Stresses in the zone of process openings in the shell of a vertical cylindrical steel tank

Diverse process openings have to be provided in the bottom parts of cylindrical tanks for the storage of liquids. Pipelines have to be connected to each tank product. These are mostly 700 mm in diameter in the case of tanks with a capacity of 50 000 m³ and more. An opening in the tank shell for a pipeline of such a diameter weakens the shell significantly. Furthermore, this occurs in the region where the material is exposed to the greatest stresses — in the zone of boundary disturbances at the junction between the cylindrical shell and the bottom plate.
In the shells of tanks for the storage of crude oil, a large opening in the form of a rectangle, joined at the top to a half-ellipse, have to be formed in the same zone of boundary disturbances, directly at the shell/bottom junction. Very often such openings are more than 900 mm high, and they can be up to 1500 mm wide (depending on tank capacity). The opening is required for a so-called cleanout door, designed for the removal of deposits precipitating from crude oil and lying on the tank bottom. To be effective during the cleaning of a tank, such clean-out doors should have their lower edge flush with the tank bottom. That causes a large change of stresses in the zone of boundary disturbances both in the shell and in the tank bottom. For that reason, both of these structural components have to be significantly strengthened in the proximity of the clean-out door.
This paper is concerned with the analysis of stresses in the region of strengthening at both the aforementioned openings in the tank shell. Since the nature of stress changes — and therefore also the strengthening around these openings — is different, this paper has been divided into two parts, each of them having a separate introduction and presenting separate conclusions. Those conclusions are fit for practical applications.