1660317324228 Worstcaseloadsimulations

Can your plant assets withstand a catastrophe?

April 20, 2022
Worst-case load simulations might not always be 100% precise but can still provide insight on how your physical assets will behave under extreme pressure.

Facilities, equipment, machineries, and piping in certain operational and environmental scenarios could be under extreme and accidental dynamic loads such as earthquakes, impact/dynamic loadings, or blast loads (blast pressures). Although these extreme and unusual scenarios might not be normal operating cases and may only happen one or a few times during the life of a facility, they still should be considered for overall safety and reliability of the plant.

In other words, the protection of different equipment and machineries require proper engineering safety and hazard assessments. This article discusses the effects of extreme dynamic loads on facilities, equipment, and machineries. The focus is on practical notes, rules of thumb, and useful guidelines.

Dynamic / impact loads

Many cases and scenarios of dynamic and impact loads exist as a result of unusual, unexpected, and accidental events. An example is earthquake loading, whereas the equipment or facility is loaded by sudden ground motions. Other scenarios could be liquid-hammer (water-hammer), dynamic forces due to the release of fluids (such as PRV/PSV), an item dropped from a crane, or a truck hitting equipment during the overhaul of the facility.

How equipment and machinery behave under major extreme loads depends on the quality of the design and the engineering/fabrication criteria met. This requires going beyond code prescriptions and accurately predicting how an equipment, machinery, facility, or assembly will respond during extreme events.

Experimental investigations are invariably limited, due to their difficulty, very high cost, and time consumption (such as the careful preparation of the test set-up). Thus, alternative and complementary approaches can evaluate the real behavior and ultimate strength of any equipment or facility under dynamic/impact loads. This requires the use of realistic models/methods that, although may not be entirely accurate, provide some realistic estimates of load carrying capacity.

There has been a trend toward experimental testing of small-scale models when dealing with equipment or structures under large dynamic loads. However, too often scaling does not work due to the high nonlinearities involved, time-dependency, and the complexity of phenomena. Many of sub-elements and phenomena involved in dynamic loading events (such as fracture mechanics) do not satisfy the requirements of the elementary scaling laws too.

For certain cases such scaling might be possible. For instance, low-velocity perforation of ductile metal plating struck by relatively heavy objects does appear to satisfy the elementary scaling laws within experimental scatter in a certain range. However, in general, geometrically similar scaling laws cannot be applied for extreme/dynamic loads.

Simple assessment methods, general guidelines, and rules of thumb have been very popular for initial estimations, preliminary checks, and rapid estimates of the overall behavior of equipment and facilities under dynamic/impact loads. Sometimes, a simple energy balance using the rigid-plastic method provides valuable guidance on the overall response of the equipment, structure, or facility and is helpful in selecting the overall dimensions, particularly when acknowledging any uncertainty in the details of dynamic or blast loading.

For instance, a large blast case emanating from an explosion might be approximated as a uniformly distributed blast loading. This simplified method is effective when pressure pulses having a peak value much larger than the corresponding static collapse pressure, particularly when considering the uncertainties in the characteristics of many explosion events.

Failures under dynamic / impact loads

Three types of failures are worth mentioning. The first type of failure is defined as permanent ductile deformations of an equipment or structure without any major incident such as leakage, fracture, or rupture. For instance, the thin shell of a tank or equipment might be deformed or wrinkled due to an earthquake. This is a failure and should be repaired even if there is no leakage/rupture.

A second type of failure is when rupture, tears, or disconnection happens. In other words, this is when the maximum strain reaches the rupture/tear value for the material, usually in localized area of equipment or facility. An example is permanent leakage of a flange joint due to large exerted loads.

The third type of failure is transverse shear failure. This more readily occurs at boundaries and other hard points (or supporting points) of dynamically loaded equipment/structures. For example, anchor bolt(s) of an equipment might be cut due to dynamic loadings.

Dynamic properties of materials. At large strains under dynamic loadings, the dynamic properties of materials are often characterized as material behaviors. The flow stress typically increases with strain (i.e., strain hardening) and strain-rates. However, the increase in the flow stress due to strain-rate is usually reduced significantly with the increase in the strain for commonly used steels and metals. In other words, typically, there is a reduction in the material strain-rate sensitivity with the increase in the strain.

Each steel type and material may behave differently under dynamic loads. For example, some high-strength steels may absorb more energy during impact/dynamic loads, over mild steels when loaded quasi-statically. But, in the dynamic range, the material strain-rate-sensitive strengthening of those high-strength steels might be less significant than that for mild steel. The failure strains are smaller as well, so that overall, any higher energy absorption observed for quasi-static loads might be lost in the dynamic range.

