Natural Disatsters/Pipes How to Make your Process Pipework Earthquake–Proof
Engineering design is a very stringent process and it involves various considerations including natural calamities, especially earthquakes. The use of this engineering design even in process plant piping is no exception. This article that gives insight into analyzing the seismic effects on the design of pipes. Using an assortment of analysis techniques, the results are presented herein.
Rocks, being elastic in nature, store elastic strain energy during the gigantic tectonic plate action that occurs in the earth. However, the material content in rocks is small. Hence, when the rocks along a weak region on the surface of the earth reach their strength, a sudden movement takes place causing release of shock waves. These waves are mainly classified as ‘Body waves’ and ‘Surface waves’. Body waves comprise primary waves (P-waves) and secondary waves (S-waves) whereas surface waves comprise ‘Love waves’ and ‘Rayleigh waves’.
P-waves produce tension or compression strain along the direction of energy transmission whereas S-waves oscillate at right angle to it (refer Figure 1). Love waves are similar to S-waves but without a vertical component. A combined effect of the S-wave and Love wave cause maximum damage to a structure due to their racking motion in both the vertical and horizontal directions.
Estimating the Effects of Earthquakes on Pipes
In a typical process plant, a pipe either rests on a steel structure or on a concrete support. The pipe can be fixed to the structure either through supports like U-bolts or can slide freely. However, for this seismic analysis, an assumption that no relative movement occurs between the pipe and the support has been made.
Consider a pipe attached to a support. Under the action of seismic event on the system (pipe + structure), the pipe experiences an excitation force due to seismic acceleration which is based on the properties of the support structure. Each structure has its own response to seismic excitation based on material, mass, geometry, stiffness etc. Accordingly, the response spectrum needs to be modified before applying to any system.
As per Newton’s first law, the inertia of the pipe resists its movement due to seismic acceleration and results into a force, called ‘inertia force’, which acts against the direction of earth motion. This force changes magnitude as well as direction based on seismic motion creating unbalanced forces and vibrations in the pipe. If frequency of pipe reaches in the close vicinity of the natural frequency of the support then it can produce higher amplitude movements and even cause resonance.