ty le ca cuoc 188bet Stray current interference and mitigation for underground pipeline

Summary

Stray currents can have a wide range of interference effects on a pipeline, ty le ca cuoc 188bet most harmful being ty le ca cuoc 188bet creation of localised electrochemical corrosion cells that attack any buried metallic structure. Stray currents come from different sources, for example, DC rail transit systems and high voltage power transmission systems. It is important for ty le ca cuoc 188bet pipeline operators to determine whether stray currents are affecting ty le ca cuoc 188bet pipelines, identify ty le ca cuoc 188bet possible source(s) of stray currents, evaluate ty le ca cuoc 188bet severity of ty le ca cuoc 188bet stray current interferences on ty le ca cuoc 188bet pipelines and mitigate ty le ca cuoc 188bet interference effects. As urbanisation proceeds and more large-scale infrastructures are invested, oil and gas pipelines are threatened by stray current corrosion risks throughout their service life due to interference issues, which are not nuisance but rather major safety hazard that may take much effort to be taken care of. This article introduces two types of DC stray current interferences caused by DC rail transit systems and high voltage direct current transmission systems, and also gives a review on ty le ca cuoc 188bet state- of-ty le ca cuoc 188bet-art of best practices on mitigating these interferences for protection.

Key words: Stray current, corrosion, dynamic interference, high voltage direct current, transit system, mitigation, remote monitoring. 

1. Introduction

Stray currents are caused by sources of current flowing through unintended paths. These can cause electrical interference and have significant effects on buried pipelines and other metallic structures. It is important for ty le ca cuoc 188bet pipeline operators to be aware of this as a type of risk as these stray currents can initiate and/or accelerate ty le ca cuoc 188bet corrosion of pipelines [1]. In this context, it is also obligated for ty le ca cuoc 188bet pipeline operators to determine whether stray currents are affecting ty le ca cuoc 188bet pipelines, identify ty le ca cuoc 188bet probable source(s) of stray currents, evaluate ty le ca cuoc 188bet severity of stray current interferences on ty le ca cuoc 188bet pipelines and mitigate ty le ca cuoc 188bet interference effects to protect ty le ca cuoc 188bet pipelines.

In ty le ca cuoc 188bet principle of electrochemistry, stray currents flow at ty le ca cuoc 188bet interface between electrolytes and metallic structures sitting in them through charge transfer reactions, governed by Faraday’s Law. Stray currents have varied deleterious effects [2]. At ty le ca cuoc 188bet locations ty le ca cuoc 188bet current leaves ty le ca cuoc 188bet pipeline, oxidation reactions occur to convert electronic current to ionic current. If metallic matrices are involved in ty le ca cuoc 188bet oxidation process by releasing electrons directly, they are transformed into metallic ions and dissolved in ty le ca cuoc 188bet sitting electrolyte, which is known as corrosion attack. Often this phenomenon is found in close proximity to ty le ca cuoc 188bet generation locations of stray currents or a holiday in ty le ca cuoc 188bet coating. At ty le ca cuoc 188bet locations ty le ca cuoc 188bet current flows into ty le ca cuoc 188bet pipeline, reduction reactions occur to convert ionic current to electronic current. Depending on what metallic structures are picking up stray currents, there might be different consequences. If ty le ca cuoc 188bet structure is coated and has its own cathodie protection (CP) system, stray current pickup, causing additional cathodic polarisation, may result in cathodic disbondment of ty le ca cuoc 188bet coating, and meanwhile, threaten ty le ca cuoc 188bet structural integrity by increasing ty le ca cuoc 188bet susceptibility of hydrogen embrittlement to sporadic hard spots of ty le ca cuoc 188bet pipeline (such as girth welds, plastically strained gouges, etc.) and ty le ca cuoc 188bet pipeline itself if made of high-strength steels. If ty le ca cuoc 188bet structure is made of amphoteric metals, ty le ca cuoc 188bet high pH niche developed at ty le ca cuoc 188bet structure/electrolyte interface caused by ty le ca cuoc 188bet reduction of water molecules can effect “cathodic” corrosion.

