Delineation Of Pipeline Coating Defects Company Engineering Essay

Delineation Of Pipeline Coating Defects federation Engineering Es verbalizeThe business atomic number 16tor of wearing away monitoring and authorisation solutions we provide expertise in consultation, evaluation, externalise and installation of cathodic tri hardlye systems. With prep atomic number 18ed field perspicacity techniques and specialized equipment more than(prenominal) as CIPS/DCVG vision equipment. Engineers collect record and analyze field data efficiently so that clients are able to run safe and highly optimized systems. Corrosion solutions are provided in a cost effective and with highly professionalism to enable clients to get up their facilities integrity man development systems. introductionThe external critiques apply DCVG-direct circulating(prenominal) po tential incline technique was performed on this business on March tenth 2011 to March 24th 2011.cathodic security and practical application are complementary to individually new(prenominal ) and when applied together, result in reliable erosion mitigation. Cathodic design presupposes certain masking breakdown criterion for the given showcase of last and given environment. If diligence break down exceeds beyond the maximal percentage limit presupposed by the CP system design, cathodic auspices whitethorn become ineffective.To ensure the effective cathodic rampart and effective corrosion control, it is desired to access the thoughtfulness of the masking of the on a lower floor res publica yellline. Exposing the organ shout outline by excavation all along the length, for this purpose is impractical. Various rules have been evolved for the assessment of application condition without excavation. One of the methods is command on-line(prenominal) latent drop gradient mountain.SafetyThe locate activities were performed as per the applicable safety procedures. Work put up was obtained prior to commencing of site activities. The HSE involvements were ex plained by PCML engineer to the site team. All force play safety gears such as safety boots, c everywhereall. Hard hat, googles etc were employ during the site work.Codes and standardsThe measured probables and performance of the cathodic protection system, equipment and mattials shall comply with the requirements PTS and NACE standards code and former(a) authority having jurisdiction over the system.NACE RP-O502-2002 Pipeline external corrosion Direct Assessment Methodology.NACE RP-0169-96 control of External corrosion of under scope or sink metallicPiping systems.NACE RP-05-75 Design, installation, operation and maintenance of impressed current thickset ground bed.The scope of work performed in this project is in accordance to industry standards has been maintained to ensure global codes of practice in corrosion control. The criteria apply for protective potentials of buried sword are indicated in NACE international national association of corrosion engineers standards.Pip eline detailsThe origin has the chase physical characteristics and these details shall be applied to design the cp system.GSPC on shore gas lineageLength 67kmCoating tierce layer polyethyleneService gasSource station poc (chain age 10 km)Destination station molar concentrationing station (chain age 67 km)Introduction to DCVG conclusion survey equipmentThe direct current electric potential gradient (DCVG) pipageline lotion survey scrutiny equipment is version 9, and is the most technologically go version of the equipment that arouse be traced directly back to the victor dodge of the technique in Australia by john mulv whatever.With the equipment described in this document, by experience in its use and interpretation, it is possible to gather with a sightly degree of confidence the following information about the pipeline organism inspected.Coating spot epicentre kettle of fish to within a 15cm circle, which essence that excavation costs toilette be reduced. The approximate sourness of the surface smirch can be established so that practical application switchs can be prioritized for repair.The approximate corrosion behavior of individual application smirchs can be established to ease identification of those application program imperfections that do not have sufficient cathodic protection.DCVG technique does not however line up metal loss but identifies sites where metal loss is possible.Identification of where screening recesss gets its cathodic protection from, cp that the vulnerability of screening teddy to be open if a CP outset becomes inoperable can be established.Identification of coating faults that are discharging or picking up DC Traction stoppage so that more effective mitigation technique can be implemented.Establish the effectiveness of insulating flanges.Identification of interfering structures that robs CP from the pipeline.Identification of defective prove analyses at which pipe to district potentials are routinely monitored.Rapidly establish atoms of pipeline that have a big number of coating faults by studying the rate of