FAULT DETECTION SYSTEM AND METHOD, AND POWER SYSTEM FOR SUBSEA PIPELINE DIRECT ELECTRICAL HEATING CABLES

Field of invention The present invention relates to the field of electrical heating of pipeline systems. More particularly the invention relates to a fault detection system, a power system, a fault detection method, program element and computer-readable me ^¬ dium for subsea pipeline direct electrical heating cables.

Background

Formation of hydrates is a well-known problem in subsea production systems for oil and gas. Several options are avail- able to solve this problem. Traditionally, chemicals have been used. Recently a more effective direct electric heating method is used for heating of the pipeline by forcing a high electric current through the pipeline itself. Heretofore a subsea pipeline direct electrical heating cable is installed parallel to and connected to a distant end of the pipeline as shown in WO 2004/111519 Al for example.

A power system for providing a subsea pipeline direct elec ^¬ trical heating cable with power from a three-phase power grid has been described in WO 2010/031626 Al for example.

The subsea pipeline direct electrical heating cable has a linearly decreasing voltage, from an input value at its power in-feed end to zero at the grounded, remote end. Conse- quently, the electric filed stress on the cable insulation also decreases linearly, from a normal operating stress at the power in-feed end to zero at the remote end. A cable fault in the remote region may be initiated by a me ^¬ chanical damage, e.g. a cut extending through the outer sheath and the insulation system, thus exposing the copper conductor to seawater. As the conductor is connected to ground at the remote end, the fault will shunt its remaining length from fault location to grounded end. The corresponding change in conductor current will be minute and extremely dif ^¬ ficult to detect at the opposite end of a subsea pipeline di ^¬ rect electrical heating cable. A current measurement will normally be done even further upstream, making small changes even harder to detect. The conductor current in a subsea di ^¬ rect electrical heating system is typically larger than 1000 A, and a fault current of 10 A through the physical fault will translate into a far smaller change at the in-feed end (due to phase shifting) . Even with the best available current measuring equipment, cable faults near the remote end will therefore pass on undetected.

An electric current flowing out from the surface of a copper conductor and into seawater will cause rapid (alternative current) corrosion of the copper conductor, even at small current levels or voltage differences. If such a fault goes undetected, the final outcome will be a complete corrosion break of the copper conductor. A seawater filled gap is thus introduced between the two "conductor stubs", but the elec ^¬ tric impedance of this gap may not be sufficiently large to cause a detectable change in current at the in-feed end of the DEH system. As the gap will not be capable of withstand ^¬ ing the source voltage, an electric arc is then formed be- tween the two "conductor stubs". The temperature associated witch such arcing is several thousand degrees Celsius, so a rapid meltdown of the copper conductor as well as any polymer in the vicinity will occur. The boiling temperature of sea ^¬ water at most relevant water depths will be above the polymer melting points, so "water cooling" will not prevent the de ^¬ scribed melt-down from taking place. The subsea pipeline direct electrical heating cable is com ^¬ monly placed as close to the thermally insulated pipeline as possible. The thermal insulation will thus also be melted down by a fault as described above. Once the steel pipeline is exposed to seawater it will appear as an alternative, and probably low-impedance, return path for the fault current. As the copper conductor is continuously eroded away and widening the gap between the "stubs", the pipeline will at some point in time become the lowest impedance return path. At that time, a new arc will be established between the conductor stub (in-feed side) and the steel pipeline. A rapid melt through of the pipeline's steel wall may result and the pipe ^¬ line contents may escape implying severe environmental pollu ^¬ tion .

In WO 2007/096775 A2 there has been proposed a fault detec ^¬ tion system for subsea pipeline direct electrical heating ca ^¬ bles. The system proposed is based on fiber optic elements comprised in the subsea pipeline direct electrical heating cables. Accordingly, the known fault detection system is not suitable for existing installations.