2. Sources of stray currents

Stray currents come from different sources and impose impacts on pipelines or other underground structures in varied manners, as summarised in Table 1. There are two types of stray currents, static and dynamic, both of which can be either direct currents or alternating currents. Static stray currents maintain a steady current  flow and are often from other cathodic protection systems on pipelines or other underground structures conducting current flow via soil. Dynamic stray currents fluctuate over time and can come from natural or manmade sources.

Table 1. Categorisation of stray current interferences

Stray currents interfere with underground pipelines as a function of ty le ca cuoc 188bet way how they are generated.

a. High voltage direct current (HVDC) transmission can be performed using monopolar earth-return systems for regular inspection or accidental fault for a sustained period of time, during which thousands of amperes of current are injected into ty le ca cuoc 188bet earth. ty le ca cuoc 188bet monopolar earth- return operation mode causes ty le ca cuoc 188bet pipeline segments to be subject to cathodic or anodic interferences at different locations. ty le ca cuoc 188bet interference lasts a short term depending on ty le ca cuoc 188bet duration of ty le ca cuoc 188bet monopolar operation, normally a few hours, and is identified with an abrupt potential excursion on ty le ca cuoc 188bet potential-time plot.

b. Long-term interferences are often detected on ty le ca cuoc 188bet pipelines that are collocated with other infrastructures, such as DC-electrified transit systems, high voltage AC transmission systems, low voltage AC distribution systems, AC-electrified high-speed railway systems, and foreign cathodic protection systems. Long-term interferences can harm ty le ca cuoc 188bet integrity of ty le ca cuoc 188bet pipeline due to their accumulative effects throughout their service life. This necessitates not only ty le ca cuoc 188bet monitoring of ty le ca cuoc 188bet interference status on ty le ca cuoc 188bet pipeline but also ty le ca cuoc 188bet mitigation of ty le ca cuoc 188bet interferences.

c. Welding operations generate DC stray current intermittently as ty le ca cuoc 188bet welding machine is normally grounded. This poses ty le ca cuoc 188bet underground structures in proximity to ty le ca cuoc 188bet welding site in a situation of suffering from accelerating metal loss.

d. Telluric currents, natural stray currents, caused by ty le ca cuoc 188bet variations in ty le ca cuoc 188bet earth’s magnetic field are well known to present difficulties to pipeline operators. These currents manifest themselves as pipe-to-soil potential fluctuations which are dependent on both location and time. Two mechanisms are generally considered when discussing telluric current effects: classical electromagnetic induction that elucidates local changes in ty le ca cuoc 188bet earth’s magnetic field will induce electrical current in stationary conductors such as pipelines; alternately, geomagnetic disturbances may result in large voltage gradients created in ty le ca cuoc 188bet earth’s crust in which long metallic conductors such as pipelines merely carry ty le ca cuoc 188bet charge [3].

In ty le ca cuoc 188bet next paragraphs, this article will focus on two types of ty le ca cuoc 188bet above interferences - dynamic DC transit interference and HVDC interference - per ty le ca cuoc 188bet authors’ experience in recent years.

3. Dynamic DC transit interference

3.1. Occurrence

Nowadays, ty le ca cuoc 188bet construction of electrified DC rail transit systems in modern cities, especially in developing countries, has been a movement to alleviate congested highways and to provide efficient mass transportation. However, dealing with corrosion and its detrimental effects, attributable to stray-current corrosion due to ty le ca cuoc 188bet DC rail systems, is, at ty le ca cuoc 188bet same time, becoming a challenge in terms of public safety, fortunately, with increasing awareness. ty le ca cuoc 188bet operation of DC rail systems causes stray current interference problems to ty le ca cuoc 188bet underground pipelines for transporting product oil, natural gas, water, etc.

Figure 1. Generation of stray current from DC rail transit systems [2].

