To ensure the stability of offshore drilling platforms, the structural integrity of production risers, catenary risers, tendons, platform legs, and braces must be constantly monitored. This means that the monitoring devices must be capable of functioning in seawater and delivering decipherable signals back to the surface, often from great depths. Macro Sensors, specialists in manufacturing LVDT-based linear position sensors and gauging probes, has designed a pressure-balanced, oil-filled, double bellows-sealed LVDT assembly to enhance the reliability of multiple redundant LVDT-based extensometers mounted longitudinally around the circumference of the structural member to monitor structural integrity.
The LVDTs enable 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 2 mm. Because required measurement must be obtained in seawater depths down to 7,500 ft, with external pressure of approximately 3,800 psi, 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.
Getting a Signal
Macro Sensors’ LVDTs 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 minimize the number of pressure-sealed connections, a 4 to 20 ma loop-powered I/O was used, which the customer converted to an RS485 digital signal for transmission to the surface. The two-wire I/O also minimized any noise going to the digital converter, which was contained in a high-pressure sealed bottle located nearby.
A major advantage of choosing an LVDT for this task is its extraordinary 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 favorably compared to results obtained from lower reliability alternative technologies such as resistance strain gauges bonded to the same structural members.