Ductile fractures pose a risk for high-energy pipelines carrying sources such as rich natural gas or carbon dioxide (CO2). These fractures have the potential to run long distances in high-energy pipelines and occur when there is insufficient pipe toughness. Where an existing operating pipeline does not have sufficient toughness to arrest a running crack, mechanical crack arrestors may be needed.
Mechanical crack arrestors include valves, tees, heavy-wall installed fixtures, and full-encirclement sleeves that aim to contain the fracture and prevent the catastrophic failure of the pipeline. One method to arrest a running unstable crack is to install a PetroSleeve.
Optimising mechanical crack arrestor design
For ductile material having sufficient toughness, experimental burst tests have revealed that the typical crack speed in the steel during ductile fracture propagation does not exceed 350 m/s (F. Van den Abeele, M, Di Biagio, J.F. Kennedylaan, 2004).
Since the acoustic velocity of gas (such as lean gas or rich methane) under usual operating conditions is in the range of 350–500 m/s, the decompression of the pipe is faster than the crack speed.
This implies that the driving force at the crack tip, over time, continues to decrease, becoming less than the initial driving force that caused the rupture. Consequently, the “running crack” loses its driving force and stops.
For brittle fracture, the crack speed fracture is equal to or higher than the acoustic velocity of the gas. This means the crack tip experiences the full initial driving force as it propagates. Consequently, due to the difference in speeds, the crack becomes a live long running brittle fracture.
The design of a crack arrestor must be such that it prevents the crack from travelling past itself. The paper ‘How to Optimize the Design of Mechanical Crack Arrestors’ (G. Wilkowski, D. Rudland and B. Rothwell, 2006) reviews steel sleeve crack arrestors with different radial spacings (with and without grouting) and axial lengths that had the same thickness and strength as the mainline pipe.
Wilkowski, Rudland and Rothwell found that there are two arrestor conditions: ‘hard’ and ‘soft’. For a hard arrest, the crack is stopped at the edge of the arrestor. For a soft arrest, the crack advances inside the arrestor but should not exit.
The PetroSleeve falls under the category of ‘hard arrestor’. To stop a running crack, the stress at the crack tip must be reduced. Increasing the effective pipe wall thickness decreases the stress at the crack tip. This can be accomplished by installing a steel sleeve. In addition, if the stress level under the steel sleeve is negative (as it is with a PetroSleeve), this would further decrease the stress at the crack tip.
Wilkowski et al. carried out modelling and testing on 152 and 304 mm (6 and 12 inch) diameter pipes pressurised with nitrogen, rich gas, and liquid CO2 that produced radically different crack-driving forces.
The tests measured the crack velocity as it entered a tight steel sleeve. The result was that the velocity reduced to zero within a very short distance in the steel sleeve. In the test, the pipe under the sleeve was in tension (the sleeve was “loose” on the pipe when it was installed). For the PetroSleeve case, the underlying steel would be in compression, further reducing the distance.
Various tests have been undertaken to determine the optimum design. The PetroSleve style, or ‘hard’ steel arrestors seemed to be superior to the steel loose sleeves, which leave a space between the pipe and steel arrestor.
PetroSleeve applicability
For a PetroSleeve installation, the following is achieved:
•The sleeve has equal to or greater pressure carrying capacity than the pipe;
• The stress in the underlying pipe is negative;
•The sleeve length is at least equal to the pipe diameter;
•The epoxy between the sleeve and pipe prevents corrosion between the pipe and sleeve; and
•The sleeve is easily installed while the pipeline is in operation.
The PetroSleeve technology was identified as having exceeded all the minimum requirements listed in the paper as being required to function as a crack arrestor.
For more information visit the Tremco Pipeline Equipment website.
This article appeared in the January 2022 issue of The Australian Pipeliner magazine. Click here to view the digital edition.