ABSTRACT

Seafloor vertical deformation is important during the methane hydrate production. A monitoring system for seafloor vertical deformation based on Micro Electro Mechanical Systems (MEMS) accelerometer is proposed. The output of the accelerometer is transferred to tilt angle. Then the amount of the seafloor vertical deformation is calculated. A lab experiment and error analysis are carried out. The maximum absolute error is 8.552 mm, the minimum absolute error is 0.795 mm, and the mean absolute error is 4.16 mm. The results demonstrate the system has a good monitoring effect.

INTRODUCTION

Seafloor vertical deformation monitoring plays an important role in gas hydrate exploitation. According to a simulation from OYO Corporation (Japan), the range of subsidence would be from 10 cm to 30 cm around the production well during Methane Hydrate production test. The estimated total subsidence would be approximately 10 cm for 10-day production (Yokoyama et al., 2012; Yokoyama et al., 2013). Therefore, it is necessary to develop the in-situ system for monitoring seafloor vertical deformation.

OYO Corporation has developed two devices for the seafloor vertical deformation monitoring. The first one is a 3-component servo- accelerometer system. The method is to measure acceleration (Yokoyama et al., 2007). The acceleration waveform data are used to calculate displacements by double integrals with noise reduction. The second uses pressure gauge. The selected pressure gauge is quartz crystal resonator, measuring water pressure on the seafloor. The system evaluates subsidence by measuring the differential pressure between two points. One point is deployed around the production well (measurement point), and the other one is deployed 100m or more away from the production well (reference point). The accuracy of the sensor is 10mm (Yokoyama et al., 2012; Yokoyama et al., 2013). However, the most quartz pressure gauges undergo a drift at unpredictable rates that typically equivalent to 20 cm/year (Polster et al., 2009).

Due to the fast development of Micro Electro Mechanical Systems (MEMS) in the last two decades, the MEMS sensors have been widely used, such as target tracking (Bitsch Link et al., 2010) and unstable slope monitoring (Uchimura et al., 2015; Zhang et al., 2018; Segalini et al., 2013). Li et al. (2014) used MEMS accelerometers to monitor the ground subsidence which was caused by underground excavation in the construction of "South Hongmei Road" super high-way tunnel in Shanghai. Five sensor nodes were separately placed with an interval of 10 meters to 20 meters on the surface along with or perpendicular to the excavating direction. The system considered only the change of tilt angles at the survey points rather than the subsidence displacements. A shape accel array (SAA) was manufactured in Canada by Measurand Inc. and consists of a rope-like array of triaxial MEMS-chip accelerometers used to measure deformation or vibration. SAA measures the shape of a path in boreholes and structures, and to monitor deformation of land-based structures and slopes (Wang et al., 2017; Abdoun et al.,2008; Dasenbrock et al., 2011).

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