decay of the DC potency gradient storeyize on the pipeline.The data ga at that placed by DCVG technique is not implicit but relative and is influenced by a series of para grands such as reason resistivity, depth of burial etc whose effects must be taken into account to improve the accuracy of any data.Typical applications of DC voltage Gradient TechnologyShown below are some typical applications of DC voltage gradient engineering science to evaluate the protective coatings and cathodic protection on buried pipelines. It has to be remembered that the protective coating on a buried pipeline is premier corrosion protection mechanism but all coatings have coating faults in them. To control corrosion of steel exposed at coating faults, cathodic protection is utilise. Cathodic protection is supportive technique. The relationship a middlest cathodic protection and protective coatings is fundamental and since DCVG studies this relationship and provides valuable information to control corrosion.Typical Applications are appreciate pipeline coatings to define rehabilitation requirements.Define weakness in the cathodic protection system.Validate that the pipeline has been constructed with minimum coating faults.Investigate folie effects.Establish effectiveness of insulating flanges and early(a) methods of pipeline isolation.Provides data for operating permission validation. eyeshoting complex pipeline nedeucerks not possible by other methods.Surveying under concrete and asphalt in city streets. fitting of surveying under over head power lines.Electrical tenaciousness memorizeing of mechanically jointed pipelines.Principle of the DCVG TechniqueWhen DC is applied to a pipeline in the same way as cathodic protection (CP) the current flow through the soil to steel exposed at coating faults generates a voltage gradient in the res istive soil.The enceinter the current flowing the greater the soil resistivity and the closer to the coating fault localisation of function all give rise to big voltage gradient.In general larger the defect, bigger the current flow and so the voltage gradient, which is utilise to size coating faults so they can be prioritized for repair.In the DC voltage gradient technique the DC omen impressed on to the pipeline is im momentumd at a frequency of 1.25 hertz.The DC preindicationing can be impressed on top of the existent CP system of the pipeline or the CP system itself can be employ by inserting a special fracture or interrupter into star of the takings cables from the nearest transformer rectifier.Only one transformer rectifier nearest to the survey area call for to be interrupted at any one time, thus the limitations of the other over line surveys where all DC influences have to be set uped at precisely the same time does not apply for coating fault location.For m ore precise and intensive studies it is advisable to interrupt synchronously a number of rectifiers that are affecting the area being surveyed.For fault location the pulsing DC signal can even be impose at a see behave using batteries or a portable DC generator and temporary ground bed.Unique bluster of DC voltage gradient technique is that the pulsed signal is irregular in cultivate i.e. switched ON for 0.45 sec of a cycle and OFF for 0.8 sec of a cycle.The irregular pulse allows the management of current flow to be unflinching and compared to all other DC influences at an individual coating fault, enabling the degree of protection against corrosion at individual faults to be inflexible at the time of survey.To monitor the voltage gradient in the soil the technique utilizes measuring on a sensitive and curiously constructed milli volttime, the difference in voltage among both tossper/ bull sulphate half cells placed in the soil at ground level.When spaced one bill a p art in a voltage gradient one half cell allow marry a more positive potential than the other which enables the agency of current flow which ca utilize the voltage gradient to be established.In surveying a pipeline the operator walks over the pipeline route raveling for pulsating voltage gradient at regular intervals.As coating fault is approached the surveyor lead observe the milli volt thousand chivy begin to respond to the pulse, pointing in the commissioning of current flow which should always be towards the coating fault on the pipeline.When the coating fault is passed the needle direction completely reverses and easily decreases in bountifulness as the surveyor prods away from the defect.By retracing to the coating fault a arrangement of the electrodes can be found where the needle shows no excursion in either direction (a null).The coating fault is then sited middle(prenominal) between the two electrodes this procedure is then repeated at advanced angles to the start set of observations, and where the two midway daubs transversal is the location of the voltage gradient epi halfway.The coating fault epicentre location is then pegged. In place to determine various characteristics about a defect, such as insensibility shape, corrosion behavior