DataSheet2_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，管道内部压力的监测至关重要，这不仅有助于监控提取过程，而且能够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）的形成。传统的解决方案通常依赖于与流体直接接触的压力传感器进行监测，因此每个传感器都需要一个孔，这不仅削弱了管道结构，还可能过早导致重大泄漏。尽管尝试开发基于声波检测或甚至应变测量（管道壁在某些方面类似于传统侵入式传感器的膜）的非侵入式压力传感器，但目前这些尝试尚未完全令人满意，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（例如：Ø 4” NPS Sch. 160 钢管）的压力测量偏差超过26%。因此，如果此类非侵入式但存在偏差的解决方案被考虑用于监测，例如核电站（NPP）的初级冷却回路，那么其后果可能是灾难性的，因为流体温度可能高达320°C。另一方面，本文所述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任何其他影响，前提是每个影响对所有传感器具有相同的作用。该方法首先依赖于对管道在静水压力下行为的更深入理解，这种理解得益于专门开发的一个模型，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，压力测量的温度依赖性可以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这一基本原理，并将模型扩展到更复杂的现场条件。文章以在压力回路中安装的FBG传感器进行的实验室测试验证结束，该压力回路经受的温度变化超过数十摄氏度，并总结了这种新颖的非侵入式传感方法的优缺点及其在其他测量技术中的潜在扩展。