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1、Addressing Challenging Environments Advanced In-Line Inspection Solutions for Gas PipelinesThomas Beuker1, Dr. Stephan Brockhaus1, Dr. Ralf Ahlbrink1, Michael McGee21. ROSEN Technology and Research Center, Lingen, Germany2. ROSEN USA, Houston, USAKeywords: 1. In-Line Inspection; 2. Corrosion; 3. Den

2、ts; 4. Stress Corrosion Cracking1 AbstractWhile the transportation of gaseous media in pipelines worldwide is a routine occurrence, the in-line inspection of these assets is more demanding than for liquid pipelines. The most common demands - either low flow and low pressure conditions or high flow a

3、nd high pressure conditions - require a more specialized configuration of the in-line inspection equipment as a prerequisite for high quality inspection data. Accordingly, low friction sealing elements and robust intelligent bypass valves are commonly incorporated into the ILI tool configurations to

4、 help guarantee a safe passage and optimal run conditions.Other demanding requirements for gas lines can include the utilization of liquid batches while using UT tools and accurately identifying pipeline features at unsteady and/or high velocities and pressures. More recently in response to these ne

5、eds, newly developed ultrasonic technologies are available to detect and characterize cracking without requiring a liquid coupling. Also, high-quality geometry inspection technology offers reporting of small dents and shallow internal corrosion even under highly dynamic run conditions inherent in ma

6、ny pipelines. Current in-line inspection capabilities and options related to gas pipelines are explored in this paper.2 IntroductionThe ultimate goal of any in-line inspection is to accurately determine the state of the asset. Reaching such an objective involves well matched ILI tools, knowledgeable

7、 analysts, and examining the structure for any anomalies or material defect which could lead to a deterioration of the asset. Based on the inspection results, prescribed measures addressing common threats such as mechanical damage, corrosion, and cracking are in place to help ensure a safe operation

8、 of the pipeline system. While these threats can and do occur in both liquid pipelines as well as in gas pipelines, the specific form of appearance and the frequency can be different.The threat of Stress Corrosion Cracking (SCC) is a good example. Although not specific to gas pipelines only, one nee

9、d only look to the extensive evidence from dig findings in both Canada and the United States to support the fact that SCC is as a predominant threat to gas pipelines 1. Another example is the so called top of the line corrosion (TOL). This phenomenon is normally not relevant for liquid pipelines due

10、 to the corrosion mechanism, however TOL is commonly found in gas pipelines located in mature gas fields. In these cases the pipelines will often include aggressive wet gas and/or contain droplets of condensed gas, both of which initiate the build-up of colonies of deep interacting pitting corrosion

11、 2.In keeping with the goal of best assessment using the above examples, it is realized that an adequate inspection technology must first be selected. Once an established technology is chosen, we must then focus on optimizing the inspection parameters to include a smooth and controlled travel throug

12、h the desired pipeline. Here the ILI tools mechanical design must address the specific demands of the specific gas pipeline to meet this expectation. It makes sense then that the most common demands - either low flow and low pressure conditions or high flow and high pressure conditions - require a m

13、ore specialized configuration of the in-line inspection equipment as a prerequisite for high quality inspection data. Therefore, to optimize the ILI tool configuration, low friction sealing elements and robust intelligent bypass valves are commonly incorporated into the ILI tools to help ensure a sa

14、fe tool passage with optimal run conditions.3 In-Line Inspection Run BehaviorIn-line inspection tools are typically pumped along with the pipeline product through the pipeline incorporating an arrangement of some type of sealing plane that sets-up a pressure differential between the front end and th

15、e rear end of the in-line inspection tool. Mostly independent from the selected inspection technology, the sealing plane often uses a specific configuration of disks and/or cups to control the runbehavior of the inspection tool during its travel. Once the resulting force of product overcomes the fri

16、ction between the inspection tool and the inner pipe wall, the tool will navigate in the direction of the product flow.When running ILI tools in pipelines with liquid service, the product-flow in the vicinity of the tool is identical to the tool speed as expected. This is due to the incompressible b

17、ehavior of liquids. Delving deeper into fluid dynamics we also see that the inertial mass of the liquid is preventing instantaneous changes of the speed profile. Consequently, changes in medium velocity are also experienced by the inspection tool in fairly close correlation. This is why pipelines wi

18、th liquid service generally provide smooth velocity run profiles.In contrast, gas is very compressible. The dynamic velocity of the ILI tool is coupled to the gas flow although objects such as weld penetrations, wall-thickness changes, installations, and even the inertial mass of the inspection tool

19、 can influence run speeds. The effects of these variables may even contribute in creating tool vacillations because of the changing localized gas pressures and the motion of the inspection device. So we must consider counteracting design measures to control the bypass flow across the tool and to con

