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Condition Monitoring of Inaccessible PipingC.H.P. Wassink, M.A. Robers, J.A. de Raad, T. Bouma
Röntgen Technische Dienst bv, Rotterdam, The Netherlands
Standard non-destructive inspections such as ultrasonic wall thickness measurements can be obstructed by various objects and attachments, whether functional or structural. Examples of obstructions for the inspection of components that are potentially subject to corrosion are supports, road crossings and insulation.
Two recent developments, Long Range Guided Wave Piping Inspection and RTD-INCOTEST that provide screening of such inaccessible sections of pipe are discussed. The first, an ultrasonic technique, inspects stretches of pipe 5-50 m in length from a single location. The other technique, RTD-INCOTEST, is a pulsed eddy current tool to detect wall loss in steel components without removal of the insulation or direct surface contact.
These screening tools provide information for condition monitoring and planning of further (future) action. They can result in considerable cost reductions for the plant owner, or enable inspection of areas that would otherwise not be inspectable at all.
Potential applications and field experience using these techniques are discussed.
Inaccessible places, like insulated pipes, pipes at supports or at road crossings, are the locations were corrosion often is more likely to strike. While this may not be a place where inherent risk is higher, ways to assess these (like opening, lifting or digging them up) are often costly or time consuming. This means that in some cases they are left uninspected for a long time.
Some of the new non-destructive testing methodologies are screening tools with characteristics specifically applicable to inaccessible piping. These are intended to obtain information on large areas in little time. At this stage detection of a corrosion area is more important than obtaining an accurate measure of the remaining wall thickness. Still, these tools give an indication on severity, making them suitable for a monitoring approach.
Monitoring tools are suitable to keep a closer look at the locations that are of interest because of their susceptibility to corrosion or a known defect area. The information obtained on the condition and its history can assist in prioritising and planning maintenance at these locations.
Screening tools aim at detecting defects in a high inspection volume per day, mostly while working on stream. An important parameter required by maintenance engineers for assessing a pipe's condition is often the remaining wall thickness. This quantitative information can be used in e.g. strength calculations and standards to determine the maximum allowable operating pressure. But first relevant areas have to been detected.
Screening can be used in two ways that look similar but derive from a different approach. The first is maintenance selection. The NDT method, by screening 100% of the area of interest, identifies the places were something is out of the ordinary. These places will be selected for follow up inspection or maintenance. The second is qualification. The NDT method will identify places that are in a good condition. These places will not be included in near future maintenance, or with a lower priority. Both these approaches primarily need a high probability of detection and low false call rate, while sizing is left to other, more local methods.
For inaccessible areas both these approaches can be used. For instance all supports under a pipeline can be inspected and suspect supports are indicated or a full piping system is screened periodically, leaving good stretches out of closer evaluation.
One of the advantages of screening can be a reduction of the total cost of maintenance. This has several reasons. In some cases less actions have to be taken or less preparation is needed due to the selection offered by the screening result. Furthermore screening can result in a decrease of down time, because the volume of inspection has been larger, and there is a higher possibility that defects have been found prior to shutdown. Moreover, in price per inspected volume these tools are very competitive. Thus they offer a low cost alternative to inspect large portions of installations, rather than spot checks. Looking at the total involved cost or lost income these screening methods offer a new opportunity in modern day maintenance.
Monitoring can be defined as the continuous or periodical inspection of a certain object. The period between succeeding inspections can be anything from days to many years. Monitoring requires a relatively cheap, reproducible inspection. Another prerequisite is that the results of succeeding measurements are stored in a format that can easily be accessed and compared over time with newly obtained data. Obviously, this format must be easily understandable for someone who is not familiar with the method used to obtain the results.
Monitoring has several advantages. First of all, it means that a record of the current condition and the history of piping is available. This information can be useful in planning and prioritising maintenance. Secondly, the overall sensitivity of inspection for processes like corrosion can be increased. Some of the new digital technologies have a good repeatability and are less operator dependent for the measurement. As a result of this, a comparison of two measurements on the same location can easily be made. The change over time of a known defect can be monitored. This gives the ability to act on growing defects, while leaving the non-progressing areas as they are.
One of the new NDT services RTD provides is Guided Waves Piping Inspection. The technology used for this is also known as Long Range Ultrasonics or Lamb waves. Lamb waves actually is the specific name for guided waves in plates.
The principle of Guided Waves Piping Inspection is based on an ultrasonic pulse being sent through the pipe around the whole circumference. Because of the excitation around the whole circumference, there is no geometric spreading of the wave and thus low attenuation of the sound travelling along the pipe. In this way inspection ranges can be achieved of 5 - 50 meters along the pipe from a single probe position, in both directions. The practical range is usually around 20 meters, in both directions.
|Fig 1: Guided Waves equipment with inflateble ring in the field||Fig 2: Data representation: Note the schematic of the pipe over the chart|
The tool is operated by placing a probe ring around the pipe at a location where it is clean and accessible. This probe ring, linked to electronics and a computer, will excite the pipe with a low frequency ultrasonic guided wave. By processing all the reflected signals a pseudo-A-scan representation is obtained indicating features and defect areas along the pipe.
