Non-Invasive Magnetic Inspection of Flexible Riser Structural Integrity
Authors: McCarthy John C. , Buttle David J. 
Published: July 2009

The determination of the stress in the axial load carrying wires of unbonded flexible risers has not been possible without invasive techniques. Trials of a new non-invasive technique are underway in Brazil and early results are promising. This paper describes how the actual real-time load distribution throughout the wires in the armour layers of a flexible riser can be continuously monitored by equipment that may be mounted on any riser after it has been installed.

The system not only indicates load in the wires but it also identifies if a wire has broken. Most importantly the detection system does not have to be positioned at the location of the break in the wire; the break can be many metres away. Because the axial armour wire is broken it is not carrying its normal share of the axial load, hence it has less stress than its neighbours. The system uses unique magnetic stress measurement technology that is completely non-destructive and non-intrusive. For the flexible riser application it can be mounted externally to the riser surface as the load in the wires can be sensed through the outer polymer protective layers.

The technique is useful for the management of progressive degradation. By monitoring the number and frequency of wire breakages it can be used as an input to the assessment of whether or not a riser needs to be replaced immediately, before failure occurs, or at a later date determined by the point where degradation has progressed to an unsafe condition. Additionally the system has the potential to be extended to traverse the full length of any riser (flexible or SCR) and yield information on real-time integrity. A further major benefit of in-service stress measurement is the potential to capture actual load history on an individual riser basis. Such information, used in conjunction with a fatigue analysis, can lead to a less conservative approach to the assessment of remnant life.

The test results on laboratory risers in Brazil are presented. The observed test results are all from a full-size riser in an asbuilt condition both undamaged and also with deliberately broken wires. The system is shown to be suitable for the condition monitoring of in-service flexible dynamic risers in a way never before possible, since wire load distribution is directly monitored.

The use of the magnetic, non-invasive stress imaging of flexible risers will significantly reduce the risk of failure of those risers. Using this inspection technique, operators have the maximum possible time in which to plan an intervention before catastrophic failures occur. This saves on downtime and minimises safety and environmental risk. Additionally, service life may be managed with greater confidence having knowledge of the material condition of the riser.

Introduction

Flexible risers are generally used to link sub-sea pipelines to floating offshore installations, such as FPSOs. In deep water oil and gas exploration flexible risers are used for oil and gas production, water and gas injection, and well control and monitoring. They are multi-layered structures; typically comprising an inner flexible metal carcass surrounded by polymer layers and spiral wound steel ligaments, all within an outer polymer shield layer (Figure 1). This complexity means that inspection is a challenge for current NDT technologies.