Introduction to Kirchoff's Law & Network Analysis

The following paper was presented at CEOCOR – Brussels, Belgium – 19th – 22nd May 2014.

The paper was authored by Ken Lax, and presented by Ken at the conference.

DISCLAIMER: All information, references to Standards and / or criteria correct at time of paper.

Introduction

Kirchhoff’s Laws are the foundation for many network analysis techniques. 

Cathodic Protection systems are essentially d.c. electrical networks and the distribution of current follows these laws.

Most CP applications will not require any knowledge of these laws but sooner or later every CP practitioner will need to apply and understand them.

By the end of this presentation you will be able to apply Kirchhoff’s Laws and resolve the simultaneous equations using matrix algebra.

An understanding of Kirchhoff’s Laws, and the ability to apply them, will enable you to understand:

  • I-V equivalent circuits (source transformations)
  • Superposition applications
  • Mesh networks
  • LaPlace transformations (not covered in this presentation)
  • Node Voltage Analysis (NVA)
  • Thevenin and Norton Theorems (not covered in this presentation)

We will only be considering d.c. circuits in this introduction.

AGENDA

  • Circuit definitions (branch, junction, node)
  • Definition of Kirchhoff’s Current Law (KCL)
  • Definition of Kirchhoff’s Voltage Law (KVL)
  • Some warm-up exercises with Ohms Law
  • Simple rules for applying KCL and KVL to produce independent simultaneous equations
  • KCL and KVL exercises with increasing complexity
  • Simple way to solve simultaneous equations using Excel
  • Solving simultaneous equations using augmented or inverted matrix operations

OVERVIEW

OVERVIEW.png

Kirchoff's Current Law (KCL) and Kirchoff's Voltage Law (KVL) will add to the existing knowledge of CP engineers and designers and help to explain some of those mysterious situations that cannot be explained just by Ohms Law.

DEFINITIONS

Passive sign convention:  Current enters a passive device at its POSITIVE terminal.

Current:  Measurement of the electric charge passing through a given point within a certain amount of time.  Units are Coulombs per second (amperes).

Voltage:  measures the amount of energy required to move a given amount of charge as it passes through a circuit.  Units are Joules per Coulomb (Volts).

Junction:  Any place where two or more wires come together.

Branch:  Anything that connects two junctions

Node:  A point in the circuit

KIRCHOFF'S CURRENT LAW (KCL)

Also known as the JUNCTION rule.  Conservation laws tell us that electrons must behave in a certain way when connecting devices to make a circuit.   The electron behaviour governs the voltage and current around the loops and at the nodes.  So Kirchhoff’s Laws apply whatever devices are used in the circuit since it is a law of conservation of charge (KCL) or energy (KVL).

The sum of the currents entering any junction must equal the sum of the currents leaving that junction.

KIRCHOFF'S VOLTAGE LAW (KVL)

OVERVIEW.png

Also known as the LOOP rule. 

The sum of the potential differences across all elements around any loop must be zero.

The Importance of Electrical Isolation

Defining "Electrical Isolation" for a Cathodic Protection System

With respect to cathodic protection systems, the term "electrical isolation" relates to confining the (cathodic) protective current to the structure being protected.

In terms of electrical separation this could mean isolating;

  • Two (or more) cathodic protection systems from one another
  • Buried / immersed structures from above ground appurtenances
  • Cathodically protected structures from earthing systems
  • Owner / Operator interfaces

Electrical isolation is a key factor in the successful application of cathodic protection where it has been included at the design stage. For the purposes of this blog article we assume that the structure is intended to be electrically isolated.

That is not to say that electrical isolation is always required, as long as the designed system has taken this into account.

There are times when it may not be desirable, or even practical, to isolate protected from unprotected structures. Examples are refineries, industrial plants, large tank farms and similar complex facilities. 

How to Electrically Isolate the Cathodically Protected Structure

Commercial fittings for providing electrical isolation of pipework include;

  • Insulating Flange Kits (IFK)
  • Monolithic Isolation Joints (IJ)
  • Non-metallic pipe sections

Cathodically protected structures can be electrically isolated from earthing systems via;

  • Decoupling Devices

Other solutions, e.g. non-conductive membranes, are available as methods of electrical isolation depending on the system in question.

Insulating Flange Kits (IFK)

Insulating Flange Kit (Exploded View) resized.jpg

The kit comprises of the following;

  • Insulating Gasket
  • Non-Metallic Sleeves for Assembly Bolts
  • Non-Metallic Washers
  • Metallic Washers
  • Nuts & Bolts

Monolithic Isolation Joints (IJ)

Monolithic Isolation Joint resized.jpg

The isolation joint is fabricated as a ready to install section of pipework.

The joint consists of;

  • Forged Rings
  • External Coating
  • Internal Coating
  • Adhesive Sealant
  • Di-Electric Filler
  • Insulating Rings
  • O Rings

Decoupling Devices

Corroconsult recommend only solid-state devices, as this eliminates the maintenance requirements and hazardous electrolytes of electrochemical polarisation cells.

