WO2007055587A1

WO2007055587A1 - Subsea uninterruptible power supply system and arrangement

Info

Publication number: WO2007055587A1
Application number: PCT/NO2006/000405
Authority: WO
Inventors: Geir Aalvik; Bernt Bjerkreim; Asbjørn ERIKSEN; Karl Olav Haram
Original assignee: Norsk Hydro Produksjon AS
Current assignee: Hydro Energi AS
Priority date: 2005-11-11
Filing date: 2006-11-10
Publication date: 2007-05-18
Anticipated expiration: 2008-05-11
Also published as: NO20055353D0; NO20055353L; NO326936B1

Abstract

An uninterruptible power supply UPS system for subsea application is disclosed. The UPS comprises an energy storage means, a power input port, a power output port, and an input power conversion system to charge the energy storage means. The UPS comprises further an output power conversion system to feed the power output port. Even further the UPS comprises at least one control system CTRL and a cooling system. The control system is equipped with a dedicated communication channel for specifically controlling operations of the UPS.

Description

"Subsea uninterruptible power supply system and arrangement"

Field of the Invention

The present invention relates to offshore installations. Specifically it relates to a local uninterruptible power supply for subsea installations.

Background of the Invention Offshore oil and/or gas fields may be developed with seabed (or subsea) installations which are tied back to a terminal onshore or an existing platform also generally called 'topside'. The seabed installation comprises one or more production templates where each template produces well fluid through manifold headers which are connected to one or more pipelines. The pipelines transport well fluid to an onshore terminal or an existing platform (receiving facility) for further processing. Processed gas and condensate are exported to the market. One or more umbilicals for power, control and utility supplies are installed from the receiving facility to the subsea installations.

For the initial production phase, well fluid may flow to the receiving facility by means of the reservoir pressure. Later in the production phase or at start-up of the production, well fluid boosting is required in order to maintain the production level and to recover the anticipated gas and condensate volumes. The conventional solution for such well fluid boosting facility is an offshore platform. However, a subsea compression system may be an alternative to or in combination to the platform solution. A subsea compression system has a number of advantages compared to a booster platform solution.

The system is safe to human injuries due to remote operation, and is also reliable, cost effective, environmental friendly and comprises few parts which make the system less complicated and easy to operate.

The long step-out power supply is defined from the connection point at the receiving facility to and including the main subsea transformer.

Such long step-out power supply comprises subsea main transformer with pressure compensation system, high voltage penetrator(s) umbilical termination head, combined or separate power and control umbilical including main electrical supply, utility power if required (also called auxiliary power or control power), fiber optic lines for control signals, hydraulic lines, barrier lines.

The compression station is connected directly to at least one subsea production template and is designed for boosting the product from the production templates. The well product is routed via a template manifold header, infield flow lines and to the suction side of the compression station.

High voltage power, low voltage power (if required), hydraulic, control and utilities are supplied from receiving facilities via the combined power and control umbilical. The umbilical is connected to the subsea compression station at the umbilical termination head. The high voltage (HV) power cables will be connected to the subsea main step-down transformer and the transformer will be installed on the subsea compression station with the umbilical attached. Figure 1 shows the power distribution system for the main subsea electrical consumers including topside, umbilical and compressor modules.

The subsea compression station comprises one or more compressor trains, one or more circuit breaker modules, inlet and outlet manifolds, inlet coolers (if supply pipelines are not sufficient for cooling the well stream), inlet sand trap

(for accidental sand production), parking location for main transformer and power umbilical termination head, required installation tools, high voltage electrical system, process system, utility power system, control system, hydraulic system, and barrier system.

The compressor train is the main equipment required for boosting the well stream. The compressor train comprises compressor module, compressor Variable Speed Drive (VSD), anti-surge valve and actuator, anti-surge cooler, separator/scrubber module, pump module, pump VSD, remote and manually operated valves, and a control system including control modules.

Common to the compressor trains is a power and umbilical connection system and a valve manifold fitted with flow line connection systems.

The station power distribution system consisting of removable circuit breaker and variable speed drive modules are arranged together at one end of the station structure adjacent to the main inlet transformer and the main umbilical tie-in.

The modules are provided with local guiding/docking and are locked into position by dedicated mechanisms. Intervention for ROV is designed for minimum top and side access. Access to modules for vertical removal/installation is provided from the top and sides of the protective structure.

