Guided Waves monitoring

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1 Guided Waves monitoring A. Demma, K. Vine, B. Pavlakovic, D. Alleyne Guided Ultrasonics Ltd 30 Saville Road London, UK W45HG Tel. +44 (0) Abstract: Lo screening di tubazioni utilizzando il metodo ad onde guidate è una realtá assodata sia in Italia che nel resto del mondo. I metodi PND tradizionali sono metodi per il controllo di punti o di aree ad estensione limitata. Inoltre il target della ispezione diventa di anno in anno più complesso a causa dell invecchiamento delle strutture e delle carenze tecniche di alcuni metodi PND classici, evidenziate solo dopo anni di incidenti (dalle piccole perdite ai disastri ambientali). Il sistema di monitoraggio Guided Wave PIMS (Permanently Installed Monitoring System) permette di effettuare un fingerprinting di alcune decine di metri di tubazione a partire da una posizione assegnata e di analizzare i cambiamenti negli anni. In questo modo è possibile evidenziare sia la presenza che il rateo della corrosione presente nelle tubazioni. Il monitoraggio è inoltre particolarmente efficace nei casi in cui l accesso alla struttura comporti costi elevati (ad esempio nel caso di attraversamenti di strada, tuberie interrate o tubazioni offshore). Il sistema PIMS, una volta installato nell area di difficile accesso, è cablato in modo da effettuare i test successivi a partire da una area facile da accedere (senza ulteriori costi di accesso). La sensibilitá del sistema PIMS è superiore alla sensibilitá del metodo di screening classico ad onde guidate ed i risultati di alcune prove di sensibilitá sono riportati in questa memoria. Questo articolo inoltre riporta alcuni dati di campo del sistema PIMS in applicazioni sia onshore che offshore.

2 Introduction The increased safety and maintenance requirements of petrochemical plants and oil and gas transportation lines has increased awareness in the current capabilities of NDT and inspired a number of research programs with the goal of enhancing the performance of existing NDT tools and methods. Concurrently Health and Safety organizations worldwide have concentrated their effort into evaluating the cost/benefit of two different types of inspection approach, namely Risk Based Inspection (1) and Direct Assessment (2). Both approaches utilize several inspection methods (including Guided Waves (GW)) for the final target of verifying the integrity of the inspected component. The screening of pipes using the GW method has been accepted by the oil and gas industry and several government bodies around the world as a good approach in several inspection situations (for example screening of bare pipes, road crossings and buried pipes). In many typical applications where the GW testing method is applied a large percentage of the total cost of the screening is due to gaining access to the test location. In these cases an attractive option is to use a PIMS (Permanently Installed Monitoring System) where a one time access is needed and the transduction system is permanently attached to the pipe. When using GW in monitoring mode a transducer ring is permanently attached to the pipe and must be protected from degradation due to environment and/or operational conditions. As reported in the remainder of this paper, the performance both in terms of range and sensitivity of the G- PIMS TM is generally superior to the performance achieved using the GW screening transducer rings. GW screening and monitoring Guided waves propagate along a pipe and enable the coverage of a large area from a single test position (see Figure 1). Transducer (a) Inspected region Transducer array (b) Inspected region Figure 1 Difference between traditional NDT methods (a) and GW method (b).

3 The generation of these waves is obtained using a special transducer array (3). The contact between the pipe and the transducers is dry and a mechanically or pneumatically applied force is used to ensure good coupling. After the transducer ring is positioned around the pipe the operator starts a rapid test which automatically sweeps several frequencies collecting data from both sides of the ring with a single test. The propagation of the GW depends on the conditions of the pipe under test. A range of several tens of meters in either direction from the transducer ring position can be obtained when the pipe is in generally good condition and there is a low density of features (such as change of directions, drains, vents, valves, welds etc..). The range decreases for pipes generally corroded. The system has been designed to detect defects that remove about 5% of the pipe cross sectional area although defect dimensions well below 5% (e.g. 1-2%) can be identified in pipes with isolated defects. Due to the large area screened from a single inspection point GW screening can increase the POD of defects in a pipeline compared to other NDT approach that rely on point measurements (4). GW monitoring can offer a further improvement of the POD due to the high sensitivity to changes when in monitoring mode. GUL G-PIMS TM transduction system The G-PIMS TM has been developed to utilize all of the software features available for GW screening, including the frequency animation and C-scan tools (5). Transducer The transducer is produced as a low profile flexible array which is bonded and clamped in place on the pipe surface. The whole transducer is then sealed in a polyurethane jacket to provide complete environmental protection. The sealed unit height is only about 10mm giving a wide range of possible install locations (see Figure 2.a). Transducers have been made to fit pipes from 2 diameter up to 42, although larger sizes can be produced. Connection box The connection box is a sealed weatherproof box (see Figure 2.b). This stores the test parameters such as pipe size, orientation and the identification of the reference file that should be used to compare the most recent data with. This allows the software to select all of the appropriate collection parameters without the need for any operator input, which ensures that the same data set is collected regardless of the operator.