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Solid-state DC decoupling devices can be used for;

  • Electrically isolating from from utility earthing (grounding) systems
  • Electrically isolating from electrical equipment earthing (grounding) systems
  • Induced AC voltage mitigation
  • Isolation joint protection

How can electrical isolation be compromised?

Incorrect Installation

An incomplete kit installation for an IFK will result in the flanged joint being electrically continuous.

Earthing / Grounding

The most common failure of installed electrical isolation is through incorrect earthing of cathodically protected structures.

DCVG Survey - A Beginner's Guide [Part 1]

Direct Current Voltage Gradient [DCVG] Survey

Introduction

This non-intrusive, above ground survey allows the operator to identify the location of coating defects (holidays).

We are pleased to announce the first uploaded animation to our YouTube channel (shown within this post).

The YouTube video represents the first instalment within a series. Its purpose is to give an introductory overview to people not familiar with the technique e.g. asset owners, operators, supervisors.

Recognition within International Standards for Buried Pipelines

The technique is an approved ECDA (External Corrosion Direct Assessment) method as detailed in ANSI / NACE Standard Practice SP 0502-2010.

DCVG is also listed as an above-ground survey used to assess the coating condition and to locate coating defects within:

  • BS EN 13509:2003 - Cathodic protection measurement techniques
  • BS EN ISO 15589-1:2017. Petroleum, petrochemical and natural gas industries. Cathodic protection of pipeline systems. On-land pipelines

All DCVG surveys should be undertaken by competent and qualified personnel in accordance with BS EN ISO 15257:2017 - Cathodic protection - Competence levels of cathodic protection persons.

 

This short animation shows the technique to be utilised for a DCVG (Direct Current Voltage Gradient) survey using an analogue voltmeter and matched electrodes.

The video covers the basic methodology in identifying, centering and measuring coating defects (holidays) in the field [OL/RE Potential]

This "How To" guide is intended for asset owners, operators and supervisors that may not be familiar with the technique.

 

An Overview of the ECDA Process

What does ECDA mean?

ECDA is the commonly used acronym for External Corrosion Direct Assessment. The process has been developed for buried onshore ferrous pipeline systems. ECDA was created as a process for improving pipeline safety. Its primary purpose being to prevent future external corrosion.

Whilst this post attempts to provide an overview of the ECDA process, it is imperative that it is tailored each time to address specific requirements of individual pipeline systems.

What are the main steps of the ECDA Process?

There are four main steps to the ECDA Process, these are;

  • Pre-Assessment [Desktop Study]
  • Indirect Assessment [Above Ground / Non-Intrusive Surveys]
  • Direct Assessment [Bellhole Excavation & Testing]
  • Post Assessment [Calculation & Reporting]

The flow diagram below shows a general approach to the ECDA Process - evaluation methods may include, but are not limited to those shown here.


Pre-Assessment

Pipe Related

  • Material
  • Diameter
  • Wall Thickness
  • Year Manufactured
  • Seam Type
  • Bare Pipe Sections

Construction Related

  • Year Installed
  • Route Changes / Modifications
  • Construction Practices
  • Locations of Valves, Clamps, Insultaing Joints etc
  • Locations of and Construction Methods used at Casings
  • Locations of Bends including Miter Bends and Wrinkle Bends
  • Depth of Cover
  • Underwater Sections / River Crossings
  • Locations of River Weights and Anchors
  • Proximity to other Pipelines, Structures, HV Transmission Lines, Rail Crossings

Soils / Environmental

  • Soil Characteristics / Types
  • Drainage
  • Topography
  • Land Use (Current and Past)
  • Frozen Ground

Corrosion Control

  • CP System Type (Anodes, Rectifiers and Locations)
  • Stray Current Sources / Locations
  • Test Point Locations (or Pipe Access Points)
  • CP Evaluation Criteria
  • CP Maintenance History
  • Years without CP Applied
  • Coating Type – Pipe
  • Coating Type – Joints
  • Coating Condition
  • Current Demand
  • CP Survey Data / History

Operational Data

  • Pipe Operating Temperature
  • Operating Stress Levels and Fluctuations
  • Monitoring Programs (Coupons, patrol, Leak Surveys etc)
  • Repair History / Records
  • Leak / Rupture History
  • Evidence of Microbiologically Influenced Corrosion (MIC)
  • Type / Frequency of Third Party Damage
  • Hydrotest Dates / Pressures

Indirect Assessment

TR & Groundbed Functionality Checks

  • TR Output Current
  • TR Output Voltage
  • Drain Point Potential measurement
  • Groundbed Output Capabilities

CIPS (Close Interval Potential Survey)

  • Synchronously Interrupted
  • On Potential Measurement
  • Instant Off Potential Measurement

DCVG (Direct Current Voltage Gradient) Survey

  • Synchronously Interrupted
  • OL / RE Measurement
  • % IR at Defect Calculation

Additional Information Collated

  • GPS Coordinates recorded for key features and locations
  • Distance measurement of key features and locations
  • Photographic records taken for key features
  • a.c. potentials measured at all test facilities
  • Overview of condition of test facilities and associated connections
  • Data logging at identified / possible sources of Stray Current

Direct Assessment

Locations for excavation shall be selected based on the information gleaned from the pre-assessment and indirect assessment procedures.