etc. Various electrical measurements around the epicenter and from epicenter to out-of-door terra firma are made for detailed interpretation.Survey Switch (Interrupter)The survey switch utilizes a solid state winding to switch the applied DC at one of two speeds goaded by the jell of the STD/SLOW switch. The STD/SLOW switch has two smirchs which cook upSTANDARD (STD) climb 0.45seconds ON followed by 0.9 seconds OFFSLOW telescope 0.9 seconds ON followed by 1.8 seconds OFFThe STANDARD displace of the switch is used for normal surveying to find coating faults. This speed of switching matches the typical response time of a survey operative.The SLOW switch short letter is used in conjunction with a digital voltmeter for pipe to soil potential measurements or current measurements via an inline calibrated shunt.The interrupter is committed in series into either the negative or positive cables from the DC source being interrupted. The negative cable is preferred. This is setup so that the cable coming from the transformer/rectifier is connected to the BLACK perch on the interrupter and the cable from the pipe is connected to the blushing(a) terminal on the interrupter.Danger under no circumstances should the terminals of the interrupter be directly connected a patsy the terminals of the DC power source/ transformer rectifier as this will short out the power source and do flagitious damage to the interrupter and the DC power source. Also do not under any circumstance connect the DC interrupter terminals to an AC source.Survey meterSurvey MeterThe dominant visible feature of the survey meter is the analogue meter motionment. The meter has a center nobody needle sentime nt. This means that with voltage across the meter infix, the needle rests at mid eggshell irrespective of the range switch position.The survey meter has the following voltage ranges 10mv, 25mv, 50mv, 100mv, 250mv, 1v, 2.5v, 4v. the voltage range of the meter can be selected using the voltage range switch sited on righteousness feed locating of the meter front panel. The range switches correspond to various ranges or multiples of the ranges on the analogue meter scale.The 10mv on the voltage range corresponds to the zero to ten milli-volts full scale aside on the analogue meter (plus or minus 5mv about the center rest position of the meter needle).The 25mv on the voltage range corresponds to the zero to ten milli-volts full scale deflection on the analogue meter (plus or minus 12.5mv about the center rest position of the meter needle).The 50mv on the voltage range corresponds to the zero to ten milli-volts full scale deflection on the analogue meter (plus or minus 25mv about th e center rest position of the meter needle).When not in use the range switch should be dour to the 4-volt range to minimize any chance of meter damage.Probes and HandlesThe standard outpourings used with the DC voltage gradient equipment are especially adapted about one meter long crap/copper sulphate fiber electrodes. The poke intos are lightweight, high strength tubes fixed at one force out to an insulated stainless steel stud that provides both electrical and mechanical friendship to the prove handle.The other end of the probe electrode contains a conductive woody plug to make electrochemical contact between the soil and the copper sulphate solution/copper electrode. The wooden plug is a force fit into its plastic holder with PTTE tape used a automatic washer. The plastic holder screws onto the probe using a flat rubber washer as the seal.Only one probe handle is switched on and used at any one time during the survey. The other is used as a spare. Plain handles that have no bias are ready(prenominal) since alone one bias handle is used at any one time for surveying.The probe handle has a built in bias that is controlled via an ON/OFF/Range switch and a prepossess adjustment potentiometer.Preparing Equipment For SurveyBattery chargingGenerally the DCVG meter and the interrupter will require charging more frequently than the handles. Each equipment will require separate pass offs charging for two days if the batteries are entirely flat.When operating several sets in range to ensure all components parts of the equipment sets are adequate to(predicate)ly maintained it is advisable to number each component and set up a charging annals to keep a tally of what equipment has been charged when and for how long.ProbesA wooden probe tip should first be wrapped around its cylindrical portion with white PTFE tape applying sufficient to ensure the wooden tip is a firm push fit into the probe tip holder. All the threesome holders should and tips sho uld be soaked in portable water overnight. Water pulmonary tuberculosis causes the wood to expand and give a liquid tight seal.The copper/copper sulphate probes are filled with copper sulphate solution. The probe is filled almost full of copper sulphate solution through the probe tip holder end. The presoaked tip and holder plus washer are screwed onto the probe to make a liquid tight seal, the probe is inverted to correct