Skip to main content
European Commission logo print header

Development of Advanced Shearography System for On-Site Inspection of Wind Turbine Blades

Final Report Summary - DASHWIN (Development of Advanced Shearography System for On-Site Inspection of Wind Turbine Blades)

Executive Summary:
Wind energy is a fast growing industry worldwide and according to the European Wind Energy Association European companies have two-thirds of the market share. Due to the increased number of wind farms, new inspection techniques with reduced operating costs and enhanced reliability are now required. It is estimated that the business opportunity for wind turbine blade (WTB) inspection is at least €1bn per annum, and increasing rapidly.

The DashWin project has developed a novel NDT system incorporating advanced shearography inspection techniques and a robotic deployment platform. It will enable inspection of in-situ composite WTBs, so that blade degradation due to fatigue or natural incidents can be determined, before a breakdown or catastrophic failure occurs. This is the first time that a shearography system has been used for inspecting in-situ WTBs. It will provide significant advantages over existing inspection systems.

The DashWin project has achieved the following main results:

(1) A novel shearography system and associated procedure has been developed which is able to inspect WTBs in-situ on a wind turbine tower.

(2) Comprehensive algorithms and procedures have been developed for phase extraction from shearography fringe patterns and for addressing rigid body motion of the WTB during inspection.

(3) A robotic climbing platform has been developed which is able to deploy the shearography system along the wind turbine tower to inspect the WTB without contacting it.

(4) The shearography system is integrated with the robotic platform so that the inspection can be carried out remotely by an operator on the ground.

(5) A software package for remote control of the whole system, data acquisition and transfer, image processing, result interpretation and information storage has been developed.

(6) Laboratory and on-site tests have been performed on various WTB samples which demonstrated the capability of the system.

(7) A field trial has been conducted on a Vesta V47 wind turbine tower which showed that the integrated DashWin system was working as expected.

The project website (www.dashwin.eu) is available and a promotional video was produced and distributed on Youtube.

The project has achieved the main objectives with a field trial carried out in a wind farm in Greece showing that the whole DashWin system is working on a wind turbine tower. Both the developed shearography system and the robotic platform can find wider applications beyond WTB inspection. For example, the shearography can be used for inspection of other non-stationary engineering components where environmental vibration is inevitable e.g. in a production line. The robotic platform has a potential for to deploy other NDT techniques for WTB inspection, and can be used for maintenance.

Project Context and Objectives:
1. THE MARKET NEEDS

Wind energy has become one of the fastest growing sectors in the world’s energy markets. According to a report of the European Wind Energy Association (EWEA), the annual installations of wind power have increased over the last 13 years, from 3.2 GW in 2000 to 11.2 GW in 2013, a compound annual growth rate of 10%. A total of 117.3 GW is now installed in the European Union, an increase in installed cumulative capacity of 10% compared to the previous year. According to EWEA, approximately €340 billion will be invested in wind energy in the EU-27 between 2008 and 2030. This can be broken down into €31 billion in 2008–2010, €120 billion in 2011–2020 and €188 billion in 2021–2030.

The European Commission has long recognised the indispensable role the wind energy sector plays in Europe. In its Strategic Energy Technology Plan (SET-PLAN) formulated in Nov 2007, the European Commission proposed to launch six Industrial Initiatives, the first being the European Wind Initiative, focusing on large turbines and large systems validation and demonstration (relevant to on and off-shore applications). In January 2008, the European Commission proposed a new legal framework for renewables in the EU, including a distribution of the 20% target between Member States and national action plans containing sectorial targets for electricity, heating and cooling, and transport. To meet the 20% target for renewable energy, the European Commission expected that ‘wind could contribute 12 per cent of EU electricity by 2020’.

All this indicates that there will be more wind farms to be built in the coming decade. However, more wind turbines in operation means more accidents that may occur. This has been demonstrated by the data collected by the Caithness Windfarm Information Forum (CWIF). Up to 31 March 2014, the total number of accidents that have been reported is 1549. Figure 1 shows the yearly distribution of these accidents.

Therefore, there is a clear market need to develop various inspection techniques that can meet the requirements of inspecting wind turbine installations so that such structural failures and catastrophic consequences can be avoided. These inspections will be carried out at various stages, including during manufacturing, before installation and during operation. According to a report (Scottish Enterprise 2013), the global market of annual Operation &Maintenance of wind turbines excluding equipment and grid costs in 2020 will be over €1.3bn and this number will increase as the total number of wind turbines grows and the existing turbines age.

2. The needs of SMEs

In Europe, organisations carrying out mechanical testing, NDT inspection and maintenance services for end-users in the wind energy industry are typically SMEs supplying labour intensive services at low margins. Wind energy is one of the few sectors in Europe that has experienced rapid growth in the last two decades. Entering into this fast growing market is clearly the aim of many SMEs. One way of achieving this is to acquire a cost effective NDT tool to conduct inspections of wind turbine blades.

Firstly, for inspection service SMEs, a novel non-contact inspection system would allow them to carry out NDT of wind turbines on-site in a cost-effective way. It will enable them to be in a better position to penetrate into the high value-added global market for on-site inspection. By putting a premium on a workforce with higher level skills for relevant SMEs, it is likely that those SMEs can attract quality focused investment and create more and better jobs.

Secondly, for wind energy operators, the identification of potential defects within WTBs at an earlier stage will help them avoid breakdown of the installation, and can save costs of repairing or replacing a failed blades, which can run to hundreds of thousands of Euros. According to a report (Moan 2002), the lifetime inspection costs are estimated at 30% of initial capital investment, and the annual inspection costs account for 40% of operating costs. Currently, inspections of WTBs are mainly carried out in factories or within an inspection facility. The techniques used include ultrasound, radiography, thermography and shearography. Once installed and in operation, it is very difficult to carry out WTB inspections using these existing techniques because of the complex structural conditions. Therefore the development of an advanced NDT technique will have significant benefits to wind energy industry.

3. The technological needs

Statistics of the wind energy market showed that large scale manufacturing of wind turbines only started in 1990s. As wind turbines are expected to work 90% of the time during a typical lifetime of 20 years, structural flaws are of great concern, particularly for blades (Drewry and Georgiou, 2006). Repairing or replacing a blade is always difficult, especially for large wind turbines. The downtime of a wind turbine installation due to a blade failure usually lasts 4-6 days, longer than that due to electrical or mechanical failure within the nacelle. Cracks in the blades sometimes appear soon after manufacture. Defects also can be produced during transportation.

Manufacturing flaws

Manufacturing flaws can cause problems during normal operation. For example, blades can develop cracks at the edges, near the hub or at the tips. Fibreglass rotor blades are regarded as the most vulnerable components of a wind turbine. Typical manufacturing flaws on the blades may be summarised as delamination, adhesive flaws and resin-poor areas. Specific flaws at the following locations are of particular concerns:

• Skin/adhesive: bad cohesion between the skin laminate and the epoxy or the epoxy is missing.

• Adhesive/main spar: no cohesion between the adhesive and the main spar.

• Delamination in main spar laminate.

• High damping in skin or main spar laminate, which could be caused by porosities or change of thickness of laminate.

Operating conditions

The magnitude and orientation of wind change constantly. A gust may exert a much greater dynamic load on a wind turbine in addition to the averaged one. Virtually all components of a wind turbine are subject to damage, including everything from the rotor blades to the generator, transformer, nacelle, tower and foundation. Wind turbines do have regular maintenance schedules in order to minimise failure. They undergo inspection every three months, and every six months a major maintenance check-up is scheduled. This usually involves lubricating the moving parts and checking the oil level in the gearbox. It is also possible for a worker to test the electrical system on-site and note any problems with the generator or hook-ups.

In-service flaws

The environmental temperature change as a result of wind flow will add strong noise to the captured thermal images. Shearography has long been recognised as a powerful inspection technique, and has found wide application in a range of industries including aerospace, automotive and shipbuilding sectors. However, conventional shearography still needs a relatively stable environment, thus is difficult for on-site WTB inspections. With the successful execution of this project, a novel shearography inspection system will be developed within two years. As a result of this project, regular on-site inspection of WTBs will become practical for wind energy operators. This requires a novel non-contact inspection system.

Competing technologies

There are a variety of NDT techniques that have been widely used in industry, but few of them can be applied to inspect a WTB on-site. Ultrasonic testing (UT) is a pointwise contact inspection technique, thus is difficult to use to inspect a WTB on-site. Radiography has safety issues because of the use of radiation. Thermography is a promising NDT technique, but its capability of inspecting a WTB on-site is not proven, because the environmental temperature change due to wind flow will add strong noise to the captured thermal images.

