Inspecting the world’s longest subsea pipeline

Figure 1. Route of Langeled pipeline.

In the spring of 2008, ROSEN was selected by STATOIL to carry out a very important and challenging project: inspect the world’s longest subsea gas pipeline. The Langeled gas pipeline runs some 1,173 kilometers between Nyhamna in Norway and Easington in England (Figure 1). The job was to inspect this pipeline in a single in-line inspection (ILI) run. The Langeled pipeline is designed to carry more than 70 million cubic meters of dry gas per day, thus supplying the equivalent of more than 20% of the United Kingdom’s gas needs. The pipeline had never before been inspected in a single ILI run.

Design and operating conditions
The planned inspection was challenging not merely because of the pipeline’s unparalleled length but also on account of its challenging design and operating conditions. Langeled is a multi-diameter pipeline with high wall thickness. For the first 632 km (up to some kilometers after the Sleipner platform), the pipeline diameter is 42-in. before increasing to 44-in. for the remaining 541 km. Similarly, the pipeline’s wall thickness varies from 29.1 mm to 62 mm in the 42-in. section, and from 23.3 mm to 50.6 mm in the 44-in. section.

Although constant internal diameters (ID) are given in the specifications, the intermediate subsea installations (valves and tees) in the vicinity of Sleipner platform restrict tool passage. The ID is 1,016 mm in the 42-in. section and 1,066 mm in the 44-in. section. The minimum bend radius is 5D. The pipeline is coated with epoxy on the inside and asphalt and concrete on the outside. The pipeline’s operating conditions make the inspection task even more complicated: Langeled is characterized by high product flow velocity and pressure. Operated at design capacities, the gas velocity increases over the pipeline length from 4 m/s to 9.5 m/s, with associated gas pressures of 220 bar at Nyhamna and down to 75 bar at Easington. Corresponding gas densities vary between approximately 61 kg/m3 and 185 kg/m3.

Combining ILI technologies
Given the challenging nature of the project, STATOIL and ROSEN thought carefully about the most suitable technologies for the planned baseline survey. The two companies agreed to use ROSEN’s RoCorr·MFL/SIC. This in-line inspection tool combines the two complementary corrosion measurement methods: Magnetic Flux Leakage (MFL) and Shallow Internal Corrosion (SIC) with high-resolution geometry inspection. MFL, which is used for measuring relative wall loss, is a versatile and reliable method for determining the geometry of metal loss in pipelines. The second technology incorporated by the tool is SIC, an Eddy Current (EC)-based technology enabling absolute geometric measurements of internal corrosion defects.

The combination of MFL and SIC permit a close approximation of corrosion growth, providing an effective tool for monitoring the degradation process. Since both corrosion measurement technologies complement each other, the simultaneous application of MFL and SIC ensures an exceptionally high Probability of Identification (POI) for internal and non-internal discrimination, notably providing high accuracy in depths, length and width sizing of internal corrosion defects. When inspecting heavy wall pipelines in particular, the SIC measurement provides precise depth sizing results for shallow internal corrosion (detection threshold ? 1 mm) while at the same time assisting MFL defect identification and depth sizing. The new tool accomplishes this by permitting better distinction of individual pits in dense clusters due to its high lateral resolution in defect surface measurement.

Moreover, the simultaneous use of both corrosion measurement methods results in such high sensitivity as to make the combined method ideal not only for high-accuracy measurement of defects but notably early feature detection. Last but not least, high-resolution geometry measurement is provided by combining EC-technology-based touchless deformation measurement with systematic monitoring of the mechanical position of caliper arms. Full coverage of the internal walls of the pipeline is ensured by means of two inspection planes with a total of 108 caliper arms, plus the associated SIC contour-following proximity sensors.

Dealing with multiple diameters

Figure 2. Customized RoCorr·MFL/SIC in-line inspection tool prior to launch.

To meet the specific challenges of Langeled pipeline, a customized combo tool was designed, equipped in the front segment with MFL technology for general corrosion measurement. In the rear segment, the tool was equipped with caliper arms featuring SIC sensors for shallow internal corrosion and high-resolution geometry measurements (Figure 2). Before this specially designed RoCorr·MFL/SIC tool was launched, however, it was necessary to first ensure that it could definitely pass through all parts of Langeled pipeline.

For this purpose, a customized gauging tool was first sent through the Langeled pipeline. Featuring both a gauge and a bend plate, the design was adapted to the multi-diameter application to confirm the bend sizes and ID of the pipeline. To mark the special significance of the project, and to represent both the countries and companies involved, the tool featured the Norwegian and British flag as well as a “mailbox.” The “mailbox” was used by the chairmen of the Norwegian administration and the collaborating companies Shell and Gassco to send letters to their counterparts in England. The subsea mail safely reached its recipients.

