Ultrasonic in-line inspection enhances deepwater pre-commissioning

Mark Slaughter

January 15, 2014

New technology Improving baseline survey accuracy was required to respond to challenges associated with Israel’s Tamar project.

The cost of deepwater pipeline repair makes inspection accuracy critical to assessing pipeline integrity. Advanced ultrasonic wall measurement (UTWM) technology was recently used to achieve a new level of baseline survey accuracy in a Mediterranean pipeline project. Conducted in conjunction with precommissioning operations, the in-line inspection (ILI) also helped eliminate logistical and scheduling constraints for overall project success. These deepwater operations are logistical and technical challenges that typically require a significant amount of vessel time, support, and budget. For the Tamar pipeline project, a new Weatherford solution was success- fully executed using ultrasonic in-line inspection tools and specialized subsea commissioning technology to mechanically displace and introduce pipeline fluids.

Image Caption: A topside pipeline  launcher receiver at Tamar

Photo coutesy of Weatherford

Deepwater challenge

The long-distance, deepwater Tamar pipeline project for Noble Energy is a subsea gas production and transportation system that connects the deepwater gas field in the Mediterranean Sea to an offshore receiving and processing platform linked to the existing Mari-B Platform.

Gas in the Tamar field comes from five high flow rate subsea wells produced through separate infield flowlines to a subsea manifold. Dual subsea pipelines transport production from the subsea manifold approximately 149km to the Tamar Offshore Receiving and Processing Platform. The processed gas is delivered to the existing Ashdod Onshore Terminal (AOT) for gas sales into the Israel Natural Gas Line (INGL).

Weatherford’s Pipeline and Specialty Services (P&SS) group was contracted to provide the pipeline pre-commissioning and inspection, including tieback pipelines, monoethylene glycol (MEG) pipelines, infield flowlines, gas and condensate injection pipelines, Tamar sales gas export pipeline and utility pipelines. Integrating these services through a single contractor was key to reducing logistical and scheduling constraints for overall project success.

The Tamar project’s challenges and solutions involved subsea flooding, testing and MEG injection; dewatering, MEG conditioning and nitrogen purging; and baseline inspection of the system’s 16-in. tieback. The inspection was suc- cessfully conducted using Weatherford’s latest generation of highly accurate UTWM tools.

Pre-commissioning operations were successfully conducted using Weatherford’s Denizen subsea pre-commissioning system. Flooding, cleaning, and gauging the twin 147km x 16in. pipelines were completed from a vessel at the shallow end of the 240m to 1700m water depth run.

In-line inspection surveys were conducted during the flooding operations. A caliper tool was pumped to verify minimum bore, followed by a UTMW tool for the wall thickness baseline survey. After the inspection, dewatering operations were conducted for all 5 km of the Tamar in-field and tieback pipelines.

Tight scheduling constraints for the subsea launch presented a challenge for the 16in. UTWM in-line inspec- tions. Normally, there would have been sufficient battery life to run the inspection tool. However, in this case, the time needed for a subsea launch required a delayed activation.

The ILI tool first had to be inserted into the pipeline launcher receiver (PLR) onboard the vessel. A vessel crane moved the launcher with the ILI tool to the pipeline end manifold (PLEM) and a hydraulic lock secured the pipeline end termination (PLET) to the pipeline. An ROV was used to turn the subsea valves and launch the pig.

The time consuming process increased the risk of delays that could drain battery life and cause a failed run. To account for the time and unforeseen delays, a two- hour window was built into the schedule. This resulted in a 12-hour delayed activation from the time the tool was inserted into the PLR on board the vessel. The delay was programed into the system and the inspection was successfully conducted.

Ultrasonic inspection

Non-destructive ultrasound testing has been used for in-line inspection since the 1980s. The technology measures wall thickness based on ultrasound compression waves directed into the pipe wall. Ultrasonic transducers positioned 90° to the pipe wall use an impulse-echo mode to transmit an acoustic wave and receive return echoes. The echoes represent the locations of the internal and external pipe wall, and other metallurgical anomalies such as laminations.

