Abstract

This paper deals with a problem of distance measurement inside of pipeline by the "pig" inspection device. The motives of a measuring error appearance are surveyed. The scenario's behaviour of an odometer wheel at collision with a welding seam is offered. The factors which have an influence on the measuring error magnitude are classified and analysed. Thesimulator of an odometer wheel movement in a pipe was described. The novel system and correction algorithm for distance computation is offered. The test results on a simulator are adduced.

Introduction

The problem of pipeline integrity is very important to prevent product losses and environmental pollution. Loss of product through liquid hydrocarbon, gas or basic petrochemicals can be a serious problem. In reviewing the incident statistics we findthat for natural-gas transmission and gathering pipelines for a 10-year period the average number of incidents reported was 82 with property damage $17,155,238. For the 12-years period (1984–1995) there was an average of 200 incidents reported ofhazardous-liquid pipeline with property damage cost of $24,657,904.1

It is necessary to carry out periodic inspection of pipelines for reducing of the above-mentioned losses. There are variousmethods for detecting pipeline damages. The most common is inspection by means of special instrument pig.

There are different types of pigs. Pigs can be classified by their maintenance aim: routine pigging for keeping a pipeline free of sediments and/or moisture; geometry inspections to insure that a pipeline is free from mayor physical damages; intelligent pigging, for delivering detailed information about a pipeline.

The last two types have various sensors (magnetic, ultrasonic, etc.) for definition of a tube condition. But, in each case when these sensors detect any damage, it is necessary to determine its location. For this purpose and for determining position, orientation and pipeline's plan reconstruction, the navigation system which is mounted on a pig is assigned.

Distance Measuring

For the constructing of pipeline plan it is necessary to have the information about the distance and slope angles.2 The distance can be measured by means of three ways:

  • By integration of the acceleration measured during of the experiments.

  • With help of Doppler system.

  • By means of an odometer.

In the first case, the basis of measuring is the inertial navigation system that has the same type as in aerospace industry. This system uses the angular speed (rate gyro) and acceleration sensors to determine the passed distance, pig attitude, as well as the inclination and direction's angles of a pipe. The basic advantages of new inertial sensors allow applying them for determining the pig position. Firstly, these sensors have small sizes which permit to design the system with the sizes those are less than the minimum diameter of a pipe. Secondly, they have small power consumption.

The disadvantage of this system is the algorithm for distance computing. This algorithm has double integration of an output signal of the acceleration sensors. It is the reason of the error accumulation.

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