Addressing high product velocity
To overcome the specific challenge of the high gas velocities in Langeled pipeline and to compensate for tool accelerations due to friction changes, the combo tool was equipped with a Speed Control Unit (SCU). The SCU enables the inspection run to be conducted at a pre-programmed target tool speed by providing controlled gas bypass to reduce tool velocity.

In controlling the bypass flow, different factors must be taken into account, since not only the tool design (e.g. bypass area, differential pressure) determines the capacity of the SCU, but also pipeline characteristics such as wall thickness, changes in diameter, coating type and gas density. Whereas a high tool differential pressure and a large bypass area enhance the performance of the SCU, high gas densities, as present in Langeled pipeline, have the opposite effect.

To prevent the front segment of the combo tool from being pushed through the pipeline by the trailing unit, the bypass area on the rear segment must be remarkably larger. Despite this large bypass area, the rear segment must still be capable of accommodating indispensable components (electronics, batteries, storage etc.). The specially designed RoCorr·MFL/SIC in-line inspection tool meets this challenge as follows: the flow circulates around and through parts of the tender’s tool body before diversion through the tool body at the front segment and final release through the SCU.

Since flow simulations indicate that the combination of high bypass velocities and high gas density lead to increased loads on the measuring unit of the rear segment (80 – 90 kg), the flow circulating around the tender must be systematically guided. Utilizing short caliper arms in combination with effective notches and holes strategically placed in the cups, as well as specially designed lamella fins between the rear cup and the measurement unit, permits partial diversion of the flow away from the measurement unit (Figure 3). With this measure, the load on the caliper arms was reduced by 60%, guaranteeing secure and reliable measurement, even though the space is additionally used to provide the required bypass.

If the tool is allowed to pass through the pipeline at product flow velocity, this may well mean that its speed is too high to warrant precise measurements, as in the case of Langeled. Whereas tool speed can be diminished by means of flow rate reductions, this results in costly throughput losses for the duration of the inspection. For both reasons, it was extremely important to overcome the high flow velocity challenge posed by Langeled without compromising either measurement accuracy or pipeline capacity. The tool velocities achieved during the actual combined ILI run which took place in August 2009 ranged from 1.5 m/s to 3 m/s, resulting in a total travelling time of 127.3 hours. The average flow rate during the inspection was around 40 million standard cubic meters per day.

Overcoming the distance challenge
Due the enormous length of the pipeline, tool wear was another major challenge posed by the Langeled inspection project. Because the tool’s differential pressure increases due to higher friction in the smaller section of the pipeline, this problem was exacerbated by the pipeline’s dual-diameter design.

To overcome this challenge, customized polyurethane cups were designed to limit tool wear during the passage through the 42-in. section while also providing sufficient sealing and carrying capabilities in the 44-in. section. Since the differential pressure of the tool is determined by the magnet forces applied at different wall thicknesses and by cup friction, the magnet strength was optimized for Langeled Pipeline conditions, thereby ensuring full magnetic saturation while limiting the differential pressure and hence cup friction and wear.

The total number of polyurethane cups mounted on the front segment of the combo tool provided three sealing planes, giving a certain safety margin with regard to cup wear and unintended bypass. Additional tool centralization to reduce cup wear in the first segment was provided by the magnet forces themselves as well as by a supporting guiding disc. In the rear segment, wheels were used to support centralization and to improve run behavior (Figure 4). The average differential pressures measured during the actual inspection were 2.0 bar in the 42-in. section and 0.8 bar in the 44-in. section. The tool constantly rotated and was in a good condition upon receiving (Figure 4).

Figure 3. Flow simulations; left illustration: with guided flow; right illustration: without guided flow

Conclusion
A customized RoCorr·MFL/SIC inspection tool was built specifically to overcome the great challenges posed by the length, multi-diameter design, high wall thickness, high pressure and high gas velocity of Langeled gas pipeline. The SIC measurement provides accurate depth sizing of shallow internal corrosion while simultaneously enhancing MFL defect identification and depth sizing through its high-lateral resolution of defect surface measurement. These combined systems make the RoCorr·MFL/SIC a very effective and reliable tool for monitoring corrosion growth rates. In general, the tool’s combination of these two complementary corrosion measurement methods ensures high sensitivity for early feature detection, a high POI for internal / non-internal feature discrimination, accurate sizing capabilities, and notably high effectiveness in the inspection of heavy wall pipelines. Overcoming all the exceptional difficulties posed by Langeled pipeline with ease, the RoCorr·MFL/SIC successfully completed an in-line inspection run on an unprecedented scale in August 2009.

Dr. Stephan Brockhaus, Dr. Hubert Lindner, ROSEN Technology & Research Center, Germany; Tom Steinvoorte, ROSEN Europe, The Netherlands; Holger Hennerkes, ROSEN, Switzerland; and Dr. Ljiljana Djapic Oosterkamp, Statoil, Norway