Smaller removable modules such as control pods, control valves and certain instrumentation units are provided as individual units and/or included within one of the main modules as removable items, these modules/items are run on dedicated intervention running tools.

The compressor is directly driven by a high-speed motor. The electrical motor is cooled with hydrocarbon gas with a pressure regulated to be equal to or as close to the suction pressure as possible. The gas source can either be conditioned gas supplied to the subsea compression station from an external source, discharge gas from the compressor module or suction gas to the compressor module. The hydrocarbon gas for electrical motor cooling might be conditioned prior to entering into the electrical motor and the hydrocarbon gas might also be replaced by other suitable gases. Alternatively the motor may be fully canned with main cooling from the gas flow.

The compressor is able to meet the design operational conditions over the production period with declining production wellhead pressure. Re-bundling of the compressor can be performed as part of a maintenance program.

A magnetic bearing system is used for each of the subsea compressor modules. The system includes magnetic radial and axial bearings as well as run-down bearings.

The compressor station manifold is equipped with a remote operated isolation valve facilitating by-pass of the compression trains. The compressor(s) have anti-surge control recycle line designed for full recycle flow at maximum continuous speed (105%). The anti-surge control valve is electric actuated, axial stroke and is located close to the compressor discharge at high point. An anti-surge re-cycle cooler is included downstream of the anti- 5 surge valve in the re-cycling pipe loop.

The compressors have a discharge pipe equipped with a remote operated isolation valve. A non-return valve is fitted in the compressor discharge pipe upstream of the isolation valve.

10

The compression station comprises the following subsea process coolers:

Anti-surge cooler/recycle cooler to cool gas flow in anti-surge line (compressor recycle loop)

Input coolers to cool the flow from template(s) (optional)

I₅

The condensate pumps are able to handle the liquid production and boost it up to the required discharge pressure. The pumps are variable or fixed speed driven.

20 The compressor station has tie-in connection for well fluid discharge. Each of these is equipped with ROV (remote operated vehicle) operated valves for routing of the well fluid to the different pipelines.

The process in the subsea compression station is envisaged in the following s paragraphs. The compression system allows recirculation for anti-surge protection and startup/shut-down operations. The recycle cooler and recycle loop is designed for full recycle flow at compressor maximum continuous speed (105%).

The liquid boosting system consists of condensate pumps with variable speed drives. The pump discharge pipes are equipped with a non-return valve upstream of the discharge isolation valve.

Anti-surge control is made possible by monitoring the compressor suction flow rate, temperature, pressure together with compressor discharge pressure and temperature.

A control and monitoring system is designed for control and monitoring one or more subsea compression trains with associated anti-surge controllers. As a base case for operating the subsea compression trains, including compression station valves, a multiplexed electro-hydraulic system is included.

The subsea facilities comprise remotely actuated valves to control the flow of produced gas and the injection of chemicals. The manifold valves may be hydraulically actuated and the anti-surge control valve may have an electrical actuator.

Local instruments (transmitters) are provided to measure pressure, temperature, gas flow rate and record the anti-surge valve position.

The different types of valves, the condition monitoring system and the transmitters are interfaced via the subsea control modules. Interfaces with subsea variable speed drives and circuit breakers, distributed control system and emergency shut down systems are foreseen.

Interface and closing of control loops between the variable speed drives circuit breakers and compressors control system may be via the receiving facilities control system main bus. All information, alarms and interlocks between the two systems should be handled by the distributed control system.

The receiving facilities distributed control system controls all control loops defined "slow". This is typically opening and closing of manifold valves and condition monitoring systems. The subsea control system has interconnection links to handle potential subsea shutdown requirements.

Dynamic control loops, which require quick response, are the anti-surge controller and the magnetic bearing controller. These loops shall be closed subsea if required.

Anti-surge algorithms are identically implemented for all compressor stages. The control algorithms include features for suction and discharge pressure override, i.e. limiting the discharge pressure or increasing the suction pressure.

Operator interface is through a distributed control system for all compressor stages, and includes, but is not limited to:

Display process inputs: Os, Is, Ps, Td, Pd - Display valve position.

Display controller output — independent of operator forcing. Possibility for operator to force the valve more open than required. Display status of fast path solenoid.

