A Study On Strength And Stability Of Large-Scale Crude Oil Storage Tanks

Unanchored floating-roof tanks which are used for the large storage amount of crude oil,are the most critical equipment in China's strategic petroleum reserves plan.Once accident of oil tanks is triggered off,that could cause devastating consequences.To keep oil tanks working well,study on strength and stability of liquid storage tanks has been an essential topic in the petroleum and chemical industry and the field of safety engineering.Stress test of oil tank with a capacity of 100,000 m~3 is implemented in an oil tank farm in China.Based on the previous experiment research,finite element numerical simulation and theoretical study are carried out for strength and stability of high-capacity oil tank.Field stress measuring & testing of open-topped oil storage tanks with a capacity of 100,000 m~3 is performed during the hydrostatic testing by resistance strain gauge technology.Large amounts of data in this testing which are valid and reliable by adopting various measures have been collected and the stress levels and distributions of tank shell and bottom have been surveyed.That provide valuable material for the safety assessment of oil tank operation.A three dimensional numerical simulation is carried out for largescale,unanchored,tapered and external floating roof tanks using ANSYS.Various items of tanks geometry that involve tank foundation with concrete ringwall,unanchored bottom with slope,annular plates and sketch plates,tapered shell thickness,wind girders,top angles,stiffening rings and supports,other attachments and appurtenances are taken into account thoroughly in the non-symmetrical full finite element model.The distribution characteristic of secondary stress at a gross structural discontinuity is reflected owing to creating of real geometrical structure of wind girders,stiffening rings and supports in FEA.The effects of changes in some factors including thickness of bottom annular plates,width from the toe of the fillet weld to the outside edge of the annular plates and width of concrete ringwall of tank foundation,on stress of shell-to-bottom fillet welds are analyzed by FEA and then reasonable design values are proposed.The measured settlement values when plotted relative to the tilt plane have an irregular shape that can be expressed as a Fourier series.In order to investigate effects of foundation settlement on structural performance of unanchored cylindrical stell storage tanks,the finite element in which harmonic settlement is loaded into lower surfaces of foundation,has been adopted to study stress of tank subjected to foundation single harmonic settlement and the utmost danger areas for oil tanks are determined.According to a large number of experimental data of foundation settlement,Chinese and foreign standards referring to tank settlement evaluation are compared.The out-of-roundness of shell caused by foundation harmonic settlement is calculate by finite elements.The relationships between parameters such as fluid level,differential settlement and roundness of shell are researched and settlement tolerances of cylindrical steel storage tanks unanchored to the foundation are proposed while single harmonic settlement happens.The fitting formulas of simulation data on radial deformation of shell caused by combination harmonic settlement when the tank has been filled to the maximum design height,are put forward.The results show that the formula calculation compared well with numerical simulation,hence it can be applied to out-of-roundness evaluation of shell induced by foundation settlement for this kinds of oil tanks with a capacity of 100,000 m~3 in the engineering practice.On account of Ramberg-Osgood material model,nonlinear stabilization is adopted in FEA to analyze the effects of foundation harmonic settlement to the axial critical buckling stress of tank shell under seismic loads.The most common earthquake damages of tanks which include elephant foot buckling of the tank wall at the base and top shell buckling,have been simulated in this paper.Elephant foot buckling critical stress calculation formulas of tank wall is given which introduces the impacts of material property,including ratio of tensile strength and yield strength,yield strength and hoop tension,on the elastic plastic stability of a large oil storage tank wall under the interaction of high hydraulic pressure and axial compression.It can provide aid for the seismic design.