For several years the ROSEN Group has worked closely with SNAM’s S.p.A. (an Italian natural gas infrastructure company) to support the integrity assessment of their distribution network, and have asked us to provide an inspection solution for a low flow pipeline. In order to establish suitable conditions for a standard ILI run, SNAM depends on the product’s actual consumption.
A complex planning process would have been needed to schedule an inspection service in such a way that the required operating conditions were available, while the gas supply remains guaranteed. However, in the case of this specific 16” gas pipeline these conditions where not attainable, ruling out the possibility of applying a standard approach. Ultimately, ROSEN’s Challenging Pipeline Diagnostics Division was faced with the unique challenge of inspecting a pipeline with a gas velocity as low as 0.1 m/s.
The ultra-low operating conditions are created by the various offtakes distributed along the pipeline to supply product to several customers. Therefore, the gas velocity is stronger at the beginning of the line but then reduces after every offtake. After the offtake at kilometer 22 the velocity reduces to an expected 0.1 m/s, which is far below the 0.5 m/s that traditional MFL tools require.
One inspection option for SNAM would be to increase the velocity to 0.5 m/s and apply a standard approach. However, this would require the excess gas to be flared off at the end of the pipeline. Implying not only an expensive loss of product, but also a significant environmental impact. This means the permits would not easily be obtained for such an operation.
The pipeline operated at approximately 45 bar pressure, hence excessive speed variations typically observed during the inspection of low pressure lines were not to be expected. Instead the risk was a stationary tool caused by insufficient sealing. Due to the low flow conditions even the smallest leakage over the tool can create an unwanted by-pass and cause the tool to stop.
The ROSEN solution therefore concentrated on designing a customized pull unit which provided optimum sealing in all diameters present. In addition, a low friction magnetizer was used to reduce the risk of a stationary tool due to bypass. Next the total inspection time was extended by installing additional batteries in the pull unit.
In the past, corrosion was measured with electrodynamic coils located between the MFL yokes on a primary sensor ring. These coils require movement to generate a voltage that can be measured. Since the MFL principle is based on volumetric measurement, a secondary ring was used discriminate between internal and non-internal defects. If a defect was recorded by both sensors it was located on the inner pipe surface, and if it was recorded by the primary sensor ring only than the feature was non-internal.
Today, most ILI vendors use hall sensors to detect corrosion. The hall sensor itself can measure in a static case. However, depending on the electronic connection between sensors and main electronic board a speed limitation may still apply.
ROSEN’s Digital extended Density Sensor has no lower speed limit. For the internal and non-internal discrimination of defects ROSEN uses an eddy current based system. Also, this system is able to measure in a static case and is integrated in the primary sensor carrier.
As with every Challenging Pipeline Diagnostics project one of the key golden rules is: what goes in must come out. ROSEN’s solution officers were confident that the tool would come out. However, given the innovative nature of the inspection it was agreed that, in the event of a stationary tool, a temporarily increased flow would be created by depressurizing the line downstream of the tool. This procedure is commonly known as ‘kicking’.
The Field Work
In order to confirm free passage for the ILI tool to be run a gauge tool combined with a Pipeline Data Logger (PDL), transmitter for accurate tracking and locating of equipment, was run first.
Assessment of the gauge tool confirmed free passage for the MFL inspection system . Furthermore the differential pressure of the PDL revealed no unexpected run behavior.
However, the total run time of 168.15 hours revealed that the flow conditions were more challenging than expected: the gas velocity in the last 6.5 km was as low as 0.01 m/s.
ROSEN accepted the challenge nevertheless and decided to launch the MFL system. It was received 145 hours later. The runtime, as well as the system’s condition indicated that excellent sealing was achieved. Unfortunately, the data was not complete due to electronic failure resulting in sensor loss and a rerun to deliver the contractually agreed data quality was recommended.
On February 16th, 2015 the ROSEN field crew returned to Italy to launch the MFL system for the second time. Because of a higher gas consumption especially in the first 22 km, the system was received two days later, on February 18th, 2015. Figure 2 shows the velocity profile of the system over the complete line. The increased velocity over the first 3 km directly correlates with the higher gas flow. From km 3 onwards the system did not exceed the specified maximum velocity of 3 m/s.
For the last 6.5 km the average velocity was 0.08 m/s. Finally, towards the end of the line the velocity further reduced to below 0.05 m/s. As can be seen in Figure 3 the overall run behavior in these extremely challenging conditions was very impressive.
The MFL system was received in good condition and the inspection was completed within the pre-agreed contractual requirements.
With the successful inspection valuable integrity data has been collected which will allow SNAM to continue the safe operation of their pipeline. Compared to a previously existing solution, ROSEN’s low flow solution did not impact the operation of the pipeline at all, while no compromise was made on the inspection quality.
In addition, the in-service inspection allowed greater scheduling flexibility as it could be carried out with less dependency on production planning.