Pipeline mapping in GASCO
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
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.
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.
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
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
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.
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
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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