All this shows that there is a clear need to develop an advanced non-contact NDT technique for wind energy. With such a technique, SMEs will benefit enormously, strengthen their competitiveness and increase their share in the NDT inspection market.

4. Objectives of the project

This project aimed to provide wind energy operators with a quick and effective means to inspect wind turbine blades on the monopile. It would consist of an advanced shearography kit and a robotic deployment platform. The system was expected to be able to inspect a composite WTB on-site without dismantling it, so that degradation of WTBs due to fatigue or natural incidents can be found before a breakdown or catastrophic failure occurs. The system to be developed will have the following main scientific and technical objectives:

Objective-1: To develop a shearography system that will be able to inspect a wind turbine blade in situ without dismantling it.

Objective-2: To develop a flexible dynamic phase extracting technique that will be able to detect phase of the speckle pattern of a WTB on-site.

Objective-3: To develop a technique to cope with the in situ rigid body motion of a WTB due to wind.

Objective-4: To develop a robotic manipulation system that can carry and deploy the integrated digital shearography system along the wind tower. The robotic system will be designed to withstand the strong winds that typically exist in such fields. The robotic platform and the shearography system will be controlled by an operator on the ground, thus eliminating the danger of operating at height for the inspection engineers.

Objective-5: To develop a software system with high-level functions comprising image signal processing, phase extracting, rigid body motion compensation, defect reconstruction from shearographic fringes, information storage and user interface.

Objective-6: To carry out a field trial to validate the system on a wind turbine installation.


REFERENCES

1. EWEA: ‘Wind in power: 2013 European statistics’, February 2014

2. European Commission: ‘A European Strategic Energy Technology Plan (SET-PLAN)’, 22 Nov 2007.

3. GWEC: “Wind Energy – The Facts”. http://www.wind-energy-the-facts.org/en/home--about-the-project.html

4. CWIF: ‘Summary of Wind Turbine Accident data to 31 March 2014’, http://www.caithnesswindfarms.co.uk/AccidentStatistics.htm

5. Innovation in Offshore Wind: Installation, Operation & Maintenance, Scottish Enterprise, 2013, http://www.scottish-enterprise.com/~/media/SE/Resources/Documents/MNO/OW%20Innovation%20-%20IOM%20v1.0.pdf.

6. Moan T., ‘Recent Research and Development relating to Platform Requalification’, Trans. ASME, V122, P 20 2002

7. Drewry MA and Georgiou GA, ‘A review of NDT techniques for wind turbines’, Annual British Conference on NDT, Stratford-upon-Avon, UK, September 2006


Project Results:
The main scientific and technological results arising from the project are as follows:

- In Work Package 1, system specification and sample preparation was carried out, which included sample procurement, a user requirement survey and production of the functional specifications of the DashWin system.

Firstly, information concerning the end-user requirements (working conditions for WTB inspections, types of defect found in WTBs, etc.) was collected and analysed (see Figure 2 and Table 1 in attached document). For WTBs, the defects can be categorised into seven types [1]:

Type 1: Damage formation and growth in the adhesive layer joining skin and main spar flanges (skin/adhesive debonding and/or main spar/adhesive layer debonding).

Type 2: Damage formation and growth in the adhesive layer joining the up- and downwind skins along leading and/or trailing edges (adhesive joint failure between skins).

Type 3: Damage formation and growth at the interface between face and core in sandwich panels in skins and main spar web (sandwich panel face/core debonding).

Type 4: Internal damage formation and growth in laminates in skin and/or main spar flanges, under a tensile or compression load (delamination driven by a tensional or a buckling load).

Type 5: Splitting and fracture of separate fibres in laminates of the skin and main spar (fibre failure in tension; laminate failure in compression).

Type 6: Buckling of the skin due to damage formation and growth in the bond between skin and main spar under compressive load (skin/adhesive debonding induced by buckling, a specific Type 1 case).

Type 7: Formation and growth of cracks in the gel-coat; debonding of the gel-coat from the skin (gel-coat cracking and gel-coat/skin debonding).

Using shearography, most of these defects can be detected. Table 2 shows the detection capability that the DashWin shearography system was expected to achieve. It was produced via detailed discussions with the project consortia, with particular input from the end-user and the SME partners. Main requirements of the DashWin inspection system include:

• Avoid any contact with turbine blades during deployment.
• Minimise time spent by personnel working at height.
• Capture digital images of the specimen in normal daylight conditions.
• Cope with rigid body motion of the WTB relative to the inspection unit.
• Generate and interpret fringe patterns identifying potential defects.

For the robotic platform, it was decided that it would be a Cable Climber based upon a manually operated platform designed by BSR (shown in Figure 3). This was because the Cable Climber design met all of the requirements for on-site WTB inspection.

Furthermore, two WTB test samples were procured which had various surface breaking defects (Figure 4 and 5). An additional test board was also obtained (Figure 6) which has a dimension of 900mm x 600mm. Six artificial defects were embedded within the test board:

>Defect-1: Dry laminate
>Defect-2: Epoxy Filler in Laminates
>Defect-3: Crack Core Material
>Defect-4: Foreign Object in Laminates (Masking Tape)
>Defect-5: Foreign Object Causing Laminate Void (Wood Splinter)
>Defect-6: De-lamination.

These samples were used to validate the DashWin inspection techniques under a ground working condition.


- In Work Package 2 an advanced shearography system and associated phase extraction algorithms were developed.

In Task 2.1 a comprehensive numerical simulation relating to the use of digital shearography to detect potential defects within a WTB was carried out.

Firstly, finite element models of a 10m whole WTB (Figure 7) and a 1m WTB section (Figure 8) were generated to investigate the mechanical behaviour of the WTB under wind load conditions. Table 3 shows the material properties used in the simulation, and Figures 9-11 show the boundary conditions of the models. Figures 12-14 show the finite element mesh. The deformation and stress distribution of the WTB models under load was obtained as shown in Figures 15-18.

Secondly, artificial defects including void, delamination and debonds were generated in the model by means of designating a specific layer with significantly reduced mechanical properties, as shown in Figures 19-20 and Tables 4-5. The maximum principal strain E and shear strain E12 along a surface path are plotted in Figure 21 and 22. For a surface where there is no tangential force, the surface shear strain should be zero. However, a close look at the surface shear strain E12 showed that this was not the case in the path plot. This means that the simulation accuracy of composite materials does not match the requirement of shearography test. Therefore further simulations using simplified model but with high mesh density were performed. The simplified WTB models were based on a 3D square plate element and a 2D axisymmetric circular plate element. A series of systematic numerical simulations was carried out. Figure 23-26 shows some typical results, which demonstrated that thermal loading can produce the required magnitude of deformation for shearography to inspect a WTB in situ.

Thirdly, shearography fringe patterns associated with different subsurface defects were simulated. Figure 27 shows a shearography fringe pattern on the whole surface of a 200mm x 200mm plate with a shearing distance of 1mm. When the carrier fringes are excluded, a typical butterfly pattern will appear (Figure 28), which highlights a subsurface defect. Figure 29 shows the shearography fringe patterns produced with different image shearing distances. These simulation results led to a greater understanding of the defect detectability for the DashWin system.


In Task 2.2 various optical set-ups were tested in conjunction with both a conventional camera and a high-speed camera to develop a spatially phase shifted shearography. Figure 30 shows a Wollaston prism which was used as an image shearing device, Figure 31 shows two set-ups with a conventional camera and a high speed camera.

A continuous 532nm wavelength (green), 100mW laser source is used to illuminate the specimen under inspection. Two types of heating lamps (halogen lamp and infrared lamp, Figure 32) were used to heat the sample during inspection.

When a slit or a double-hole aperture is placed in front of the imaging lens (Figure 33), spatially phase shifted shearography pattern can be obtained with improved fringe quality. For a double-hole aperture, the recorded speckles are modulated by carrier fringes as shown in Figure 34. The resultant shearography fringe pattern (Figure 35) has improved quality. However, most of the illumination laser light is filtered out because of the extremely small aperture (less than 1mm), resulting in greatly sacrificed image intensity. When no slit or hole apertures are used, the F-number of the imaging lens should be set between F4 and F11. A large F-number setting will result in a large speckle size, which is beneficial to the correlation tolerance, but detrimental to fringe visibility.

