Indirect Assesment

DIRECT CURRENT VOLTAGE GRADIENT (DCVG)

indirect1 Direct Current Voltage Gradient (DCVG) Survey is a surveytechnique to evaluate the coating condition on buried pipelines. Theprinciple of this technique involves detecting a voltage difference onthe earth surface just above the buried pipeline by using two or more portable reference electrodes separated by a short distance. In aDCVG Survey, a DC Signal is typically created by interrupting thepipeline’s Cathodic Protection current, and the voltage gradient in thesoil above the pipeline is measured. Voltage gradients arise as a resultof current pickup/discharge at coating defect locations.

Above Ground Survey
Coating Defect Location
Foreign Contact Location
Coating Defect Severity

CLOSE INTERVAL POTENTIAL SURVEY (CIPS)
Close Interval Potential Survey (CIPS) is a survey technique that provides adetailed continuous pipe to soil potential profile. In normal operation, Pipe toSoil Potentials measured only at test points. When measuring a potential atone location (test point), the amount of pipe sampled in the measurement isconsidered the length of the pipe encompassed by a 120° arc entered on thereference electrode. To determine the potential over the entire pipelinesurface require that the reference electrode be moved along the centerline ofthe pipe route and place at regular intervals (@ every 2m). The purpose of aClose interval Potential Survey (CIPS) is to evaluate the criteria of the cathodicprotection, to identify local deficiencies in a cathodic protection system, toassist in identifying the causes of such deficiencies, and to aid in determiningthe equirements to enhance the level of cathodic protection to the structure.

Assess adequacy of Cathodic Protection System
Stray Current identification
Above ground survey
Able to detect interference from foreign structure
Pipe-to-soil potential readings between test station does not accurately reflect the pipe potential between test station.
CIPS takes potential readings at interval of 2 meters.
There are measurement error when taking potential with current flowing, known as IR error.
To overcome IR error, TRs are interrupted using GPS Current Interrupter, true pipeline potential are taken when current stops flowing.

PIPELINE CURRENT MAPPING (PCM)
Pipeline Current Mapping (PCM) is relatively new technology in locating faults and poor coating that isdraining CP Currents, resulting in pipelines not adequately protected by its CP system.

The PCM advantage over traditional DCVG Survey is mainly on its ability to conduct survey for pipelinesburied under Concrete and Road Surface. It detects accurately the location of faults when conducted by highlytrained operators. The PCM machine also comes with A-Frame which can be used to perform CIPS Survey.

Detect exact location of buried pipelines
Detection of Coating Defect location
Most acceptable method for pipelines buried under tarmac and concrete
Above ground survey

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SOIL RESISTIVITY

Soil resistivity influences the degree of corrosion of buried metals and also partly determines the design configurations of cathodic corrosion protection system. Soil resistivity measurement is normally conducted using Wenner 4-pin

Method. Developed by American National Bureau of Standards, this specify insertion of four pins into the ground at even spacing and in a straight line. The two outer pins are current electrodes and the two inner pins are
potential electrodes, which measure the potential drop due to the resistance of the soil path when an alternating current is passed between the outer pins.

The resistance can be measured and the resistivity calculated according to the formula. It should benoted that
measurements made in this manner indicate average resistivity over a depth of soil corresponding to the spacing between adjacent pins.

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Measurement using Wenner 4 Pin Method
Soil resistance is measured and resistivity is calculated
Soil resistivity is an indication of how corrosive the local soil conditions above ground survey

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SURVEY FOR NEARSHORE

METHOD STATEMENT
Inspection utilizing two reference electrodes has been widely accepted by the oil & gas
industries all over the world to inspect their vast network of subsea pipelines. It has several
advantages over other methods:

Low Cost
Does not involved expensive Remote Controlled underwater vehicle
Does not required divers
No trailing wire that may caused problems to other vessels

During the survey, the boat position will be aligned with a pre-set DGPS coordinates of the subsea pipelines. Through this method, the subsea pipeline can be traced down to length with higher accuracy. With the DGPS system in place, there is no requirement to use marking buoy along the subsea pipelines route.

As mentioned, divers are not required for this inspection method. To bring the reference electrode as close as possible to the pipeline, the reference electrode will be weighted using concrete.

The reference electrode, Silver-Silver Chloride will be housed in a heavy concrete casing weighted approximately 60kg. This heavy concrete casing is meant to minimize drifting during survey. It will also bring the reference electrode as close as possible to the pipeline.

Prior to the start of the survey, an array of coordinate points spaced at 200 meters interval from one chainage to another will be plotted on the DGPS screen. These pre-set coordinates will be used as guidance for the boat driver
to navigate in a zigzag format to cross perpendicularly with the pipelines. Data collected through this method will be more comprehensive and accurate.

At every crossing point with the subsea pipeline, the weighted reference electrode (RE-2) will be released to the bottom of the sea, at the same time, another set of reference electrode (RE-1) will be lowered to the seawater but its depth is approximately 1 meter only.

survey At this point, potential difference between the two electrodes will start to drift. Once the measurement is stable, the reading will be recorded using datalogger, if not manually. Subsequently, the weighted reference electrode will be lifted up approximately 2 meters using the boat winch.

OFFSHORE PIPE LINE SURVEY

PRINCIPLE THEORY OF SURVEY

Local stresses (either from physical defect or non physical) will cause changes in magnetic field strength.
Measurement of magnetic field strength to evaluate the local stresses degree of danger.
Assessment of Stress Deformed State base on Magnetoelastic effect (Villary effect)
Villary Effect – The Higher the mechanical stress, The Higher the magnetic field intensity.

MAGNETIC PROPERTIES OF FERROUSMAGNETIC PIPE – With stresses

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REMOTE OPERATED VEHICLE (ROV)

FIXING OF EQUIPMENT SKIF TO ROV

SERVICE VESSEL WITH ROV

LAUNCHING OF ROV

INSPECTION ON PROGRESS

AC/DC STRAY CURRENT & MITIGATION

Stray currents in a rail transit environment may consist of direct current componentsassociated with the operation of dc electric traction systems and alternating current componentsassociated with the operation of ac electric traction systems and electric utility systems. The impact onthe infrastructure will be different for the unidirectional flow of iron ions from a structural steelelement due to dc stray currents than for the flow of iron ions from a structural steel element on onehalf-cycle and the reversal of current into the structural steel element on the other half-cycle due to acstray currents. The consideration of this issue will yield functional and economic benefits in designingstray current mitigation measures that are commensurate with the electrical environment.

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