Ingenuity Summer 2019

failure modes are primarily driven from high winds and flooding. Each category of hurricane is given an effective wind speed to calculate internal and external loads and associated damage. As previously mentioned, inundation models associated with facility elevation will be reviewed to evaluate flooding. Flooding in the containment dike due to rainfall will also be evaluated to determine buoyancy forces on each storage tank. How to Evaluate Failure Modes Assessing a storage tank for overturn requires evaluating the total horizontal and uplift pressure of given wind speeds. These variables are used to calculate moment forces based on tank characteristics and configuration (diameter, height, specific gravity of product, yield strength, and material thicknesses, etc). Roof uplift is evaluated by comparing the roof weight and connection to the shell to the negative pressure as a result of hurricane force winds moving over the structure. Types of connections that need to be assessed are welded and bolted connections for fixed roofs and geodesic domes. Tank sliding is evaluated by comparing the horizontal force on the side of the tank and a given sliding coefficient provided in API 650 with relation to the weight of the storage tank. Shell compressive failure is mostly a concern for tank currently under construction without a fixed roof or shell stiffener. The compressive failure is sometimes called “fish mouth” due to the curling of the top of the shell. It is also known as buckling, creasing, or staving. The windward and leeward sides of the tank shell must be evaluated utilizing equations provided in API 650. Tank buoyancy must also be considered. Tanks can experience floating due to excessive rain in a containment dike and/ or surge in coastal areas. When evaluating surge, inundation models are consulted to determine the surge for each hurricane category. Containment dike flooding is evaluated based on dike depth and foundation height of the tank. In all cases, the weight of the tank is compared to the buoyant forces created by submerged structures of the tank. Other factors must also be evaluated before providing recommendations for mitigating action or tank upgrades. Equipment may be in place to combat the effects of the hurricane. These include hold-down straps for horizontal tanks, anchorage for vertical tanks, and fixed roof configuration (aluminum geodesic, fixed steel, external floating roofs, etc.). For most damage mechanisms, tank anchorage must be evaluated to combat the effects of hurricane effects. Where the anchorage is designed and installed correctly, most of these effects can be mitigated or completely resolved. However, it is imperative to compare the stresses experienced during these events to the strength hold-down strength of the anchors. There have been numerous cases where existing anchors have been undersized or improperly installed, yielding negative results. Mitigative Actions to Take Actions can be as easy as filling the tank to a predetermined level to protect against damage mechanisms as identified above. These liquid levels are provided to the client in feet of water or product stored to stabilize the tank during a hurricane. Other recommendations can be made that would include installation of anchors, design and installation of intermediate wind girders. The development and actionable items that result from a HPP can provide the terminal with peace of mind. Preventing the catastrophic failure of a storage tank and release of product to the environment is paramount in coastal operations of storage tank terminals.  Brian Lunsford, PE, API 510/570/653, ASNT Level III, AWS, Sr. CWI, STI-SP001 Vice President of Mechanical Integrity Shell Compressive Failure Overturn Diagram Tank Sliding Diagram The most common failure modes are overturning, roof uplift, sliding, tank shell compression, and buoyancy. Pond Ingenuity Summer 2019 11