A minimum of at least one trial pit should  be excavated to clarify the effectiveness of the first two procedures.

 
 

Post Assessment

All data obtained during the survey activities is provided, generally in both graphical and tabular formats (where appropriate), as well as photographic records, etc.

Updated Website Released

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Corroconsult UK Limited are pleased to announce the release of our updated website.

The new site includes expanded sections regarding our services and material data sheets as well as new client and project gallery sections.

We hope that you enjoy the new content, and will be looking to add further information over the coming months.

ISO & OHSAS Accreditation Renewed

 
 

Corroconsult are proud to announce the renewal of their ISO and OHSAS certification following today's audit.

Following on from last years successful audit to the latest ISO standards for Quality and Environmental management systems, Corroconsult remain one of the few corrosion companies to have already been recognised with this accreditation.

Full versions of our current certificates can be found using the links below;

New ISO 20313 Standard Released - Cathodic Protection of Ships

The new ISO 20313 standard for the Cathodic Protection of ships chaired by Ken Lax was published on 30 January 2018.  This is a major achievement for the working group and especially for the UK mirror group participants (Pat Lydon, Winston Shepherd and Andrew Willis). 

 
Marine_Ship_01 resized.jpg
 

The plenary meeting of ISO Technical Committee (TC) 8 and Sub Committee (SC) 8 – Ship design met on 31 January 2018 and complimented the working group on meeting all the deadlines and producing a good standard. 

The working group has now been disbanded, although the Chairman of TC8 SC8 requested Ken to remain on the sub-committee to provide assistance.

The new Standard is available for download here: https://www.iso.org/standard/67645.html

Ken Lax Presents to Annual FKKS "Infotag"

Ken Lax participated in the fkks annual “Infotag” held in Esslingen, Germany in January 2018.  In spite of the bad weather there were more than 130 cathodic protection experts there from all over Germany as well as members from Austria, Norway, Switzerland and the UK.  Ken presented a brief paper on the progress in the new ISO standard for pipeline electrical interference.

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The standard is underway with a good international participation.  Some new topics that will be covered in this standard are:

  • Tidal effects on potentials at landfalls
  • Telluric currents (with some calculation examples)
  • Effects of third harmonics on the ac waveform shape and an example calculation of the true rms value (as opposed to the value that might be given by some digital voltmeters)

Above Ground Data Collection and Evaluation for Pipeline Rehabilitation

The following paper was presented at BHR Group's 1st Interational Conference for Pipeline Integrity – Schipol, The Netherlands – 5th – 7th November 2008.

The paper was authored jointly by Richard Lindley and Ken Lax, and presented by Richard at the conference.

DISCLAIMER: All information, references to Standards and / or criteria correct at time of paper.

ABSTRACT

A description is provided of the above ground measurements that should be made in the evaluation of pipelines that are to be rehabilitated. Simple measurements, carried out by trained and competent personnel, can provide quality data to enable decisions to be made for pipeline rehabilitation. 

The importance of data integrity is considered and the use of competent personnel to collect data.

Data management and data analysis methods are discussed and an example of the weighting factors for a Priority Matrix to provide an overall risk for each section of the pipe is provided.

INTRODUCTION

When a pipeline is considered for rehabilitation it is inevitably because the pipeline is believed to be in a poor condition, usually due to neglect over the operating life.

On this basis it is usually safe to assume that:

  • Whatever coating there was has now deteriorated to the point where it is of no benefit
  • Cathodic protection was never properly applied, maintained, or implemented
  • Construction records are no longer available

In certain countries you will also be safe to assume that the field joints are suspect and were never properly coated.

Add to this the unrealistic expectations of lay personnel regarding the cost of pipeline rehabilitation and repairs and there is a huge uncertainty over the prospects for rehabilitation.

Contrary to the opinion of many large pipeline contractors and consultancy groups the best way to rehabilitate a pipeline may not be to dig, expose it all and re-wrap it.  Aged pipelines, even those suffering from neglect, are not usually uniformly bad and in need of replacement.

The above ground measurement and inspection techniques described in this paper will enable firm engineering decisions to be made regarding pipeline repairs, coating replacement, and cathodic protection.

International standards and recommendations are not always helpful in providing guidance.  These documents should be treated with caution and recognised for what they are.  In the case of standards they are a guide for specialists to employ, not text books.  Recommended practices are the opinions of a committee based on their experience and do not have the provenance of a standard.

A good in-line inspection tool can provide the best available information on remaining wall thickness but some tools exaggerate the capabilities of the analysis to provide details on wall thickness, coating disbondment and cathodic protection statements.  As in all cases the tool capabilities should be carefully reviewed and verified before committing to major expense.

This paper deals with the options available for above ground inspection.  Above ground inspection is sometimes the only tools that is available to prepare an assessment for rehabilitation and can be used as a stand-alone or as a complimentary set of data for rehabilitation.

It is always good for research and progress to develop new techniques and advanced analysis but we can achieve a great deal if we properly employ the known and existing techniques.

There is no universal solution to pipeline rehabilitation works.  Each section of the pipeline has to be considered on its own merits. 

