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The integrity of flexible pipe: search for an inspection strategy
Engineering Structures, Vol. 17, No. 4, pp. 305-314, 1995
I~IUTTERWORTH I1~1E I N E M A N N
0141-0296(95)00028-3
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0141-0296/95 $10.00 + 0.00
The integrity of flexible pipe: search for an inspection strategy J. M. M. Out*, D. A. Kronemeijer, P. J. van de Loo and A. de Sterke Shell Research, Amsterdam, The Netherlands
Flexible pipe is composed of polymeric cylindrical elements, reinforced by layers of helical steel wires. Its integrity is threatened by a number of processes, ranging from imperfections during manufacturing, impact during installation to chemical ageing and mechanical deterioration during service. Managing the integrity of flexible pipe should address all of these processes. Inspection is part of integrity management. Shell's research efforts in this field have concentrated on mechanical deterioration, i.e. fatigue and wear of the steel components. The relevant processes are briefly described. Considerations when judging inspection techniques are the detection capability, whether they can be applied in-service and whether monitoring is continuous or intermittent. A difficulty is that not all of the constituent layers can be reached directly. A number of techniques are described which are aimed at detecting fatigue cracks or fractures. They include radiography, magnetic strayflux, electric reflectometry, ultrasonic guided waves and thickness measurement, eddy current, photogrammetry and acoustic emission. Their virtues, limitations and potential for field application are discussed. Other techniques consider wear of the sliding metal surfaces in contact, i.e. the amount of wear or the type of process. Recent efforts have been devoted to acoustic emissions (AE), for detection of active wear or fatigue cracking. The principles of AE, how to qualify and implement it are described. The paper concludes with the state-of-the-art of flexible pipe integrity management.
Keywords: flexible pipe, integrity, inspection techniques Flexible pipe is a high pressure pipe that is compliant in bending and strong and :~tiff in axisymmetric loading, such as pressure, tension and torsion. This is possible because the sealing elements are compliant polymeric cylinders and the load bearing elements are steel helical wires wrapped around them. Two different generic types have emerged in the last two decades: the unbonded flexible pipe (Figure 1), without adhesive agents between the layers, and the bonded flexible pipe with the reinforcing bonded to an elastomeric matrix. An important p~rt of flexible pipe is the end fitting in which all elements are anchored. If we limit ourselves to risers and towlines, the predominant type is the unbonded flexible pipe. Unless otherwise noted, we shall limit ourselves to this type. When integrated with electro/hydraulic umbilicals or other flexible pipes, they are known as integrated service umbilical,; (ISU) or multibores. The life of a flexible pipe can be separated into four
phases; design, manufacturing, transportation and installation, and service. In the design stage the constituent elements of flexible pipe, their shapes, lay angles, thicknesses and materials are chosen to suit the application. In principle, the design of pipe (and that of the system in which it functions) determines the in-service failure mode if we assume that no loads are forgotten. During the manufacturing phase the design is realized. The product is then transported from the factory to the site and installed. Finally, the flexible pipe is used for a period of time. At all times, the user will want to know that the flexible pipe is fit for its purpose. Inspection means using a certain technique to look at the structure and assess its suitability. This paper concentrates first on what conditions should be satisfied, before an inspection strategy can be proposed. The type of defects and degradation in all phases of the life are discussed and the role of inspection is highlighted. The scope is then limited to in-service inspection for mechanical deterioration. The criteria for judgement of in-service inspection techniques are discussed. Next, several tech-
* Presently with Shell U K Exploration and Production, Aberdeen, UK
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The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
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there is to be done, which may reach into the impractical. We assume that qualification testing has revealed the order of magnitude of (iii). Inspection will now revert to dealing with how it is done, if possible: a technique, a procedure and an inspection interval. If impossible, the only options are to change out conservatively or to redesign the flexible pipe or the system in which it functions.
Defects and degradation processes
Figure I
Schematic of unbonded flexible pipe
niques are evaluated. Finally, the paper discusses where integrity management of flexible risers stands and where it may go from here.
Role of inspection Inspection prerequisites Before inspection can commence the following questions about the defect or degrading process will have to be answered (i) What kind of defect or process are we to look for, in which element of the flexible pipe; what is the critical size or stage in the process? (ii) At which position around the circumference, along the pipe do we expect the damage? (iii) When does the damage initiate? What is the propagation rate? The more uncertain the answer to (i), the more impossible the task. The less one knows about (ii) the more work
Inspection of flexible pipe has a role to play during all phases of its life. During the design of the pipe or the system one should consider whether the relevant pipe section, the relevant layers are accessible to the considered technique. For example, it is hard to receive acoustic emission through a highly absorbent layer, such as the electro/hydraulic peripheral layer on an ISU. It is impossible to inspect a layer with an eddy current, if it is shielded by another, ferromagnetic layer. The use of high strength aluminium for reinforcing may reduce the inspectability with eddy-current; the use of glass-fibre reinforced epoxy (GRE) reinforcing and eddy-current are totally incompatible. It may improve the distinction to wind two adjacent layers in opposite directions. In the extreme case, one may choose to alter the design in order to make the system inspectable, even at the expense of part of the expected service life. During manufacturing, inspection, in the form of quality control (QC), aims to ensure that one produces what one expects. Examples are establishing the absence of breaks in the continuously extruded polymeric sealing sheath and checking the welds in the continuous steel reinforcing. QC is always in the hands of the manufacturer, but witnessed by first and third parties, after their quality assurance (QA) and/or QC has been reviewed. At this stage, the acessibility of each individual layer is at its best and finding factors that reduce the future service life is easiest and cheapest. The importance of proper end fittings does not always receive the attention it deserves. The suspicion expressed in Reference 1 is recognized. Shell have always insisted that the end fitting should be at least as strong as the pipe, statically and dynamically. A robust assembling procedure, combined with thorough QC (e.g. proper epoxy bonding in the end-fitting, avoidance of notches in the parts with varying stress), should prevail, since inspection in-service will be next to impossible. Transportation and installation has often been quoted as the critical phase, during which the pipe is threatened by misuse and impact by other objects. The result will be tears in the external polymeric sheath and/or permanent deformation of a section of pipe. More often than not, these accidents are known to have happened and their effects will be revealed by visual inspection, radiography or hydrotesting. Whether or not the effect is acceptable is not always easy to tell and detailed inspection may help to decide. The section may be discarded and the remaining pipe be mended by installing field end-fittings. Damage or deterioration during service strongly depends upon the application. For flowlines, ageing of the polymeric liner and corrosion of the steel reinforcing play a role, as well as interference with trawler gear, if unburied, or upheaval buckling, if buried. For risers, ageing and corrosion should also be considered, as well as interference with anchors or mooring lines, mechanical overloading and so-called mechanical deterioration, i.e. wear and fatigue of
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al. the steel elements. Mechanical deterioration is extensively discussed later in this paper.
Criteria for in-service inspection In addition to its cost, detection capability and accuracy, an in-service inspection technique is judged by the following operational aspects: • Early warning: a defi,~ct or process should be detected sufficiently early, so that remedial action (to satisfy safety and economics) can be taken. • Is it applicable on-stn~am or does it require a shutdown of operations? For example, internal inspection of an oil riser by an intelligent pig is on-stream; using a crawler pig it requires suspension of normal operation. If the ultrasonic inspection of a gas riser requires the internal medium to be water, then operations must be suspended. • If on-stream, does it monitor continuously or measure intermittently? Continuous monitoring means being online all the time, being triggered by an anomaly and then giving warning. Intermittent measuring means, for example, an annual measurement run by a pig, or continuous monitoring which is switched on and off to take 'fingerprints'. • Is it a global or a local technique? A global technique covers a large area relative to the sensor size, whereas a local technique makes a spot measurement, which has to be repeated if a larger area needs to be covered. • Accessibility. Can the technique be applied where needed? If external inspection is required at the riser top, the bending stiffener is in the way. The same holds true for flexible pipes in ISUs and multibores. Also, a crawler on the outside may be hindered by 'piggy-back' umbilicals.