20、trol the friction of the tool to achieve an ideal inspection environment.a. Controlling the Inspection SpeedIt is generally accepted that the ideal velocity range for ILI tools rests between 1 - 5 m/s (211 mph) using current sensor design and capabilities. Yet these limitations in many cases do not

21、reflect the actual gas flow velocities offered in transportation pipelines or as desired by the pipeline Operators. Speeds are much higher, even up to 15 m/s (34 mph) for instance. ILI vendors understand this and some have responded by developing ILI tools equipped with an active bypass valve allowi

22、ng the gas in the pipeline to flow at a higher speed than the tool would normally travel. Such a device attached to a 40” MFL tool is shown in Figure 1.Incorporating active bypass valves allows appropriately equipped ILI tools to inspect pipelines without requiring a substantial reduction in product

23、ion rates. The simple design is analogous to a thermostat at home. In effect, sensors measure actual velocities, set values and regulate by adjusting the control device accordingly. In the active bypass system, velocity readings are measured from odometer data and the onboard electronics execute inc

24、remental adjustments affecting the valve position. A basic control scheme is presented in figure 2. In this case the target velocity for the tool had been set to 2 m/s (4.5 mph).Figure 1: 40” In-line inspection tool. Magnetic Flux Leakage implementation for detection and sizing of axial-oriented fla

25、ws. The tool is equipped with an active speed control unit at the front. The bypass vale is in full open position.The maximum differential speed difference between the gas flow and the inspection tool depends mainly on the available cross sectional area of the bypass system. The varying valve positi

26、on regulates tool velocity similar to a sail on a boat, capturing more or less volume depending on the needs. Other constituents of tool velocity include tool friction, gas pressure and specific medium characteristics. All variables must be considered as a whole prior to inspection. A comprehensive

27、review of the relevant pipeline parameter including the consideration of finite element modeling to avoid turbulent flow conditions is referenced in 3.Figure 2: Control Scheme for an active bypass valve system. The valve position is triggered by the tool speed. In this example, the inspection speed

28、has been set to 2 m/s (4.5 mph). Once the gas flow exceeds 2 m/s, the active bypass valve opens making incremental adjustments to help keep the tool speed constant. At a flow speed exceeding 5 m/s (11 mph), the active bypass valve is fully open and therefore the tool will accelerate proportional to

29、the gas flow.b. Controlling the Tool DynamicsIn addition to maintaining a predefined inspection velocity independent from a higher gas flow, the active bypass valve is also used to diminish speed fluctuations caused by oscillating differential pressure across the tool. In these instances, the valve

30、position is dependent on the measured differential pressure and responds to support a smooth run with minimal variations in velocity. The corresponding algorithm has been developed based on a simulation model also discussed and presented in 3.The efficiency of the control algorithm in practice can b

31、e demonstrated by a speed profile obtained in two subsequent runs (Figure 3).Figure 3: Comparison of inspection runs in gas pipelines with and without active speed control. Without the ability to equalize the differential pressure across the inspection tool, the inspection speed is well above the ta

32、rget speed for a period of 1-2 Km.The first run (blue) has been conducted without speed control while the subsequent run (purple) incorporates active speed control. From the graph it can be seen that the tool stopped at two installations correlating to the log distance just before the two velocity p

33、eaks. After a sufficient back-pressure is established the tool speeds up out of each installation accelerating to a high rate.Without speed control the differential pressure across the tool requires 1-2 km (1.6-3.2 mi) of travel before equalizing and resuming normal velocity. With speed control and

34、once the tool begins to accelerate above the preset 2 m/s (4.5 mph), the valve position opens allowing a fast equalization of the differentialpressure and achieves the target speed within only 0.050 km (0.08mi). We can see that utilizing speed control in this example represents a 95% reduction in th

35、e deceleration distance as compared with not using speed control 4.c. Reduced Pressure and Flow ConditionsThe performance of the speed control units pressure equalization is further influenced by the overall friction of the in-line inspection tool against the surface of the internal pipe wall. The f

36、riction can actually be beneficial as usually found during the inspection process of common gas pipelines with an operating pressure of 30 bar (435 psi). When the tools drag offsets the gas flow past a certain level, this in turn permits a higher gas flow during ILI.However, in cases of reduced pres

37、sure and flow conditions the scenario is very different. Due to a reduced absolute pressure, a high differential pressure across the inspection tool cannot be achieved. Consequently, low pressure applications require a low friction configuration, although the sealing capabilities of the tool must no

38、t be compromised since the low flow pipelines require a “zero bypass” drive system.From an inspection point of view, low pressure (PL) pipelines are divided into two categories with different pressure ranges for the first category adaption-kits are applied that adjust the tools configuration to the

39、established operational condition and the second category simply requires a specially optimized tool configuration:Low Pressure Cat. 017 bar PL 15 bar(100 psi PL 218 psi)Low Pressure Cat. 0215 bar PL 30 bar(218 psi PL 435 psi)Cat. 02 Tools are also used to cope with low flow rates (velocity =VL):Low