The paper does not make any prescriptive recommendations, its purpose is to emphasise that when above ground measurements are made by professionally competent personnel then the data can be used to determine the suitability of each section of pipeline for rehabilitation.

The techniques have been successfully applied to many pipelines around the world, even those that the “experts” said could not be rehabilitated.

EXTERNAL CORROSION

Whilst above ground measurements alone will provide no indication as to the internal condition of the pipe, with no available alternative, an assessment of the external condition of the pipeline can provide vital information as to the feasibility of rehabilitation.

Mechanisms

The following section details the principal mechanisms for external corrosion to occur on a pipeline.

Coating Failure

The purpose of a coating is to provide a high resistance barrier between the pipe and its surrounding environment.

Where the coating is damaged, active corrosion will occur in the absence of an operational cathodic protection system.

Where effective cathodic protection is applied, the corrosion process at a coating defect, or holiday, can be slowed to an almost negligible rate.

Soil Conditions

Soil resistivity measurements are made to provide an indication of the soil corrosivity and for use in anode groundbed and attenuation calculations.

Soil corrosivity is classified by the British Standard BS7361 (1) as:

·        0 – 10 ohm·m                    severely corrosive

·        10 – 50 ohm·m                  corrosive

·        50 – 100 ohm·m                moderately corrosive

·        > 100 ohm·m                     slightly corrosive

Resistivity is not the only factor affecting corrosion, other factors which may be considered include;

  • pH
  • total acidity
  • aeration
  • moisture content
  • soil type
  • soil permeability and composition
  • heterogeneity

Mechanical Damage

It is not uncommon that during construction, or through third party excavation, the pipeline can be damaged. At best this can mean removal of the coating, at worst it can mean scoring of the metal itself.

If this damage is not identified at the time, or subsequently reported and repaired, these locations will be subject to corrosion.

On pressurised pipelines, the maximum allowable operating pressure is calculated using the pipe material and its initial wall thickness. If metal loss occurs, either directly, or through corrosion, than naturally there is a reduction in the allowable operating pressures.

DC Stray Currents

DC stray currents will create metal loss, not at the point where the current enters the pipe, but at the point that it leaves the pipe.

1 Amp DC leaving a steel pipe will cause 9.1 kg of metal loss per year. And whilst the current leaving the pipe is usually of a significantly smaller magnitude than this, they can be increased due to sources of DC stray current, including;

  • DC transit systems, such as railways, trams etc
  • Third party cathodic protection systems
  • Welding activity
  • Mining
  • Telluric currents

AC Influence

AC interference can present two main issues on a pipeline.

Firstly, is one of safety, as voltages above 15 VAC (2) can be considered as hazardous to personnel, through “Touch Potential”.

Secondly, it is only relatively recently that AC interference has truly been recognised as a potential cause for corrosion on well-coated pipelines.

The propensity for AC corrosion to occur can be classified by;

  • Proximity to AC source
  • Magnitude of AC source
  • AC current density
  • Ratio between DC and AC current densities
  • Soil Type / Resistivity
  • Levels of cathodic protection

ABOVE GROUND MEASUREMENTS

The purpose of the above ground measurements is to obtain enough quantifiable information with regard to the pipeline without the need for excavation.

The following section details several methods that can be combined to assess the pipeline condition, although their individual selection depends on both Client requirements and feasibility.

DCVG (Direct Current Voltage Gradient)

The purpose of a DCVG survey is to identify coating defect locations and, once located, to benchmark them against other coating defects on that pipeline.

The technique requires a source of DC current to be interrupted cyclically, either from an existing cathodic protection system, or a temporarily installed system. The pipeline is then traversed using two equally matched electrodes, and an analogue voltmeter, as shown in Figure 1.

 
 Figure 1

Figure 1

 

The sequence of indications on the meter are shown below;

DCVG survey Diagram.png

This method gives a clear and precise method of locating coating defects with the pipe in-situ, and without the need for intelligent pigging or excavation.

The coating defects can also be determined to be anodic (current leaving) or cathodic (current joining) the pipeline. Whilst this is not an exact science, in that the defects can alternate their nature, it does give a baseline indication as to whether the pipe is likely to be losing metal.

Additionally the attenuation of the DC current is also measured, giving an early indication to the current requirements for a cathodic protection system.

Soil Resistivity

Soil resistivity measurements can be taken using the 4 Pin Wenner Method. This method allows for data to be collected in and around pipe depth without need for excavation.

The 4 pins are spaced equally apart, in a straight line, the spacing of which is equivalent to the depth of the measurement required. Figure 2 shows the equipment set-up, with the pins spaced at 1 metre.

 Figure 2. Soil resistivity measuring equipment set-up

Figure 2. Soil resistivity measuring equipment set-up

An AC current is applied to the outer pins, and the voltage drop between the inner pins is measured. The resistivity is a function of current, voltage drop and the spacing of the pins.

The measurement taken by the equipment is one of resistance, in ohms, in order to calculate the resistivity, the following calculation must be made;

res1.png

Where;   

res2.png

The Wenner 4 Pin method provides an average resistivity to the depth of the measurement, though in reality the resistivity of individual layers can vary dramatically.