WO 2006/130722 A2 discloses an apparatus and method for de ^¬ termining a faulted phase resulting from a fault in a three- phase ungrounded power system. The known method includes com ^¬ paring a phase angle of an operating phasor to a phase angle of a fixed reference phasor. The operating phasor is derived from a digitized signal sample of a plurality of measured signals of the power system. The known method also includes comparing a phase angle difference between the operating phasor and the fixed reference phasor to at least one thresh ^¬ old to determine the faulted phase. The fixed reference phasor may be a phase-to-phase voltage or a positive sequence voltage of the plurality of measured signals of the power system. The operating phasor may be a zero sequence current, a zero sequence voltage or a combination of a zero sequence current and a zero sequence voltage of the plurality of meas ^¬ ure signals of the power system. Moreover, EP 0 079 504 Al describes a method and an apparatus for detecting a single-phase-to-ground fault on a three-phase electrical power system, and for identifying a faulted phase. A single-phase-to-ground fault is correctly distinguished from other faults, including phase-to-phase-to-ground faults, even with transmission lines, which utilize series capaci ^¬ tors, by taking into consideration the phase-to-phase volt ^¬ age, which is in quadrature with the voltage to ground of the monitored phase.

Furthermore, US 2004/0032265 Al discloses a double-ended dis ^¬ tance-to-fault location system using time-synchronized posi- tive-or-negative-sequence quantities for a three-phase trans- mission line.

However, the fault detection systems proposed in

WO 2006/130722 A2 , EP 0 079 504 Al, and US 2004/0032265 Al do not take into account the special requirements for subsea electrical heating installations. In particular, these fault detection systems do not relate to subsea power consumers like direct electrical heating cables.

Hence, there may be a need for a fault detection system for subsea pipeline direct electrical heating cables, a power supply for subsea pipeline direct electrical heating cables, a fault detection method for subsea pipeline direct electri ^¬ cal heating cables, a fault detection program element for subsea pipeline direct electrical heating cables and a corre- sponding computer-readable medium which are suitable for ex ^¬ isting as well as new installations.

Summary

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims. According to a first aspect of the invention there is pro ^¬ vided a fault detection system for subsea pipeline direct electrical heating cables, the fault detection system com- prising a first ammeter for measuring a first phase current, a second ammeter for measuring a second phase current, a third ammeter for measuring a third phase current, a first calculation unit for calculating a negative sequence current from the first phase current, the second phase current, and the third phase current, and a first detection unit for de ^¬ tecting a change in the negative sequence current.

In this way a defect even at the distant end of a subsea pipeline direct electrical heating cable may be detected. The method even may be used for already existing subsea pipeline direct electrical heating cables as all measuring equipment may be installed above the surface. This may also reduce costs as no subsea operations may be necessary. According to a first embodiment of the fault detection sys ^¬ tem, the fault detection system further comprises a second calculation unit for calculating a positive sequence current form the first phase current, the second phase current, and the third phase current, and a third calculation unit for di- viding the negative sequence current by the positive sequence current to obtain a relative negative sequence current, wherein the first detection unit is adapted to detect a change in the relative negative sequence current. The positive sequence current may only experience small changes in case of subsea pipeline direct electrical heating cable failures and may be essentially dependent on the oper ^¬ ating voltage of the subsea pipeline direct electrical heat ^¬ ing cable. Using the relative negative sequence current as control may allow defining a single threshold value for de ^¬ tecting a subsea pipeline direct electrical heating cable fault even when the subsea pipeline direct electrical heating cable is operated with different operating voltages. Operat- ing the subsea pipeline direct electrical heating cable with different operating voltages may allow a safe transport of fluids trough the associated subsea pipeline, when the fluids have varying compositions and/or the ambient temperature of the subsea pipeline changes.

According to a second embodiment of the fault detection sys ^¬ tem, the fault detection system further comprises a fourth calculation unit for calculating a sequence impedance matrix Z _s . The sequence impedance matrix Z _s may be derived from the negative sequence current and the positive sequence current. The sequence impedance matrix Z _s then may be used to estimate the location of the subsea pipeline direct electrical heating cable fault.

According to a further embodiment of the fault detection sys ^¬ tem, the fault detection system further comprises a second detection unit adapted to receive a signal indicating a mal ^¬ functioning of a symmetrizing unit.

