NO20160575A1

NO20160575A1 - Voltage Regulation for a Subsea Control System

Info

Publication number: NO20160575A1
Application number: NO20160575A
Authority: NO
Inventors: Gunleiv Skofteland; Nicola Chiesa; Steinar Midttveit; Øyvind Holm Snefjellå; Trond Benny Bertmand
Original assignee: Statoil Petroleum As
Priority date: 2013-09-18
Filing date: 2016-04-11
Publication date: 2016-04-11
Also published as: GB2537482B; GB201604171D0; WO2015039682A1; GB2537482A

Description

Voltage Regulation for a Subsea Control System

FIELD OF THE INVENTION

The invention relates to voltage regulation for a subsea control system, and particularly to a sensorless inverter-based solution for subsea control voltage regulation.

BACKGROUND OF THE INVENTION

Electrical power is required for subsea control systems. Such subsea control systems may include a subsea control module (SCM), which may comprise a computing device and actuators requiring electrical power. The SCM may for example control the actuation of valves for a subsea oil and gas production well. The subsea control system may also include sensors, such as a multiphase flow meter, well instrumentation, data communications and modems, all of which require electrical power.

Electrical power is provided to subsea control modules by umbilical cables comprising one or more cable pairs. Umbilicals can be a number of kilometres in length, and a voltage drop occurs along the length of the cable. The voltage drop on the cable vanes as function of the load on the cable, and preferably the voltage at the receiving end of the cable should be guaranteed to be within specified limits suitable for subsea connectors and subsea equipment. In particular, a high voltage from the host facility may be required to transfer more power during full-load, but this voltage at the subsea end may be too high when the load is reduced. Power delivered by an umbilical cable pair may be typically in the region of 250 to 1,500 W (Watts).

In today's subsea control systems, the topside sending voltage is fixed and may be determined by an evaluation of the requirements of maximum power and maximum voltage at minimum-load conditions.

The trade-offs between maximum power and maximum voltage at minimum load conditions pose severe restrictions on the maximum transferable power and maximum transmission distance.

Wet-mate connectors are connectors which allow connection of electrical cables subsea, often using a remote operated vehicle (ROV). If wet-mate connectors are consistently operated at a voltage above a required maximum value (Vreq), for example above about 600 volts, the degradation rate of the connectors' insulation may increase, resulting in a decreased lifetime of the wet-mate connectors, and an increased failure rate.

A "brown field" is an oil or gas field which has matured typically to a production plateau (ie flat production), or even declining production. In such brown fields the infrastructure, including subsea control systems, is al ready in place and, currently, upgrading the control system often requires an increase in control power that may result in excessive upgrade costs when using a standard solution, or may limit the upgrade possibilities.

SUMMARY OF THE INVENTION

The invention provides a subsea control system and method as set out in the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows an embodiment of a subsea control power system in accordance with the invention; and

Figure 2 is a flow diagram showing a voltage regulation method.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments will now be described, by way of example only, with reference to the accompanying drawings. We describe a system which allows dynamic regulation of voltage supplied to a subsea control system.

Figure 1 shows a subsea control power delivery system 2, which supplies power to a Subsea Control Module (SCM) 4. An uninterruptible power supply (UPS) 6 provides power to a voltage regulation device 8, which in turn provides an AC electrical output to a step-up transformer 10. Preferably the power supply 6 is a single phase power supply. The alternating voltage from the step-up transformer 10 is transmitted by a step-out umbilical cable 12 to a first subsea template 32 where it is routed forward to one or more in-field umbilical cables 14, delivered to a second subsea template 34, and through a step-down transformer 16 to the Subsea Control Module (SCM) 4. The step-out umbilical cable 12 may have a length for example in the range 20 km to 80 km. The in-field umbilical may be shorter in length..

The step-out umbilical cable 12 is connected to the in-field umbilical 14 by means of wet-mate connectors 26 and 36 at the input and output of the first template 32 respectively. The infield umbilical 14 is connected to the second template 34 by wet-mate connector 28. The number of templates is not limited to two and series-connected in-field umbilicals can be used for connection between templates using wet-mate connectors. A template is a structure which is used as a base for various subsea structures such as wells and subsea trees and manifolds. For the purpose of this patent, the first and second templates 32 and 34 can be thought of as nodes, at which wet-mate connectors 26 and 36 are exposed to a voltage V2 and wet-mate connector 28 to a voltage V3.