Display distance from operating point to surge/control line.

Display surge control line margins with respect to pressure, flow and speed.

However, the control system and associated anti-surge system have to take into consideration the different operating scenarios.

As shown above a number of signal- and power lines have to connect the subsea installation to the receiving facility (topside). When distance from topside to the subsea installation increases and the one or more umbilicals get longer, problems concerning power supply increase. Reactive power produced by the cable must be compensated. Harmonic distortions and electrical transients during energizing and disconnection of equipment are other well known challenges. By installing a subsea uninterruptible power supply

(hereinafter UPS) system many of these challenges will be solved or reduced.

Also the cost may be reduced since there are less cables required compared to having the UPS located at topside because the UPS can be fed from subsea main power. The short circuit level of a UPS is low and the challenge to have enough short circuit power available in the subsea installation in order to achieve correct relay protection and discrimination philosophy can be solved by having the UPS subsea close to the power consumers.

UPS systems are well-known in industry, office and today even at home in situations where a power consuming device must not suffer from being cut off the public (or more local) power supply. They are available in small versions being able to provide power from about 100W up to several hundred kW for from a few minutes up to many hours. During this time span the critical equipment supplied with power has either to be transferred into a power-off tolerable state or external power has to be re-supplied, i.e. grid power has to return or alternative power has to be provided. A UPS is always located as close as possible to the power consuming device to avoid as many as possible fault sources and is usually under control of the responsible operator of the power consuming device.

In general a subsea UPS can be used in all applications where distribution of low voltage (typically 400V) is required subsea. Typical consumers of low voltage power subsea supplied by a UPS comprise:

• Several control systems located in a geographically small area. • Electric actuators for valves

• Magnetic bearings

• Switchgear monitoring and control

• Measuring devices for current and voltage in switchgear, transformers, VSDs, motors and other electrical installations.

Referring to Figure 3 a conventional UPS comprises an energy storage means (ES = energy store) and two power converters. The first power converter (PWRC = power converter) takes the input power through the input power port IN from the grid (there might be a transformer between the input and PWRC for galvanic isolation) and converts it into a form which is suitable to be fed into the energy store ES and into the second power converter - called inverter (PWRI = power inverter). This inverter PWRI brings the power back to a form which the consuming device can use. Since the inverter PWRI has full power supply from both the energy store ES and the converter output any malfunction of the input power line, which would result into the converter PWRC shutting down, would only result into the inverter PWRI taking energy from the energy store ES instead of from the converter PWRC. Until the energy store ES is drained, the inverter output (OUT = output) will provide power to the connected loads by taking energy from the energy store. A control and monitoring system (CTRL = control system) is also a part of a UPS. And since power conversion involves losses resulting in heat, UPS systems may need cooling systems (COOL = cooling system) to transfer the heat to a heat sink (HS = heat sink).

Summary

An uninterruptible power supply UPS system for subsea application is disclosed. In a first aspect the invention provides a UPS comprising an energy storage means ES, a first power input port IN, a power output port OUT, and an input power conversion system PWRC to charge the energy storage means ES with energy taken from the first power input port IN. The UPS comprises further an output power conversion system PWRI to feed the power output port OUT using energy taken from the energy storage means ES and/or the input power conversion system PWRC. Even further the UPS comprises at least one control system CTRL. The control system CTRL is equipped with at least one dedicated communication channel CC for specifically controlling operations of the UPS.

The UPS system may further comprise any of a start-up function and a close- down function. The communication channel (CC) may be a fiber-optical communication channel.

In a further embodiment the control system (CTRL) is separately retrievable from the subsea installation. The energy storage means (ES) may be at least one of a mechanical storage means, a chemical storage means and an electrical storage means.

Further, said UPS system may comprise a further power input port (INB) connected to a selector circuit (SEL) wherein the power output port (OUT) receives its power from said selector (SEL), the selector being also fed by said power output conversion system (PWRI) thus enabling said UPS system to select a power path bypassing and relieving, said input power conversion system (PWRC), said output power conversion system (PWRI), and said cooling system (COOL). The UPS system may comprise a further power input port (INC) to said power input converter (PWRC), wherein said further power input port (INC) may be equipped with an additional transformer. Further power input port (INC) may be fed from one of an offshore platform, a shore based facility, another subsea installation, a submarine, a surface floating vessel or a remotely operated vehicle (ROV).