When dealing with fast dynamic deformation, the introduction of a high-speed camera would improve defect detection. However, since thermal stressing is the only practical excitation method for on-site WTB inspection, the deformation of the WTB is not fast and so a high-speed camera is not required. Replacing the conventional camera with a high-speed camera did not significantly improve defect detection. Consequently, only the IDS µEye conventional camera was used in subsequent tests. Experimental tests showed that a conventional digital camera with a frame rate of 10-20fps was sufficient to record the dynamic speckle images both during the heating period (when the heating lamps were switched on) and the cooling period (when the heating lamps were switched off).


In Task 2.3 Flexible dynamic phase extracting techniques have been developed which can extract phase from a single specklegram.

Firstly, two conventional phase extraction methods (Fourier Transform Method (FTM) and Improved Fourier Transform Method (IFMT)) have been designed and implemented to extract phase information from single shearograms.

FTM is a technique suitable for phase analysis of continuous deformation and requires only a single fringe pattern. By using a Fourier Transform for phase evaluation, the multiple (at least 3) phase shifts that are normally required in the phase-shifting technique are no longer needed. However, FTM needs to generate a high frequency carrier fringe, which usually requires a complex optical set-up to tilt the wavefront. This causes difficulties in applying the FTM technique in cases where the deformation is changing over time. For WTB inspection, if the defect is too small, its effect on a shearography fringe image will also be very small. Quite often such small signals are treated as noise by the FTM method and hence are filtered out. Therefore, the FTM method is only suitable for inspection of defects that will have a sufficient influence on the surface deformation (e.g. a small but near surface defect, or a large but relatively deep defect).

If the shearography fringe pattern to be analysed does not contain a carrier fringe (or if the carrier frequency is not high enough), such as is the case with closed fringes, then the spectrum in the frequency domain cannot be isolated from the background spectrum. In this situation, the FTM technique is not applicable. However, it is possible to apply the IFTM which uses a band-pass filter to obtain one half of the spectrum. The improved technique then follows the same approach as the standard FTM.

A case study is provided in Figure 36. Figure 36(a) shows a typical shearography fringe pattern obtained. It contains such a high level of noise that the effect of the defect is not discernible. A Butterworth low-pass filter was used to filter this fringe pattern, and the result is shown in Figure 36(b). Figure 36(c) shows the central area of Figure 36(b) (i.e. the pixels at the margins have been excluded). Figure 36(c) contains a high spatial carrier frequency, hence it is possible to separate the background spectrum and extract the phase of Figure 36(c), as shown in Figure 36(d). The wrapped and unwrapped phases of Figure 36(c) are shown in Figure 36(e) and (f), respectively. For the FTM technique, a low-pass filter (such as a Hanning or Hamming window filter) can be applied in the frequency domain. However, the resulting resolution will be low and small defects will be filtered out.
Figure 36(g) is a 3-dimensional version of Figure 36(f). The tilt components along the x and y axes in Figure 36(e-f) have been removed, making it easier to identify potential defects in the phase map. However, it is not possible to see any clear signs of the 10mm subsurface defect in the WTB section under test.

Figure 37 shows the phases extracted from a series of shearography fringe patterns which were captured in quick succession (100 fps). In theory, the phase near a defect should change more rapidly than the phase in other areas. Hence it was anticipated that it would be possible to observe the defects via such a series of phase maps. However, no obvious signs of the defect were observed. It was concluded that the effect of the defects had been filtered out by the FTM.

The process of applying IFTM to process a shearogram without a carrier term is illustrated in Figure 38. A shearogram containing closed fringes is seen in Figure 38(a), and the filtered Fourier transformations in the x and y directions are shown in Figure 38(b) and 38(d), respectively. The wrapped phases from the filtered spectra are displayed in Figure 38(c) and 38(e). Finally, the actual wrapped phase can be seen in Figure 38(f). Clearly, the wrapped phases shown in Figure 38(c) and 38(e) contain the correct phase information (albeit inverted).

Both the FTM and IFTM methods can only deal with shearograms where large spatial carrier fringes are present.

A third algorithm for phase extraction is a two-step approach which is based on Spiral Phase Transformation and optical flow. The optical flow approach is a standard method used in computer vision for obtaining the distribution of apparent velocities of objects, surfaces, and edges in an image set. The optical flow method calculates the motion field at every pixel position between two image frames that are taken at adjacent times. When the movement between the two adjacent times is small, the brightness of an object can be assumed not to change over time. This can help determine the components of the movement in the direction of the intensity gradient, but it is not possible to obtain the components along the direction of the iso-intensity contours. As a consequence, the flow velocity (u; v) cannot be computed locally without introducing additional constraints. One way of addressing this issue is to impose smoothness in the displacement field. A regularized optical flow method has been developed, which involves minimising the energy of the image field through a regularizing parameter that weighs the smoothness of displacement components. This minimisation process is carried out by an iterative method using the Gauss–Seidel method. Detailed description of the two step phase shift interferometry (PSI) including mathematical expressions are provided in the attachment. A case study was provided to show the effectiveness of the methods. Figures 39(a&b) are two shearography fringe patterns captured in succession by a high-speed camera via post-processing. The direction map from the two shearography fringe patterns is shown in Figure 39(c). By using the SPT operator, we can obtain the wrapped phase, as shown in Figure 39(d). The sign of the wrapped phase map is correct. Figure 39(e) is the unwrapped phase of Figure 39(d). Figure 39(e) clearly shows the phase of Figure 39(a).

Comparison tests have been performed to evaluate the three phase extraction methods. The results have shown that the two-step PSI algorithm is the most effective method for the DashWin system, as it can correctly extract phase information from a single shearogram and it can also process multiple dynamic shearography fringe patterns.


In Task 2.4 The first version of a compact shearography system was designed and produced. It was primarily for laboratory applications, and consisted of the following main parts:

• An IDS μEye conventional camera manufactured by Image Development Systems GmbH.

• An image shearing device. The image shearing device consists of a Wollaston prism, a polariser and a band-pass filter at 532nm wavelength. The filter only allows green light (532nm) to pass through the shearing device, thus enabling shearography in daylight conditions.

• A continuous 532nm wavelength (green), 100mW laser source.

• Other elements such as imaging lens and infrared heating lamps.

To meet the safety requirements, the prototype shearography system is mounted on a 250x300mm aluminium breadboard. A protection housing case was constructed to enclose all of the above components (as shown in Figure 40). The dimensions of the compact system are 300x250x170mm and the overall weight is approximately 7kg. It can be mounted on a tripod for ease of inspection. Based on standard BS EN 60825-1: 2007, the system was classified as a Class 2 laser product.

The operation procedure is as follows:

(1) Start the shearography control software
(2) Activate the laser
(3) Adjust focus to display clear speckle image
(4) Start recording video clips just before activating the heating lamps.
- Option: Live shearography fringe display may be activated if necessary at the cost of reduced speckle video recording speed
(5) Heat up the target area for a predefined period of time, e.g. 30s
(6) Switch off the heating source
(7) Keep video recording for some time (e.g. 30-60s)
(8) Change inspection target area and repeat Step 3-7.
(9) Switch off the laser when inspection is complete
(10) Post-processing the recorded video clips to identify potential defects

The compact system was trialled at a workshop of a wind turbine operator in Scotland on a static WTB section (positioned on the ground), as shown in Figure 41. During the trial, an infrared heat lamp was used to heat the WTB section for approximately 30 seconds. Shearographic images were displayed on the laptop screen in real-time. Since no specific defects were present in the test sample, no conclusive inspection results were obtained. Impact damage was then introduced into the test sample by striking the specimen with a hammer. As the impact damage produced almost no visible effect on the WTB surface, it would have been impossible to detect via visual inspection. However, using the compact DashWin shearography system, clear signs of the impact damage were displayed via the live shearography images, which were in the form of fringe disturbances, created by the subsurface defects. In addition to the impact damage other geometric discontinuities were also clearly shown in the shearography fringe patterns.

This preliminary demonstration of the compact DashWin system was considered to be a successful trial. It also served as useful preparation for the full on-site WTB inspection field trial under planning.


- In Work Package 3, techniques to cope with the rigid body motion of a stopped WTB on-site of a wind tower due to wind have been developed.

In Task 3.1 a high speed DIC system for measuring in-plane rigid body motion has been developed. It has the following measurement capabilities:

• Displacement accuracy: 0.02 pixels. The absolute measurement accuracy depends on the magnification of the imaging lens. A sub-micrometre accuracy can be achieved by the use of an appropriate optical lens (such as a Canon EF macro lens).

