Flexible pipes have been increasingly used in the last 10 years for dynamic risers. Their multi-layer composite structure offers the flexibility of polymers and the strength of steel.
The present methods to assess their integrity is by periodic pressure testing or to remove a test pipe section and subject it to destructive evaluation. The alternative to these techniques is to devise a non-destructive inspection and monitoring system to determine the in-service condition of the flexible pipe.
Due to the complex construction of flexible pipe, the data fiom most inspection techniques is difficult to evaluate. Adequate interpretation of the data can be enhanced by advanced signal processing with automated detection and characterisation routines.
This paper reviews some of these issues and identifies possible defect characterisation techniques to reduce the ambiguity of data from flexible pipe inspection.
Flexible pipes are complex composite structures which provide the attractive features of strength and flexure capability. In the offshore oil industry this offers installation benefits and the ability to link a floating production unit to a fixed seabed unit or other floating unit.
The cost attractiveness of floating production systems for marginal and deep - water developments has catalysed a rapid increase in the use of flexible pipe for risers and the associated increase of subsea tiebacks has also led to an increase in flexible flow lines. The engineering feasibility of such developments identified the need to develop condition assessment methodologies and techniques for flexible pipe and several studies and technology development initiatives have addressed this area since 1990.
There are numerous systems for the condition monitoring of solid pipes but these systems are not directly applicable to the composite structure of flexible pipes. As flexible pipe is still a relatively new technology there are few case studies with which to build a specific knowledge for failure modes, to determine life integrity. The lack of in service condition data may generate conservative life span predictions, which can result in costly replacement at end of design life.
The present condition assessment techniques for flexibles comprise of periodic pressure testing and external visual inspection. These are the techniques used to monitor the effects caused by many diverse factors that could have an adverse affect on the pipe integrity. Such factors include:
extreme weather conditions
exceeding minimum bend radius and other detrimental strain effects by mishandling
frequency of exposure to extremes of temperature and pressure
use of incompatible fluids
Due to the effects of these diverse factors the designated design life is not a completely reliable prediction without condition monitoring.
There are three main areas where inspection methods can contribute to a condition monitoring system for flexibles:
method for monitoring the internal metallic liner
method for monitoring the interlocked hoop stress wire and tensile armour
method for monitoring the ageing of the polymer materials
This paper addresses the tensile armour, but the detection and characterisation techniques are equally applicable to other metallic layers.