WO2009122174A1

WO2009122174A1 - Underwater power supplies

Info

Publication number: WO2009122174A1
Application number: PCT/GB2009/000869
Authority: WO
Inventors: Steven Lewis Charles Simpson; Peter John Davey
Original assignee: Vetco Gray Controls Limited
Priority date: 2008-04-04
Filing date: 2009-03-31
Publication date: 2009-10-08
Also published as: GB0806098D0

Abstract

An underwater hydrocarbon production installation including a hydrocarbon production well, comprises an electrical power source. A method of providing power to an underwater hydrocarbon production installation including a hydrocarbon production well, comprises the steps of locating an electrical power source at the underwater hydrocarbon production installation and connecting at least one power source to the installation.

Description

Underwater Power Supplies

This invention relates to an underwater hydrocarbon production installation including a hydrocarbon production well and a method of providing power to such an installation.

In current underwater hydrocarbon extraction systems, such as subsea production control systems, electrical AC power is generated at a topside facility such as a rig, vessel or land facility, and is then transmitted to the subsea field via copper cores in an armour-clad umbilical cable. The subsea field may typically include, for example, four subsea "Christmas trees" and two subsea manifolds. Each manifold accommodates a subsea control module (SCM) with up to two internal subsea electronics modules (SEM) in each SCM. The SEMs are supplied with AC power from the topside facility via the umbilical cable and connectors on the body of the SCM.

Part of such a system is schematically shown in Fig. 1. Electric power is supplied from a control platform (not shown) to an SCM 1 which is mounted on a Christmas tree 2, via an umbilical cable 3. The umbilical 3, which may for example be as long as about 200 km, carries copper conductors for the transmission of power and, typically, fibre optic cables for carrying communication signals. The umbilical cable 3 is terminated in a subsea distribution unit (not shown) which splits the power supply and communications signals to feed the SCM 1 at each well. This supply is fed in turn to an SEM, internal to the SCM 1, whereby it is converted to the appropriate voltage and distributed, via a distribution unit 5, to process sensors 4. Electrical communications signals are routed between the SCM 1 and the process sensors 4 by the same route.

Due to the resistive, inductive and capacitive properties of the copper in the umbilical cable, power is lost which is dissipated as heat. The subsea field may for example be positioned up to around 200km from the topside facility, which means that in some cases about half the power generated at the topside facility is lost as heat into the sea water. As a result, operating costs are relatively high and there is considerable energy wastage. It is an aim of the present invention to overcome the disadvantages of the conventional power supply systems described above and dramatically reduce the amount of wasted power lost as heat to the surrounding water, which in turn would reduce operating costs considerably. This aim is achieved by the utilisation of local power generation, i.e. at the underwater installation.

In accordance with a first aspect of the present invention there is provided an underwater hydrocarbon production installation including a hydrocarbon production well, the installation comprising at least one electrical power source.

hi accordance with a second aspect of the present invention there is provided a method of providing power to an underwater hydrocarbon production installation including a hydrocarbon production well, comprising the steps of locating at least one electrical power source at the underwater hydrocarbon production installation and connecting the or each power source to the installation.

There could be at least one dedicated such power source to supply electrical power to respective components of the installation.

The or each respective component could comprise a sensor.

The power source or at least one of the power sources could comprise a battery.

The power source or at least one of the power sources could comprise a capacitor.

The power source or at least one of the power sources could comprise a generator, hi this case, the generator or at least one of the generators could comprise means for generating electrical power from kinetic energy of the surrounding water. The generator or at least one of the generators could comprise means for generating electrical power from thermal energy and/or kinetic energy associated with produced hydrocarbon.

The generator or at least one of the generators could comprise means for generating electrical power from thermal energy differentials at different depths of the surrounding water.

The power source or at least one of the power sources could comprise means for generating electrical power utilising electrical potential differences between metals present in the surrounding water, for example from a cathodic protection system of the installation.