Mechanical deterior~ttion of flexible risers Mechanical deterioration of flexible risers has been the main target of Shell Research's efforts, both in terms of testing and analysis on the one hand 2,3 and inspection on the other. We shall limit the scope of the paper to mechanical deterioration of the reinforcing, excluding the collapse carcass. The critical area was determined to be the top of the riser with bending stiffener. The mechanical deterioration process is considered independently, assuming no interaction with, for instance, c,orrosion (sour service). Until recently, unbonded flexible pipe was only available without a polymeric tape between the tensile armours or with just a thin one. Mechanical deterioration of such flexible pipe was observed to be due to the wear of these armours, followed by fatigue and fracture 3-5. Initially, Shell's inpection research efforts were directed towards detecting fractured tensile armours. When it became clear that the time period between the first fracture and final failure of the pipe may be shorter than a practical inspection interval, attention was shifted to the wear process. Here one might choose to measure the amount of wear incurred. Alternatively, one may follow the wear process itself and detect when it changes ti~om mild to severe wear. Presently, flexible pipe is also available with a solid lubricating layer, that is sufficiently thick to prevent all wear of the tensile armours. It is natural to assume that this increases the service life,. Given that operators judged the
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design life of the old flexible pipe acceptable, the consequence of this may be that mechanical deterioration ceases to be of practical relevance. It may also lead to more demanding applications, so that the next weakest link arises. For example, fatigue of the pressure reinforcing may occur. In that case, inspection continues to be an issue. The fatigue process should be well characterized to set the inspection requirements. This means that the crack size that leaves a certain life margin (at least a year) should be known. In general, qualification testing and/or model development are needed, whenever major design changes have been made.
Assessment of inspection techniques for flexible pipe A number of techniques are now reviewed, that are aimed at fractured tensile armours, thickness measurements of the tensile armours, fractured pressure reinforcing and the detection of the wear or fatigue process. They are listed in
Table 1. Radiography Holes were drilled through the thickness of the external armours of a 4 in ID flexible pipe with diameters ranging between 1 and 6 mm. The pipe was filled with water. Conventional radiography (CR), using an X-ray source and a radiographic film, and real time radiography (RTR), using an X-ray (more accurate) or a "y-ray source and a screen (real time image intensifier) were applied, both in double wall exposure. Both CR and RTR (best out of four RTR contractors), using an X-ray source, detected holes down to 2 mm (Figure 2). The assessment was performed late in 1987 and the field of RTR may have developed further (e.g. linear arrays) since then. The detection of broken tensile armours, and to a lesser degree Zeta wires, with gaps down to 2 mm should be possible. The detectability falls when the location is off centre. Radiography in single wall exposure would be more accurate, but impractical. Regarding operations, RTR seems preferable because it would allow remote in-service inspection. An external crawler with circumferential freedom would need to be designed. The use of RTR underwater, however, is not an established method. CR, with a slightly higher accuracy, needs diver interference for application of the film and would appear practical only when applied to certain areas of limited extent, e.g. at damaged areas, the top of the riser or the end-fitting.
Magnetic strayflux When one applies a magnetic field along a flexible pipe, a stray flux field will emerge from irregularities, such as fractures and pits, which can be detected by sensor coils or Hall sensors. On a 1.5 in ID flexible pipe, three adjacent broken wires of outer tensile layer were detected by our wire cable inspection instrument (Figure 3(a),(b)); five breaks in the inner tensile layer were not detected. An area of wear is not detectable with magnetic strayflux (unless very concentrated). Such metal loss could be detected by measuring the total flux, as has been demonstrated on wire rope. However, the allowable metal loss forms such a small percentage of the total metal area (including pressure reinforcing) that it is below the detection limit.
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The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out e t al. Table 1 Assessment of the feasibility of inspection techniques for mechanical deterioration of flexible pipes. The numbers 0-5 indicate the likelihood (0 = next to impossible, 5 = high). This assessment of likelihood includes whether the technique may be applied in the field, but not the extent to which it is operationally acceptable or can be applied sufficiently frequently Wear of tensile armour Radiography Magnetic strayflux Eddy current E-reflectometry US guided waves US wall thickness Acoustic emission Photogrammetry
1
Fracture tensile armour
Fatigue pressure armour
Cracked pressure armour
End-fitting
3 3 4 1 1
2 3 0
Figure2 X-ray real-time radiography images of holes in tensile armours Time domain electric reflectometry Time domain electric reflectometry means that a short ( 101000 ns) electric pulse is applied to two parallel wires (twisted pair or coaxial). The delay of the reflection is a measure of the distance to the wire end, e.g. at a fracture. For wires made of ferromagnetic material, the difficulty is that the signal is highly attenuated, limiting the inspectable length to less than 50 m. An experiment was conducted on a short 1.5 in ID flexible pipe using the tensile armours as the screen and the collapse carcass as the conductor. Three fractures in the outer tensile armour layer were not detected. Only when all the wires were cut was detection possible. Other schemes such as taking the tensile armours one by one, would not work because they are not insulated from each others. Ultrasonic guided waves Introducing ultrasonic (US) waves in the steel tensile armour wires, using them as acoustic wave guides, and timing the reflections from discontinuities would allow the detection of fractures. The wire would be excited by one
Figure3 (a)Wire cable inspection instrument and 1.5in ID flexible pipe at Shell Research, Amsterdam US transducer and the reflection pattem picked up by the same (pulse-echo mode) or a second transducer (pitchcatch mode). For the pitch-catch mode, one could imagine placing them on either side of the suspected area or at the same side. Attenuation of the signal will take place as a result of energy lost to the environment, i.e. the other armour layers, lubrication layers, grease etc. The zero-order longitudinal wave mode was found to have the least attenuation: 16 dB/m and over, increasing (slightly) with contact pressure between wires. Given a dynamic range of approximately 100dB, this means a travel distance of 6 m (implying a distance between the transducer and the defect of 3 m at the most, when using the pulse-echo mode). For this principle to be used in practice, one would need direct access to the suspected tensile armour wires at at least one location. For monitoring the external armour layer at the riser top, excitation at the end fitting seems an option and measurement on the pipe below the bending stiffener, if access to the tensile armours were obtained.
Ultrasonic wall thickness measurements Ultrasonics (US) can be used to measure the thickness of outer tensile armour wires. A screening study was perfor-
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
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EC has proved to be able to detect 0.2 mm local erosion and a O 1 mm hole in the collapse carcass. In the ferromagentic pressure reinforcing severed faces of the wire were detected with gaps down to 1 mm, as well as holes down to O 5 mm 8. When concerned about the mechanical deterioration of flexible pipe, fatigue cracking of the pressure reinforcing is a potential failure mechanism, particularly when wear of the tensile armours has been prevented. EC is a potential internal inspection method for the pressure reinforcing, provided that the collapse carcass is not ferromagnetic. In addition, since the distance between probe and wire is at least 10 mm, a low AC frequency is called for, with an associated large probe size (order: 20-30 mm) and matching low resolution. Multifrequency EC is currently being developed so as to enhance detectability. If we set the minimum detectable crack at 10 inm, oriented along the wire, we feel that EC has potential to detect the crack at a sufficiently early stage for safe operation. External inspection with EC can detect broken tensile armours and general disarrangement, the presence of which would urge immediate action.