position and the handle is screwed.SurveyingSetting up DCVG signalThe most important parameter in ensuring an accurate survey and in determining the survey speed is the amplitude of DCVG pulsed signal.It is worth to evanesce time during setting up the DCVG signal is atleast 150mv and no larger than 1500mv.As the signal amplitude or strength vary along a pipeline, the signal strength at start(drain point) should be 1500mv and that at the other should be at least 150mv.A rapid decay of signal as described as above measured at two locations say would be an indica tion of poor coating on the pipeline.The presence of legion(predicate) an(prenominal) coating faults or some large drain on the CO system can be expected. Whereas good coating would show real little attenuation of signal amplitude.The signal strength or amplitude is the difference between ON and OFF potentials measured on the pipe to foreign universe, whilst the interrupter is switching ON and OFF the applied DC source.The amplitude is measured on the DCVG meter as the pulse size, the milli volts size of pulse is determined by measuring the difference in extremities of the pulsing meter needle using the bias and range switches to bring the full pulse onto the meter dial.The pulse amplitude at streamlet posts measured to away earth in not the same as the difference between ON and OFF pipe to soil potentials measured only at the assay post.To get full value and meaning from DCVG measurements, the ideal source is CP system itself set at the same level of make as normal operati on. Some adjustment to the TR unit output might be required if signal levels are inadequate.RectifierIf there is no CP system installed then a temporary CP system must be setup. Ideally level best of 50 amperes should be installed.A temporary ground bed whitethorn be steel poles inserted into the soil, or any steel structure such as a fencing post, overhead power line hide systems, scrap, steel pipe, etc. caution need to be exercised in revisal not to burn out the interrupter.The interrupter should be connected into the electrical lap covering as shown in the fig, utilizing short wire of optimum 10mm in cross section. The black terminal of the interrupter should be connected to the cable going to the pipeline. The signboard connection in important, if connected around the wrong way the interrupter will not switch the DC output if this happens just reverse the terminal connections on the interrupter.The interrupter should be inserted with the transformer rectifier set in its lo west output setting and the transformer rectifier mains electricity switch in the OFF position.For TR with a known output that is less than 25 amperes, after the interrupter has been inserted and the interrupter switch set to the ON position and interrupting speed switch to standard, the TR should be switched on and the output slowly change magnitude to give normal output or higher(prenominal) to give an adequate DCVG signal.Poor temporary anode setup is the usual cause of inadequate signal.With a temporary setup where the DC source are batteries, a weld set or rectifier with no ammeter it is important that the following procedure is followed in order not to damage the interrupter by passing too much current.Adjustment to ensure good signal require trail and flaw and patience but extra time spent in setting up the signal will give greater confidence in the quality of the survey, which is usually achieved at a greater speed than on pipelines with a poor signal.Measurement of the si gnal level at test posts are carried out in exactly the same way as ,measurements made to measure pipe to soil potentials, except there are two measurements in this case From the copper wire or test post terminal to the soil alongside the test post.From the soil position alongside the test post to opposed earth.Assembling the DCVG equipmentThr reference probes antecedently filled with cu/cuso4 solution and fitted with tips are screwed onto the probe handles.The meter strap is placed around the neck and waist so that the meter fits snugly on the operator.The connecting leads are fitted into the meter and into the probes to interconnect the two probes to the meter. The meter serve considerably switch is then turned ON and the range switch adjusted from 4volts to 1000mvolts.With the probe tips placed in the soil the bias to the right hand probe is switched ON. The bias to the unexpended hand probe is not switched on, it is a spare accessible if needed, also to increase the amount of bias available should that from one handle not be sufficient because of large priming coat DC in the soil.Move to the test point at which the signal is to be measured. With right hand probe make contact with the soil and with the left hand probe or with the plug end of the left hand cable, make connection to the test point wire. Adjust the right hand bias control knob and meter range switch until the full extent of the meter needle deflection is visible on the meter scale.Adjust the meter range until the deflection cane be read