Report of Simonet Seminar held on April 14^th  2005

 

Professor Bill Dover opened the meeting by welcoming those attending and explained that the theme of this seminar included offshore, vehicles, networks and sensors.

 

1. The story of Buchan Alpha,  David Linkens, Atkins

This talk gave an overview of the Buchan Alpha production platform, and the structural issues which have been dealt with, to enable it to operate successfully and safely for the last 25 years.

Originally constructed as a drilling rig, Drillmaster, it was converted by BP to a production platform from 1978-1981.  It is now under the ownership of Talisman Energy. During the conversion work, in 1980, the worst offshore disaster of the decade occurred.  The Alexander Kielland, a sister platform, broke up in a storm, and overturned within 30 minutes, with the loss of 123 lives.

An urgent and thorough review was carried out by BP, and independently by Sir High Ford.  A number of necessary improvements were made, to ensure structural safety.  The floating pentagon design of the 1970's has a lack of redundancy in the tubular bracework which connects the five pontoons.  To operate safely it is necessary to apply a stringent ongoing inspection regime, to detect any fatigue cracking well before it becomes critical. 

Several photographs of the original construction, and transport of various node sub-assemblies, and assembly afloat at Normandy were shown.  The range of inspection approaches which are used to manage the integrity were described, including installation of an acoustic monitoring system in recent years.

In this talk it was shown how vigilance and ongoing attention have enabled a critical structure to operate safely, and to continue to do so. 

 

2. Ageing Offshore Structures,  Dr.John Sharp (Cranfield University) and Dr.Alex Stacey (HSE),

 

Dr.Sharp said that many offshore structures were now beyond their original design lives and that life extension was now being demonstrated. There were a number of hazards to ageing structures which included fatigue, extreme  weather, subsidence and accidental damage. He also showed the ‘bath tub’ curve, with three phases (early life failures, constant failure rate and the wear out phase) and indicated that although an increase  in failures would be expected in the wear-out phase due to ageing it was not clear on what timescale this would occur. Life extension was addressed in a number of codes and standards and in particular the draft ISO 19902 standard on fixed steel structures had a new section on assessment of existing structures.  Reassessment included condition monitoring, and this was an area where structural monitoring could play a significant part. Structural vibration monitoring and acoustic emission had been sued in the past to monitor defects and there was a new development of fatigue sensors which could be useful in life extension assessment. It was concluded that SIM techniques were improving and new developments in vibration monitoring were indicating significant promise. However it was felt that the offshore industry did not yet fully appreciate the benefits of SIM monitoring and that SIMoNET could play an important part in publicising these techniques.

 

3. Measurement of Marine Fouling, Dr. Martin Lugg, TSC Inspection Systems

Dr. Lugg explained that TSC Inspection Systems are best known for the ACFM technique for crack detection and sizing.  However, the company has also developed a couple of spin-off applications using the same electromagnetic technology. One is the measurement of stress in steel, the other is the measurement of lift-off (the distance between a probe and a metal surface).

 

ACFM probes are normally designed to minimise the effect of lift-off, but for spin-off applications, probes are instead designed to maximise the effect.  This is done by using a field direction perpendicular to the metal surface and using balanced differentially-wound sensor coils.

 

TSC have produced probes suitable for a variety of applications from pipe ovality measurement, where sub-millimetre accuracy is required up to marine fouling thickness measurement where separation from the metal surface can be up to 300mm.

 

Marine growth is a problem for offshore structures, especially in warm waters, because of the increased loading on the structure caused by increased drag.  Conventional methods of measuring marine growth thickness (encircling tape measure by diver or calipers by ROV) only provide an average thickness.  However, marine growth is usually unevenly distributed and local thickness variations influence both the loading and the procedure for removal.

 

The probe developed by TSC, deployable by diver or ROV, provides spot measurements of marine fouling thickness to an accuracy of about ±10% on any tubular steel member.  The system has been used in waters off Australia and Thailand and is expected to be used in similar waters more widely in the future.

 

 

4. Fatigue Damage sensor for welded structures,  Peter Tubby, TWI

Peter Tubby described CrackFirst™ which is a new fatigue damage sensor system specifically designed for welded steel structures. It was developed within a DTI funded collaborative project managed by TWI, involving Caterpillar Peterlee, Micro Circuit Engineering Ltd, Fatigue Monitoring Bureau and UMIST. The CrackFirst™ sensor itself consists of a thin metallic coupon containing a central fatigue crack, which is attached to the structure adjacent to a critical welded joint. Stress cycles in the structure cause extension of the fatigue crack, the magnitude of which is proportional to the cumulative fatigue damage in the target joint. Sensor readings therefore provide an estimate of the rate at which the structure's design life is being consumed, allowing the remaining safe life to be estimated and service inspection intervals to be defined on the basis of usage.

A combination of capacitor discharge welded studs and adhesive bonding is used to attach the sensor to the structure. A tensile pre-stress induced in the coupon during attachment ensures that it responds to both tensile and compressive applied load cycles, in the same way as a welded joint containing tensile residual stress. Sensor electronics with an expected battery life of ten years are used to interrogate the sensor and store results, which can be downloaded to a PC for further analysis. Twelve sequential step changes in the electrical output occur during the life of the sensor.