Figure 2. Pipe-to-soil potential vs time plot displaying fluctuations.

ty le ca cuoc 188bet most common DC rail systems are traction powered by DC current supplied from substations via a positive feeder circuit (e.g., overhead power conductor in Figure 1) and use ty le ca cuoc 188bet running rails as ty le ca cuoc 188bet negative return circuit to return ty le ca cuoc 188bet power current to ty le ca cuoc 188bet substation. ty le ca cuoc 188bet running rails are in resistive or conductive contact with earth, rather than being ideally isolated from it, causing current leakage problem. Essentially, stray-current leakage is a result of ty le ca cuoc 188bet resistance relationship between ty le ca cuoc 188bet rail-to-earth return path and ty le ca cuoc 188bet running-rail return path. A high resistance of ty le ca cuoc 188bet running-rail negative return increases ty le ca cuoc 188bet voltage drop along ty le ca cuoc 188bet rails and, therefore, makes ty le ca cuoc 188bet rail-to- earth return circuit a more favourable path for ty le ca cuoc 188bet return current, thus causing stray-current leakage. These leakage currents in ty le ca cuoc 188bet earth tend to pick ty le ca cuoc 188bet paths like tunnel structure steel bars and underground metallic piping and cables with low resistance and then flow back to ty le ca cuoc 188bet substations [4]. ty le ca cuoc 188bet current leakage off ty le ca cuoc 188bet running rails can be 5 - 10% of ty le ca cuoc 188bet power current for railcar running. This may seem a small percentage, but ty le ca cuoc 188bet stray currents can be substantial because ty le ca cuoc 188bet start-up load current can be several hundred amperes for a single railcar and several thousand amperes for a subway train. If ty le ca cuoc 188bet traction power is at ty le ca cuoc 188bet level of 2,000 - 3,000 amperes, ty le ca cuoc 188bet stray current leaking to ty le ca cuoc 188bet earth can reach up to 100 - 300A, which is significantly higher than ty le ca cuoc 188bet magnitude of current required by in which ty le ca cuoc 188bet cathodic protection systems for ty le ca cuoc 188bet underground pipelines, typically, at ty le ca cuoc 188bet level of several amperes to tens of amperes.

ty le ca cuoc 188bet stray current leakage behaves in a fluctuating manner. Not only will ty le ca cuoc 188bet magnitude of ty le ca cuoc 188bet stray current vary with time of day and whether ty le ca cuoc 188bet vehicle is accelerating or decelerating, but ty le ca cuoc 188bet location of stray current pick- up on ty le ca cuoc 188bet underground pipelines will also change as ty le ca cuoc 188bet railcar moves along ty le ca cuoc 188bet rails. Thus, a pipe-to-soil potential recording will have a dynamic appearance, as seen in Figure 2 from our team’s project. ty le ca cuoc 188bet potential- time recording of stray current effects from a DC transit system has a distinctive pattern. There are considerable potential fluctuations during ty le ca cuoc 188bet daytime when ty le ca cuoc 188bet transit schedules are busy, while virtually no changes during ty le ca cuoc 188bet early morning hours. In Figure 2, ty le ca cuoc 188bet potential data was obtained with a data logger from a product pipeline with route intersected with a metro line. ty le ca cuoc 188bet plot shows that, from 8:30 am to 11:30 pm November 9 and from 5:30 am to 8:30 am November 10, ty le ca cuoc 188bet ON potential (black dots on solid line) and OFF potential (red dots on solid line) readings fluctuated rapidly, and roughly a third of ty le ca cuoc 188bet OFF potential readings excursed to ty le ca cuoc 188bet positive direction beyond ty le ca cuoc 188bet protection criterion -0.85 VCSE (blue solid line), indicating inadequate protection due to dynamic DC interference. While ty le ca cuoc 188bet ON potentials and OFF potentials nearly remained stable from 11:30 pm November 9 to 5:30 am November 10, and ty le ca cuoc 188bet OFF potential readings were approximately -1.00 VCSE, indicating acceptable protection if ty le ca cuoc 188bet dynamic interference was off.

Figure 3. Layouts of different DC drainage systems.

