DataSheet5_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.

管道内部压力测量在石油天然气工业中至关重要，用于监测开采流程，从而预防特定温压条件下可能形成的水合物堵塞。传统解决方案通常采用与被测流体直接接触的压力传感器，因此每个传感器需开设一个孔洞，但此举会削弱管道结构，可能过早引发严重泄漏。此前针对非侵入式压力传感器的研发尝试，例如依赖声波检测或应变测量（在某种程度上，管壁可充当传统侵入式传感器的膜片），截至目前尚未完全达到理想效果，主要原因在于温度交叉灵敏度补偿不足。以光纤布拉格光栅（Fiber Bragg Grating, FBG）换能器为例，1℃的温度补偿误差通常会导致其在100巴压力下的压力测量偏差超过26%，如公称通径4英寸、壁厚等级Sch.160的钢管。因此，即便这类非侵入式但存在测量偏差的方案曾被考虑用于监测核电站（Nuclear Power Plant, NPP）一回路冷却剂系统，也可能因流体温度可达320℃而引发灾难性后果。另一方面，本文详述的方案真正实现了温度交叉灵敏度的抵消，只要所有换能器受到的各类影响一致，即可消除压力测量中任何额外的干扰因素。该方案首先基于对静水压下管道行为的更深入理解，辅以专门开发的专用模型，证明通过至少两个方向敏感型换能器，即可恢复封闭管道的内部压力与表面温度变化，且通过简单的补偿流程即可彻底消除压力测量的温度依赖性。本文通过仅基于少量假设构建的形式化模型阐释了其核心原理，并将其拓展至更复杂的现场工况。最后，本文通过实验室测试验证了该方案：将FBG换能器附着于承受数十摄氏度以上温度变化的压力回路中，并总结了这种新型非侵入式传感方法的优势与局限，以及其向其他测量技术拓展的潜力。