Flexible Pipe Integrity Analysis Using Event Trees
Authors: Dos Santos Ramon Loback , Matos BC!rbara , Raposo Celso Velasco , Kumar Rajesh 
Published: July 2010

Flexible pipes present several failure mechanisms part to their complexity and part to gross human errors due to the lack of knowledge on how to operate them properly. The present article assess some of these failure mechanisms through the application of event trees in order to identify all parameters involved in a given flexible pipe failure. Additionally, available inspection methods have been revisited but are still considered not as reliable as an instrumented pig technology applied in rigid pipeline inspections campaigns for instance, leaving the analyst with few alternatives to assume the stage of deterioration of the different layers.

The findings of the present work are the result of extensive review of specific initiatives on failure analyses and laboratory results collected from DNV experience and from operators, manufacturers and contractors literature within the flexible pipe industry.

An important insight from the project was that failure of any specific layer does not necessarily require immediate replacement of the pipe and understanding the sequence of steps involved in a given failure mechanism and their signals can help operators to prevent failure and anticipate the necessary provisions in terms of inspection and maintenance plans.

Specific recommendations are listed to overcome the regular difficulties in data tracking and operational procedures observance found during the integrity management of oil & gas fields.

Finally, the article identifies technological gaps in flexible pipe deterioration models which are not shared among the industry players, as the majority of them are proprietary of flexible pipe manufactures. Subsequent degradation tests would help developing quantitative ageing models for flexible pipes.

Introduction

Unbonded flexible pipes are largely used in many oil and gas offshore production systems worldwide. They were introduced in the 70's as a solution to extreme dynamic conditions. They additionally offer relatively good insulating and chemical compatibility properties when compared with carbon steel pipes.

The technology qualification of flexible pipes was not capable of anticipating the variety of failure mechanisms which followed after the first years in use and it is still considered unsuitable to deliver a high reliable product. Possibly, there are two main reasons for that: the first is the high patented environment surrounding their technology development, where algorithms and degradation models are closed to the market and the second is the lack of high level independent design verification. Here, high level regards the completeness of the verification, with no judgement to its quality.

On the other side, the rigid carbon steel pipe industry operates in an opened scenario, where the different failure modes are checked against opened formulas included in offshore standards and recommended practices. This is possible, among other reasons, due to the fact that a steel pipe is a simple structure and can be easily obtained for research purposes. So, many universities and research development centers can reproduce independent tests and find their own conclusions. Here, the engineering part is more concerned to the global behaviour of the pipe in terms of global buckling, free-spans and installation loads.