Photogrammetry
Figure 3 (b) Magnetic stayflux field around broken outer tensile wire
med on a 4 in ID flexible pipe with wires of 6 × 3 mm 2 (width xthickness). A 5 MHz focused probe (0.5 in Q~) was identified to be optimal in terms of attenuation versus resolution. The focusing of the US bundle was achieved by using a curved probe with a PVDF piezo-electric layer. With regard to alignment, an angle of tilt with the pipe surface of less than 5 ° could be tolerated. Figure 4 shows radial measurements (B-scan presentation) on a pipe segment, taken by moving circumferentially around the pipe. It should be noted that the pipe was not pressurized. This makes matters worse since under that condition the tensile armours are slack and slightly tilted. The reflections of the wires in the outer layer are easily visible, but an accurate determination (within 5 10%) of thickness is not possible. It was concluded that further development of the probe configuration would be needed for establishing the feasibility of the method, and that the ultimate accuracy in the field would, at best, be 10%.
Eddy current An electrically conductive material can be inspected with eddy current (EC) probes moving along its surface. EC is sensitive to: firstly, obstructions to currents such as cracks; or secondly, distance variations from, for instance, corrosion pits or erosion. The geometry of EC probes can be adapted to the application. The penetration of EC into the material is greatly reduced if the material is ferromagnetic, which generally limits the detection to surface defects. EC is well established in the field of heat exchanger tubes, including ferromagnetic ones 6. In the course of the I?PS2000 project7, single frequency
Photogrammetry is the technique of measuring the geometry of a (curved) surface, with a grid of markers attached to it, by taking and analysing advanced stereo photographs. Doing this at different times, would make changes in the geometry visible. The method is not sufficiently sensitive that it can detect the slight distortion of a mechanically deteriorated flexible pipe when a few reinforcing wires have broken. We have, however, used this technique to assess the top of a 16 in ID flexible riser sample with bending stiffener. The objective was to detect the influence of either the deteriorated pipe or a bending stiffener on the curvature distribution. The conical part of the bending stiffener and the adjacent pipe section below were equipped with markers. Surveys were made of the riser top at high curvature at different times. A thorough analysis allowed us to derive the curvature distribution along the bending stiffener and the pipe immediately below (Figure 5). Neither the flexible pipe nor the bending stiffener had undergone significant deterioration during the test. The result was in line with our expectations, indicating the proper functioning of the bending stiffener throughout the test. For this purpose, photogrammetry appears to have potential.
Acoustic emission Principle Acoustic emission (AE) is the technique of detecting the acoustic activity of a mechanical process, such as wear or fatigue crack growth. In principle, it is a passive technique. It is a global technique in that an AE sensor registers signals from a surrounding area. AE is therefore a good candidate for continuous monitoring of flexible pipe in dynamic applications. In this discussion, we use resonant transducers as AE sensors. They have the following properties. An acoustic event that is strong enough and contains a frequency component near to the transducer's resonant frequency is registered. The transducer's response to the event is characterized by amplitude, rise time and duration (number of counts) (Figure 6). Given the amplitude, the response contains no direct information on the time shape of the event. The time shape is weakly reflected in the rise
310
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Qualification AE is well established for the inspection of pressure containing fibre-reinforced plastics and is gaining acceptance in the field of steel pressure vessels, such as those used in the nuclear and oil industry, in which the presence of cracks can be 'heard' when applying a proof load. In general, AE requires a good choice of the sensor (frequency), sensor layout and detection threshold on the one hand and a good knowledge of the characteristics of the relevant acoustic events, as a function of parameters such as structure geometry, loading amplitude and frequency, pressure, temperature etc. It is certainly a challenge to avoid mistaking noise for relevant events. AE is considered for field monitoring the condition of flexible risers that are subjected to 'random' curvature changes as a result of floater and wave motion. For qualification, a thorough, but painstaking approach (if only due to the need for many test samples) would be to set up a programme of: (a) small-scale wear or fatigue tests; (b) laboratory tests on full (or stripped) pipe to failure, during which pipe would be nondestructively checked at intervals for the stage of damage; (c) full-scale verification tests on a flexible riser top section; and (d) field verification on a real application. The final deliverable would be a qualified AE system and a known influence of the parameters. For all practical purposes, this approach to qualification is impossible. AE was applied in full-scale testing
vVlllVllvvlvvvvvvv ,,,,o Figure 6 Typical AE event and its parameters
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al. conducted at Shell Research Rijswijk 2 (steps (b) and (c) of the ideal programme). Monitoring of rotary bending tests to failure showed that a significant increase of acoustic activity was apparent from approximately 90% of the test life. Subsequently, a number of tests on a flexible riser top section, under variable bending and high static tension (Figure 7), were monitored.
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mitted along the tensile armours, thought to serve as good wave guides for this frequency, whereas when detected by the 30 kHz sensor fully isotropic behaviour was apparent • Signals generated on the tensile armours and detected on the sheath with the 175 kHz sensor were attenuated less as one moved away from the site of generation than with the 30 kHz sensor
Testing Before monitoring a full-scale 4 in ID flexible riser top section which was expected to fall from wear of the tensile armours, the characteristics of the flexible pipe were studied in laboratory experiments with a 30 kHz and a 175 kHz transducer. Signals were generated by the standard ASTM pencil lead break. It wa,; found that • Signals generated and detected on the external polymeric sheath by the 175 kHz sensor were preferentially trans-
From this it was concluded that the 175 kHz sensor would be relatively insensitive to accidental noise generated on the external sheath and best for detecting activity at the tensile armours. The additional use of the 30 kHz sensor would allow identification of such accidental noise on the external sheath and support the interpretation of the 175 kHz response. The resonant frequencies mentioned are specific for the flexible pipe in question. It should be realized, however, that acoustic waves transmitted along the tensile layer may have originated in another layer
(Figure 8). The high tension/bending test on the 4 in ID flexible riser top section was far advanced (with hindsight: approx. 90% of the test life) when the AE sensors were installed. The sample failed from wear between the two tensile armour layers within a number of days. Although this is unfortunate, it is plausible that the wear process would have deteriorated during the monitoring period. The loading is expressed in terms of the range of cycling top angle. Six classes of constant range cycling during hours to days were alternated. The root mean square (RMS) and the number of counts per time were continuously monitored. It was observed that the RMS value of the amplitude of acoustic activity peaked about 2 - 3 h after the angle range was changed from a lower class to a higher one and that the reduced level at which the RMS finally stabilized was reached after some 30 h. The RMS value was positively correlated with internal pressure. Figure 9 shows the results for the 30 kHz sensor. The peak RMS tended to increase both with time and with the loading class, whereas the number of counts increased only slightly with time and not at all with the loading. This suggests that the active process was not correlated with the loading and that with time little to no increase of active sources took place, but each had a higher energy. The results for the 175 kHz sensor are shown in Figure 10. Neither the peak RMS nor the number of counts increased with time, but both increased with the loading. The process appears to be constant with time, but more active sources emerged at higher loading. Interpreting both trends, it seems that the 175 kHz was indeed aimed at the wear process, since this should correlate with the loading, but that after all the wear process did not change with time. It is .'lot certain which process the 30 kHz sensor was sensitive to. It could be the rubbing between the bending stiffener and the flexible pipe. Because of the brief monitoring time, the statements made are tentative. Nevertheless, AE was considered promising. Receiver External polymer sheath Tensile reinforcing Pressure reinforcing ~ ,
Figure 7 High tension/bending rig at Shell Research, Rijswijk
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Figure8 Possible paths for transmission of acoustic energy from pressure reinforcing
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out e t al.
