Ductile fracture and structural integrity of pipelines & risers

Abstract

The Oil and Gas (O&G) industry has recently turned its interest towards deep and ultra-deep offshore installations in order to address the global increase of energy demand. Pipelines and risers are key components for the production and transportation of oil and gas both in the offshore and onshore environment. The structural integrity and fracture control of pipes, which are major components for the exploration, production and transportation of fossil fuels have been the subject of extensive analysis in the past decade using classical fracture mechanics approaches, especially for the offshore case. The main driving force for this line of research was the fact that both the cost and the technical challenges increase disproportionally with water depth. In the deep and ultra-deep water environment the technical challenges include higher permanent and operational loads, extreme environmental conditions and the presence of corrosive agents. All the above mentioned parameters demand the use of modern fracture mechanics approaches. At the same time, the inaccessibility to structures located at depths of two to three kilometers, results in extreme repair costs. Due to the magnitude of environmental and financial consequences in the event of failure, the industry has established extremely conservative safety requirements resulting from outdated approaches for those types of structures. Furthermore, the O&G industry is reluctant to adopt novel fracture models, unlike other industries, such as the automotive and aerospace. Pipelines and risers need to be evaluated both from a structural and a financial perspective. The current thesis is proposing a new physics-inspired technology and computational capability for the prediction of fracture and structural failure of pipelines and risers operating in extreme conditions, such as deep and ultra-deep water environments subjected to extreme conditions and accidental loads. The computational tool employed in the current study is derived from a variational principle, combined with a cumulative measure of damage that is developed to control the fracture initiation. The calibration process of this methodology is achieved through a hybrid numerical experimental procedure. The material selection for this study was chosen naturally from the O&G and pipeline community. Traditionally, the O&G and pipeline industries have been using not only conventional fracture methods, but also conventional low-grades of steels for pipelines and risers, such as X60 and X70. However, deep and ultra-deep applications and the demand for increase of daily flow production pose new challenges in terms of harsh environmental conditions, increase of external diameter and higher operational loads. The industry is well aware of the fact that Advance High Strength Steels (AHSS), such as X100 and X120, can address those issues, but is not yet ready to introduce them, due to incomplete understanding of their material properties and structural behavior in the plastic and near failure range. Therefore, the current thesis offers a comprehensive study of two representative grades from both categories (X70 and X100), comparing their mechanical properties and completing a preliminary analysis quantifying the financial difference between the two for pipeline construction. Pipeline and riser installations are extremely capital intensive. They need to be evaluated both from a structural and a financial perspective, so that operating companies can quantify the integrity of their investments. The proposed thesis will develop a method using representations of oil prices and material costs along with a fracture mechanics model to improve the decision process of the material, the design, and the operating conditions of pipeline installations. This technique will not only attempt to account for the mechanical properties and structural integrity of the tubular component of interest but also to quantify the financial benefit of AHSS in the Oil and Gas community.