A monitoring system from AV Technology is guaranteeing the largest BHP Billiton offshore oil storage installation in Liverpool Bay remains in place and safe
A monitoring system from AV Technology is guaranteeing the largest BHP Billiton offshore oil storage installation in Liverpool Bay remains in place and safe. The development off the North Wales coast includes six offshore oil and gas fields as well as offshore and onshore facilities used for extracting, transporting and processing the reserves. Once the oil is processed and blended it is pumped through a 20km pipeline to the OSI - a double hulled, 870,000 barrel-capacity tanker permanently moored outside shipping lanes in the Irish Sea.

The OSI safety features include the 10 cargo tanks being segregated by 4.8 metre wide seawater ballast tanks.

The vessel is also manned around the clock and is protected by an 800-metre exclusion zone that is patrolled 24 hours a day.

To complement the security system, AVT was asked in 2003, to install a trial system to monitor the strain on the vessel mooring lines and has since introduced a permanent system.

The Mooring Line Monitoring System is fitted to the OSI buoy to measure and record the forces on the nine anchor chains that attach the facility to the seabed.

Neil Parkinson, AVT divisional manager, said the location of the vessel and its cargo presented a number of challenges to the system designers.

'The environment in and around the buoy is classified as a hazardous area and we had to create a micro safe area within the buoy.

But, we are used to solving tricky structural monitoring problems, especially in awkward environments'.

The AVT Monitoring system uses strain gauges to detect the effects of chain load variation by measuring shear force variations in the four vertical spider sheer webs, which are located adjacent to each set of anchor chains.

The 12 sets of strain gauges, which have been installed with nine sections of the lower buoy, have been sealed with three separate layers of environmental protection, cabled into the termination enclosure and connected through the barriers to the CR10X data logger.

Readings from the strain gauge sets are stored in CR10X, which is set to sample every 300 seconds.

Parkinson said AVT was able to implement a simple but effective solution: 'In projects such as this one, the working environment is far from perfect and the space inside the buoy is quite cramped adding to the complexity of the project.

We found from the initial evaluation exercise that we could monitor the integrity of the mooring lines which are in seawater in the hostile splash zone, using simple strain gauges mounted inside a clean, dry environment'.

Once data is downloaded, AVT assess the integrity of the nine mooring lines by comparing the relative load changes as the vessel moves 360degrees twice a day with the tides.

With the results generally consistent abnormal deviations are obvious, however, AVT engineers visit the OSI regularly to perform routine condition monitoring work and an electric report is emailed quarterly to BHP to show results.