Tài liệu PIPELINE PIGGING TECHNOLOGY- P13 doc

Data processing Calliper processing The sonar ring measures distances from the pig carrier to the pipe wall, thus capturing a cross-section of the pipe.. Velocity processing Velocity inf

The effect of the latest technology is even more pronounced in data

analysis Initially, the IBM AT's graphics' speed was slow and annoying for the

evaluating technician The desired work flow speed was only realized with

the introduction of the Compaq Deskpro 386 series PC, which has a 20MHz

clock frequency The story is similar for the various storage mediums The

floppy discs, streamers, and Winchester discs available at the beginning of the

pig's development were not suitable to store and handle the vast amount of

data subsequently made available The problem was solved only when the

optical disc with a storage capacity of SOOMBytes was introduced This disc

has a large storage capacity, random access, and is handy for archiving

purposes

The situation becomes even more problematic when pigs for smaller

pipelines, e.g 6-in (150-mm), are designed Only those electronic

compo-nents which are based on SMD, Hybrid, and LSI technology can be used The

magnetic tape recorders that are used in these smaller-diameter pigs are based

on relical scan recorders which have appeared only recently on the market

CONCLUDING REMARKS

The UltraScan corrosion pig is the first internal pipeline inspection tool

that permits direct quantitative measurement of remaining wall thickness and

scans the entire inner surface area of a pipeline The development was well

timed, with the 16-bit microprocessor technology and other advanced

com-ponents required for the building of small-diameter tools having only just

appeared on the market

HIGH-ACCURACY CALLIPER SURVEYS

WITH THE GEOPIG PIPELINE

INERTIAL GEOMETRY TOOL

IN THE DEVELOPMENT of the inertial geometry pig, Pigco recognized the

need for relating the pig position to the pipe wall The sensors used for this

during some of the initial runs provided a very good picture of the inside of

the pipe wall To meet the need for a high-accuracy calliper with the ability

to accurately locate features, Pigco has improved its Geopig To provide data

in a useful form that facilitates interpretation, Pigco consulted with its clients

and developed a software-analysis system; and a test-dig programme verified

the accuracy of location and feature measurement The paper describes the

pig hardware, the analysis software, operations, and the results of the survey

Potential for structural analysis and the scheduling of maintenance is also

discussed

INTRODUCTION

Previous inertial pig development

The Geopig was designed to meet a large variety of user requirements

using a modular system which integrates a number of different sensors The

Geopig can be customized and adapted to fluid or gas lines with minor

modifications The current versions can inspect pipelines of diameter NPS

lOin (254mm) and above

The strapdown inertial measurement unit (or SIMU) produces a

three-dimensional measurement of inertial acceleration and angular rate directly

from orthogonal triads of accelerometers and gyroscopes Two inertial

systems are currently in use: one uses an orthogonal triad of

single-degree-of-Fig.l NFS 30 tool configuration.

freedom gyros; the other uses a pair of two-degree-of-freedom gyros In the

SIMU with two-degree-of-freedom gyros, a redundant or combined axis

measurement is available The SIMU accelerometers and gyros are

comple-mentary sensors which, when coupled, deliver the measurements for

com-puting pipeline curvature, orientation of that curvature, and the positioning

capability for location of features

The Geoptg is suspended in the pipeline by rubber disks fore and aft of

each carrier; this restricts the Geoptg to moving close and parallel to the pipe

centreline However, this guidance is not accurate enough to ensure that the

trajectory of the pig coincides with the pipe centreline, and that the pig's

pitch and heading coincide with the slope and azimuth of the pipeline,

respectively The actual deviations have to be determined continuously

which is achieved by two rings of sonars mounted on each end of the inertial

system Combination of these sonar readings yields the pig-to-pipe translation

and attitude

The initial data set from the alignment calliper showed the ability to

observe very small features in the line It was therefore decided to expand on

this capability by increasing the number of sonars The NFS 30-in tool was

designed with 72 sonars on the back ring and eight on the front for alignment

These give a full picture of the internal shape of the pipeline, with each sonar

covering about 3cm of circumference

The Geopig is completed by some other sensors and devices: odometers,

which measure the distance travelled, tracking transmitter for location of the

Geoptg, and a storage device and power supply which allow independent

operation for long measurement periods

Fig.l is a schematic of the Geopig for NFS 30-in sizes and larger For a

detailed description of the development of the Geopig, see Adams etaL, 1989.

For detailed description of the applications, see Price etaL, 1990.