The effect of the invention is that a conventional electrical power supply to a subsea well complex via an umbilical, is replaced by at least one underwater electrical power supply, located at the underwater installation, close to the well complex. This not only reduces the power loss dissipated as heat to the surrounding water, with a consequential reduction of operating costs, but also substantially reduces the capital and installation costs, since the provision of local power generation is likely to be substantially less for both capital plant and installation costs. Local power generation in accordance with the invention is becoming more attractive as the current tendency for the distance from the installation to the topside platform, which is currently already up to about 200 km, increases. Currently, power to the process sensors is provided by the SEMs in the SCM, as it is a convenient location for voltage conversion of the main supply from the umbilical cable. A local electrical power supply (whether AC or DC) for these sensors is beneficial in that power would not have to be provided by the SEMs. This improves their reliability, as there is less heat generation, with fewer components, including expensive connectors, and as a result will result in smaller, more manageable modules. Typical requirements of subsea well installations are:-

1. Low Power Applications (< 1 OW DC) : -A-

- Provide power to wireless self-powered sensors mounted on a subsea Christmas tree or manifold (which forms the subject of a co-pending application).

2. Medium Power Applications (>10W DC, <100W DC):

- Provide power to wireless self-powered multi-phase flowmeters (approximately 5OW DC).

3. High Power Applications (>10OW AC and/or DC)

- Provide approximately 250W of continuous AC power to an SEM mounted in an SCM;

- Provide approximately 250W of time-constrained AC backup power to an SEM mounted in an SCM during a field failure (i.e. so that there is enough power to allow graceful shutdown of the field if topside power is lost); - Provide power to Christmas tree-mounted pumps, motors and compressors.

hi order to be feasible, an underwater power supply would be required to deliver continuous power to the SEM(s) in the SCM for a substantial length of time, for example around 3 years. This is a reasonable period of time without requiring unscheduled intervention by the operator (i.e. maintenance). Such a power supply could be achieved in several ways, for example:

- Large battery technology able to deliver continuous power at about 250W AC for around 3 years without recharging. Recharging would then be performed via an intervention vessel, i.e. a ship with a remote operated vehicle (ROV), a battery charger and a relatively short umbilical. Such charging could be carried out at the operator's convenience, for example during routine maintenance.

- Large battery technology trickle-charged using energy generated from renewable sources; and

- Subsea battery technology generating energy from seawater. In the case of generation from renewable sources, there are various practical sources available:

- Thermal, utilising the substantial heat available from the production fluid, and / or the temperature differentials at different water depths;

- Corrosion protection (CP) system, utilising the electrical potential difference between metals in the sea water;

- Seabed currents, using low speed turbines to harness the energy of seabed currents;

- Subsea batteries. These would provide storage of electrical energy particularly from renewable sources which are not guaranteed to be constant, such as seabed current generators. For example, it is practical to power process sensors from conventional batteries which would provide an operational duration of, typically, three years before requiring replacement. The recent developments in so-called "Super Capacitors", using nanotube technology is making them an attractive alternative to conventional primary and secondary batteries, since it is claimed that they have no significant charge cycle time or electrical current limitations.

The present invention provides various advantages over prior art systems. These include:

Cost Savings:

- The majority of topside and subsea power distribution equipment can be removed from the system because power is generated and distributed locally, i.e. at the underwater installation;

- Costs associated with the manufacture of umbilical cable are reduced due to the reduction in the required copper core diameter as a result of lower topside to installation power transmission, or the complete removal of the copper core; - Costs associated with the handling of the umbilical are reduced due to the smaller umbilical necessary;

- The power loss by heat dissipation is reduced due to the power being generated and delivered locally rather than having to be transmitted along a copper umbilical; - Equipment required to handle and install the umbilical is smaller and, therefore, cheaper to hire or purchase;

- Topside to installation communications may be effected using highly reliable optical fibre, thus requiring no copper umbilical and suffering zero electrical power loss.

Time Savings:

- The umbilical is easier and quicker to handle due to its reduced size and weight;

- The equipment required to handle and install the umbilical is smaller and therefore quicker to mobilise.

Reliability:

- Power is generated and delivered locally (i.e. within the installation) rather than having to be transmitted along a copper umbilical; - The majority of topside and subsea power distribution equipment can be removed from the system because power is generated and distributed locally.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 schematically shows part of a conventional underwater installation;

Fig. 2 schematically shows part of an underwater installation according to a first embodiment of the present invention;

Fig. 3 schematically shows part of an underwater installation according to a second embodiment of the present invention;

Fig. 4 schematically shows a hydrocarbon production system including an underwater installation according to a third embodiment of the present invention; and Fig. 5 schematically shows a hydrocarbon production system including an underwater installation according to a fourth embodiment of the invention.

Fig. 2 schematically shows in a simplified manner a first embodiment of the present invention. A control platform (not shown) is connected to an SCM 7, which is mounted on a Christmas tree 2, via an umbilical cable 3. The umbilical 3 includes fibre optic cables for carrying control and monitoring communication signals, but unlike the system shown in Fig. 1, there is no power supplied via the umbilical 3, and therefore no need for copper conductors to be included therein.

