Problem Solving in the North Sea
By: Stefan Papenfuss, Vice President, Pipeline Resources
"The challenge to the client was to complete the inspection and integrity assessment of a low-flow production line with limited access in the North Sea. The fact that the line is offshore and subsea always creates another level of complexity compared to onshore operations,” explains Jason Tuer, Quest Integrity’s Technical Advisor for the project.
The pipeline inspection project presented two major challenges. First, the pipeline flow was exceedingly slow, by about a factor of ten. Quest Integrity Group overcame this challenge using the InVista™ ultrasonic in-line inspection technology. "Most of the ILI tools on the market cannot run at such a slow pace,” said Tuer, "but Quest Integrity’s ILI tool does. The inherent features of the technology allow the equipment to work extremely well in low-flow conditions.”
The second challenge was the vertical launch. Since the tool is very compact, it doesn’t take up a lot of space on the platform deck and it is very lightweight, which allows it to be launched from a vertical position with no specialized handling required. "Fortunately, no newly engineered, developed or designed tool was needed,” says Tuer. "Quest Integrity was able to complete the inspection and integrity assessment using the standard design InVista tool.”
Prior to engaging with Quest Integrity, the client set the scope of services for the project, which included geometry data on the line. The client assumed that would require a separate caliper inspection, which would have been difficult given the operational conditions. However, Quest Integrity’s ILI tool provides complete wall loss and geometry dataÂ, therefore there was no need to perform a separate caliper survey.
Quest Integrity traveled to the offshore platform and met with the client’s staff to begin preparations for the inspection. As part of standard operating procedure for offshore projects, Quest Integrity arrived in advance of the actual inspection start date. The early arrival allowed the project manager to witness the final stages of the cleaning campaign and assist in final cleaning tool configurations to ensure that the line was sufficiently clean prior to introducing the ILI tool into the line.
The tool was propelled at a pressure of 6/7 barg, which translates to a speed of 91.44 mm per second (0.30 feet per second). It traveled the entire length of the pipeline and was received at the wellhead platform receiver.
After determining that the recorded data was mostly free of anomalies associated with scaling, gas pockets or other foreign material, the data was downloaded and prepared for the preliminary API 579-1/ASME FFS-1 2007 fitness-for-service assessment. This preliminary report was delivered to the client within hours of the inspection.
Inspection Results and Fitness-for-Service Assessment
Following the field inspection data verification and preliminary report, the pipeline inspection data was analyzed for wall thinning and anomalies such as corrosion, denting, and ovality and the fitness-for-service of the pipeline was determined. The assessment was based on the longitudinal extent of thinning found in the pipeline and in accordance with a Level 2 Assessment described in Part 5 of the API 579 standard. The data can be viewed and assessed using LifeQuestTM Pipeline software, and the remaining strength factor (RSF) and reduced maximum allowable operating pressure (MAOPr) were also reported for the pipeline.
Table 1 represents a summary of the quality of data analyzed, the calculated RSFs, and compares computed MAOPr values for the pipeline to the maximum operating pressure provided by the exploration and production company.
Table 1: Summary of the Quality of Analyzed Data
The MAOP was provided as 12,400 kPa (1,798 psi). The pipeline was constructed of 3620.12 m (11,877.0 feet) of DIN 17172 StE 360.7 and 219.09 m (719.1 feet) of API 5L Grade X-52 carbon steel, 10-inch pipe with reported nominal thicknesses 15.1 mm (0.594 in.) and 20.6 mm (0.811 in.).
Highlights of the inspection report included:
· A total of 19 external and manufacturing related metal loss anomalies were individually identified in the inspection data. The minimum measured thickness due to external metal loss was 13.1mm (0.516 in.). Based upon a nominal wall thickness of 20.6mm (0.811 in.), this metal loss corresponds to a 36.4% wall loss.
· No internal metal loss anomalies were individually identified in the inspection data.
· A total of 17 dents in excess of 0.5% of the nominal outer diameter (OD) were identified in the inspection data. The maximum dent size was 2.2% of nominal OD and was located at 2966.22m (9731.7 ft.), between girth welds #2700 and #2710.
· The minimum reduced MAOP calculated, according to the Part 5 Level 2 assessment methodology in API 579, was 28,220 KPa (4,093 psi).
Based upon the inspection data, the pipeline was found to satisfy the API 579 Part 5 Level 2 fitness-for- service criteria for any maximum operating pressures equal to or below the listed MAOP of 12,400 kPa (1,798 psi).
Also, the inspection identified no dents on the pipeline in excess of 6% of the nominal OD, no metal loss in excess of 80% wall loss, and no dents with associated metal loss. The inspection revealed that the ASME modified B31G maximum depth criterion of 80% was not exceeded for the pipeline. The lowest Psafe value, based on the original B31G equation, was 30,588 KPa (4,436 psi), which was greater than the listed design MAOP of 12,400 kPa (1798 psig).
The report noted that the assessment calculations were performed without any future corrosion allowance and that all calculations stated in the report did not reflect tool tolerance. The final report recommended future inspection to monitor corrosion rates to the client, and that additional assessment to analyze the damage mechanisms or to more accurately quantify the stresses (e.g. Level 3 analysis) experienced by the thinned areas, could provide greater confidence for the continued safe operation of the pipeline.
"Using the InVista ILI tool, we were able to
gather an abundant amount of data,” says Tuer. "The project resulted in a
detailed final data set for the client’s service department.”
Benefits to the Client
By utilizing the InVista tool, Quest Integrity created additional value for the client because no front-end tool, line or operational modifications were required in order to run the tool. Heavier and longer ILI tools requiring specialized handling would have increased project costs and HSE concerns. Caliper tool runs would also be required if both the geometry and wall loss data could not be collected by the same tool in one run. In addition, a gauge tool run is traditionally a safety measure ahead of an ILI run to make sure the line is unobstructed, but due to the high collapsibility factor of the tool, it was not necessary.
"We could literally get onto the platform
without special permits or concessions that would have had to be made with
other larger or unwieldy tools,” explains Tuer. "The fact that the tool is
extremely lightweight and easy to handle means there is limited risk to
inspectors involved in the operation. This translates into an increased
emphasis on safety, because the operator did not have to use the type of
cumbersome tool that is traditionally used in pipeline inspections.” By utilizing
lightweight and compact equipment within confined spaces in an offshore
environment, operators can mitigate risk for in-line inspections.