The cooling system may be built into a waterproof, pressure resistant housing, said housing being used as a heat dissipating body to dissipate heat from said UPS system.

In one preferred embodiment the cooling (COOL) system is a passive cooling system comprising heat pipe technology or boiling cooling technology. However, said cooling system may also be an active cooling system.

The UPS or parts of said UPS system may be separately retrievable from said subsea installation.

In a second aspect the invention provides an auxiliary-power supply arrangement, comprising a main-power supply, an auxiliary-power distribution point for non-critical loads fed by said main-power supply, an auxiliary-power distribution point for critical loads, wherein said auxiliary-power distribution point for non-critical loads is divided into at least two parallel, mutually redundant auxiliary-power distribution points for non-critical loads, each being fed by an individual transformer which in its turn is fed from said main-power supply, wherein each of said auxiliary-power distribution points for non-critical loads feeds said auxiliary-power distribution point for critical loads through at least one of said UPS systems. Drawings

Below embodiments of the present invention will be described mainly with reference to the accompanying drawings in which

Fig. 1 shows an overview electric power circuit scheme of subsea compression station, Fig. 2 shows the electrical arrangement around a UPS inside a subsea installation,

Fig. 3 shows the block diagram of a conventional UPS, Fig. 4 shows the block diagram of a UPS according to the present invention, Fig. 5 shows a UPS with additional charging input within the environment of the seabed station,

Fig. 6 shows seabed installations with auxiliary charging cables between them, and

Fig. 7 shows the arrangement used to provide a redundant auxiliary-power supply.

Detailed description of preferred embodiments Preferred embodiments of the present invention will now be described here with reference to enclosed figures.

Abbreviations and definitions

Main power: Power supplied from a topside power generating system or grid connection. Typically supplied through subsea cables from 11 kV up to several hundreds of kV. Power supplied for the main subsea consumers such as VSDs, motors and distribution transformers.

Auxiliary power: Power used for small power consumers such as control systems, magnetic bearings, electrical actuators, measuring devices UPS input power, etc. Typically between 230 V and 690 V. The word control power and utility power is sometimes used in replacement for auxiliary power. Subsea main-power distribution switchboard: Switchboard where the subsea main power loads are connected. For the subsea compression system main power loads will be VSDs for compressors, VSDs for pumps and auxiliary power distribution transformers.

Non-critical loads: Auxiliary power loads that are tolerant towards voltage variations and loss of power. The loads will be disconnected if there is a failure in the topside power system.

Critical loads: Auxiliary power critical loads are loads that require power for a given time period after loss of main power. Typically magnetic bearings and control equipment in order to achieve safe shutdown after loss of main power.

Figure 4 shows a block diagram of the UPS according to an embodiment of the invention. First of all the diagram shows the dedicated communication channel CC to the control system CTRL. This communication feature can be used to control the behavior of the UPS in situations when a standard behavior is not wanted. For instance would prior art UPS system take over the power supply of the connected loads when main power goes down and 'hope for' main power coming back before the energy storage means ES being discharged. This behavior is however not wanted in some cases. In the subsea compression station case, when main power is disconnected, there will be a sequence of operations being performed by different modules inside the station on using power from the UPS and controlled by different other control systems. This can be for instance bringing down magnetic bearings into a state where they can tolerate being without power without being harmed. Valves could be closed, switchgear run into a desired power-off state, 'final' measurements being reported to topside, and more.

When all these tasks are performed there is no need for the UPS to supply further power to the last still working systems and the UPS can be instructed to close down by instructions sent over the communication channel. From now on the remaining energy in the energy store ES can be saved.

Further when main power is available again, a series of start-up procedures in a number of other control systems in the subsea station will normally be performed. Magnetic bearing have to be turned on, initial measurements must be reported to the receiving facility, valves have to be opened, and more. Here an instruction is sent to the UPS, which supplies those control systems with power taken from the energy storage means ES. Finally main power is switched on and supplies the seabed station.

The preferred implementation of the communication channel CC is a fiber optical communication line avoiding any disturbance by magnetic and electrical fields inside the umbilical, which may be very long (>100km) or outside.

