A Post-Installed Subsea Monitoring System for Structural and Flow Assurance Evaluation

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Abstract This paper describes major advancements for deploying sensing systems on existing subsea structures as part of the Clear Gulf study. In the past, a major shortcoming of post-installed monitoring systems has been poor coupling between the sensor and structure; this study provided methods to overcome this problem. Both subsea field testing and laboratory testing were performed to verify these advancements. The Clear Gulf study is a collaboration between the offshore energy industry and NASA with the goals of risk mitigation, safety, efficiency and environmental concerns in the offshore energy industry. The study is ongoing and has investigated new methods to improve deepwater monitoring and addresses installation of advanced sensors on already-deployed risers, flowlines, trees, and other subsea assets. These highly sensitive monitoring systems will provide operators a cost-effective method for detecting and responding to potential failures, flow assurance issues, and catastrophic events. A significant achievement was recently made in bonding methods in subsea environments. Prior to installation on Tension Leg Platforms (TLPs), subscale test articles were fabricated and tested. There was a slight decrease in bonding strength, however, the methods demonstrated adequate adhesion for structural coupling integrity on subsea components. Aging tests were conducted to ensure long-term service life potential. Full-scale implementation was accomplished on two TLPs in West Africa. Two challenges in calibration of the post installed sensors are determining the baseline or zero state of stress and sensitivity of the fiber Bragg gratings on the clamp to actual load changes on the structure. The effectiveness of the monitoring system was enhanced by calibration methods employing fundamental predictive methods to establish both baseline and sensitivity values. The transition layer between surface and deep water levels (thermocline) required temperature compensation of the strain sensors. Vastly changing temperature profiles were measured and analyzed at this moving boundary. A method to isolate temperature effects from structural data was developed and integrated into the software algorithm. A multiplexing method was incorporated with minimum cabling and connectors to make the system redundant and fault-tolerant to inadvertent cable damage. With this unique design, severing the cable in one location leads to no loss of data.

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