Fracture Control and Arrest in Gas Pipeline

When designing the gas pipeline system, every pipeline engineer would be keen to avoid having their new pipeline fail. There are several control mechanism that being used as stated in ASME B31.8 :

Fracture Toughness Criterion

The purpose of Fracture Control is to prevent pipeline system from leaks and ruptures caused by crack growth. A fracture toughness criterion or other method shall be specified to control fracture propagation when meet one of the following :

1. A pipeline is designed to operate either at a hoop stress over 40% through 80% of SMYS in sizes NPS 16 (DN 400) or larger.
2. A pipeline is designed to operate at a hoop stress over 72% through 80% of SMYS in sizes smaller than NPS 16 (DN 400).
3. A pipeline is designed with a minimum design temperature below −20°F (−29°C) 

When a fracture toughness criterion is used, control can be achieved by ensuring that the pipe has adequate ductility and either by specifying adequate toughness or installing crack arrestors on the pipeline to stop propagation.

Brittle Fracture Control

To ensure that the pipe has adequate ductility, fracture toughness testing shall be performed in accordance with the testing procedures of supplementary requirements SR5 or SR6 of API 5L, or other equivalent alternatives. If the operating temperature is below 50°F (10°C), an appropriate lower test temperature shall be used. The appropriate lower test temperature shall be taken to be at or below the lowest expected metal temperature during pressure testing (if with air or gas) and during service, having regard to past recorded temperature data and possible effects of lower air and ground temperatures. The average shear value of the fracture appearance of three Charpy specimens from each heat shall not be less than 60%, and the all-heat average for each order per diameter, size, and

grade shall not be less than 80%. Alternatively, when drop-weight tear testing is specified, at least 80% of the heats shall exhibit a fracture appearance shear area of 40% or more at the specified test temperature.

Ductile Fracture Arrest

To ensure that the pipeline has adequate toughness to arrest a ductile fracture, the pipe shall be tested in accordance with the procedures of supplementary requirements SR5 of API 5L. The all- heat average of the Charpy energy values shall meet or exceed the energy value calculated using one of Charpy V-Notch equation.

Mechanical Crack Arrestors

Mechanical crack arrestors consisting of sleeves, wire-rope wrap, heavywall pipe, or other suitable types have been shown to provide reliable methods of arresting ductile fracture. The mechanical crack arrestors shall be placed at intervals along the pipeline.