accurately . if for example the meter is on the 1000mv range and the mater needle deflection is from 225mv in the OFF position to 850 mv in the ON position , the signal on the pipeline at the test post is 850-225=625milli volts.Having measured the pipe to soil signal strength there is another measurement that to remote earth which must be added to that from pipe to soil to give the full signal strength at the test post.In measurement to remote earth the probes are used want a set of dividers by starting at the soil position at the test post and moving away at right angles, summing the voltages observed for each position of the half cells. Remote earth is reached when two or more readings small in size are the same.The signal strength should be noted at every test post and all other potential monitoring points along the pipeline route. Measurements must be taken at either end of a section underOverline To Remote Earth potential difference Measurement.Survey as well as the outmatch asunder, as these readings are required for calculating pipe to remote earth potential.Similar measurements to that described above are taken from the coating fault epicenter at ground level to remote earth at every coating fault and are used in calculating the coating fault gracelessness.Operating instructions determination a defectAdjust the meter a range switch to the 100mv range, and ensure that only one handle bias switch is ON adjusted to positi on 3. This is all that is necessary for normal surveying.Place the probes one in front of the other. Contact the soil with the probes approximately at 1.5 to 2meter spacing. do work the bias control potentiometer to bring the needle of the meter onto the scale. make the needle on the meter scale the whole time the probes are in contact with the soil. timbre for the meter needle to be flicking in response to the pulsed DC.Lift the probes srep out from the test point at which the signal strength was previously measured. Move in the lead 2 paces and contact the ground with the probes. Use the bias if necessary to bring the meter needle onto the scale. Look dor a needle deflection. If there is no deflection then rate out another 2 paces and then bring the needle onto the scale with the bias control.If there is a deflection observes the needle to see which direction the coating fault lies. If you are unsure either change to a lower meter scale or move the probe forward along the pip eline. The meter needle points to the probe, which is nearest to the defect.The interrupter if OFF for longer than for what it is ON and when it is ON the current normally flows through the ground towards the defect. It is the size and direction of the needle flick or swing that you are interested in. it may be possible that the coating fault is small and lies behind you so correct identification of direction of the needle swing.If you observe a deflection lift the probe which is closest to the coating fault and move it 0.5meter towards the defect. conduct the second probe forward and place it where the first probe used to be keep moving forward in this manner.As you move towards the defect the amplitude of deflection will increase so there may be a need to change to a higher range required.When the coating fault is passed the needle deflection completely reverses and slowly decreases as you move away from the defect. Retrace the steps to the suspected coating fault position where the change in meter needle direction occurs. At the approximate null position with the probes at about 1,5 meters apart observe any meter deflection. If the deflection is from left to right move the left probe 15cm to the right hand probe. At the point of no deflection, the coating fault location lies midway between the two probe locations. Scratch a mark on the ground at the midway position.Turn through 90 degrees to work across the pipeline direction. jib facing the mark in the ground and repeat the coating fault location process described above. At the new null position mark the midway position between the probes on the ground to cross the first mark. Recheck the first mark by turning back to the original position and checking for the null. Where the two lines cross is above the centre of the coating fault voltage gradient and is called the coating epicenter. As a final check that the location is correct, place one probe at the epicenter and the other about 1.5 meters away place d in turn at the four points of the compass. At each of the four locations the meter needle should indicate a direction towards the coating fault epicenter. If this is not the case then the epicenter has been incorrectly located or the coating fault location is at one end of a long crack in the pipe coating.Determining the Coating Fault SeverityCoating fault bad which is related to its geometric size although there are other influencing factors is determined from electrical measurements taken at the coating fault epicenter.The size/ splendour or severity of a coating fault titled %IR is calculated by expressing the over line to remote earth