Therefore the measurement is taken at several depths, and then, using the Barnes Layer Method, analysed.

The Barnes Layer Method was developed to make use of the average resistivities of subsurface data. This method endeavours to distinguish the resistivity of layers of the earth. The thickness of the layer is assumed to be equal to the incremental increase in the spacing of the pins.

res3.png

Where;

res3.png

Note:    mhos are the inverse of ohms, as conductance is the inverse of resistance

Visual

Direct visual inspection of the pipe condition can be carried out at above ground facilities, such as AGIs and river crossings, or where the pipe is already exposed due to erosion or landslides.

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Such areas allow for additional information to be obtained including;

  • Presence / absence of coating and its condition
  • Condition of clamps / supports

 There is a clear advantage in walking the entire length of the pipeline route, this enables information to be gathered such as;

  • Road river and rail crossings with indications as to whether or not they are sleeved
  • Crossings and parallelisms of overhead high voltage power lines
  • Crossings and parallelisms of ac and dc electrical traction systems
  • Obvious changes of soil conditions such as arable to clay soils
  • Areas subject to mechanical risk from landslips and avalanches
  • Sites of special scientific interests
  • Critical sections of the pipe such as hospital and power feeder sections
  • Evidence of pollution, either from the pipe or from other sources

Potential Measurements

Structure to soil potential measurements will provide indications of:

  • Cathodic protection status
  • Presence of stray currents
  • Presence of AC interference
  • Active corrosion

Buried steel has a natural potential (also known as rest potential, free corrosion potential or native potential) that is a function of the steel and the soil composition.

All structure to soil potentials onshore are made with respect to a saturated copper / copper sulphate (Cu/CuSO4) reference electrode.

Test Posts

Test posts are installed at specified intervals along the pipeline, or at key locations, and facilitate a pipe-to-soil potential to be taken using the cable connection terminated inside.

Facilities may also be available to measure connections to;

  • Foreign services (crossing or parallel)
  • Casings
  • Sacrificial anodes
  • Coupons
  • Current Spans

Above Ground Features

Above ground features, such as AGIs, valves and crossings allow a direct connection to the pipeline. On some pipelines these may be the only source of a direct pipe connection, when test posts have been vandalised, stolen or not previously installed.

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Potential Gradients

Where there is no available connection to the pipeline, either through a test post, or an above ground feature, two matched electrodes can be placed in the ground, suitably spaced, in order to determine the magnitude and direction of DC currents.

CIPS (Close Interval Potential Survey)

CIPS is only really applicable to a pipeline where an existing CP system is in operation, as the purpose of the survey is to determine the effectiveness of the applied cathodic protection. There is seldom any value in carrying out a CIPS for a temporarily applied cathodic protection system.

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Any source of cathodic protection current should be synchronously interrupted if an IR Free (sometimes referred to as an Off) Potential is required.

The survey relies on a connection to the pipeline being made, through a cable reel, potentials are then measured at short intervals typically 1 – 2 metres along the route of the pipeline. The connection is remade at each available pipe connection.

MANAGING DATA

Careful management and handling of the data collected during the above ground surveys will provide valuable insights into the pipe condition.  It is easy to overlook vital clues if the data is not managed correctly.

Above ground surveys rely upon visual reports, above ground measurements, and bell hole measurements.

GPS locations should be provided for each feature and measurement location.

This data needs to be segregated into a format that permits marrying up to the pipeline route.  Due to the large amount of data that is collected it is easy to overlook critical information for a particular section because the data has not been related to the correct section.

For this reason it is better to divide the pipeline up into sections so that the various data can be easily cross related.  These sections do not necessarily need to refer to convenient pipeline operating sections, or even electrical sections, but they must be able to be easily identified.

Data Analysis

Data analysis needs to be carried out on a section by section basis and needs to cover all of the information.

A Priority Matrix should be developed to list all of the contributory features along with a weighting, or factor, to categorise the overall risk of external damage.

Visual reports should be analysed to determine whether there is any evidence of previous leaks or repairs to the pipeline or if there are any factors that make the risk of damage higher than usual.  Such factors may include signs of pollution, evidence of landslides or construction activities.

Potential measurements will provide useful, and definitive, information concerning ac and dc interference that can significantly increase the corrosion rates of bare steel.  Guidance is given within the relevant cathodic protection and stray current interference standards with regard to the interpretation of the results.

In isolation the various measurements will not provide enough evidence to determine excavation and repair locations.  The data needs to be combined into a Priority Matrix that adds weighting to the data to develop a total risk factor.

Over the years Corroconsult have developed a weighting system to encompass all of these features, and an example is shown in the Appendix.

The Priority Matrix has to be adjusted for each new pipeline, since the factors may change depending upon the environment and local regulations.

Priority Matrix

A Priority Matrix is utilised to assist in selecting suitable locations for excavations and further measurements based on the information gleaned from the Above Ground Measurements phase of the works

Weightings are applied to various features of the pipeline route to provide an initial assessment. The higher the index then the higher the priority i.e. 0 = low priority, 80 = high priority.

After the initial priority has been established then further key factors are included to provide a final weighting. This final weighting will then be used to supply the final recommendations for excavations.

