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Product category: Testing and Calibration Services and Information
News Release from: AV Technology | Subject: Offshore structural monitoring
Edited by the Engineeringtalk Editorial Team on 17 December 2007

Prevention is better than cure offshore

Neil Parkinson, Technical Director of AV Technology, outlines some of the requirements for offshore structural monitoring.

On March 27th 1980, the Alexander Kielland oil rig, operating in the North Sea's Ekofisk field, broke up and capsized with the tragic loss of 123 lives Subsequent investigations showed that the disaster had been caused by fatigue cracks emanating from a small and apparently innocent hydrophone port on one of the support legs

The tiny bracket had been welded at what appeared to be a noncritical point of the structure and unfortunately no problems had been envisaged.

Evidence suggested that there had been a small crack present from the outset and constant pounding from winds and seas gradually extended the cracks to the critical point when the leg fractured, initiating a rapid and catastrophic structure collapse.

It is clear that any structural failure can have far reaching effects impacting on critical factors such as safety and cost.

Far too often when a problem occurs, structural engineers are left to literally "pick up the pieces" in order to analyse what went wrong.

Although sophisticated computer design techniques are now used extensively for structural design it is still very difficult to predict the effects of every operational and environmental eventuality, especially if local conditions change with time.

This highlights the importance of employing effective structural monitoring as both a predictive and preventive tool.

Carrying out structural monitoring on offshore installations presents its own particular challenges, especially in terms of accessibility and operating environment.

The corrosive environment mandates the use of effective sealing for the sensors and instrumentation, whilst offshore structural monitoring applications typically involve working in certified hazardous areas.

In parallel strict health and safety requirements must be met when installing equipment and retrieving data.

The key to successful structural monitoring is directly dependent on: effective sensor selection, location and installation; effective measurement and data collection; rapid data analysis and presentation; reliable interpretation of results; and effective situation prediction.

Stockport based AV Technology has extensive experience in both onshore and offshore applications, typically involving the measurement and analysis of dynamic strains, loads, vibrations and displacements.

Sensors used include multiple- and single-axis accelerometers, linear displacement transducers (LVDTs), strain gauges, vibrating wire gauges (VWGs) and tilt meters.

Not only does AVT have the expertise to fit the diverse range of sensors, but the company has developed comprehensive monitoring equipment and offshore software packages, which enable them to collect and analyse large amounts of data.

At one end of the spectrum, monitoring may be on a continual long term basis, whereas at the other tests may last only a few hours or days.

AV Technology's clients need accurate structural data in a format to suit their specific requirements.

The offshore environment is very demanding and unforgiving therefore our installations must be reliable and robust.

You can't just "pop out" to an oil rig if there are equipment problems.

We are used to working in challenging environments and often have to install equipment in awkward and confined spaces.

To meet hazardous area regulations we have pioneered the development of micro safe areas for our equipment using certified enclosures and intrinsically safe Zener barrier protection systems.

In one application a permanent mooring line integrity monitoring system supplied by AVT is playing a crucial role in ensuring the ongoing safety of the floating offshore oil storage installation (OSI) for BHP Billiton as part of their Liverpool Bay Development.

The explosion proof system, which is fitted inside the OSI mooring buoy, is designed to measure and record the forces on the nine anchor chains which attach the buoy to the sea bed.

This is achieved by measuring shear force variations in the four vertical spider shear webs adjacent to each set of anchor chains using spot weldable strain gauges coated with three separate layers of environmental protection.

Over a thousand miles away in the Caspian Sea a quite different AVT structural monitoring system is helping ensure the safety of the Chirag-1 oil production platform.

Situated in the Chirag field 120km from the Azerbaijan coastline, the platform is operated by the Azerbaijan International Operating Company (AIOC), an international consortium, headed by BP.

Although the area beneath the Caspian Sea is rich in oil deposits, it is also an area of high seismic activity and lies on a major fault line running through the Caucasus Mountains.

It is clear that earthquakes pose a serious threat to oil production and above all the safety of the people working on the platforms.

As a result, establishing methods to assess the possible dangers to equipment and personnel has the highest priority.

Although Chirag-1 has been specially modified to withstand normal seismic activity, the operators want to ensure that the likelihood of any damage resulting from earthquakes can be assessed quickly and effectively.

AVT was commissioned to install a PC based natural frequency and seismic monitoring system so that platform displacements and response modes to the changing environmental conditions can be recorded on a permanent basis.

The explosion proof Exd system uses high sensitivity servo accelerometers to collect data on a real time basis.

These are housed within explosion proof Exd certified cast-iron enclosures fitted to the module support frame (MSF) in the North-East and South-West corners of the platform to measure the motions of the rig in three axes.

All the data are logged onto a dedicated data collection system inside one of the control rooms and sent back to AVT for processing approximately every three months.

The system operates simultaneously in dual modes.

In the natural frequency mode it routinely collects data once a week for 3000 seconds, at a sampling rate of 100 samples per second.

In the seismic mode, the system is constantly waiting for a seismic event, triggered by acceleration levels above preset thresholds.

When this occurs, the system collects higher speed data at a rate of 1000 samples per second for 300 seconds, plus 10 seconds pretrigger data.

The processed data is assessed by Bomel, which has extensive experience in structural analysis and risk assessment in the offshore industry.

The natural frequency information is used to identify the natural frequencies of the platform can then be compared with natural frequency predictions made by Bomel using finite element models and used to fine-tune these models for future use.

It is also used to identify any in-explicable changes in modal frequencies, which could provide an indication of structural damage to the jacket or a change in foundation conditions.

The seismic data are used to quantify the platform's response to earthquakes.

This three dimensional information, combined with other seismic data from around the world allows Bomel to build up a knowledge base from which it will be possible to assess the likely effect of earthquakes in terms of damage to the platform.

A similar natural frequency monitoring system is used for the ongoing structural monitoring of the British Gas Armada Platform situated in the North Sea.

The explosion proof Exd system uses four high sensitivity servo accelerometers fitted in pairs to the top of two diametrically opposite jacket legs, just below where they join the main deck.

Platform displacement and response mode data are combined with eight channels of wind and wave data to form an overall picture of how the structure is responding to the environmental loads.

AVT prepares periodic reports based on this information which is then again analysed by Bomel, which compares the recorded data with the assumed original design data to ensure the successful ongoing structural integrity management of the platform.

As Bomel's Head of Technology, Dr Phillip Wicks, and Head of Structural Integrity Management, Richard Turner, conclude: "The data supplied by AVT in both these applications is of vital importance".

"The information allows us to monitor the life cycle of the platforms and from this we can put together detailed inspection routines, appropriate to the changing conditions".

"Although there are already standard inspection procedures, including annual or biennial underwater examinations, being able to provide an accurate ongoing predictive risk assessment on the condition of such complex structures is paramount in assuring safety for the platform personnel and the environment". Request a free brochure from AV Technology ...

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