DK180544B1

DK180544B1 - Subsea Hydrocarbon Production System

Info

Publication number: DK180544B1
Application number: DKPA201770136A
Authority: DK
Inventors: Stenevik Karl-Atle
Original assignee: Statoil Petroleum As
Priority date: 2010-05-28
Filing date: 2017-02-23
Publication date: 2021-06-14
Also published as: NO20200471A1; GB2497841B; NO345364B1; CN103025994A; GB2497841A8; MX362064B; MX2012013838A; AU2010353877A1; GB201221223D0; RU2553757C2; AP4004A; DK180281B1; BR112012030170A2; WO2011147459A1; CA2800702C; RU2012155005A; DK201770136A1; NO20121553A1; AP2012006629A0; BR112012030170B1

Abstract

A subsea oil and/or gas production system comprises a host production facility and a plurality of subsea wells. A fluid conveying network is provided that connects each subsea well to the host production facility. A separate electrical power and data network is provided for conveying direct current electrical power and data, operatively connected to each subsea well for providing each subsea well with data transfer and electrical power services. The use of direct current ensures that the electrical power and data network can provide power over much greater distances than currently available, and the use of separate networks for conveying fluids and for providing electrical power and data transfer allows for a much more flexible system.

Description

DK 180544 B1 q Subsea Hydrocarbon Production System Technical Field The present invention relates to subsea hydrocarbon production, and in particular to systems for connecting a host production facility to subsea wells.

Background of the Invention In subsea hydrocarbon production systems, a host production facility such as shore based production facility or an offshore platform is typically connected to one or more subsea wells.

Production pipelines connect each subsea well to the host production facility.

Production pipelines are used for conveying extracted hydrocarbons back to the host production facility.

In addition to the production pipelines, it is necessary to provide each subsea well with certain services.

These may include electric power, data transfer, hydraulic fluids and wellstream service fluids, such as chemical additives.

These four services are normally provided in a subsea umbilical.

Referring to Figure 1 herein, there is illustrated schematically a network of subsea wells connected to a host production facility 1. Several subsea umbilicals 2, 3, 4, 5 are connected to the host production facility 1. Each umbilical connects several subsea wells (shown in Figure 1 as circles) to the host production facility 1. Taking umbilical 5 as an example, this connects subsea wells 7, 8 and 9 to the host production facility 1. Note that each subsea well, for example subsea well 7, could comprise several physical wells.

In addition to the umbilicals, hydrocarbon-carrying pipelines are also shown as thick black lines that follow substantially the same route as the umbilicals 2, 3,4,5. Figure 2 is for the purposes of illustration only to show the services provided in an umbilical.

An umbilical 2 typically includes several hydraulic fluid conduits 10 (four are shown in Figure 2), for providing hydraulic power for opening and closing valves.

Furthermore, an umbilical typically includes several wellstream service fluid conduits 11 (four are shown in Figure 2) for carrying wellstream service fluids of different compositions, depending on the wellstream composition and the purpose of the wellstream fluid.

Seven electrical cables 12 are shown in Figure 2, which provide

DK 180544 B1 2 electric power to subsea control modules. The subsea control modules in turn provide hydraulic power to subsea wells. Furthermore, four data transfer cable 13 bundles are shown in Figure 2. Of course, these numbers are illustrative only, and different umbilicals may have different numbers of hydraulic fluid conduits 10, wellstream service fluid conduits 11, electrical cables 12 and data transfer cables 13. This depends on various factors, such as the number of subsea wells served by the umbilical, the distance that the umbilical must traverse and so on. A protective sheath 14 is also provided.

Subsea umbilical 2 connects the subsea wells 7, 8 and 9 to the host production facility 1 in series, typically by ‘daisy-chaining’. In other words, in order to provide electric power and data transfer between the host production facility and subsea wells, these services must pass subsea wells 7 and 8. A typical subsea umbilical may serve 12 or more subsea wells, but there are practical limits on the number of subsea wells that can be served by a subsea umbilical.

As control systems for subsea wells become more sophisticated, the electric power and data transfer requirements for each subsea well increase. In time, this will either reduce the number of subsea wells that can be connected to each subsea umbilical, or require subsea umbilicals with a larger capacity to transfer electric power and data. This could greatly increase the costs of subsea umbilicals.

