Abstract
This paper explores the application of the required loads while performing fitness-for-service (FFS) assessments on an in-service storage tank. Specifically, API 579 – 1, Level 3 FFS assessments involving finite element analyses are studied. In the authors’ experience, due to the absence of specific details about the application of loads within the design codes and post construction standards such as API 579 - 1, the chosen load application is purely at the discretion of the analyst. Often times, choosing one methodology over the other can result in non-conservative assessments. For example, in order to apply wind load pressure distributions on tanks and vessels for plastic collapse and buckling behavior, API 579 – 1 directs the user to ASCE 7 – 16 for wind load calculations. However, previous versions of ASCE 7 did not specify a circumferential pressure distribution for cylindrical structures which can significantly vary around the circumference of a large diameter storage tank. In addition, a few changes in ASCE in the recent edition affect the assessment of in-service tanks for seismic loads. The authors assessed various load cases on tanks of varying diameter to thickness (D/t) ratios and diameter to height ratios. The results show that the choice of loading can have a significant impact on the buckling characteristics of the tanks in-service. This is an attempt to develop some assessment guidelines to support the proposed Tank Assessment Section in a future Edition of API 579 – 1 Standard.