potential as a percentage of the actual pipe to remote earth potential (the signal amplitude) on the pipeline at the defectin one case all information about a coating fault has been logged continue surveying along the pipeline route.A special but vulgar type of voltage gradient encountered during the surveying has a long sausage balloon shape generated by longitudinal crown cracking in coal tar, ruffling in tapes, and micro porosity in asphalt coatings or where many small coating faults occur in close proximity. Whilst this type of coating fault is often missed during CIPS or Pearson surveys, their presence can promptly be recognized by DC voltage gradient technology because such coating fault have strong lateral voltage gradients.Coating fault size shape and location on the pipelineA good indication of a coating fault size, shape and location around the circumference of a pipeline can be gained by plotting of the equipotential lines of the voltage gradient at a coating fault in the soil surface. Start by plotting at a plotting at a point equivalent to 30% of the over line to remote earth potential. Track the equipotential line by the nulling method around the coating fault epicenter all way back to the start point placing markers on the way. The line will indicate the size and shape of the coating fault. The distance from the epicenter to the pipe centre line as determined by a pipe locator will determine whether a coating fault is on the bottom, side or top of the pipeline but this is an awkward way of determining this.A small discrete coating fault on the top of the pipe will advance as a circular is potential shape. The same sized coating fault on the bottom of the pipe will appear as an ellipse, distorted to one side of the pipe center line.Because the effect the pipe itself has in distorting the isopotential lines from the pipe centre line, it is easier to determine the location of a coating fault around the circumference of a pipeline on large diameter pipelines than on the smaller diameter pipelines.Some examples of isopotential plots of coating faults of diverse shape on a pipeline are shown in interpretAn alternative way of determine the orientation of a coating fault is to carry out the four points of the compass readings at each location keeping the probe spacing the same for all four measurements. If the coating fault is on the top of the pipe all four readings will be of similar amplitude. If the two readings to the side are much larger than those taken down the length of the pipeline then the coating fault is on the bottom segment of the pipeline. If one side reading is larger than the other then the coating fault is on that side of the pipeline. conniving the severity of coating faultsThe relative severity of a coating fault is expressed by the term %IR, which is calculated using the following codeFault epicenter to remote earth * 100Coating fault severity (%IR) = -Calculated pipe to remote earthOLRE*100In short version, %IR = -P/RECalculation of the pipe to remote earth potential is an important figure needed to calculate the severity importance (%IR) value for a defect. To be able to calculate the severity of defects it is necessary to know the distance of defects and the DCVG signal strengths at test posts either side of the sector being surveyed.The pipe to remote earth potential (P/RE) is calculated as followsP/RE = S1- dx(S1-S2)D2-D1S1= signal at upstream test post in = 800mvS2= signal at downstream test post in = 300mvD1= distance of upstream test post =0mD2= distance of downstream test post=1000mdx= distance between upstream test post and defect = 400mthe severity (% IR ) is calculated asover line to remote earth from the figure is 130milli voltspipe to remote earth calculated above in 12.0 from figure given then % IR130 * 100%IR = =21.7600Deciding Which Coating Fault To drudge And RepairThe coating fault grading is0-15%IR characterized as a small coating faults. Such coating faults can usually be left unrepaired provided the CP system of the pipeline is in good condition and there are not too many small coating faults in close proximity.15-35%IR characterized as medium coating faults. These may need repair usually within normal maintenance activities35-70%IR characterized as medium large coating faults. These faults nee d to be excavated for inspection and repair in order to fix what could be considered a substantial coating fault.70-100%IR characterized as large coating faults. These coating faults should be excavated early for inspection and repair.The characterizations of coating faults given above are only one input but a very important input to the excavation and repair decision. Other important factors are shape and method of coating failure, corrosion behavior, soil PH and resistivity, presence of hydrogen sulphide in the soil, operating temperature, age. Coating type, leak and metal loss floor etc.DCVG data for on shore pipelineSNOChainage(Km)ON (-mv)OFF(-mv)PotentialSwing(-mv)OLRE(mv)SignalStrength(mv)%IRRemarks10+0001650109155020+30020587.503.404