Loads like subsea direct electrical heating cables may be symmetrized by a symmetrizing unit before connecting them to a three phase network. The symmetrizing unit may be posi ^¬ tioned above sea level and accordingly may easily be moni- tored. If a malfunctioning of the symmetrizing unit is detected this may be communicated via a signal to the fault de ^¬ tection system. The fault detection unit may therefore pre ^¬ vent that a change in the negative sequence current due to a malfunctioning of the symmetrizing unit is erroneously at- tributed to the subsea pipeline direct electrical heating system. A fault detection system with a second detection unit adapted to receive a signal indicating a malfunctioning of the symmetrizing unit may be more reliable. According to a still further embodiment of the fault detec ^¬ tion system, the fault detection system further comprises a third detection unit adapted to receive a signal indicating a malfunctioning of a balancing unit. Electrical loads may comprise resistive impedances as well as reactive impedances. Such a load may be balanced by a balanc ^¬ ing unit to reduce cable cross sections to the power network. A malfunctioning of the balancing unit may affect the nega ^¬ tive sequence current, too. Adding a third detection unit adapted to receive a signal indicating a malfunctioning of the balancing unit may therefore further ameliorate the reli ^¬ ability of the fault detection system.

According to a second aspect of the invention there is pro ^¬ vided a power supply for subsea pipeline direct electrical heating cables, the power supply comprising a symmetrizing unit for symmetrizing a load, and a fault detection system as has been described as hereinbefore.

A power supply according to the invention may provide an easy and reliable means for energizing a subsea direct electrical heating cable with a three phase power grid. In particular, the symmetrizing unit may reduce the load experienced by the power grid.

The symmetrizing unit may comprise a first capacitor means and an inductor means both adaptable to the impedance of the subsea direct electrical heating cable. The direct electrical heating cable may be connected to the first phase and the third phase of the power grid, the first capacitor means be ^¬ tween the first phase and the second phase of the power grid and the inductor means between the second phase and the third phase of the power grid.

According to a first embodiment of the power supply, the power supply further comprises a balancing unit for balancing the load.

A balancing unit may comprise a second capacitor means com ^¬ pensate the reactive part of the load and thus may help to enhance power transmission efficiency. According to a second embodiment of the power supply, the power supply further comprises a local fault detection de ^¬ vice. The local fault detection device may allow the detec- tion of a malfunctioning of the symmetrizing unit and/or the balancing unit. In particular, the local fault detection device may provide the fault detection system with signals in ^¬ dicating a malfunctioning of the symmetrizing unit and/or the balancing unit. As has been described hereinbefore such sig- nals may prevent an erroneous detection of a subsea pipeline direct electrical heating cable.

According to a third aspect of the invention a fault detec ^¬ tion method for subsea pipeline direct electrical heating ca- bles is provided, the fault detection method comprising meas ^¬ uring a first phase current, measuring a second phase cur ^¬ rent, measuring a third phase current, calculating a negative sequence current from the first phase current, the second phase current, and the third phase current, and detecting a change in the negative sequence current.

This fault detection method for subsea pipeline direct elec ^¬ trical heating cables may be applied to already installed subsea pipeline direct electrical heating cables. All method steps may be performed above sea level. Subsea measuring or detection devices may be omitted.

According to a first embodiment of the fault detection method, the fault detection method further comprises calcu- lating a positive sequence current from the first phase cur ^¬ rent, the second phase current, and the third phase current, dividing the negative sequence current by the positive se ^¬ quence current to obtain a relative negative sequence cur ^¬ rent, and detecting a change in the negative sequence current by detecting a change in the relative negative sequence cur ^¬ rent . Such an embodiment may allow selecting only one threshold value for fault detection even if the subsea pipeline direct electrical heating cable is subjected to different operating voltages .

According to a further embodiment of the fault detection method, the fault detection method further comprises calcu ^¬ lating a sequence impedance matrix Z _s from the first phase current, the second phase current, and the third phase cur- rent, calculating a change of sequence voltages based on the sequence impedance matrix, and calculating a load impedance change based on the change of sequence voltages.

A determination of the load impedance change may help to es- timate the location of the subsea pipeline direct electrical heating cable fault. Accordingly, the subsea pipeline direct electrical heating cable may be repaired faster.

According to a fourth aspect of the invention there is pro- vided a fault detection program element for subsea pipeline direct electrical heating cables.

Said program element may be easily adaptable to new types power supplies for subsea pipeline direct electrical heating cables. Furthermore the program element may be executed by a data processor of an existing power supply for subsea pipeline direct electrical heating cables thus providing a facile way to improve the reliability of subsea pipeline direct electrical heating.

