Slug Flow Analysis Using Dynamic Spectrum Method in Caesar II

For dynamic analysis Caesar II software provides a very nice module, dynamic module where we have to simply provide the input parameters to get the output result. Before you start the dynamic analysis you have to perform conventional static analysis of the system (without using any slug force) and qualify the system from all criteria. To open the dynamic module in Caesar II click on dynamic analysis button

When you click on the dynamic analysis button following window (Fig.2) will open. Select Slug Flow (Spectrum) from drop down menu. The window will be filled with some pre-existing data. For clarity simply select all those and delete. Now we have to provide inputs for analysis

During dynamic analysis our first input will be the generation of spectrum profile. Slug load is one type of impulse load. So the magnitude of load varies from zero to some maximum value, remains constant for a time and then reduces to zero again. The force profile can be represented by a curve

So from the profile it is clear that in addition to slug force (Refer Static method of Slug Flow using Caesar II for calculation of slug force), we need to calculate two additional parameters, a) Slug Duration and b) Slug Periodicity.

Slug Duration: Slug duration is defined as the time required for the slug to cross the elbow. Mathematically it can be denoted as, Slug Duration=Length of Liquid Slug/Velocity of Flow.
Slug Periodicity: Slug Periodicity can be defined as the time interval for two consecutive slugs hitting the same elbow. So mathematically it can be denoted as, Slug Periodicity = (Length of Liquid Slug + Length of Gas Slug)/Velocity of Flow.

Let’s assume that the calculated slug duration is 8 milliseconds and periodicity is 400 milliseconds as shown in Fig. 3. We will use these data for generation of spectrum profile.

When you click on Enter Pulse data it will open the window where we have to enter the data for spectrum profile generation. From the above curve at time 0 the force is 2120 N the same force will be active for next 8 milliseconds till the slug crosses the elbow. Then at time 8.1 forces will be reduced to zero. And the same zero force will be there till 400 milliseconds. Then the next cycle will start. i.e, at time 400.1 seconds the force will be again 2120 N. That way enter data for at least two cycles

Clicking Save / Continue button will convert the time history into its equivalent force response spectrum in terms of Dynamic Load Factor versus Frequency and the screen “Spectrum Table Values “as shown in Fig. 5 will appear.
Be sure to specify a unique spectrum name, as this processor will overwrite any existing files of the same name.
By clicking OK, the processor will load the appropriate data in the Spectrum Definitions tab in Dynamic Input and move the data to the dynamic input

Once the spectrum profile is generated click on force sets button and enter the slug force with proper direction in the fields

Click on the + button to add more rows and – button to delete rows.
In force set field input a numeric id which will be used to construct dynamic load cases.

After that click on Spectrum load cases menu and create the required load cases for dynamic analysis. You have to specify at least two load cases as shown.