Brittle fracture assessment of the API specification 12F shop welded flat bottom tanks

Abstract

Above ground storage tanks (AST) are essential equipment in the oil and gas industry. In general, large field erected steel storage tanks have been studied and evaluated to understand their behavior under different loadings and identify possible design flaws. However, deep research is necessary to improve the operation of shop-built storage tanks.Particularly, API 12F storage tanks are commonly used in the upstream, exploration, and production segments of oil and gas projects. The environmental conditions of these projects will depend on their locations and might be harsh and threatening for the equipment operation. This paper investigates the brittle fracture failure of the API 12F shop-welded tanks. An elastic stress analysis using finite element models was developed to determine the response of these equipment to extreme environmental conditions and the API 579 “Fitness-for-Service” rules were implemented to perform the brittle fracture assessment.A brittle fracture evaluation is conducted to inquire the propensity of a material to fail without appreciable plastic deformation. In case of the API 12F tanks, several parameters such as ambient temperature, residual stresses, and material toughness can play significant roles in the behavior of the equipment. Complete 3D models of the current API 12F shop-welded, flat bottom tanks as well as axisymmetric and solid submodels were created to address the brittle fracture evaluation and determine whether these storage tanks can operate safely under extreme conditions. Through-wall and surface crack flaws were considered in the most critical joints of the storage tanks and toughness relative ratios between the response of the steel material and the acceptance criteria of the API 579 standard were calculated to describe the behavior of the equipment.