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Copyright © 2001 Scientific Surveys Ltd. All rights reserved. 1

ScoutScan: first experience in a Ruhrgasline with a new pipeline-mapping toolby P Loef

1, U Schneider

2, and R Vetterer

2

1Ruhrgas AG, essen, Germany

2Pipetronix GmbH, Stutensee, Germany

Contents of this Paper:

• Introduction

• Description of the ScoutScan tool train

• Data interpretation

• ScoutScan survey

• Figure 1: Correction of errors by reference points

• Figure 2: TBMS box at the reference point

• Figure 3: 20" ScoutScan

• Figure 4: Interpretation proceedings

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Pipeline Pigging and Integrity Monitoring Conference: Stavanger, 1999

2 Copyright© 2001 Scientific Surveys Ltd. All rights reserved.

Inspections of on- and offshore pipelines using intelligent in-line inspection tools (intelligent pigs) are a more andmore common method used by pipeline operators to monitor their systems and to assist in the decision-makingprocess for the maintenance of the lines. In addition to the corrosion detection and geometry measuring tools, newmapping tools were developed, thoroughly tested and approved. These tools produce a continuous line of co-ordinates and information on the trajectory of a pipeline. So, besides the general information on the course of thepipeline, there is information on deviations caused by movements and the stress they provoke on the line. Thispaper describes the technique of the tool, the conditions for the survey, the data evaluation, the informationprovided by the tool and the experience in a Ruhrgas pipeline. It includes a comparison with the performance of atraditional (geodetical) survey.

Introduction

The world-wide oil, gas and product pipeline network spans several million kilometres and is still growing everyyear. For the maintenance and supervision of this major investment, different monitoring systems were developed.A nowadays very common proceeding is the use of intelligent in-line inspection tools (intelligent pigs) for thedetection of geometrical defects like dents, ovalities and other ID reductions, for metal loss or wall thicknessmeasurement, for crack detection and leak location.

One further requirement of pipeline operators was and is to know the exact position of their underground orsubsea pipeline, especially if the line is in an area where it can move, like in mining areas, Perma frost areas, rivercrossings and offshore. In these areas it is advantageous to have the digital data of the trajectory (mapping andprofile), and even essential to know the deviation in order to calculate the stress on the line. For that reason,Pipetronix started the development of the first ScoutScan (gyrotool) in 1991, supported by the company Dr.Veenker in Hanover.

Since then, the tool was continuously used in different pipelines mainly in Germany, the Netherlands, the RussianFederation, the U.S. and Canada, and steadily improved.

Several inspections over the last 2 years have given the opportunity to test the accuracy of the by now thirdgeneration of the tool with the help of traditionally surveyed reference points in the line, like in RRP lines in theNetherlands, the GRI (Gas Research Institute) testloop and in a newly constructed Ruhrgas line in Germany.

Description of the ScoutScan tool train

General information on the ScoutScan system

The conventional way to determine the course of a buried pipeline by digging and subsequent measuring providesinformation on isolated spots only and, what is more, at considerable costs.

The Pipetronix ScoutScan system is an intelligent tool for the determination of geodetic data on pipelines in on-line operation. Changes in the direction the pipeline is running are continuously recorded by inertial navigation at

intervals of a few millimetres.

Rarely available on old pipelines, precise information on a pipeline’s course is required for the keeping of maintenance plans.

Precise information on changes in the direction the pipeline is running permits to calculate the mechanical loadcaused by deformation of the line (tension analysis). The operational parameters ‘pressure’ and ‘pumping rate’can be optimized by pipeline inspections and the operational safety can be improved at the same time, particularlyso in subsidence areas.

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ScoutScan: first experience in a Ruhrgas line with a new pipeline-mapping tool


In addition to the above mentioned, the bend angles are precisely determined. This information can be of utmostimportance for the use of other intelligent tools for pipeline inspection.

The geodetic data of every girth weld can be entered into the pipe book. As the girth welds are entered into thepipe book for any other intelligent inspection as well, the costly markering can be done without for the followingsurvey runs.

So the ScoutScan does not only provide a cost-effective, steady course measurement and tension analysis, but also

a way to reduce the costs of corrosion and crack detecting inspections.

Measuring method

Inertial navigation

The measuring method is based on the inertial navigation by means of a fiber optical gyro, known from aerialnavigation.

The essential item is a gyroplatform consisting of three gyros and three acceleration sensors.

The three gyros record any change of direction of the tool and thus of the pipeline in any of the three levels. Therespective gyro indicates the change of direction as a rotational rate in degree per second [°/s]. The ScoutScanrecords these rotational rates in constant intervals in time. Scanning rates of approx. 200 cps are reasonable, solocal resolution of about 5 mm is theoretically feasible. This very high resolution, however, is sensible only forshort pipeline sections.

