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Development of an advanced medium range ultrasonic technique for mooring chains inspection in water
Graham R Edwards*, Stefanos Kokkorikos**, Alvaro Garcia,PMP*** , Christopher Patton***, Prof. Dr. Tariq Sattar***.
*Plant Integrity Ltd
Cambridge CB21 6GP, UK +44 (0)1223893994
**IKnowHow, Athens, Greece
(+)30 210 6041 425 [email protected]
*** InnoTecUK Ltd, London, SE1 6LN, UK
+44 (0) 203 2861168 [email protected]
[email protected] [email protected]
Abstract MoorInspect is a collaborative project within the European Unions (EU’s) Framework Programme (FP) 7 research programme for Small-to-Medium sized Enterprises (SMEs). The 2-year project began in October 2011 and is being co-ordinated by Plant Integrity Ltd. This paper will describe the project’s aims and the technology involved, highlighting the importance of robot applied NDT within the burgeoning offshore oil and gas industry. The NDT technique being deployed is guided ultrasonic waves (GUWs), a technology that is used extensively for screening pipe for corrosion. GUW testing of chain links is providing new challenges including wave propagation around bends in a ‘race-track’ and the skin effect apparent when propagating in solid cylinders. The GUW transducers will be applied above and below water from a robot that is able to climb the chain. Current results are proving to be very promising, but new signal processing algorithms are needed to extract relevant features from the test signals. 1. Introduction Mooring chains are safety critical structural components of many floating structures, including oil and gas production platforms and Floating Production, Storage and Offloading (FPSO) systems. In the future, they may play an equally important role in offshore platforms for deep-water wind farms and perhaps in the very distant future, with bursting world populations and rising sea levels, for floating ‘cities’. The most pressing current application is for FPSOs, where mooring chains anchor the central ‘turret’ around which the ship rotates in the sea current (Figure 1). There may be 14-20 mooring chains around one turret, extending down to the sea floor. In modern
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deep-water oil and gas fields, such as those found off the coast of Brazil, these may extend down to a depth of 2Km and for weight reasons, only the top and bottom 30-50m are made of chains, the rest being wire rope, or special composite cable. It is the chains however, which take the greatest loads and complex twisting motions can lead to premature failure, particularly near the hawse, where the chain enters the turret. Failures do occur. With one chain failure there is sufficient redundancy to maintain stability, but once one chain has broken, others may soon follow, when their structural integrity has been weakened by the presence of cracks. Indeed, there have been instances where the FPSO has broken free. This can lead to rupture of risers bringing oil or gas up from the sea-bed. The calamity of the ‘BP Horizon’ exploration rig shows what can happen if safety cut-offs fail.
Figure 1. FPSO with external turret
Figure 2. Typical mooring chain and crack locations.
The structural integrity of the chain to withstand the complex loads that exist on floating structures must be maintained. To reduce the probability of failure, chains are inspected periodically. Dimensional checks for wall loss due to corrosion or elongation can be done by divers using special sub-sea callipers. Generally however, divers are not allowed near chain links for reasons of safety and the inspection is no more than a ‘swim-by’ with a diver or Remote Operated Vehicle (ROV). Chains can be removed and taken to dry-land for inspection, which is the usual case for drilling rigs that can come on shore, but for fixed platforms and FPSOs this is an onerous task, which is rarely undertaken. It is well just to replace the chain. Any in-situ inspection system for chain links must therefore be robotic. It must be able to detect flaws and discontinuities that create a significant risk of failure ie defects, in the presence of other innocuous flaws and discontinuities. This balance between high defect sensitivity and low false-call rate is a common one in Non-Destructive Testing (NDT). NDT relies on a physical process such as reflection of ultrasound or disruption of magnetic fields to detect discontinuities and the relationship of this process to size of defect is never a directly proportional one. For this reason NDT procedure development is a crucial task. In the MoorInspect project Medium Range Ultrasonic Testing (MRUT) has been selected as the NDT method. Defects in chains normally occur in the surfaces between the chain links where fretting and out-of-plane twisting can lead to fatigue cracks (Figure 2). These slowly propagate
