Subsea production control systems are instrumented to constantly monitor flowline pressure and temperature at key locations to prevent plugging and introduce mitigating control strategies. New fiber optic sensors with ruggedized construction and non-electrical components are subjected to accelerated aging tests and deployed in several installations with long service life. An overview of current progress with fiber optic technology is provided for fatigue monitoring, temperature, pressure, and strain sensing. Recent developments include improved service life, novel bonding methods, pipeline sensor station improvements, sensor calibration, and long-term fatigue analysis.
The latest advancements are validated on multiple installations on a subsea tieback in the deepwater Mississippi Canyon of the Gulf of Mexico at 6,500 ft depth. A prior third-party sensor design experienced multiple non-recoverable sensor failures. A new sensor station design is employed on two Flowline Terminations to monitor pressure and temperature at a rate of 100 Hz. Subsea tiebacks are susceptible to flow assurance issues caused by plugging events such as hydrate formation. The system was originally designed to track pig location but transitioned to pressure and temperature sensing. An issue with the transition was the lack of calibration relating the fiber Bragg grating (FBG) strain levels to the actual process conditions. A novel method is presented for in situ adjustment of the sensor array calibration.
During the calibration procedure, the sensors produced unanticipated results during pipeline flow shut-in and later startup operations. The sensors helped uncover a configuration of the flowline and sensor locations that is valuable for detecting hydrate forming conditions at a key junction location. The sensors are located before and after the junction of two flowlines in the mixing zone of the pipeline streams. The novel contributions of this study are the high speed data collection, in situ fiber optic calibration, review of advancements in fiber optic sensing technology, and a field case study with multiple sensing arrays.
The developments are part of the Clear Gulf study, a collaboration between the offshore energy industry and NASA that was formed in 2010. The objective of the Clear Gulf study is to employ space technology and testing facilities for use in the up-stream industry to advance subsea sensor technology. The highly sensitive monitoring systems developed as part of this study are used to give early warnings for flow assurance issues, structural failures, or catastrophic events.