Subsidence Monitoring Sensors Deployed at Scale in Norway

A new breed of underwater sensor that is able to self-calibrate, enabling precise, long-duration subsidence monitoring at all depths, has been deployed at scale for the first time.

The 20-plus Fetch Ambient-Zero-Ambient (AZA) pressure monitoring transponders (PMTs), developed by Sonardyne, will support an ongoing long-term, large-scale seabed monitoring project at Ormen Lange–Norway’s second largest gas field–for A/S Norske Shell.

Unlike traditional pressure sensors, which suffer from drift over time, Sonardyne’s AZA technology autonomously recalibrates in situ. A unique control system periodically cycles the pressure sensor from ambient seabed pressure to near zero, enabling comparison to a highly accurate low-pressure reference sensor for calibration. The reference sensor is never subjected to ambient pressure and is accurate to changes of less than a millibar, or about 1 cm of head of water.

In-situ calibration unlocks the ability to monitor seafloor subsidence with centimetric accuracy for up to 10 years, without a loss of precision or any need for retrieval and recalibration of the sensors.

This is the latest deployment of Fetch PMTs at the Ormen Lange field, 120 km offshore, in 800- to 1,100-m water depth. Each sensor accurately collects pressure, temperature and inclination data at the seafloor, at pre-programmed intervals. The data are periodically harvested from an integrated high-speed acoustic modem contained within each Fetch PMT or Fetch AZA PMT, using a choice of Sonardyne’s acoustic systems deployed from a USV or conventional ship. The data are used to calculate any vertical displacement of the seabed at the Ormen Lange field.

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