• Displacement measurement range: up to 1000 pixels or 60% of the size of the field of view (1600x1200 pixels) of the camera.

• Strain accuracy: 50µε in global measurement or 100µε in local measurement. This accuracy could be enhanced further with the use of a telecentric lens (although not demonstrated in this work).

Laboratory tests were carried out to validate the system (software and optical hardware set-up), which showed that for a field of view of 1 m, an in-plane displacement resolution of 0.04mm (0.05 pixels) can be achieved. Each single calculation is performed in a fraction of a second.

Several tests (indoor and outdoor) were conducted, in which a conventional digital camera and a Canon macro lens were used to capture a sequence of images of the target object over a period of time. The DIC software was then used to calculate the in-plane rigid body motion of a suspended WTB section (Figure 42-43), a crane, a flag pole (Figure 44-45) and a lamp post (Figure 46-47). The results obtained were shown to be accurate (accuracy of 0.05mm) and reliable, as the results were both consistent and stable.

In Task 3.2 techniques for coping with rigid body motion of the target component (WTB) under inspection were developed. This is an important part of the project, because considerable rigid body motion (from multi-millimetres to multi-centimetres) is inevitable even the WTB is stopped for inspection due to wind load. In order that the shearography technique can work on a wind turbine tower, it is essential that the rigid body motion can be accommodated.

Two direct motion compensation approaches have been designed. The first is based upon direct motion compensation via intensity rearrangement, the second on rearrangement of phase maps. Direct intensity rearrangement method is simple and fast, but can only cope with a rigid body motion up to 0.5mm. The phase based motion compensation is implemented by two approaches: the Phase-of-Differences method and the Difference-of-Phases method. The flow charts are shown in Figure 48 and 49, respectively. In the Difference-of-Phase approach, the phase is calculated by two techniques: FTM (Figure 50) and spatial carrier phase sifting (SPCP) method. However, the phase based methods also yield a maximum rigid body motion tolerance of 0.5mm.

A correlation analysis based on second-order speckle statistics revealed that the tolerance of 0.5mm in rigid body motion represents the inherent limitation of any interferometric techniques, as shown in Figure 51. Nevertheless we continue striving to tackle this problem, and finally the tolerance has been extended to at least 5mm. This is achieved by using an innovative image processing approach which was developed in the second period of the project and was reported in deliverable D5.2.

Figure 52 shows the result of a laboratory test using a suspended WTB. The WTB was slightly pushed at the beginning of the test, which caused it to oscillate around its balance position. The motion consisted of both in-plane and out-of-plane displacements. Speckle images were recorded during the heating and cooling period of the WTB sample by using an infrared heating lamp. Figure 52a shows a typical speckle image extracted from a 94 second video clip which contained 796 images. Figure 52b shows an uncorrelated shearography fringe pattern which is simply a noise pattern. By screening the image subtraction, clear fringe patterns were obtained at certain occasions, as shown in Figure 52c. This proved the effectiveness of the modified procedure. Further detailed assessment of the procedure is required and is on-going, which includes the possibility of analysis to determine, among other things, how long the video recording should be to guarantee obtaining two correlated speckle images and the vibration magnitude the modified procedure can tolerate. Nevertheless, the modified procedure constitutes a major achievement in tackling the inherent problem associated with non-contact NDT of an unstable component using shearography. It will open up a variety of application for shearography beyond the WTB inspection.


In Task 3.3 the fast digital image correlation algorithm was integrated with the shearography system, forming a DashWin IPS (Image Processing System) software which was developed on Microsoft Visual C++ platform. This software includes shearography speckle image acquisition, fringe processing and other image processing functionality. Figure 53 shows the main interface of the new IPS software system developed.


- In Work Package 4, a robotic climbing platform was designed and fabricated. It was then integrated with the shearography system to form the whole DashWin system, allowing non-contact inspection of the WTB to be carried out as the robot climbed along the wind tower.

In Task 4.1 a climbing robotic platform was designed and fabricated.

Prior to the design of the robotic platform, the technical specifications relating to the shearography system, the environmental requirement and the workspace specification were determined, as listed in Table 6-8. Based on the specifications and the BSR model (Figure 3), a concept was formed, as shown in Figure 54.a stability analysis was performed, which led to the following conclusions:

• There is just one narrow area for the selection of length L3 where both F1 and F2 are positive, meaning that both wheels are safely touching the monopile’s surface.

• The force values (10-20Kg range) seem quite small.

• For a very small L3, the rear wheels are very well attached on the monopile, while the opposite happens with the front wheels, where the reaction force is negative (up to 30Kg for instance). This could cause a small tilting of the vehicle body around the X-axis, until a new balance point is reached. The system will not tip over, but the problem is that it will become non-operational and it could potentially touch on the WTB surface.

• In the opposite case, the upper wheels are stable but the rear part will lose contact.

• The L3 selection should be different for the first 14m, than for the remaining length of the monopile.

• The more the ladder opens, the more the center of mass of the robot moves away from the monopile and the moment imposed on the rear wheels increases significantly. As the equilibrium forces dictate, the opposite happens with the reaction force of the front wheels. The more the ladder opens the greater adherence force is required to keep the front wheels in contact with the monopile.

• There is a small area (very small ladder inclination), where both forces are positive, but not all along the monopile’s length. The front wheel reaction forces become negative at heights larger than almost 50% of the monopile’s length (~22m).

• The reaction force of the front wheels becomes slightly bigger as the height of the motion increases. This takes place because the slope of the main pulling cable decreases as the platform moves upwards and its tension contributions to the upper force gradually weakens

Based on these conclusions, L3=~580mm was selected. Other parameters for the magnets, winches etc. were then also decided.

Detailed design of the robotic platform was implemented through the following blocks:

1. VEHICLE DESIGN, which consisted of 5 main subsystems:

(1) Main chassis.
(2) Rear wheel assembly.
(3) Front wheel assembly.
(4) Vertical drive system.
(5) Sensor system.

The chassis comprises: 3 main parts (upper, middle and bottom, Figure 55), a custom truss made from off-the-shelf tubular aluminium beams welded together, and appropriate interfacing provisions for all relating equipment (4 wheel assemblies, a main winch, a ladder winch, a ladder and a main electrical panel). Figure 56 shows the principle of winch operation.

The selected beam is made of aluminium grade 6061 and its external dimensions are 80mm x 80mm x 4mm (wall thickness). It is quite stiff as its large external dimensions offer a high second moment of area, making it stiff to bending. It is also relatively lightweight, 3Kg/m. Given time, cost and market availability restrictions, it was considered a good option.

The sensors system is used to monitor the position of the platform. It is achieved via an encoder assembly. This assembly is mounted on the front wheel and is spring loaded for traction. The encoder measures the rotations of the wheel that passively rotates against the monopile surface. Figure 57 shows a CAD snapshot of the assembled “vehicle”.

2. LADDER DESIGN.

The ladder consisted of the main chassis, drive system and sensors.

The main ladder chassis comprised a parallel four-bar mechanism that kept the scanner mounted at its tip, parallel to the ground. An interfacing structure was positioned at the tip in order for the scanner to be firmly mounted. Driving of the ladder was achieved by means of a winch, accompanied by a safety mechanism (Figure 58). The sensors used an encoder, Figure 59 (left), which was mounted on the shaft of the parallel mechanism in order to monitor the ladder’s absolute angle of inclination. Limit sensors, shown in Figure 59, are used to monitor the ladder’s extreme position

3. SCANNER DESIGN.

It is a crucial need to constantly measure the distance between the shearography system and the WTB, so that no collision will occur between the two. All motor drives have incorporated encoders in order to allow each drive to perform position control. As the distance between the WTB and the NDT system can vary considerably, we need a sensor that has a long-range detection ability. However, during the inspection process, the stand-off distance is expected to be constant, thus a sensor with high accuracy and resolution is needed. The two distance sensors were selected as shown in Figure 60 and Figure 61.

4. CONTROL SYSTEM.

The control system includes motor controllers, magnet controller, feedback sensors, and CPUs, see Figure 62. The CPU used is a BeagleBone® board based on the AM335x 720MHz ARM Cortex-A8 processor. The EPOS2 DC motor controllers for the three scanner drives were used. The Wagner Magnete magnets employed are controlled by their own controller and power supply. We can control the power to the magnets and thereby regulate their output pulling force by up to 16 levels. They are supplied with single phase 220V AC.