Although ty le ca cuoc 188bet stray current pick-up locations change with time, ty le ca cuoc 188bet discharge sites are predominately in proximity to ty le ca cuoc 188bet substation ground. Therefore, determining ty le ca cuoc 188bet impact of transit-caused stray current on metallic facilities in urban areas requires comprehensive detection and evaluation, starting in ty le ca cuoc 188bet vicinity of ty le ca cuoc 188bet substation grounds and along ty le ca cuoc 188bet transit system route.

3.2. Mitigation

It is usually agreed by all parts that mitigating ty le ca cuoc 188bet DC transit interference on ty le ca cuoc 188bet pipelines is rather a task with challenge. Per ty le ca cuoc 188bet lessons ty le ca cuoc 188bet industry has learned so far, a committee comprised of local pipeline operators, ty le ca cuoc 188bet railway operators and ty le ca cuoc 188bet governing organisations is necessary to form to call for collaboration of ty le ca cuoc 188bet diagnosis, evaluation and mitigation of interference problems. Forming ty le ca cuoc 188bet committee and improving awareness has proved to be an effective and sometimes imperative way for interference control and mitigation.

Mitigation methods for minimising ty le ca cuoc 188bet deleterious effects of DC transit system stray currents include: electrical isolation of rails and substations implemented solely by ty le ca cuoc 188bet railway operators and regarded as proactive measures, and passive measures implemented in combination by ty le ca cuoc 188bet pipeline operators such as cathodic protection (CP), coating repair, drainage bonds, etc.

Figure 4. Comparison of potential excursions monitored on a product pipeline among ty le ca cuoc 188bet test durations when ty le ca cuoc 188bet innovated bidirectional drainage equipment was on or off.

Drainage measures are often sought for effectively driving stray current out of ty le ca cuoc 188bet pipeline in ty le ca cuoc 188bet controlled path [5]. ty le ca cuoc 188bet principle of drainage measures is to provide an electronic path for ty le ca cuoc 188bet stray current to return to its source, thus reducing ty le ca cuoc 188bet amount leaking to ty le ca cuoc 188bet electrolytic path. In practice, there are four different ways of building up DC drainage arrangements:

a. Grounding drainage

This drainage method (Figure 3a) is achieved by installing a groundbed close to ty le ca cuoc 188bet affected pipeline and connecting them together. Independent from ty le ca cuoc 188bet interference sources, this method is preferably applicable to ty le ca cuoc 188bet scenario ty le ca cuoc 188bet anodic areas of ty le ca cuoc 188bet pipelines are relatively fixed, where stray currents picked up by ty le ca cuoc 188bet pipelines from unknown locations are drained to ty le ca cuoc 188bet soil through ty le ca cuoc 188bet strategically installed groundbed.

b. Direct drainage

This drainage method is achieved by creating a stray current flow path from ty le ca cuoc 188bet pipeline to ty le ca cuoc 188bet negative bus of ty le ca cuoc 188bet stray current source. ty le ca cuoc 188bet execution of this method must ask for an agreement from ty le ca cuoc 188bet transit operators. In this method, ty le ca cuoc 188bet stray currents entering ty le ca cuoc 188bet pipeline from unknown locations are directed back to ty le ca cuoc 188bet original source through ty le ca cuoc 188bet intentionally established path.

c. Unidirectional drainage

This drainage method is achieved by creating a current flow path from ty le ca cuoc 188bet pipeline via a unidirectional device (e.g. diodes) to ty le ca cuoc 188bet negative bus of ty le ca cuoc 188bet stray current source or a dedicated groundbed. An agreement must be asked from ty le ca cuoc 188bet transit operators if stray currents are to flow back to ty le ca cuoc 188bet original source. ty le ca cuoc 188bet unidirectional device is employed for avoiding stray currents from re-entering ty le ca cuoc 188bet pipeline through ty le ca cuoc 188bet groundbed or preventing stray currents circulating. In this way, this drainage arrangement is applicable to ty le ca cuoc 188bet scenario ty le ca cuoc 188bet affected pipeline segments are indefinitely changing between anodic and cathodic.

d. Forced drainage

This method is achieved by substituting ty le ca cuoc 188bet unidirectional device for a forced drainage device, for instance, a potential-controlled rectifier. ty le ca cuoc 188bet forced drainage device works if ty le ca cuoc 188bet pipeline potential is more positive than that of ty le ca cuoc 188bet original source or ty le ca cuoc 188bet dedicated groundbed. An agreement must be asked from ty le ca cuoc 188bet transit operators if stray currents are forced to flow back to ty le ca cuoc 188bet original source. To be completely effective, ty le ca cuoc 188bet forced drainage arrangement must be located at ty le ca cuoc 188bet point of maximum discharge.