The umbilical cable 3 is terminated in a subsea distribution unit (not shown) which splits the communications signals to the SCM 7 at each well. They are fed in turn to an SEM, internal to the SCM 7, and distributed, via a distribution unit 5, to process sensors 4.

A local electric power generator 6 is provided at the well tree 2, which is connected to, and therefore powers, the SCM 7. Electric power and signals are then supplied by the SCM 7 to the process sensors 4 via distribution unit 5.

Fig. 3 shows a second embodiment, in which process sensors 8 are self-powered by associated respective batteries 9. The sensors 8 are wireless-linked to the SCM 7 via respective antennae 10 fitted to each process sensor 8. This arrangement removes the need for all copper conductors, and their associated expensive connectors, in the system.

Fig. 4 shows a third embodiment, illustrating a complete hydrocarbon production system. A topside control platform 11 is connected to a subsea distribution unit 12 via an umbilical 13. This includes a multi-channel optical fibre and no copper content. The distribution unit 12 connects to each Christmas tree 14, here four trees are shown, in the well complex, via respective copper-free optical fibres 15. Each Christmas tree 14 is powered by a local electrical power generator 16. Process sensors (not shown) on each tree are self-powered as in the second embodiment. Fig. 5 shows a fourth embodiment in which: reference numeral 17 designates a topside facility in the form of a control platform; reference numeral 18 designates a main production flowline umbilical; reference numeral 19 designates a subsea Christmas tree or manifold; reference numeral 20 designates a subsea hydrocarbon well; reference numerals 21 designate tree or manifold sensors; reference numeral 22 designates a remote sensor; reference numeral 23 designates an SCM; reference numeral 24 designates well bore tubing; reference numeral 25 designates a subsea turbine; and reference numerals 26 designate power generators.

Topside facility 17 is connected to the tree or manifold 19 by main production flowline umbilical 18. Hydrocarbons flow along the umbilical 18 from the well 20 through the well bore tubing 24 and the tree or manifold 19 creating a flow to the topside.

The production flow is used by power generators 26 to create electric current to the power the tree/manifold mounted sensors 21, remote sensor or sensors 22 or any other device connected to the system. Methods of power generation include conversion of heat from the flowline, conversion of flow of the hydrocarbons and current generated by a cathodic protection system.

In addition, subsea turbine 25 is used to generate power for the entire tree or manifold 19 including the SCM 23. This converts the flow of subsea water currents into electricity.

The above-described embodiments are exemplary only, and various alternatives will be apparent to those skilled in the art, within the scope of the claims.

Claims

1. An underwater hydrocarbon production installation including a hydrocarbon production well, the installation comprising at least one electrical power source.

2. A method of providing power to an underwater hydrocarbon production installation including a hydrocarbon production well, comprising the steps of locating at least one electrical power source at the underwater hydrocarbon production installation and connecting the or each power source to the installation.

3. An installation according to either of claims 1 and 2, or a method according to claim 2, comprising at least one dedicated such power source to supply electrical power to respective components of the installation.

4. An installation or a method according to claim 4, wherein the or each respective component comprises a sensor.

5. An installation according to any of claims 1, 3 and 4, or a method according to any of claims 2 to 4, wherein the power source or at least one of the power sources comprises a battery.

6. An installation according to any of claims 1, 3, 4 and 5, or a method according to any of claims 2 to 5, wherein the power source or at least one of the power sources comprises a capacitor.

7. An installation according to any of claims 1, 3, 4, 5 and 6, or a method according to any of claims 2 to 6, wherein the power source or at least one of the power sources comprises a generator.

8. An installation or a method according to claim 7, wherein the generator or at least one of the generators comprises means for generating electrical power from kinetic energy of the surrounding water.

9. An installation or a method according to claim 7 or 8, wherein the generator or at least one of the generators comprises means for generating electrical power from thermal energy and/or kinetic energy associated with produced hydrocarbon.

10. An installation or a method according to any of claims 7 to 9, wherein the generator or at least one of the generators comprises means for generating electrical power from thermal energy differentials at different depths of the surrounding water.

11. An installation according to any of claims 1 and 3 to 10, or a method according to any of claims 2 to 10, wherein the or at least one of the power sources comprises means for generating electrical power utilising electrical potential differences between metals present in the surrounding water.