ibp1086 09 pipel ine mapping and strain assessment …

______________________________ 1 B Eng (Hons). MIMechE. C. Eng., Chartered Mechanical Engineer, Principal Consultant (Pipeline

Integrity) - GE PII Pipeline Solutions 2 Dipl.-Ing. Head of Pipelines Competence Center - E.ON Ruhrgas AG

IBP1086_09

PIPELINE MAPPING AND STRAIN ASSESSMENT USING ILI

TOOLS

Brian Purvis, Principal Consultant GE PII, Dr. Thomas Hüwener,

Pipelines Competence Centre E.ON Ruhrgas AG

Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009, held between September,

22-24, 2009, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event

according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented,

were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is

presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, or that of its Members or

Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings.

Abstract - Pipeline Mapping and Strain Assessment Using ILI Tools

GE PII IMU Mapping inspection system measures pipeline location coordinates (x, y, z) and provides data for

determing pipeline curvature and consequential pipeline bending strain. The changes in strain can be used in the

application of structural analyses and integrity evaluation of pipeline systems. This paper reviews the Inertia Measuring

Unit (IMU) system and field investigation works performed on a high-pressure gas pipeline for E.ON Ruhrgas AG.

The Inertial Measuring Unit (IMU) of the pipeline inspection tool provides continuous measurement of the pipeline

centreline coordinates. More than one inspection run was performed which allowed a more accurate strain comparison

to be made. Repeatability is important to establish the reasons for increasing strain values detected at specific pipeline

sections through in-line inspection surveys conducted in regular intervals over many years. Moreover, the flexibility

resulting from a combination of different sensor technologies, makes it possible to provide a more complete picture of

the overall situation.

This paper reviews the work involved in detecting, locating and determining the magnitude and type of strain

corresponding to the pipeline movement in field.

1. Introduction

The as-laid position of pipelines is not always constant, since movement can occur for several reasons.

Earthquakes, permafrost thaw or heave, landslides, ship anchors and other third-party influences can move and bend

pipelines. Any movement of the pipeline position can lead to increased curvatures, which can in turn impact pipeline

integrity as it can lead to excessive strain in the pipe, severe pipe wall deformations as well as fatigue that can cause a

pipeline to fail.

The major sources of pipeline movement are:

• Soil instability, a result of landslides, earthquakes, permafrost thaw, frost heave, excessive overburden,

seabed scouring, waterway crossing erosion and insufficient support in soft soils

• Operational changes in temperature, pressure and/or increase in pressure cycling

Rio Pipeline Conference and Exposition 2009

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• External interference such as impact by construction equipment, ship anchors, fishing gear as well as long

term effect of a new construction in the pipeline right of way, e.g. new parallel pipelines, bridges, dykes,

berms etc

Pipeline movement can lead to severe wall deformations, excessive strain and fatigue. It redistributes the axial force

along the pipe, increasing tension in some locations and compression in others. It can also give rise to pipeline

curvature, which induces tensile strain in half of the pipe cross-section and compressive strain in the other half. The

excessive tensile strain can lead to pipeline rupture, particularly at girth welds. The large compressive strain may cause

buckling (Upheaval/Lateral or localised pipe wall buckling - wrinkles).

Pipeline bending can be regional or local in character. Both defect classes can be detected and analyzed with

specific GE PII in-line inspection modules. The latest metal loss and geometry sensors developed by GE PII can be

combined with proven inertial navigation systems.

This combination improves sensitivity, repeatability and confidence when detecting pipeline curvature-bending

strain while, also taking into account the influence of bending strain on existing pipeline features.

GE PII Pipeline Solutions has recently performed pipeline curvature/strain assessment on high-pressure gas

pipeline for Eon Ruhrgas AG and carried out pipe bending and strain investigation on the basis of pipeline mapping and

IMU data obtained. As this pipeline had been previously inspected in both 2006 and 2007 with an Inertial Measuring

Unit (IMU) it was possible to perform a run-to-run comparison resulting in greater accuracy of the pipeline

curvature/strain assessment results.

