Development of an Optical Monitoring System for Flexible Risers
Authors: Andersen Martin , Berg Arne , Saevik Svein 
Published: October 2008

The feasibility of using fibre Bragg grating (FBG) sensing technology for strain monitoring in flexible risers has been demonstrated. The FBG sensing technology has the potential of increasing the safety of flexible riser applications. Strain and temperature sensors have been integrated in the inner tensile armouring layer in the annulus of a full-scale test riser. The monitored strain during testing has been used to verify and update riser simulation models to improve design predictability. In future applications various types of FBG sensors integrated in operational flexible risers can be used to continuously monitor riser integrity and predict remaining lifetime.

Introduction

The offshore application of flexible risers is increasing. Larger water depths and frequent use of floating production facilities require highly reliable riser systems. Riser reliability and safety can be improved using optical monitoring systems based on fibre Bragg grating (FBG) sensors. In particular, strain, temperature and other parameters of importance to riser integrity can be measured to improve the understanding of riser behaviour and performance and to optimise the design. Furthermore, it is possible to verify the predictions from riser simulation programs.

The prediction of riser service life is primarily based on expected fatigue and corrosion of the armour wires and deterioration of the polymer layers. By providing a riser with an accurate monitoring system for measuring both the annulus temperature and tensile armour wire strain variations, a more precise service life prediction can be made. Accurate measurements of a riser in service can provide a detailed load history that can be used for verification of the riser design and material utilisation factors. Furthermore, a monitoring system will increase confidence, since any unexpected riser loading can be detected, and necessary adjustments made.

Projects to develop and evaluate the use of optical fibre technology for increasing the safety of flexible risers have been supported by Demo 2000 (Norwegian Research Council), BP Amoco, Norsk Hydro, Norske Shell and Statoil. The first project was started in 1997, and the present one will be completed this year. The projects have included sensor development and testing, development of sensor integration and termination techniques, test riser manufacturing, full scale testing and evaluation of riser simulation models.

OPTOPLAN has manufactured the sensors integrated in the riser and the read-out unit to monitor the measurands. NKT FLEXIBLES has manufactured the riser with integrated sensors and mounted the end fittings. The full-scale riser has been tested at MARINTEK, and the test results have been used to verify and update their riser stress analysis program.

Concept

The basic concept for the monitoring system is to integrate FBG sensors along grooves in the tensile armouring wires in the riser annulus as can be seen in Figure 1. Packaged FBG sensors can be made with a diameter in the order of 1 mm. A small groove is made in the armour wire, and the packaged sensors are placed in the groove.