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During the above test, only the RMS of the amplitude and the number of counts/time were (continuously) recorded. In order to discriminate between different AE sources and damage mechanisms, it was necessary to gather more detailed information in subsequent tests. Instead of considering averages over a period, individual AE events would be recorded. In order to limit the potentially vast amount of data, a choice had to be made between raising the detection threshold (useful when the aim is to detect the snap of a breaking wire) or switching from continuous monitoring to intermittent (or trend) monitoring. The latter was preferred since the gradual wear process (or high cycle fatigue when wear is prevented) was being investigated. In addition, equipment was used that allowed six measurement sensors. The sensors may be distributed evenly around the circumference (Figure 11) but other schemes are also possible for better locating the origin. A data collection procedure was devised as follows. Every few cycles per 2000, AE data was collected and subsequently reduced to so-called fingerprint files, which have histograms of the statistics (average, mean, median, minimum and maximum) of all of the AE parameters against the number of cycles and the prevailing test condition. Four parametric signals were also recorded: internal pressure, temperature, static offset and the RMS of the top angle. Figure 12 shows an example of the number of hits/time registered by a 160 kHz sensor during a test on a 16 in ID
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sample that did not reach failure. It suggests that after an initial period a plateau was reached. Since the associated energy showed a similar trend, we conclude that the process monitored was stable. These and other results have neither produced firm conclusions nor disproved the potential of AE. Clearly, to qualify AE along this route, as opposed to the qualification process of the preceding subsection is arduous. Reference 10 discusses the experience with a single fullscale test to failure. Again, steps (a) and (b) of the ideal qualification programme were omitted. In this test, service conditions were not directly simulated since a shallow angle along the pipe was set and a strongly varying tension caused varying bending. Compared to service, the small slip amplitudes in the area of failure are likely to have modified the wear process. AE recording was done in short periods. Very little signal was received from a transducer on the external sheath below the bending stiffener, quite close to where the wear occurred, whereas a transducer on the top end-fitting, well removed from the wear area, was very active• The evolution with time of the distribution of AE during a single load cycle was suggested to be indicative of the damaging process. Initially, AE concentrated at the extremes of the load cycle, but gradually the middle of the load cycle started to dominate. This investigation also leaves questions that still need to be answered, before AE can be considered to be proven•
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
313
mately allow automation. The AE response depends upon the intensity of the variable loads. In order to have a basis for comparison, the sea state should also be monitored and the AE response during a defined 'three-monthly', 'biannual' storm would form the riser's signature. This makes field verification in the given case a difficult, but perhaps a necessary task. nd ictor
Discussion
Inspection for mechanical deterioration damage of unbonded flexible risers is faced with the following dilemma • On the one hand the (still evolving) design of flexible riser systems and flexible pipe is not fully proven and in-service inspection is desirable to ensure safety and availability • On the other hand the unproven design is associated with a complex failure process, so that inspection is faced with an ill-defined task
Figure 11 Configuration of AE sensors 31111 •
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Figure 12 Number of hits/time throughout flexible risers test
The potential of acoustic emission The experience with AE monitoring of the flexible riser tests is limited and data evaluation of some tests is still in progress. Proper qualification of the method is hampered by the fact that not all tests are conducted to failure and that no duplication tests on a given pipe structure were performed. A few preliminm~¢ remarks can be made. The likelihood of AE being suitable for monitoring ISUs is small. Whether or not, in the case of flexible pipe with a thick solid lubrication layer between tensile armours, the amount of energy associated with fatigue cracking of the pressure reinforcing is sufficient to be detectable is currently being investigated. Current studies on the propagation routes and attenuation of acoustic energy (Figure 8) are intended to elucidate this and may suggest that embedding sensors in the pipe wall enhances the feasibility. If the above research effort were to be successful, what would an AE system look like in practice? We may be able to discard part of the 'fingerprint files', if those parameters are not correlated wih tke failure process, or are strongly correlated with other parameters. That might reduce the amount of data produced, make it manageable, or ulti-
What measures do the current and prospective operators of flexible risers take5,9? The current state-of-the-art is that the integrity management does not include inspection, other than periodic visual inspections by ROV, since there are no readily available inspection services. In the light of this, the QA/QC should be stringent. Some advocate the monitoring of riser motions in order to establish the conservatism of the input to the design of the flexible pipe. Lastly, pressure testing may be used to confirm the integrity periodically. For the other important failure mechanism, ageing of the polymeric liner, monitoring of the diffusion rate into the annulus at the top end-fitting and installation of spool pieces or test pipes are proposed. The criteria for rejection or remedial action in the above approaches are uncertain. Work is continuing, within Shell and elsewhere, to rationalize the application of such 'conventional' approaches. Hence costly, conservative replacement may be required. This can only be avoided if monitoring techniques become available during the life of the riser or other evidence to support the design of flexible risers appears (e.g. a design model supported by full-scale testing). How should the industry go forward? Table 1 summarizes the assessment made in the preceding section on the different inspection techniques that we have considered. If there remain dynamic applications of flexible pipe without a thick solid lubricant between the tensile armours, acoustic emission (AE) remains a good candidate for monitoring the wear process between these armours. We then need to investigate the relation between the AE central frequency and the flexible pipe structure and that between AE level and the loading parameters. Suitable candidates to detect the amount of wear have not yet been identified. The development of techniques to detect broken tensile armours will not contribute to giving an early warning, but is useful for confirmation. Radiography, magnetic strayflux and eddy current (EC) are likely to fulfil this task. For applications of flexible pipe for which wear of the tensile armours has been prevented and where fatigue of the pressure reinforcing may be the governing factor, continuous monitoring by AE may also have limited potential. EC inspection appears to be a more suitable candidate. The time frame of this fatigue process must be determined before this can be decided. Inspection by EC is intermittent and may only be
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The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
used on-stream- if the riser is pigable and EC is sufficiently robust to withstand pig motions.
Conclusions The design of flexible pipe and flexible riser systems for mechanical deterioration is not fully proven and the governing failure modes are quantitatively uncertain. Qualification testing and model development remain necessary as input for the inspection of flexible riser systems. Acoustic emission has potential for the inspection of flexible pipe for wear damage, but a significant development effort is still necessary. Radiography, magnetic strayflux and eddy current should be considered for the inspection of flexible pipe for fractured outer tensile armours. This inspection is useful to verify suspected damage, it does not give an early warning. For the inspection of flexible pipe for fatigue cracking of the pressure reinforcing eddy current shows the most promise. Acoustic emission may also have potential here. For the near term, effective use of conventional technology, such as monitoring of riser motions, pressure testing and annulus monitoring is recommended.
References 1 Macfarlane, C. J. 'Flexible riser pipes: problems and unknowns', Engng Struet. 1989, 11, 281-289 2 Kastelein, H. J., Out, J. M. M. and Birch, A. D. 'Shell's research efforts in the field of high pressure flexible pipe'. Deepwater Offshore Technology Conf., Monaco, October 1987 3 Out, J. M. M. 'On the prediction of the endurance strength of flexible pipe' Offshore Technology Conf. 1989, Houston, TX, paper 6165 4 Feret, J., Boumazel, C. and Rigaud, J. 'Evaluation of flexible pipe life expectancy under dynamic conditions' Offshore Technol. Conf. 1986, Houston, TX, paper 5230 5 Barthelemy, B. 'Inspection and monitoring of flexible pipes- why and how?' Noroil, February 1988, p44 6 Berg, W. H. van den and Bakker, H. L. M. 'Eddy current inspection of the internal bore of ferromagnetic heat exchanger tubes', in Nondestructive testing (Proceedings of 12th World Conference), Elsevier, 1989, p 315 7 Berge, S. and Olufsen, A. (eds) Handbook on design and operation offlexible pipes SINTEF, STF70 A92006, Trondheim, Norway, 1992 8 Midtgaard, O. and Eriksen, M. 'Eddy current inspection of flexible pipe', Flexible Pipe Technology Seminar FPT 92, Marine Technology Centre, Trondheim, Norway, February 1992 9 Marine Technology Centre, Proceedings of Flexible Pipe Technology Seminar FPT 92, NTH, Trondheim, Norway, 1992 10 Berge, S., Eide, O. I., and Hval, M. 'Acoustic emission monitoring of a flexible pipe during fatigue testing', 12th Offshore Mechanics and Arctic Engineering Conf. ASME, Glasgow, UK, 1993
I~IUTTERWORTH I1~1E I N E M A N N
0141-0296(95)00028-3
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All fights reserved 0141-0296/95 $10.00 + 0.00
The integrity of flexible pipe: search for an inspection strategy J. M. M. Out*, D. A. Kronemeijer, P. J. van de Loo and A. de Sterke Shell Research, Amsterdam, The Netherlands
Flexible pipe is composed of polymeric cylindrical elements, reinforced by layers of helical steel wires. Its integrity is threatened by a number of processes, ranging from imperfections during manufacturing, impact during installation to chemical ageing and mechanical deterioration during service. Managing the integrity of flexible pipe should address all of these processes. Inspection is part of integrity management. Shell's research efforts in this field have concentrated on mechanical deterioration, i.e. fatigue and wear of the steel components. The relevant processes are briefly described. Considerations when judging inspection techniques are the detection capability, whether they can be applied in-service and whether monitoring is continuous or intermittent. A difficulty is that not all of the constituent layers can be reached directly. A number of techniques are described which are aimed at detecting fatigue cracks or fractures. They include radiography, magnetic strayflux, electric reflectometry, ultrasonic guided waves and thickness measurement, eddy current, photogrammetry and acoustic emission. Their virtues, limitations and potential for field application are discussed. Other techniques consider wear of the sliding metal surfaces in contact, i.e. the amount of wear or the type of process. Recent efforts have been devoted to acoustic emissions (AE), for detection of active wear or fatigue cracking. The principles of AE, how to qualify and implement it are described. The paper concludes with the state-of-the-art of flexible pipe integrity management.