2. A Partnership – E.ON Ruhrgas AG & GE PII Pipeline Solutions

E.ON Ruhrgas AG wanted to prove whether pipeline movement might have occurred within the vicinity of a

number of its high-pressure gas transmission pipelines. E.ON Ruhrgas AG understands the importance of monitoring

both the pipeline movement and the deformations caused by it. Therefore E.ON Ruhrgas AG has embarked on a

programme of determining the status of ground movement for these suspected pipelines. This can be accurately and

efficiently achieved by performing in-line inspections using the proven GE PII IMU mapping system, which can

incorporate metal loss and/or calliper modules if required. The inertial mapping system used in this application allows

for accurate 3D measurement of the pipeline centreline position and curvature that is used for calculation of the bending

strain and provides high accuracy pipeline geometry data for any further Finite Element Modelling and analysis.

Comparing two surveys at different times are used for precise monitoring of the pipeline movement and bending strain

between inspection runs. The tool used also records caliper measurements of the pipe wall shape, which allows for

detection of pipe anomalies that may have developed as a result of the pipeline displacement.

This paper describes the GE PII pipeline strain assessment technique and a recent experience with E.ON Ruhrgas AG

pipeline strain assessment.

3. Inspection Philosophy

GE PII IMU has been in use for over 10 years and has inspected pipelines in many locations around the world.

With each deployment both GE PII and pipeline operators have increased the capability for safe pipeline operations.

This particular deployment involved a pipeline cleaning operation followed by pipeline inspection tool survey.

A typical pipeline caliper/mapping inspection tool can be seen in the photograph below which highlights the IMU

module within the inspection tool.

Figure 1 - Inertia Measurement Unit (IMU) Module


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Figure 1. Inertia Measurement Unit (IMU)

The 1st photograph shows the location of the IMU module within the inspection tool. The 2

nd photograph shows

the IMU within the IMU inspection module.

The in line inspection tools used for E.ON Ruhrgas AG were equipped with the following sensors systems:

3.1 IMU – The Inertial Measuring Unit (IMU)

The unit comprises angle rate gyros and linear accelerometers. The system measures the precise path the pig

has taken during its traverse of the pipeline. This system is used to produce a detailed plot of the pipeline, measure

curvature, and to identify any potential serious abnormal centreline deviations from the as laid or previous inspection

condition. It can also be used to locate welds and dents


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Figure 2 – Illustrates the Gyroscopes and Linear Accelerometers within the IMU

Figure 2. IMU Gyroscopes and Accelerometers

The IMU 3D measurement system employs gyroscopes for “Angular Rate (Degrees/sec)” measurement and

Accelerometers (m/sec2) for linear acceleration measurement. Data is acquired at 50 Hz or 50 sample points per second.

3.2 Odometer

This is a wheel that measures distance travelled by the tool along the pipeline and instantaneous pig speed.

The odometer wheel rolls along the pipe wall. Distance is found by counting the wheel’s revolutions. The distance

information is used to: (1) improve accuracy of the estimated route from the IMU, and (2) determine the distance of

each feature.

3.3 Weld Detection Sensors

These are used to provide data on weld location and joint length.

3.4 High Resolution Caliper

This is a device for measuring the pipeline internal diameter, ovality and dent size as well as shape.

The data collected by the sensors is stored in an on-board solid-state memory. The inspection tool systems and

memory are battery powered. Integral to the capability of the GE PII IMU is the computer software used to analyze the

survey results.

4. Inspection Deployment

This particular tool configuration as seen in figure 1 was used as it enabled E.ON Ruhrgas AG to integrate the

various inspection data and strain assessment results into it’s operations and maintenance systems. Also accurately

identifying where to excavate and over which approximate area, thereby significantly reducing excavation costs.

E.ON Ruhrgas AG In Line Inspection strategy has been motivated by the need to improve efficiency in both

pipeline operations and maintenance. Mapping/IMU inspection has helped deliver this by providing improved pipeline

GIS information, reduce maintenance costs as a result of accurate feature location and therefore lessen excavation

activity. It also proved a solution for enhanced monitoring of pipeline movements.

Figure 3 – The E.ON/GE PII Partnership Learning Curve


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Figure 3. The E.ON/GE PII Learning Curve

The illustration demonstrates how improvements in equipment and deployment philosophies have improved the

GE PII services offered. In recent years this has improved accuracy, quality of the inspection data and consequently the

strain assessments results.