Finally, for maximum safety we have designed a straightforward manual override for the system in case of a malfunction (i.e. any kind of failure in the control system, scanner, shearography system or sensors). The manual override is a handheld control panel that will be used by an operator at ground level in case of an emergency situation. In such a case the electromagnets will be deactivated, the ladder will be closed (via the small winch) and the system will be lowered to ground level (via the big winch) for investigation.

Figure 63 shows the 3D CAD viewing of the full system adhered to WT monopole. The transparent grey cone shown on the tip of the NDT system is the theoretical field of view of the camera. Moreover, the two brown transparent prisms present the theoretical illumination area of the two thermal sources (IR lamps). Figure 64 shows the physical system at its short version (short vehicle chassis) at an early assembly stage, hanged from Innora’s lab crane for preliminary testing. The system is in contact with a flat metallic plate, simulating the WT’s monopole. Figure 65 shows the extension of the robotic platform chassis.

5. SAFETY CONSIDERATIONS

The most serious safety issues result from failure of the electronic control components. All the reasonably foreseeable failure scenarios have been considered and solutions were provided. Finally, the climbing robot is designed to be compatible with a 0.66MW wind turbine which will be used in field trial.


In Task 4.2 the shearography system was integrated with the robotic platform, allowing WTB inspection to be carried out remotely by an operator on the ground.

The integration involves three aspects: mechanical integration, electrical integration, software integration.

The mechanical interface between the shearography system and the robot platform is achieved by an interfacing flange (Figure 66), which was positioned at the end of the robot’s scanning device for this purpose.

Aluminium alloy 7075 was chosen specifically for the interfacing flange, because of its high mechanical strength. The flange has four holes for mounting the shearography system. The shearography system features the exact same pattern of holes on its base plate. Consequently, mounting and detaching the shearography system can be performed with ease.

Regarding the prior actions taken, the weight, shape and functionality of the shearography system were taken into account when designing the scanner axis, so that the shearography system could perform as required. A study was undertaken to ensure that no collisions could occur between the shearography system and the components of the scanner during the scanning operation.

The scanner’s long range laser distance sensor was mounted at the bottom side of the shearography system enclosure, ensuring in parallel a clear field of view in the direction of the WTB. This laser was positioned directly on the main plate of the shearography system.
The electrical integration covers three main components: a digital camera, a laser light source, and two infrared heating lamps.

Electrical integration was achieved with the following specifications:

1) Provision of DC power was achieved via the power supply installed in the Ladder’s electrical panel (electrical panel adjacent to the scanner).

2) Provision of AC power for the laser light source was achieved through the mains input of the Ladder electrical panel.

3) Provision of AC power for the infrared heating lamps was achieved through a separate power supply cable routed from the Vehicle electrical panel (mains panel). This is a safety precaution.

4) The control signal of 5V TTL for the laser light source was provided by the BeagleBone board in the Ladder’s electrical panel (CPU LAD).

5) The control signal for the infrared heating lamps’ high power relay was provided by one of the motor controller’s digital outputs, as it has the necessary power to drive the relay coil. The motor controller is also installed on the Ladder’s electrical panel.

The integrated software for the DashWin system does not feature a single Graphical User Interface (GUI) through which an operator can control/monitor both the robot and the shearography system. Instead, two operators are required to operate the robotic platform and the shearography system separately follow the procedure below:

A. When the robot is located in the correct position to perform an inspection, it notifies the NDT operator through the NDT GUI.

B. Until further input is given, the robot waits for the operator’s next command.

C. When the operator sends the ‘Start inspection’ signal via the NDT GUI, the NDT process begins.

D. The robot automatically switches ON both the laser light source and the infrared heating lamps.

E. When the operator decides that the inspection process is complete he/she sends an ‘End inspection’ command via the NDT GUI.

F. The robot automatically switches OFF both the laser light source and the infrared heating lamps.

G. The robot advances to the next required position (across WTB’s width or length) and the inspection process is repeated.

Figure 67 depicts the electrical integration of the DS system with the robot’s infrastructure. Figure 68 shows the DashWin system following integration in laboratory.


In Task 4.3 controlling software and interface was developed.

The software package for remote control of the robotic platform has the following functionalities:

(1) Actuator control and coordination.
(2) Robot status monitoring and safety procedures.
(3) Communication with external agents.

The main functional features of the central control GUI used by the ground-level operator comprise:

(1) Robot status monitoring.
(2) Issuing of warnings/alarms.
(3) A set of manual control options.

Figure 69 illustrates the detailed structure of individual operations.

To ensure the safety of the whole system, a further high-level operations and coordination has been designed and implemented. This high-level operation comprises move up/down, perform inspection, and manual control.

• Move Up/Down

When the robot is instructed to move up/down the following operations are performed in sequence:

a. The controller closes the ladder.
b. The electromagnets are disengaged.
c. The controller drives the large winch in the required direction.
d. The large winch is stopped:
i. Via closed loop control (fixed step).
ii. Via the operator’s manual command.

The following safety interlocks ensure safe operation during the above sequence of actions:

(1) The robot CANNOT be moved Up/Down if ANY of the following conditions are met:
a. The ladder is NOT fully closed.
b. The electromagnets are switched ON.

(2) The robot CANNOT disengage the electromagnets if:
a. The ladder is NOT fully closed.

(3) The robot CANNOT open the ladder if:
a. The electromagnets are switched OFF.

• Shearography inspection

In this mode of operation the robot performs inspection of the WTB via the following sequence of operations:

(1) The ladder is opened using feedback from the long-range sensor.
(2) The ladder is stopped at a predefined stand-off distance from the WTB (pre-set in the software).
(3) The Y-axis controller is activated and set in the ‘Following’ mode. So that the robot will track the motion of the WTB and avoid possible collision.
(4) The NDT inspection process proceeds following a signal from the NDT operator.
(5) The laser light source is activated.
(6) The shearography software is initiated remotely (see Section 3.3 for details).
(7) The heating source is activated.
(8) The heating source is switched off (following a predefined period of time).
(9) The laser light source is switched off.
a. Either after the operator’s command.
b. Or automatically after a predefined time has passed.
(10) The next step of the inspection process is determined, the NDT system is moved either:
a. Horizontally across the width of the WTB.
b. Vertically up/down the WTB length.
(11) In the case of 10.2 the ladder is fully closed prior to movement.
(12) The system moves either horizontally or vertically by a predefined step in readiness for the next iteration of the inspection process.

Figure 70 shows the control graphical user interface (GUI).

The following safety interlocks ensure safe operation during the inspection sequence:

(1) The NDT process CANNOT commence, until the robot is in WTB ‘Following’ mode.

(2) The robot CANNOT open the ladder to an angle greater than a pre-set safety value.

(3) During ladder opening, IF the stand-off distance between the WTB and the DashWin system reaches a pre-set safety distance THEN the driving winch will automatically stop.

The software package for the shearography system has the following functionalities: image acquisition, live image subtraction and fringe display, video recording, data transfer, fringe and phase processing, result interpretation and information storage. Figure 71 shows a screen shot.

During a WTB inspection, the shearography software is primarily used to record sequential laser speckle images in the form of video clips via remote control through an optical cable. Live image subtraction and fringe display shall be deactivated. This will ensure that the image acquisition time is minimised, along with the cost of carrying out an on-site inspection. Image subtraction, phase extraction, speckle fringes reconstruction and other image processing functionalities are implemented in a separate software package, designed for off-line operation (previously developed and tested in the laboratory).


In Task 4.4 both the robotic platform and the shearography system were further optimised following laboratory tests and an initial field trial.

Firstly, the shearography system was optimised through the upgrading of the laser source, the protective enclosure and the heating lamps, and the installation of a surveillance camera for remotely monitoring the inspection scenario on a wind tower.

Secondly, the robotic platform was improved and optimised so that it was operational for a full inspection cycle on the WTB..

The optimisation of the shearography system involved the following aspects:

• Upgrading of the laser source

The original100mW continuous wave 532nm green laser was sufficient in laboratory application since the ambient illumination is mainly from the lighting which can be controlled. However, when outdoor tests were carried out, it was found that the ambient illumination was much stronger even under a cloudy weather condition. As a result, the useful coherent green laser light is contaminated by the green spectrum of the natural white light, reducing the signal to noise ratio of the shearography speckle patterns. Apparently this situation will become even worse when the system is working in a sunny weather condition. To address this problem, a higher power laser was required for outdoor application especially the field trial. Considering the health and safety issues of a laser, a 368mW green laser was purchased.

