Pipeline mapping in GASCO

Authors
Paul Shaw, GeoVision
Magdi Tawfik, GASCO on line inspection manager

The Egyptian Gas Company (GASCO) is responsible for over 3000km of gas transmission pipeline in Egypt (left). With a network covering mainly areas of rapid urbanisation, both the accurate positioning and integrity of the gas carrying pipes are critical components in ensuring international safety standards are met. GASCO tackled these problems in an innovative and unique way. This paper outlines the technologies used and advantages gained over traditional pipeline mapping techniques.

 

1. Introduction

GASCO was formed in the late 1990’s by the Egyptian General Petroleum Company to bring control of the gas transmission network under a single body. An immediate priority for the new company was to assess the integrity of the pipeline network. No pipeline checks had been carried out prior to the formation of GASCO and some of the pipes were over 25 years old. Traditionally a pipe is surveyed using pigging methods for the pipe integrity checks and land survey techniques for positioning. Chainage markers are then built at typically 1 kilometre interval and major crossings to assist pipe location and repair. Faced with over 3000km of pipeline needing to be surveyed in populated urban and rural areas, and the inevitable problems that direct access would create, GASCO took the risk of using a new pipeline mapping technology - an Inertial Navigation System (INS) integrated within the Inspection Pig tool – in order to minimise land access requirements.

2. Pipeline Mapping

The Inspection PIG (below) checks pipeline integrity using Magnetic Flux Leakage (MFL). This enables pipe thickness variations, dents, rust, supports, weld positions, bends and other internal and external pipeline information to be gathered through the magnetic sensor arrays. The distance of these points from the launch trap is recorded by three Odometer wheels spaced 120 degrees apart around the circumference of the Pig. Chainage data is recorded every 3.3mm. The processing software integrates the distance and MFL data to enable defects or assets to be located by a chainage. In GASCO’s case the Inspection Pig was also fitted with an Inertial Navigation System (INS) developed by Honeywell. This measures rate of change of distance and direction with time using three accelerometers and three gyroscopes. INS data is sampled and stored at a nominal 50Hz rate. Since the Pig travels along the pipe at an average speed of 4.0metres/sec during data collection, INS position is recorded every 8cm. It is important to note here that an INS does not give absolute position, only change of position, and that it also suffers from drift. Drift is normally measured as an angular movement away from the expected position over time, in this case specified as 0.005 degrees per hour. Control of the position and drift is carried out by referencing tie points to a co-ordinate system. The frequency of tie points will determine the positional accuracy. Since GASCO required the absolute accuracy of its assets and defects to ± 1m, control points would be required every 3km along the pipe.

The Global Positioning System (GPS) was used to control the INS. A high accuracy reference network had been established for the Egyptian Survey Authority in 1995 called NED95. This is based on WGS84 with a modified Transverse Mercator projection using 3 central meridians. This network was transformed to UTM Zone 36N since the entire pipeline network fell within this Zone. This would maintain co-ordinate consistency for hard copy plans and on-line inspection work.

Static methods of GPS observation were used to bring control nearer each pipeline. From this control real time kinematic measurements were taken to exposed welds (left) and valves at the launch and receive stations and static measurements to magnets placed directly on the pipe. (below) The MFL, INS, Odometer and GPS data is later processed and merged to produce co-ordinates for each weld. Those pipelines that were too small in diameter to accommodate the Inspection Pig with INS were surveyed by above ground survey techniques. Measurements were taken to all exposed sleeves, valves, supports on pipeline crossings and any other data covered in the MFL report. These surveys took typically 10 times longer than using the INS and it proved difficult to correlate accurately the MFL data with the above ground survey measurements.

3. GIS

The GIS was specially developed for pipeline asset management. It uses an open GIS for geographic data handling, can be stand-alone or network based using Oracle and allows multiple user access to different levels of security. (below) Pipeline data from the MFL report is entered into the GIS using the geographic co-ordinate tie-points at each of the welds. The GIS then uses the chainage data between the welds to assign co-ordinates to each defect and pipeline detail. UTM co-ordinates are therefore available for every asset and defect. The GIS allows user-defined assets and sub-assets. For instance, the main asset may be a 24” diameter pipeline and the sub asset a valve on the pipeline. For each of these assets, relevant pipeline information can be attached to the record, like valve inspection reports for the valve and Cathodic Protection readings for the pipeline

A variety of functions allow the integration and analysis and output of a wide range of pipeline data in user defined formats. This includes Population Density Mapping, Pipeline Integrity Maintenance and Monitoring, Land Use/Landowner Management, Environmental Management and other data integration and analysis facilities. One feature of particular note is an automatic safety calculation function based on the standards set by the American Society of Mechanical Engineers. This selects the density of buildings, building types and distance from the pipeline and calculates the safety measures required for each pipeline section.

Mapping backdrops are provided by 1:50,000 scanned and 1:5,000 digital vector maps. In the GIS, the different scales of mapping are automatically generated depending on the scale the user is zoomed into. Unfortunately the mapping is at least 10 years old and requires updating, especially where GASCO’s pipelines cross areas in which of rapid urbanisation has taken place. The use of high-resolution satellite imagery has been researched since imagery can be easily sourced and integrated into the GIS. This would allow the update of the mapping database and therefore ensure continuous compliance with international safety standards.

At present, data transfer between the main office and elsewhere is dependent on multi-user 56k telephone lines. This requires some data to be stored on individual PC’s or on network servers within individual buildings. A new high-speed microwave network is being developed. This will allow all data to be stored in one location, in turn, enabling better control of the asset database.

4. Pipe Maintenance

The gain from a GIS referenced to an accurate GPS control network is in pipe maintenance. Traditionally, the point of excavation is found by measuring from marker posts using chainages from the pipeline reports. The accuracy of the location is dependant on the frequency of chainage markers and correlation with the Odometer readings. In GASCO, on-line inspection is carried out using a real time differential GPS system (left) which can accept both beacon and OmniSTAR signals. The co-ordinates of the point to be located are entered into a pen-computer connected to the GPS. The software has a navigation facility that directs the user to the defect. It is simple, quick and effective. The inspectors can also use the receivers to survey detail along the pipe route to add to the GIS as required.

 

5. Conclusion

GASCO’s mission of being a pioneer in the use of modern technology for its gas transmission network is reflected in the way it has developed a unique method of pipeline inspection and maintenance. Critical to the success of the project have been the following.

· Establishment of an accurate co-ordinate network based on GPS observations.
· An Inspection Pig with an integrated INS for accurate pipeline mapping.
· A multi-functional and adaptable GIS to allow pipeline analysis and maintenance.

Quality Control checks recently carried out using the differential GPS system to assets along the pipeline network gave an average positional accuracy of ± 0.6m. The risk of using new technologies has therefore paid dividends, saving time and money. GASCO now has a solid foundation for all its future pipeline asset management requirements.

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