In service, flexible pipes are exposed to harsh environments and challenging conditions that subject the structure to demanding physical rigour that is largely unknown or unnoticed throughout the service life of the asset. Resistance to extremes and large fluctuations in strain, temperature and pressure are built into the pipe design, however the pipes are often breached due to accidental damage to the outer shield caused by external objects. Early knowledge of this damage is vital to limit the degradation of the metallic wires due to contact with seawater, and with the early knowledge of breach prompt remedial actions can be triggered to prevent further deterioration of the pipe that may lead to expensive repair or replacement. Using fibre optics as a distributed temperature sensor, the entire length of the asset can be continuously monitored, and the incidence of a breach in the outer shield of the pipe can be efficiently captured.
This paper describes a fibre optic-based breach detection system developed to monitor offshore flexible pipes. A brief outline of the development programme is presented. The work involved selection of fibre/monitoring equipment, physical encasement of the ‘harsh environment’ fibre optics in the pipe structure, subsequent termination in the pipe end-fitting, and the procedures for interrogation and interpretation of the Raman-based optical signals. A prototype system has been implemented on mid-scale pipe samples and the transient and steady state thermal behaviour of the pipe has been investigated under a simulated and wide range of controlled breach test conditions. A summary of the test results is presented.
Numerical models of the pipe geometry have been developed to predict the transient thermal behaviour of a shield breach. Good correlation of experimental results with the predictions shows the sound adaptation of fibre optic-based sensing technology in offshore flexible pipes.