Abstract

Polyamide PA-11 is a workhorse polymer within the flexible pipe industry, performing many important functions, in innersheath, outer sheath and other applications. Its use is, however, limited by degradation at elevated temperature, in the presence of water and acidic environments. Over the past decade, there have been multiple industry efforts to advance the understanding of this degradation process in order to optimise flexible pipe design and cost, whilst assuring technical and operational integrity.

This paper provides an update of the recent activities of an industry work group-the so-called Rilsan User Group - which has been co-ordinating these activities. This Group has been a non-funded, in-kind activity on the part of a large number of end users, manufacturers & other interested third parties. The main aim of the Group has been to provide more detailed guidance on the use and limitations of PA-11, and develop a common, industry accepted procedure for predicting the service life of PA-11 pressure sheathes.

Introduction

In the construction of a flexible pipe, an inner polymeric pressure sheath is the only conveyed fluid sealing layer within the pipe structure. Therefore, its integrity is crucial to safe and reliable operation. Polyamide 11 (PA-11) is the most commonly used sealing layer (1).

Almost two thirds of the current flexible pipes in service have Polyamide 11 (PA-11) pressure sheathes, and ageing of this PA-11 has been responsible for a number of pipe failures over the years (1). The main concern regarding PA-11 is the ageing effect of water on polyamide materials at elevated temperature. Existing standards documents relating to flexible pipe - API RP17B and Specification 17J - contain only limited information in this respect PA-11 (2,3).

Towards the end of 1998, BP Exploration took the lead in organizing a series of meetings of interested operators to discuss the issues facing PA-11 materials in flexible pipes. Of the many issues discussed within this forum, the wide range of life prediction models, available at that time, was perhaps the most important operationally. As illustrated in Figure 1, for any given temperature, life estimates over several orders of magnitude were possible, depending on the prediction method and end point criteria employed.

Figure 1- Comparison of life prediction curves available to the industry in 1998.(Available in full paper)

Over the last four years, the resolution of this technical issue has became the major objective of the so-called "Rilsan User Group" which expanded to include a much larger number of operators, flexible pipe manufacturers, university research groups, and materials suppliers. In 1999, the Group also took on responsibilities as an API Working Group.

This User Group has seen unprecedented co-operation between the different sectors of the industry and between commercial competitors, all for the common good. The collective goal of all contributors continues to be the prevention of the premature failure of PA-11 pressure sheathes, through ageing and associated loss of mechanical properties, by determination and dissemination of the appropriate scientific and practical understanding.

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