WO2007055594A1

WO2007055594A1 - Arrangement for external black start of subsea power system

Info

Publication number: WO2007055594A1
Application number: PCT/NO2006/000412
Authority: WO
Inventors: Geir Aalvik; Bernt Bjerkreim; Asbjørn ERIKSEN; Karl Olav Haram
Original assignee: Norsk Hydro Produksjon A.S
Priority date: 2005-11-11
Filing date: 2006-11-10
Publication date: 2007-05-18
Also published as: NO20055360D0; NO325437B1; NO20055360L

Abstract

An arrangement for external black start of a subsea power system is described. The subsea power system comprising a subsea main power distribution switchboard fed from a topside power supply, at least one main power switchgear control module controlling the subsea main power distribution switchboard, at least one subsea auxiliary power distribution switchboard, and at least one subsea auxiliary power switchgear control module controlling said auxiliary power distribution switchboard. The subsea power system further includes at least one critical switchgear control module connected to an external power source, said critical switchgear control module being operable to control the main power switchgear and /or auxiliary power switchgear in order to perform a black start.

Description

'Arrangement for external black start of subsea power system"

INTRODUCTION

The present invention concerns power systems in a subsea offshore installation. Especially, the invention concerns an arrangement for external black start of a subsea power system.

BACKGROUND

An offshore gas field may be developed with seabed installations which are tied back to a terminal onshore or an existing platform. The seabed installation comprises one or more production units where i.e. template produces well fluid through manifold headers which are connected to one or more pipelines. Said 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 umbilical for power, control and utility supplies are installed from the receiving facility to said 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 said platform solution. Providing a compression system subsea is safe to human injuries and fatalities due to remote operation, reliable, cost effective, environmental friendly and comprises few parts which make the system less complicated and easy to operate. Subsea equipment, e.g. a subsea compressor system, is normally supplied from topside power sources. Main power and control-power is supplied through separate cables, in order to ensure that both systems are not failing at the same time. Other power failure situations may also arise. Another problem with power interruption is that magnetic bearings in the subsea compression system normally only tolerates 5-7 power loss situations before the run down bearings must be replaced. Refurbishment of bearings is a time consuming and costly operation which involves shut down of the gas/oil production. Uninterruptible power supplies (UPS) are therefore provided in the topside auxiliary power systems.

The power cables are arranged in one or more umbilicals. The following explanation is based on only one umbilical. If there is a failure in one of the power cables in the umbilical, the umbilical must be cut open and the power cable cut and spliced and the umbilical mended. Sometimes the power cable must be replaced with a new power cable over a length of the umbilical. The umbilical together with the rest of the supply lines inside must then also be cut and replaced over the same length. All the cables and lines must then be spliced. Splicing always provides reduced reliability, which is certainly not desired in oil and gas field production facilities. In such a situation, there will be no power supply at all to the seabed installation. When the umbilical is repaired and reinstalled, the operator on the topside will not know anything about the state of the seabed installation and will have to perform a black start of the subsea system. Umbilical failure may lead to loss of control power and main power simultaneously. This can lead to damage on equipment because the shut down process will be without control.

The main power is in many cases provided from a land based facility. This implies high voltage power transmission in a long cable (e.g. 120 km in the Ormen Lange subsea production wells). When the power suddenly is switched on, there will be voltage transients in the power supply system. These voltage transients may destroy equipment subsea. Providing power from the topside to the seabed installation on long distances also has the problem of having small power transmission capability compared to the cable cross section.

s US 6,595,487 describes a control system using a battery as the principal power source for a motor. The control system controls an undersea valve. To provide redundancy the valve may be operated independently by two motors, each motor having their own control unit and their own battery. The motors can then be supplied power from the power supply topside or the attached battery. The o batteries may be charged from the topside power supply. This system is for valves and cannot supply the AC auxiliary power required for the whole subsea compression system.

SUMMARY OF THE INVENTION s The present invention is conceived to provide a solution to the power interruption/failure problems provided above.

When feeding a subsea system from topside power systems, using common main and control power (auxiliary power) cable, it is desirable to have control voltage 0 available before the main power loads are energized to be able to perform controlled start.

