Towards a New Limit State Function for Determining the Failure Pressure of a Pipeline Containing Mechanical Damage

Abstract

Mechanical damage is generally considered to be damage that occurs to a pipeline when mechanical excavation, drilling, or boring equipment impinges on a buried pipeline creating scrapes, abrasions, gouges, punctures, and/or dents in the pipeline. Above ground pipelines may also be damaged in a similar manner from impacts by vehicles or projectiles or by willful acts of vandalism. In some cases, immediate failure will occur resulting in potentially catastrophic consequences. It is thus important to understand the conditions that would lead to such a failure in order to ensure that design parameters are selected such that immediate failures occur very rarely. In cases where the damage does not create an immediate failure or the release of gas, the concern generally is that a delayed failure will occur because the integrity of the pipeline has been significantly compromised. In such cases, the possibility is that repeated pressure fluctuations, small increases in pressure, or time-dependent creep will erode whatever margin of safety remains and a failure will ensue. Particularly unsettling are the cases in which damage of this nature is encountered through some form of inspection where the source of the damage and its time of creation are unknown. In such cases, the operator of the pipeline will generally not know what margin of safety remains. There are a number of models in existence that may be used to predict both instantaneous and delayed failures due to mechanical damage and indeed these have been used quite extensively as the basis of repair criteria and for determining safe pipeline operating conditions. Nonetheless, there are significant elements of uncertainty associated with these models and for this reason an adequate reserve factor needs to be incorporated or recourse must be made to probabilistic approaches that address such uncertainty. However, since pipelines are getting older and in some cases are being operated at higher pressures than they were previously, there is a requirement to obtain a better understanding of the significance of mechanical damage. In view of this Pipelines Research Council International (PRCI) and other research bodies, such as European Pipelines Research Group (EPRG), are taking a keen interest in this topic. To this end, PRCI have commissioned an extensive research program to investigate all key aspects of both instantaneous and delayed failures. Kiefner and Associates Incorporated (KAI) and Andrew Francis and Associated Ltd (AFAA) were commissioned to investigate the conditions that cause instantaneous failures. The purpose of this paper is to describe the approach that was adopted and the formulation of the new model that emerged from study. This model is being validated through testing which is currently ongoing.

Análise de métodos para avaliar dutos com dano mossa e sulco

“…Dutos não enterrados (aparentes ou aéreos) podem também ser danificados de maneira similar devido a impactos de veículos, projéteis ou atos de vandalismo [7].…”

“…Al. [3], Seevam et al [6], Jandu et al [7], API 579-1/ASME FFS-1 [8], Cosham e Hopkins [9], Cosham [11], Spiekhout et al [16], Jones e Hopkins [18], Bood et al [21], Roovers et al [22], Bai e Song [24], citam e/ou descrevem os métodos analíticos semi-empíricos abaixo relacionados, em ordem cronológica, que visam avaliar a adequação ao uso de duto com dano mossa e sulco. Detalhes gerais sobre cada um destes métodos encontram-se descritos, respectivamente, nos itens 3.1 ao 3.6.…”

“…(2) Onde: 1 Como o método desenvolvido por Kiefner e Andrew Francis é referido no artigo técnico de Jandu et al [7] apenas como o "Novo" modelo, essa mesma denominação foi adotada neste trabalho.…”

“…O desenvolvimento do "novo" método proposto por Kiefner e Andrew Francis [7] iniciou-se por uma revisão dos modelos existentes para avaliação de dano mecânico em dutos. Foi observado que vários aspectos destes modelos não poderiam ser harmonizados com a moderna teoria da mecânica da fratura elasto-plástica.…”

“…Foi observado que vários aspectos destes modelos não poderiam ser harmonizados com a moderna teoria da mecânica da fratura elasto-plástica. Em virtude disto, foi procurado elaborar um modelo que poderia justifi car tanto esta teoria quanto os dados disponíveis encontrados para os testes de ruptura de dutos com dano mecânico [7].…”