Another problem with existing subsea umbilicals is that the cost of extending a subsea umbilical to a new subsea well in a new marginal prospect can be prohibitively expensive, especially if the subsea well must be connected to a host facility with a new umbilical.

A further problem associated with existing subsea umbilicals is that when a subsea umbilical develops a fault, it can affect all of the subsea wells ‘downstream’ of the fault, and so repairs and upgrades can be expensive in terms of lost production.

Existing subsea umbilicals use alternating current. Each power transmission cable in a subsea umbilical acts as a capacitor, and so where alternating current is used the capacitance properties appear in parallel with the electrical load. This leads to losses

DK 180544 B1 3 in power transmission, which limit the practical length of a subsea umbilical to around 150 km.

GB2299108 discloses a method and system for production of hydrocarbons from offshore reservoirs in which production flow from a number of wells is collected and transferred to a vessel at the sea surface for processing of the well flow and temporary storage of hydrocarbons in tanks on board the vessel before unloading of the hydrocarbons to an adjacent tanker.

Summary of the Invention The inventor has realised that the existing system of providing services to subsea wells using subsea umbilicals has several problems as described above.

It is an object of the present invention to mitigate some of these problems and provide an improved system of providing data transfer and electric power services to subsea wells.

According to a first aspect of the invention, there is provided a subsea oil and/or gas production system as set out in claim 1. Further aspects and preferred features are provided as set out in claim 2 et seq.

The invention differs from GB2299108 at least in that the electrical power and data network is for conveying direct current electrical power and data, and that the electrical power and data network is separate from the subsea fluid conduit network.

The invention thereby provides an improved subsea connection system for hydrocarbon wells.

Brief Description of the Drawings Figure 1 illustrates schematically a plurality of subsea wells connected to a host production facility; Figure 2 illustrates schematically a cross-section view of a subsea umbilical;

DK 180544 B1 4 Figure 3 illustrates schematically a plurality of subsea wells connected to a host production facility according to an embodiment of the invention; Figure 4 illustrates schematically in a block diagram a subsea node according to an embodiment of the invention; and Figure 5 illustrates schematically a cross-section view of an electric power and data transfer cable.

Detailed Description of the Invention The inventor has realised that many of the problems associated with prior art subsea umbilicals can be mitigated by separating out the services. According to the present invention, subsea umbilicals are primarily used to convey hydraulic fluid and wellstream service fluids, and are not used to convey electric power to subsea wells or data between a subsea well and a host production facility. A separate electric power and data network is provided. The following description introduces the term “subsea fluid conduit” to refer to a subsea conduit that is used for conveying fluids such as hydraulic fluids and wellstream service fluids, but is not used to convey electric power or data.

Referring to Figure 3 herein, a schematic diagram of a plurality of subsea wells connected to a host production facility 15 is illustrated. Pipelines carrying hydrocarbons are illustrated as thick black lines. Subsea fluid conduits are illustrated as thick dashed lines, and a separate network carrying data and electric power is illustrated as dotted lines. The separate network provides cables for electric power and data transfer, distributed in different ways. This type of network is referred to herein as a DC/FO network, as it provides DC current and data transfer using Fibre Optic (FO) cables.

The DC/FO network can typically provide a total system power of 100 kW and a total data bandwidth of 120 Gb/s, although it will be appreciated that any power and bitrates may be used within the constraints of the subsea environment. The DC/FO network in this description is served by hubs 16, 17 and 18, each with a typical output power of 10 kW and a data bandwidth of 1 Gb/s (limited by the subsea control system), although it will be appreciated that any power and bitrates may be used within the constraints of

DK 180544 B1 the subsea environment. The DC/FO network carries high voltage direct current from the host production platform 15 (or another source), and a function of the hub is to convert this voltage to a lower voltage usable by the subsea wells 19, 20, 21 (typically it must be converted from around 10,000V DC to below 1,000V DC). In the example of 5 Figure 3, hub 16 provides electric power and data to subsea wells 19, 20 and 21, and may also be connected to hub 17. The DC/FO network is separate from the subsea fluid conduit network and does not necessarily follow the same path as the subsea fluid conduit network.