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through the chain link until rupturing suddenly. There is still great uncertainty as to the initiation and critical size of these cracks. This is the subject of a research project at TWI, Plant Integrity’s parent company. Therefore chain link design has a great deal of redundancy and cracks may grow to 50% of through-wall depth before failure occurs. The critical crack size is not of the order of millimetres, but rather of centimetres. This is fortunate, because the conventional ultrasonic techniques used to measure crack sizes in the order millimetres require high ultrasound frequencies, short pulse lengths and narrow ultrasound beams, all of which are dependent on a smooth surface on which couple the ultrasound probe with the test object. This is clearly not possible in the case of chain links in-situ, where surfaces are rough and often encrusted, as well as tightly curved. Compared with conventional ultrasonic testing, MRUT and the Long Range Ultrasonic Test (LRUT) method from which it is derived, use lower frequency ultrasound; KHz rather than MHz, use guided waves rather than bulk waves that flood the whole inspection volume with ultrasound, and are essentially a ‘dry-coupled’ technique that will not suffer from coupling problems to the same degree. This is at the expense of reduced sensitivity however. Existing LRUT is limited to detecting discontinuities of the order of several centimetres in size and so an improvement in sensitivity is being sought for chain inspection. This will be achieved by increasing the ultrasound frequency from a range 20-100KHz to a range 100-500KHz. This will limit the test range to less than 10m, compared with up to 100m for LRUT. Therefore the term MRUT is being used to distinguish it from LRUT. The range limitation should not be a problem for testing chains, since the perimeter of an individual chain link is less than a metre. It should be stated that the MRUT technique developed in MoorInspect will test chain links individually. It is not possible to couple the chains together ultrasonically. The inspection robot must therefore be able to climb the chain and place the ultrasound transducers on individual chain links. Because the individual chains are linked in planes at right angles to each other, the design of the climbing mechanism for the robot is not a simple task. Other test procedural challenges arise from the need to propagate the guided waves around the tight bends at either end of the chain. Guided waves used in LRUT test long straight lengths of pipe using waves that are symmetrical around the pipe. These will be distorted when propagating around a bend. Moreover, while pipes are hollow cylinders, chains are solid cylinders. Guided wave propagation around chain links is therefore very complex compared with guided wave propagation along pipes. This must be investigated in order to develop appropriate procedures for testing chain-links. However our level of understanding is unlikely to reach levels where definitive analysis of A-scan data collected from tests is possible. Interpretation will rely on pattern recognition techniques, which are being developed in the project. 2. Project aims and objectives Previous experience had shown that it is possible to propagate guided ultrasonic waves around a chain link and moreover detect a machined notch 50% through the chain wall. However, for any new NDT method to gain acceptance, it must show significant improvements upon the detection capabilities and accuracy of existing inspection methods. The aim of MoorInspect is therefore to demonstrate a prototype MRUT
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system that can achieve this acceptable level of defect sensitivity and measuring accuracy. To achieve this aim, the project was set the following scientific and technical objectives:- 1. To develop an MRUT technique using guided waves to detect cracks in mooring chains, particularly in the ‘hidden’ area between the chain links. 2. To investigate the use of alternative ultrasound transducers to current Lead Zirconium Titanate (PZT) ceramic ones. i.e. Electro-Magnetic Acoustic Transducers (EMATS) 3. To develop signal processing and data analysis methods for interpreting ultrasonic test data (A-scans) from the chains. 4. To develop a graphical user interface that includes a representation of the link within a length of chain and the location of any defect detected. 5. To develop a robot vehicle for climbing the chain and deploying the MRUT transducers. 3. Project work programme. MoorInspect is divided into eight work packages, beginning with a full MRUT system specification drawn with the help of potential ‘end-users’. The project consortium includes two end-users, Single Buoy Moorings and Vicinay Cadenas, especially for this purpose.