E.ON Ruhrgas AG experience over the past years has confirmed that the use of calliper support with IMU has

been fully justified by the increased quality of the inspection data obtained. The MFL pig sometimes detects dents in the

pipeline without specifying dimensions. Therefore from an economic approach it’s efficient to have the additional

calliper pig runs analysed and have a closer look at the dents mentioned in the MFL report. Often, the strength

characteristics of the relevant pipeline sections are good so that costly confirmation digs and examinations on site can be

avoided.

The quality of the defect data listed in the MFL reports is very high. In many cases defects listed were found

immediately when the line was excavated and uncovered and the defect dimensions determined by the pig were fully

confirmed. The E.ON Ruhrgas Operations Department and Pipeline Technology Centre of Competence therefore

consider intelligent pigging a very valuable inspection method.

5. How Does a Mapping IMU Inspection Tool Measure Strain?

Initially we need to understand what we mean by pipeline mapping. Pipeline mapping is a recording of the as-is

status of a pipeline. Inertial Measurement Units (IMU) utilising gyroscopes inside the pipeline measure the X-, Y-, and

Z-coordinates. The data can be used directly in a GIS environment, displaying the pipeline route and used in correlation

with data obtained from other in-line inspections tools.

In situations where global bending leads to a buckle or wrinkle in the pipe wall, both IMU and calliper, when

available, must be combined to fully assess pipeline integrity.

5.1 Pipeline Mapping

The pipeline location is given in terms of Latitude, Longitude (or Northing, Easting in local co-ordinate

systems or Universal Transverse Mercator, “UTM”) and height above sea level. To reduce the accumulation of the

absolute position error the pipeline plan and profile are rotated and scaled using the known coordinates of selected

control points on the pipeline. The control points must be identifiable both in the GE PII IMU mapping system data and

in the field (such as valves, bends, tees, magloggers etc) and the recommended spacing between them less than 2 Km.

The coordinates of those points are usually obtained from the Differential Global Positioning System (GPS) survey,

which has centimetres accuracy. The standard accuracy of the inertial survey is 1:2,000 of the distance from the closest

control point. If the control points are spaced not more than 3km, the position accuracy is better than 1.5m. The


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accuracy of 1:2,000 is usually exceeded when the control points are spaced more frequently, because of the smaller

effect of the nonlinear terms on the error.

5.2 Pipeline Curvature

We determine pipeline curvature from the in-line inspection IMU data. Curvature is the angle that the pipeline

turns through over each metre of pipe. Curvature is high on bends and, theoretically, it should be zero for a perfectly

straight section of pipeline.

The IMU gyroscopes measure change in angle, and the odometers measure distance so the curvature

calculation is relatively straightforward:

Figure 4 – Illustrates the Pipeline Centreline Curvature

Figure 4. Pipeline Centreline Curvature

Figure 4 shows how using two IMU data points the centreline curature angle can be determined.

Figure 5 – Illustrates How Pipeline Curvature (rad/m) is Determined

Figure 5. Curature Radians per Metre

The units of curvature are radians per metre. Bend radius in metres is 1 divided by curvature.

The real strength of curvature measurement is finding low-level bending caused by external factors. Clients

have asked GE PII to measure bending caused by many external factors: floods, earthquakes, ships dragging their

anchors, sea bed scouring / movement and permafrost.

We can determine whether a pipeline has moved by comparing curvature data from successive pipeline

surveys. Curvature is essentially a quantitative measure of how “curved” the pipeline is, so a survey comparison tells us

whether the pipe has become more curved or less curved between surveys or in line inspections. Any change in

curvature implies that the pipeline is experiencing an external force resulting in a bending moment.

Fortunately change in curvature is directly proportional to peak bending strain in the pipeline:


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The pipeline position and bending (Pipeline curvature) are derived from the odometer and IMU measurements.

The odometer measures the distance travelled by the GE PII IMU, while the IMU provides the acceleration and rotation

of the inspection tool about three orthogonal axes (with respect to an inertial reference which is fixed in space).