Example weighting factors are:

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BELL HOLE EXCAVATIONS

Bell hole excavations provide a direct assessment, and vital insight as to the condition of the pipeline. The following section outlines the basic tests that should be performed at any location where the pipe is exposed.

Relative Information

With the pipeline exposed, the following information should be recorded;

  • Individually assigned bell hole number
  • Inspectors
  • Serial numbers of inspection equipment
  • Excavation location
  • GPS coordinates and elevation
  • Actual depth of cover
  • Level of water table (if present in excavation)
  • Weather conditions
  • Dimensional checks (circumference and ovality)

Comprehensive photographical records are immensely useful for later reference, and cataloguing all of the information in a visual format.

Physical Measurements

Alongside the relative information listed above there are a number of physical measurements that should be performed in order to obtain the most comprehensive record for the excavated location.

Coating Evaluation

Prior to the removal of the coating at the exposed bell hole the following data shall be recorded, where applicable:

  • condition of the existing coating (number of defects, mechanical damage, disbondment and cracks)
  • type of insulation
  • number of insulation layers
  • presence of adhesive primer
  • percentage of bare metal exposed
  • location and size of corroded sections and depth of metal loss
  • type of corrosion
  • evidence of sulphate reducing bacteria
  • coating pull-test on sample

In addition to written and photographic records, a trace of the pipe circumference may be taken using marker pens and transparent plastic sheet.

UT Wall Thickness

Ultrasonic wall thickness measurements provide an indication of corrosion and / or manufacturing defects.

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By drawing a grid pattern both along and around the pipeline, spot measurements are taken. Further measurements are taken at sites of external corrosion and either side of weld seams.

The percentage of wall loss allowable is determined through calculation.

Pit Depth

Pitting caused by corrosion on the pipeline, should be measured and quantified. Information that should be recorded is;

  • Maximum Depth
  • Longitudinal (axial) Length
  • Orientation (with respect to the pipeline)

This information can then be calculated using ASME B31:G, to obtain a maximum allowable operating pressure (MAOP) for the pipeline at that location.

This calculation is crucial in determining whether the pipeline is fit for purpose at the desired operational / testing pressures.

Soil Resistivity

Actual soil from the pipeline / soil interface can be retrieved from the bell hole and tested. This gives a 100% accurate measurement for the layer of soil that the pipeline is within at this location.

Instead of the Wenner 4-Pin Method, a slightly different technique is employed.

A soil box is constructed of known dimensions and pin spacings. By placing the soil sample within this box and testing, the result is returned directly in ohm.cm, without the need for any further mathematical formula.

Potential Measurements

A potential measurement at the pipeline / soil interface removes the error associated with the measurement taken at the surface, giving a truly representative indication of the cathodic protection status / natural potential.

WHAT TO DO NEXT

With all the data collated, analysed and presented, it is between the Contractor and the Client to decide what the next steps should be.

The following section outlines the main possibilities available, but ultimately the decision falls down to;

  • Overall costs
  • Logistics
  • Time constraints

Cathodic Protection

There are two types of cathodic protection that can be applied;

  • Sacrificial
  • Impressed Current

The selection of which type would be best suited to the pipeline is clear from the data collected in the previous phases. This information would not only determine which system to use, but would provide enough information to a design company to ensure a working system is installed.

Sacrificial protection is best used where corrosion / coating damage is localised to small distinct areas, and the resistivity of the soil is below 100 ohm.m.

Impressed current protection is preferred where the general condition of the coating is poor along the entire pipeline length and / or the current demand is higher than expected due to foreign contacts, stray current interference etc.

The two systems are not exclusive from one another, it is common for sacrificial anodes to augment an impressed current system when;

  • Design criteria cannot be achieved at both mid-point and drain point
  • A source of stray current interference is present

Coating Repair

For short sections of poorly coated pipeline, it may be desirable to excavate the length, and recoat it.

Corrosion will only occur where there is a holiday in the coating and therefore repairing the coating at these locations would inhibit the corrosion process from occurring further.

Recoating extensive lengths of pipeline may well be unfeasible due to budgetary constraints.

Mechanical Repairs

Should sections of the pipeline have been deemed to fall below the threshold levels for the operating / testing pressures, than mechanical repairs are possible in order to restore the integrity.

These repairs could entail a direct removal and replacement of a pipe section, or strengthening through reinforcement.

Whilst there are a number of reinforcement products presently on the market, when the length of the section under repair exceeds a certain limit, the repair / replace option can become more economically viable.

Nothing

Always a preferred option for the accountants. This has the benefit of no capital expenditure, and leaving a huge risk for someone else to deal with.

Certainly not recommended.

AC Corrosion Demystified

The following paper was presented at BHR Group’s 18th International Conference for Pipeline Protection – Antwerp, Belgium – 4th – 6th November 2009.

The paper was co-authored by Richard Lindley and Ken Lax, and presented by Ken at the conference.

DISCLAIMER: All information, references to Standards and / or criteria correct at time of paper.

 
AC_Mitigation_01 - Resized.jpg
 

ABSTRACT

AC corrosion is a pressing issue with an increasing number of reports of serious corrosion damage.  Paradoxically the problem exists only on modern well coated pipelines.  The paper will explain why modern coatings are “responsible” for the increase in ac corrosion damage.

