Vibration Monitoring Technique to Detect Failure in Armour Wires of Flexible Risers
Authors: Braga Arthur M.B. , Morikawa Sergio , Camerini Claudio Soligo , Camerini Murilo Giron , Ribeiro Alexandre Santanna , Simoes Tiago Baltar 
Published: October 2011

In the present contribution, we address the use of continuous vibration measurements to detect rupture of wires in the tensile armor layers of flexible risers. The interest in structural health monitoring of these components of deep and ultradeep water petroleum production systems has been growing steadily in the last few years. For pipes that are already reaching the limit of their service life, monitoring systems may provide early warnings of failure and also assist the operator in optimizing maintenance stops or scheduling top end retermination operations. There are already a number of different techniques, such as the use of video cameras, fiber optic sensors, acoustic emission, or magnetic stress measurement, which are being tested or deployed in the field in order to monitor progressive failure of the armour wires. These techniques either provide direct measurements in the wires or indirect signals that allow the identification of rupture of single or multiples wires. Monitoring vibration falls in the latter category. As a wire in the external or internal layer of the tensile armour breaks, a very distinguishable vibration signal, both in frequency and amplitude, is picked up by accelerometers placed on the risers' outer sheath. In the paper we report results of four full scale tests that have demonstrated the feasibility of deploying the system in a first field trial. Results have shown that the probability of detection significantly increases when the vibration monitoring system is employed in conjunction with measurements of longitudinal and torsional strains in the outer sheath. We discuss the architecture and signal processing strategies that have been developed to implement the proposed vibration monitoring system in the field.

Introduction

One of the main mechanisms of failure in flexible risers is the disruption of the armour wire traction [1]. Experience has shown that this class of damage tends to occur primarily in the submerged part of the riser, close to its termination. The rupture of the wires occurs gradually and may be caused by different processes such as: corrosion by inflow of fluid in the annular space between the cover and armor, excessive deterioration associated with contact and friction between adjacent wires or between the different layers of metal armor or even the presence of high levels of stress produced by mechanical loads to which the riser is subjected during operation. The progressive deterioration can lead to localized defects that act as stress concentrators and lead the wire to break through a process of fatigue.

The flexible duct can remain operational even with some of their armor broken wires [2,3], but a string of disruptions may lead to the occurrence of leaks or even catastrophic failures. The continuous monitoring, in real time, is one of the main alternatives to prevent the progressive damage to the armor of the riser resulting in accidents with severe environmental and economic consequences [4,5]. The vibration system presented in this paper was developed and tested by technological cooperation between the Petrobras Research Center (CENPES) and laboratories at PUC-Rio. In this article, the results of laboratory tests carried out with full-scale prototypes of these systems are described in good details.