Hazardous area instrumentation is all at sea

A permanent mooring line integrity monitoring system plays a crucial role in ensuring the ongoing safety of a floating offshore oil storage installation in Liverpool Bay.

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 its 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.

Liverpool Bay is BHP Billiton's largest operated asset and comprises six offshore oil and gas fields, together with significant offshore and onshore facilities used for extracting, transporting and processing these reserves.

Offshore operations are centred on the Douglas complex sited 24km from the North Wales coast.

This is a three-platform facility that monitors and controls the development's three unmanned satellite platforms at Lennox, Hamilton and Hamilton North.

Oil from all four fields is received at Douglas where it is then processed and blended before being pumped through a 20km pipeline to the OSI, which takes the form of a double-hulled, 870,000-barrel-capacity tanker permanently moored outside shipping lanes in the Irish Sea.

Designed with safety as a key priority the vessel's ten cargo tanks are flanked by segregated 4.8m-wide seawater ballast tanks.

As well as being manned around the clock, it is protected by an 800 metre exclusion zone, which is continuously monitored by radar and patrolled 24 hours a day by a high-powered support boat.

Initially AVT were commissioned in March 2003 to install a trial system to monitor the effects of chain load variations as functions of strain variations in the structural components of the buoy.

Having successfully demonstrated the capability of a strain-gauge based method for assessing the integrity of the mooring lines, the system has been upgraded during 2004 and is now a permanent installation, replacing both the temporary monitoring system and the original inclinometer based system.

The installation presented a number of intriguing challenges for AVT but as Divisional Manager Neil Parkinson explains: "We are used to solving tricky structural monitoring problems, especially in awkward environments".

"The environment in and around the buoy is classified as a designated hazardous area and to meet the ATEX requirements, we created a micro safe area within the buoy".

"This consists of a novel dual Exd-Exi environment using explosion proof enclosures (EExd IIC T6) to house the Campbell Scientific CR10X Data Logger, multiplexer and the MTL zener barriers".

"Everything outside of these enclosures, including the cabling and the strain gauges, was then protected by an intrinsically safe certified ATEX system".

The AVT monitoring system uses strain gauges to monitor the effects of chain load variations by measuring shear force variations in the four vertical spider shear webs adjacent to each set of anchor chains.

The 12 sets of strain gauges, which have been installed within nine sections of the lower buoy, are configured as full Wheatstone bridge circuits, each consisting of four single element spot weldable strain gauges.

After installation, the strain gauges have been sealed with three separate layers of environmental protection and 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.

Collecting data is only a small part of the work AVT carries out and as Neil Parkinson continues: "The key to successful structural monitoring is being able to analyse and interpret large amounts of information and then present it to the client in the optimum format".

"All round good communication and the ability to work closely with other interested parties are also vital ingredients".

"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".

"However, we are used to working in difficult and often remote areas with limited facilities and have developed a high level of ingenuity to ensure that our equipment can operate without problems".

"In this application, the solution adopted by AVT is attractive because it is so simple".

"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".

The dynamic nature of the working environment requires periodic visits to the buoy to download the data.

From these data, AVT can clearly assess the integrity of all nine mooring lines using a novel data processing method which makes use of the "weather-vaning" behaviour of the OSI vessel.

Basically, AVT simply compares the relative load changes on each of the mooring lines as the vessel moves through 360 degrees twice per day due to tide changes.

Any loss of integrity of a mooring chain will result in a reduction of the normal tidal variation in load due to vessel direction changes, resulting in a corresponding reduction of strain in the supporting web plates.

Similarly, there may be a corresponding increase in loads in the remaining mooring lines.

Such effects will be identified from a separation in the trended "relative tidal load distribution ratios" (RLD) for each of the four web plates per lobe.

These RLD ratios have initially been normalised to unity based on the data from the temporary installation.

At any point in time, the values of four RLD ratios for the four web plates per lobe will average to unity.

The system has been operating successfully since it was commissioned in February 2004.

Data from the system are downloaded quarterly by other AVT engineers who regularly visit the OSI to perform routine machinery condition monitoring work.

RLD trends to date show excellent results with only short term variations in loadings on the mooring chains in severe weather.

Long-term trends are extremely consistent and therefore any permanent abnormal deviations would be very obvious.

After each quarterly analysis a brief electronic report is distributed by e-mail to all relevant parties within BHP Billiton, both onshore and offshore.

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