AC corrosion can be difficult for non-specialists to understand.  This paper presents an understandable description of the phenomenon and how the effects can be mitigated.  It provides guidelines for pipeline operators and constructors to assist in determining whether or not there is a risk of ac corrosion damage to the pipeline.

There have been examples of unacceptable levels of corrosion damage caused by induced ac with voltages as low as 4v ac whilst cathodic protection is applied.

The paper will show, strange as it may seem, that over-zealous application of cathodic protection current can actually exacerbate ac corrosion.

No complex mathematical or chemical formulae are included in the paper.

BACKGROUND

In the last 10 years or so there has been a marked increase in the number of incidents of ac corrosion reported.  There is a good chance that the risks of ac corrosion will increase even further now that so called “energy corridors” have been established in crowded countries and that means the overhead and buried ac power lines run close to buried coated high pressure gas and oil lines.  In addition to the proximity issue it also likely that increased energy demands will result in higher ac voltages on the overhead lines, which will in turn result in higher induced voltages and hence increased corrosion risk.

External corrosion protection is usually provided to high pressure gas and oil pipelines by a combination of coatings and cathodic protection.  Although it is hard to realise from coating specifications the main function of coatings is to provide a high electrical resistance between the steel and the surrounding environment.  A high resistance will impede the flow of electrons and hence reduce the risk of corrosion. (This is explained in more detail in the Basic Theory section).  Cathodic protection is provided to prevent electron flow from the pipe at locations where the coating is damaged or does not provide a high enough resistance.  Theoretically bare pipelines can be protected using cathodic protection but the current demands will be high.  Current density at a coating defect is the main driver for corrosion, the higher the current density then the higher the corrosion rate.  So a small coating defect in an otherwise well coated pipeline will mean that all the corrosion will take place at one spot.  From an ac corrosion point of view bare pipelines are less prone to corrosion than coated pipelines!

Cathodic protection is a method of stopping external corrosion by forcing electrons (a flow of electrons is an electric current) onto the pipeline.  These electrons can be provided by galvanic (sacrificial) anodes such as zinc or magnesium or from impressed current anodes such as silicon iron.  Galvanic anodes do not require an external power source to release the electrons but impressed current anodes need an external power source (usually a transformer-rectifier).

At the coating defect there is an increase in the steel alkalinity caused by the cathodic protection reaction.  This can play an important part in the ac corrosion process.

SOME BASIC THEORY

If you have a voltage and a resistance then you can have a current.  For direct current this can be expressed as ohms law.

Current (I amps)  = Voltage (V volts) divided by Resistance (R ohms)

Often written as I = V/R amps

In simple terms the voltage is the power that drives the current through the resistance.

The voltage has to be high enough to overcome the resistance in order to drive the current through the resistance.

So, for a given voltage the lower the resistance the higher the current.

This law shows us that the current that flows depends on the voltage and the resistance.

AC is a bit more complicated because we talk about impedance (another word for resistance really) and impedance is a combination of capacitive reactance, inductive reactance and resistance.  Reactance is influenced by the capacitance and inductance of the circuit and the frequency, so it is not a simple as a pure d.c. resistance.

SINE WAVE.png

 

The sine wave drawing shows one complete cycle.  There are normally 50 of these cycles per second. (60 in the USA).

 

 

SINE WAVE DIODE.png

AC is converted to dc via a unidirectional device called a diode.  A diode can be considered as a one way valve.  When certain conditions exist either side of the diode the diode will have a low resistance and will allow voltage through. 

At all other times the diode exhibits a high resistance and does not allow anything through.

This means that an ac voltage will permit the diode to conduct each half cycle. This results in a pulse of direct current every half cycle.

This is what happens to an ac voltage on a pipeline at a coating defect.  The ac voltage is rectified and the resulting direct current causes corrosion as it leaves the structure and enters the soil.

The “diodes” in this case are created on the steel surface by the chemical reactions that are taking place.  The overall process is known as Faradaic Rectification.

Faradaic rectification may be defined as “A component of the current that is due to the rectifying properties of an electrode reaction and that appears if an indicator or working electrode is subjected to any periodically varying applied potential while the mean value of the applied potential is controlled.”  In other words it is what you can get at a coating defect that is subject to ac interference whilst a cathodic protection potential is also present.

The big question, however, is how does the ac voltage get onto the pipeline anyway?  To understand this we need to consider some basic electromagnetic principles.

  • Every electric current has a magnetic field associated with it.
  • The magnetic field strength is directly proportional to the magnitude of the electric current.
  • In the case of ac current this magnetic field will vary in exactly the same way as the ac current (i.e. 50 times per second in Europe and 60 times per second in the USA)
  • A metal conductor placed in a magnetic field will have a voltage induced in it that is directly proportional to the strength of the magnetic field and the speed at which the magnetic field changes.