40、 Flow Condition0.5 m/s VL 1.0 m/s(1.1 mph VL 2.2 mph)The most common inspection method for measuring corrosion uses magnetic flux leakage technology (MFL). In short, a strong magnetic field is applied to the pipe wall from permanent magnets coupled beneath wear brushes that provide protection agains

41、t debris and damage inside the pipe during an inspection survey. Any deviation from the established nominal magnetic field is used to identify wall loss using the tools sensor readings. While the brushes serve a valuable purpose they also increase the friction between the pipe wall and the inspectio

42、n tool. One common way to reduce this drag effect in low pressure pipelines is to incorporate non-magnetic rollers instead of standard brushes (see figure 4).Figure 4: 12 Intelligent In-Line Inspection tool for low flow / low pressure pipelines . The friction between the tools cup material and the p

43、ipe wall is minimized which supports smooth run performance. The different cup material is color coded to avoid confusion (red, black, & blue).Utilizing low flow / low pressure designs (LF/LP) lessen the ILI tools drag resistance and create a transition from static friction to dynamic friction. The

44、wheeled magnet unit is obviously easier to move than a design incorporating standard brushes, and achieves friction reductions by as much as a factor of 2.Particularly relevant for smaller diameter inspections (16”), the selection of sealing and drive cups for the inspection tool is critical. The cu

45、ps are designed for minimal but constant friction over the course of an ILI survey. This is key because any sizeable change in friction caused by variations in the internal bore, such as girth weld penetrations or non-transitioned wall-thickness changes could cause the tool to temporarily stop until

46、 the back-pressure overcomes the static friction of the tool. This can result in oscillating velocity changes, though with proper cup selection this behavior is mitigated.Three different cup designs are presented in Figure 5 below. Besides the standard design which is intended to carry high loads, a

47、 low friction design and a wheel design is shown. The low friction design is incorporated into the sealing components of the tool, while the wheel design carries the additional measurement and electronic units. The low-friction and wheel design are demonstrated in Figure 4.Figure 5: Dynamic friction

48、 force demonstrated for the three different cup designs (red, black, & blue).The friction behavior of the wheel design illustrates the preferred low change in friction force as a function of the wall thickness. Still, at higher wall thickness (16mm in this case) the friction force will eventually in

49、creases at a disproportionate rate. So similar to the other cup configurations, the application range with regards to the wall thickness should not be exceeded during an inspection beyond the certain limitations. The pipe bore must be known in advance before selecting the optimized tool set-up for l

50、ow pressure, low flow pipeline conditions.4 In-Line Inspection Characterization of Pipeline AnomaliesThe ultimate goal for an in-line inspection is the detection and characterization of anomalies in pipelines that originate during operation and may develop to an integrity threat. The latest developm

51、ents and adaption of more sophisticated inspection technologies have enlarged the available inspection options for difficult and challenging environments.Some improvements have been made for the better assessment of dents by means of an innovative eddy current based geometry sensor. Similar technolo

52、gy has also been optimized for very precise sizing of shallow internal corrosion where before the well-known robust Magnetic Flux Leakage technology was standard for these types of corrosion assessments.In addition, a novel technology using the electromagnetic excitation of ultrasonic waves is now a

53、vailable for the detection and characterization of cracking in gas pipelines, thereby negating the prerequisite of a liquid coupling normally required for UT sensors. The following section elaborates on these unique approaches and applications of for gas pipelines.a. Dents and Pipeline GeometryInspe

54、cting a pipeline to determine the geometry of the internal bore is routine. However, reliably characterizing dents compounded with the known challenges of gas lines requires some additional considerations.The traditional mechanical caliper tool design using purely mechanical movement of the caliper

55、arm typically leads to inspection speed restrictions because of the dynamic behavior of the arms under these run conditions. Above a critical tool speed, the caliper arm will lose contact with the internal surface of the pipe- line and also at low speeds abrupt changes at the internal pipe surface m

56、ay not be monitored correctly 5.These mechanical designs try to overcome lift-off difficulties by using a lightweight design. But these configurations are quite fragile. So, these systems do not sufficiently extend the operational range of this inspection task.A method solving this problem is provid

57、ed by a adding a touch less measurement component. To both achieve a high measurement accuracy and satisfy circumferential resolution, a mechanic caliper arm system equipped with an electronic distance measurement were combined to a create a mechatronic solution (ref. Figure 6).Figure 6: The picture

58、 shows the concept of having a touch less electronic sensor integrated inside the sensor head and a position sensor attached at the bottom monitoring the mechanical position of the sensor arm.The picture shows the touch less electronic proximity sensor integrated inside the sensor head and an arm position sensor attached to the bottom that monitors the mechanical position of the sensor arm. The touch less electronic sensor is based on eddy current technology (EC) and used to compensate measured data obtained from the dynamic behavior of the caliper arm. In operation, the unwanted iner