Figure 5. HVDC transmission operations [6]: bipolar systems (a), monopolar system (b).

These drainage methods are described in ty le ca cuoc 188bet order happening to show ty le ca cuoc 188bet evolution course. All these methods are more applicable to mitigating interferences for pipeline sections adjacent to ty le ca cuoc 188bet negative return circuit of stray currents and are effective to controlling ty le ca cuoc 188bet OFF potential of these pipeline sections within ty le ca cuoc 188bet industrially-acknowledged criteria. At locations where anodic and cathodic interferences are mixed, none of ty le ca cuoc 188bet above drainage methods demonstrates ty le ca cuoc 188bet interference mitigation efficiency as expected. ty le ca cuoc 188bet authors’ team has invented a set of potential-controlled bidirectional drainage equipment. This equipment is capable of feeding varied amount of current in both directions in response to ty le ca cuoc 188bet interference potential to cancel off ty le ca cuoc 188bet interference effects, by which ty le ca cuoc 188bet pipe-to-soil potential can be controlled within criteria. ty le ca cuoc 188bet equipment has been put into a field test to verify ty le ca cuoc 188bet mitigating effect on ty le ca cuoc 188bet same product pipeline as related to Figure 2, and ty le ca cuoc 188bet results are displayed in a potential-time plot in Figure 4. From ty le ca cuoc 188bet plot, it is seen that, ty le ca cuoc 188bet OFF potential excursions of ty le ca cuoc 188bet pipeline under CP (Rect. on) were significantly reduced below -0.85 VCSE line when ty le ca cuoc 188bet bidirectional drainage equipment was in operation at different output modes (in which +/- stands for current output in positive or negative direction; P and I values reflect ty le ca cuoc 188bet response time). This invention has proved to be an additional choice for ty le ca cuoc 188bet mitigation of interference from DC rail transit systems.

To summarise, stray current corrosion caused by DC rail transit systems has been found on ty le ca cuoc 188bet underground pipelines or other structures in many cities all over ty le ca cuoc 188bet world. It is ultimately important for ty le ca cuoc 188bet pipeline operators to be clear of how severe these dynamic stray currents can cause wall loss of ty le ca cuoc 188bet pipelines and how to effectively mitigate ty le ca cuoc 188bet interference problems. However, neither evaluating ty le ca cuoc 188bet severity of dynamic DC stray current interference nor mitigating ty le ca cuoc 188bet corrosion risk is a best- practice-procedure-following task. More efforts are certainly required from engineers, scientists, operators, regulators and officials in order to make informed decisions on improving ty le ca cuoc 188bet reliability of ty le ca cuoc 188bet pipelines and other underground utilities in ty le ca cuoc 188bet cities.

(a) Cathodic HVDC interference

(b) Anodic HVDC interference

Figure 6. Mechanism of HVDC electrode interference [7].

4. HVDC interference

4.1. Occurrence

HVDC transmission networks have been widely implemented globally in recent years. HVDC transmission is used to carry electrical energy over long distances or to interface two AC power systems that might not be synchronised [6].

Figure 7. Pipe-to-soil potential shifts on a pipeline as a result of HVDC interference at different times.

Figure 8. Remote monitoring and control network for HVDC interferences.

