SIMoNET Seminar Report

Report of Simonet Seminar held on 5 May 2004

 

Professor Bill Dover opened the meeting by welcoming those attending and explained that the theme of this seminar included sensors, data handling and risk based inspection.

 

1. Structural monitoring with optical Bragg gratings – T.Gibbs, Smart Fibres Ltd

Mr. Gibb of Smart Fibres presented an overview of fibre Bragg grating (FBG) sensing for structural health monitoring applications. He described the background with an FBG structural health monitoring system comprising three primary
components, an array of FBG sensors, an interrogation system to illuminate
the array and monitor changes to the reflected Bragg wavelengths, and a data
processing unit to calculate and log resulting measurands such as strain,
temperature, stress, force, pressure. The miniature fibre sensors (1 - 10 mm long and 0.15 mm in diameter) can be embedded within materials such as composites and concrete, or attached to the surface of a structure using a range of robust packaging solutions
developed by Smart Fibres to meet the needs of different applications.
Dozens of optical sensors can be written into a single fibre, making
multiple point sensor arrays easy to handle and quick to install. FBG arrays
range from the miniature, with sensors only mm apart, to multi-km arrays for
monitoring pipelines, oil risers, tunnels and the like. Since Fibre Bragg
grating sensors do not drift, require no electrical power and are immune to
electromagnetic interference, they are perfect for long-term structural
health monitoring.

Multiple sensor arrays are illuminated and interrogated by a single
opto-electronic unit, offering simultaneously data analysis of several
hundred sensors with high precision. With different data handling regimes, such systems can be used for real-time condition monitoring, or periodic "health checking" (weekly, monthly, yearly etc.) Under a periodic regime, the interrogation unit need not be
permanently connected to the sensors and, due to the excellent stability of
fibre optic sensors, no recalibrations are needed throughout the monitoring
period. Recent advances in interrogator technology have resulted in a significant
reduction in unit size and cost making this type of structural health
monitoring system commercially viable and rugged enough for use in a range
of hostile applications.

Speaking of the future Mr. Gibb said that with the many benefits of structural health monitoring using fibre optic technology becoming more widely understood, and with lowering system costs, reduced size and greater availability, Smart Fibres believe that this
technology will become adopted in more and more industries. Their vision is of
a future where it is the norm to manage high value or safety critical structures with condition based maintenance inspection regimes using such structural health monitoring techniques.

 

 

 

2. Fibre Optic Monitoring – David Hill, Qinetiq Ltd.

David Hill explained their application of fibre optic sensors for road and rail vehicle monitoring. They worked for the Highways Agency as well as for the rail industry. Fibres had the advantage to transmit signals over long distances with a high bandwidth, and with protection for electromagnetic radiation. The system is being used for advanced traffic management, with the capability of detecting moving vehicles and their speed and being able to interrogate form remote locations. Four sensors are used – two in each lane of a highway. Weight measurement is also possible. At present 12 in-road sensors have been deployed, with the ability to detect traffic flow and vehicle weight.

 

 

3. Structural monitoring of the construction of a new building using stress memory units, Dr Bart de Leeuw, University College London

 

Bart de Leeuw presented the details of monitoring a new eight storey building in London during construction.  Real time continuous monitoring of key structural elements was carried out using Stress Memory Units, a UCL patented technology*.  The wireless units remained in place for six months measuring and recording every structurally significant event.  Results correlated exactly with the construction log of events and highlighted an unforeseen anomaly which was also observed through deflection measurements.  The SM units provided clear and accurate results over this period without interfering with the surrounding building operations.

 

As part of the construction, a major jacking operation was also monitored using additional units at various points of structural interest.  During this operation, the units were responsible for identifying the parts of the structure that were experiencing predicted loads and those that showed where the jacking sequence needed to be modified.

 

Dr de Leeuw explained the ease and cost effectiveness of monitoring in this way and applied to construction and civil infrastructure provides an improved level of information regarding the load distribution within a structure.  Reduction of the uncertainties associated with load model predictions increases the overall safety of construction operations.

 

*   http://www.fiostec.com

 

 

4. Monitoring using the Stressprobe, Nicholas Stone. TSC Inspection Systems

 

 

TSC Inspection Systems has developed the StressProbe system for measuring stresses in a number of components including threads, welded components and forgings. In order to measure stress in these components the system takes advantage of magnetic property changes in the material, due to the application of mechanical stress. The system can detect the change in magnetic properties, or more specifically permeability, by the use of multiple induced fields and sensor coils encased in a non-contacting probe. The sign of the change in permeability depends on the relative directions of the stress & applied magnetic field, and on whether the stress is tensile or compressive.

