A steel pipeline was under construction for offloading naphtha from ships arriving at a jetty. A surge analysis was conducted to evaluate the pipeline’s capability under various transient upset scenarios. Following this, a dynamic stress and flexibility study was performed based on the calculated pressure surges.
Analysis
A transient one-dimensional fluid model of the entire pipeline, from the loading arm to the storage tank, was created. BOSfluids, a software package designed to model one-dimensional fluid transients, was utilized to simulate and analyze different transient upset scenarios. From this fluid model, the transient unbalanced forces on the piping were extracted and applied to a piping model in the dynamic time-history solver of CAESAR II. For the dynamic analysis, a detailed stress model was developed, incorporating the steel bridge structures to accurately represent their stiffness.
The dynamic stresses, displacements, and support reactions caused by various transient upset conditions were calculated and assessed.
Results
The surge analysis results indicated that the most critical transient scenario was the rapid closure of the emergency valve located in the loading arm. In this scenario, the pressure downstream of the closed emergency valve dropped rapidly to vapor pressure level. This pressure decrease resulted in the vaporization of some downstream fluid, occurring predominantly in the elevated parts of the system due to pressure differences with elevation.
By coupling the closure of different valves in the system, it was possible to reduce the amplitude of the pressure surges. When the flow was sufficiently decelerated, the pressure recovered from vapor pressure, and the vapor cavities began to condense. The implosion of these cavities in the elevated sections led to the collision of two fluid columns, causing large amplitude pressure waves that traveled through the system.
These pressure waves generated significant unbalanced forces between elbow-pairs of the piping system. The dynamic stress calculations revealed that, without modification, these forces would lead to excessive stresses, which did not conform to the design code. The solution proposed by DRG was to reduce the speed at which the liquid columns collided with each other. This could be achieved by activating the closure of a downstream valve shortly after the emergency shutdown valve was closed. This solution eliminated the large unbalanced forces and the resulting stresses within the system, ensuring compliance with the design code limits.