In addition to hubs, the DC/FO network in this example is also provided with branching units 22, 24. A branching unit is effectively a cable splitter where a high voltage cable is input and two or more high voltage cables are output. The advantage of using branching units is that in the event of a fault or failure, individual portions of the DC/FO network can be isolated without affecting the rest of the DC/FO network. A subsea well can receive electric power from a hub, but not directly from a branching unit as the voltage supplied by a branching unit is too high. In this case, there will be a subsea node (hub) integrated in or in the near vicinity of the subsea well. This hub is not shown in Figure 3 to maintain readability.

In order to extend a network, one or more cable pigtails (shown as unconnected dotted lines at branching units 22 and 24 are retrieved from a branching unit and a new cable is spliced in. This is then connected to a new hub, and can then be used to connect the hub to new subsea structures such as subsea wells. Further distribution of the DC/FO network to subsea wells or other subsea structures can then be performed by daisy-chaining the DC/FO cable from another subsea well. For example, the distance between the host production platform 15 and hub 16 is greater than 150 km. The DC/FO network therefore uses a high voltage direct current power supply between the host production unit 15 and hub 16. Hub 16 steps down the voltage to a level that is useable by the subsea wells but is limited to a transmission length of a few kilometres.

Subsea wells 19 and 20 connect directly to the hub, and subsea well 21 is daisy- chained from subsea well 20.

A subsea end of the DC/FO Riser or landfall cable 23 may include several DC/FO cables which are typically connected to the DC/FO network in one of two ways; subsea retrievable pigtails may be provided on the seabed or as coiled cables in a frame,

DK 180544 B1 6 where each cable pigtail is retrieved for further expansion of the DC/FO network as required. Alternatively (or additionally), the subsea end of the DC/FO Riser or landfall cable 23 is provided with wet-mate high voltage connectors allowing direct connection of a subsea DC/FO cable to the DC/FO Riser or landfall cable 23.

In addition to connection to the DC/FO network, a subsea well must be connected to a subsea fluid conduit in order to receive hydraulic fluids and wellstream service fluids. A separate subsea fluid conduit network is provided. These may follow a pipeline, as shown by subsea wells 25, 26 and 27. These subsea wells obtain electric power and data services from hub 18, and hydraulic and wellstream service fluids via a conduit that could follow the pipeline connecting the subsea wells 25, 26, 27 to the host production platform. However, the subsea fluid conduits may be provided in a separate network that does not follow the pipelines between the subsea wells and the host production platform. One way to facilitate this is to provide hydraulic riser bases 28, 29. In this case a subsea fluid conduit extends from connected to the host production platform 15 to a hydraulic riser base 28. The hydraulic riser base 28 in this example has three outputs of subsea fluid conduits, allowing it to serve several different subsea wells that are not connected by the same pipeline to the host production facility. Hydraulic riser base 28 receives hydraulic fluids and wellstream service fluids from the host production platform 15 and provides a first subsea fluid conduit that serves subsea wells 30, 31, 32, a second subsea fluid conduit that serves subsea wells 33, 34 and a third subsea fluid conduit that serves subsea wells 19, 20 and 21. If necessary, a subsea fluid conduit could be extended from subsea well 21 to marginal prospect 35, and electric power and data transfer could be provided to marginal prospect 35 from hub 16. It will be appreciated that marginal prospect 35 could be developed using data and electric power transfer from any suitable hub, and obtain a subsea fluid conduit extended from any suitable subsea well or directly from a hydraulic riser base.

A hydraulic riser base need not be fully utilised, but may be available for future expansion. For example, hydraulic riser base 29 is shown having one input subsea fluid conduit and only one output subsea fluid conduit, but a further two subsea fluid conduits could be connected to the hydraulic riser base 29 in the event that future

DK 180544 B1 7 expansion is required.

Similarly, branching unit 24 does not split the electric power and data transfer into more than one branch, but could be used to do so in the future.

Note that most of the subsea wells are illustrated as receiving electric power and data transfer services directly from a hub.

However, this is not always necessary.

Subsea well 21 is illustrated as receiving electric power and data services extended from subsea well 20. Owing to the low voltages, this would limit the allowable distance between subsea wells 20 and 21, unlike a cable between the host production platform and a hub which owing to the high voltage direct current can have a length of greater than 150 kilometres without a significant loss of electric power.

However, advantages of connecting electric power and data services to a subsea well 21 from another subsea well include reducing the number of hubs required, which reduces costs.

In addition, it is envisaged that whilst DC/FO networks are being implemented, some subsea wells could retain existing umbilicals that provide data, electric power and fluids, whilst others are connected to the new DC/FO network and the new subsea fluid conduit network.