When we consider voltage induction in buried pipelines and add the knowledge of the corrosion process we can conclude that if a voltage is induced in the pipeline and the pipeline has a resistance (which it always has) then there will be a current flow.  If this voltage is converted to direct current then there will be corrosion where the current leaves the pipe.  This will occur at the point of lowest resistance i.e.  a coating defect.  The smaller the defect the greater the corrosion current density.  The greater the corrosion current density the greater the metal loss.

Putting all these facts together we can see that high voltage and high current ac cables close to a pipeline with small coating defects has the possibility to cause rapid corrosion at the coating defects.

WHAT CAUSES CORROSION ANYWAY?

There are a wide range of corrosion mechanisms that result in loss of metal from buried pipelines.  One thing that they all have in common is that the consequence of the corrosion process is direct current leaves the pipe and that there is a relationship between the magnitude of the current and the metal loss.  1 amp dc for one year will result in 9.1 Kgs of carbon steel consumption.

HOW DO WE KNOW IF A PIPELINE IS AT RISK FROM AC CORROSION?

The scientific community have yet to agree on a single mechanism for ac corrosion.  That is probably because they are striving to find one theory whereas there may be several different mechanisms.

External corrosion on buried pipelines is caused by a current exchange between the soil and the pipe.  No current exchange, then no corrosion. 

Because there is no universal acceptance of the mechanism there is also no consensus on the criteria for ac corrosion. 

It would be nice if we could just measure the induced ac voltage and use that to determine whether or not there is a corrosion risk.  Unfortunately the voltage by itself is not usually a good indicator of the risk.  Knowledge of the soil resistivity close to the coating defect, the pH, the defect size, ac current density and dc current density are also required.  These cannot all be measured in the field.

If the ac powerline is more than 110kV and the pipeline is within 150m of the pipeline, and the pipeline is more than 2Km long then the risks of harmful levels of induced ac voltage increase.  Areas of particular concern are where there is a change of direction of either the cable or the pipeline route.

As a guideline the following criteria have been used:

  • If the ratio between ac and dc current densities is greater than 0.5 then the risk is low
  • If the ratio between ac and dc current densities is less than 0.5 the risk is greater
  • The risk increases if the ac current density is greater than 30A/m²
  • The risk increases if the pipe-to-soil off potential is more positive than -0.850
  • The risk increases if the pipe-to-soil off potential is more negative than -0.950

The measurements cannot be made directly on the pipe and they are, therefore, made on a corrosion coupon.

Practical field measurements have shown that by increasing the cathodic protection current density the pH at the surface of the defect changes, and actually accelerates the ac corrosion.  So just increasing the cathodic protection current is not always beneficial.

The recommendation of the authors is that a special ER corrosion probe is used to quantify the actual corrosion rates.  These probes are placed in areas of risk, and control areas where no risk is perceived, and provide an absolute measure of the corrosion rate regardless of current densities, pH levels etc.

The corrosion probes work by monitoring the extent of corrosion on a known surface area of metal exposed to the soil and connected to the pipe.  Using the relationship between resistance and metal surface area they can accurately calculate the metal loss based on the measured resistance.  The better quality systems will also measure the on and off potentials and ac and dc current densities to provide additional information.  The probes can also be used to validate the effectiveness of any remedial measures.

Another method is to use a mathematical model to predict the ac induction and hence the risks.  Not surprisingly this method is popular with the companies that offer the service!  Many operators find it comforting to be able to produce calculations and maps to show the risks.  It is always worthwhile to consider the true costs and benefits of these models versus the practical application of remedial measures.

CP < 0.85 VDC              CP > 0.95 VDC

HIGH DC INTERFERENCE            HIGH RISK                      LOW RISK

HIGH AC INTERFERENCE            LOW RISK                       HIGH RISK

METRICORR CHART.png

Graph courtesy of MetriCorr

The graph shows that as the cathodic protection is increasingly negative the ac corrosion rate increases. 

When the potential is set to -0.85v the ac corrosion rate reduces.

WHAT CAN WE DO IF WE HAVE AC INTERFERENCE?

Simple.  Make the pipeline earthy, so that the induced ac will return to earth via a dedicated earthing system.

Not quite so simple, however, since we spend a lot of time and money to provide the pipeline with a high integrity coating to isolate it from the earth!

Practically speaking we need to provide a low resistance (impedance) path for the ac that is a high resistance path to the cathodic protection current, which is dc.   This is best achieved with a polarisation cell that has special characteristics that provide a low resistance to earth for the ac but blocks the dc.

The earth can be provided by zinc ribbon, copper earthing, or any other low resistance earth (e.g. silicon iron groundbed).

CONCLUSIONS

  1. Well coated pipelines near to high voltage cables (>110kV) are at greatest risk.
  2. Typically the high risk areas are where the pipeline or the ac cable changes direction.
  3. Just measuring the ac voltage is not an indication as to whether or not there is a risk of ac corrosion
  4. Increasing the cathodic protection current may increase the ac corrosion.
  5. Coupons are required to determine key characteristics such as ac and dc current density.
  6. Practically speaking electrical resistance probes are the best way to monitor the effectiveness of ac mitigation.
  7. If the pipeline is going to run near ac high voltage cables then provision should be made in the cathodic protection design for ac mitigation.
  8. The location of the mitigation can be determined pragmatically or from mathematical models.