The Industry Challenge
The CNG Optimum System, a compressed natural gas (CNG) containment carrier developed by Global Energy Ventures Ltd. (GEV), required approval from the American Bureau of Shipping (ABS) prior to commercial construction. This program was part of a larger project, which included the design of the ship on which the CNG Optimum System would be installed.
As part of the CNG Optimum System approval process, testing was required to validate the fatigue resistance and burst capacity, as per the table below.
Two additional tests were also performed to evaluate structural aspects of the container:
- a bend test and
- a friction test.
The bend and friction tests were used to verify that the pipe bundle would act as a rigid unit and stiffen the ship’s hull.
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Test specimens were fabricated from 20-inch outer diameter (OD), API 5L X80 PSL2 line pipe that was 15 m to 18 m in length. C-FER coordinated girth weld procedures with a local fabricator, RMS Welding Systems. We utilized our experience working to exacting standards with fabrication and welding companies on various challenging fabrication projects to coordinate girth weld procedures with RMS Welding Systems.
Artificial notches were saw cut into the outer pipe wall to evaluate the fatigue resistance and burst capacity of the containers in the presence of flaws either missed during non-destruction inspection (NDI) or generated during operation. We performed a fracture analysis to size the notches and the notches were cut into the pipe wall using custom built equipment.
C-FER’s Cyclic Test System (CTS), a self-contained system that utilizes a dedicated hydraulic power unit, consists of a hydraulic accumulator and an oil/water separator, with 5000 psi pressure capacities, to cyclically apply pressure. All specimens were subjected to internal pressure loading without end restraints.
The working fluid of the CTS is hydraulic oil; however, water was used in the specimen to reduce environmental impact in the event of a rupture. Barriers, a floor liner, and a top canopy were installed to ensure safety and provide containment during testing.
The test program presented significant challenges due to the size of the specimens and their contained energy during burst testing. Considerable work went into designing the safety containment system as the predicted energy released at rupture was approximately 2 MJ.
Achieving the Joule-Thomson (J-T) temperature, necessary to evaluate burst capacity in the event of a through-wall leak, presented another challenge. C-FER used a computational fluid dynamic (CFD) analysis to determine the J-T temperature, of -80°C, for this program.
The artificial notches were locally cooled using a custom system designed to uniformly dispense liquid nitrogen onto the notch.
Following the completion of testing, GEV attained their approval letter from ABS to allow for commercial construction of the CNG Optimum System.