Fiber-Optic Gas Monitoring for Flexible Risers
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 19901, "Fiberoptic Gas Monitoring of Flexible Risers," by Nick Weppenaar, NKT Flexibles, and Anatoliy Kosterev, Lei Dong, David Tomazy, and Frank Tittel, Rice University, originally prepared for the 2009 Offshore Technology Conference, Houston, 4-7 May. The paper has not been peer reviewed.
The full-length paper presents a new advance within the field of optical gas measurement, with applications for monitoring of gases inside the annulus of flexible risers used in the offshore industry. This advance is based on the novel quartz-enhanced photoacoustic-spectroscopy (QEPAS) technology. Specifically, the first demonstation of such a gas-sensor system using a spectraphone (a module for detecting laser-induced sound) consisting of a quartz-tuning-fork (QTF)/microresonator assembly and two commercial single-frequency diode lasers for detecting hydrogen sulfide (H2S), carbon dioxide (CO2), and methane (CH4).

Introduction
The need for monitoring of flexible risers is becoming ever more apparent as oil exploration moves to greater depths and wells can be hotter and more sour than seen so far. In this situation where the limits of pipe design need to be expanded reliably, continuous monitoring of pipe health becomes a priority.
Internal and external monitoring of pipe strain and temperature are becoming established technologies, with all the operational safety and lifetime extension these technologies enable. However, the field of annulus-gas monitoring so far has been lagging, in that no accurate sensing technology has existed that is continuous, accurate, and free of electrical leads. Monitoring of annulus gas levels can be vital to a reliable estimate of pipe health because the presence of gases such as H2S and CO2 can influence corrosion-fatigue levels dramatically. In fact, knowledge of strain and temperature conditions in a pipe can be insufficient for a pipe operating under sour conditions when it comes to accurate calculation of remaining pipe lifetime.

Existing Methods
Because of the demands for explosion proofing, transmission lengths, and operational robustness, fiber-optical sensing methods are an excellent way of monitoring flexible risers. However, while the methods of fiber Bragg gratings and Brillouin scattering are relatively easy to adapt for use in the annulus environment, no optical gas-sensing method has been feasible for use in flexible pipes because of a number of problems, such as insufficient sensitivity and large optics. Therefore, gas monitoring has been carried out in two ways. One is to install a traditional electrically based gas sensor such as a gas chromatograph close to the pipe and enclose it in explosive protection, which is an expensive and bulky option. The other way has been to sample the annulus gas manually in bags and have these sent to a laboratory for analysis. This approach has the downside that it requires manual work near the pipe, sampling is infrequent, the accuracy is low, and the results can be several days in returning. Therefore, a real-time, autonomous optical gas-sensing method that can replace existing inspection methods would be highly desirable.