Inspection and monitoring techniques for un-bonded flexible risers and pipelines

Abstract

Un-bonded flexible pipelines and risers are an alternative to conventional rigid steel pipes. The use of flexible pipes has enabled development of several offshore fields that seemed unfeasible with the use of rigid pipes due to extensive seabed preparation and large dynamic motions. The lack of knowledge and integrity management tools for flexible pipes is a limiting factor and cause pipelines and risers to be replaced before their service life has been reached. This thesis aims to discover the critical failure modes for flexible pipes and explore conventional and novel techniques for performing inspection and monitoring. By understanding the failures and having access to the necessary technology the remaining lifetime and lifetime extension calculation will be more accurate than to date. Degradations and failure mechanisms will be detected at an early stage giving the operators better time to initiate mitigating and repair measures.

By contacting industry experts and performing a study of the latest literature this thesis presents the possible failure modes for flexible pipes, as well as a screening of conventional and novel inspection and monitoring techniques. Reported incidents for Norway, UK and the rest of the world are presented to reveal the greatest risks for flexible risers. An integrity management strategy is based on the reported incidents, including recommendations and purpose of inspection and monitoring techniques.

Based on incident reports and inputs from stakeholders the most frequent and critical failures to flexible pipes caused by to damage to and degradation of the internal and external polymer sheath. Breach of the outer sheath creates a hazardous environment in the annulus leading to an increased risk for several failure mechanisms. Degradation of the internal pressure sheath threatens the fluid containment integrity, and is difficult to inspect for. It is recommended to have a well-functioning annulus vent system attached to a monitoring system to control the annulus environment. Integrated fiber optics is considered as the most promising technique currently under development. This would provide continuous temperature monitoring throughout the riser and pipeline which can be used for outer sheath breach detection and temperature degradation calculations. For existing pipelines a number of solutions are under development for integrity management, such as radiography, ultrasonic testing, and magnetic stress measurement among others. Individually they are useful, but if combined they might act as powerful multipurpose tools.