Background for feature reporting

In the initial stages of development, delivery of data was in hard-copy form

A typical report would consist of several three-ring binders; for several

reasons, this was unsatisfactory It was difficult and time-consuming to go

through the data, and analysis by manual techniques did not always result in

correct answers With the large volume of data, many important features

could be missed Storage of the information was expensive

To solve many of these problems and provide a system that would allow

easy and precise analysis, Pigco developed a PC-based software package By

using the new optical disk technology, the processed data from a 300-km line

section would fit on one cartridge Although streaming tape could be used, it

would not allow random access of the data points Access times for the optical

drive are only slightly slower than for the normal hard disks found on many

PCs

By automating many of the search functions, the software allows rapid

screening or, if desired, afeature-by-feature step-through the line Calculation

of effective dent height was made uniformly and consistently, and not subject

to interpretation error

All the data is contained on the optical disk record; any point can be called

up and viewed By interfacing with any of a number of hard-copy devices,

prints in colour or black can be produced as desired

HARDWARE

Calliper sonar

The sonar devices are mounted in a ring and spaced at precisely-machined

constant angles around the ring on the pig An accurate offset is added to these

ranges to give the actual distance from the centre of the carrier to the pipe

wall These observations are in a polar coordinate system and are converted

to a rectangular coordinate system to form the pseudo-observations

Two rings of sonar sensors in liquid, or ultrasonic sensors in gas, scan the

wall of the pipe and determine the pig-to-pipe translation and attitude The

use of sonar or ultrasonic technology increases the reliability and accuracy

without the dependency on mechanical detectors and without contact on the

wall

Configuration of the sensors

The sonar or ultrasonic devices that range to the pipe wall are designed to

stand-off 10 to 15cm (4 to 6in) Shorter distances cause an inability to read the

transit time with sufficient accuracy for precise distance measurements If the

distances are much longer, it is difficult to obtain sufficient signal strength to

correctly pick the return time

In the NFS 30-in tool, the sensors were spaced to cover 3cm (1,25in) of the

pipe wall circumference At a run speed of Im/s (3.3fps) and a recorded

sample rate of 32Hz, the length of the sample window is 3cm (1.25in) The

footprint of a sonar on the wall has a diameter of about 1cm (0.4in)

Timing

To minimize interference from one sensor with the others and the effect

of reflecting signals, the sensors are pulsed opposite the last one plus one

Originally, the Geopig sonar sensors were sampled at 32Hz using a

medium-strength power level The return signal was recorded regardless of

the amplitude In the early runs, single sensor spikes occurred on one or two

scan lines These were most probably reflections off particles in the oil or

refraction off the side of a small dent

On subsequent runs the sampling rate was increased to 64Hz The first

pulse was low power; if the return signal amplitude was too low, a more

powerful second pulse was sent out If the amplitude of the second return

signal was stronger, it was recorded as the pipe wall return

This sequence of power pulsing significantly reduced the false returns on

the later runs

Strapdown inertia! navigation system

The selection of a particular SIMU was based primarily on size, accuracy,

power requirements, and cost The size requirement was dictated by the

smallest pipeline diameter and the ability to negotiate bends in the line The

accuracy requirement was to provide radius of curvature measurement to

better than 100m There are basically two ways to determine the curvature of

a pipeline by a SIMU: via cross-track acceleration (centrifugal force), or via

cross-track angular velocity

Low-accuracy SIMUs are not accurate enough to use their acceleration to

determine the curvature However, the acceleration is necessary to orientate

the curve with respect to the vertical (see Knickmeyer etal, 1988) Power

requirement was an important consideration due to the duration of pipeline

runs of a week or more

Weld detection

Circumferential weld-detection sensors are mounted in one of the pig

rubbers, and sense the change in material at the weld The output of each of

the three or four sensors is an electrical pulse At any time, one of the sensors

may pick up the long seam weld or other changes in the metal, but only at the

girth weld will they all fire simultaneously The resulting girth-weld indication

is used to correlate the pig data to as-built plans The welds also build a log of

pipe joints for future comparisons In epoch-to-epoch measurements, the

historical information on weld separation provides an indication of the axial

forces acting on the pipeline

Odometers

Velocity information computed from odometer wheels bounds the errors

which occur in the time integration of the inertial data At the same time, these

sensors provide a system chainage for the pig through its travel down the

pipeline The hinged wheels maintain contact with the pipe wall by spring

tension; the pivot allows the wheels an additional degree of freedom to

maintain a tangential orientation when the pig is negotiating bends

Data processing

Calliper processing

The sonar ring measures distances from the pig carrier to the pipe wall,

thus capturing a cross-section of the pipe These ranges are processed using

adjustment techniques to compute the centre of the pipe with respect to the

pig, and the pig-to-pipe attitude, using circular and ellipsoidal models

Deviations from the model (adjustment residuals) give the cross-sectional

picture of the pipe with determination of dents and ovality, as shown in Fig.2

The on-board processing consists mainly of packing the data to take as little

tape space as possible On recovery, the processing consists of:

spinning the data to the correct clock position, based on the

accelerom-eter output from the inertial system;

correcting for offset of the sensor from the centre of the pipe;

correcting for changes of the velocity of sound in various media

Strapdoum inertial unit processing

A SIMU is ideally suited to the task of providing trajectory information in

the local sense for several reasons Firstly, it experiences rotations due to

curvature of the pipeline directly, because its movement is constrained by

rubber disks Secondly, output is at a high rate, typically 16 to 64Hz, hence

profiles can be analyzed at a very high resolution based on pipeline fluid or

gas-flow rates For a structural analysis of critical pipeline curvatures, accurate

local measurement is required This local accuracy characterizes inertial

instruments, so that a low-accuracy SIMU (gyro drift 10°/hr) is sufficient for

the problem at hand (see Schwarz etal, 1989).