The presence of the pipe-features like welds and welded attachments makes it easy to overlay separate measurements, because defects can always be reported relative to a specific geometric feature. In monitoring this can be helpful if the exact measurement location can not be reproduced on a repeated measurement. It is possible to distinguish geometric features from defects.
The combination of a long inspection range and a short measurement time per location (5-15 minutes per location) make this tool highly suitable for a screening approach. Under field conditions a productivity of up to 7 measurements per hour has been achieved in a pipe support screening action, which can mean about forty supports inspected per hour.
One of the screening programs RTD has done was the screening of piping under a jetty. In total, about 4 km of piping was inspected in 13 working days, under conditions that would have made inspection with any other method very hard. The piping involved was suspended over seawater, with support locations every 6 meters. Although the original scope primarily called for the inspection of these support locations, the result was a prioritised mapping of all indications found on all of the piping. The result presented to the client was a schematic drawing of the pipe, with the relative axial position of known features and corrosion indications, as well as report sheets of every measurement.
|Fig 3: Jetty piping inspected with Guided Waves||Fig 4: Guided Waves equipment with fixed ring at a road crossing|
|Fig 5: RTD-INCOTEST equipment||Fig 6: Principle of operation RTD-INCOTEST|
The area over which the measurement is taken is referred to as 'the footprint'. Probe design is such that the magnetic field focuses to a minimal area at the surface of the object. The result of the INCOTEST measurement is an average reading over this area, which makes the tool suitable for rapid detection of corrosion areas, like Corrosion Under Insulation (CUI). Detection of more irregular corrosion types like pitting is not reliable with this tool.
Although originally developed for CUI detection INCOTEST is well suited to detect internal erosion like Flow Accelerated Corrosion, again without removal of the insulation. But also detection of corrosion through coatings, concrete and marine growth were identified as good applications, as well as surveying heavily corroded pipes on which grinding (needed for UT measurements) was not safe. New applications are constantly emerging, as people realise the big benefits from this technology.
One of the attractive features of INCOTEST is its insensitivity with regard to coupling the signal to the object. Holding the probe at the right location is sufficient, so with a minor amount of training and under tough circumstances the measurement can be performed. This has led to applications e.g. on offshore riser pipes, using rope access or with a probe attached to a fishing rod.
This non-contact characteristic makes it possible to detect corrosion on high temperature surfaces without many probe adaptations. A simple thermal shield protects the probe from extreme temperatures allowing measurements up to +240ºC and up to +500 ºC on large wall thickness (>15mm). Above this temperature the reduction of the magnetic permeability in the object prohibits the use of INCOTEST.
In general on piping an inspection grid is laid out along the pipe or in areas of specific interest. This area of interest can for instance be indicated using knowledge about the installations and the corrosion processes or by a previous survey using Guided Waves Pipeline Inspection. Typically a measurement of a single gridpoint is done in five seconds and up to 1000 measurements per day can be taken under favourable (field) conditions.
Two characteristics make INCOTEST suitable to monitor inaccessible piping. There is no need to remove insulation or coating, which reduces cost and increases the inspection speed. The other characteristic is that coupling the signal to the object is not critical. This allows the technique to be used under uncommon circumstances.
The reproducibility of the tool at a single location is about 2%. Field experience in a monitoring situation showed that keeping track of the exact measurement locations is of major importance; as paint can be washed away and the insulation or sheeting material is not necessarily fixed to a location on the pipe.
Flow accelerated corrosion (FAC) can cause internal thinning of feedwater, condensate and heater drain carbon steel piping. These objects often are insulated. If the operating conditions have been such that FAC can be expected, an on stream screening tool like INCOTEST offers the opportunity to select the locations where the insulation should be removed first for a closer UT inspection. Together with partners in the power industry this application has been evaluated and is currently succesfully applied. Mainly those areas that are most susceptible to this type of degradation are inspected, e.g. downstream of flow control valves, reducers and bends. INCOTEST indicates which of these areas is affected significantly and if required a follow up action can be set-up.
|Fig 7: INCOTEST operation on an insulated bend||Fig 8: INCOTEST measurement on rough surface|
Screening and monitoring of installations requires different approaches and techniques than usual in maintenance inspection. Inspection speed, overall cost and data handling are key issues to be dealt with.
Two examples of new screening tools have been described, specifically looking at their application for monitoring inaccessible piping:
Guided Waves Piping Inspection is a technique that gives a very high inspection volume per measurement and has a high potential to inspect an inaccessible area due to its long inspection range. RTD-INCOTEST is a tool allowing fast screening of objects of which the surface can not be accessed directly. Field experience with both tools has indicated their suitability for monitoring and opened a wide variety of applications.
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