The idea is to connect one or several of the subsea switchgear/switchboard control modules to a topside power source enabling topside control of the subsea 5 switchgear before energizing the rest of the subsea system.

In a first aspect the invention provides an arrangement for external black start of a subsea power system, the subsea power system comprising a subsea main power distribution switchboard fed from a topside power supply, at least one main power o switchgear control module controlling the subsea main power distribution switchboard, at least one subsea auxiliary power distribution switchboard, and at least one subsea auxiliary power switchgear control module controlling said auxiliary power distribution switchboard. The subsea power system further includes at least one critical switchgear control module connected to an external power source, said critical switchgear control module being operable to control the main power switchgear and /or auxiliary power switchgear.

In a further embodiment the critical switchgear control module comprises a communication system. The subsea auxiliary power distribution switchboard may include a subsea power distribution switchboard critical loads, and a subsea power distribution switchboard non-critical loads. The critical switchgear control module may also be directly connected to the subsea power distribution switchboard critical loads.

The external power source may be a land based system, a topside system, a neighbouring template, subsea equipment or interfaces towards floating vessels or ROV/submarines.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described with reference to the following drawings, where Fig. 1 shows a schematic overview of a subsea compression system, Fig. 2 shows a subsea main power system single line diagram, Fig. 3 shows an arrangement for external black start of a subsea power system using control power to subsea switchgear control modules from topside solutions, according to an embodiment of the invention, and Fig. 4 shows an arrangement for external black start of a subsea power system using control power to subsea critical loads switchboards from topside solutions, according to another embodiment of the invention.

DETAILED DESCRIPTION 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, e.g cooling pumps, pre energizing systems for VSDs. 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 1 illustrates a subsea compression system in which the present invention may be implemented. The invention may however also be implemented in other subsea systems to provide safe black start.

The compression system in Figure 1 comprises one or more subsea compression stations and one or more long step-out power supplies. 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 the following subsea components: 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)

- Fiber optic lines for control signals

- Hydraulic lines (if required)

- Barrier lines (if required)

The compression station is connected directly to at least one subsea production templates (A, B, C or D) and is designed for boosting well fluid from said production templates. Well fluid from the production templates is routed via one of the template manifold headers, via the infield flow lines and to connectors on the suction side of the compression station.

The compression station is connected to export pipelines with flow spools to each pipeline. Compressed gas will be transported in said export pipelines to the receiving facility.

Figure 2 shows an example embodiment of a main power and auxiliary power distribution system single line diagram for the subsea compression system in Figure 1. High voltage power, low voltage power (if required for black start), hydraulic, control and utilities are supplied from receiving facilities with 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.

A subsea compression station, where well stream is compressed, comprises typically the following modules and parts: 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. Common to the compressor trains are a power and umbilical connection system and a valve manifold fitted with flow line connection systems. 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.

This description is related to the subsea compression station layout, but several other configurations can be made.

Electric power is supplied through an approximately 120 km cable from shore. The operational voltage on the cable is approximately 120 kV. On the subsea end of the cable there is a dry penetrator connected to a subsea three winding transformer. The transformer secondary and tertiary winding is connected through wet mateable connectors to a subsea main power distribution switchboard operated at 22 kV. The switchboard is used to connect and disconnect the main consumers which are the VSDs for compressor and pumps.

One distribution transformer for auxiliary power is connected to each of the subsea main power distribution switchboards. The switchboard connected to the auxiliary power distribution transformer G in Figure 2 is called the non-critical loads distribution switchboard. A UPS is connected to the non-critical loads distributions switchboard in order to provide uninterrupted power to the critical loads. Note that the non-critical loads and critical loads distribution switchboards are rated to supply the whole subsea installation. The configuration shown in Figure 2 is thereby a redundant system. The non-critical loads distribution switchboard could be omitted if all auxiliary loads are connected to the critical loads switchboard.

The UPS would then be connected to the subsea main-power distribution switchboards through a transformer. In Figure 2 the UPS includes a transformer on its power input. This transformer is used for galvanic isolation and can be omitted.