 

Until recently the use of StressProbe was limited to those applications where the material was of ferromagnetic type and so therefore, obviously, metallic. With new research it has been possible to develop a sensor patch system whereby a suitable material can be inserted into or on the surface of non-metallic structures. This patch can then convey the representative stress from the component to the StressProbe device in the conventional manner. During tests on the new composite riser technology, being developed by a number of oil and gas companies, this new patch sensor system performed exceptionally well.  The implications of the success of this approach means that the StressProbe system is no longer limited to ferromagnetic applications and can be employed on a much larger selection of components in service.

 

 

5. Influence of antenna orientation on radar testing of post tensioned concrete beams, Prof. Mike Forde, University of Edinburgh.

Following the 1992 moratorium on post tensioned bridges in the UK there was considerable drive to develop improved inspection techniques. A time domain radar system had been developed, using a ‘bow-tie’ antennae. Prof. Forde described recent progress with development of the technique.

 

 

6. Thermographic inspection of surfaces, John Rudlin, TWI

John Rudlin described a thermal method for surface inspection, based on a project funded by ITF in Aberdeen. He reviewed existing crack detection methods, all of which needed good access. The thermal method could be used at a distance and was based on cracks distorting the local heat pattern. The method used a collimated heat/light source (e.g. laser).  In phase 1 of the study test samples with slots had been prepared and were examined at distances from 50-100 mm. In phase 2 surface condition, presence of coatings and the effect of welds are included. In addition several different heat sources are to be tested. The results showed that changing the surface condition could have a significant effect, particularly with problems from shiny surfaces. Paint coatings seemed to be acceptable. In the ongoing programme welds were being examined. Additionally light sources of different frequency were being tested, with the aim of developing a workable instrument. The main restriction of the method was that line of sight was required, it did not work with thick coatings and there were restrictions on the laser power that could be used.

 

 

7. Data handling developments in a pipeline condition assessment process, Malcolm Wayman, Advanced Engineering Solutions Ltd (AESL).

AESL worked primarily for the water and gas industry, both on low and high pressure pipelines and vessels. The main inspection tool was  non-intrusive, based on magnetic flux leakage. Data was processed on site using a PDA, linked to a GPS location. Malcolm Wayman described the data handling and transfer processes. 3-D models were also used to allow visual interpretation of the data.

 

 

8. Risk based inspection (1), Dr. Toula Onofriou, Surrey University

Dr.Onofriou described the background to risk based inspection (RBI), showing the use of target reliability levels, with the ability to update from inspection data and allowing considerable cost savings in inspection costs. She also demonstrated the benefits of on-line monitoring inputs, using probability based modeling and whole life optimization.

 

 

9. Risk based inspection (2), Daniel Straub,

Dr.Straub outlined the benefits of combining monitoring with inspection, to provide an improved level of knowledge of the structure. Their team had developed a generic approach based on fatigue/corrosion damage. Monitoring helps reduce the uncertainties in modeling parameters. Full integration of monitoring into a risk based inspection approach requires an integrated asset management strategy.

 

10. Risk based inspection (3), John Wintle, TWI

Mr.Wintle reviewed the status of risk based inspection (RBI), using pressure equipment as his example. New UK regulations were introduced for pressure systems in 1989, (updated in 2000) that allowed duty holders the flexibility to plan inspection according to the risk of failure (“so as to avoid danger”). HSE guidance was provided on the process of RBI within HSE Research Report 363/2001, and this contained an audit tool that duty holders could use to test the effectiveness of their process.

 

He also mentioned the European RIMAP project that was addressing the procedures and practices for risk based inspection and maintenance in several industries: power, petrochemical, chemical and steel. RIMAP aims to harmonize RBI procedures across Europe and set a benchmark for good practice. Its results will be published next year. Many major users in the petrochemical and power sectors now use the RBI for planning inspection and maintenance.

 

RBI processes have been evaluated in an HSE sponsored project with 7 different participants planning inspections for 4 different items of process equipment, using the same data. This was reported in 2002 and the results showed a considerable variation in damage predictions and inspection requirements. As an example, inspection periods ranged from 3-10 years for one case study and 1-6 years for another. It was concluded that proprietary software was not necessarily an advantage. The study was useful in demonstrating the sensitivity of the process and the need for further standardization.

 

Mr Wintle concluded by saying that RBI had many potential benefits, but was currently variable in its application, difficult to control and audit, and the competency of those applying it was not always assured. Where was it going? He suggested that other industries including in rail and offshore could benefit from the RBI approach. More intelligent software and supporting data was needed to assist users to evaluate their risks. Overcoming the availability of plant specific information is one of the main challenges, and the increased use of on-line structural health monitoring could make an important contribution. Eventually on-line structural monitoring could be seen as a substitute for RBI by removing most of the uncertainty that the RBI process addresses.

 

Professor Dover closed the meeting by thanking the speakers and reminding those attending that there would be another seminar later in the year.

 

J.V.Sharp, Dec.2004