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Feasibility Study for Fitness-for-Service Assessment of Reinforced Thermoplastic Pipes

By Dr. Rob Kulka, Section Manager – Asset Integrity Management at TWI Ltd. This article appears in the March/April 2021 issue of Inspectioneering Journal.
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Introduction

Globally accepted fitness-for-service (FFS) procedures for non-metallic components, such as reinforced thermoplastic pipes, do not exist. A study has identified that significant gaps exist in the current literature and has warranted an initial roadmap for the development of a fitness-for-service methodology. This methodology will be suitable for pipes in hydrocarbon as well as other applications. The current work summarizes a review of literature and experience with existing techniques, and information that can be used to support future developments to FFS procedures based on an understanding of the possible damage mechanisms.

It was found that very little information on the strength characteristics of composite components (especially in the defective condition) is openly available. Procedures for determining damage tolerance are non-existent and assessments are currently carried out in a very conservative way. There may be opportunity in adapting existing FFS procedures for metallic components to composite components and their typical damage mechanisms.

Background

The general tendency worldwide towards exploration and production from unconventional resources has been pushing the oil and gas industry to consider new material solutions for corrosion protection in such demanding environments. Non-metallic pipes (a term used in the oil and gas industry to refer to reinforced polymer pipes) have gained a great deal of attention and acceptance for being a reliable alternative to traditional carbon steel pipes. While initially deployed mostly in non-critical applications such as fire water systems and utilities, their increasing adoption in water injection, oil and gas flowlines, and risers are giving the industry a good level of confidence for broader deployments. The availability of well-established standards and procedures for the integrity management (including but not limited to FFS assessment) of non-metallics will enable a broader deployment of non-metallic piping systems. 

FFS procedures are based upon engineering approaches to evaluate the safe functionality of engineering components and structures under defined loading and environmental conditions. 

Conventionally, for metallic components, the main objective of an FFS assessment is to assess operational safety in the presence of defects. Common occurrences of these defects include areas of corrosion damage, cracks, lack of fusion, fatigue damage, misalignment, or other shape defects (perhaps as a result of mechanical damage), as well as to requalify the component for lifetime extension or for utilization in other services. 

Well validated FFS procedures exist for metallic components and structures in various standards and codes such as BS 7910, API 579-1/ASME FFS-1, DNVGL-RP-F108, ASME B31G, and API 1104. When a component experiences damage that is outside of the design limits, there is a need to repair or replace it. However, in some instances, it may be preferable (i.e., for accessibility or cost) to conduct an FFS assessment to evaluate the remaining strength of the component and determine whether repair and/or pressure de-rating is required. It should be noted that the FFS acceptance criteria is typically different than the original design criteria. FFS studies can also be very beneficial when considering the possibility of plant life extension. An evaluation of the operating history, materials degradation, and in-service defect initiation may serve to permit operation of the plant beyond design life, perhaps with modified operating conditions. An FFS study is based on knowledge of the mechanical behaviour of a flawed component under loading and precise understanding of the flaw type, size, and orientation. It also may be supported by numerical stress analysis and materials testing. The most important elements of a FFS assessment are the geometries of the component, loading history including pressure and temperature, material’s properties including dependencies on the environment, and identification of the damage mechanism. Understanding of these elements permits an accurate assessment of suitability for continued service, the need for de-rating, or the need for repair or replacement.

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