Under normal operation the subsea equipment is supplied with power only from s the main power system. If there is a failure in the main power, the compressors drives will stop within 2-3 minutes. During this time period the UPS must supply power to the magnetic bearing and control system for the subsea compression station. When the shut down process is finished, the power demand is low (only control systems) until the system is going to be restarted. However, if there is a o failure in the main power supply lasting for such a long period that all the stored energy is taken from the UPS system, there will be a so called "blackout" in the subsea system, meaning that no power at all is available.

During normal operation the UPS will have stored energy for safe startup and s shutdown of the system. Black start is defined as a situation when:

• The main power is out of operation.

• There is no power available in the UPS energy storage system.

In the following description of Figures 3 and 4, the symbol x denotes a breaker or o fuse pending on final protection philosophy for the system. The number and type of breakers/fuses may vary, and some of the breakers shown may also be omitted. Figure 3 and 4 are therefore only example embodiments. Also, the number of consumers and equipment rating will vary for different applications.

s An arrangement of a subsea power system according to an embodiment of the invention is shown in Figure 3. In Figure 3, a main power bus bar is connected to topside through a main step-down transformer, and a non-critical loads (NCL) bus bar is connected to the main power bus bar through a second step-down transformer. An UPS is connected to the NCL bus bar directly or through an o isolation transformer. The UPS has at least one charging port connected to the non-critical load switchgear/switchboard. It is also possible to connect the UPS to the NCL switchboard through a bypass port. A bus bar critical loads (CL) is connected to an output port of the UPS.

In Figure 3, the different loads are arranged on a bus bar with switches for connection/disconnection of the attached loads. These switches are controlled by the different switchgear control modules (A, B) in Figure 3. At least one switchgear/switchboard control module is provided to control the switches in the main power switchboard, and at least one switchgear control module is provided to control the switches in the auxiliary power switchboards, i.e. non-critical loads and the critical loads bus bar. In addition to the control systems used during normal operation (A and B in Figure 3), the main power and/or non-critical loads switchgear modules are provided with an additional switchgear control module C

(the critical switchgear control module). This is to enable control from topside in a situation of power failure, and when a black start of the subsea system must be performed. The additional switchgear control module C provides a control system for monitoring and control of the switchgears for main power and/or non-critical loads.

In Figure 3, the switchgear control modules C are connected to a topside control power system. The connection to topside is enabled through a dedicated communication system in switchgear control module C, powered by the same topside power source as the control power for the switchgear control modules C.

This is to ensure topside controlled start-up of the system. It is however sufficient that only one of the subsea switchboard control modules C for the subsea main power modules is connected to the topside energy source.

The sources for connection of control power as described above for switchgear control modules C may be land based systems, topside systems, other available subsea equipment, e.g. a neighbour template, or interfaces towards floating vessels or ROV/submarines. The extra feeder line from topside provided to the switchgear control modules C can also be used to charge the UPS batteries. When the UPS batteries are fully charged the UPS output can be switched on thereby providing normal start of the system through the control systems (A, B in Figure 3). A transformer may be arranged on the control power feeder line if required, as indicated by dotted lines in Figure 3.

In a normal situation the switchgear control modules A and B together with the UPS systems are powered from the topside through the topside main power as shown in Figure 3. These switchgear control modules controls normal operation and shutdown of the non-critical loads. The UPS control modules controls the normal operation of the UPS system and the critical loads. As mentioned above, switchgear control modules C are provided to enable control from topside in a situation of power failure, and a black start of the subsea system must be performed.

If there is a failure in the power supply and the system is to be restarted, or at first start-up, topside control of the subsea breaker modules when energizing the system must be provided. In Figure 3, this is achieved through the additional switchgear control module(s) C. In a black-start situation, or at start-up, an operator topside energizes the additional switchgear control module(s) C. The control module(s) transmits a confirmation signal to the operator and also information regarding the status of the different loads, both critical and non-critical, are provided to the operator. Based on this information, the operator may decide how to best handle the black-start situation, and sequentially energize the normal control modules A, B, UPS and switches for the different loads. When the main power again is available, the UPS system(s) will be available and other control equipment, like switchgear control modules A and B in Figure 3 will take over during normal operation and shutdown.