Correction of errors by reference points

Like any other measuring system, the measurement of rotational rates has its fault. The precise manufacturing of the gyros and the internal error correcting system keep the absolute error of the individual rotational rate valueswithin the limits.

The error is, however, considerably increased by the addition of the rotational rates for the calculation of theresulting changes of direction. Assuming a realistic recording of 200 values per second (scanning rate of 200 cps)and a tool velocity of 1 m/s, there are as much as two hundred thousand (200,000) values added up per kilometreof the pipeline!

As a consequence, the error increases continuously. Reference points are introduced at pre-defined distances todecrease the absolute error.

The geodetic data of these reference points are precisely known by conventional measurement and the applicationof Differential GPS (Global Positioning System). These reference points and the measured data can be mapped oneach other by time correlation after the survey run, in the course of the post processing.

A reference point can be considered as a new starting point for the summing up of the rotational rates. The

absolute position is determined with reference to this point, so the error is reset.

Owing to the data structure the absolute position can be determined in both directions. Summing up between tworeference points is done from the first reference point to the middle and then virtually backwards from the secondreference point. As a result, the error is largest in the middle between two reference points. See Figure 1.

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Figure 1: Correction of errors by reference points

Markering

The reference points have to be adjusted to the measurement data. For this purpose, the exact time the ScoutScanpasses a reference point is determined. This is carried out by the Time Based Marker boxes, which aresynchronized with the extremely precise time (atomic clock) of the GPS (Global Positioning System) satellites.

The TBMS boxes are placed exactly on the reference points along the track (see Figure 2). On these points theyreceive a triggering signal at the very moment the tool is passing. The passing time is recorded.

Figure 2: TBMS box at the reference point

The ScoutScan as well is synchronized with the GPS time. The time is recorded with the data. So the timesynchronisation permits to adjusted every reference point to the data.

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Line measurement

The direction of the tool can be indicated at any moment owing to the measurement of the changes of thedirection in a fix time pattern. The gyro data are, however, not sufficient to determine the displacement along thepipeline axis in longitudinal direction, as the tool does not run at a constant velocity.

Measuring systems basing on inertial navigation as it is in common use in aviation, measure the longitudinal

displacement by means of acceleration sensors. The displacement is computed by double integration. For thispurpose, there is one acceleration sensor mounted for each axis on the gyroplatform of the ScoutScan.

In aviation, this computation is constantly supported by GPS measurements (Global Positioning System) amongothers. This correction method is, however, not practicable for survey runs, as GPS signals cannot be received inpipelines.

ScoutScan acceleration data are confirmed by odometer wheels. The velocities and the distance determined fromthe acceleration values are checked with the odometer wheel information and their plausibility is verified.Distance is measured by three separate odometer wheels. As the wheels cover different distances in bends, thedistance along the pipe’s centre line is computed with the information of all of the three wheels.

Configuration of the tool train

The 20" ScoutScan designed by Pipetronix consists of two units (see Figure 3).

Figure 3: 20" ScoutScan

The towing unit is sealing the pipeline with its cups. The differential pressure created in this way moves the toolthrough the pipeline with the electronic unit being pulled by the towing unit.

The electronic unit is kept as parallel as possible to the axle of the pipeline by the guiding wheels, with three of

the rear wheels being designed as odometer wheels.

The electronic unit contains the gyroplatform, the recording electronics, the data memory, the power supply andthe transmitter triggering the Time Based Marker System (see ‘Markering’).

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Specification of the tool train

tool size : 20"

total length : 2,010 mm

weight : 250 kg

minimum bend radius : 1.5 x D / 90°

safe maximum pressure in the pipeline : 120 kp/cm2

locating system : Time Based Marker

safe temperature of the medium : - 25°C to + 60°C

mass storage capacity : 1.1 GBytes

maximum running time : 75 hours

recommended range of velocity : 1 m/s to 3 m/s

longitudinal resolution (1 m/s, scanning rate 200 Hz) : 5 mm

number and diameter of the odometer wheels : 3 x 99.8 mm

number of odometer pulses per turn of wheel : 8

maximum rotating rate : ± 1000 °/s

resolution of the rotating rate : 0,03 °/s

maximum acceleration :± 40 g (g = acceleration due to gravity )

resolution of the acceleration : 2.4 mg

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Data interpretation

Interpretation proceedings

There are two major steps in data interpretation. The first step, post processing, comprises the computation of angles and co-ordinates from the rotational rates. In the second step, data interpretation, the course of the pipeline

is indicated on the basis of these values, the pipebook is prepared, bends are determined and further datainterpretation work, like for example a tension analysis, is carried out (see Figure 4).

Figure 4: Interpretation proceedings

Post processing

The raw data (rotational rates, acceleration, odometer information and time) provided by the tool are mapped on

the starting and receiving co-ordinates and the co-ordinates of the reference points.

