The experimental validation of a numerical technique for temperature/strain profile reconstruction based on Brillouin optical-fibre time-domain analysis (BOTDA) sensors is presented. In this approach, we search directly for the Brillouin frequency shift profile along the fibre that matches the measured data. The algorithm is based on a harmonic expansion of the unknown profile, whose coefficients are determined by means of a multidimensional minimization. Experimental measurements have been carried out in order to reveal the influence of nonlocalities in Brillouin measurements, and to prove the capability of the proposed algorithm to compensate for these effects.

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Monitoring of the effects of hydrocarbon pipeline blockages such as may be caused by hydrates and waxes is receiving a higher level of consideration as the distributed sensing capability offered by the use of fibre optic technology matures. The extent of the hydrate or wax formation problem increases with pipeline length through the effects of cooling. The challenge is significantly greater when assuring flows in deep water and remote subsea locations. Commercially available strain and temperature sensing equipment such as discrete FBGs (Fibre Bragg Gratings) and fully distributed sensing techniques such as Raman DTS (distributed temperature sensor) and Brillouin OTDR (optical time domain reflectometry) typically offer sensing lengths of the order of 20–30km. Whilst this is in many instances a useful length, it is not sufficient to be able to monitor the whole of a pipeline which may be several hundreds of kilometres in length. The authors have developed and demonstrated a method for extending the reach of a Brillouin OTDR interrogating system such that sensing sections of conventional length (approximately 25km) can be successfully interrogated from distances well in excess of 100km without having to compromise on the performance. With a single instrument, more than 250km of sensing fibre can be monitored to within 1.5 metre resolution. By this means, temperature and strain profiles may be measured for the entire pipeline length which will enable active flow assurance measures to be taken including identifying the presence, nature and extent of blockages as they form. Consequently, any corrective action taken by the pipeline operators will be on an informed basis (such as the injection of an optimised quantity of inhibitor), and will incur a significantly lower level of risk than is currently possible. This paper describes the technology which has been developed to meet this requirement and provides results of simulated pipeline blockage effects which demonstrate this.