A Study Into the Plasticity Response of Compression Tube Fittings During Swaging and Influencing Factors

Abstract Small bore tubing (SBT) assemblies are process containing lines that are critical for the safe and reliable operation of sensing and control instrumentation in the oil and gas industry. They are used extensively in industry including onshore and offshore plants in the oil and gas for the purpose of plant monitoring, control and safeguarding. However, SBT assemblies are prone to various failure modes. The failure of SBT assemblies would typically lead to a loss of containment (LOC) event, which in the absence of further controls, can ultimately lead to fire, explosions and/or fatality in oil and gas. One of the most common failure modes of SBT assemblies is Vibration Induced Fatigue (VIF) which is not well understood in industry. While there exists well-established guidance on the fatigue life of welded small bore piping components in the EI Guidelines, the fatigue properties at the swaged zone of compression SBT fittings are limited. The key objective of this study was to enhance the body of knowledge required in the development of a well-informed guideline. More specifically, the primary objectives of this study are: - To evaluate the influence of the swaging effect on the fatigue properties of the tubing material; - To evaluate the influence of common installation errors (e.g: overtightening) on the fatigue properties of the tubing material. The study was limited to tube material made of stainless steel 316 and for tube sizes in the range of 1/4″ to 1/2″, which are considered to be most commonly prone to VIF. The study revealed that the swaging mechanism results in a significant reduction of fatigue life by up to 43%. In addition, nut overtightening was found to have a detrimental effect on the fatigue properties of the tubing.