Real time continuous structural integrity monitoring of flexible risers with optical fiber sensors
Authors: Morikawa Sergio , Camerini Claudio Soligo , Braga Arthur M.B. , Llerena Roberth Angulo 
Published: July 2010

Even though the armor layers play a major role in the structural integrity of flexible risers, operators and service providers worldwide are still in search of reliable inspection or monitoring techniques capable of detecting the presence of cracks or other defects in the metallic wires that compose the internal and external armor layers of the pipe. In the present contribution, we report the effort undertaken by Petrobras in order to face this challenge. Novel inspection and monitoring systems, based on acoustic emission, measurement of residual magnetic field, optical sensing, or employing visual monitoring cameras, are being developed and tested. Among these techniques, the one relying on optical fiber sensors has provided the best results so far. The use of fiber Bragg grating sensors for directly measuring strains in each wire of the external traction layer offers an accurate and reliable real-time, continuous monitoring technique, providing the operator with valuable information and early warning if any deviation from the expected wire's structural behavior is detected. The paper describes the technique and presents results from full scale laboratory tests as well as from the first field trial conducted in an offshore platform operated by Petrobras in Campos Basin.

Introduction

With most of its productive assets located in deep and ultra deep waters off the Brazilian coast, Petrobras has become the largest flexible pipeline operator in the world. The first flexible pipes in Brazil were installed in 1977. Today, Petrobras operates a 7,800 km network of flexible pipes including more than 1,200 risers. Some of these risers are now reaching their design life, and the ability of timely detecting appearance and growth of structural damage has become a significant concern in order to assure their integrity and extend service life beyond the original design.

A number of distinct failure modes have been identified in unbonded flexible pipes [1-6]. Here, we are concerned with the rupture of the wires in the traction armor layers of the pipe. Failure in the armor is often caused by mechanical deterioration of one or more wires due to corrosion, erosion from sliding contact between wires, inter-layer friction, presence of fatigue cracks generated by overloads, or excessive deformations such as bending, torsion or axial compression [3]. Localized deterioration increases stress concentration in the wire, which is then followed by fatigue leading to a rupture. The flexible pipe may be still operational after one or a few wires in the armors are ruptured [4,7]. However, with less load resistance and increased wear in the remaining wires, the armor may experience successive wire ruptures leading to a loss in pressure containment or even catastrophic failure. This type of progressive damage is more likely to occur in the riser's top section, from 30 m water depth up to the platform, often near the end fitting and at the I-tube [4,6,7]. Detecting initial rupture of the wires through continuous monitoring will therefore provide an early warning and allow the operator to mitigate the progression of damage in the riser's armor layers.