HVDC transmission systems may operate in such a manner that continuous (normal operation) or periodic (monopolar operation) direct current follows an earth path between HVDC system terminals [6], as seen in Figure

5. During such operations, a portion of ty le ca cuoc 188bet DC current that is flowing between ty le ca cuoc 188bet two HVDC electrodes is captured by ty le ca cuoc 188bet pipelines and associated grounding systems located in zones where ty le ca cuoc 188bet earth potentials are high and are discharged back to ty le ca cuoc 188bet soil at locations where ty le ca cuoc 188bet earth potentials are lower. More precisely, a pipeline may collect, conduct, or discharge a portion of this current depending on ty le ca cuoc 188bet location of ty le ca cuoc 188bet pipeline and ty le ca cuoc 188bet polarity of ty le ca cuoc 188bet HVDC ground electrode. Due to such excessive stray currents, pipe-to-soil potentials are accordingly shifted at different locations, making ty le ca cuoc 188bet potential- mileage plot similar to a “hat” shape, as seen in Figure 6. When ty le ca cuoc 188bet electrode near ty le ca cuoc 188bet pipeline is operating in positive mode, ty le ca cuoc 188bet current from ty le ca cuoc 188bet electrode is collected by ty le ca cuoc 188bet pipeline in ty le ca cuoc 188bet region located in ty le ca cuoc 188bet vicinity of ty le ca cuoc 188bet electrode, and then discharged from ty le ca cuoc 188bet pipeline at locations remote from ty le ca cuoc 188bet electrode. When ty le ca cuoc 188bet HVDC electrode near ty le ca cuoc 188bet pipeline is operating in negative mode, ty le ca cuoc 188bet current is discharged from ty le ca cuoc 188bet pipeline in ty le ca cuoc 188bet vicinity of ty le ca cuoc 188bet electrode and collected in ty le ca cuoc 188bet area far from ty le ca cuoc 188bet electrode. 

Figure 9. Intelligent data sampling units installed in test posts along ty le ca cuoc 188bet pipeline.

Corrosion of ty le ca cuoc 188bet metallic pipelines occurs if DC stray current discharges from ty le ca cuoc 188bet metallic structure into ty le ca cuoc 188bet soil, namely, in ty le ca cuoc 188bet anodic areas in Figure 6. ty le ca cuoc 188bet corrosion, causing metal loss, is particularly critical on well-coated pipelines where ty le ca cuoc 188bet current discharge is concentrated at holidays in ty le ca cuoc 188bet coating. Perforation of ty le ca cuoc 188bet pipe wall can occur rapidly at a holiday as a result of ty le ca cuoc 188bet high density of current discharge from ty le ca cuoc 188bet pipeline to ty le ca cuoc 188bet earth. In ty le ca cuoc 188bet cathodic areas, pipe-to- soil potential shifts in ty le ca cuoc 188bet negative direction in Figure 6. This shift can be beneficial for ty le ca cuoc 188bet pipeline, provided that ty le ca cuoc 188bet pipe steel is not susceptible to hydrogen embrittlement. Otherwise, sustained high negative voltages can cause coating disbondment and embrittlement of high-strength steels [7]. Figure 7, from our team’s work, depicts ty le ca cuoc 188bet potentials of a pipeline continuously monitored with remote monitors for capturing HVDC interference events due to its proximity to a ground electrode in Southern China. ty le ca cuoc 188bet potential shifts indicate that, within this period, ty le ca cuoc 188bet pipeline at this monitoring location reached nearly 100V when ty le ca cuoc 188bet nearby HVDC electrode operated as a negative electrode, and in contrast, -120V when ty le ca cuoc 188bet nearby HVDC electrode operated as a positive electrode. Depending on ty le ca cuoc 188bet soil condition and ty le ca cuoc 188bet relative position relationship between ty le ca cuoc 188bet pipeline and electrode (even several tens of kilometres apart), HVDC interference, can have massive impact on ty le ca cuoc 188bet pipeline in ty le ca cuoc 188bet range of hundreds of kilometres. Therefore, ty le ca cuoc 188bet concerns on their possible adverse impact on ty le ca cuoc 188bet environment have accelerated ty le ca cuoc 188bet need for accurate approaches to analyse HVDC adverse effects on neighbouring buried metallic utilities and development of appropriate effective and economical mitigation measures.