The presence of a network, and possibly also interconnected hubs and branching units, in a developed field allows much easier development of marginal prospects, as separate DC/FO network allows the provision of electric power and data transfer services to subsea wells regardless of where they are located.

It is much less expensive to extend existing subsea fluid conduits, as data and electric power need not be provided in a subsea umbilical as with prior art methods, but can be separately provided to each subsea well.

This makes the extension of subsea fluid conduits to a potential new subsea well that may be a marginal prospect more attractive.

Marginal prospect 35 can easily be provided with electric power and data transfer via hub 16 if required.

A further advantage of using a network of hubs is that electric power and data transfer is be standardized, leading to lower costs and greater reliability.

Currently, subsea umbilicals are typically custom made and subsea control systems typically use proprietary protocols for data transfer.

The use of a DC/FO network allows marginal prospects to be more easily investigated and exploited, as described above.

DK 180544 B1 8 The invention can also be used in existing operating fields to replace faulty or damaged umbilicals. This is normally an expensive process, but costs are reduced by replacing existing umbilicals with a DC/FO and subsea fluid conduit networks separately, as and when required (since the lifetime is expected to be longer for a fluid conduit network). As existing umbilicals come to the end of their life, the DC/FO and subsea fluid conduit networks could gradually replace all of the umbilicals. Furthermore, the provision of a subsea DC/FO network having hubs can reduce the costs of other aspects of subsea hydrocarbon recovery. For example, in addition to providing electric power and data transfer to subsea wells, the hubs can provide electric power and data transfer for condition monitoring of a subsea production system, subsea environmental observatories 36, Autonomous Underwater Vehicles (AUVs), 4D seismic grids on the seafloor and so on. AUVs are typically used for operation of valves, inspection of infrastructure, and in some cases maintenance and repair of infrastructure. Currently the operational time of an AUV is limited by battery life, but the provision of a subsea DC/FO network would allow the AUV batteries to be recharged in the subsea environment: Furthermore, the DC/FO network can be used to provide electric power and data transfer services to production pipelines that are used to convey hydrocarbons from a subsea well to the host production facility 15. Electric power and data transfer may be required, for example, for monitoring the wellstream flow in a pipeline branch, condition monitoring of production pipelines, monitoring of valve positions, actuating valves and so on.

The DC/FO network and subsea fluid conduit network can also be used to provide electric power and hydraulic fluids to a subsea processing unit 37. Examples of subsea processing units include units for wellstream boosting or units for wellstream separation (typically oil, gas or water) As described above, the electric power is provided as direct current. An advantage of using direct current is that electric power can be transmitted over longer distances than using AC. For example, using High Voltage Direct Current (HVDC), losses can be reduced to around 20% per 1,000 km, which makes direct current more attractive for supplying electric power to subsea wells from a remote source. Furthermore, the use

DK 180544 B1 9 of direct current reduces capital costs, as fewer and thinner conductors can be used compared with electric power transmission using AC. This is because the root mean square (RMS) voltage measurement of an AC conductor is only around 71% of the peak voltage, which determines parameters such as the insulation thickness and minimum conductor sizing. Another advantage of using direct current rather than AC is that, in a distributed electric power network, there is no need to synchronise AC sources, further reducing capital costs. The use of direct current allows the transfer of electric power over distances greater than 150 km.

Fibre optic communications are preferred for data transfer as there will be no electromagnetic interference from the DC power part of the cable, and they have a higher bandwidth than electrical transmission and can transmit data over longer distances than electrical transmission owing to very low losses.

Turning now to Figure 4, there is illustrated schematically in a block diagram an example of a subsea hub node 17. The hub 17 is provided with a unit 137 for receiving electric power and data from an external source. A processor 38 is used to control the operation of the hub 17. A computer readable medium in the form of a memory 39 is provided on which is stored a computer programme 40. When executed by the processor 38, the program 40 controls the operation of the hub 17. The memory 39 may also be used to store data such as a log of the hub’s 17 operation. A unit 41 for providing electric power in the network is provided, and a unit 42 for providing data transfer in the network is provided. It will be appreciated that these units operate most efficiently if they are physically combined and comprise a plurality of connectors, each connector having connections capable of providing both data and electric power transfer. There could be separate electric power and data transfer connection points, or a hybrid connector with connection points for electric power and connection points for data.