In addition to the points mentioned above, the systematic errors like the gyroscopic drift and the banking of thetool in the pipeline are corrected to a large extent by rigorous filters being applied in the computation of the co-ordinates (see Figure 4).

As a result, there are co-ordinates with a statistic error of +/- 0.05%. This means, the error is +/- 0,5 m to theclosest reference point at a distance of 1,000 m between these points.

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ScoutScan survey

The survey was carried out in in Ruhrgas’ Werne-Walstedde 20" x 17,2 km pipeline.

Ruhrgas strategy and intention

Ruhrgas are carrying out conventional geodetical surveys with a surveyor every time a new line is constructed andis still in an open ditch.

Course information and profile maps are not always complete nowadays, and sometimes even very rough andincorrect. Therefore Ruhrgas were interested in the development of a mapping pig, the costs of such design work and, of course, the measuring accuracy and reliability.

Ruhrgas awarded a contract on the following items to Pipetronix / Veenker in September ’97:

• Measurement of the course of the line.

• Provision of the 3D coordinates of the line in 1 m intervals (resolution) – better in bends – in digitalformat.

• Check of pipebook and geometry of the installations.

• Determination of all bends ³ 5° (angle and radius) in two selected 1 km areas.

Unlike in the provision of new documentation for old lines with the accuracy requirement for the coordinates of ±0,5 m for x and y and ± 0,3 m for the height (z coordinate), Ruhrgas asked for an accuracy for x + y + z of ± 0,2m for new lines.

The use of the traditional surveyor and the ScoutScan on the same line were supposed to offer an opportunity of objective comparison and assist in the decision for further surveying measurements.

Pipeline preparation

As there were no pig traps on the line, a temporary launcher and receiver were installed.

The passage clearance was checked and confirmed by a BIDI pig with gauge plate.

Product for the inspection was air. 2 compressors pushed the pig with approx. 5 bar through the line.

34 reference points in total (marker points) at approx. 500 m intervals along the pipeline track were selected andmeasured by a traditional surveyor and marked. The coordinates of these points had to have an accuracy of ± 1 –5 cm.

ScoutScan preparation

The PTX ScoutScan was adjusted to the pipe diameter, the batteries were installed and all functions were finallytested. A sufficient number of TBMS boxes (Time Based Marker System) were prepared and checked as well.After the transport to the launcher, the ScoutScan interval clock time was adjusted to the exact GPS time receivedby the TBMS.

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ScoutScan run

After launching the ScoutScan into the starting trap, the pig was kept still for approx. 5 minutes in order tocompensate the gyro drift later on.

Now the pipeline was pressurized with the 2 airs compressors. Approx. 24 hours later the tool was started. Thetool reached the receiver approx. 8 hours later. It was kept still for 5 minutes in order to have the final position

stored.

Afterwards the tool was synchronized again to the TBMS time in order to correct time deviations.

The data were transferred and checked for completeness and plausibility.

30 of the 34 passage times could be determined.

At two marker points the wooden pecks had unfortunately been removed in the night before the run.

Data interpretation

The data interpretation was carried out as described above with a scanning rate of 200 (Hz) cps and an averagetool speed of 0,6 m/s, with coordinates being recorded every 3 mm.

A Final Report was prepared and presented, which included 3D coordinates in a 1 m interval, better than 1 minterval in bends, stated in the Gauß-Krüger system (local German coordinate system). A pipebook was created,which included installations and bends ³ 5° with angles and radii for 2 x 1 selected km. In addition to the above,the pipeline course and the profile were entered in a GIS (geographic information system) System with maps indifferent scales, satellite photographs and aerial views. Other data like right of ways and owner information, pipematerial, girth weld information, data from cp (cathodic protection) surveys and last not least inspection data fromall kind of intelligent pigs as for example geometry, corrosion and crack pigs can be entered in this softwareprogram called Pipeline Surfer .

Only this combination of data which is easily visible on the screen can give the complete picture of the condition

of a pipeline and is more than helpful for risk analysis and lifetime evaluation.

As only a part of the reference points coordinates were given to Pipetronix before the evaluation, Ruhrgas had thechance to check the achieved accuracy and reliability with the other part of the reference points. This verificationshowed that the required accuracy was met by this ScoutScan run, although the tool speed varied extremely due tothe low pressure level (approx. 5 bar). When a tool is run in gas in an operating line, the pressure level usually ismuch higher, which results in a more continuous speed. In liquid lines, the speed variation can be nearly zero.

It was a surprise to everybody that the table of costs calculated by Ruhrgas demonstrated that the traditionalsurveyor was within the same price range as the combination of ScoutScan plus surveyor (for the reference pointsand information on the course) providing more information. As a result, Ruhrgas decided to continue to use thePipetronix ScoutScan not only for old lines but also for newly constructed lines (baseline survey).

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