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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 ∗Address all correspondence to this author. 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 1 Copyright c © 2018 by ASME 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 upstream 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. NOMENCLATURE BAR Barataria Davg Average pipeline diameter (m) Din Inside pipeline diameter (m) Dout Outside pipeline diameter (m) D Diameter DCS Distributed Control System E Young’s Modulus (GPa) EMI Electromagnetic interference FG Relation to photo-elastic constant FBG Fiber Bragg Grating FLET Flow Line End Termination FPS Floating Production System ILS In-Line Sled, (Flowline) L Pipeline length (m) MC Mississippi Canyon MPM Multi-Phase Meter Pin Internal pressure (Pa) Pex External pressure (Pa) P0 Initial pressure (Pa) SSC South Santa Cruz T Temperature (K) T0 Initial Temperature (K) T LP Tension Leg Platform αΛ Thermal expansion coefficient ( με K ) αn Thermo-optic coefficient ( με K ) β Slope of pressure correlation δλ Change in wave-length from the nominal condition λ0 ε Strain ratio of deformation per original length (m m ) γ Intercept of pressure correlation λp Bragg wavelength (nm) λ0 Baseline wavelength με μ-Strain ( 10−6 m ) μεhoop μ-Strain due to hoop oriented FBG μεM μ-Strain due to other mechanical forces μεQ μ-Strain due to thermal expansion of pipeline μεT μ-Strain due to fiber optic temperature changes FLOWLINE OVERVIEW Deep Gulf Energy developed a subsea tieback to the Blind Faith floating production system (FPS) deepwater platform in the Gulf of Mexico. It is located in the Mississippi Canyon (MC) Blocks 695 and 696. The South Santa Cruz (SSC in MC 563 at 6,550 ft water depth) and Barataria (BAR in MC 521 at 6,770 ft water depth) tiebacks to the Blind Faith FPS at 6,480 ft water depth utilize a 21.9 cm (8.625 inch) outer diameter with a 2.54 cm (1 inch) thickness on the inner pipe of a pipe-in-pipe configuration. The tieback extends 24 km (15 mi) with a flowline to transport production fluid and gas from the well heads to the platform [1] as shown in Figure 1. Production commenced in 2017. Fiber optic monitoring of subsea equipment began 21 years ago on the Troika project in the Gulf of Mexico in 1997. The Troika project used FBG sensors with a prototype signal conditioning unit. This sensing system monitored the pressure, temperature, and strain in a pipe-in-pipe 22.5 km (14 mi) subsea tieback without pipewall penetration. Since the initial deployment, other deployments have monitored steel catenary risers, drilling risers, tension leg platforms (TLPs), umbilical installations, touchdown zones, slugging mitigation, and subsea tiebacks ( [2–8]). Originally implemented in high temperature rocket motor applications, recent studies also extend this sensing technology to distributed monitoring of cryogenic liquified natural gas transfer pipelines [9]. For the Deep Gulf Energy application, pressure and temperature are key pieces of information to maintain flow assurance and event detection. Pig passage monitoring is also important for pipeline inspection and maintenance. This paper gives information about several monitoring stations installed during the initial construction and deployed for high-speed monitoring of conditions that predict potential flow assurance issues such as hydrate formation during startup and shut-in of the flowline. To obtain data from the fiber optic sensors, an umbilical was installed with fiber optic, hydraulic, and electrical lines. Only the fiber optic strands are used for the instrumentation and the sensors are nonpenetrating into the flowline and non-electrical with no signal repeaters. Fiber optic sensors are appealing for deepwater applications because of ruggedness, immunity to electromagnetic interference, multiplexing capability, and low vulnerability to water. For long tiebacks, signal degradation is anticipated and included in the design to ensure that signal strength is above a 1 decibel (dB) threshold for detection [7]. The source signal is in the range of 40-60 dB that is attenuated over splicing connections, through the fiber, and reflected for a round-trip journey. The umbilical includes fiber optic strands in a stainless steel tube. The tube void is filled with a hydrogen scavenging gel to reduce hydrogen embrittlement. The gel lessens hydrogen infusion by absorbing free hydrogen before it reacts with the silicon glass fibers. If hydrogen diffuses into the fiber it can cause attenuation 2 Copyright c © 2018 by ASME

New Flow Assurance System With High Speed Subsea Fiber Optic Monitoring of Pressure and Temperature