Keywords: flexible pipe, integrity, inspection techniques Flexible pipe is a high pressure pipe that is compliant in bending and strong and :~tiff in axisymmetric loading, such as pressure, tension and torsion. This is possible because the sealing elements are compliant polymeric cylinders and the load bearing elements are steel helical wires wrapped around them. Two different generic types have emerged in the last two decades: the unbonded flexible pipe (Figure 1), without adhesive agents between the layers, and the bonded flexible pipe with the reinforcing bonded to an elastomeric matrix. An important p~rt of flexible pipe is the end fitting in which all elements are anchored. If we limit ourselves to risers and towlines, the predominant type is the unbonded flexible pipe. Unless otherwise noted, we shall limit ourselves to this type. When integrated with electro/hydraulic umbilicals or other flexible pipes, they are known as integrated service umbilical,; (ISU) or multibores. The life of a flexible pipe can be separated into four
phases; design, manufacturing, transportation and installation, and service. In the design stage the constituent elements of flexible pipe, their shapes, lay angles, thicknesses and materials are chosen to suit the application. In principle, the design of pipe (and that of the system in which it functions) determines the in-service failure mode if we assume that no loads are forgotten. During the manufacturing phase the design is realized. The product is then transported from the factory to the site and installed. Finally, the flexible pipe is used for a period of time. At all times, the user will want to know that the flexible pipe is fit for its purpose. Inspection means using a certain technique to look at the structure and assess its suitability. This paper concentrates first on what conditions should be satisfied, before an inspection strategy can be proposed. The type of defects and degradation in all phases of the life are discussed and the role of inspection is highlighted. The scope is then limited to in-service inspection for mechanical deterioration. The criteria for judgement of in-service inspection techniques are discussed. Next, several tech-
* Presently with Shell U K Exploration and Production, Aberdeen, UK
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The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
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there is to be done, which may reach into the impractical. We assume that qualification testing has revealed the order of magnitude of (iii). Inspection will now revert to dealing with how it is done, if possible: a technique, a procedure and an inspection interval. If impossible, the only options are to change out conservatively or to redesign the flexible pipe or the system in which it functions.
Defects and degradation processes
Figure I
Schematic of unbonded flexible pipe
niques are evaluated. Finally, the paper discusses where integrity management of flexible risers stands and where it may go from here.
Role of inspection Inspection prerequisites Before inspection can commence the following questions about the defect or degrading process will have to be answered (i) What kind of defect or process are we to look for, in which element of the flexible pipe; what is the critical size or stage in the process? (ii) At which position around the circumference, along the pipe do we expect the damage? (iii) When does the damage initiate? What is the propagation rate? The more uncertain the answer to (i), the more impossible the task. The less one knows about (ii) the more work
Inspection of flexible pipe has a role to play during all phases of its life. During the design of the pipe or the system one should consider whether the relevant pipe section, the relevant layers are accessible to the considered technique. For example, it is hard to receive acoustic emission through a highly absorbent layer, such as the electro/hydraulic peripheral layer on an ISU. It is impossible to inspect a layer with an eddy current, if it is shielded by another, ferromagnetic layer. The use of high strength aluminium for reinforcing may reduce the inspectability with eddy-current; the use of glass-fibre reinforced epoxy (GRE) reinforcing and eddy-current are totally incompatible. It may improve the distinction to wind two adjacent layers in opposite directions. In the extreme case, one may choose to alter the design in order to make the system inspectable, even at the expense of part of the expected service life. During manufacturing, inspection, in the form of quality control (QC), aims to ensure that one produces what one expects. Examples are establishing the absence of breaks in the continuously extruded polymeric sealing sheath and checking the welds in the continuous steel reinforcing. QC is always in the hands of the manufacturer, but witnessed by first and third parties, after their quality assurance (QA) and/or QC has been reviewed. At this stage, the acessibility of each individual layer is at its best and finding factors that reduce the future service life is easiest and cheapest. The importance of proper end fittings does not always receive the attention it deserves. The suspicion expressed in Reference 1 is recognized. Shell have always insisted that the end fitting should be at least as strong as the pipe, statically and dynamically. A robust assembling procedure, combined with thorough QC (e.g. proper epoxy bonding in the end-fitting, avoidance of notches in the parts with varying stress), should prevail, since inspection in-service will be next to impossible. Transportation and installation has often been quoted as the critical phase, during which the pipe is threatened by misuse and impact by other objects. The result will be tears in the external polymeric sheath and/or permanent deformation of a section of pipe. More often than not, these accidents are known to have happened and their effects will be revealed by visual inspection, radiography or hydrotesting. Whether or not the effect is acceptable is not always easy to tell and detailed inspection may help to decide. The section may be discarded and the remaining pipe be mended by installing field end-fittings. Damage or deterioration during service strongly depends upon the application. For flowlines, ageing of the polymeric liner and corrosion of the steel reinforcing play a role, as well as interference with trawler gear, if unburied, or upheaval buckling, if buried. For risers, ageing and corrosion should also be considered, as well as interference with anchors or mooring lines, mechanical overloading and so-called mechanical deterioration, i.e. wear and fatigue of
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al. the steel elements. Mechanical deterioration is extensively discussed later in this paper.
Criteria for in-service inspection In addition to its cost, detection capability and accuracy, an in-service inspection technique is judged by the following operational aspects: • Early warning: a defi,~ct or process should be detected sufficiently early, so that remedial action (to satisfy safety and economics) can be taken. • Is it applicable on-stn~am or does it require a shutdown of operations? For example, internal inspection of an oil riser by an intelligent pig is on-stream; using a crawler pig it requires suspension of normal operation. If the ultrasonic inspection of a gas riser requires the internal medium to be water, then operations must be suspended. • If on-stream, does it monitor continuously or measure intermittently? Continuous monitoring means being online all the time, being triggered by an anomaly and then giving warning. Intermittent measuring means, for example, an annual measurement run by a pig, or continuous monitoring which is switched on and off to take 'fingerprints'. • Is it a global or a local technique? A global technique covers a large area relative to the sensor size, whereas a local technique makes a spot measurement, which has to be repeated if a larger area needs to be covered. • Accessibility. Can the technique be applied where needed? If external inspection is required at the riser top, the bending stiffener is in the way. The same holds true for flexible pipes in ISUs and multibores. Also, a crawler on the outside may be hindered by 'piggy-back' umbilicals.
