 32.

Chemical storage (e.g. storage of TEG) is provided at 33.

The conduits associated with oil storage (i.e. between conduit 13 and tanks 30) and TEG storage and regeneration are not shown in this figure for clarity. Gas motor 34 is supplied by gas from the platform via line 35. The motor drives generator 36 and the electricity from generator 36 is transmitted to the platform via a subsea cable within the utilities conduit (see Figure 1, 12).

Seawater treatment unit 37 removes sulphates from seawater as discussed above. Treated water from that unit is pumped by seawater injection pumps 38 via water conduit 14a to injection wells 27 at the sea bed.

Shuttle tanker 15 comprises oil storage tanks (not shown). These are connected via conduit 40 and valves 41 which are connectable to a flexible conduit 16, which leads to the oil storage tanks 30 of the FUSO vessel. The connectable arrangement allows for the shuttle tanker to be moored close to the FUSO vessel for side-by-side offloading in the known manner.

The superstructure 3 of the platform 1 provides the hydrocarbon processing apparatus, in particular the further separation of the produced fluid into its components (oil, water, gas) following the first stage separation carried out subsea. (As is well known in the art, a single stage of separation will not completely separate the fluids and so references to the flow of a specific fluid from a separator refers to a flow that is primarily of that fluid, but which may contain other components).

It will be appreciated that the specific details of the separation apparatus shown in Figure 3 are not critical to a proper understanding of the invention described herein and so the discussion below focuses on the main components.

Oil (i.e. oil-rich fluid) from riser 10b flows to second stage separator 50. The oil component of this then passes to third (and final) stage separator 51, with the oil from the outlet of the final stage flowing via pump 52 and oil transfer riser 13 to the oil storage tanks 30 on the FUSO vessel 7.

Gas from the first stage separator 9 flows via riser 10a to gas cooler 53 and then to first stage scrubber 54. The gas from the outlet of scrubber 54 is split into two flow paths. The first of these flows via line 35 to the gas motor 34 on board the FUSO vessel 7 (used to drive electricity generator 36). The remaining portion flows to first stage gas compressor 55 and then via dehydration inlet cooler 56, dehydration unit 57 and gas scrubber 58 to export gas compressor 59. The gas from the compressor 59 then flows via gas export riser 60 to a gas export pipeline 61 on the sea bed. This is used to transport gas to an onshore facility or to a host vessel. It will be appreciated that in some cases it will not be economically viable to provide a gas transport pipeline and so in alternative embodiments, gas that is not used for fuel at the FUSO vessel 7 may be re-injected or flared.

Water from water riser 26 is fed via compact floatation unit 62 and walnut shell filter 63 to cooler 64 and water pumps 65 before flowing along water conduit 14b to the injection wells 27 at the sea bed. Any hydrocarbons removed from the water at the compact floatation unit 62 and walnut shell filter 63 are sent to the second 50 and/or third 51 stage separator.

It will be noted that in this embodiment, the injection water is provided both from the platform 1 (i.e. separated water for reinjection) and from the seawater treatment unit at the FUSO vessel 7. In alternative embodiments, the injection water may be provided from only one of these sources. Additionally and/or alternatively, the water separated at the platform 1 may be passed to the FUSO vessel 7 for treatment prior to injection at the wells 27. As such, the compact floatation unit 62 and walnut shell filter 63 may be provided on the FUSO vessel 7.

In addition to the flows described above, the flows of separated gas and water from the second 50 and third 51 stage separators need to be considered.

Gas from the second stage separator 50 flows via third stage cooler 66 and scrubber 67 to third stage compressor 68 before joining the flow of gas from the first stage separator 22 upstream of cooler 53.

Gas from the third (final) separator 51 flows via first stage cooler 69, scrubber 70, compressor 71, second stage cooler 72, scrubber 73 and second stage compressor 74 before joining the flow of gas from second stage separator 50 upstream of the third stage cooler 66.

It will also be noted from the figure that oil removed from the gas at scrubbers 58, 54 and 73 is returned to the previous scrubber, with oil from scrubber 70 being routed via pump 75 to join the oil flowing from second stage separator 50 to third (final) separator 51.

Finally, water from second stage separator 50 joins the flow of water from riser 26 upstream of CFU 62.

Thus, in this embodiment it will be seen that the produced fluid is fully separated and processed at the platform itself, with the oil component then being stored in tanks 30 on board the FUSO vessel 7 prior to loading on board shuttle tanker 15 for transportation to a host.

In addition, the FUSO vessel 7 provides utilities, which would conventionally be provided on board the platform 1, including chemical (e.g. TEG) storage 33, TEG regeneration 32, injection water provision and treatment 37 and 38, and electricity generation 34 and 36.