Loading/unloading lines are pipeline assets in the oil and gas industry that often present unique subsea operating conditions. Although these pipelines are distinctive in many ways, they are also subjected to metal loss threats, as is any pipeline. Identifying the best methods for detecting and managing metal loss threats for loading lines requires not only knowledge of technologies but also of best practices for conducting any inspection work. This was also the case with a 24” loading line in the Colombian Caribbean. Together with the operator, the ROSEN group created a custom solution for this challenging asset to identify any metal loss and gain knowledge of the mechanical condition of the asset. The team would face a variety of challenges. An additional goal, and also what brought along much of the challenge, was to identify the precise location of the pipeline using submarine geo referencing techniques.
Subsea Geo Referencing
The precise location of a pipeline may seem trivial, but it can have exponential cost-saving effects if in fact a feature was identified during inspection and repair work was deemed necessary. Achieving an accuracy range of one meter is very much possible, but detailed steps must be considered. Challenges faced specifically during the geo referencing aspect of the inspection included adjusting calculations based on a positioning system to correct for factors such as wave speeds created by changes in water density, streams, tides and depth along the entire length of the pipeline. These factors would all negatively affect the quality of data transmission. In addition, these factors resulted in the modification of the technology used to identify magnetic field disturbances generated by the markers installed on the pipe.
Simply coordinating the logistics of such a complex project was a challenge in itself that; it involved installing subsea markers on top of the concrete coating of the pipeline. Low visibility conditions are often encountered by divers, as are seabed sediments partially or entirely covering pipelines. All in all, a combination of technologies was required for the geo referencing activities, which included highly accurate DGPS equipment and high frequency data links as well as underwater markers, beacons, transducers and other related subsea positioning equipment. During the subsea DGPS surveying process, the density, salinity, temperature and depth of the water is measured and input into the surveying software to assure that the proper correlation of sonar data and distances is utilized. For the work, coordination between divers and ROSEN surveying specialists was vital. For example, the positioning beacons were placed by divers at pre-determined distance intervals along the line, with equipment to provide reference points for later integration into the inspection data provided to the operator. As there is no GPS signal available under water to be used in high-accuracy DGPS surveys, the sonar equipment and related software provide pinpoint distances from the pipeline reference point to the rover equipment aboard the survey vessel. The surface vessel maneuvers in a specific pattern during surveying to collect distance data transmitted by the underwater equipment, and ROSEN specialists on board the vessel monitor, validate and compute the data in real time to accomplish the required DGPS position accuracy.
These complex activities, both in terms of logistics and calculations, allowed for an accurate identification of the pipeline’s center line, and, using this, an accurate location of the anomalies detected by the metal loss inspection.
Metal Loss Inspection
The collection of data for metal loss identification also does not end with the application of a measurement technology. The processes start with selecting the best technology for the job. In this case, Ultrasound was the choice as it conducts a direct measurement and is therefore extremely accurate. This technology applies an ultrasonic transducer to generate an acoustic wave directly in the sensor probe that propagates through a liquid medium and the pipe wall. Applying this technology not only assesses the wall thickness, it also enables the distinction between internal and external metal loss.
But this technology also presents its difficulties. The most significant one in this case was the need for a very clean pipeline. UT is very sensitive to debris, requiring an extensive cleaning program to be conducted before personnel could proceed with the ILI run. In this case, this was a multi-step process. It started by circulating the accumulated Naphtha in the store tanks through the 24” pipeline using four filter units. The filters were removed, cleaned and reinstalled after each subsequent cleaning run. The cleaning process was ultimately conducted by using four cleaning pigs with high-quality polyurethane discs, which provide optimal sealing capabilities for smooth and efficient cleaning runs.
Once the cleaning program was completed and on-site teams were confident all elements were sufficiently clean, the UT inspection was next in line. ROSEN had previously modified the UT inspection tool by adding tri-axial MFL sensors, which allowed for the identification and recording of the magnetic field disturbances emitted by the markers installed on the pipeline exterior, later to be appropriately filtered out of the UT data evaluation. For the propulsion of the ILI tool, the now cleaned Naphtha was once again applied, providing the liquid medium required for the measurement technology to work.
Despite the difficult offshore conditions, the activity was carried out during the tight operations window – provided by the arrival of tanker vessels. The process of gathering the Ultrasound data may have been the smallest element of this project, but the process of what came before – the accurate positioning and the in-depth cleaning – provided reliable confidence in the data. Not only did the inspection provide insights into the current state of the asset, it also allowed for the calculation of corrosion growth rates when combined with data from a previous inspection. This facilitates the optimization of the entire integrity program and the ideal management of the asset.