Multi-phase slug flow forces in pipelines at the Kuparuk production facilities have caused fatigue damage to piping components and vessel nozzles. A program was implemented to modify the existing inlet systems to accommodate slug forces for current and forecast flow conditions. Data acquisition systems were installed to quantify the effect of modifications on vessel stresses. The systems are also providing justification to avoid more costly modifications while allowing continuous improvement of operating parameters in the field.
Multi-phase slug flow, specifically two-phase flow, has been evident in piping systems at the Kuparuk River Unit, North Slope, Alaska since shortly after startup in 1981. The forces associated with slug flow have caused excessive movement of partially restrained piping. Recently, the magnitude and number of stress reversals has caused fatigue cracking in piping branch connections and a pressure vessel nozzle. In early 1991 a program was initiated to accelerate research on the impacts of slug flow on the operating facilities and concurrently develop design strategies to mitigate undesirable effects on piping, vessels, and support systems.
Two-phase flow, as discussed in this paper, is movement of a product which consists of both a liquid and gas. Under stratified conditions the flow in a pipeline is well-behaved, with liquid flowing at one velocity in the bottom of the line and gas flowing at a higher velocity above the liquid. Elevation changes or other sources of flow disruption in the pipeline can cause liquid holdup, or blockage of the gas flow by liquid "plugs". The higher velocity gas behind the liquid accelerates the liquid plug and a "slug" results.
Slugs have varying lengths, densities, and configurations. At each change in direction of flow a resultant force must be resisted by the piping and pipe support system. Unrestrained elbows and tees in piping networks are subject to deformations and cyclic stress. Similarly vessel nozzles and internals are subject to cyclic stress and fatigue resulting from the dynamic fluid forces.
The 1991-92 program incorporated design and retrofit of pipe support restraint systems, vessel nozzle modifications, piping reinforcement, and limited pipe replacement. Design work was undertaken at three production facilities to control undesirable movement and stresses in the inlet systems. The construction work was accomplished with a minimum of facility downtime and cost impact to operating facilities.
The normal sequence of events in a design project begins with the definition of design loads, proceeds to analysis, and is completed after several iterations of analysis/design. This project incorporated a unique design sequence and an integrated analysis approach for facility modifications. As the analysis and design matured, data collected from realtime pressure and strain monitoring systems was used to quantify and confirm design loads and thus qualify retrofit designs.
An integral part of the program was the implementation of data acquisition systems at each production facility to monitor pressure and strain. Strain is monitored at critical locations in the inlet system and dynamic pressure is monitored at the flow lines upstream of the inlet header.