Maintaining structural integrity is an ever increasing problem, particularly as installations get older. Structural failure can lead to loss of life, loss of asset value and environmental pollution, and the risk of this occurring needs to be reduced to an acceptable level. Many bridges, offshore installations and other structures are approaching or have exceeded their original design life, with increasing maintenance and repair costs. Therefore life extension is a regular requirement. As an example Network Rail (formerly Railtrack) is responsible for managing more than 40,000 bridges, with an average age of more than 130 years. Their annual maintenance cost is around £700 million. Many offshore structures in the UK sector of the North Sea are more than 20 years old, with a good proportion seeking extensions to their original lifetime of 25 years so that oil and gas production can continue, often via adjacent subsea wells. Consequently, there is a growing interest in techniques that remotely monitor the state of a structure. These can provide both a cost-effective solution to maintenance, as well as providing a continuous output on the state of the structure, which is often important when regulatory approval for life extension is needed.
The Structural Integrity Monitoring Network ( SIMoNET )
This is where SIMoNET comes in. SIMoNET, the Structural Integrity Monitoring Network, aims to bring together those involved and interested in structural integrity monitoring, whether users, providers or consultants, for the dissemination and exchange of information on the most up-to-date methods and developments in this field.
Applications Covered by the Structural Integrity Monitoring Network
Typical applications covered by the network include oil and gas platforms, floating offshore structures, jack-ups, industrial process plant, bridges, transport (rail, road vehicles, aircraft and ships) and power generation. SIMoNET commenced its second phase at the beginning of 2002 and is now totally funded by industry and government organisations. It was originally an Engineering and Physical Sciences Research Council funded network, set up five years ago to facilitate communication between industrial participants, researchers and all those interested in structural integrity monitoring (SIM). The current steering committee, which meets every two to three months, includes members from industry (Atkins, BNFL, Mecon, Ove Arup, BP, Fugro, HSE, Network Rail, TSC Inspection Systems, Galbraith Consulting) and is managed by University College London (UCL).
Figure 1. This bridge over Oxfordshire (UK) has a composite sub structure and is wired with fibre optic sensors to provide a real time picture of its condition.
Structural Integrity Monitoring Network Services
The networks’ internet site, contains information on equipment, suppliers, technical review papers, reviews of previous workshops, structural monitoring activities and links to other bodies involved. The website has become a central information point for the network and, for example, providing information on forthcoming seminars and reporting on these and other events. There is also a notification service for new items on the website as well as a discussion forum. Plans are underway to extend the activities to Europe, via other existing networks with which the group is in contact. Regular newsletters are posted on the website, with articles relevant to structural integrity monitoring. Many of these are posted by the group's member companies.
Workshops held in London form a very important part of the group’s activities. The last workshop was held in April 2003, with free access for members. Its theme was the monitoring of composites and the broader use of photogrammetry. Key papers presented included one from Mouchel on the monitoring of the first composite highway bridge in Oxfordshire installed in 2002, the use of acoustic emission for monitoring of composites (Pancom) and the monitoring of structures using close range photogrammetry (UCL). More general papers included a presentation of the latest status of the 1SO standard for offshore structures (Galbraith Consulting) and an outline of new regulations on structural safety (HSE) The previous workshop held last November included 11 papers on topics such as condition assessment of pipelines and steel structures, novel stress and crack measurement instruments (ACSM, accelerometers, ACFM and eddy currents), remote sensing of rail track deterioration, and instrumentation of rail tracks, tunnels and embankments. Previous seminars have included topics such as the Millennium Footbridge, monitoring in the Channel Tunnel, monitoring of ship hulls and bridge bashing.
Origins and Evolution of Structural Integrity Monitoring Network
The roots of SIMoNET lie back in the 1970s, when there was considerable interest in structural monitoring, particularity for offshore installations. Several major projects were undertaken but with the limited technology available at that time only major damage was detectable, with significant limitations for managing structural integrity. Improvements in sensors, better signal processing and minimisation of background levels from wave loading now make it possible for on-line monitoring to be effective. In the case of offshore structures, an improved understanding of structural behaviour at the system level make it possible to rely on detection of larger cracks and more major damage earlier than was previously possible, thanks to the benefits of using on-line monitoring.
Online Monitoring for Damage
Online monitoring for damage or crack growth involves the application of tools to measure changes in vibration, displacement, strain, acoustic emission, eddy currents or to measure stress by changes in magnetic properties. During the past decade the number of tools available has increased and their sensitivity improved, enabling more sophisticated monitoring to be undertaken.
Remote Monitoring of Bridges
Bridge bashing is one area where online monitoring could be of value, as three rail bridges in the UK are hit, on average, each day by vehicles. A large proportion of collisions cause no damage to the bridge but visual inspection by a trained engineer is usually necessary before allowing rail traffic to continue using the bridge. This can cost a significant amount of time and money, as well as frustration to the travelling public. Remote monitoring systems enable decisions to be made earlier with much less loss of time and effort.
Monitoring of Ships
The wider-scale use of monitoring in the ship industry has been addressed at one of the SIMoNET workshops by BMT In-service inspection of ships is difficult due to their large structure, poor access and limited access time. Operational benefits are an ongoing assessment of operational conditions as well as a record of the structural loads experienced by the vessel. For example it is possible to provide warnings to slow the vessel down as a result of high stresses occurring within the structure.
Assessment of Remaining Life and Life Extension
Life extension of many types of structures is now an important and a growing topic. This applies to many different types of structure and plant, and previous workshop presentations have included papers on the life extension of pressure vessels and pipework as a result of corrosion, the regulator's approach to the topic and the use of online monitoring for offshore platforms. The assessment of remaining life can be enhanced with knowledge of the structural performance during a period of time, resulting from the use of online monitoring instrumentation.
New Technologies for Remote Monitoring
An important theme for the network is to identify the latest available technology for remote structural monitoring, with presentations at the workshops from companies using new technologies in a range of different industrial applications. The cross industry nature of the workshops has an advantage in enabling members to see where new technologies are being applied and whether these might be relevant to their own field. As an example, the measurement of stress levels in structures is now possible using technology developed by TSC and UCL. This can be important when damage is present and an assessment of the significance of the damage is required. The ACSM technique is based on the anisotropic changes in magnetic permeability produced by mechanical stress, and has led to the introduction of a new system. Known as the StressProbe and manufactured by TSC, it can measure stress without contacting the metal surface.
The Future for Structural Integrity Monitoring
Structural integrity monitoring is now being used in a broad range of applications, particularly where remote monitoring has significant advantages, where regular information on the state of the structure is important (e.g. when damage is known to be present), and when cost is a significant factor in using more conventional technologies. For these reasons it is expected that the value of the SIMoNET network will grow as its membership increases.