(a) (b) Figure 2 G-PIMS TM transducer (a) and connection box (b). GUL PIMS stability of results The stability of results is a basic requirement for any monitoring technology.

Figure 3 shows an example of monitoring results on a PIMS ring exposed to weather changes.

scale in this case. The horizontal axis shows both negative and positive direction corresponding to the two directions of propagation along the pipe from the transducer location.

The grey zone covering a short distance to the left and right of this indicates the "near field", in which measurements can be interpreted but not as accurately as elsewhere.

4 (a) (b) Figure 2 G-PIMS TM transducer (a) and connection box (b). GUL PIMS stability of results The stability of results is a basic requirement for any monitoring technology. Stability of results can be achieved by reducing the possibility of changes in the performance of the transduction system. Figure 3 shows an example of monitoring results on a PIMS ring exposed to weather changes. As in standard GW screening, the "A-scan" display has a horizontal axis which shows the distance of any reflectors from the transducer and a vertical axis which shows their amplitude, on a linear scale in this case. The horizontal axis shows both negative and positive direction corresponding to the two directions of propagation along the pipe from the transducer location. The vertical green band at zero distance indicates the "dead zone" at the transducer ring, within which no interpretation can be done. The grey zone covering a short distance to the left and right of this indicates the "near field", in which measurements can be interpreted but not as accurately as elsewhere. The black and red traces are the torsional and flexural wave respectively at time zero and the blue and pink traces are the torsional and flexural wave after 12 months. The data displayed confirm that the variation of the recorded trace with time are well below 0.5% of the cross section change. This essentially demonstrates very good stability of the PIMS results and assuming that the pipe had no changes (which is unlikely) the PIMS stability is 0.5% or less. In order to minimize changes due to variation of transducer performance some compensation algorithms ought to be used.

5 -F5 -F4 -F3 -F2 -F1 +F % CSC 15.0 Amp (mv) 1 10% CSC Figure 3 G-PIMS TM stability of results. Black and red traces are the torsional and flexural wave respectively at time zero and the blue and pink traces are the torsional and flexural wave after 12 months. Black dotted line represents a 22.5% CSC (cross section change) and blue dotted line represents a 10% CSC [this is valid for all other test results in this article] GUL PIMS sensitivity To demonstrate the sensitivity that can be obtained from the PIMS a trial was carried out on a 16 diameter test pipe. This had a PIMS transducer installed toward one end of the 6m sample and a reference data collection was performed. After this a defect was machined into the sample 3.5m from the transducer. The size of the initial defect was 0.5% change in pipe cross section. This defect was then increased in size to 0.7% and then 0.9%. The results from this trial can be seen in Figure 4. This shows that both the initial defect and the incremental changes are clearly identifiable. The result shown has traces taken from the initial data collection and three subsequent sets of data from different defect sizes. The section of trace between 1m and 3m shows how repeatable the PIMS results are when there are no changes to the cross-section of the pipe. Please note that the results are shown here for a specific frequency value whereas a range of frequencies and bandwidths collected within a single test sequence are available to the operator analyzing the data.