Figure 3. Technology readiness levels
The research and development is divided between 3 Work Packages (WPs), each allocated to a Research and Technology Developer (RTD) with relevant expertise:
1. Development of marinised transducer housing’. – Plant Integrity Ltd. 2. ’ ‘Development of MoorInspect software for control, data collection and
automated defect detection’. – IknowHow. 3. ’ ‘Development of prototype marinised inspection capsule for deployment of
transducer housing in-water’. – InnoTechUK. There is a work package devoted to dissemination activities, including a web-site (moorinspect.eu) and a plan for use and dissemination of foreground knowledge, ie
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knowledge and expertise as well as any products and services resulting from the project. Using the concept of Technology Readiness Level (TRL) (Figure 3), the project will end with successful demonstration of a prototype, either TRL5 or TRL6 depending upon the environment in which it is tested. To rise to a TRL of 10, additional funding will be sought. Although some funding may be made available through the EC’s so-called ‘Demonstration’ projects, the most likely source of funding will be from potential end-users. It is important to note that the last TRL, TRL10, is being introduced specifically for NDT, because of the need to thoroughly qualify procedures, equipment and operators. Finally, as part of the dissemination activities, a ‘Road Map’ is being created for the MoorInspect system as a whole and for its individual components. The SMEs sponsoring the project have specific interests in each of the individual components. Presently, at the mid-term stage of the MoorInspect project, work that has been carried out in the three RTD work packages as follows:- 3.1 Development of marinised transducer housing. The MRUT technique will require a transducer housing that can be clamped around the chain link by the climbing robot above or below water. To begin with, a fundamental investigation of guided wave propagation around chain links was undertaken, before moving onto procedure and transducer development. 3.1.1 Investigation of guided wave propagation around chain links. As part of the NDT technique development, numerical modelling has been done of guided wave propagation around chain links, supported with experimental work using PZT transducer collars. To begin with, guided wave propagation along straight solid bars was considered. Guided waves are complex. They exist in specific modes determined by ultrasound wavelength and cylinder diameter. In effect, guided waves only form at particular ‘resonances’, dependent on the wavelength/diameter ratio. So-called ‘dispersion curves’ for a specific cylinder diameter can be generated showing which wave modes exist within a specific range of ultrasound frequencies and how the velocity varies with frequency. Modelling showed that there were many more wave modes possible in a solid cylinder (Figure 4) than in a hollow one (pipe) in the ultrasound frequency range normally used in LRUT (20-100KHz). This has important consequences on the guided wave technique to be developed for chains. 1. The abundance of L-wave modes that can exist at all practicable frequencies makes T-wave mode more practicable. 2. Only at low frequencies (<50KHz) can comparative wave mode purity be achieved. 3. At the medium range frequencies envisaged in the original project plan, only Rayleigh (surface) waves exist.
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Figure 4. Dispersion curves for a 110mm diameter solid rod
Transmitted wave
Wave past bend
Figure 5. Guided wave propagation past a bend
On chain links, distortion of the symmetrical wave, transmitted from the ultrasound transducers that encircle the straight side of the chain link, was expected. However, numeral models showed there remained a sizable symmetrical wave proportion after the wave had gone around the bend (Figure 5). This was confirmed experimentally using a conventional LRUT collar (Figure 6). These data were collected in a frequency range 30-100KHz, well below the target range for MRUT. It showed that useable data could be collected at this low frequency, which could be matched with the main geometric ultrasound reflector in the chain; the flash weld that joins the link together.
Figure 6. A-scan collected from a 110mm chain link.
Figure 7. EMAT designs investigated.