The program element may be implemented as computer readable instruction code in any suitable programming language, such as, for example, JAVA, C++, and may be stored on a computer- readable medium (removable disk, volatile or non-volatile memory, embedded memory/processor, etc.). The instruction code is operable to program a computer or any other programmable device to carry out the intended functions. The program element may be available from a network, such as the World Wide Web, from which it may be downloaded.

The fault detection method may be realized by means of a com- puter program respectively software. However, the invention may also be realized by means of one or more specific elec ^¬ tronic circuits respectively hardware. Furthermore, the fault detection method may also be realized in a hybrid form, i.e. in a combination of software modules and hardware modules.

According to a fifth aspect of the current invention there is provided a computer-readable medium on which there is stored a computer program for processing a physical object, the computer program, when being executed by a data processor, is adapted for controlling and/or carrying out the fault detection method as described hereinbefore.

The computer-readable medium may be readable by a computer or a processor. The computer-readable medium may be, for example but not limited to, an electric, magnetic, optical, infrared or semiconductor system, device or transmission medium. The computer-readable medium may include at least one of the fol ^¬ lowing media: a computer-distributable medium, a program storage medium, a record medium, a computer-readable memory, a random access memory, an erasable programmable read-only memory, a computer-readable software distribution package, a computer-readable signal, a computer-readable telecommunica ^¬ tions signal, computer-readable printed matter, and a com ^¬ puter-readable compressed software package.

It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with ref ^¬ erence to method type claims whereas other embodiments have been described with reference to apparatus type claims. How ^¬ ever, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the method type claims and features of the appa ^¬ ratus type claims is considered as to be disclosed with this document.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ^¬ ment but to which the invention is not limited.

Brief Description of the Drawings

Figure 1 shows a general overview of a system for direct electrical heating of a subsea pipeline. Figure 2 shows a schematic representation of a power supply according to the invention.

Figure 3 shows effective voltage, effective current and sub- sea impedance curves changes due to a subsea short circuit.

Figure 4 shows negative sequence voltage and negative currant changes due to a subsea short circuit.

Detailed Description

The illustration in the drawings is schematically.

Figure 1 shows a general overview of a system for direct electrical heating of a subsea pipeline 100. The subsea di ^¬ rect electrical heating cable 101 comprises a pipeline por ^¬ tion 102 extending along the subsea pipeline 100 and being electrically connected to the subsea pipeline 100 at a con- nection point 103. The other end of the pipeline portion 102 is, in particular integrally, connected to one end of a first riser portion 104 of the subsea direct electrical heating ca ^¬ ble 101. The subsea direct electrical heating cable 101 fur- ther comprises a second riser portion 105 is electrically connected with its one end to the subsea pipeline 100 at a connection point 106. The other ends of the first riser por ^¬ tion 104 and the second riser portion 105 are connected to a power supply 107. The power supply 107 provides the subsea direct electrical heating cable 101 with power from a main power grid 108. Due to the large distance between connection point 103 and connection point 106 an insulation defect 109 of the pipeline portion 102 near the connection point 103 is difficult to detect by conventional means. However, it may not only prevent heating of the remaining portion of the subsea pipeline 100 but may as well seriously damage the subsea pipeline 100.

The subsea direct electrical heating cable 101 and the part of the subsea pipeline 100 through which the current passes are represented in Figure 2 as a single phase direct electri ^¬ cal heating load 210. This single phase direct electrical heating load is connected via a 2-wire-connection 215 to a power supply 207 which itself is connected to a main power grid 208 via a 3-wire-connection 216. The power supply 207 comprises a balancing unit 211, a symmetrizing unit 212, a fault detection system 213, and a three-phase transformer 214. The power supply 207 is implemented with an IT (Isole Terre) earthing scheme. Accordingly, none of the three internal phases 217, 218, 219 of the power supply 207 is connected to earth. Accordingly, a single insulation fault within the power system is unlikely to cause hazardous high currents.

The three-phase transformer 214 comprises a high voltage side 220 and a low voltage side 221 wherein a first tap changer 222 is connected to the high voltage side 220 of the three- phase transformer 214 and a second tap changer 223 is connected to the low voltage side 221 of the three-phase trans ^¬ former 214. The voltage to be provided to the direct electri ^¬ cal heating load can be selected in the range from minimum to maximum load by operating the first tap changer 222 and the second tap changer 223. By changing the voltage level the heating power level may be augmented or reduced. The first tap changer 222 and the second tap changer 223 may be oper ^¬ ated while the direct electrical heating load is fully ener- gized.

