Abstract
Fibre optic cables can be used as sensors to monitor temperature changes through the analysis of back scattered light. This can be linked to changes in the ambient conditions surrounding the fibre optic cable. Active distributed temperature sensing relies on an external heat source relative to the fibre optic cable to measure the properties of, and changes in, the surrounding medium. An experiment was conducted using distributed temperature sensing technology to monitor changes in sediment overburden for the purpose of determining whether scour could be measured above buried power cables containing fibre optic cables. Fibre optic cables were buried in a channel containing saturated sand and water with an external heat source. The depth of overburden sediment above the fibre optic cables was reduced, whilst the associated temperature response along the fibre optic cable was monitored. The data was matched to a finite element model so that the heat transfer taking place could be simulated and then the thermal conductivity of the soil modified to observe the potential changes in heat detected by the fibre optic cables. This paper explains the characteristics of heat transfer from an active heat source to the surrounding soil medium providing a means to translate the temperature measurement to the associated overburden thickness and to model the same response in different materials.
Highlights
Sediment scour is an important concern in river and marine environments especially in relation to buried assets
Because a small error in the installation of fibre optic cables can cause a large variation in the temperature detected by OFS, the model calibration was performed by varying the distance of optical fibre sensor to heating tape so that the simulated results were similar to the experiment data
The results show that the temperature change is greatly affected by the thermal conductivity of soil
Summary
Sediment scour is an important concern in river and marine environments especially in relation to buried assets. To determine whether seabed scour can be detected in real-time above a buried power cable containing fibre optic sensors a controlled experiment using Brillouin optical time domain analysis (BOTDA) is presented in this paper It follows on from the experimental work by Ouyang et al (2017) by introducing a numerical code specific to heat transfer applications. This paper is set out in three main parts each containing separate results and discussions: Part 1 Experiment details and evaluation: describes the setup of the experimental model and the fibre optic sensors, verification of baseline temperature under steady state conditions, method to remove sediment overburden and corresponding OFS measurement and analysis; Part 2 Numerical model and validation: introduces the numerical model and how it is applied to simulate the physical model; Part 3 Parametric study: discusses the effect on temperature by changing soil thermal conductivity
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