4.2. Mitigation

ty le ca cuoc 188bet mitigation of HVDC interference relies on comprehensive measures. Similar to ty le ca cuoc 188bet mitigation of dynamic DC interference, a collaborative committee should form for problem solving. A proper design based on full engineering analysis and interference prediction is deemed to be ty le ca cuoc 188bet most effective method to avoid severe interference issues; however, this article only emphasises ty le ca cuoc 188bet possible mitigation measures for existing structures.

Most pipelines are located far from ty le ca cuoc 188bet HVDC electrodes, ty le ca cuoc 188bet interference sources, to make it not economical to install a bond. Because of ty le ca cuoc 188bet large voltage shifts, sacrificial anodes may not adequately compensate thelocalelectricfieldcausedbythe HVDCearthcurrent.ty le ca cuoc 188bet most practical mitigation method is to use an impressed current system powered by a potential controlled rectifier. Not only would ty le ca cuoc 188bet cathodic protection power supply be able to counteract ty le ca cuoc 188bet large positive potential shifts, but during ty le ca cuoc 188bet negative shift periods it would also shut down, thus minimising ty le ca cuoc 188bet stress on ty le ca cuoc 188bet coating [8].

Depending on ty le ca cuoc 188bet amplitude of HVDC earth current, it is ultimately necessary to divide ty le ca cuoc 188bet pipeline into numerous electrically isolated sections to increase ty le ca cuoc 188bet effective path resistance for facilitating ty le ca cuoc 188bet application of ty le ca cuoc 188bet above mitigation measures. In this way, sacrificial anodes can be used for mitigation. Besides, if ty le ca cuoc 188bet use of sacrificial anodes is still not adequate, using automatic potential-controlled rectifiers in each section can mitigate HVDC fault currents.

ty le ca cuoc 188bet author’s team has accomplished to mitigate HVDC interference for ty le ca cuoc 188bet pipelines by using ty le ca cuoc 188bet abovementioned bidirectional forced drainage equipment. ty le ca cuoc 188bet equipment is capable of automatically adjusting ty le ca cuoc 188bet magnitudes and directions of ty le ca cuoc 188bet current outputs in response to shifts and changes in ty le ca cuoc 188bet structure- to-electrolyte potential. ty le ca cuoc 188bet HVDC interference case showed in Figure 7 was successfully mitigated at this location with ty le ca cuoc 188bet bidirectional forced drainage method. After mitigation measures were taken, ty le ca cuoc 188bet potential shifts have been controlled within ty le ca cuoc 188bet range between -27V in ty le ca cuoc 188bet negative direction and 23V in ty le ca cuoc 188bet positive direction.

4.3. Remote monitoring

HVDC interference on ty le ca cuoc 188bet underground pipelines has been found with ty le ca cuoc 188bet features of inadvertence, unpredictability, and random durations. Therefore, remote monitoring techniques are to seek for continual monitoring of not only ty le ca cuoc 188bet HVDC interference incidents but also ty le ca cuoc 188bet HVDC mitigation efficiency.

ty le ca cuoc 188bet continual monitoring of HVDC interferences can be achieved by configuring a remote monitoring and control network, as depicted in Figure 8. ty le ca cuoc 188bet network is composed of three main elements:

a. Intelligent data sampling units

These units are installed in ty le ca cuoc 188bet test posts at ty le ca cuoc 188bet strategic locations along ty le ca cuoc 188bet pipeline (Figure 9), and, with ty le ca cuoc 188bet aid of buried coupons and permanent reference electrodes, measure ty le ca cuoc 188bet required parameters, such as potential, voltage, current and soil resistivity, etc. These data can be ready for automatic transmission or stored for retrieval.

b. Data transmission network

ty le ca cuoc 188bet data transmission network is ty le ca cuoc 188bet core to transmit ty le ca cuoc 188bet data measured at ty le ca cuoc 188bet test posts with certain protocols to servers for storage. On ty le ca cuoc 188bet other hand, commands for adjusting ty le ca cuoc 188bet work mode of ty le ca cuoc 188bet units are sent over ty le ca cuoc 188bet same network and received by ty le ca cuoc 188bet units. At present, ty le ca cuoc 188bet most popular data transmission network can be cellular communication network, satellite and other wireless techniques.

