Acquisition system keeps track of quake-prone rigs

Specialist structural monitoring consultant AV Technology is playing a vital part in ensuring the safety of the Chirag-1 oil production platform, situated in the Gunashli field in the Caspian Sea.

Specialist structural monitoring consultant AV Technology is playing a vital part in ensuring the safety of the Chirag-1 oil production platform, situated in the Gunashli field in the Caspian Sea, 100km from the Azerbaijan coastline.

Extensive development of the oil fields in this region has been taking place since the late 1990s and they are now 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.

The Azerbaijan authorities have set up over 10 ground monitoring stations to provide real-time records of seismic activity on a permanent basis.

The network registers between two and ten small events every day and two to three major earthquakes per month over magnitude 3.

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

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

The Chirag-1 platform has undergone extensive refurbishment and the original modules have been removed, stripped of all internals, enlarged, strengthened and equipped to international standards.

The additional modules which had to be built included a 1035t module support frame (MSF).

Unlike most conventional rigs, the MSF, which supports the top size deck structure, is supported on two separate jackets, each with 10 legs.

Seismic isolators have been installed on the top of each leg to minimise problems occurring during seismic activity.

Although the 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.

Based on its expertise in the structural monitoring of offshore equipment, Stockport based AV Technology was commissioned by AIOC to install a PC-based, natural frequency and seismic monitoring system on the Chirag-1 so that platform displacements and response modes to the changing environmental conditions can be recorded on a permanent basis.

Not only does AV Technology have the expertise to fit the diverse range of sensors but the company has developed comprehensive monitoring equipment and offshore packages which enables it to collect and analyse huge amounts of critical data.

The explosion-proof Exd system installed on the Chirag-1 uses Sundstrand QA700 high sensitivity servo accelerometers to collect data on a real-time basis.

These are housed within explosion-proof Exd certified cast-iron enclosures and strategically fitted to the MSF in the north-east and south-west corners of the platform to measure the motions of the rig in the x, y and z axes.

All the data are logged onto a Somat eDaq dedicated data collection system, housed in a wall-mounted enclosure, located inside one of the control rooms.

Data are then downloaded by existing platform personnel via Ethernet connection to a PC and sent back on CD to AV Technology 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 3000s, at a sampling rate of 100sample/s.

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 1000sample/s for 300s, plus 10s pretrigger data.

AV Technology has developed powerful data analysis macros using nCode International's nSoft-E software.

For natural frequency monitoring, these macros use nCode's advanced frequency module to enable the semi-automatic data processing to firstly validate the data and then to automatically detect, identify and trend all modal peaks (frequency, amplitude, phase and coherence).

The processed data are assessed by Bomel, an advanced technology company with extensive experience in structural analysis and risk assessment in the offshore industry.

The natural frequency data are used to identify the natural frequencies of the platform which are a combination of fundamental north-south and east-west sway modes, plus torsional modes.

These will then be compared with natural frequency predictions made by Bomel using finite element models and used to fine-tune these models for future use.

They are also used to trend the frequencies to identify any inexplicable changes in modal frequencies, which could provide a global indication of structural damage to the jacket or a change in foundation conditions.

It should be noted that expected changes in natural frequency will also occur due to platform weight changes.

For example, the removal of a drilling module on a platform in the North Sea recently, caused the fundamental north-south sway mode to increase from 0.48 to 0.51Hz.

The seismic data will be used to quantify the x, y and z components of the platform's response to earthquakes.

This 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 the earthquake in terms of damage to the platform.

As Keith Ward of Bomel concludes: "The data supplied by AVT is of vital importance in this project.

The information allows us to monitor the life cycle of the Chirag-1 and from this we can put together detailed inspection routines, appropriate to the changing conditions.

Although there are already standard inspection procedures, including an annual underwater examination, being able to provide an accurate ongoing predictive risk assessment on the condition of this complex structure is paramount in assuring safety for the platform personnel and the environment".

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