SIMoNET Seminar Report

Report of SIMONET Meeting held at UCL on Nov. 5^th 2004 with a focus on seismic, alternative energy & marine technologies

 

Contributed by Professor J.V.Sharp, February 2004

 

Professor Dover welcomed those attending to the meeting and briefly explained the background to Simonet. This was the 8^th meeting organised by Simonet, all of which had been well attended and provided a useful opportunity to exchange ideas between different industry sectors.

 

1. Monitoring bridges using wireless mems, Paul Greening, Dept. of Civil Engineering, UCL

Paul Greening explained that new types of transducers for measuring acceleration (micro electronic mechanical systems -mems) are being developed at a fast rate, and units can now be bought off the shelf for ~$10 each. They have sensitivities for measuring acceleration of 2mg at 60 Hz, require low power and are robust in operation. A combination of these units with Bluetooth type technology enabled good communication from operational sites. He demonstrated a working prototype and indicated some potential applications, particularly on bridges.

 

2. Structural monitoring of wave and tidal energy devices – Prof. J.V.Sharp (Cranfield University)

Prof. Sharp outlined the background to the growth of renewable energy in the UK, with a government target of 10% contribution by 2010. Offshore wind was an area of very fast growth whereas wave and tidal were still at the research and demonstration stage, but would be expected to contribute to the UK supply by 2020. Several wave and tidal devices were now at large or close to full scale and he explained the application of sensing systems in these. These included Limpet (Wavegen) , which is an oscillating water column device, based on Islay, already producing power to the local grid. In this case wave pressure was being measured at the front of the collector and up to 4 bars had been recorded from wave slam. The Pelamis wave energy unit, being developed by Ocean Power Delivery Ltd has been tested at several different scales, and was now being developed at full scale for testing at the Orkney test site, early in 2004. This included extensive structural monitoring, with 16 strain gauges at each joint. Data would also be collected from the mooring system. In future machines a fatigue logging system was planned, based on strain gauges at each joint. Two UK tidal stream devices were currently being tested at close to full scale. These included Stingray (Engineering Business Ltd). This was a hydroplane device, with an advanced control system to optimise the power developed on each stroke. It had been deployed in Yell Sound (Shetlands) in September 2002 and generated power. Further testing was underway in 2003. Sensors were being used to measure loads between the hydroplane and the main arm, as well bending loads in the hydroplane structural tubes.

The second large scale tidal stream device is Seaflow (Marine Current Turbines Ltd) which is a 300kW unit installed in the sea ,off Lynmouth, in the summer of 1993. It is an underwater turbine installed on a 2m diameter pile. Strain gauges are located in the pile base, on the rotor blades and accelerometers are positioned at the top of the pile and in a rotor tip blade. Dynamic and static loads are being monitored on the pile. Lift and drag forces are being monitored on the rotor blades, as inputs to the assessment  of fatigue and the onset of significant cavitation. Data collection is continuing now the turbine is installed. There are plans to develop a larger 1 MW unit, with two turbines on a single pile. Overall structural monitoring is playing a key part in the development of wave and tidal systems.

 

3. Structural Instrumentation for the Renewable Energy industry – lessons from offshore oil and gas, A.Dougan, Fugro Structural Monitoring.

 

Alan Dougan explained how on-line instrumentation had been used to benefit the development of offshore oil and gas structures. Projects such as Shell’s Tern and BP’s Magnus had provided a much better understanding of the loading on these platforms, enabling more efficient designs to be developed for future structures. Monitoring of damage on offshore structures, such as that on the Ninian Southern platform, gave confidence that operations could continue whilst repairs were being planned and undertaken. He also outlined the background to an ongoing research project relevant to wind farms, which was the development of suction piles. These were an alternative to piled foundations, with significant potential costs savings per foundation. . Fugro was one of the companies involved in the project.

 

He outlined the large planned growth of the offshore wind industry, in similar offshore environments and proposed that on-line monitoring could also be very beneficial in this industry as well and hopefully the developers would take the opportunity at an early stage. Key issues with offshore wind towers are their dynamic response and strength, whilst for piled foundations their stiffness and possible scour were important factors. The addition of simple structural instrumentation to enable their response to be measured would provide confidence and valuable feedback to enable future structures to be optimized, with potential costs savings.

