Fatigue evaluation of API 12D tanks with a semicircular-topped rectangular cleanout and stepped shell design

Abstract

Ten tank designs with nominal capacities ranging from 500 bbl (79.5 m3) to 10,000 bbl (1590 m3) are provided by the American Petroleum Institute Specification 12D (API 12D) to accommodate the needed temporary storage of extracted product in the upstream segment of the oil and gas industry. These tanks are designed to withstand the operational conditions of the storage process and the operating pressure ranges, and are field-welded and mass-produced to accommodate the storage needs of the upstream segment of the facility. The fatigue evaluation, based on ASME BPVC Section VIII, Division 2 is performed in this study for these ten tank designs with a new cleanout detail that is an extended-neck rectangular cleanout with a semicircular top and is surrounded by a reinforcement pad. The stresses found in the junctions connecting the cleanout neck to the bottom plate, the shell to the bottom plate, and the roof to the top angle that is attached to the shell are investigated under the influence of internal pressure, vacuum pressure, and hydrostatic pressure. The tanks were idealized as axisymmetric models using a finite element analysis approach to study the stresses in the roof-to-shell and shell-to-bottom junctions, while a three-dimensional, solid-element, sub-model driven by a three-dimensional shell model of the entire tank was used to idealize the cleanout junction. The stress classification in the junctions of interest were obtained to evaluate the fatigue life by simulating the modeled tanks and loading pattern using elastic stress analyses. The number of permissible design cycles for each of the ten API 12D tanks is presented and discussed in this work.