Fatigue tests on fillet welded plate specimens under constant amplitude loading were conducted to check sensor performance. In all cases the sensors failed before the test joints, the sensor giving a characteristic fatigue endurance similar to the ‘Class F’ design curve in the UK fatigue design rules for welded steel structures. The sensor therefore matches the design fatigue strength of typical fillet welded details such as stiffeners or other welded attachments commonly used in plate structures.

Variable amplitude loading measured by Caterpillar on an articulated truck undergoing endurance trials in a quarry was also used in the validation. For the particular sequence investigated, test lives of the welded joints were significantly greater than predicted by Miner’s rule, due to stress cycle interactions resulting in crack growth retardation. A similar effect was observed in the sensors, although in all cases they still failed conservatively, i.e. before the test welds. These results suggest that the CrackFirst™ sensor will provide a more accurate assessment of damage accumulation than conventional methods based on strain cycle counting and Miner’s rule. Further tests under different loading sequences are planned to explore this.

Field trials on earth moving equipment, ships and offshore structures are planned. TWI is in the final stages of development and in bringing the system to market. Companies interested in using the system are invited to contact the author.

 

5. National Composites Network, Dr.Deborah Pullen, NCN

Dr. Pullen explained that the National Composites Network (NCN) is a new and unique Knowledge Transfer Network jointly funded by government and industry that will embrace the entire UK Composites industry and its supply chain. The NCN will connect all segments of UK composites expertise, fill key technology gaps for the future and signpost and facilitate end user access to the most appropriate source of assistance. This will enable the development of a national strategy for promoting the composites sector in the UK and abroad.

 

NCN is forming working relationships with a number of existing networks and trade bodies across relevant sectors and technology platforms to increase facilitation of new collaborations and maximise the impact of its services. The recent formal link between NCN and Simonet is an excellent example where the communities of the two networks can come together to maximise the benefits of both and seek to develop common goals in unison.

 

A significant part of the programme will focus on active technology transfer, particularly to SMEs, where the NCN will respond to specific queries made via the helpdesk and host a series of interactive workshop events covering a range of technical themes. The technology transfer service also offers one to one technology audits with the possibility of follow-on 5 day practical feasibility studies. There will also be a number of cross-sector demonstrators to showcase the true impact of successful technology transfer. A pool of relevant expertise and nation-wide facilities within this network are made available through NCN Technology Transfer services. Predominantly free of charge, the services provide excellent opportunities for forward-thinking companies to improve their products and manufacturing processes.

 

Deborah Pullen, the NCN Programme Director is keen to talk with anyone from the industry or who has interests in expanding into composites to hear their views on how the NCN can be effective and all-encompassing.

 

The NCN can be accessed via the helpdesk 01223 891284 or by e-mail to info@ncn-uk.co.uk

 

 

6. Vehicle Condition monitoring,  Dr.Andrew Halfpenny, nCode

Dr.Halfpenny explained that the company was set up in 1982 to deal with fatigue failure and software for assessing fatigue with military ground vehicles. The HUMS system on board can monitor loads using sensors (transducers, strain gauges) and can thereby help to optimise future designs and reduce life cycle costs. HUMS not only is diagnostic (identifying presence of fault) but also prognostic (predicting residual lie). In the prognostic mode the software includes an analytical model of the vehicle. From measuring the load history it can predict the accumulated damage and provide immediate feedback to the crew. In addition there is an off-board management system, which includes long term storage of measured data, which enables fleet management to be undertaken.

 

 

7. Stesses in Tunnels,  Peter Wright, Tubelines

Peter Wright’s talk gave an overview of Tube Lines Company and its responsibilities as defined within the Public Private Partnership PPP scheme.  For Tube Lines their responsibilities are for maintaining the Northern, Jubilee and Piccadilly lines of the London Underground network. Under the PPP scheme  all deep tube tunnels have to be assessed. The assessment consists of an inspection as well as structural/geotechnical calculations to ensure that adequate safety factors exist at all points in an ageing network consisting of some 350km of tunnels. One of the principal problems is the indeterminate nature of the old cast iron tunnels that make up most of the network. In most cases it is not possible to accurately predict the direct and bending stresses that these tunnel segments are subject to, especially as the ground loading does not normally change. However, using a combination of 3D finite element analysis and NDT measurements using a newly-developed Alternating Current Stress Measurement tool called StressProbe, it has been possible to directly understand these stresses. The talk illustrated the use of both techniques on specific sections of tunnel lining, and presented the benefits of using the newly developed non-contacting stress measurement tool, the TSC StressProbe in completing tunnel assessments. The results presented for work carried out on three tunnels using the StressProbe showed very good correlation with calculated Over Burden loads at these sites.