Besides the UTM coordinates, the azimuth and pitch of the pipe centreline in the local frame are also

computed. The pitch P describes the pipeline tilt with respect to the horizontal plane at the pipeline chainage S, while

the azimuth A specifies the angle between pipe direction and North.

Equation 1 – Pipeline Total Curvature

κ = Square Root (κν

2 + κh

2) (1)

Equation 2 – Pipeline Curvature Vertical Component

κν = δP / δS (2)

Equation 3 – Pipeline Curvature Horizontal Component

κh = - δA / δS Cos (P) (3)

The changes δP and δA of pitch and azimuth over a distance δS along the pipe centreline allow the

determination of the equation 1 pipeline total curvature κ and its vertical κν equation 2 and horizontal κh equation 3

components.

The pipeline bend curvature is used for the calculation of the pipeline bending strain. There are two principal

components of the strain in pipelines; hoop and axial strain. Axial strain is also considered as having two components,

which are axial and bending. The neutral axis of bending comes through the centre of the pipe the relationship between

the bending strain and pipe centreline curvature as described by the following equations 4, 5 & 6.

Equation 4 – Total Pipeline Bending Strain

ε = (D/2) κ (4)

Equation 5 – Pipeline Bending Strain Vertical Component

εν = (D/2) κν (5)

Equation 6 – Pipeline Bending Strain Horizontal Component

ε h = (D/2) κh (6)

Where D is the pipe nominal diameter, ε is the total bending strain, εν and ε h are the vertical and horizontal components.

The total bending strain ε describes the maximum longitudinal strain in a pipe cross-section induced by bending. The

vertical bending εν corresponds to the axial strain at the bottom of the pipe (the strain induced by bending at the top of

the pipe is the same magnitude as at the top, but of the opposite sign). The horizontal component of the bending strain

describes the axial strain at the most outer surface. Equation 7, bending strain ε (α) in any point around the outside

surface of the pipe cross-section is:

Equation 7 – Pipeline Bending Strain

ε (α) = εν cos(α) + ε h sin(α) (7)

Where (α) is the roll angle of that point measured clockwise from the top of the pipe cross-section. The distance δS over

which the curvature is averaged is assumed to be 3 pipe diameters. This is the best way of processing the inertia data to


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obtain the bending strain because it filters out a high frequency noise caused by pipe wall deformation without distorting

the curvature component induced by bending

6. Strain Assessment

This technique has proved to have several advantages over traditional methods of monitoring pipeline

movement, such as geotechnical surveillance or installation of strain gauges on the pipeline. The major benefit is a cost-

effective collection of data for the entire pipeline, not just pre-selected areas of concern. The inspection revealed that the

pipeline movement could occur in the areas that were not identified by routine geotechnical patrols. The ILI readily

collects data in the areas, which are difficult to access. Another advantage is a direct measurement of the cumulative

effects of the pipeline movement on it’s integrity by accurate location and sizing of the weakest points in the pipeline, in

terms of pipe wall deformations and bending strains

The strain analysis process has three elements

1. Inertial measurement unit (IMU) measures the inspection tools centreline motion in 3D: both acceleration

and rotation rates.

2. Data combined with distance wheel to generate “free route”

3. Drift in free route corrected by GPS survey

Inputs to Strain Assessment software are the PMB files, which contain raw & processed IMU data. Euler

angles (roll, pitch, heading) defining the IMU’s orientation relative to the Earth are calculated for each time step (50 /

100 Hz depending on IMU) when the free route is produced. PMB may also contain co-ordinates (latitude-longitude-

altitude) at each time step.

The StrainCom software then enables configurable XY plot interface shows data aligned on distance. Boxes highlight

features (deformation / change in deformation), bends, red rings, etc. The boxing is automatic but the classification is

checked manually and feature sizing is automatic.

StrainCom exports all data and graphics for the report into CSV and BMP files. These are then assembled into a report.

Figure 6 – Illustrates The Strain Assessment Data Flow

Figure 6. Strain Assessment Data Flow

Figure 7 – E.ON Ruhrgas Strain Assessment Data 2006 & 2007


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Figure 7. Strain Assessment Data

Figure 7 shows the IMU data between two inspections 2006 & 2007 and illustrates the amount of pipeline

movement detected.