The voltage regulation device 8 may be positioned topside (ie before the step-out umbilical cable 12, which generally means above the sea) and comprises a power electronic inverter 18 which receives an AC supply from the UPS 6 and converts this to a constant frequency alternating voltage output V1 with controllable amplitude. This output may pass through a filter 20 before reaching the step-up transformer 10 to ensure compatibility with data communication over the power line. The maximum transmission voltage at the output of the transformer 10 is in the range of 1,000 Volts and determined by the insulation level of the cable and wet-mate connectors. Where we refer to an alternating voltage value in this specification it should be assumed that we are referring to the RMS (root mean square) value of the voltage waveform.

The current is measured at the output of filter 20 by a current measuring device 22, and this current measurement is fed to a regulation system 24 which uses this current measurement, together with knowledge of the topside voltage V1 at the inverter output, to estimate the voltage V2 at the first wet-mate connector 26. This estimate is then used by the regulation system 24 to control the voltage output V1 of the inverter 18. The regulation system 24 can be a digital system based on a processor card, FPGA card, or both. Although in this embodiment we have described the current measurement as being made at the output of filter 20, and a voltage measurement at the output of the inverter 18, it should be understood that the current and voltage measurements could be made at a number of different places in or near the voltage regulator 8, or at the output of the step-up transformer 10. It is advantageous that the current and voltage measurements are made before the beginning of the step-out umbilical cable 12 råtner than subsea at the other end to avoid long distance communication requirements.

The SCM 4 is generally part of a subsea control system, which may comprise other SCMs and other sensors and components as described above. The power consumption of the subsea control system will generally vary over time, for example as different components and different SCMs are switched on and off. These different power consumptions result in different voltage drops along the length of the system from V1 to V3. In particular, a higher power consumption results in a larger voltage drop from V1 to V3, and a lower power consumption results in a smaller voltage drop from V1 to V3.

Although the wet-mate connectors 26, 36 and 28 can tolerate voltages above a required maximum value Vreq, it is generally preferable to keep the voltages V2 and V3 below Vreq in order to prolong the lifetime of the wet-mate connectors 26, 36 and 28, as described above.

The voltage V3 also determines the input voltage for SCM 4. It is more critical to keep this voltage within specified limits than it is to keep the voltage V2 below Vreq. This is because the SCM 4 is more sensitive to voltage changes at its input, and can only operate correctly for a specified range of input voltages. The SCM 4 will also usually have a minimum and maximum voltage which cannot be exceeded at any time.

The power system 2 described above allows topside sensorless inverter control for the regulation of voltage at a subsea template. The system is sensorless in the sense that it is not necessary to provide sensors subsea to measure the voltages V2 and V3. Instead the knowledge of the topside voltage V1 and the topside current is used to estimate the subsea voltages V2 and V3.

Voltage regulation is achieved using the power electronic inverter 18 placed topside. The filter 20 on the output may be required to allow data communication over the power line, which allows data to be transferred between a process control unit (PCU) 30 and the SCM 4 through the umbilical cables 12 and 14 together with the control power.

The power system 2 described above is particularly suitable for providing an upgraded power supply system to brown fields. The voltage regulation device 8 is added before the step-up transformer 10; otherwise the system is identical to the existing brown field system. The only system upgrade required is the connection of the voltage regulation device 8, comprising the inverter 18, filter 20 and regulation system 24, and eventually an upgrade of the topside transformer 10 if existing tapping and ratings is not adequate. No modification to subsea equipment is required unless the subsea transformer power rating is not suitable.

The power system 2 also allows a multi-objective voltage regulation to be implemented using voltage estimation at different subsea nodes, ie at different points in the subsea power system, for example V2 and V3. For example the conflicting requirements of maximum voltage at the first wet-mate connector (loose requirement) and the minimum voltage at the SCM 4 (strict requirement) at the end of in-field umbilical 14 can be optimized. At the same time the lowest admissible voltage at the wet-mate connectors could be chosen at any time to prolong their lifetime.

One or more in-field umbilical cables 14 can be connected in series to the main step-out umbilical cable 12 passing through subsea templates. Howeverthe subsea voltage should be controlled at the end of the main umbilical 12 (V2), preferably to keep this voltage below Vreq , due to the wet-mate connector requirements discussed above.

For certain operation conditions and long in-field umbilical cables, the voltage at the far end SCM 4 (V3) may be too low for correct operation if V2 is maintained below the Vreq limit. In this case, a multi-objective optimization of the voltage allows the voltage V2 at the wet-mate connectors to be temporarily increased above Vreq in order to meet the more strict requirement of a minimum voltage at the SCM 4. At the same time, the regulation system 24 could be arranged to maintain the voltage V2 at the wet-mate connectors as low as possible, whilst meeting the voltage requirements of the SCM 4, in order to prolong the life of the wet-mate connectors.

