Assessment of a Jetty Pipeline
The results of the surge analysis show that the most critical transient scenario is the rapid closure of the emergency valve located in the loading arm.
In this situation the pressure downstream of the closed emergency valve drops rapidly to vapor pressure level. As a result of this pressure decrease some of the downstream fluid is vaporized. On account of the pressure difference with elevation, this vaporization occurs predominantly in the elevated parts of the system.
When the flow is sufficiently decelerated the pressure recovers from vapor pressure and the vapor cavities begin to condense. The implosion of these cavities in the elevated sections leads to collision of two fluid columns. These collisions cause large amplitude pressure waves that travel through the system.
These pressure waves cause large unbalanced forces in between elbow-pairs of the piping system. The dynamic stress calculations showed that without modification these forces would lead to excessive stresses which did not conform with the design code.
The solution proposed by DRG was to reduce the speed at which the liquid columns collide with each other. This could be achieved by activing the closure of a downstream valve shortly after the emergency shut down valve was closed. This solution eliminated the large unbalanced forces and the stresses arising within the system conforming with the limits from the design code.