Numerical modeling of above-ground storage tank subject to multi-hazard event
Above-ground storage tanks are typically thin-walled, cylindrical structures that can store large amounts of oil or chemicals. While this shape is efficient in sustaining large pressure from inside, it is susceptible from outside pressure such as loadings from hurricane storm surge and/or winds. For instance in the Houston Ship Channel there are over 4000 above-ground storage tanks, nearly 35 percent of these tanks lie within or in close proximity to the FEMA's 500 year return period surge estimates. Hurricane Katrina alone in 2006 caused over 7 millions gallon of oil leakage, causing significant economic and social consequences. Risk assessment of AST subject to storm event is of great importance considering its vulnerability to natural hazards.
Before thoroughly assessing the failure risk of AST during hurricanes, understanding the possible failure mechanism and numerically predicting the tank response to multi-hazard event is inevitable. Existing literature primarily focuses on the post-event reconnaissance or one-way deterministic bucking analysis, the dynamic responses of AST subjected to surge flows is not well understood. Other knowledge gaps include the combined strong wind and wave effect and AST geometry imperfection etc, are also poorly characterized.
In this dissertation, we are going to model the storm event using a two-phase air-water flow model and investigate the fluid-structure interaction(FSI) with the above-ground storage tank. Compared to one-way coupling, FSI considers the effect of structural response to fluid and vice-versa, thus making the predication more accurate and reliable. We use the FEniCS library to implement a two-phase Navier-Stokes solver, where the level-set method was utilized to distinguish the air and water. Incremental pressure correction scheme(IPCS) is used to decouple the two-phase Navier-Stokes equation, allowing the use of iterative solver for large scale computation. To recover the distance property during level set advection, a re-distance equation is solved to re-initialize the level set. For the tank part, we model it as cylindrical shell and use Calculix as a black box structure solver. Non-linear geometry and J2 flow plasticity can be included in the structural deformation. To reduce the computational time, semi-implicit coupling of two-phase flow and tank is proposed where only the pressure correction equation and structure solver are strongly coupled. To flexibly couple the fluid and structure solvers, or even more participant solvers, we use an open source coupling code called preCICE, which provides minimal invasive communication and coupling API for fluid and structure solvers.
The numerical simulation of two-phase flow and cylindrical shell indicates that there are different time scales that need to be resolved in the FSI process. Constant time stepping is not optimal. Instead of complicated time adaptive method, we use the simple yet effective truncation error based method to adjust the time step size. To reduce the fluctuation of dt, I-control or PI-control, which is popular in control theory and explicit ODE problem, is used for the smooth change of time step. In the end, the dynamics of shell subject to dam break are analyzed and some parametric studies are carried out.