In order to estimate the voltages V2 and V3 from V1 and current, a cable emulator in the regulation system is used to model the cable. The system provides inverter-based voltage regulation based on a cable emulator to control system power to boost power transfer capability.

The voltage regulation device 8 allows the voltages V1, V2 and V3 to be dynamically and quickly regulated, thus allowing a high sending voltage during high load demand to boost power transfer, and adjusting the voltage to a lower level during low load demand to avoid excessive voltage on subsea components.

The voltage regulation device 8 is a topside device, which allows a simple, compact and fast upgrade to a brown field system. The voltage regulation device 8 also allows multi-objective optimum voltage regulation and extended lifetime of wet-mate connectors.

Figure 2 is a flow diagram showing a voltage regulation strategy which can be followed by the regulation system 24 of Figure 1.

In Figure 2, V1, V2 and V3 are the voltages at the points shown in Figure 1. V1ref is a reference voltage calculated by the regulation system 24 and transmitted to the inverter. V1ref is effectively a target value for V1, and at step 64 (described below) the regulation system operates to bring the value of V1 to the value of V1ref.

V1n is the nominal value of V1 used as initialization of V1ref. V1max is the maximum allowed value of V1, which is the maximum output voltage of the inverter 18 topside (dictated by cable, connectors, and inverter 18). V1max can for example by 1,000 V.

Vreq is the maximum value of V2 required for a low failure rate of the wet-mate connector 26. This is not a strict limit, because voltage requirement Vreq can be exceeded as wet-mate connectors are designed to operate up to V1max, albeit with reduced longevity. A typical value for Vreq is between 50% and 80% of V1max, for example 600 V.

V3min and V3max are respectively the minimum and maximum allowed values of V3, which define the voltage range for operation of SCM 4.

Referring to Figure 2, the process begins at the START 40. First the required data and parameters must be input into the regulation system 24. In step 42 the lengths and properties, such as electrical resistance, of the umbilical cables 12 and 14 are input into regulation system 24. At step 44 the values of V1 n, V1 max, Vreq, V3min and V3 max are input into regulation system 24. These values are defined above. V1ref is initialized to V1 n in step 68.

Operation of the voltage regulation device 8 begins at step 46, where the topside voltage and current measurements are provided to regulation system 24. As noted above, the current is measured at the output of filter 20 by a current measuring device 22, and this current measurement is fed to the regulation system 24. The control system is also provided, by any suitable means, with the value of voltage V1 at the inverter output. Current and voltage measurements could be made at a number of different places in or near the voltage regulator 8, or at the output of the step-up transformer 10.

At step 48 the regulation system 24 uses the current and voltage measurements provided in step 46, together with the data parameters provided in step 42, to calculate an estimate of the subsea voltages at V2 and V3 by means of a cable estimator. In one of its simplest implementations, a cable estimator can be based on following equation where capacitive cable parameters are neglected:

V2=V1 - Zeq<*>I

with Zeq the equivalent impedance of the system between V1 and V2, and I the measured current. More advanced cable estimation approaches exists to account for the distributed parameter nature of cables and should be used for increased accuracy of the voltage estimation.

At step 50 the regulation system 24 tests whether V3 is between V3min and V3max. If not, the method moves to step 52, which gives a high priority to regulating V1ref so that V3 is between V3min and V3max and hence within the specified range for the SCM 4.

If V3 is within the required range, the method moves to step 54, at which the regulation system 24 tests whether V2 is less than Vreq. If not, the method moves to step 56 at which the regulation system 24 gives a mid-level priority to reducing V1ref so as to reduce V2 below Vreq, thus meeting the preferred requirements of the wet-mate connector 26.

If V2 is below Vreq, then the regulation system 24 gives a relatively low priority to gradually reducing V1ref, and hence also gradually reducing V2, in order to increase the life of the wet-mate connectors 26 and 28, whilst keeping V3 above V3min, and also meeting system stability criteria (ie the operating point should be above the voltage stability limit point in the nose-curve).

The use of the different priorities described above, allows the voltage regulator 8 to balance a number of sometimes competing objectives, thus allowing multi-objective voltage regulation. More advanced strategies than the one of Figure 2 may be used where all objectives can be tested simultaneously.

At step 60 the regulation system 24 tests whether V1ref is below V1max. If not, V1ref is reduced to V1max at step 62.

At step 64 the regulation system 24 updates the input of the inverter 18 such that the new operating voltage becomes V1 = V1 ref. Transition to new values of V1 may be gradual to avoid disturbances.

At step 66 the current measurement at point 22 and the voltage V1 are monitored by the regulation system 24 in order to detect any load change. The method then returns to step 46.