7. E.ON Ruhrgas Field Pipeline Movement Verfication Works

During November 2008 E.ON Ruhrgas performed some Strain Dig Verification and excavated a number of

reported strain features to compare the results delivered by GE PII. An example of one of these is Strain Feature 41

(122m long) and the results are described here.

In practice there is no easy way to measure the curvature and strain data directly. Surveyors using high-

accuracy GPS equipment can determine position of girth welds. Displacement is related to curvature (although the

relationship is not straightforward), so confirming the girth weld displacements establishes confidence in the IMU

performance and curvature and strain data.

The original pipeline position was assumed to be a straight line between the girth welds upstream and

downstream of each deformation feature was used as a reference for displacement. Horizontal displacement for each

girth weld was defined as the perpendicular distance from the line to the top of the weld (12 O’clock position) in the

horizontal plane. Similarly the vertical displacement was the difference in altitude between the straight line and the top

of the girth weld.

Horizontal and vertical displacements were calculated from tool data for each girth weld in a feature using the

tool data. The figure below shows an example of calculated offsets from this straight line for Feature 41. This data

was compared with field measurements.

To avoid any movement of the pipeline during the excavation and to reduce dig cost only five access holes

were dug up on site (start and end point and each three verification points). The comparison surveying was performed by

PLEdoc / Ruhrgas with GPS.

The GPS-surveying equipment used was a Leica System 1200 directly connected to the German online

correction service ASCOS. The expected accuracy of this system was 2cm in horizontal and 3cm in vertical direction.

Figure 8 – Strain Feature: 41, GE PII supplied data


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Figure 8. Strain Feature 41, GE PII Supplied Data

The local deviations are based on the straight line bewteen the start and end point calculated by GE PII.

Figure 9 – E.ON Ruhrgas/PLEdoc Field Results for Strain Feature: 41


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Figure 9. E.ON Ruhrags/PLEdoc Field Results for Strain Feature 41

The local displacements results from PLEdoc/Ruhrgas in the field verification works are shown in the above

graphs.

Table 1 & 2 – Displacement Comparison Tables showing the GE PII supplied data for Strain

Feature: 41 and E.ON Ruhrgas field results

Table 1. Strain Feature 41 Horizontal data table

Data point Displacement Data Difference (m)

Start

2 0.02

3 0.00

4 -0.02

End


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Table 2. Strain Feature 41 Vertical data table

Data point Displacement Data Difference (m)

Start

2 0.00

3 -0.04

4 -0.02

End

The local displacement results from PLEdoc/Ruhrgas field investigation works are shown in the above table

listing. The verification work described above illustrated that the displacements reported by GE PII were in deed

extremely accurate with report derivation of approximately 0.02 m.

8. References

DR. THOMAS HUWENER – Strain Assessment – A Tool for Pipeline Integrity Evaluation –GE Oil & Gas Pipeline

Conference Sitges, Spain 7-9 May 2008

DR. THOMAS HUWENER E.ON Ruhrgas – Inline Inspection from an Operator’s Point of View – PTC 2007

RALF LORENZEN E.ON Ruhrgas/PLEdoc Strain Dig Verification Report –Nov 2008

COX M., GARRIGUS A., WALKER W., WADE R.L., 1995. Pipeline monitoring and remedial action from inertial

geometry surveys in buried pipelines. Pipeline and Inspection Technology Conference, Houston, Texas, Feb. 1995.

CZYZ J.A., FRACCAROLI C., SERGEANT A.P., Measuring Pipeline Movement in Geotechnically Unstable Areas

Using An Inertial Geometry Pipeline Inspection Pig, ASME 1-st International Pipeline Conference, Calgary, June

1996.

JAROSLAW A. CZYZL, SERGIO E. WAINSELBOIN Monitoring Pipeline Movement and Its Effect on Pipe Integrity

using Inertial/Caliper In-Line Inspection - IBP575_03

DAVID HEKTNER Pipeline Out- of - Straigntness Assessment Using Pipeline Inertial Geometry Survey (GEOPIG)

TECHNOLOGY Alaska Pipeline Workshop Anchorage, Alaska November 08, 1999

ibp1086 09 pipel ine mapping and strain assessment …

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