Mechanical deterior~ttion of flexible risers Mechanical deterioration of flexible risers has been the main target of Shell Research's efforts, both in terms of testing and analysis on the one hand 2,3 and inspection on the other. We shall limit the scope of the paper to mechanical deterioration of the reinforcing, excluding the collapse carcass. The critical area was determined to be the top of the riser with bending stiffener. The mechanical deterioration process is considered independently, assuming no interaction with, for instance, c,orrosion (sour service). Until recently, unbonded flexible pipe was only available without a polymeric tape between the tensile armours or with just a thin one. Mechanical deterioration of such flexible pipe was observed to be due to the wear of these armours, followed by fatigue and fracture 3-5. Initially, Shell's inpection research efforts were directed towards detecting fractured tensile armours. When it became clear that the time period between the first fracture and final failure of the pipe may be shorter than a practical inspection interval, attention was shifted to the wear process. Here one might choose to measure the amount of wear incurred. Alternatively, one may follow the wear process itself and detect when it changes ti~om mild to severe wear. Presently, flexible pipe is also available with a solid lubricating layer, that is sufficiently thick to prevent all wear of the tensile armours. It is natural to assume that this increases the service life,. Given that operators judged the
307
design life of the old flexible pipe acceptable, the consequence of this may be that mechanical deterioration ceases to be of practical relevance. It may also lead to more demanding applications, so that the next weakest link arises. For example, fatigue of the pressure reinforcing may occur. In that case, inspection continues to be an issue. The fatigue process should be well characterized to set the inspection requirements. This means that the crack size that leaves a certain life margin (at least a year) should be known. In general, qualification testing and/or model development are needed, whenever major design changes have been made.
Assessment of inspection techniques for flexible pipe A number of techniques are now reviewed, that are aimed at fractured tensile armours, thickness measurements of the tensile armours, fractured pressure reinforcing and the detection of the wear or fatigue process. They are listed in
Table 1. Radiography Holes were drilled through the thickness of the external armours of a 4 in ID flexible pipe with diameters ranging between 1 and 6 mm. The pipe was filled with water. Conventional radiography (CR), using an X-ray source and a radiographic film, and real time radiography (RTR), using an X-ray (more accurate) or a "y-ray source and a screen (real time image intensifier) were applied, both in double wall exposure. Both CR and RTR (best out of four RTR contractors), using an X-ray source, detected holes down to 2 mm (Figure 2). The assessment was performed late in 1987 and the field of RTR may have developed further (e.g. linear arrays) since then. The detection of broken tensile armours, and to a lesser degree Zeta wires, with gaps down to 2 mm should be possible. The detectability falls when the location is off centre. Radiography in single wall exposure would be more accurate, but impractical. Regarding operations, RTR seems preferable because it would allow remote in-service inspection. An external crawler with circumferential freedom would need to be designed. The use of RTR underwater, however, is not an established method. CR, with a slightly higher accuracy, needs diver interference for application of the film and would appear practical only when applied to certain areas of limited extent, e.g. at damaged areas, the top of the riser or the end-fitting.
Magnetic strayflux When one applies a magnetic field along a flexible pipe, a stray flux field will emerge from irregularities, such as fractures and pits, which can be detected by sensor coils or Hall sensors. On a 1.5 in ID flexible pipe, three adjacent broken wires of outer tensile layer were detected by our wire cable inspection instrument (Figure 3(a),(b)); five breaks in the inner tensile layer were not detected. An area of wear is not detectable with magnetic strayflux (unless very concentrated). Such metal loss could be detected by measuring the total flux, as has been demonstrated on wire rope. However, the allowable metal loss forms such a small percentage of the total metal area (including pressure reinforcing) that it is below the detection limit.
308
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out e t al. Table 1 Assessment of the feasibility of inspection techniques for mechanical deterioration of flexible pipes. The numbers 0-5 indicate the likelihood (0 = next to impossible, 5 = high). This assessment of likelihood includes whether the technique may be applied in the field, but not the extent to which it is operationally acceptable or can be applied sufficiently frequently Wear of tensile armour Radiography Magnetic strayflux Eddy current E-reflectometry US guided waves US wall thickness Acoustic emission Photogrammetry
1
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Figure2 X-ray real-time radiography images of holes in tensile armours Time domain electric reflectometry Time domain electric reflectometry means that a short ( 101000 ns) electric pulse is applied to two parallel wires (twisted pair or coaxial). The delay of the reflection is a measure of the distance to the wire end, e.g. at a fracture. For wires made of ferromagnetic material, the difficulty is that the signal is highly attenuated, limiting the inspectable length to less than 50 m. An experiment was conducted on a short 1.5 in ID flexible pipe using the tensile armours as the screen and the collapse carcass as the conductor. Three fractures in the outer tensile armour layer were not detected. Only when all the wires were cut was detection possible. Other schemes such as taking the tensile armours one by one, would not work because they are not insulated from each others. Ultrasonic guided waves Introducing ultrasonic (US) waves in the steel tensile armour wires, using them as acoustic wave guides, and timing the reflections from discontinuities would allow the detection of fractures. The wire would be excited by one
Figure3 (a)Wire cable inspection instrument and 1.5in ID flexible pipe at Shell Research, Amsterdam US transducer and the reflection pattem picked up by the same (pulse-echo mode) or a second transducer (pitchcatch mode). For the pitch-catch mode, one could imagine placing them on either side of the suspected area or at the same side. Attenuation of the signal will take place as a result of energy lost to the environment, i.e. the other armour layers, lubrication layers, grease etc. The zero-order longitudinal wave mode was found to have the least attenuation: 16 dB/m and over, increasing (slightly) with contact pressure between wires. Given a dynamic range of approximately 100dB, this means a travel distance of 6 m (implying a distance between the transducer and the defect of 3 m at the most, when using the pulse-echo mode). For this principle to be used in practice, one would need direct access to the suspected tensile armour wires at at least one location. For monitoring the external armour layer at the riser top, excitation at the end fitting seems an option and measurement on the pipe below the bending stiffener, if access to the tensile armours were obtained.
Ultrasonic wall thickness measurements Ultrasonics (US) can be used to measure the thickness of outer tensile armour wires. A screening study was perfor-
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
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EC has proved to be able to detect 0.2 mm local erosion and a O 1 mm hole in the collapse carcass. In the ferromagentic pressure reinforcing severed faces of the wire were detected with gaps down to 1 mm, as well as holes down to O 5 mm 8. When concerned about the mechanical deterioration of flexible pipe, fatigue cracking of the pressure reinforcing is a potential failure mechanism, particularly when wear of the tensile armours has been prevented. EC is a potential internal inspection method for the pressure reinforcing, provided that the collapse carcass is not ferromagnetic. In addition, since the distance between probe and wire is at least 10 mm, a low AC frequency is called for, with an associated large probe size (order: 20-30 mm) and matching low resolution. Multifrequency EC is currently being developed so as to enhance detectability. If we set the minimum detectable crack at 10 inm, oriented along the wire, we feel that EC has potential to detect the crack at a sufficiently early stage for safe operation. External inspection with EC can detect broken tensile armours and general disarrangement, the presence of which would urge immediate action.
Photogrammetry
Figure 3 (b) Magnetic stayflux field around broken outer tensile wire
med on a 4 in ID flexible pipe with wires of 6 × 3 mm 2 (width xthickness). A 5 MHz focused probe (0.5 in Q~) was identified to be optimal in terms of attenuation versus resolution. The focusing of the US bundle was achieved by using a curved probe with a PVDF piezo-electric layer. With regard to alignment, an angle of tilt with the pipe surface of less than 5 ° could be tolerated. Figure 4 shows radial measurements (B-scan presentation) on a pipe segment, taken by moving circumferentially around the pipe. It should be noted that the pipe was not pressurized. This makes matters worse since under that condition the tensile armours are slack and slightly tilted. The reflections of the wires in the outer layer are easily visible, but an accurate determination (within 5 10%) of thickness is not possible. It was concluded that further development of the probe configuration would be needed for establishing the feasibility of the method, and that the ultimate accuracy in the field would, at best, be 10%.