Claims

1. A subsea control system comprising: at least one electrical cable håving first and second points spaced apart along the length of the electrical cable or cables; a power supply electrically connected to said first point, and arranged to supply a first alternating voltage to said first point; at least one subsea control device, for assisting in the control of a subsea oil and gas production system, said subsea control device being electrically connected to said second point so as to receive at said second point a second alternating voltage as a result of transmission along said cable or cables of said first alternating voltage; and a voltage regulator for varying the magnitude of said first alternating voltage in dependence on the electrical load applied to said second point.

2. A system as claimed in claim 1, wherein: said at least one electrical cable comprises a step-out umbilical cable; and said voltage regulator com prises: a regulation system for regulating said first alternating voltage output by said power supply; and a current measurement device arranged to measure the current at a position between said first point and the beginning of said step-out umbilical cable, and wherein said regulation system determines changes to the magnitude of said first alternating voltage on the basis of said current measurement and the magnitude of said first alternating voltage.

3. A system as claimed in claim 2, wherein said step-out umbilical cable has a length of between 20 km and 80 km.

4. A system as claimed in any preceding claim, wherein said voltage regulator comprises an inverter for converting a AC voltage into said first alternating voltage.

5. A system as claimed in any preceding claim, wherein said at least one electrical cable comprises one or more series connected cables håving a combined length of at least 20 km.

6. A system as claimed in any preceding claim, wherein said at least one electrical cable comprises one or more cables håving a combined length of no more than 150 km.

7. A system as claimed in any preceding claim, wherein said at least one electrical cable comprises a step-out umbilical cable which is connected, directly or indirectly, to said subsea control device by a wet-mate connector.

8. A system as claimed in claim 7, wherein the voltage at said wet-mate connector is regulated by said voltage regulator to ensure that it remains below a maximum specified value.

9. A system as claimed in any preceding claim, wherein said voltage regulator is positioned topside.

10. A system as claimed in any preceding claim, wherein the magnitude of said second alternating voltage is maintained by said voltage regulator within a specified range which is suitable for said at least one subsea control device.

11. A system as claimed in any preceding claim, wherein said power supply is a single phase power supply.

12. A method of regulating voltage within a subsea control system, where the subsea control system comprises: at least one electrical cable håving first and second points spaced apart along the length of the electrical cable or cables; a power supply electrically connected to said first point, and arranged to supply a first alternating voltage to said first point; and at least one subsea control device, for assisting in the control of a subsea oil and gas production system, said subsea control device being electrically connected to said second point so as to receive at said second point a second alternating voltage as a result of transmission along said cable or cables of said first alternating voltage; said method comprising: adding to said subsea control system a voltage regulator, and using said voltage regulator to vary the magnitude of said first alternating voltage in dependence on the electrical load applied to said second point.

13. A method as claimed in claim 12, wherein said at least one electrical cable comprises a step-out umbilical cable and wherein said method further comprises: making a current measurement to measure the current at a position between said first point and the beginning of said step-out umbilical cable, and varying the magnitude of said first alternating voltage on the basis of said current measurement and the magnitude of said first alternating voltage.

14. A method as claimed in claim 12 or 13, which further comprises: estimating the voltage at the input of said at least one subsea control device; and regulating the voltage at the input of said at least one subsea control device so that the voltage at the input of said at least one subsea control device remains above a specified minimum and below a specified maximum.

15. A method as claimed in any one of claims 12 to 14, wherein said step-out umbilical cable is connected, directly or indirectly, to said subsea control device by a wet-mate connector, and wherein said method further comprises: estimating the voltage at said wet-mate connector, and regulating the voltage at said wet-mate connector to ensure that the voltage at said wet-mate connector remains below a maximum specified value.

16. A method as claimed in claims 14 and 15, which further comprises: assigning a first priority to said regulation of the voltage at the input of said at least one subsea control device; assigning a second priority to said regulation of the voltage at said wet-mate connector; wherein said first priority is higher than said second priority; and performing said step of regulating said voltage at the input of said at least one subsea control device, and said step of regulating the voltage at said wet-mate connector, in accordance with said first and second priorities respectively.

17. A method as claimed in claims 14 and 15, which further comprises: gradually reducing said voltage at said wet-mate connector, whilst at the same time maintaining the voltage at the input of said at least one subsea control device above said specified minimum.

18. A method as claimed in claims 16 and 17, which further comprises: assigning a third priority to said step of gradually reducing said voltage at said wet-mate connector; wherein said third priority is lower than both said first and second priorities; and performing said step of gradually reducing said voltage at said wet-mate connector in accordance with said third priority.