The control system may be implemented as a separately retrievable unit to permit repair and maintenance work. Even any combination of modules belonging to the UPS can be mounted inside separately retrievable container for this same reason. For instance during installation of the subsea station some or all modules of the UPS system including a fully charged energy storage means ES could be connected to the subsea station as the last step before starting up the station.

The energy storage means ES can be implemented choosing from a number of technologies, such as electrical (e.g. supercapacitors, superconductors), chemical (e.g. batteries, fuel cells) or mechanical (e.g. flywheels).

Figure 2 shows the inventive arrangement with main power supply from topside and a UPS system embedded inside that arrangement. From topside an umbilical supplies the installation with high-voltage power. After a first transformer the power is fed into a first switchboard, the main power switchboard. Several large power consuming units can be connected by breakers to this switchboard. At least one of these consumers is a second transformer which in turn feeds a second switchboard for non-critical loads auxiliary power. Several consumers can be connected to this second switchboard, the consumers characterized in that they generally would not suffer in case the power supply is switched off unexpectedly. Among the consumers supplied with power from this switchboard for non-critical loads is the UPS. The output of the UPS supplies power to a third switchboard for critical loads. Some electrical power consuming equipment, typically magnetic bearings and control systems might lead to damage from an unexpected, sudden loss of power. Such consumers constitute "critical loads", and must be supplied with power from a UPS. Any such "critical load" consumers in the installation are connected to the switchboard fed by the UPS.

Further with reference to Figure 4 the UPS according to the present invention could be equipped with a further power input port INB. INB can be used to bypass and. relive the input power conversion system (PWRC), the output power conversion system (PWRI) and the cooling system. The INB and selector switch (SEL) could however also be omitted in the system design. The UPS output (OUT) would then be connected directly to the output power conversion system (PWRI).

Further with reference to Figure 4 the UPS according to the present invention could be equipped with a further power input port INC. Power supplied to this port is used to charge the energy storage means ES additionally. The objective with an additional port is that it could be supplied with power from an independent source. Even if the amount of power available is far too low to run the subsea station auxiliary power system, it can be sufficient to keep some subsystems running.

Figure 5 shows as a block diagram parts of the substation with a UPS equipped with such an additional input port in place. As an example and not limitation the figure indicates that the port receives this power from a neighbor subsea station. A transformer could be added to avoid direct electrical contact between both stations and/or to adapt voltage levels. The transformer could be a part of the UPS even if the figure shows it outside the UPS. Figure 6 shows a situation with 2 seabed installations SBH , SBI2. Both have their own topside supplies TS1 , TS2 which might be independent concerning main power supply, and supply power through the umbilicals U1 , U2. The neighbor connection NC cable can be used to supply a minor amount of power from SBH to the UPS in SBI2 in case TS 1 looses its main power supply. Besides being connected to the neighbor seabed installation the connection could also be made to an offshore platform, a second shore based receiving facility, a submarine, a surface floating vessel, a remotely operated vehicle (ROV) or any other suitable source of power.

Another feature is the cooling system COOL indicated in Figures 3 and 4. Any power conversion process involves heat losses. A subsea station comprises a multitude of electrical or electronic power conversion units. The heat from the losses of these power conversion units must be removed from the system to a heat sink HS to avoid extensive temperature increase which ultimately would lead to failure or destruction of the system. The conditions for a subsea cooling system are special from at least three points of view:

(1) the cooling system is - as a part of a subsea installation - not easily accessible for maintenance and repair

(2) there is a good heat sink HS consisting of seawater at reasonably low temperature available (3) a close-down for repair and maintenance on a subsea station is expensive

These conditions make it necessary to design the cooling system correspondingly. First, anything which requires repair and/or maintenance during the expected operational lifetime of the installation must be avoided. In conventional onshore- or platform-based power conversion systems, an active air cooling is commonly used which involves the application of fans. Fans however are prone to wear, thus needing replacement and/or maintenance. The cooling system to cool mainly the power converters of the UPS would be built into a waterproof, pressure resistant housing, and the housing itself would be used as a heat dissipating body submerged into the seawater as it is. To achieve a technical solution with minimal wear of parts of the cooling system, a passive cooling system based on heat pipe technology and/or boiling cooling technology is preferred. The heat is removed typically from the power electronic components (e.g. IGBTs) in the UPS power converters PWRC, PWRI and dissipated to the ambient seawater. An active cooling system could also be used if the components have a sufficiently long MTBF (mean time between failures).

