Session: DA-03-02 Fatigue-2

Paper Number: 154500

154500 - 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. The mechanical features of SBT assemblies make them economically attractive because of their ease of installation and by design, they should provide the required integrity over the life of the plant. As such, they are widely deployed in modern processing plants, and feature in large numbers of control and monitoring instruments. 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 studies into the fatigue life of welded piping components, the fatigue properties at the swaged zone of compression tube fittings is limited. The key objectives 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:

- Evaluate the influence of the swaging effect on the fatigue properties of the tubing material via non-linear plasticity FEA.

- Evaluate the influence of common installation errors (e.g: overtightening) on the fatigue properties of the tubing material.

The study is limited to tube material made of stainless steel 316 and for size range of 1/4" to 1/2", which is considered to be the most common installations. The study revealed that the swaging mechanism results in a reduction of fatigue life by up to 43% when compared against the design curve fatigue limit extracted from API 579. In addition, nut overtightening was found to have a detrimental effect on the fatigue properties of the tubing. 

Presenting Author: Ian Ty Cheong QGC Pty Ltd

Presenting Author Biography: Ian is a mechanical engineer with more than 10 years of industry experience, with areas of specialization in metallurgical failure analysis, strain gauging, piping vibration and fitness for service of pressure equipment and piping. Ian's current role is team lead for mechanical static in Shell QGC asset, and asset technical authority in mechanical static discipline, responsible for technical assurance for projects, operations and maintenance, and leading the resolution of threats to asset integrity and reliability.

Authors:

Ian Ty Cheong QGC Pty Ltd
Raj Singh WOOD AUSTRALIA