• Upgrading of the digital camera

The original IDS camera uses an USB cable for data transmission. Although the data transmission speed of an USB cable is generally sufficient for a conventional camera, the USB cable is limited to 5m in length. To work on a wind turbine tower, a higher data transmission speed and longer distance cable should was required. Therefore a new μEye monochrome camera (Model UI-6250SE-M, manufactured by Image Development Systems GmbH) with 2 Megapixel spatial resolutions (1600x1200 pixels) was purchased. It is a highly compact GigE camera with gigabit Ethernet transmission over extended distance which was essential to ensure that the vast amount of speckle image data acquired during an on-site inspection can be transmitted and recorded completely for subsequent image processing.

• Upgrading of the protective enclosure

The original shearography system was designed for laboratory use. In order to operate in the harsh environment of a wind turbine the shearography components must be housed in a firm enclosure with a high level of ingress protection against water/dust incursion. An electrical box with dimensions 300x300x210mm was selected and modified such that the components of the shearography system was enclosed in the box to protect it from rain and dust. Two exit windows were inserted, one for the divergent laser beam and another for the imaging camera. The specialized glass mounts were also sealed using EPDM (ethylene propylene diene monomer (M-class) rubber). The second version shearography system is shown in Figure 72.

• Upgrading of the heating lamps

The original heating lamp had a power of 1.2kW but only worked indoor. To meet the requirement of working on a wind turbine tower, two new infrared heating lamps were purchased to enhance the heating efficiency of the shearography system. Each lamp has a power of 1500W (1.5kW). These super-efficient infrared lamps provide targeted heating via low frequency waves which heat an object directly without wasting energy warming the air between. The lamps are completely weatherproof and therefore are suitable to be used outdoors in any application and are perfectly safe to be used in the rain.

• Installation of a surveillance camera

Since the DashWin system is expected to be operated remotely by operators on the ground, a requirement to closely oversee the working scenario on the platform was identified. This thinking has led to the decision to install a surveillance camera on top of the shearography protective box. A D-Link model DCS-2332L surveillance camera was purchased and integrated. This HD Wireless Cloud Camera is suitable for outdoor use, with weatherproof housing that protects it from dust and rain. It can record both high-quality snapshots and video clips with resolutions of up to 720p HD. However, as we intend to use an optical cable to transmit image data from the shearography camera, the wireless function of this surveillance camera was not used. The optical cable was used for data transmission and camera control.

After the above upgrading, a second version of the compact DashWin shearography system was produced, as shown in Figure 73. It is equipped with two heating lamps on the left and right hand sides, and a surveillance camera on the top. The shearography system is classified as a Class 3R laser product.

The optimisation of the robotic platform primarily concentrated on addressing the stability of the platform to ensure that safety is maintained at all times. Mechanical, electrical and software testing was also performed both in laboratory and on a wind tower in order to identify any issues and then to rectify them.
One of main stability issues related the magnets. To address the problem, the front magnets were adjusted to remain at their extreme point of their stroke when inactive, via a low force returning spring. This is around 5-6mm from the monopile’s surface; a sufficient safety gap from the monopile. It is important not to apply a big force via the spring because it is necessary for the magnet to be able to overcome this opposing force and clamp on the metallic surface. It is known that these type of magnets are highly dependent on the air gap. A 2mm gap decreases the pulling force by 85%, compared with the one when the magnet is in contact with the metallic surface. When activated the magnets end their stroke around 1mm before they touch the monopile’s surface. This way the opposing force caused by the deflected pneumatic wheels is relatively low. Finally, a rubber band of 1 millimetre was applied on the two poles of the electromagnets as a protective means for the monopile’s surface, see Figure 74.

The rear wheels magnets was examined in a similar way but with the difference that the rear magnets must not touch the surface as they are very strong permanent ones. These are not so sensitive to the air gap.

The electric control panels (Figure 75) were also tested and validated on

• Power supplies
• Grounding check
• Interfacing with all components / controllers
• Communication with the CPUs.

After various design modifications, the whole DashWin system was successfully deployed to an expected height of the wind tower in a preliminary field trial (Figure 76), which meant that the whole system was ready for a full field trial.

- In Work Package 5, validation and field trial were carried out to demonstrate the viability of the DashWin system and technique..

In Task 5.1 validation testing both on the shearography system and the robotic platform in laboratory conditions was performed, so that any potential issues of the whole DashWin system can be identified and rectified prior to carrying out the subsequent field trial on a wind turbine tower.

The effectiveness of the shearography system in detecting subsurface defects in WTB samples had been proven in the work carried out in WP2 and WP3. Hence the laboratory test here mainly involved (1) an outdoor test using the previous shearography with a 100mW green laser source, and (2) a connection test on the second version shearography. This connection test was aimed at verifying that the integrated shearography system works smoothly as expected through remote control and that the vast amount of image data acquired during an inspection can be transmitted reliably up to 300m via an optical cable

Figure 77 shows the outdoor test scenario in TWI in December 2013 where a 4m long WTB sample was erected outside a building for inspection. Figure 78 shows one example of the shearography fringe pattern. The contrast of the fringes appeared low. This was because the natural day light (non-coherent white light) contains the full spectrum of visible light, including the green light that the laser source emitted. The existence of this non-coherent green light tended to dilute the useful coherent green laser light (100mW). As a result, the shearography fringe pattern that was generated via image subtraction has a poor fringe contrast. This meant that a higher laser (e.g. the new 368mW laser) should be used in field trial.

Data connection and transmission is crucial for the whole DashWin system to work on a wind turbine tower. Significant efforts were made to design and implement the data transmission scheme. The central part of this scheme is the use of a 300m optical cable through which all the image data as well as the camera control are carried out. Figure 79 shows the 300m optical cable used in a laboratory testing. Figure 80 provides details of the connections between different cables and sockets. The TTL (Transistor-Transistor Logic compatible signal) wires on top of the connector are used to control the power supply of the laser source. The connector has four data channels, allowing four data cables to be connected at the same time.

Further laboratory tests were performed to assess the second-version integrated shearography system. Figure 81 shows the experimental set-up, where a WTB sample was inspected. Figure 82 shows a shearography fringe pattern in the test, which revealed the uneven distribution of the glues that bonded the spar and the shell skin of the WTB. This result demonstrated that the new shearography system is working as expected.

Laboratory testing was performed on the robotic platform after it was fully assembled to its short version (without the 2.5m extension chassis part) due to space restrictions. The robot was suspended from an overhead crane and its wheels were placed in contact with a large flat metal plate, simulating the monopile’s outer surface. Details are similar to the optimisation work described in Task 4.4 above.


In Task 5.2 a field trial has been conducted on a Vesta V47 wind turbine tower which showed that the integrated DashWin system was working as expected.

The field trial was carried out at CRES wind turbine test facility, near Athens, Greece, on 28-29 April, 2014. The operational procedure of the field trial was as follows:

1) The robotic platform and the shearography system were unloaded from a lorry. The shearography system was mechanically disassembled from the robotic platform and mounted temporarily on the robot’s chassis, for safety reasons during transport. The system was placed near the WTB at a convenient position for lifting and mounting it on the WTB monopile.

2) The shearography system was assembled on the 3-axes scanner’s head by connecting four bolts with the base plate of the shearography system, as shown in Figure 83.

3) Power supply was provided by an electrical generator and connected to the mains power supply of the system. Data communication was achieved by means of a fibre optic. It was connected to a switch on the ground station. This hub was used to connect the two control GUIs (robot & SHEAROGRAPHY system control), running on two laptops, to enable communication with the system.

4) The main steel cable was connected on the main winch. First, the steel cable was passed through the main winch pulley assembly, Figure 84a. In this figure the cable route is depicted by the red arrows and the relevant numbers.

Next, the steel cable was pushed inside the winch’s cable inlet until no further motion was possible, Figure 84b. Finally, to make sure that the cable will be pulled by the winch, the manual button Up of the winch’s handheld device was pressed, while in parallel the cable was pushed towards the winch’s inlet. Eventually the steel cable hanging from the nacelle was tightened and the lifting of the front part of the robotic platform, had started, see Figure 85 and 86.

5) The system was then lifted by activating (Upwards motion) the system’s main winch, Figure 86 via the override manual handheld device. The motion was stopped when the system was positioned at a perpendicular position. For easier and safer handling during lifting, two persons were pulling the rear part of the system via two belts.

