This recent event is an excellent example of why vessels have Minimum Design Metal Temperatures (MDMT) and what can happen when they are exceeded. It also is a nice example of how API RP 579-1 / ASME FFS-1, Fitness-For-Service standard can be used to ensure a vessel is fit for continued service.
On January 22, 2018, workers discovered a release of liquefied natural gas (LNG), which is natural gas having methane (CH4) as its major constituent and that is converted to a liquid (or “cryogenic”) state of -260°F from Tank S-103. Tank S-103 experienced a release of LNG into the annulus (i.e., the space between the inner and outer tank walls), which eventually caused cracks in the outer tank wall and the pooling of LNG in the secondary containment area surrounding the tank (Incident).
Exposure of LNG to the carbon steel outer tank resulted in the cooling of the outer tank wall to a temperature far below its design temperature of -25°F and the formation of four separately-identifiable cracks. These cracks propagated to a length of approximately one to six feet in length in a short amount of time.
After launching an investigation into the release, PHMSA learned that Tank S-101 had also experienced releases of LNG from the inner tank into the annular space. This raises the possibility that the conditions which resulted in the Incident may be present in multiple tanks.
The facility reported no injuries or fatalities as a result of the Incident, and there were no reported fires or explosions. The cause of the Incident has not yet been determined. PHMSA has initiated an investigation of the Incident. The preliminary findings of the agency’s ongoing investigation are as follows: