Integrity Management of Flexible Pipe: Chasing Failure Mechanisms
Authors: Out Johannes M.M. 
Published: April 2012

Integrity management of flexible pipe has been a tenuous undertaking ever sinceflexible pipe was introduced, notwithstanding the substantial effort that hassince been devoted to develop inspection and monitoring techniques. This ispartly because flexible pipe as a composite structure has many failureprocesses and some of them are deeply buried. It is partly because theapplication boundaries are ever advancing and new failure modes manifestthemselves.

The paper examines some of the new failure mechanisms, such as those associatedwith gas applications at high pressure, temperature and velocity (erosion, FlowInduced Pulsations, PVDF applications near the design limits.)

It further examines the current understanding of the risks that needs to bemanaged and what the means are to fulfill this task. These means start withstandards and proper design specification, continue with the requirements ofQA/ QC during manufacturing, monitoring and inspection during installation andfinally during the service life.

The safeguarding and monitoring methods and inspection techniques thatsignificantly reduce the risk are discussed and so are the gaps that remain. Inspection only reduces the risk if the results can be interpreted well and theresulting course of action is clear. Corrosion and corrosion-fatigue of thesteel reinforcing are highly uncertain processes, however. The consequences ofthis are treated.

Finally, Life Time Extension (LTE) of flexible pipe is becoming topical. Thepaper addresses what should be demonstrated during LTE, what can bedemonstrated and how any gaps should be addressed.

1. Introduction

There are those that perceive flexible pipe as an overly risky engineeringsolution and on the other extreme those that blindly accept it based on itssubstantial trackrecord. The message to the former should be that properintegrity management is possible, to the latter that it is verynecessary.

Steel is the quintessential structural/ mechanical engineering material. It ishas been known as such for centuries, along with its failure modes. Flexiblepipe has gained prominence as an engineering solution since the late-70s. It isclearly not of a simple steel construction. The facts that it consists of alarge number of layers, that it contains polymeric layers, that the steelreinforcing wires are helically wound and partly consist of complex shapes makethe failure modes many and diverse and the integrity management difficult. Thismeans that preventing failure is key and that therefore no opportunity shouldbe lost to specify the requirements and to design the product well, to exerciseexcellent QAQC during manufacture, to install the product as intended and in anundamaged condition. During service Integrity Management should first andforemost concentrate on verifying that the pipe operates within the designenvelope. Where uncertainty remains, Inspection and NDT is a means to gauge theactual condition of the pipe during service.

Fig. 1 shows the common make-up of flexible pipe. It is important to identifythe so-called annulus as the space between the internal and external polymericsheaths. This space assumes some 10% of the total volume between these sheaths. It is central to the integrity and integrity management.

Integrity Management is aimed at managing the risk of failure. This paperbriefly discusses the chief failure mechanisms, including some that have cometo the fore recently.