 

 

8. Substrate Preparation for the application of fibre composites – can monitoring help?  Brian Bell, Network group for Composites and Construction

Brian Bell described some of the current surface preparation specifications used in connection with the application of FRPs to existing structures.  He pointed out that there were inconsistencies between the finishes specified for different substrates (e.g. concrete, steel and cast iron) and also with the finish provided on manufactured FRP plates protected by peel ply.  He concluded that these inconsistencies could be responsible for some of failures experienced with retro fitted FRP plates and suggested that an investigation into these questions could be beneficial.  Brian finally posed this question to the monitoring community – “Can you come up with simple monitoring or inspection systems for use on site that will confirm when appropriate surface preparation has been undertaken?”

 

 

9. Acoustic Emission monitoring and its application on structures,  Jon Watson, Physical Acoustics

Jon Watson gave a presentation on the use of Acoustic Emission (AE) monitoring on civil and offshore structures. He gave a brief summary on the wide variety of steel structures that Physical Acoustics have worked on which included (amongst other things) steel box girders, welded I beams, orthotropic bridge decks, various bridge bearings and shear studs.

 

Differences in AE monitoring strategy and use were defined.

Global monitoring – used to identify damage hotspots and also strategically to screen large numbers or large areas of structures for active defects

Semi Global monitoring – used for detailed investigations of structures, commonly those that have already been identified as having potential problems.

Local monitoring – highly accurate active source location used on structural details and areas of concern, commonly applied on welds and bearing.

 

Tying in with the days earlier presentation by Atkins, Jon summarised Physical Acoustics long term continuous AE monitoring project on the Buchan Alpha platform, shown pictures of the installation and data from the commissioning. The AE system on Buchan Alpha automatically updates a secure website accessible to authorized persons on and offshore with the latest monitoring information. The system has now been running continuously for almost 4 years.

 

In the final slides of the presentation another example of long term monitoring on Thelwall Viaducts roller bearings was shown. This system monitored the cracking occurring within the rollers and utilised “feature data extraction” which enabled differentiation between noise from bearing movement and that from fracture. Transient signal feature extraction is essential for monitoring on bridge structures and is available on all Physical Acoustics AE systems.

 

The presentation was concluded with reference to the Highways Agency’s Advice note BA 79 “Management of Substandard Structures” saying that AE monitoring met the requirements for monitoring and that the technical application of AE on civil structures would be specifically covered within an extension to the current Highways Agency Advice Note BA 86/04 on Non Destructive Testing, which is expected to be published in 2006.

 

Further information can be sought from the website www.pacuk.co.uk or by contact Jon direct at jonwatsonuk@aol.com

 

 

 

10. Structural Health Monitoring of Civil infrastructures,  Prof. James Brownjohn,  Vibration Engineering Section, University of Sheffield, Dept of Civil and Structural Engineering

 

 

Professor Brownjohn explained that structural health monitoring (SHM) has a range of definitions but in this talk, for civil infrastructure, SHM was defined as the collection and interpretation of signals for assessing health for diagnosis and prognosis of bridges, dams. buildings and other structures. The term ‘health’ used in civil engineering implies a spectrum from imminent failure through to high levels of performance; hence civil SHM systems are installed for a variety of reasons to do with correction operation, safety, loading and economic operation.

 

Civil structural monitoring is not a new idea; it was particularly fashionable in the 1970s and 1980s but despite a number of technological advances, there were limits on the information and knowledge that could be extracted from the data. Possibly too much was expected; in particular the promise of vibration techniques to detect, locate and even quantify damage were not delivered, as difficult operational conditions and noise resulting from ambient and non-structural effects showed such techniques to be unreliable.

 

This over-reliance on vibration-based tools and the resulting loss of faith seems to have subdued interest in ‘monitoring’ in the UK. Not just in the UK, because researchers and practitioners in structural monitoring have tended to come from dynamics backgrounds, there is still a tendency to focus on such methods.

 

Outside the UK, over the last decade, there has been a resurgence of interest in the science of structural monitoring and much of the emphasis has shifted to a holistic approach involving management and interpretation of data, which is the point at which it becomes structural health monitoring. Research groups without bias towards dynamics have adapted and developed a wide range of mathematical tools for data mining, pattern recognition and system identification. Supported by and in parallel with leaps in technology of sensors (fibre optics, GPS, PZT patches etc.), miniaturization (for low power consumption and on-board or embedded processing) and communications (GSM, GPRS and 3G) there is an acceleration of interest in SHM internationally. The greatest progress has been made in the Far East (Taiwan, Korea, Japan) as well as in Canada and to a lesser extent USA. The UK, possibly hindered by previous experience, has been slow to jump on the bandwagon.

 

The talk presented the philosophy of civil infrastructure SHM, with the personal viewpoint that off-line activities such as system identification and updating are required to produce a reliable model for data interpretation. A few quick examples (a tall building, a concrete box girder bridge and tunneling works) illustrated the range of SHM technologies and applications.

 

The talk emphasized practical applications rather than academic research with the sentiment that there is a gulf between what operators/managers need and what researchers can offer. Such a gap needs to exist to drive technology but there are needs for owners and consultants to have a little  faith in the new technologies and provide opportunities for research so that potential can be realised. Meanwhile researchers have to provide a little of what the users need immediately and take every chance to evaluate systems on live structures.

 

Closure. Professor Dover thanked the presenters and those who had contributed to the questions for an interesting and useful seminar.