Dacon Inspection Services (Dacon) provide intelligent pigging solutions using a proprietary ultrasonic system to all the major companies in the oil, gas, petrochemical and mining industries globally. There is a constant thirst for new technology amongst our clientele, and Dacon are constantly being asked to produce solutions for unpiggable pipelines, internally lined pipelines, and non metallic pipelines.
One such request came from a multinational mining company in Indonesia. The mine had been given permission to offload their treated tailings to a deep sea trench approximately 3.5 km offshore, but as the tailings pipeline would traverse protected fisheries the Indonesian government insisted they use non-metallic pipelines to reduce the environmental impact.
The mine management team opted to go with HDPE pipelines, but this meant there would be a high erosion rate from the abrasive tailings mixed with water. The government stated that the company should inspect these lines on a regular basis, and that once they reached a minimum threshold they must be replaced. The mining company then approached Dacon to develop a system capable of inspecting these pipelines.
There were many challenges in developing such a system. Firstly the pipelines were 48” in diameter, with a nominal wall thickness of 110 mm (4.3”). HDPE has a very high attenuation of ultrasonic signals, and it is very hard to get a signal from the outside diameter of a normal thickness HDPE pipe, so this wall thickness was on the boundaries of possibility. Due to the design and technology utilized by Dacon Ultrasonic Intelligent Pigs it was possible to develop a tool that was able to address this issue and gather repeatable results.
The Dacon UT intelligent pig uses an Internal Rotating Inspection System (IRIS), which contains a horizontally placed transducer contained within the central body of the pig. The transducer sends its sound wave forward where it hits a 45° rotating mirror, which then directs the sound wave perpendicular on to the pipe wall. From the inner pipe wall the next signal will travel to the outer pipe wall. Every time the signal hits a surface it will reflect back the same way it was sent allowing the computer to calculate the time of travel. This time of travel is then converted to distance and measurement.
An ultrasonic reflection (echo) will take place at the inner wall (ID) and outer wall (OD) of the pipelines. These echoes are reflected back, received and processed by the Data acquisition and Central processing units. The time between these two echoes represents the wall thickness of the pipe. The remaining wall thickness can be calculated by using the sound velocity in the material, the sound velocity of any liquid which used as a propellant medium (this liquid will be used as a couplant between the transducer and the pipe wall) and time of flight between two echoes.
Using the above technology Dacon developed a highly sensitive low frequency system, which experienced less signal attenuation and was able to get reliable signals from the OD of the pipe. Specialist software was also developed to apply complex algorithms to the data, to be able to visualize the erosion in the pipe, and give accurate remaining thickness measurements.
The tools are always run in triplicate to provide the ability to cross correlate results to confirm the accuracy of the data.
HDPE pipelines fall outside of the normal codes and standards such as ASME B31.4 and B31G, but these standards were applied to calculate wear rates and remaining life of each pipeline, allowing the company to accurately estimate when the pipelines would need to be replaced.
It has been found that each pipeline lasts generally between 1-2 years depending on the ore and production output. Working with Dacon, the company was able to establish a system of when to install reserve lines, when to plan the next inspection, and when they must remove a line, seamlessly switching to a strategically installed reserve line to avoid interrupting production.
Due to the nature of the pipelines use (transporting slurry) it was expected that a significant erosion groove would be encountered on the pipeline in the 6 o clock position. This can clearly be found in the below illustration:
In addition to the expected erosion groove it was also found that additional erosion occurred in the HDPE weld join areas due to turbulence in the pipeline caused by the water and slurry mix travelling over the internal beading at the weld joints which is encountered when HDPE lines are heat welded.
The pipeline is open to the sea at the end, with the exit of the pipe approximately 130 meters below the surface. A system had to be devised to safely retrieve the pigs, so Dacon added pressure resistant buoyancy foam to the external bodies of the tools to allow them to float to the surface and be retrieved by a support vessel and divers.
The pigs are fitted with air activated GPS systems to ensure they cannot be lost and flashing beacons. The support vessel remains onsite above the end of the pipe via GPS, but the tools can move underwater due to ocean currents and surface several hundred meters away. The tools are all float tested before every inspection run. Given that the tools weigh approximately 550 kg, it takes a considerable amount of buoyancy force to keep them afloat.
The system has now been developed for smaller diameter HDPE pipes, and HDPE internally lined steel pipes where both the liner and the steel can both be measured using advanced algorithms and analysis tools. All data is downloaded immediately and scanned for major defects, a site report can be issued within a few hours highlighting any major anomalies in the pipeline, although full analysis takes longer and can normally be completed within two weeks of demobilization.