Study Explores Integration of Subsea Optical Distribution Systems
Authors: Carpenter Chris 
Published: August 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 32645, “All-Optical Subsea Sensing and Communications,” by Glenn Wilson, SPE, Halliburton, and Mauricio Uribe and Sigurd Moe, TechnipFMC, et al. The paper has not been peer reviewed. Copyright 2023 Offshore Technology Conference. Reproduced by permission.

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Subsea control systems use electric or optical communication channels within subsea optical distribution systems for redundant, duplex telemetry between topside facilities and subsea control systems. Downhole fiber-optic sensing (DFOS) systems use the same subsea optical distribution systems for establishing transmission paths between the same topside facilities and downhole sensing fibers. At the time of writing, subsea fiber-optic control and sensing systems have been operated on independent subsea optical distribution systems. This redundancy introduces complexity and cost into the overall subsea optical distribution system required for subsea developments. In the complete paper, the authors describe the systems that combine fiber-optic communications for subsea control systems and DFOS systems into the same subsea optical distribution system.

Need for Systems Integration
While maturity of, and demand for, DFOS technology has grown for offshore projects with dry-tree installations, operators have expressed the need for solutions to address fiber-optic technology gaps for subsea development projects. To this end, the topside DFOS interrogation of subsea wells requires optical engineering solutions to compensate for the insertion losses and back-reflections accumulated through umbilicals, multiple wet- and dry-mate optical connectors, splices, optical feedthrough systems (OFS), and downhole-sensing-fiber and optical wet-mate connectors (Fig. 1).
To obviate these problems for distributed acoustic sensing (DAS), the authors previously introduced a subsea fiber topology with two transmission fibers from the topside to a remote optical circulator deployed in the optical flying lead (OFL) at the subsea tree. This limits the sensing fiber portion of the total fiber length to the fiber located below the remote circulator and eliminates all back-reflections from the multiple subsea connectors in the subsea infrastructure above the remote circulator. The DAS pulse-repetition rate is constrained only by the fiber length below the remote circulator, thus enabling dry-tree equivalent pulse-repetition rates (i.e., acoustic bandwidth) regardless of the tieback distance. This yields significant signal-to-noise ratio improvement through stacking and selective amplification while further enabling sensing of high-frequency acoustic events occurring inside, or in the vicinity of, the wellbore. The initial application for subsea DAS was installing fiber from the tubing hanger to the production packer for vertical seismic profiling. The ability to maximize acoustic bandwidth irrespective of tieback distance, however, enables a wider range of interventionless reservoir diagnostic applications. This has increased operator demand for installing fiber across the reservoir section.