Floating Roof Storage Tanks Life Extension: A Novel Risk Revealed

Abstract

Abstract External floating roof crude oil storage tanks are widely used in the Oil & Gas industry due to many advantages. Floating roofs in storage tanks are well-known in minimizing product losses due to evaporation. The floating roof systems also mitigate fire risk by eliminating vapor space beneath the roof. However, the operation of such massive structures brings different challenges. This paper reveals the relation between floating roof mechanical deformation and tank floor settlement. This research studies a real case of a floating roof storage tank. The double deck floating roof experienced an excessive mechanical distortion in form of bulges and undulations at roof lower deck. A Finite Element (FE) Analysis studied the relation between tank bottom plate settlement and roof deformation while roof is in parking position. The roof bulges were dimensionally surveyed and compared with FE results. The FE Analysis results of stresses and strains were utilized to decide the integrity condition of the tank roof and to study the behavior of the existing bulges for continued operation of the tank roof. During storage tank major overhaul, bulges were reported in the lower deck of the floating roof with 50-100 mm depth. The bulges are in form of waviness and they are more severe in the roof middle pontoons. The simulation results showed that while the roof is parked over a floor with non-uniform settlement, some of the roof supporting legs will be engaged with the tank floor earlier. This will lead to a non-uniform weight distribution of the tank roof over the legs, and overloading the roof lower deck with bending stresses. The FE results were able to define the location of the induced high stresses that may lead to cracking in the welding lines of the roof lower deck plates. It was decided to conduct a magnetic particle inspection at those critical lines to ensure they are crack-free. Results showed that the stress levels in the roof will reduce dramatically after refilling the tank and floating the deformed roof, this is due to the redistribution of the roof weight uniformly over the product surface. After refilling the tank, plates with elastic bulges will recover and plastically deformed bulges will have lower stress levels. The assessment concluded that storage tank floating roof is fit for service. The relation between storage tank floor settlements and floating roof deformations has never been analyzed to this depth of engineering analysis. The storage tanks international codes state that bulge sizes in the floating roof shall be kept as minimal without providing any acceptance criteria. This research quantifies the acceptance limits of floating roof bulges based on an advanced engineering simulation.