SIMU processing consists of calibration, alignment, mechanization, and

Kalman filtering modules Various updates stabilize the computation of

position and attitude The error state is comprised of misorientation, position,

velocity, accelerometer bias, and gyro-drift parameters

The processing provides:

position (latitude, longitude, height, or UTM or local coordinates on any

datum) of the trajectory;

attitude of the pig (pitch, roll, yaw), and consequently of calliper and

other sensors;

statistical information to qualify the computed quantities

The SIMU processing is, apart from the sensor error compensation,

independent of the actual unit used

Velocity processing

Velocity information computed from Doppler sonar and odometer wheels

bounds the errors which occur in the time integration of the inertial data At

the same time, these sensors provide a system chainage and continuous

checking between the two sensors to eliminate odometer slippage and

provide scale-change estimation The velocity-processing module combines

the velocity data from the odometer wheels and the Doppler sonar, and yields

the best velocity possible for use in Kalman filter processing

Continuous checking between the two odometer wheels (or four,

depend-ing on configuration) determines odometer-wheel slippage and is corrected

The redundancy also allows for relative scale estimation between the wheels

The velocity processing for the odometer wheels makes use of the

redun-dancy to compute the best velocity possible for use as input observations for

the Kalman filter The along-track velocity is computed by using the recorded

times of the reflectors passing by the proximity sensor The measured

circumference of the wheel over time interval yields the velocity for each

wheel Opposing wheels are averaged to compute the centre line chainage

and velocity of the pig

DATA PRESENTATION: THE GEODENT

SOFTWARE

Description

The Geodent program is designed to assist in analyzing the status of the

inside of an oil or gas pipeline using the data collected from the Pigco Pipeline

Services Ltd Geoptg The data is collected and processed at intervals along and

around the pipeline for its entire length; included are the coordinates in

Northing and Easting, chainage, the elevation, the inside diameter, the ovality,

and the weld to weld distances By examining this data in detail using the

powerful graphics in Geodent, anomalies can be identified and analyzed to

quantify the size, shape and location of dents, buckles, and so on

The major features of the Geodent program that enhance its use are:

analysis is conducted on a standard PC-compatible computer using

menu-driven displays that minimize the learning curve to efficiently

and effectively analyze the pipeline condition;

all features and anomalies are located with accurate coordinates and

chainages for field location and correlation to the as-built drawings;

the Scan feature grades all features for the entire length of the pipeline,

prioritizing the analysis and allowing quick access to problem areas;

multiple windows facilitate viewing of a potential anomaly in many

different perspectives, scales and colours, including

three-dimen-sional, contour or section;

interactive measurements yield rapid determination of the size, shape

and extent of any anomaly;

computer-generated reports provide automatic grading of dents, listing

of pertinent details, and incorporating engineering comments;

the program can be interfaced with over 200 printers and plotters for

presentation and analysis

Fig.3 Typical dent statistics.

The utility programs WELDREPTand DENTREPTprovide reports of all the

welds and summaries of dents meeting user-defined specifications

Reporting functions

Dent report

DENTREPT produces a summary of the features (Fig.3) identified during

the run of the Geoptg The search variables are the height of the feature and

the number of scan lines The number of along-track scans that a feature

covers is related to its length, and depends on the pig speed and the sampling

rate, which are variables used to determine the data points included in the

search

The program provides an output that is summarized in Table 1 below Each

feature is identified with a number that is used in more detailed analysis

Table 1 Typical output parameters.

Feature Time Chainage Length Maximum Average Clock

number (sec) (m) (m) ovality ovality position

Weld report

The Geopig system measures with the weld detect sensors each girth weld.

