Many mature oil fields use compressed gas to force the oil out of underground oil bearing strata; the gas is compressed using reciprocating piston gas compressors. The gas flows through a scrubber to remove liquids out of the gas before it enters the gas compressors and immediately before the gas compressors, the gas flows through a pulsation damper to reduce the gas velocity and smooth out the flow.
Whittaker Engineering was asked to design and manufacture nozzles to fit inside the damper to ensure that no liquids made their way through the damper outlets into the compressors. A drain was then fitted with an outlet pipe to a collection bottle to gather any liquids found, as liquids in a gas compressor can have serious effects to the mechanical parts of a compressor.
Analysis
IDAC was contracted to verify and modify the design of the outlet nozzles by taking all the output data from the compressors and simulating the gas flow using CFD techniques. The work was carried out in two parts as follows:
- CFD analysis on the existing design of the gas damper
- CFD analysis on the new design of the gas damper
The geometry of the gas damper comprises of a cylinder with two outlets at the bottom and one inlet at the side. The geometric model was defeatured using ANSYS DesignModeler and then meshed using prisms and tetrahedral meshes. A boundary layer mesh, based on the global length scale Reynolds number was also placed on the walls. Using the volumetric mesh, a transient solve was set up in CFX-Pre using a single-phase fluid, properties of which were specifically created to model the saturated gas.
The CFD domain was set up to run with the same atmospheric pressure as when in operating conditions, with pressure and mass flow boundary conditions defined at the outlets and inlet. Other details pertinent to the analysis are listed below:
- Using CFX Expression language, outlets were kept 90° out of phase
- A specific material was created for the saturated gas, which included molecular weight, density and outlet 1 and 2 properties
- No buoyancy effects were studied
The original design showed unequal velocities and mass flow rates through each outlet. The flow was biased towards outlet 1, rather than making it all the way to outlet 2. In order to force the flow onto Outlet 2, the geometry was changed, such that each outlet protruded further into the cylinder.
On analysing the new model, it was found that both outlets were performing better in terms of ‘sharing’ the total fluidic loading. When each outlet is opened to its maximum flow rate condition, both situations show similar mass flow rate values exiting the gas damper.
CFD analysis
The down time of the compressors had direct financial implications. To reach a quick and effective solution, it was decided to conduct a CFD analysis, as several models could be created, edited and run at the same time.
Once the basic model had been established and verified, changing the geometry marginally, while keeping all other parameters constant meant very little time was needed for each different set-up. More time was dedicated to analysis solving and post processing. The client also gained further understanding into some of the other issues surrounding the old design, such as unequal mass flow rates between the two outlets.
With both outlets performing equally in the new design, the compressors downstream of the gas damper were also being stressed equally. Having met the design criteria, the redesigned gas damper is once again operational onboard a vessel in the North Sea.
Founded in 1983, the last 25 years has seen Whittaker Engineering develop into a respected and reliable contributor to the North Sea oil industry. Whittaker Engineering provides in-house manufacturing, engineering design and analysis for the offshore oil and gas industry.