DataSheet3_Optimally Temperature Compensated FBG-Based Sensor Dedicated to Non-Intrusive Pipe Internal Pressure Monitoring.PDF

Pipe internal pressure measurement is of utmost importance in the oil & gas industry to monitor the extraction process, and thus to prevent hydrate-plugs formation which may occur in specific temperature and pressure conditions. Traditional solutions usually rely on pressure sensors in direct contact with the fluid to monitor, therefore requiring one hole per sensor, but they also weaken the pipe structure, which may prematurely lead to significant leaks. Attempts to develop non-intrusive pressure sensors relying, for instance, on acoustic waves detection or even strain measurements (the pipe wall acting, in some way, like the membrane of a traditional intrusive sensor), are up to now not fully satisfying, mainly due to poor temperature cross-sensitivity compensation. Thus, 1 °C temperature compensation error typically leads for Fiber Bragg Grating (FBG) transducers to pressure measurement biases greater than 26% at 100 bar (e.g.: Ø 4” NPS Sch. 160 steel pipe). Consequently, if such non-intrusive, but biased, solutions could possibly have been considered to monitor, for instance, a Nuclear Power Plant (NPP) primary coolant circuit, it was with the risk of dramatic consequences since the fluid can reach temperatures up to 320 °C. On the other hand, the solution detailed here truly achieves to cancel the temperature cross-sensitivity, and potentially any additional effect on pressure measurement, provided that each effect has the same influence on all transducers. It first relies on a better understanding of the pipe behavior under hydrostatic pressure, supported by a dedicated model developed on purpose, which demonstrates that the internal pressure and the surface temperature variations of a closed pipe can be recovered with at least two direction-sensitive transducers, the temperature dependence of the pressure measurement being simply removed by a straightforward compensation process. This paper explains the underlying principle, thanks to a formal model established with only few hypotheses, but extended to more complex field conditions. It ends with a lab-test validation involving FBG transducers attached to a pressure circuit submitted to temperature variations greater than several tens of °C, and concludes about the advantages and limitations of this novel approach for non-intrusive sensing, and its potential extensions to other measurement techniques.

在石油与天然气行业中，管道内部压力的测量至关重要，这不仅有助于监控提取过程，还能有效预防在特定温度和压力条件下可能出现的hydrate-plugs（水合物堵塞）现象。传统的解决方案往往依赖于与流体直接接触的压力传感器进行监控，因此每个传感器都需要一个孔，这不仅削弱了管道结构，还可能导致管道提前出现重大泄漏。尽管尝试开发基于声波检测或甚至应变测量（管道壁在某种程度上类似于传统侵入式传感器的膜）的非侵入式压力传感器，但迄今为止这些尝试并未完全令人满意，主要原因是温度交叉敏感性补偿不足。因此，1°C的温度补偿误差通常会导致光纤布拉格光栅（Fiber Bragg Grating，简称FBG）传感器在100 bar压力下产生超过26%的压力测量偏差（例如：Ø 4” NPS Sch. 160钢管道）。因此，如果这种非侵入式但存在偏差的解决方案可能被考虑用于监控核电站（NPP）的初级冷却回路，那么由于流体温度可能高达320°C，其后果将是灾难性的。另一方面，本文详细介绍的解决方案真正实现了消除温度交叉敏感性，以及可能对压力测量产生的影响，前提是每种效应对所有传感器的影响相同。该方案首先依赖于对管道在静水压力下行为的更深入理解，这得益于专门为该目的开发的模型，该模型表明，通过至少两个方向敏感的传感器可以恢复封闭管道的内部压力和表面温度变化，而压力测量的温度依赖性则通过简单的补偿过程消除。本文通过建立仅包含少数假设的正式模型来解释其背后的原理，并将该模型扩展到更复杂的现场条件。最后，通过在经历数十摄氏度温度变化的压力回路中附着FBG传感器的实验室测试进行验证，并就这种新颖的非侵入式传感方法的优势和局限性进行总结，同时探讨了其向其他测量技术的潜在扩展。