Eddy current An electrically conductive material can be inspected with eddy current (EC) probes moving along its surface. EC is sensitive to: firstly, obstructions to currents such as cracks; or secondly, distance variations from, for instance, corrosion pits or erosion. The geometry of EC probes can be adapted to the application. The penetration of EC into the material is greatly reduced if the material is ferromagnetic, which generally limits the detection to surface defects. EC is well established in the field of heat exchanger tubes, including ferromagnetic ones 6. In the course of the I?PS2000 project7, single frequency
Photogrammetry is the technique of measuring the geometry of a (curved) surface, with a grid of markers attached to it, by taking and analysing advanced stereo photographs. Doing this at different times, would make changes in the geometry visible. The method is not sufficiently sensitive that it can detect the slight distortion of a mechanically deteriorated flexible pipe when a few reinforcing wires have broken. We have, however, used this technique to assess the top of a 16 in ID flexible riser sample with bending stiffener. The objective was to detect the influence of either the deteriorated pipe or a bending stiffener on the curvature distribution. The conical part of the bending stiffener and the adjacent pipe section below were equipped with markers. Surveys were made of the riser top at high curvature at different times. A thorough analysis allowed us to derive the curvature distribution along the bending stiffener and the pipe immediately below (Figure 5). Neither the flexible pipe nor the bending stiffener had undergone significant deterioration during the test. The result was in line with our expectations, indicating the proper functioning of the bending stiffener throughout the test. For this purpose, photogrammetry appears to have potential.
Acoustic emission Principle Acoustic emission (AE) is the technique of detecting the acoustic activity of a mechanical process, such as wear or fatigue crack growth. In principle, it is a passive technique. It is a global technique in that an AE sensor registers signals from a surrounding area. AE is therefore a good candidate for continuous monitoring of flexible pipe in dynamic applications. In this discussion, we use resonant transducers as AE sensors. They have the following properties. An acoustic event that is strong enough and contains a frequency component near to the transducer's resonant frequency is registered. The transducer's response to the event is characterized by amplitude, rise time and duration (number of counts) (Figure 6). Given the amplitude, the response contains no direct information on the time shape of the event. The time shape is weakly reflected in the rise
310
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Qualification AE is well established for the inspection of pressure containing fibre-reinforced plastics and is gaining acceptance in the field of steel pressure vessels, such as those used in the nuclear and oil industry, in which the presence of cracks can be 'heard' when applying a proof load. In general, AE requires a good choice of the sensor (frequency), sensor layout and detection threshold on the one hand and a good knowledge of the characteristics of the relevant acoustic events, as a function of parameters such as structure geometry, loading amplitude and frequency, pressure, temperature etc. It is certainly a challenge to avoid mistaking noise for relevant events. AE is considered for field monitoring the condition of flexible risers that are subjected to 'random' curvature changes as a result of floater and wave motion. For qualification, a thorough, but painstaking approach (if only due to the need for many test samples) would be to set up a programme of: (a) small-scale wear or fatigue tests; (b) laboratory tests on full (or stripped) pipe to failure, during which pipe would be nondestructively checked at intervals for the stage of damage; (c) full-scale verification tests on a flexible riser top section; and (d) field verification on a real application. The final deliverable would be a qualified AE system and a known influence of the parameters. For all practical purposes, this approach to qualification is impossible. AE was applied in full-scale testing
vVlllVllvvlvvvvvvv ,,,,o Figure 6 Typical AE event and its parameters
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al. conducted at Shell Research Rijswijk 2 (steps (b) and (c) of the ideal programme). Monitoring of rotary bending tests to failure showed that a significant increase of acoustic activity was apparent from approximately 90% of the test life. Subsequently, a number of tests on a flexible riser top section, under variable bending and high static tension (Figure 7), were monitored.
311
mitted along the tensile armours, thought to serve as good wave guides for this frequency, whereas when detected by the 30 kHz sensor fully isotropic behaviour was apparent • Signals generated on the tensile armours and detected on the sheath with the 175 kHz sensor were attenuated less as one moved away from the site of generation than with the 30 kHz sensor
Testing Before monitoring a full-scale 4 in ID flexible riser top section which was expected to fall from wear of the tensile armours, the characteristics of the flexible pipe were studied in laboratory experiments with a 30 kHz and a 175 kHz transducer. Signals were generated by the standard ASTM pencil lead break. It wa,; found that • Signals generated and detected on the external polymeric sheath by the 175 kHz sensor were preferentially trans-
From this it was concluded that the 175 kHz sensor would be relatively insensitive to accidental noise generated on the external sheath and best for detecting activity at the tensile armours. The additional use of the 30 kHz sensor would allow identification of such accidental noise on the external sheath and support the interpretation of the 175 kHz response. The resonant frequencies mentioned are specific for the flexible pipe in question. It should be realized, however, that acoustic waves transmitted along the tensile layer may have originated in another layer
(Figure 8). The high tension/bending test on the 4 in ID flexible riser top section was far advanced (with hindsight: approx. 90% of the test life) when the AE sensors were installed. The sample failed from wear between the two tensile armour layers within a number of days. Although this is unfortunate, it is plausible that the wear process would have deteriorated during the monitoring period. The loading is expressed in terms of the range of cycling top angle. Six classes of constant range cycling during hours to days were alternated. The root mean square (RMS) and the number of counts per time were continuously monitored. It was observed that the RMS value of the amplitude of acoustic activity peaked about 2 - 3 h after the angle range was changed from a lower class to a higher one and that the reduced level at which the RMS finally stabilized was reached after some 30 h. The RMS value was positively correlated with internal pressure. Figure 9 shows the results for the 30 kHz sensor. The peak RMS tended to increase both with time and with the loading class, whereas the number of counts increased only slightly with time and not at all with the loading. This suggests that the active process was not correlated with the loading and that with time little to no increase of active sources took place, but each had a higher energy. The results for the 175 kHz sensor are shown in Figure 10. Neither the peak RMS nor the number of counts increased with time, but both increased with the loading. The process appears to be constant with time, but more active sources emerged at higher loading. Interpreting both trends, it seems that the 175 kHz was indeed aimed at the wear process, since this should correlate with the loading, but that after all the wear process did not change with time. It is .'lot certain which process the 30 kHz sensor was sensitive to. It could be the rubbing between the bending stiffener and the flexible pipe. Because of the brief monitoring time, the statements made are tentative. Nevertheless, AE was considered promising. Receiver External polymer sheath Tensile reinforcing Pressure reinforcing ~ ,
Figure 7 High tension/bending rig at Shell Research, Rijswijk
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The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out e t al.