The subsea station is not easily accessible for maintenance and repair, but one can not avoid that parts can be damaged or must be replaced for some other reason. To make this possible, using for instance a ROV, some modules should be designed to be exchangeable in situ. The most likely configuration is to put all the elements shown in Figure 4 in one pressure resistant housing with wet matable connectors to connect the different inputs and outputs. However, other configurations could also be made. Typically the energy storage means ES of the UPS is a candidate for replacement during the operational life-time of the subsea station, and could therefore be separately retrievable from the other elements of the UPS. However, when parts are designed for replacement subsea anyway, some combinations of modules can be mounted inside a common enclosure. For instance, the power input converter PWRC and/or power output inverter PWRI can be located in separate enclosures, but also in the same enclosure as the energy store ES.

The present invention relates also an arrangement of the UPS system as shown in Figure 7. By using two or more UPS systems, each feeding an auxiliary- power switchboard, those switchboards are mutually redundant. Modules being supplied with power from at least two switchboards could continue to work even if one switchboard fails. Further, even the charging of the two or more UPS systems could be achieved by also separating the switchboard for non-critical loads into at least two separate switchboards, each supplied with power by a separate transformer fed from the main power switchboard.

Finally it should be emphasized that many variations could be applied to the principles disclosed here. However these modifications and variations should be considered being a part of the scope and idea of the present invention.

Claims

1. An uninterruptible power supply (UPS) system for subsea application, comprising - an energy storage means (ES), a first power input port (IN), a power output port (OUT), an input power conversion system (PWRC), an output power conversion system (PWRI), and - at least one control system (CTRL) characterized in that said control system (CTRL) is equipped with at least one dedicated communication channel (CC) for specifically controlling operation of said UPS. .. ^■ , .

2. A UPS system according to claim 1 , characterized in that said operation comprises any of a start-up function and - a close-down function

3. A UPS system according to claim 1 or 2, characterized in that said communication channel (CC) is a fiber-optical communication channel.

4. A UPS system according to one of the preceding claims, characterized in that said control system (CTRL) is separately retrievable from said subsea. installation.

5. A UPS system according to claim 1 , characterized in that said energy storage means (ES) is at least one of a mechanical storage means, 5 - a chemical storage means, and an electrical storage means

6. A UPS system according to claim 1 , characterized in that io said UPS system comprises a further power input port (INB) connected to a selector circuit (SEL) wherein the power output port (OUT) receives its power from said selector (SEL), the selector being also fed by said power output conversion system (PWRI) thus enabling said UPS system to select a power path bypassing and relieving i5 - said input power conversion system (PWRC), said output power conversion system (PWRI), and a cooling system (COOL).

7. A UPS system according to claim 1 , 20 characterized in that said UPS system comprises a further power input port (INC) to said power input converter (PWRC).

8. A UPS system according to claim 7, 25 characterized in that said further power input port (INC) is equipped with an additional transformer.

9. A UPS system according to claim 7 or 8, 30 characterized in that said further power input port (INC) is fed from one of an offshore platform, a shore based facility, another subsea installation, a submarine, a surface floating vessel, a remotely operated vehicle (ROV).

10. A UPS system according to claim 1 , characterized in a cooling system (COOL), said UPS system beeing built into a waterproof, pressure resistant housing, said housing being used as a heat dissipating body to dissipate heat from said UPS system.

11. A UPS system according to claim 10, characterized in that said cooling (COOL) system is a passive cooling system comprising one of heat pipe technology boiling cooling technology.

12. A UPS system according to claim 10, characterized in that said cooling system is an active cooling system.

13. A UPS system according to claim 1 , characterized in that said UPS or parts of said UPS system are separately retrievable from said subsea installation.

14. An auxiliary-power supply arrangement, comprising a main-power supply an auxiliary-power distribution point for non-critical loads fed by said main-power supply - an auxiliary-power distribution point for critical loads characterized in that said auxiliary-power distribution point for non-critical loads is divided into at least two parallel, mutually redundant auxiliary-power distribution points for non-critical loads, each being fed by an individual transformer which in its turn is fed from said main-power supply, wherein each of said auxiliary-power distribution points for non-critical loads feeds said auxiliary-power distribution point for critical loads through at least one of said UPS systems.