Preventing potential failure in pipework due to fatigue induced vibration provoked by slug flow is a critical part of detailed engineering design phase as these types of failures have been evidenced across the industry, and particularly in the oil and gas sector leading to significant incidents.
Slug forces are generated at change of direction of piping systems (i.e: elbows, tees, branches, and laterals) due to change of momentum of the fluid in two-phase flow regimes; The order of magnitude of such slug forces depend on the process stream properties, and in the relation between the two phases liquid and gas converging into the same pipework.
In order to address this concern, the piping designer typical approach is to conduct the static equivalent method where dynamic loads are converted to static loads in the piping flexibility analysis. In this approach, the designer estimates the slug force by selecting the most conservative combination of both variables, density and velocity of the slug, which is multiplied by the internal sectional area of the pipe, where the slug load is expected to occur. The resulting slug force is then typically multiplied by 1.5 to 2.0 in order to take into account the dynamic load factor.
The resulting slug forces are applied at all the affected changes of directions as a constant force in the piping flexibility analysis. While this approach is routinely followed as it is typically conservative from the piping flexibility static load cases perspective, it does not simulate the real dynamic conditions of the piping system, as it does not consider the influence of the slug forces occurring as a function of time, wherein reality, not all the slug forces are occurring at once at any given time, but occurring sequentially as the fluid travels through the length of the pipe, impacting the elbows in a progressive order, which directly dependents on the velocity of the fluid and the estimated length of the slug.
Therefore, this approach may not be reliable to evaluate existing systems with high vibration due to the high level of confidence required before proceeding to perform field modifications.
This paper describe over a real case example, an alternative methodology which allowed to determine the dynamic interaction of the slugging forces by utilizing the Time-History assisted by Response-Spectrum, both available within common piping flexibility analysis software.
The methodology allowed to resolve the piping vibration problem on subject by re-designing the associated spring and supports, without reducing the gas treatment plant production rates nor shutting down the facility.
The original design and modification of NPS 30 piping inlet to a Rich Amine Column is discussed along with results of both flexibility analysis and field vibration measurements before and after the modification.