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During the above test, only the RMS of the amplitude and the number of counts/time were (continuously) recorded. In order to discriminate between different AE sources and damage mechanisms, it was necessary to gather more detailed information in subsequent tests. Instead of considering averages over a period, individual AE events would be recorded. In order to limit the potentially vast amount of data, a choice had to be made between raising the detection threshold (useful when the aim is to detect the snap of a breaking wire) or switching from continuous monitoring to intermittent (or trend) monitoring. The latter was preferred since the gradual wear process (or high cycle fatigue when wear is prevented) was being investigated. In addition, equipment was used that allowed six measurement sensors. The sensors may be distributed evenly around the circumference (Figure 11) but other schemes are also possible for better locating the origin. A data collection procedure was devised as follows. Every few cycles per 2000, AE data was collected and subsequently reduced to so-called fingerprint files, which have histograms of the statistics (average, mean, median, minimum and maximum) of all of the AE parameters against the number of cycles and the prevailing test condition. Four parametric signals were also recorded: internal pressure, temperature, static offset and the RMS of the top angle. Figure 12 shows an example of the number of hits/time registered by a 160 kHz sensor during a test on a 16 in ID
b
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Figure 10 175kHz sensor. (a)Peak RMS of AE amplitude; (b) counts/time. (# indicates invalid results because of insufficient run-in time)
sample that did not reach failure. It suggests that after an initial period a plateau was reached. Since the associated energy showed a similar trend, we conclude that the process monitored was stable. These and other results have neither produced firm conclusions nor disproved the potential of AE. Clearly, to qualify AE along this route, as opposed to the qualification process of the preceding subsection is arduous. Reference 10 discusses the experience with a single fullscale test to failure. Again, steps (a) and (b) of the ideal qualification programme were omitted. In this test, service conditions were not directly simulated since a shallow angle along the pipe was set and a strongly varying tension caused varying bending. Compared to service, the small slip amplitudes in the area of failure are likely to have modified the wear process. AE recording was done in short periods. Very little signal was received from a transducer on the external sheath below the bending stiffener, quite close to where the wear occurred, whereas a transducer on the top end-fitting, well removed from the wear area, was very active• The evolution with time of the distribution of AE during a single load cycle was suggested to be indicative of the damaging process. Initially, AE concentrated at the extremes of the load cycle, but gradually the middle of the load cycle started to dominate. This investigation also leaves questions that still need to be answered, before AE can be considered to be proven•
The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
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mately allow automation. The AE response depends upon the intensity of the variable loads. In order to have a basis for comparison, the sea state should also be monitored and the AE response during a defined 'three-monthly', 'biannual' storm would form the riser's signature. This makes field verification in the given case a difficult, but perhaps a necessary task. nd ictor
Discussion
Inspection for mechanical deterioration damage of unbonded flexible risers is faced with the following dilemma • On the one hand the (still evolving) design of flexible riser systems and flexible pipe is not fully proven and in-service inspection is desirable to ensure safety and availability • On the other hand the unproven design is associated with a complex failure process, so that inspection is faced with an ill-defined task
Figure 11 Configuration of AE sensors 31111 •
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Figure 12 Number of hits/time throughout flexible risers test
The potential of acoustic emission The experience with AE monitoring of the flexible riser tests is limited and data evaluation of some tests is still in progress. Proper qualification of the method is hampered by the fact that not all tests are conducted to failure and that no duplication tests on a given pipe structure were performed. A few preliminm~¢ remarks can be made. The likelihood of AE being suitable for monitoring ISUs is small. Whether or not, in the case of flexible pipe with a thick solid lubrication layer between tensile armours, the amount of energy associated with fatigue cracking of the pressure reinforcing is sufficient to be detectable is currently being investigated. Current studies on the propagation routes and attenuation of acoustic energy (Figure 8) are intended to elucidate this and may suggest that embedding sensors in the pipe wall enhances the feasibility. If the above research effort were to be successful, what would an AE system look like in practice? We may be able to discard part of the 'fingerprint files', if those parameters are not correlated wih tke failure process, or are strongly correlated with other parameters. That might reduce the amount of data produced, make it manageable, or ulti-
What measures do the current and prospective operators of flexible risers take5,9? The current state-of-the-art is that the integrity management does not include inspection, other than periodic visual inspections by ROV, since there are no readily available inspection services. In the light of this, the QA/QC should be stringent. Some advocate the monitoring of riser motions in order to establish the conservatism of the input to the design of the flexible pipe. Lastly, pressure testing may be used to confirm the integrity periodically. For the other important failure mechanism, ageing of the polymeric liner, monitoring of the diffusion rate into the annulus at the top end-fitting and installation of spool pieces or test pipes are proposed. The criteria for rejection or remedial action in the above approaches are uncertain. Work is continuing, within Shell and elsewhere, to rationalize the application of such 'conventional' approaches. Hence costly, conservative replacement may be required. This can only be avoided if monitoring techniques become available during the life of the riser or other evidence to support the design of flexible risers appears (e.g. a design model supported by full-scale testing). How should the industry go forward? Table 1 summarizes the assessment made in the preceding section on the different inspection techniques that we have considered. If there remain dynamic applications of flexible pipe without a thick solid lubricant between the tensile armours, acoustic emission (AE) remains a good candidate for monitoring the wear process between these armours. We then need to investigate the relation between the AE central frequency and the flexible pipe structure and that between AE level and the loading parameters. Suitable candidates to detect the amount of wear have not yet been identified. The development of techniques to detect broken tensile armours will not contribute to giving an early warning, but is useful for confirmation. Radiography, magnetic strayflux and eddy current (EC) are likely to fulfil this task. For applications of flexible pipe for which wear of the tensile armours has been prevented and where fatigue of the pressure reinforcing may be the governing factor, continuous monitoring by AE may also have limited potential. EC inspection appears to be a more suitable candidate. The time frame of this fatigue process must be determined before this can be decided. Inspection by EC is intermittent and may only be
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The integrity of flexible pipe: search for an inspection strategy: J. M. M. Out et al.
used on-stream- if the riser is pigable and EC is sufficiently robust to withstand pig motions.
Conclusions The design of flexible pipe and flexible riser systems for mechanical deterioration is not fully proven and the governing failure modes are quantitatively uncertain. Qualification testing and model development remain necessary as input for the inspection of flexible riser systems. Acoustic emission has potential for the inspection of flexible pipe for wear damage, but a significant development effort is still necessary. Radiography, magnetic strayflux and eddy current should be considered for the inspection of flexible pipe for fractured outer tensile armours. This inspection is useful to verify suspected damage, it does not give an early warning. For the inspection of flexible pipe for fatigue cracking of the pressure reinforcing eddy current shows the most promise. Acoustic emission may also have potential here. For the near term, effective use of conventional technology, such as monitoring of riser motions, pressure testing and annulus monitoring is recommended.
References 1 Macfarlane, C. J. 'Flexible riser pipes: problems and unknowns', Engng Struet. 1989, 11, 281-289 2 Kastelein, H. J., Out, J. M. M. and Birch, A. D. 'Shell's research efforts in the field of high pressure flexible pipe'. Deepwater Offshore Technology Conf., Monaco, October 1987 3 Out, J. M. M. 'On the prediction of the endurance strength of flexible pipe' Offshore Technology Conf. 1989, Houston, TX, paper 6165 4 Feret, J., Boumazel, C. and Rigaud, J. 'Evaluation of flexible pipe life expectancy under dynamic conditions' Offshore Technol. Conf. 1986, Houston, TX, paper 5230 5 Barthelemy, B. 'Inspection and monitoring of flexible pipes- why and how?' Noroil, February 1988, p44 6 Berg, W. H. van den and Bakker, H. L. M. 'Eddy current inspection of the internal bore of ferromagnetic heat exchanger tubes', in Nondestructive testing (Proceedings of 12th World Conference), Elsevier, 1989, p 315 7 Berge, S. and Olufsen, A. (eds) Handbook on design and operation offlexible pipes SINTEF, STF70 A92006, Trondheim, Norway, 1992 8 Midtgaard, O. and Eriksen, M. 'Eddy current inspection of flexible pipe', Flexible Pipe Technology Seminar FPT 92, Marine Technology Centre, Trondheim, Norway, February 1992 9 Marine Technology Centre, Proceedings of Flexible Pipe Technology Seminar FPT 92, NTH, Trondheim, Norway, 1992 10 Berge, S., Eide, O. I., and Hval, M. 'Acoustic emission monitoring of a flexible pipe during fatigue testing', 12th Offshore Mechanics and Arctic Engineering Conf. ASME, Glasgow, UK, 1993