The system shown in Figure 3 is redundant, and there are two parallel systems which can act independently of each other, shown in the top and bottom half of Figure 3 accordingly. Each of the two systems has got the capacity to supply the whole subsea installation (e.g. subsea compression station) with the required auxiliary power. The switchgear control modules critical loads are also cross connected to provide further redundancy. In Figure 3, the switchgear control module B in the upper system half is connected to the critical loads switchboard in the lower system half, and the switchgear control module A in the lower system half is connected to the critical loads switchboard in the upper system half. When power is down in the upper system, the critical loads may be controlled by the lower system half and vice versa. As mentioned above the number of switchboard control modules for the different bus bars and switchgears may vary, but will always be symmetric in the two redundant systems.

An alternative embodiment of the present invention is shown in Figure 4. In Figure 4, a main power bus bar is connected to topside through a main step-down transformer, and a non-critical loads (NCL) bus bar is connected to the main power bus bar through a second step-down transformer. An UPS is connected to the NCL switchboard directly or through an isolation transformer. The UPS has at least one charging port and a bypass port connected to the non-critical load switchgear. A switchboard critical loads (CL) is connected to an output port of the UPS. In the embodiment in Figure 4, a switchgear control module critical loads is connected to a topside power source other than the topside main power. The switchgear control module controls the critical loads switchgear directly. In a black- start or start-up situation, the critical loads can then be controllably energized through the topside power source and switchgear control module.

The switchgear control module is in Figure 4 connected to a control power cable from a topside system. If a black start needs to be performed, the control power cable will be used in order to supply power for control of the switchgear control module.

A control system controlling the voltage at the subsea end of the control power cable may be implemented. This control system and the corresponding signal transmission system will be energized from the control power cable itself. The extra feeder line provided to the switchgear control modules C can also be used to charge the UPS systems. A transformer may be arranged on the control power feeder line if required.

Again the subsea system is made redundant by having two parallel systems for subsea control power on non-critical loads, the UPS and the critical loads. There may be at least one critical switchgear control module, but all the critical loads of the subsea systems are controllable by the at least one module.

The sources for connection of control power as described above for the switchgear control module C may be land based systems, topside systems, other available subsea equipment or interfaces towards floating vessels or ROV/submarines.

The embodiments for black-start or start-up described above will provide the following:

• Control power is available from topside system (onshore, platform, ROV, boat, other template etc,)

• Control power is available for control of the subsea main-power distribution switchboard and non-critical loads switchboards (C in Figure 3).

• When main power is again available topside, the subsea equipment can be energized because the status on the subsea main-power distribution switchboard and non-critical loads switchboards are known and controlled through the dedicated control system (C in Figure 3). • When the main power and the non-critical load switchboard have been energized, the UPS can be energized. Alternatively the UPS can be connected directly when the subsea main-power distribution switchboard is energized.

• When the UPS has been energized normal start up of the rest of the system is available. Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Claims

Claims:

1 . Arrangement for external black start of a subsea power system, the subsea power system comprising a subsea main power distribution switchboard fed from a topside power supply, at least one main power switchgear control module controlling the subsea main power distribution switchboard, at least one subsea auxiliary power distribution switchboard, and at least one subsea auxiliary power switchboard control module controlling said auxiliary power distribution switchboard, characterized in that the subsea power system including: at least one critical switchboard control module connected to an external power source, said critical switchboard control module being operable to control the main power switchboard and /or auxiliary power switchboard.

2. Arrangement according to claim 1 , characterized in that the critical switchboard control module comprises a communication system.

3. Arrangement according to claim 1 or 2, characterized in that the subsea auxiliary power distribution switchboard comprising a subsea power distribution switchboard critical loads.

4. Arrangement according to claim 3, characterized in that the subsea auxiliary power distribution switchboard comprising a subsea power distribution switchboard non-critical loads.

5. Arrangement according to claim 1 , characterized in that the critical switchboard control module is connected to the subsea power distribution switchboard critical loads.

6. Arrangement according to one of the previous claims, characterized in that the external power source is a land based system, a topside system, a neighbouring template, subsea equipment or interfaces towards floating vessels or ROV/submarines.