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Macro Sensors has designed a pressure-balanced, oil-filled and double-bellows-sealed LVDT assembly to enhance the reliability of multiple redundant LVDT-based extensometers.

The extensometers are mounted longitudinally around the circumference of the structural member to monitor structural integrity.

The LVDTs enabled measurement of the extension of the structural members to a fraction of a microstrain.

For the huge loads on oil platforms, total movement being measured is typically less than 2mm.

Because required measurement must be obtained in seawater depths down to 7,500ft, with external pressure of approximately 3800psi and where service life requirements are a minimum of 20 years, Macro Sensors designed its LVDT assembly from 316 SS and Inconel 625 for pressure and corrosion resistance.

These materials enhance the high reliability required of this instrument so that it will continue to operate for at least 10 years, even if the device is fully exposed to seawater.

The LVDTs from Macro Sensors also resolved the problem of getting a useful signal back to the surface, even in great water depths, for monitoring, recording, and controlling thrusters and ballast transfer pumps.

The extensometer housing can be outfitted with a variety of mechanical terminations to facilitate connection to the structural members.

To minimise the number of pressure-sealed connections, a 4-20ma loop-powered I/O was utilised, which the customer converted to a RS485 digital signal for transmission to the surface.

The two-wire I/O also minimised any noise going to the digital converter that was contained in a high-pressure sealed bottle located nearby.

An advantage of choosing an LVDT for this task is its repeatability, regardless of offsets due to pressure and/or temperature.

Offsets can be easily corrected in the data acquisition system on the platform above.

The LVDT extensometers performed favourably compared to results obtained from lower reliability alternative technologies, such as resistance strain gauges bonded to the same structural members.

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