Dexon TechnologyDexon Technology PLC’s pipeline inspection department recently completed the inspection of multiple challenging 36” loading lines in South America. The inspection posed several challenges including the need for bi-directional metal loss and ovality inspection in heavy crude oil requiring precise subsea ILI tool tracking. Dexon’s research and development department fabricated a customised tool to accomplish the inspection requiring a complete overhaul of mechanical components.
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The Challenge
Heavy Crude Oil
– Ultrasonic waves exhibit distinct behaviour in heavy crude oil due to the unique properties of the medium. Ultrasonic inspection involves emitting high-frequency sound waves through a medium to the internal diameter of the pipe wall. Pipeline characteristics such as ovality, wall thickness, and cracking can then be determined by carefully analysing the echoes and changes in wave characteristics of the returned ultrasonic signals. Heavy crude oil is characterised by increased viscosity and density, causing ultrasonic waves to experience higher attenuation—meaning the waves progressively lose energy as they traverse the medium. This accelerated perpetual loss of energy means that a revised tool design is required to complete the inspection from the traditional transducer ring set-up.
Bi-Directional Inspection
– The loading lines ran from an onshore tank terminal and storage facility to an offshore single-point mooring (SPM) after passing through a subsea pipeline end manifold (PLEM). The presence of a PLEM meant the inspection was performed from a launcher at the tank terminal to the PLEM on the sea floor and then back to the launcher.
Accurate Tool Tracking
– The bi-directional inspection and termination of the inspection at the PLEM meant that extremely accurate tool tracking was required for each inspection. Allowing the flow to be reversed before the tool passes the return flow valve at the PLEM while also ensuring the full inspection length had been achieved. A tool becoming stuck in a PLEM will result in extensive PIG rescue operations and in extreme cases pipeline abandonment.
The solution
Heavy Crude Oil
– To overcome high attenuation in the heavy crude oil the ultrasonic transducers were raised up to the pipe wall on mechanical arms. Decreasing the distance the sound waves need to travel, maintaining signal strength.
Bi-Directional Inspection and Data Repeatability
– An advantage of bi-directional inspection is the ability to compare individual measurements from data sets collected over each pass. This allows Dexon to gauge the reliability of the tool in collecting data and the ability of the analysis department to accurately interpret and report.
The Combination of Heavy Crude Oil and Bi-Directional In-line Inspection
– While ultrasonic tools capable of inspection in heavy crude oil and single directional calipe r tools are nothing new to the industry. The combination of multiple pipeline characteristics is where standard inspections can get complicated. The inspection required the complete redesign of both tool's mechanical arms. Ensuring passage in both directions while maintaining a minimum bore reduction of 80% for the passage of dents, buckling and other geometric deformations, and guaranteeing the tool's arms would not buckle when encountering penetrating pipeline features on the return run.
Accurate Tool Tracking
– Three separate tool tracking techniques were used for the inspection to ensure smooth operations.
Diver-Assisted Hand-Held Tool Tracking
– Subsea divers were utilized with handheld tool tracking devices to pinpoint the exact moment each tool arrived at the PLEM, a support vessel was on standby on the surface to relay information to the team waiting at the pumping station to initiate the reversal in flow redirecting the tool back to the storage facility.
Tethered Subsea “Above Ground Markers” (AGMs)
– AGMs are regularly used with above-ground in-line inspections on land to monitor tool movement. An AGM unit is placed at specific locations along the pipeline. An onboard GPS unit provides real-time GPS coordinates while sensors onboard the tool and AGM generate a time and date stamp when the tool passes the unit. The time and data information paired with GPS location data is then used in combination with onboard inertial measurement unit data to generate accurate 3D XYZ pipeline route mapping. Subsea environments inherently impede GPS signals due to the water's ability to absorb and attenuate satellite transmissions. This absorption effect limits the accuracy and reliability of GPS positioning systems when attempting to locate and track AGMs attached to subsea pipelines. Modified tethered above-ground markers were used to track the tool at certain points along the pipeline. The GPS unit was installed onboard a support vessel anchored above the pipeline while the tool sensor was placed on the pipeline.
Acoustic Tool Tracking
– Acoustic tracking was also used to provide a third point of location data. Acoustic tracking involves the use of underwater microphones placed on the external pipe wall, allowing teams to listen for sounds generated as the tool moves through the pipeline.