These welds are used in the database as primary identifiers and in the

kinematic analysis as boundary points The WELDREPT program gives a

listing of all the welds, the run time when they were detected in seconds, and

the chainage in meters

The valves (V), the start of wall sections (SH) and the end of

heavy-wall sections (EH) are identified in the Weld Number column:

Weld Time Chainage Chainage Length Length

number (sec) (m) (ft) (m) (ft)

Overview functions

Dent

The drag menu Dent on the main menu allows the number and extent of

the dents on the database to be summarized graphically The chainage or time,

the computed out-of-roundness (including dent height less average ovality),

and the lengths of the dents, are summarized graphically The welds,

thick-wall sections, and valves are identified on the display The detail display can

be used to zoom-in on an area in the main display The facility to locate a dent

and then exit to the analysis program allows rapid evaluation of the feature

The width in time or metres of the display may be selected for either the

Dent or Curve mode The minimum ovality or dent depth and minimum

length in scan lines may be selected as a criterion for those features that are

included in the summary

Curve

As part of the Dent mode there is also the ability to plot the curvature of

the pipeline The ratio of the detected bend in the pipeline to the pipeline

radius is indicated as potential strain according to the formula:

K = (rp r^) xlOOwhere K = curvature,

rp = pipe radius, and

rg = measured radius of centre line

Display functions

The screen

The display screen for Geodent has the following basic framework:

a main menu bar across the top of the screen, with pull-down menus

selectable by the mouse, arrow keys or keyboard letters;

a main display on the screen, where the pipeline data is displayed in a

selectable format;

a detail display on the screen, where a zoom of the main section is

displayed in a selectable format;

information panels on the left of both the main and detail displays,

where either the colour spectrum or information about the dent or

feature is displayed;

data panel at the bottom of the screen, where six lines of information

are displayed concerning the program status or data requested

Types of display

This section describes the graphic display types that are available on

menus on the main display bar under Display Each display will show the

options available for that menu

Thermal: depicts a section of the pipeline in a selectable colour scheme.

Each calliper sonar reading in the window area is colour plotted as a function

of its residual value from a circle on each scan line and for the display width

Cross section: plots all sonar ring readings within a section of the pipeline

at a distance or time along the pipeline (scan line) Each sonar reading is

plotted relative to a theoretical vertical line that would represent a circle The

plotted deviations from this line represent the magnitude of the dent, ovality

or other feature

Profile section: plots the readings from each of the calliper sonars within

a section of pipeline as it transits the pipeline Each sonar reading is plotted

relative to a theoretical horizontal line that would represent a point on a

circle The plotted deviations from this line represent the magnitude of the

dent, ovality or other feature

Round section: plots each set of sonar readings within a section of the

pipeline at a distance or time along the pipeline The view is as if looking in

the end of the pipeline at the end of the section Each sonar reading is plotted

as a residual from a circle

Contour section: is similar to the Thermal section, except that the sonar

readings are contoured and a line of constant depth is determined and colour

plotted

Gyro and diameter, plots the readings from the cross-track gyroscopes of

the inertial system This is useful in confirming welds, as the gyroscopes will

see deflections as the front and rear cups of the pig cross the weld or any dent

If, for example, there is some foreign matter clouding a particular sonar or

group of sonars instead of an actual dent, then there will be no perturbation

of the gyros This helps to distinguish real features from measuring errors or

transducer problems Also plotted is the best-fit of all the calliper sonars to

give a reading of the inside diameter of the pipeline

3D plot: provides the ability to show the feature in a three-dimensional

view The viewing perspectives in the horizontal and vertical can be changed

to give different perspectives The vertical exaggeration is five times on the

vertical (depth) that on the horizontal to make the features more

distinguish-able

Pig movement: a more quantitative correlation of the dents from the gyro

is possible using this display, which shows the horizontal and vertical

movement of the Geopig as it transits the pipeline The movements are in

centimetres, and are related to the size and location of the feature

VGA or EGA colour graphics screen

2- or 3-button Mfcrosq/fr-compatible mouse driver

For production pipeline analysis, the following PC-compatible computer

is recommended:

1Mb of memory

extended memory manager

DOS 3.0 operating system

300Mb hard disk

1.2Mb floppy disk drive

math coprocessor

VGA (640 x 480) 16-colour graphics screen

2- or 3-button Af/croso/?-compatible mouse driver

SUMO read/write optical disk drive

HP PaintJet colour plotter

ANALYSIS OF FEATURES

Preliminary evaluation

The first step in determining the extent of the out-of-roundness problem

is to set criteria and determine the number of anomalies that exceed these

levels Geodent provides two ways to do this The dent-reporting utility can

be used to provide a hard-copy listing with dent numbers assigned to each

feature The dent number can then be referred to in future analysis and

verification digs The dent display portion of the main program with a suitable

window width can also be used to identify features that require further

analysis (Figs4-7)

Detailed analysis

Occasional sonar drop-outs, refraction from dent flanks, and reflections

from particles can cause a feature to appear or be much larger than it really

is The redundancy of the Geoptg system provides several ways to verify that

there is a significant feature present and its size

The gyroscopes in the strapdown inertial unit are affected even by the

slight movement the pig experiences in crossing a girth weld The gyros will

be deflected by any dents From the geometry of the tool (Fig 1), it can be seen

Fig.4.