Dynamic energies vs. permanent deformations. External dynamic energies whether from blast, impact, or dynamic forces would be absorbed through large plastic/inelastic deformations of the parts, members, and components. Typically, large impact loadings produce plastic deformations/strains, highly nonlinear responses and severe conditions such as large displacements, ruptures, fractures, failures, leaks, and collapses. It is also desirable to limit the damages and associated repair costs.

The importance of energy-absorbing tools, parts, and systems should be insisted. This is applicable to earthquake scenarios, impact cases (accidental hits), and many others. In general, the introduced energy (energy of impact, dynamic load, or blast) should be safely absorbed, with minimum permanent deformations, preferably in low-cost and easy-to-replace parts. For instance, for scenarios involving a possible strike of heavy objects to facilities, the first measure might be proper barriers and energy absorbing devices. The second action is to properly simulate and evaluate different hit and impact scenarios to recognize vulnerabilities and consequences.

Complexities, challenges, and difficulties

The complexity of the response of practical equipment, assemblies, piping, and structures subjected to large dynamic loads involves many phenomena, some of which are at the forefront of understanding in mechanics.

Examples of such complexities and complications could be elastic and plastic wave propagation; material properties might vary with the dynamic loading (for instance, material strain rate sensitivity), elastic and plastic (inelastic) buckling, fracture, rupture, and many more. The use of different kinds of jointing techniques (welding or bolting, for example) make any simulation or assessment more challenging. Another difficulty is the time dependency of dynamic loadings and responses.

Details of large dynamic/impact loadings are often uncertain. For example, the actual impulse acting on equipment, an assembly, or structure from an exp    losion event (due to malfunction, overpressure, or other reasons) might be severely affected by interferences.

Vulnerability of piping. Piping systems, particularly small sizes or low thickness, have been vulnerable to different dynamic, impact, and blast loads. Assessments, simulations, and estimates are required for the resistance of a pressurized piping to all possible dynamic/impact loadings to ensure their safety and reliability. In many cases, proper protections and barriers should be in place to reduce risks to acceptable levels. Supports and their structures play a major role in the dynamic responses of piping systems.

Global buckling, considerable yielding & localized damages. Typically, if failure is in the form of a global buckling (i.e., localized yielding) or non-symmetric buckling/collapse modes, the absorbed energy is relatively low, and for a certain amount of kinetic energy, extensive damages would be expected. An example is the impact of a sharp metallic object on a tank or thin shell equipment, which could result in a localized tear of the shell (leakage/spillage) even with relatively low kinematic energy. Another example is a global buckling of a column/tower/silo, which can lead to catastrophic failure.

On the other hand, vast yielding involving considerable volumes of materials or plastic progressive collapses (involving massive plastic deformations) absorb a considerable amount of energy. Good examples are energy-absorbing barriers that can absorb considerable amount of kinetic energy in accidental hits. 

Stiffeners. Stiffeners have been widely used for equipment, machineries, facilities, piping supports, and others. They have been critical to stiffening the equipment and facilities to deal with normal operating loads as well as extreme/dynamic loads. To improve the stiffness and strength of equipment/structures, adding stiffeners or increasing the stiffener dimensions is usually more efficient than simply increasing the thickness of equipment/structure.

Stiffeners can be in different forms and types. They might be from plates; they could be beams/frames, or stiffening may be introduced in other forms such as corrugating the body or shell of the equipment. Stiffeners have been very effective in dealing with lateral loads, out-of-plane bending, or loads exerted from different directions.

Many equipment or structures are usually stiffened in different directions and patterns. For instance, many plate fabricated equipment/structures are stiffened both vertically and horizontally. If stiffeners and support members are relatively weak, they deflect together with the main equipment/structure. In fact, this is the case of many practical equipment/structure whereas stiffeners are not far stronger than parts and components they stiffening.

Final notes

It is usually important to simulate how machineries, equipment, assemblies, piping, structures, or frames behave under extreme loads such as those in case of a major malfunctions, explosions, blasts, transient cases, overloading, accidents, earthquake, major storms, and similar. Many such cases involve dynamic/impact loads.

There have been many challenges and difficulties to assess the effects of these dynamic/impact loading cases on facilities. A challenge is uncertainties related to mechanical behaviors under extreme loads and how the equipment/assembly/structure might be deformed or damaged. Another key challenge is the usual lack of accurate information on the dynamic material properties and on the characteristics of the external dynamic/impact/blast loadings for many practical problems.

Simple and realistic simulation models have been popular and, although they may not be entirely accurate, they still can provide some realistic and useful insights.

This story originally appeared in the April 2022 issue of Plant Services. Subscribe to Plant Services here.

About the Author: Amin Almasi

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