The modelling work has shown that usable test data can be collected at the lower ultrasound test frequencies and that distortion around the bends is less than expected. At higher test frequencies, Rayleigh waves dominate and the modelling work is now looking at ‘skin effects’ imposing upon the depth of penetration.. 3.1.2 Investigation of alternative transducers The LRUT method from which the MRUT method is derived uses a collar of ultrasound transducers that encircle the pipe. The transducers are of PZT ceramics. For MoorInspect, it was decided that EMATs would be investigated as an alternative, firstly because the rigid ceramic transducer might not sit properly on the tight curvature of the
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chain link and secondly because the chain’s surface roughness might make the inductive ultrasound coupling of the EMAT more efficient than PZT. Chains are made from highly ferro-magnetic low carbon steels. Therefore the EMATs can rely on the magneto-strictive effect to generate ultrasound waves. By applying an alternating magnetic field from a coil across a permanent ‘bias’ magnetic that aligns the magnetic dipoles in the chain, stress waves can be generated. With careful selection of the bias field direction and strength, the coil turns, applied current and AC frequency, the stress waves can be ordered to produce specified guided waves. Trials have been made with a number of EMAT configurations including encircling coil, pancake coil, meander coil and Periodic Permanent Magnet (PPM) coil (Figure 7). The encircling coil EMATs were the simplest and most effective, but they have been ruled out for impracticability when deployed from a clamp. Of the other coils, PPM has proved the most effective as transmitters. They generate Shear Horizontal (SH) waves, which should be very sensitive to surface breaking cracks. Work is continuing on improvements to the performance of EMATs as receivers, so the procedure development and marinised collar design have had to progress using conventional PZT transducers. The coupling conditions for a prototype MoorInspect transducer collar are relatively benign and PZT transducers will suffice. With the MRUT procedures developed for PZT transducers it should be possible to modify these for EMAT transducers beyond TRL5 and therefore after MoorInspect has been completed. EMATs will be written into the project’s ‘Road Map’. 3.2 Software development. The complexity of A-scans generated from guided waves propagating around a chain link require a higher level of processing than is currently necessary for LRUT of pipes. This is evident from the A-scans collected from six 110mm chain links tested in WP2 as part of the MRUT procedure development. Two of these contained machined slots in the critical inspection area. The A-scans were collected in frequency sweeps between 30KHz and 100KHz (Figure 8 & Figure 9).
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Figure 9. A-scan frequency sweep from chains without slots
There is an evident difference in the pattern of peaks, but the pattern varies with frequency, the distinction between conditions being more pronounced at some
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frequencies than at others. The figures show A-scans created from received symmetrical wave signals. When received flexural waves are taken into account, even more information can be gathered about the size and location of the slots. The steps being used in developing a pattern recognition software are illustrated in Figure 10.
Figure 10. Pattern recognition system architecture
Figure 11. A-scan (20kHz) from 10mm thick plate (top) and wavelet analysis using various mother wavelets (bottom)
The necessary work for the signal preconditioning module was determined in order to transform the LRUT raw data to a signal recognisable by the consequent signal processing algorithms. A-scans collected by propagating A0/S0 guided waves through a 10mm thick plate between two transducers 12.5 m apart were used to demonstrate the correct functionality of the routines that were developed. Matlab® and the Wavelet Analysis Toolbox® were used for the processing. At first, the envelope function was applied on the raw signal using the Hilbert Transform and then several wavelets were tested to significantly increase the signal to noise ratio of the test samples. The results for a 20kHz A-scan are shown in Figure 11. In the module of the feature extraction, a number of signal features were tested, in order to identify which of them differ more discretely among defected and non-defected samples. The selected features were the following: Estimated Central Frequency, Central Frequency Deviation, Bandwidth, Dominant Pulse Power, Deviation, and Covariance. Then, a neural network was designed using Matlab® and trained using a specialised training algorithm. In the next stage, a set of LRUT signals acquired by Pi's Teletest® equipment on 110mm diameter studless chain links were input in the software and the neural network classified them in defected and non-defected. Depending on the set of features that were used for the classification, different success rates were accomplished. In Table 1, the test results are being summarised.