 

4. Wave Dragon, Duncan Dunce, Babcocks

Duncan Dunce explained the background to the wave dragon wave energy device. It is an ‘overtopping’ type unit, fitted with a low head turbine, slack moored, with 2 deflector arms. The device originated in the 1980’s and with funding becoming available in the 1990’2 has been tested at model scale. A large scale prototype has  been built in Denmark (57m between the arms) which was launched in March 2003. There are a number of project partners, including Aalborg University and Babcocks. It is being tested at a Danish site, which has less extreme conditions than the unit has been designed for. A second site has also been selected for testing with more demanding environmental conditions. In May 2003 the unit was operating and supplied electricity to the Danish grid for a short time. Improvements have since been made to the device. Monitoring systems have been built into the device, both for condition and structural monitoring. A webcam is also available to monitor performance. For structural monitoring strain gauges and accelerometers are being used; an area of particular interest is the joint between the arm and the main body. Work has already started on a larger scale device, aimed at producing power in the MW range.

 

5. A new NDT technique for the railway industry: ACFM inspection of axles and rails, Dr Michael Smith, NDE Projects Manager, TSC Inspection Systems.

 

Alternating Current Field Measurement (ACFM) has recently been introduced to the rail industry for the inspection of safety critical components. Originally researched by UCL and developed by TSC Inspection Systems, the technique has been in regular use in the offshore oil and gas industry for the inspection of surface cracking.

 

ACFM is an electromagnetic technique which can locate and size surface breaking defects in conductive materials and can operate through coatings. It offers the advantage of recording all inspection data and produces computerised reports. Dr Smith described the theory behind the technique along with the capabilities and limitations of it in practise.

 

Three ACFM applications were described within the rail industry: the inspection of axles, bogie frames and the ACFM Walking Stick for the inspection of plain rails. Independent trials of comparative techniques were carried out by the Engineering Link on axles and the results showed that ACFM outperformed MPI for detection and could also indicate the defect depth. Extensive testing by Bombardier Transportation and Network Rail has led to ACFM’s acceptance within the rail industry.

 

6.      Beach monitoring – a strategic approach, Dr.Travis Mason, Channel Coastal Laboratory.

Dr. Mason explained her role, covering the south coast area, with concerns on cliff erosion, state of beaches etc. High quality data was needed for beach management; in some cases long period records were available (e.g. Bournemouth) whilst for some areas there was virtually no data. DEFRA is funding a five year programme to develop improved data sets. Risk based categories are being developed, associated with management actions (hold or advance current line, to doing nothing). Monitoring programmes are needed to support these. GPS control networks were also being developed. Surveys of the whole coastline were scheduled for every 5 years, with intermediate surveys being carried out selectively once or twice p.a. Hydrographic surveys were also being deployed, together with obtaining real time tidal data. She explained that this was the first scheme in the country, and possibly to be extended to the SW coastal area in the future

 

 

7.      Permanent Structural Monitoring using Guided Waves for oil Industry structures, Dr.P.Fromme, Dept. of Mechanical Engineering, UCL

 

Technical machinery and systems are subject to varying or cyclic service loads and environmental influences, like adverse weather conditions. Such operation conditions can lead to the development of faults during the lifecycle of the structure, e.g., fatigue cracks and severe corrosion damage in offshore oil platforms and oil storage tanks. This damage can lead to the malfunction and ultimately failure of such structures, endangering lives and the environment.

 

Fast inspection of large areas of plate-like structures can be achieved employing guided ultrasonic waves. Guided ultrasonic wave testing utilizes a lower frequency region than standard ultrasonic testing (UT), usually in the order of several hundred kHz. The excited wave has a mode shape through the thickness of the structure and can propagate over large distances along the structure. Such measurements have been successfully performed on pipelines, where propagation distances of up to hundreds of meters were realized. This allows the fast and cost-efficient monitoring of difficult-to-access structures from a single sensor location.

 

For the practical long-term monitoring of structural integrity, a permanently attached guided ultrasonic wave array prototype is being designed and built. The prototype is developed as a remote sensor that runs autonomously, i.e., independent of external energy supply, and transmits data about the condition of the structure wirelessly using a bluetooth connection. The array consists of a ring of piezoelectric transducer elements for excitation and reception of the guided wave. The properties and coupling of the transducers has been studied and optimized to achieve a sufficient excitation amplitude and good measurement sensitivity. The development of the compact array device for the inspection of large areas with minimum power consumption, necessary for long term operation independent of external power sources, is described. The array operation and data processing schemes are shown and first laboratory measurement results presented.

 

The sensitivity of the measurement method to typical structural defects is studied experimentally and theoretically. The reflection and mode conversion of the employed guided ultrasonic wave at model defects and structural features is calculated numerically employing a three-dimensional finite element code and verified in laboratory experiments. Corrosion damage often results in large area thickness reduction and is modelled as a circular part-through hole. The scattering and mode conversion of the first antisymmetric Lamb wave mode A₀ at such defects is studied. Good agreement with measurement results in the laboratory is found and the sensitivity of the guided wave measurement to such a kind of defect can be predicted.

 

Closure

Professor Dover thanked the speakers and welcomed those attending who were not members Simonet to join and benefit from the wider activities of the group.