6) The system was then properly mounted on the monopile. This was done by, first, making sure that the front wheels were all in contact with the monopile. Second, the rear part of the system was attached on the monopile, by pushing the two rear wheel assemblies to be clamped on it, via the magnets pulling force, Figure 87.

7) After the mechanical installation of the system was completed, a number of tests were performed to make sure everything was working properly. All actuators were tested and the scanner’s head was homed. Next, the shearography system laser and heating lamps were switched on/off. The system was then ready to climb along the monopile. This was done via the joypad, which allows full control of the robotic platform part of the system, see Figure 88.

8) When the robot was considered to have reached a suitable height to perform an inspection, the platform was halted and its electromagnets were activated.

Then, the robot arm opened until the shearography system was at the correct distance (digital camera focal length) from the surface of the WTB, Figure 89. The scanner’s “following” mode was activated. This whole procedure was successfully completed.

9) The robotic platform operator was instructed by the NDT operator to position the shearography head at the correct position across the WTB’s width (i.e. A-1 as per Figure 90). A scanning sequential pattern of the WTB was calculated, Figure 90. The individual steps were designed to overlap to ensure full coverage of the WTB’s surface. In this test, the first two rows of this scanning pattern were executed.

10) The inspection started under the command of the NDT operator. The robot automatically switched ON both the laser light source and the infrared heating lamps, Figure 90. This is also shown in Figure 91 where a greenish area can be seen in the centre of the image. This picture was captured by the surveillance camera installed on top of the shearography box.

11) Once the heating lamps were switched on, video recording of the speckle pattern started. After approximately 30s, the heating lamps were switched off, and the video recording continued for another 30s. Image processing was scheduled to be carried out later off line.

[It should be noted that due to the insufficient margin of power provided by the generator, it was found that when both heating lamps (2X1.5kW) was switched on during a ground testing prior to lifting the whole DashWin system up to the monopile, the total power consumption had almost reached the limit of the generator, causing the cut-off of the total power supply. Therefore during the trial inspection, only one heating lamp was operated which may result in insufficient deformation required for the shearography system.]

12) Before moving to the next position, the laser illumination source and the heating lamps were switched off.

13) The robot advanced to the next position required and the inspection process was repeated.

During the trial, the robot platform successfully carried the shearography system to a height of approximately 40m on the wind tower, which is 10m away from the nacelle and covered above half of the WTB length (Figure 89). (Due to the narrow gap between the WTB and the tower, the platform is not allowed to climb to the top of the tower.so as to avoid collision.)

During the field trial, the surveillance camera on top of the shearography system recorded comprehensive video clips which covered both the WTB and the far field scenes. These video clips enabled quantitative measurement of both the stability of the robotic platform and the vibration of the WTB. Since the far field scenes are stationary, any relative displacement within the video images corresponds to the motion of the surveillance camera. Two-dimensional digital image correlation is used to measure the motion history, which is a measure of the stability of the platform. The same procedure applies to the images of the WTB, where existing marks on the WTB surface or its edge features are used to calculate the vibration during the period of inspection.

One of the video clips recorded by the surveillance camera was analysed. It has a duration of 62.375s and a frame rate of 24 fps, which means a total of 1497 still images were recorded. Figure 92 shows one still image extracted from the video clip. In order to obtain the image scaling factor, a full image of the wind turbine is used, as shown in Figure 11. Since the length of the WTB is known (23m), it is easy to calculate that the distance between the two rust lines on the tower in Figure 93 (where the main girth weld was made) is 16m. The width of the WTB adjacent to the first rust line where the inspection was made was calculated as 1.4m. Using this width in Figure 92, the image scaling factor in the video clip was determined as 1.75mm per pixel.

After processing all the 1497 still images of the video clip by a 2D DIC programme and applying the scaling factor of 1.75mm/pixel the displacement of the surveillance camera relative to an infinite scenery feature was plotted in Figure 94. It shows that the displacement of the platform for the majority of time is between 0-2mm.

The vibration of the WTB was measured by the same DIC programme using the features on the WTB surface as shown in Figure 92. Figure 95 shows the motion history over 62s, where the motion is at the magnitude of 3-9mm, much greater than that of the platform. The motion difference is clearer when both vibration curves are plotted in the same diagram, as shown in Figure 96. Figure 97 shows the wind speed on the wind turbine V47 during the trial days. As the wind turbine was stationary during the trials, the vibrations on the blades were produced only by the wind.

• Assessment of the shearography speckle images

The shearography system was initially designed to inspect large WTBs for multi MW wind turbines where the blade will be typically 30-50m long. The distance between the WTB and the wind tower must be at least 5m, which allows the shearography camera equipped with a lens having a 25mm focus length to work at an ideal imaging distance of 2-3m. As the Vestas V47 WTB is only 23m long, the gap between the WTB and the tower is much smaller, i.e. from only 2.5m at the top to 3.5-4m at the WTB tip. Therefore both the robotic platform and the shearography setup needed to be modified to adapt to this wind turbine. The imaging distance of the shearography system was determined to be at 0.8m which is approximately a quarter of the original distance. Although a lens with a shorter focus length (e.g. 12mm) can be used to increase the field of view at short image distance, it requires additional changes to the laser illumination too in order that both the laser illumination and the imaging field of the camera meet on the WTB surface. Since the upgraded shearography system is housed in a compact enclosure, there is not much room to relocate the optical elements. Therefore, the original 25mm focus lens has to be used. As a result, the scanning area of 1m2 is dramatically reduced to about 0,05m2 (25cm in diameter).

Within such a small inspection area, no shearography fringe patterns are expected to be produced from the field trial. Consequently, the assessment of the shearography speckle images is focused on evaluating the correlation level among different speckle images. If the WTB is stable with minimal vibration, all the speckle images will be correlated with one another, which means shearography fringe patterns will be easily generated by image subtraction. When there is no or negligible deformation between two speckle images, the subtracted image will be a near black (due to electronic background noise), indicating a uniform or zero deformation field. If the WTB is unstable such as in vibration as shown in Figure 95, then speckle correlation will be lost most of the time. Only when a vibrating WTB returns to its original position can a correlation between the two speckle images recover. At that time, shearography fringe patterns will be formed by image subtraction if there is sufficient deformation between the two stats of the WTB, or a near black image will be produced when the deformation is negligible. By carefully processing the video clip speckle images recorded by the shearography camera, it was demonstrated that it is indeed possible to find correlated speckle images even though there are considerable vibrations of the WTB during video recording. One typical calculation is provided as follows.

A video clip of the speckle images was selected for processing. This video clip lasted 81.6s and was recorded approximately the same time as the video clip that was recorded by the surveillance camera described above in Section 4. A total of 1007 still speckle images was extracted, which gave an actual frame rate of 12.34 fps of the shearography camera at that time, close to the maximum nominal frame rate of 12.5 fps.
Figure 98 shows three speckle images, the first image (Figure 98 a) is the 11th still image from the video image sequence. The second image is the 93rd still image (Figure 98b). Both of them are randomly selected from the image sequence. The third image is the 508th still image (Figure 98 c) which is carefully selected such that it appears to have high correlation with the 11th image (Figure 98 a). Figure 99 shows two shearography patterns produced by image subtraction among the three speckle images. The first shearography image (Figure 99a) is the subtraction between 11th and 90th speckle images, which shows no fringe pattern. The second shearography image (Figure 99b) is the subtraction between 11th and 508th speckle image. Although it also shows no fringe pattern, its brightness is considerably lower than that in Figure 99a, indicating that the 11th and 508th speckle images are highly correlated, although they were acquired approximately 41s ((508-11)/12.34=41.17s) apart in time. This is also demonstrated in the histogram as shown in Figure 100, where the distribution of the correlated shearography pattern is concentrated in the lower end of the grey level scale (less than 30 among the 255 grey level scales). The distribution of grey level scale of the uncorrelated shearography pattern covers an extended range from zero to over 100. In contrast, the distribution of grey level scale of the speckle image itself encompasses almost the whole grey level scales from 0 to 255.

From the above analyses, it can be concluded that although significant vibrations of the WTB occurred during the field trial, it is still possible to obtain correlated shearography fringe patterns. The almost total black of the shearography pattern in Figure 99b indicates a uniform or zero deformation across the small scanning area due to reduced image focus distance. If the image focus distance can be increased to the normal 3m distance, it is likely that fringe pattern can be produced. Therefore the whole DashWin system is demonstrated to be working on this wind turbine tower.