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Table 1. Classification results of 7 LRUT samples for 100 kHz chain link testing Pair of features selected Correctly classified Misclassified Success rate Central frequency deviation vs Estimated central frequency 4 3 57%
Central frequency power vs Bandwidth 5 2 71% Estimated covariance vs Estimated standard deviation 3 4 43%
Correlation with base non-defected vs Covariance with base non-defected 6 1 86%
Correlation with base defected vs Correlation with base non-defected 7 0 100%
Covariance with base non-defected vs Covariance with base defected 7 0 100%
The future work for the MoorInspect software has been identified as follows: 1. More sample evaluation has to take place in order to secure the robustness of the classification software 2. Further signal processing algorithms have to be developed in order to be able to identify the type, size and location of the defect. 3. Graphical user interface along with visual representation of the chain link and any defects detected. 4. An HMI has to be developed. It will incorporate all technical aspects of the project, including the signal processing and neural network, the robot control software and the LRUT data acquisition 3.3 Inspection capsule development. The aim of this part of the work programme is to develop a robot that can climb the chain, above and below the water-line, to deploy the inspection collar at individual chain links Climbing is complicated by the fact that the plane of each chain link is at 90° degree to the next. Seven traction mechanisms were considered before deciding upon a final robot design. These included the use of magnetic wheels and ‘grippers’. Models were built and ‘brain-storming’ sessions with technical staff held to establish the advantages and disadvantage of each. The selected final version can be observed in the following picture, Figure 12.
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Error! Reference source not found.Figure 12. Inspection capsule design
The structure proposed by InnotecUK imitates the method used by a human trying to climb up/down the chain underwater and in air. The frame has been designed to embrace the chain in order to reduce the likelihood of losing the robot and assure that the robot can work when the chain has a certain inclination angle different to 90 degree in relation to the floor. Besides, this structure can be easily upgraded to work with different sizes and types of chains or even when two consecutive chain links are not perfectly placed at 90 degree between both of them. The hooks can be considerated as arms with two different types of movement: Up/down to climb up/down the robot along the chain (Using stainless steel screw bars) and Close/open to support the robot structure weight over each chain link. These two movements, per hook, have to be perfectally synchronized by the control system in order to reach the next chain link. A set of motor-gearboxes will be used to ensure smooth movement and the enough torque to counteract all external forces taking into account the worst working environment (in Air). The hook structure may be adjusted to take into account its manufacturing process. Internal tests are being currently performed by InnotecUK to verify the integrity of the mechanical structure and reduce, as much as possible, its complexity. Underwater, the robot will move more easily due to the Archimedes’ principle. InnotecUK will evaluate the possibility of including in the design a buoyancy system to reduce the weight of the robot underwater and, hence, to reduce the power needs. A set of rollers will be installed; figure 12, at two faces to slide the frame over the chain and make its movement easier. The motors and the rest of electronic devices needed will be supplied, monitored and controlled using an umbilical cable. This umbilical cable will allow installing the heaviest devices on surface and reducing the likelihood of waterproofing problems. For example: Servos, MRUT instrument, Power Supplies, etc. The motors, webcams, NDT sensors and position sensors will be placed on the robot and they will have to be sealed to protect them against seawater.
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The monitoring and control of the robot will be performed by a tele-operator using a MMI (Man Machine Interface), provided by InnotecUK, and with which all robot movements will be controlled/monitored and the sensors output will be showed. The position of the NDT collar will be also controlled using this MMI. Nowadays, InnotecUK has already started the procurement process of all control devices, including the motors and the gearboxes, and is currently designing the surface rack in where the Servos, Industrial PC and the Power Distribution Unit will be installed. Moreover, if piezo-electric transducers are to be used, then an even force of about 70 Newtons must be applied to each transducer around the ring.
3. Conclusions
MoorInspect is a 2-year collaborative project within the EC’s FP7 to support SMEs with research and development funding. The project aims to develop a prototype inspection robot that uses guided ultrasonic waves to detect fatigue cracks and wall loss in the chains that moor offshore floating platforms and FPSOs. At the half-way stage, the following have been developed; NDT procedures for a PZT transducer collar, a software architecture for recognising patterns in the test A-scans and a mock-up of the inspection capsule for deploying the transducer collar on the chain link. Acknowledgements MoorInspect is collaboration between the following organisations: ByTest, Orme, Robotnik Automation, Sonomatic, InnotecUK, iKnowHow, Plant Integrity, Vicinay Cadenas and SBM. The Project is partly funded by the EC under the Research for the Benefit of SMEs programme in Grant Agreement 286976.