Figure 5 illustrates a cross section view of a electric power and data transfer cable 43. The cable comprises a protective sheath 44 in which optical fibres 45 are provided for data transfer, and electric power cables 46 are provided for electric power transfer. Each end of the cable 43 has one or two connectors, one end for connecting to the hub 17 and another end for connecting to a subsea well (typically via a subsea control module) or another subsea asset that requires data and electric power. Figure 5 is

DK 180544 B1 10 shown schematically by way of example only to illustrate the functional components of an electric power and data transfer cable. A typical cable will have multiple cores for data and one or two electric power cores, and various layers of shielding.

The invention effectively separate the functions of prior art subsea umbilicals into two separate networks; a network for providing electric power and data transfer, and subsea fluid conduits for conveying hydraulic fluids and wellstream service fluids to subsea wells. By providing a separate electric power and data network, costs are reduced and flexibility is greatly increased, allowing easier exploitation of marginal prospects and the provision of electric power and data to other subsea assets such as 4D seismic grids on the seafloor, subsea observatories, AUVs, monitoring systems and production pipelines.

It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. Furthermore, it is possible that hydraulic power for operating valves and so on may be replaced by electrical power, in which case hydraulic fluid will not need to be transported to each subsea well.

Claims

DK 180544 B1 11 PATENT REQUIREMENTS:

A subsea oil and / or gas production system comprising: a host production facility (15); a plurality of subsea wells (19-21, 25-27, 30-34) connected to the host production facility by a plurality of hydrocarbon pipelines; a subsea fluid conduit network connected to each of the plurality of subsea wells; characterized in that the plurality of subsea wells are connected in series by the plurality of hydrocarbon pipelines, and in that the subsea oil and / or gas production system comprises a plurality of subsea supply lines for transporting electricity, data transmission, hydraulic fluid and well flow service fluids to the plurality of subsea wells. ; and an electric power and data network for transporting DC power and data, the network being operatively connected to at least one of the plurality of subsea wells to supply the at least one of the plurality of subsea wells with data transmission and power services, the DC power and data network being separated. from the subsea fluid pipeline network.

The subsea oil and / or gas production system of claim 1, wherein the fluid transport network is adapted to transport any of hydraulic fluid and well stream service fluid.

A subsea oil and / or gas production system according to claim 1 or 2, further comprising - at least one branch unit (22, 24) between the host production facility and a hub (16, 17, 18), which hub (16, 17, 18) is arranged to reduce a voltage in the electric power received from the branch unit (22, 24) before supplying electric power to at least one of the plurality of subsea wells, the branch unit being arranged to receive electric power and data cables from the host production facility and supply separate power and data cables for a plurality of hubs.

A method of providing an underwater oil and / or gas production system, wherein a system comprises a host production facility (15), a

180544 B1 12 plurality of subsea wells (19-21, 25-27, 30-34) connected in series with the host production facility (15) by means of a plurality of hydrocarbon pipelines, and at least one subsea supply line for transporting electricity, data transmission, hydraulic fluid and wellbore service fluids to at least one of the plurality of subsea wells (19-21, 25-27, 30-34), the method comprising: providing an electric power and data network for transporting direct current electric power and data, wherein electric power and the data network is in operative communication with at least one of the plurality of subsea wells to provide the at least one of the plurality of subsea wells with data transmission and power services, to provide a subsea oil and / or gas production system according to any one of claims 1 to 3.

A method of replacing / repairing a subsea supply line in a hydrocarbon production system, the system comprising: a host production facility (15); a plurality of subsea wells (19-21, 25-27, 30-34) connected to the host production facility (15) by a plurality of hydrocarbon pipelines; characterized in that the system comprises a plurality of subsea supply lines connected to the plurality of subsea wells (19-21, 25-27, 30-34), and the method comprises: replacing at least one of the plurality of supply lines with a DC power supply. and data transmission network, and a subsea fluid conduit network, wherein the subsea fluid conduit network is separated from the DC power and data transmission network.

The method of claim 5, wherein the plurality of - subsea supply lines - transport - electric power, - data transfer, hydraulic fluid and well flow service fluids to the plurality of subsea wells.

The method of claim 5 or 6, wherein the subsea fluid conduit network is adapted to transport any of a hydraulic fluid and a well flow service fluid.