6 +F1 +F2 Amp (mv) mm 0.7% hole defect 6mmhole 0.5% defect 12mmhole 0.9% defect 1.0 baseline Figure 4 G-PIMS TM sensitivity performance. Case studies Road crossing G-PIMS TM was installed on pipes at a cased road crossing position prior to back filling. Monitoring tests could be performed subsequently in the future without expensive excavations. The pipe being inspected is shown in Figure 5 and the result from one of the pipes is shown in Figure 6. The G-PIMS TM is protected from environment and external mechanical damage so backfilling does not present a risk to the operation of the transducer. In this case the range of test was longer than the length of the crossing and the signal/noise ratio confirms typical G-PIMS TM performance. Ø12, Pipe 146 Figure 5 G-PIMS TM used at a road crossing.

7 12.0 -F7 -F6 -F5 -F4 -F3 -F2 -F1+F1 +F Amp (mv) Figure 6 G-PIMS TM result for road crossing 146 in Figure 5. Offshore The G-PIMS TM transducer can be used for monitoring risers pipes or subsea lines. When monitoring riser pipes both installation from topside using rope access and subsea installation are possible. One picture of riser monitoring using rope acces G-PIMS TM is shown in figure 8.a. The related result with a test range in excess of 60m is shown in figure 8.b. A G-PIMS TM transducer was installed in 2007 on a length of pipe that has subsequently been connected to a subsea riser for a gas platform in the North Sea. Figure 9 shows the G-PIMS TM installed on the pipe before being sent offshore. Once welded to the rest of the riser pipes in situ, the G-PIMS TM cable connector was made accessible on the deck inside the control room of the platform. Data can now be collected from the control room of the platform without expensive subsea ROV or diver operations. The result in Figure 10 shows a comparison of data taken shortly after the pipe spool was connected subsea with data taken a year later. This result shows that there has been no appreciable change in the pipe condition, although it can be seen that there has been an increase in the sound attenuation. This is most clearly demonstrated by the amplitude of the flange reflection at the 18m location. This is thought to be mostly due to marine growth (the pipe is in relatively shallow water) or due to sediment collecting

8 on the pipe. However the result shows no new reflections from any defects and indicates that the pipe is still in excellent condition. (a) 3 Sea bed Water level Flange+ Caisson Amp (Linear) 2 1 (b) Figure 8 G-PIMS TM installed on an offshore riser from topside (a) and related Wavepro result (b). Figure 9 G-PIMS TM installed on an offshore riser pipe prior to pipe being tied in to the riser base with wet mate connector attached to protection frame.

9 1 -F5 -F4 -F3 -F2 -F1 +F1 +F2 +F3 +F4 G3-08#3717 (mv) F4 -F3 -F2 -F1 +F1 +F G3-35#2103 (mv) Figure 10 - Results taken from subsea PIMS after installation (bottom) and 1 year later (top). Conclusions The stability and sensitivity of the G-PIMS TM reported in this article demonstrate the general reliability of the transduction system. The capabilities of the system to detect circumferential distribution of wall loss and the operation over a wide frequency range enable good accuracy of the calls and the reduction of false calls that hugely increase the total price of the inspection (including verification) in pipes that are difficult to access. The experience with G-PIMS TM when working in relatively difficult operational environments (road crossings and offshore pipes) demonstrates the robustness and durability of the G-PIMS TM in applications where a monitoring approach provides a good solution in terms of cost/benefit. REFERENCES 1. Risk-based Inspection technology, API 581, American Petroleum Institute, 2 nd Edition, Ersoy D., Final report, GTI project Number 20386, Gas Technology Institute, Alleyne D.N. and Cawley P., Optimization of Lamb wave inspection techniques, NDT & International 25, 11 (1992) 4. Vogt T., Evans M., Reliability of Guided Wave testing, 4th European- American Workshop on reliability of NDE, 2009, in press 5. Nunez LV. M., Perez E., Demma A. and Lowe M.J.S, Guided wave testing of an immersed gas pipeline, Materials Evaluation, 102 (Feb 2009)

Testing of Buried Pipelines Using Guided Waves

Testing of Buried Pipelines Using Guided Waves A. Demma, D. Alleyne, B. Pavlakovic Guided Ultrasonics Ltd 16 Doverbeck Close Ravenshead Nottingham NG15 9ER Introduction The inspection requirements of pipes