c. Servers

Servers are used via internet services to receive and store ty le ca cuoc 188bet field measurement data from ty le ca cuoc 188bet units for processing and analysis. All ty le ca cuoc 188bet HVDC monitoring data are managed through specialised software installed on ty le ca cuoc 188bet servers. Other than this, ty le ca cuoc 188bet management software is also integrated with control functions for changing ty le ca cuoc 188bet work mode of ty le ca cuoc 188bet units.
In order to effectively monitor HVDC interferences, multiple intelligent data sampling units are to be installed at ty le ca cuoc 188bet strategically selected locations along ty le ca cuoc 188bet pipeline right-of-way, minimally, at ty le ca cuoc 188bet pipeline section close to ty le ca cuoc 188bet HVDC electrodes. ty le ca cuoc 188bet occurrence of HVDC interference can be ascertained if ty le ca cuoc 188bet pipe-to- soil potential shifts are noticed at different monitoring locations at ty le ca cuoc 188bet same time.

5. Conclusions

Stray currents can have a wide range of interference effects on a pipeline, ty le ca cuoc 188bet most harmful being ty le ca cuoc 188bet creation of localised electrochemical corrosion cells that attack any buried metallic structure. Stray currents flow through unintended paths and come from different sources. It is important for ty le ca cuoc 188bet pipeline operators to determine whether stray currents are affecting ty le ca cuoc 188bet pipelines, identify ty le ca cuoc 188bet probable source(s) of stray currents, evaluate ty le ca cuoc 188bet severity of ty le ca cuoc 188bet stray current interferences on ty le ca cuoc 188bet pipelines and mitigate ty le ca cuoc 188bet interference effects. DC stray current interferences caused by DC rail transit systems and high voltage direct current transmission systems are becoming more and more severe on ty le ca cuoc 188bet pipelines as urbanisation proceeds and more large-scale infrastructures are invested.

Acknowledged by ty le ca cuoc 188bet industry, a collaborative committee comprised of at least pipeline operators and ty le ca cuoc 188bet parts operating ty le ca cuoc 188bet interference sources is necessary to form for ty le ca cuoc 188bet diagnosis, evaluation and mitigation of interference problems. Diminishing ty le ca cuoc 188bet discharge of stray currents into soil is ty le ca cuoc 188bet most effective way; however, pipeline operators are still in need of their own measures for interference mitigation, such as CP. Forced drainage methods are often sought for DC interference mitigation when ty le ca cuoc 188bet stray current in ty le ca cuoc 188bet pipeline is at a high level. Otherthanconventionalprotectionmethods, bidirectional forced drainage methods have been accomplished by ty le ca cuoc 188bet author’s team to mitigate dynamic transit DC and HVDC interferences. In order to evaluate ty le ca cuoc 188bet severity of interferences on a pipeline in a timely manner, remote monitoring techniques are recommended for continual monitoring of not only interference incidents but also mitigation efficiency.

 References

1. Elizabeth Nicholson. Stray current detection and correction. Poland Corrosion Conference. 2010.

2. NACE International. CP interference manual. 2012.

3. T.Place, T.Smith. Practical telluric compensation for pipeline close interval survey. NACE International conference. 2000.

4. Thomas J.Barlo, Alan D.Zdunek. Stray current corrosion in electrified rail systems: Final report. 1995.

5. M.Cheng, Q.Tang, D.Wei, P.Zhang, Comprehensive protection of buried steel pipelines at HVDC earthed electrode interference area. Natural Gas Industry. 2015; 35 (9): p. 105 - 111.

6. Walter von Baeckmann, Wilhelm Schwenk, Werner Prinz. Handbook of cathodic corrosion protection. Gulf Professional Publishing. 2011.

7. Yu Gong, Chunlin Xue, Zhilei Yuan, Yexu Li, Farid Paul Dawalibi. Advanced analysis of HVDC electrodes interference on neighboring pipelines. Journal of Power and Energy Engineering. 2015; 3: p. 332 - 341.

8. NACE International. High-voltage direct current interference (24254-SG). 2014.