Image Caption: A skid mounted remote logger

Photo courtesy of Weatherford

A UTWM baseline inspection identifies and classifies non-injurious signals such as mid-wall laminations and other mill-related anomalies; a baseline corrosion survey provides wall loss sizing data. Greater accuracy is important when assessing anomalies, assigning risk, and prioritizing maintenance and expenses. Accurate anomaly classification and sizing is particularly valuable when comparing baseline data to future inspection results and integrity measures such as engineering assessments and determining growth rates. For deepwater subsea lines, where normal onshore nondestructive examination (NDE) validation practices are cost prohibitive, accuracy is key to managing costs.

Compared to magnetic flux leakage (MFL) tools, ultrasonic technology yields better sizing accuracy in determining wall loss and pipe wall thickness. This is because ultrasonic pulse echo physics are a more direct measurement of wall loss. But in some cases, MFL is a better solution because it can be more forgiving of dirt, debris, rough internal pipe surfaces, and waxy liquids. This necessitates a comprehensive pre-inspection assessment prior to selection of the appropriate technology.

Accuracy is important Accurate measurement of wall thickness directly influences the calculation of a corrosion feature failure pressure. MFL tools do not typically measure wall thickness but instead infer it from several sources, including API pipe specification, pipeline construction data, and/or estimated variations in the magnetic field. This provides only a relative assessment due to pipeline data inaccuracies, or difficultly in obtaining data because of asset ownership transfers, unavailable data, or unrecorded pipeline reroutes and modifications.

Inferred measurements also do not consider wall thickness tolerances from the pipe mill. As a result, an MFL corrosion wall loss depth measurement depends on a relative measurement of the pipe wall, which decreases the sizing accuracy beyond the normal in-line inspection (ILI) tool sizing tolerance, because in addition to toler- ances associated with tool anomaly sizing, there are also tolerances associated with the actual pipe spool wall thickness from the mill.

Acceptable tolerances from the mill can be as high as ± 10% for pipe wall thick- nesses between 5 mm and 15 mm in welded pipeline. Tolerances for pipe walls greater than or equal to 15 mm are ± 15% in welded pipe. These pipe mill tolerances and the high corrosionanomaly sizing tolerances of an MFL tool mean the calculated failure pressure from an ILI survey can be significantly over or under as the result of sizing inaccuracies caused by quantifying depths as a per- centage of the assumed wall thickness.

Greater corrosion sizing accuracy also provides better data to feed an assessment standard, such as B31G, modified B31G, or RSTRENG effective area assessment, the preferred method for determining the remaining strength of the pipe. Of the three, RSTRENG effective area assessment has the most accurate results based on actual versus predicted burst pressure tests.

New sensor technology in current Weatherford UTWM devices helps enhance detection and accuracy capabilities to limit the occurrence of echo loss due to adverse pipeline conditions. This loss has been demonstrated over many years of experience.

Engineering tests and field work data analysis to API 11636 standards have shown the new sensor technology improves sensitivity and reduces signal degradation. The advance is critical to achieving successful deepwater subsea baseline surveys. The same sensor tech- nology is used for in-line crack inspection with accurate sizing results that can be used for API 5797 integrity assessment methodologies.

Deepwater inspection solution Deepwater in-line inspection using ultrasonic technology provided the Tamar project with the accuracy demanded for subsea operations. Conducted in concert with the pre-commissioning, the inspection ensured pipeline integrity while helping to achieve high levels of logistical and scheduling efficiency. OE

Mark Slaughter is Pipeline and Specialty Services Global Product Line Manager, In-line Inspection at Weatherford in Houston. He has 24 years in the oil and gas industry with expertise in Product Line Management. Slaughter earned a Bachelors of Business Administration, Accounting and Information Systems at The University of Texas at San Antonio, and an MBA from University of Texas at Tyler.