REFERENCES

1. Sørensen B F, Jørgensen E, Debel C P, Jensen F M, Jensen H M, Jacobsen T K, and Halling K, ‘Improved design of large wind turbine blade of fibre composites based on studies of scale effects (Phase 1). Summary report’. Riso-R-1390(EN), Risø National Laboratory, Denmark. 2004

2. SAND2004-0522: ‘Design Studies for Twist-Coupled Wind Turbine Blades’ James Locke and Ulyses Valencia, Wichita State University, 2004.

3. Steinchen W and Yang L X, 2003: ‘Digital shearography - theory and application of digital speckle pattern shearing interferometry’. SPIE Press, Bellingham, Washington, USA.

Potential Impact:
Potential Impacts

The final results of the project are a WTB inspection system and associated techniques. These include:

(1) A novel shearography system and associated procedures for in-situ WTB inspection.

(2) Comprehensive algorithms and procedures for phase extraction from shearography fringe patterns and for addressing rigid body motion of the WTB during inspection.

(3) A robotic climbing platform able to deploy the shearography system along the wind tower in order to inspect the WTB without touching the blade.

(4) The integrated DashWin system comprising the shearography inspection unit and the robotic platform that enables WTB inspection to be performed by an operator at ground level.

The potential impact from this project is increased probability of detecting cracks, debonds, delaminations and other defects in WTBs, and hence reduced probability of WTB mechanical failure. Such failures not only result in costly repair and lost revenue, they can also potentially result in human casualties. The robotic platform could also be used to host other equipment to carry out WTB inspection or maintenance tasks. The shearography system could also have wide application in other industry sectors, where components having a large surface area need to be inspected, and/or the component is in an unstable condition due to environmental vibrations. Examples include the inspection of aircraft wings and the examination of bridges under traffic load.

The project has helped and will continue to improve the competitiveness of the SMEs via expansion of their existing markets and the opening up of new markets. For example, BSR UK specialise in maintenance and inspection of rotor blades; this project has enabled them to plan expansion of their areas of expertise to include non-contact NDT inspection (both in production and in in-service situations). Innora specialise in the development and supply of innovative NDT systems; this project has enabled them to acquire a new climbing robotic platform and it has expanded their system integration expertise to include optical techniques for NDT of engineering components and equipment. The project has stimulated Pro Optica to plan to enter the wind energy market by providing a new optical system for WTB inspection. It has also introduced MicroTest to shearography inspection, which will enable them to add optical inspection techniques to their mechanical testing offering.

Dissemination activities

The main dissemination activities of the project included:

• Establishment of the project website with both confidential and publically accessible areas (www.dashwin.eu).
• Production of dissemination material in the form of leaflets, presentations, posters/exhibition material, a project logo and a video.
• Presentations at industrial and academic conferences.
• Publication of articles in print and on-line magazines.
• Development of appropriate training material for in-house use in Beneficiary organisations.
• One-to-one interactions with key Wind Energy sector organisations.
• Use of ready-made networks such as LinkedIn© to identify leads to rapid exploitation of the developed technology.

The DashWin project was promoted via discussions at the following conferences, exhibitions and events:

• 18th World Conference on NDT, 16-20 Apr 2012, Durban, South Africa
• EWEA Wind Energy Event, 16-19 Apr 2012, Copenhagen, Denmark
• AWEA Windpower 2012, Conference & Exhibition, 3-6 Jun 2012, Atlanta, GA, USA
• Husum WindEnergy 2012, 18-22 Sep 2012, Husum, Germany
• Wind Turbine Blade Manufacture, 27-29 Nov 2012, Dusseldorf, Germany
• ISNDT Conference, 10-12 Dec 2012, Delhi, India
• The 10th International Conference on Condition Monitoring and Machinery Failure Prevention Technologies, 18-20 Jun 2013, Kraków, Poland
• WindPowerExp, 24-26 Sep 2013, Zaragoza, Spain
• 5th European-American Workshop on Reliability of NDE, 07-10 Oct 2013, Berlin, Germany
• R4i Research for Impact (Making Public Funding Work), 31 Oct 2013, Cambridge, UK
• 6th International Conference: Wind Energy in Romania, 20 Nov 2013, Bucharest, Romania
• 4th Annual Wind Power Romania & Eastern Markets , 21-22 Jan 2014, Bucharest, Romania

In addition, the project video was utilised on an exhibition stand at the BINDT NDT 2013 Conference,10-12 Sep 2013, Telford, UK, and a presentation was made at the 11th International Conference on Condition Monitoring and Machinery Failure Prevention Technologies, 10-12 Jun 2014, Manchester, UK.

BSR has held detailed discussions with the a number of companies and organisations in Belgium, Brazil, Denmark, Spain, Sweden, Switzerland, USA, UK and Germany in the Wind Energy sector to promote the DashWin inspection system.

An article that summaries the DashWin project and its anticipated impacts is scheduled to appear in TWI’s Connect+ magazine (Issue 5 – May/June 2014). MicroTest is currently in discussions with Energetica XXI (a print and on-line magazine for the Renewable Energy sector) regarding the publication of a DashWin news item which is scheduled for later this year. BSR is developing a promotional article for the WINDTECH International magazine. Publication is anticipated later this year.


Exploitation

Calculations were performed to compare the cost of standard visual WTB inspection with the anticipated costs of using the automated DashWin system. The calculations indicated that the cost of using the DashWin system is comparable with the cost of standard visual inspection. It was concluded that the costs associated with using the DashWin system are not prohibitive to its commercial success.

Patent searches and literature surveys performed have established that the DashWin SME Partners could potentially submit a patent relating to motion compensation for a specimen being inspected using shearographic techniques. However, at the present time there are no plans for the SME Partners to apply for a patent.

Main exploitation of results and commercialisation of the DashWin system will occur in several distinct phases, covering product development, initial exploitation, European expansion and global expansion. The exploitation strategy to be followed upon project closure is outlined below.

Year 1 Production Development – during this phase the Partners will use the existing prototype to demonstrate the technology to potential customers (wind energy inspectors and operators) and conduct extensive performance tests. This work will result in more detailed case study material. BSR UK in consultation with RWE will ensure that DashWin obtains vital exposure and industry feedback during this period. Dissemination in this period is likely to be low, due to the lack of developed results.

Year 2-5 Initial Exploitation – This stage will concentrate on establishing a market presence in key markets. The benefits data collected from RWE in the first year following the project will be used to promote the DashWin technology to the major wind farm operators. We will also look to sell the technology into engineering firms that are involved in the construction, commissioning and maintenance of wind farms. In addition, we will look to promote the take-up of the DashWin technology for aerospace and other engineering sectors. A major task will be to establish a network of wind farm operators in our target countries. The consortium members will also look to identify new applications for the DashWin project results. This will be accomplished by interaction with industrial Partners of the Consortium members (especially BSR UK). One such application is the inspection of offshore wind turbines.

Year 6-10 European Expansion – By this time we expect to have established a solid foothold in several European countries. We will now consolidate these markets and start pan-European expansion. It must be noted that the specific markets that are going to be targeted will largely depend on local wind farm operators. Over this period we will consolidate and strengthen the DashWin supply chain to prepare it for rapid growth towards year 8-10 and beyond. We should also be in a position to lobby insurers to offer discounts where DashWin technique is employed, giving our customers a further incentive.

Beyond Year 10 Global Expansion – At this stage we will consolidate the European market and start exploring market opportunities in the rest of the world, particularly the US and the South Asian region. We will achieve this by engaging with local investors and wind farm operators.

It is also envisaged that both the DashWin shearography system and robotic platform will be used for secondary applications. The DashWin shearography system will be of great interest to organisations needing to inspect stationary specimens having a large surface area. Examples include the inspection of aircraft structures, or helicopter blades. The DashWin robotic platform could also be used to carry out other types of wind turbine NDT and maintenance operations (e.g. cleaning the WTBs). These secondary uses of the main DashWin system components could provide a substantial secondary income stream for the SME Partners.


List of Websites:
Project website:

www.dashwin.eu


Contact:

Jianxin Gao

TWI Limited
Granta Park, Great Abington
Cambridge CB21 6AL, UK

Tel: 44 (0) 1223 899000
Fax: 44 (0) 1223 892588
Web: http://www.twi.co.uk
Email: jianxin.gao@twi.co.uk