The first internal phase 217, the second internal phase 218 and the third internal phase 219 on the low voltage side 221 of the three-phase transformer 214 are connected to the sym- metrizing unit 212. The symmetrizing unit 212 comprises a first capacitor means 224 and an inductor means 225 to dis ^¬ tribute the single phase direct electrical heating load 210 symmetrically among the three phases. The first capacitor means 224 is connected to the first internal phase 217 and the second internal phase 218. The inductor means 225 is pro ^¬ vided between the second internal phase 218 and the third in ^¬ ternal phase 219.

The capacitive and inductive values of the first capacitor means 224 and the inductive means 225 can be changed on-load, i.e. when the power system is energized. First capacitor means 224 and inductive means 225 may thus be adapted to the impedance of the direct electrical heating load 210 to reduce the negative sequence current. Accordingly, the power factor to the transformer may become very close to one and the nega ^¬ tive sequence current close to zero.

The balancing unit 211 comprises a second capacitor means 226 connected to the first internal phase 217 and the third in- ternal phase 218 on the one hand and to the direct electrical heating load 210 on the other hand. The balancing unit 211 compensates the reactive part of the direct electrical heat- ing load 210. The capacitive value of the second capacitor means 226 may be changed on-load.

The fault detection system 213 includes a first ammeter 227, a second ammeter 228 and a third ammeter 229. A first calcu ^¬ lation unit 230 is provided for calculating a negative se ^¬ quence current from the first phase current, the second phase current, and the third phase current measured with said first ammeter 227, second ammeter 228, and third ammeter 229. A de- tection unit 231 detects changes in the negative sequence current indicative of a subsea pipeline direct electrical heating cable fault. Furthermore, the fault detection system 213 comprises a second calculation unit 232 and a third cal ^¬ culation unit 233 to calculate a positive sequence current and a relative negative sequence current respectively. A fourth calculation unit 234 may calculate a sequence imped ^¬ ance matrix Z _s which may serve to localize a subsea pipeline direct electrical heating cable fault. Finally, the fault de ^¬ tection system 213 includes a second detection unit 235 and a third detection unit 236 to account for error signal provided by the local fault detection devices 237, 238, 239 of the balancing unit 211 and the symmetrizing unit 212.

Figure 3 shows a simulation of the development of effective subsea voltage V _s , effective subsea current I _s , and subsea impedance Z _s over time. In a time interval from ti to t ₂ an insulation defect 109 occurs near connection point 103. The distance from connection point 106 to the insulation defect 109 is approximately 97 percent of the distance between the connection point 109 and the connection point 106.

The effective subsea voltage drops from approximately 9850 volts to approximately 9750 volts at ti and rises from ap ^¬ proximately 9750 volts to approximately 9850 volts again at t ₂ · Correspondingly, the effective subsea current rises from approximately 1540 ampere to approximately 1560 ampere at ti and drops again to 1540 ampere at t ₂ . Such a change in the order of 1 percent (voltage) or 1.3 percent (current) is very difficult to detect by conventional measuring equipment. Even the subsea impedance drops only from approximately 6.4 ohms to approximately 6.2 ohms thus by only approximately 3.0 per ^¬ cent in the interval from ti to t ₂ .

Figure 4 now shows the corresponding behavior of negative se ^¬ quence current I _N and negative sequence voltage V _N for the same time interval. Even if the symmetrizing unit of the power supply may be adaptable to different single phase di- rect electrical heating loads a low effective negative se ^¬ quence current from 5 to 10 amperes and a low effective nega ^¬ tive sequence voltage may be present even if there is no in ^¬ sulation defect. However, from ti to t ₂ the effective nega ^¬ tive sequence voltage is with approximately 65 volts around 3 times as high as the effective negative sequence voltage of 22 volts without an insulation defect. In the same interval the effective negative sequence current raises from approxi ^¬ mately 9 amperes to 25 amperes. Such current and voltage changes are easily detectable.

It should be noted that the term "comprising" does not ex ^¬ clude other elements or steps and the use of articles "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

In order to recapitulate the above described embodiments of the present invention one can state: The claimed fault detec- tion system, power supply, fault detection method, and pro ^¬ gram element for a subsea direct electrical heating cable of ^¬ fers substantial advantages over known systems.